Search

Temperature affects the timing of the transformation of amorphous silica (opal-A) into crystalline (opal-CT) exponentially. Thus, in hot subseafloor environments opal-A is expected to convert into opal-CT at relatively shallow burial depths, where in situ temperatures do not exceed ∼56 °C, as it has been previously observed at various deep-sea sites and in onshore rock outcrops as well as assessed during lab experiments. The response of biosilica (biogenic opal-A) diagenesis to steep geothermal gradients (∼224–529 °C/km) at extremely high sedimentation rates (∼1 m/kyr) was examined in cores from off-axis boreholes drilled by the International Ocean Discovery Program (IODP) Expedition 385 in the actively spreading, intrusive sill-riddled Guaymas Basin at the Gulf of California (Mexico) rifted margin. At three sites drilled by IODP Expedition 385 (U1545, U1546, and U1547), the conversion from amorphous opal (−A) to crystalline opal (−CT) occurs in relatively deep (up to ∼330 mbsf) and unexpectedly hot (in situ temperatures of ∼74–79 °C) subseafloor conditions. This observation indicates a significantly slower reaction kinetics of biosilica transformation than previously reported. A compilation of empirical data that include biosiliceous basins with a similarly hot subseafloor (Sea of Japan and Bering Sea) yield new kinetic parameters that account for the slower rates of silica transformation. Thus, current kinetic models for the prediction of opal-A to −CT conversion face limitations when burial rates exceed those typical of biogenic sedimentation in open-ocean conditions. At Guaymas Basin Site U1545, where there is no evidence of sill-related metamorphic overprint, the d-spacing of the opal-CT (1 0 1) peak correlates linearly with in situ temperature between ∼75 and 110 °C throughout the opal-CT zone, thus, providing a local silica paleothermometry proxy that can be used to calculate the maximum temperature to which opal-CT sediment has been subjected. [ABSTRACT FROM AUTHOR]

Carbonate archives record a brief snapshot of the ambient Earth's surface conditions at their deposition. However, the geologically reasonable extraction and interpretation of geochemical proxy data from ancient, diagenetically altered rock archives is fraught with problems. Three issues stand out: the dichotomy between petrographic and geochemical alteration; the lack of quantitative age constraints for specific diagenetic phases resulting in a poorly constrained admixture of local, basin-wide and over-regional (far-field) features; and an often insufficient understanding of the temperatures and compositions of diagenetic fluids. Here, the archive of Devonian marine limestones exposed to multiple far-field diagenetic events is used as an example to explore the above-listed issues. Methods applied include petrography, micro XRF, fluid inclusion data, clumped isotopes, δ13C and δ18O isotopes, 87Sr/86Sr ratios and quartz trace element data. Devonian limestones studied here were overprinted by two cross-cutting regional fault zones (T ≈ 230 °C) by multiple events between the Variscan Orogeny and the late Paleogene. The following processes are recorded: (i) protolith deposition and partial dolomitisation during rapid burial in the Middle/Late Devonian (T ≈ 180 °C); (ii) deep burial to ca 6.5 km and tectonic/hydrothermal overprint during the Variscan Orogeny in the Carboniferous (T ≈ 90–230 °C); (iii) rapid uplift to 1–2 km burial depth at the end of the Variscan Orogeny and hypogene karstification (T ≈ 50 to 100 °C) initiated by regional geology in the Permian/Triassic; (iv) tectonic/hydrothermal overprint during the opening of the Proto-Atlantic Ocean between the Early Jurassic and the Early Cretaceous (T ≈ 50 to 130 °C); (v) tectonic/hydrothermal overprint including renewed hypogene karstification and hydrothermal calcite cement precipitation (T ≈ 50 to 180 °C) during Alpine Orogeny between the Late Cretaceous and late Paleogene. Despite this complex series of diagenetic events, the protolith limestones largely preserved their respective Middle/Late Devonian dissolved inorganic carbon (DIC) and 87Sr/86Sr signatures. This study documents that geochemical proxy data, placed into their petrographic, paleotemperature, and local to over-regional context, significantly increases the ability to extract quantitative information from ancient carbonate rock archives. Research shown here has wider relevance for carbonate archive research in general. [ABSTRACT FROM AUTHOR]

The Eocene‐Oligocene transition (EOT) (∼34 Ma) is marked by the rapid development of a semi‐permanent Antarctic ice‐sheet, as indicated by ice‐rafted debris and a 1–1.5‰ increase in deep sea δ18O. Proxy reconstructions indicate a drop in atmospheric CO2 and global cooling. How these changes affected surface ocean temperatures in the North Atlantic and ocean water stratification remains poorly constrained. In this study, we apply clumped‐isotope thermometry to well‐preserved planktonic foraminifera, that are associated with lower mixed‐layer to subthermocline dwelling depths from the drift sediments at international ocean discovery program Site 1411, Newfoundland, across four intervals bracketing the EOT. The thermocline/lower mixed‐layer dwelling foraminifera record a cooling of 1.9 ± 3.5 K (mean ± 95% CI) across the EOT. While the cooling amplitude is similar to previous sea surface temperature (SST) reconstructions, absolute temperatures (Eocene 20.0 ± 2.9°C, Oligocene 18.0 ± 2.2°C) appear colder than previous organic proxy reconstructions for the northernmost Atlantic extrapolated to this location. We discuss seasonal bias, recording depth, and appropriate consideration of paleolatitudes, all of which complicate the comparison between SST reconstructions and model output. Our subthermocline dwelling foraminifera record a larger cooling across the EOT (Eocene 19.0 ± 3.5°C, Oligocene 13.0 ± 3.2°C, cooling of 5.5 ± 4.6 K) than foraminifera from the thermocline/lower mixed‐layer, consistent with global cooling and an increase in ocean stratification which may be related to the onset or intensification of the Atlantic meridional overturning circulation. Plain Language Summary: During the Eocene temperatures on Earth were much warmer than today. It is generally believed that the Antarctic ice‐sheet first developed around 34 million years ago, during the Eocene‐Oligocene transition (EOT). How this change occurred is still widely debated, but it was probably caused by decreased CO2 levels and changes to heat distribution through ocean currents. Here, we study how water temperatures at the dwelling depths of planktonic microfossils in the Atlantic Ocean changed across this event. We use clumped isotopes—a way of reconstructing the temperature from fossil shells. We measured shells of planktonic foraminifera, single‐celled microscopic plankton, that lived at two different depth levels in the upper ocean. We find that the foraminifera with a shallower (thermocline/lower mixed‐layer) water dwelling depth in the North Atlantic Ocean cooled by about ∼1.9 K, while those that dwelled deeper cooled by ∼5.5 K across the EOT. The thermocline/lower mixed‐layer cooling is similar in magnitude to reconstructions from organic biomarkers. However, our reconstructed absolute temperatures are colder than previous estimates. We think that our deeper water temperature reconstructions reflect global cooling, while thermocline/lower mixed‐layer temperatures did not cool as much because a warm water current developed, similar to the Gulf Stream. Key Points: Clumped isotopes from well‐preserved planktonic foraminifera imply 1.9 K lower mixed‐layer/thermocline cooling across Eocene‐Oligocene transition (EOT) in North AtlanticOur thermocline temperatures for Eocene (20°C) and Oligocene (18°C) are cooler than organic proxy sea surface temperature estimatesIncreased subthermocline cooling compared to the thermocline indicates increased stratification across the EOT, hinting at intensified Atlantic meridional overturning circulation [ABSTRACT FROM AUTHOR]

The spatial and temporal distribution of Alpine snow is a sensitive gauge of environmental change. While understanding past snow dynamics is essential for reconstructing past climate and forecasting future trends, reliable snowpack data prior to the instrumental record are scarce. We present a novel pairing of cosmogenic paleothermometry and luminescence photochronometry which constrain the temperature and insolation history of bedrock outcrops at the Gotthard Pass, Switzerland, over the last ∼15,000 years. By coupling these results with cosmogenic 14C‐10Be chronology and modern in situ rock thermometry, we infer a ∼70‐day reduction of snowpack at the topographic mid‐slope. Our data indicate stable environmental conditions throughout the Holocene, followed by a 6.6 ± 2.9°C increase of ground surface temperature, coeval with an order‐of‐magnitude or more increase in ground surface insolation. Bracketing the onset of these changes between 1504 and 1807 CE, our findings tie the snowpack decline with the onset of human industrialization. Plain Language Summary: The extent of snow cover is shrinking in high elevation mountain environments due to climate change. However, it is challenging to determine when snow cover began to change because humans have only been monitoring snow cover for several decades, and snow cover has appeared to shrink over this entire time. We used two new geologic records of ground temperature and light exposure—both of which are impacted by the duration and amount of snow cover—to assess when snow cover began to change in the Gotthard Pass in Switzerland. We found that the middle slopes of the pass began experiencing snow cover loss three centuries ago, well before historical monitoring of snow cover in this region. This record of prolonged snow cover loss matters for the informed management of mountain water sources, rock and snow avalanche risk, and ecosystem change. Key Points: We present a novel data set of cosmogenic paleothermometry and luminescence photochronometry at the Gotthard Pass, SwitzerlandStable Holocene environmental conditions are followed by increases in ground temperature and insolation caused by snow cover declineThese environmental changes began between 1504 and 1807 CE, tying the observed snowpack decline with the onset of human industrialization [ABSTRACT FROM AUTHOR]

Analyzing the triple oxygen isotope (Δ′17O) composition of carbonates and air CO2 can provide valuable information about Earth system processes. However, accurately measuring the abundance of the rare 17O‐bearing CO2 isotopologue using isotope ratio mass spectrometry presents significant challenges. Consequently, alternative approaches, such as laser spectroscopy, have been developed. Here, we describe an adaptable dual inlet system for a tunable infrared laser direct absorption spectrometer (TILDAS) that maintains stable instrumental conditions for subsequent sample and reference measurements. We report ∆′17O measurements on three types of samples: reference CO2, CO2 derived from the acid digestion of carbonates, and air CO2. The external repeatability (±1σ) for reference‐sample‐reference bracketing measurements is generally better than ±10 ppm, close to the average internal error of ±6 ppm. Our results demonstrate that laser spectroscopy is a capable technique for measuring triple oxygen isotopes with the precision required to resolve variations in the ∆′17O values of air CO2 and to use the ∆′17O of carbonates for paleothermometry. Plain Language Summary: Tunable infrared laser direct absorption spectroscopy (TILDAS) is a technique that determines isotopologue abundances by shining a laser through the analyzed gas and measuring the amount of light absorbed. Laser spectroscopy is emerging as a competitive alternative to the more common isotope ratio mass spectrometry due to its ability to provide rapid and precise measurements of rare isotope ratios directly on CO2 gas. We present a cost‐effective dual inlet system for TILDAS to measure the triple oxygen isotope composition of carbonates and air CO2. Our setup maintains stable measurement conditions, for example, instrumental temperature, gas composition, pressure, and achieves external repeatability generally below ±10 ppm. Triple oxygen isotope measurements with such precision enable resolving variations in the 17O‐anomaly of air CO2 and using the ∆′17O values of carbonates for paleoenvironmental applications. Key Points: An inlet system using inexpensive hardware and open‐source software was developed for tunable infrared laser absorption spectroscopySpectroscopic measurements of ∆′17O in CO2 can be negatively influenced by variable temperatures, uneven CO2 mixing, which were addressedThe external standard deviation of repeated ∆′17O measurements of CO2 from carbonates and air was better than 10 ppm [ABSTRACT FROM AUTHOR]

Abstract Analyzing the triple oxygen isotope (Δ′17O) composition of carbonates and air CO2 can provide valuable information about Earth system processes. However, accurately measuring the abundance of the rare 17O‐bearing CO2 isotopologue using isotope ratio mass spectrometry presents significant challenges. Consequently, alternative approaches, such as laser spectroscopy, have been developed. Here, we describe an adaptable dual inlet system for a tunable infrared laser direct absorption spectrometer (TILDAS) that maintains stable instrumental conditions for subsequent sample and reference measurements. We report ∆′17O measurements on three types of samples: reference CO2, CO2 derived from the acid digestion of carbonates, and air CO2. The external repeatability (±1σ) for reference‐sample‐reference bracketing measurements is generally better than ±10 ppm, close to the average internal error of ±6 ppm. Our results demonstrate that laser spectroscopy is a capable technique for measuring triple oxygen isotopes with the precision required to resolve variations in the ∆′17O values of air CO2 and to use the ∆′17O of carbonates for paleothermometry.

The Mg/Ca ratio of foraminiferal calcite is a widely used empirical proxy for ocean temperature. Foraminiferal Mg/Ca‐temperature relationships are based on extant species and are species‐specific, introducing uncertainty when applying them to the fossil tests of extinct groups. Many modern species show remarkable heterogeneity in their intra‐test Mg distributions, typically due to the presence of high Mg bands, which have a biological origin. Importantly, banding patterns differ between species, which could affect Mg/Ca‐temperature relationships. Few studies have looked at intra‐test variability in Mg/Ca ratios in extinct species of foraminifera, despite the obvious implications for paleothermometry. We used electron probe microanalysis (EPMA) to investigate intra‐test Mg distributions in the fossil tests of two species of planktonic foraminifera from the extinct muricate mixed‐layer‐dwelling genus Morozovella, commonly used in Paleogene sea surface temperature reconstructions. Both M. aragonensis and M. crater show striking Mg banding patterns with multiple high and low Mg/Ca band pairs throughout the test wall in all chambers. The intra‐test Mg variability in M. aragonensis and M. crater is similar to that in modern species widely used in paleoclimate reconstructions and banding patterns are consistent with published growth models for modern forms, albeit with subtle differences. The presence of Mg bands supports the application of Mg/Ca‐palaeothermometry in extinct Morozovella species as well as the utility of EPMA for examining preservation of foraminifera tests in paleoclimatological studies. However, we emphasize the importance of rigorous assessments of inter‐ and intra‐test Mg variability when using microanalytical techniques for foraminiferal Mg/Ca paleothermometry. Plain Language Summary: It is widely accepted that the incorporation of Mg into the calcium carbonate shells ("tests") of foraminifera (single‐celled marine organisms) is dependent on temperature. As such, measurement of Mg/Ca ratios in fossil tests from deep‐sea sediments has become an important working tool for reconstructing past seawater temperatures. However, Mg/Ca‐temperature relationships are based on modern foraminifera and are species‐specific, which introduces uncertainty when we apply them to extinct groups. Furthermore, modern foraminifera tend to have distinct high Mg bands in their test walls and these bands are biologically controlled, with differences in banding patterns between species. Few studies have compared banding patterns in extinct foraminifera species with those in modern forms, despite the potential implications for the accuracy and precision of Mg/Ca paleotemperature reconstructions. We used electron probe microanalysis to investigate the distribution of Mg within the well‐preserved fossil tests of two species belonging to the extinct mixed layer dwelling genus, Morozovella, which is commonly used in Paleogene (∼66–23 million years ago) ocean temperature reconstructions. Our results show striking Mg banding patterns in M. aragonensis and M. crater similar to those described in modern species. Overall, our data support the use of these extinct species in Mg/Ca paleothermometry. Key Points: Alternating bands of high and low Mg/Ca calcite are found throughout the tests of Morozovella aragonensis and M. craterIntra‐test Mg distributions in these species are similar to those in modern species upon which Mg/Ca‐temperature calibrations are basedOur results support the use of EPMA in evaluating the preservation of fossil foraminifera [ABSTRACT FROM AUTHOR]

In this study, we have investigated the oxygen isotope compositions (δ17O and δ18O) of modern rain and ice cores using published isotopic data. We find that, contrary to existing interpretations, precipitation δ17O is influenced by two factors: mass-dependent fractionation (MDF), which occurs during ocean evaporation, and mass-independent fractionation (MIF), which happens in the stratosphere. The MDF contribution remains constant and can be understood by studying tropical rain, as the overall movement of mass in the tropics is upward toward the stratosphere. On the other hand, the MIF effect comes from the mixing of stratospheric air in the troposphere, which is a result of the Brewer–Dobson circulation. This MIF effect on precipitation 17O increases from the tropics toward the poles and is observed consistently in modern precipitation and ice cores. The relative δ17O and δ18O composition, denoted as ∆'17O, in modern precipitation can be calibrated with surface air temperature, creating a new and independent tool for estimating past temperatures. We used this calibration along with the ∆'17O of Antarctic and Greenland ice cores, and our reconstructed past temperatures are in excellent agreement with those derived from borehole thermometry or gas phase analysis of air trapped in the ice. The ∆'17O method overcomes the problems associated with using δ18O alone for paleothermometry. Our findings align with climate models that suggest a weakening of the Brewer–Dobson circulation during the Last Glacial Maximum. Furthermore, our approach could be used to monitor future changes in stratosphere–troposphere mass exchange in response to a warming climate caused by increasing greenhouse gases. [ABSTRACT FROM AUTHOR]

Close relationship exists between the Mg/Ca ratio in foraminiferal shells and the calcification temperature. Even when most paleoceanographic studies focused on near-surface property reconstructions leaving the deep ocean less explored, during the last years, new Mg/Ca-temperature calibrations were established for the benthic genera Uvigerina and Cibicidoides. Among the main conclusions, it has been established that secondary factors exist affecting the Mg incorporation into the benthic shells, particularly at low temperatures. In order to establish a potential relationship between the Mg/Ca ratios of the aforementioned genera for the western South Atlantic and tropical Atlantic, 38 core-top samples are analyzed here. Our results confirm the existence of other parameters than just temperature affecting Cibicidoides and Uvigerina Mg/Ca, suggesting that the effect of all these other parameters together produces a variable "offset" between both genera that can be linearly approached. We develop an algorithm that allows transforming the Mg/Ca data measured on one of these two genera into the other. This not only would allow performing a continuous Mg/Ca record independent of which benthic genera is available for study but also potentially leads to new and more accurate Mg/Ca-temperature calibrations allowing to get more, high-resolution bottom water temperature reconstructions. [ABSTRACT FROM AUTHOR]

How hot can we expect Greenhouse Earth to have been in the past and the future? This is a question that deep‐time paleoclimatologists have long investigated. In a new paper by Fetrow et al. (2022), https://doi.org/10.1029/2022PA004517, deconvolving the depositional environments and burial history of continental carbonates deposited during known greenhouse climates, such as the Cretaceous, highlight the way Earth scientists can embark on understanding the clumped isotope derived temperatures and properly interpreting such temperatures. Plain Language Summary: Over the last 15+ years, the development of a new geochemical analysis (clumped isotope paleothermometry) to determine temperatures from ancient rocks has allowed Earth scientists to investigate the range of Earth surface temperatures in ancient climate conditions (both greenhouse and icehouse). Earth scientists are uniquely positioned to examine in detail the history of rocks to better understand Earth's past climates so that we may better understand the potential temperature extremes of the future. In this light, a new study by Fetrow et al. (2022), https://doi.org/10.1029/2022PA004517 demonstrates the kinds of investigation needed to interpret temperature estimates from such rocks. Key Points: Investigations of past greenhouse climates can inform future climate conditions including just how hot Earth surface temperatures can getA new study from Fetrow et al. (2022), https://doi.org/10.1029/2022PA004517 demonstrates the steps needed to properly interpret paleotemperatures from clumped isotope analysesEvaluating the depositional environment, burial history, petrography, and traditional stable isotopes is necessary to interpret temperature [ABSTRACT FROM AUTHOR]

The oxygen isotopic composition of benthic foraminiferal tests (δ18O b) is one of the pre-eminent tools for correlating marine sediments and interpreting past terrestrial ice volume and deep-ocean temperatures. Despite the prevalence of δ18 O b applications to marine sediment cores over the Quaternary, its use is limited in the Arctic Ocean because of low benthic foraminiferal abundances, challenges with constructing independent sediment core age models, and an apparent muted amplitude of Arctic δ18O b variability compared to open-ocean records. Here we evaluate the controls on Arctic δ18O b by using ostracode Mg/Ca paleothermometry to generate a composite record of the δ18O of seawater (δ18O sw) from 12 sediment cores in the intermediate to deep Arctic Ocean (700–2700 m) that covers the last 600 kyr based on biostratigraphy and orbitally tuned age models. Results show that Arctic δ18O b was generally higher than open-ocean δ18 O b during interglacials but was generally equivalent to global reference records during glacial periods. The reduced glacial–interglacial Arctic δ18O b range resulted in part from the opposing effect of temperature, with intermediate to deep Arctic warming during glacials counteracting the whole-ocean δ18O sw increase from expanded terrestrial ice sheets. After removing the temperature effect from δ18O b , we find that the intermediate to deep Arctic experienced large (≥1 ‰) variations in local δ18O sw , with generally higher local δ18O sw during interglacials and lower δ18O sw during glacials. Both the magnitude and timing of low local δ18O sw intervals are inconsistent with the recent proposal of freshwater intervals in the Arctic Ocean during past glaciations. Instead, we suggest that lower local δ18O sw in the intermediate to deep Arctic Ocean during glaciations reflected weaker upper-ocean stratification and more efficient transport of low- δ18O sw Arctic surface waters to depth by mixing and/or brine rejection. [ABSTRACT FROM AUTHOR]

The genus Celtis includes widespread trees that produce drupes with aragonite endocarps, or "hackberries." These carbonate endocarps are preserved in the fossil record, often in cave deposits or packrat middens, and thus are targets for paleoclimate reconstructions. Stable oxygen isotopes in Celtis endocarps have been used as proxies for oxygen isotopic composition of past stream water and for paleothermometry. Here, we explore the suitability of hackberry carbonates for paleoclimate reconstructions based on carbonate clumped‐isotope thermometry. We sampled modern hackberries grown at sites across North America (n = 37) for stable and clumped isotope analyses. Measured clumped‐isotope temperatures are found to be within the range of measured local modern growing season surface temperatures and typically in dual clumped‐isotope equilibrium. As such, we propose that hackberry clumped‐isotope measurements can be used to reconstruct past Earth‐surface air temperatures. Plain Language Summary: Hackberries are tree‐growing fruits that create carbonate mineral structures surrounding their seeds. Geochemical properties of these minerals can give us insight into growing conditions, particularly the growing season temperature of the hackberry plants. We tested the promise and limitations of clumped isotope as a paleothermometer for carbonate growth temperatures using modern hackberries collected across North America. We find recorded clumped‐isotope temperatures are consistent with mean growing season temperatures. We propose the use of hackberry carbonate clumped isotope measurements to reconstruct past environmental temperatures using ancient hackberry deposits found in caves (formed by humans) and middens (formed by animals such as packrats) throughout the past 10,000+ years. Key Points: Hackberry plants create an aragonitic mineral endocarp within their fruit which reflects environmental conditionsClumped‐isotope analyses reconstruct temperatures comparable to mean growing season temperatures in modern hackberry endocarpsClumped isotopes in hackberries provide a potential paleothermometer for Quaternary‐aged cave deposits and packrat middens [ABSTRACT FROM AUTHOR]

Chars are useful proxies in many disciplines such as the geosciences and archaeology. Raman spectrometry is frequently used in the characterization of such materials. However, when they are ancient and potentially altered, chars have the particularity of being oxidized, which can strongly disturb the spectra and seriously distort their interpretation. In this study, we highlight the Raman parameters affected by oxidative weathering based on comparisons between the spectra of experimental and ancient chars, of vegetal and animal origin. This makes it possible to differentiate between unweathered and weathered chars and we show that the recent methods of Raman thermometry and precursor discrimination are no longer valid when applied to oxidized chars. Nonetheless, it remains possible to estimate the formation temperature of a charcoal when some fragments are preserved at the micrometre scale. Furthermore, we propose an original tool that enable us to distinguish the vegetal or animal origin of ancient and oxidized chars. We successfully apply this new tool to charred hearth residues of the Bruniquel cave (France). [ABSTRACT FROM AUTHOR]

Ammonites are externally shelled cephalopods that were common in the North American Western Interior Seaway (WIS), and as they grew their aragonitic shells via accretion, they recorded aspects of their environment in the stable isotopic composition of their shells. While the mobility of ammonites may complicate efforts to reconstruct temperatures from their shells, they remain a potentially valuable target for paleothermometry. In this study, we reconstruct the spatial and temporal variability of WIS temperatures using a suite of ammonites (n = 113) spanning the last 25 million years of the Cretaceous along a North-South gradient ranging from the Canadian WIS to the Mississippi Embayment. We present a temporally high-resolution (∼0.6 Ma) oxygen isotope record from these ammonites that indicates cooling temperatures in the WIS of comparable magnitude (∼18 °C +/‐− 4°) to the temperature change seen in global studies, most notably cooling from the Cretaceous Thermal Maximum in the Turonian until the late Maastrichtian. Studies disagree regarding the development and strength of a latitudinal (pole-to-equator) temperature gradient during the Cretaceous; we do not see strong evidence for a latitudinal temperature gradient in the WIS. We do not observe any bias driven by ammonite morphology in our isotopic data, though we suggest that researchers consider the effects of taxonomy and ecological bias on their temperature records. As our ammonite δ18O record matches the direction and magnitude of global temperature reconstructions, our data imply that ammonites are viable targets for paleothermometry. • Measured δ18O from 113 Late Cretaceous ammonites from Western Interior Seaway. • Ammonite δ18Ο tracks global temperature reconstructions of Late Cretaceous cooling. • ∼18 °C cooling from Cretaceous Thermal Maximum to Cretaceous-Paleogene boundary. • No evidence for latitudinal temperature gradient observed in this record. • Ammonites are a viable target for isotope paleothermometry in the WIS. [ABSTRACT FROM AUTHOR]

Halite deposits have long been utilized for interrogating past climate conditions. Microthermometry on halite fluid inclusions has been used to determine ancient water temperatures. One notable obstacle in performing microthermometric measurements, however, is the lack of a vapor bubble in the single-phase liquid inclusions at room temperature. (Pseudo-) isochoric cooling of the inclusions to high negative pressures, far below the homogenization temperature, has commonly been needed to provoke spontaneous vapor bubble nucleation in the liquid. High internal tensile stress in soft host minerals like halite, however, may induce plastic deformation of the inclusion walls, resulting in a wide scatter of measured homogenization temperatures. Nucleation-assisted (NA) microthermometry, in contrast, employs single ultra-short laser pulses provided by a femtosecond laser to stimulate vapor bubble nucleation in metastable liquid inclusions slightly below the expected homogenization temperature. This technique allows for repeated vapor bubble nucleation in selected fluid inclusions without affecting the volumetric properties of the inclusions, and yields highly precise and accurate homogenization temperatures. In this study, we apply, for the first time, NA microthermometry to fluid inclusions in halite and we evaluate the precision and accuracy of this thermometer utilizing (i) synthetic halite crystals precipitated under controlled laboratory conditions, (ii) modern natural halite that precipitated in the 1980s in the Dead Sea, and (iii) Late Pleistocene halite samples from a sediment core from Death Valley, CA. Our results demonstrate an unprecedented accuracy and precision of the method that provides a new opportunity to reconstruct reliable quantitative temperature records from evaporite archives. • Fluid inclusions trapped in halite host crystals are valuable paleoclimate archives. • Primary halite inclusions act as physical thermometers at time of entrapment. • Nucleation-assisted microthermometry is precise and accurate in halite inclusions. • Formation temperatures are measured for Lab-grown, Dead Sea, & Death Valley halites. [ABSTRACT FROM AUTHOR]

The stable (δ18O and δ13C) and clumped (Δ 47) isotope compositions of coral carbonate are valuable archives for paleoclimate reconstructions. However, the Δ 47 -temperature relationships of warm and cold-water corals deviate from that of inorganic carbonate precipitated at equilibrium. Dual clumped isotope thermometry of carbonates (i.e., simultaneous Δ 47 and Δ 48 measurements on a single carbonate) has the potential to achieve more accurate paleotemperature reconstruction, identifying and correcting for kinetically driven isotopic disequilibrium. Here we present the first extensive dual clumped isotope dataset of coral carbonate, spanning a broad range of cold and warm-water coral species. We confirm that corals are enriched in Δ 47 and depleted in Δ 48 relative to equilibrium, a pattern corresponding to the mixing of an equilibrium DIC pool with kinetically derived HCO 3 – produced by hydration and hydroxylation of CO 2. Dual clumped isotope measurements of cold-water corals fall on the initial linear portion of model (IsoDIC) predicted departure from equilibrium. The dual clumped isotope composition of cold-water corals, corrected by the model-predicted Δ 47 /Δ 48 offset slope (−0.78), yield accurate reconstruction of coral growth temperature with a precision of <3 °C at the 68% confidence level. In contrast, disequilibrium offsets in the Δ 47 and Δ 48 of warm-water corals correspond to precipitation from a more equilibrated DIC pool, which we attribute to the action of carbonic anhydrase in the calcifying fluid. It may be possible to correct warm-water coral growth temperatures, using an empirically derived correction (Δ 47 / Δ 48 offset slope of −0.4). Dual clumped isotope thermometry of coral carbonate opens new possibilities to reconstruct both sea surface temperatures and ocean dynamics of intermediate to deep water masses. [ABSTRACT FROM AUTHOR]

The oxygen isotopic composition of benthic foraminiferal tests (δ 18Ob) is one of the preeminent tools for correlating marine sediments and interpreting past terrestrial ice volume and deep-ocean temperatures. Despite the prevalence of δ 18Ob applications to marine sediment cores over the Quaternary, its use is limited in the Arctic Ocean because of low benthic foraminiferal abundances, challenges with constructing independent sediment core age models, and an apparent muted amplitude of Arctic δ 18Ob variability compared to open ocean records. Here we evaluate the controls on Arctic δ 18Ob by using ostracode Mg/Ca paleothermometry to generate a composite record of the δ 18O of seawater (δ 18Osw) from fourteen sediment cores in the intermediate to deep Arctic Ocean (700–2700 m) covering the last 600 kyr. Results show that Arctic δ 18Ob was generally higher than open ocean δ 18Ob during interglacials but was generally equivalent to global reference records during glacial periods. The reduced glacial-interglacial Arctic δ 18Ob range resulted in part from the opposing effect of temperature, with intermediate-to-deep Arctic warming during glacials counteracting the whole-ocean δ 18Osw increase from expanded terrestrial ice sheets. After removing the temperature effect from δ 18Ob, we find that the intermediate-to-deep Arctic experienced large (≥ 1 ‰) variations in local δ 18Osw, with generally higher local δ 18Osw during interglacials and lower δ 18Osw during glacials. Both the magnitude and timing of low local δ 18Osw intervals are inconsistent with the recent proposal of freshwater intervals in the Arctic Ocean during past glaciations. Instead, we suggest that lower local δ 18Osw in the intermediate-to-deep Arctic Ocean during glaciations reflected weaker upper ocean stratification and more efficient transport of low- δ 18Osw Arctic surface waters to depth by mixing and/or brine rejection. [ABSTRACT FROM AUTHOR]

A late Carboniferous to early Permian carbonate and evaporite succession from the Finnmark Platform (southern Barents Sea) contains nodules of anhydrite partially to fully replaced by calcite spar and native sulfur genetically linked to hydrocarbon migration and/or oxidation, analogous to processes observed in salt diapir caprocks in the Gulf of Mexico. In situ U-Pb dating of this calcite spar therefore has the potential to directly date hydrocarbon migration and provide further insight into the geochemical and temperature conditions during this event when coupled with traditional stable and clumped isotope ratios (δ13C, δ18O, and Δ47). Results indicate calcite formed ca. 50-15 Ma, postdating host-rock deposition by 250-285 m.y. Strongly negative δ13C values in the calcite spar (mean = -15‰) are consistent with a major contribution of carbon from hydrocarbons, and Δ47 paleothermometry indicates a mean precipitation temperature of 46 ± 11 °C. These geochemical results are consistent with the local burial history and suggest protracted hydrocarbon migration and/or oxidation and caprock formation spanning ~35 m.y. [ABSTRACT FROM AUTHOR]

In this study, we have investigated the oxygen isotope compositions (δ17O and δ18O) of modern rain and ice cores using published isotopic data. We find that, contrary to existing interpretations, precipitation δ17O is influenced by two factors: mass-dependent fractionation (MDF), which occurs during ocean evaporation, and mass-independent fractionation (MIF), which happens in the stratosphere. The MDF contribution remains constant and can be understood by studying tropical rain, as the overall movement of mass in the tropics is upward toward the stratosphere. On the other hand, the MIF effect comes from the mixing of stratospheric air in the troposphere, which is a result of the Brewer–Dobson circulation. This MIF effect on precipitation 17O increases from the tropics toward the poles and is observed consistently in modern precipitation and ice cores. The relative δ17O and δ18O composition, denoted as ∆‘17O, in modern precipitation can be calibrated with surface air temperature, creating a new and independent tool for estimating past temperatures. We used this calibration along with the ∆‘17O of Antarctic and Greenland ice cores, and our reconstructed past temperatures are in excellent agreement with those derived from borehole thermometry or gas phase analysis of air trapped in the ice. The ∆‘17O method overcomes the problems associated with using δ18O alone for paleothermometry. Our findings align with climate models that suggest a weakening of the Brewer–Dobson circulation during the Last Glacial Maximum. Furthermore, our approach could be used to monitor future changes in stratosphere–troposphere mass exchange in response to a warming climate caused by increasing greenhouse gases.

The Pliocene sedimentary record provides a window into Earth's climate dynamics under warmer-than-present boundary conditions. However, the Pliocene cannot be considered a stable warm climate that constitutes a solid baseline for middle-of-the-road future climate projections. The increasing availability of time-continuous sedimentary archives (e.g., marine sediment cores) reveals complex temporal and spatial patterns of Pliocene ocean and climate variability on astronomical timescales. The Perth Basin is particularly interesting in that respect because it remains unclear if and how the Leeuwin Current sustained the comparably wet Pliocene climate in Western Australia, as well as how it influenced Southern Hemisphere paleoclimate variability. To constrain Leeuwin Current dynamics in time and space, this project obtained eight clumped-isotope Δ47 paleotemperatures and constructed a new orbitally resolved planktonic foraminifera (Trilobatus sacculifer) stable isotope record (δ18 O) for the Plio-Pleistocene (4–2 Ma) interval of International Ocean Discovery Program (IODP) Site U1459. These new data complement an existing TEX 86 record from the same site and similar planktonic isotope records from the Northern Carnarvon Basin (Ocean Drilling Program (ODP) Site 763 and IODP Site U1463). The comparison of TEX 86 and Δ47 paleothermometers reveals that TEX 86 likely reflects sea surface temperatures (SSTs) with a seasonal warm bias (23.8–28.9 ∘ C), whereas T. sacculifer Δ47 calcification temperatures probably echo mixed-layer temperatures at the studied Site U1459 (18.9–23.2 ∘ C). The isotopic δ18 O gradient along a 19–29 ∘ S latitudinal transect, between 3.9 and 2.2 Ma, displays large variability, ranging between 0.5 ‰ and 2.0 ‰. We use the latitudinal δ18 O gradient as a proxy for Leeuwin Current strength, with an inverse relationship between both. The new results challenge the interpretation that suggested a tectonic event in the Indonesian Throughflow as the cause for the rapid steepening of the isotopic gradient (0.9 ‰ to 1.5 ‰) around 3.7 Ma. The tectonic interpretation appears obsolete as it is now clear that the 3.7 Ma steepening of the isotopic gradient is intermittent, with flat latitudinal gradients (∼0.5 ‰) restored in the latest Pliocene (2.9–2.6 Ma). Still, the new analysis affirms that a combination of astronomical forcing of wind patterns and eustatic sea level controlled Leeuwin Current intensity. On secular timescales, a period of relatively weak Leeuwin Current is observed between 3.7 and 3.1 Ma. Notably, this interval is marked by cooler conditions throughout the Southern Hemisphere. In conclusion, the intensity of the Leeuwin Current and the latitudinal position of the subtropical front are both long-range effects of the same forcing: heat transport through the Indonesian Throughflow (ITF) valve and its propagation through Indian Ocean poleward heat transport. The common ITF forcing explains the observed coherence of Southern Hemisphere ocean and climate records. [ABSTRACT FROM AUTHOR]

Diffusion properties of cosmogenic 3He in quartz at Earth surface temperatures offer the potential to directly reconstruct the evolution of past in situ temperatures from formerly glaciated areas, which is important information for improving our understanding of glacier–climate interactions. In this study, we apply cosmogenic 3He paleothermometry to rock surfaces gradually exposed from the Last Glacial Maximum (LGM) to the Holocene period along two deglaciation profiles in the European Alps (Mont Blanc and Aar massifs). Laboratory experiments conducted on one representative sample per site indicate significant differences in 3He diffusion kinetics between the two sites, with quasi-linear Arrhenius behavior observed in quartz from the Mont Blanc site and complex Arrhenius behavior observed in quartz from the Aar site, which we interpret to indicate the presence of multiple diffusion domains (MDD). Assuming the same diffusion kinetics apply to all quartz samples along each profile, forward model simulations indicate that the cosmogenic 3He abundance in all the investigated samples should be at equilibrium with present-day temperature conditions. However, measured cosmogenic 3He concentrations in samples exposed since before the Holocene indicate an apparent 3He thermal signal significantly colder than today. This observed 3He thermal signal cannot be explained with a realistic post-LGM mean annual temperature evolution in the European Alps at the study sites. One hypothesis is that the diffusion kinetics and MDD model applied may not provide sufficiently accurate, quantitative paleo-temperature estimates in these samples; thus, while a pre-Holocene 3He thermal signal is indeed preserved in the quartz, the helium diffusivity would be lower at Alpine surface temperatures than our diffusion models predict. Alternatively, if the modeled helium diffusion kinetics is accurate, the observed 3He abundances may reflect a complex geomorphic and/or paleoclimatic evolution, with much more recent ground temperature changes associated with the degradation of alpine permafrost. [ABSTRACT FROM AUTHOR]

Goethite (α-FeOOH) contains two structurally-distinct oxygen sites that can potentially be independently interrogated for isotopic composition by analyzing bulk goethite δ18O (δ18O bulk) and the δ18O of water extracted from it (δ18O extracted). The isotopic contrast between these two components may form the basis of a paleothermometer (Miller et al., 2020). We sought to better understand the behavior of goethite undergoing vacuum dehydroxylation, the key step in this potential paleothermometric technique. A refined method for making these measurements applied to a suite of natural and synthetic goethites revealed that δ18O extracted is extremely sensitive to extraction temperature and to details of the vacuum environment in which the goethite is dehydroxylated. This sensitivity likely arises from the fundamental mechanism by which evolved water is removed from goethite in vacuum, via nm-wide pores penetrating nascent hematite surrounding dehydroxylating goethite. These pores may provide favorable conditions for back reaction that scrambles the oxygen isotopic composition, and also for kinetic mass fractionation. The character of these pores is known to vary with goethite composition, crystallinity, and extraction temperature. We identified additional sample-specific factors that affect δ18O extracted. For example, some synthetic goethites undergo dehydroxylation at surprisingly low temperature (<90 °C) in vacuum, under conditions usually used to remove nonstoichiometric water. In addition, some of the synthetic goethites have spectroscopic characteristics suggesting a range of OH bonding environments that complicate the idea that goethite has just two structurally (and potentially isotopically) distinct oxygen sites. Instead, in fine grained or poorly crystalline goethite there may be a continuum of physisorbed water, chemisorbed water, and stoichiometric water that is simultaneously released during vacuum extraction. After investigating a wide range of extraction conditions, we determined that rapid heating to 255 ± 5 °C is optimal for achieving high degrees of water extraction with the least apparent back reaction. When applied to synthetic goethites grown between 5 °C and 70 °C, we found no compelling correlation between synthesis temperature and the contrast between δ18O bulk and δ18O extracted. Analyses of two natural samples, known to have grown at ∼5 °C and ∼ 25 °C, revealed no significant difference in measured isotopic contrast. Together these observations suggest that goethite has no resolvable formation temperature information encoded in the bulk and extracted water oxygen isotope ratios as interrogated by the method we explored. If there is temperature-dependent isotopic contrast between the two oxygen sites in goethite, the use of the dehydroxylation method appears unable to reliably reveal it. [ABSTRACT FROM AUTHOR]

Although foraminiferal magnesium/calcium (Mg/Ca) measurements are now widely used for reconstructing sea surface temperature (SST), there is uncertainty about the fidelity of different calibrations of this proxy. Whereas single‐variable calibrations suggest temperature sensitivity of about 9–10% per °C, multivariable calibrations suggest ∼6% per °C, with additional dependence on salinity and pH. Here, we apply five calibrations to six published Mg/Ca records of Globigerinoides ruber, a planktic foraminifer typically used for reconstructing low latitude SST during the Pleistocene. Reconstructed SST by the different calibrations, spanning the past 250,000 years, can be offset by a few degrees, possibly reflecting variable dissolution or hydrographic effects. However, for 4 out of 5 calibrations, the reconstructed temperature anomalies yield estimates that are consistent within the calibrations' uncertainty (<±1 °C), despite the fundamental differences in temperature sensitivity among the equations. We further propose a new seawater Mg/Ca record for the late Neogene and show that the same consistency holds for longer time scales (∼4 My) independently of the choice of the calibration or which seawater Mg/Ca record is used. These comparisons attest to the robustness of the calibrations despite all the confounding nonthermal effects, and offer an empirical basis for researchers and reviewers to judge the records without any prejudice about which calibration is the "best" and evaluate their uncertainties. Plain Language Summary: The ratio of magnesium to calcium (Mg/Ca) in the shells of foraminifera (single‐celled marine organisms) is sensitive to temperature, and therefore is used for reconstructing seawater temperatures. However, salinity and the pH exert additional influence. Recently, multivariable equations, which account for these nonthermal effects, have been developed to replace the simpler calibrations that account only for temperature variability. By applying the different calibrations to the same Mg/Ca records in several cores, we find that the inconsistencies among the calibrations are mostly below ±1 °C, despite the additional effects of salinity and pH on Mg/Ca‐derived temperatures. We further show that these results may be applicable for at least the past 4 million years. Key Points: We assess the performance of different planktic foraminiferal Mg/Ca calibrations for reconstructing sea surface temperaturesWe propose a new seawater Mg/Ca record for the late NeogeneWe assess uncertainties in Pliocene Mg/Ca‐temperature reconstruction [ABSTRACT FROM AUTHOR]

The EPICA Dome C (EDC) ice core provides the longest continuous climatic record covering the last 800 000 years (800 kyrs). Obtaining homogeneous high resolution measurements and accounting for diffusion provide a unique opportunity to study decadal to millennial variability within the past glacial and interglacial periods. We present here a compilation of high resolution (11 cm) water isotopic records with 27,000 d18O measurements and 7,920 dD measurements (covering respectively 94 % and 27 % of the whole EDC record), including both published and new measurements (2,900 for both d18O and dD) over the last 800 kyrs on the EDC ice core. We show that overlapping time series performed over multiple depth ranges over the past 20 years, using different analytical methods and in different laboratories, are consistent within analytical uncertainty, and therefore can be combined to provide a homogeneous data set. A frequency decomposition of the most complete d18O record and a simple assessment of the possible influence of diffusion on the measured profile shows that the variability during glacial periods at multi-decadal to multi-centennial timescale is higher than variability of the interglacial periods. This analysis shows as well that during interglacial periods characterized by a temperature optimum at its beginning, the multi-centennial variability is the strongest over this temperature optimum. [ABSTRACT FROM AUTHOR]

The archaeological site of Ifri Oudadane, NE Morocco, contains well-preserved marine mollusk concentrations throughout the Epipaleolithic (hunting-gathering) and Neolithic (food production) cultural phases, useful to test hypotheses driving such transition. However, the chronology and stratigraphy of harvested shells is complex due to the confluence of human activity and natural deposition processes. This work first quantifies the age and degree of time averaging of archaeological shells and then estimates sea-surface temperatures (SSTs) from the oxygen isotopes of selected specimens. Thirty-four radiocarbon-dated shells exhibited significant time averaging between 310 to 1170 yr that could not be explained by analytical error alone. This finding illustrates the need for individually dating shells in future paleoclimate investigations aiming for high temporal resolution. Nine isotopically analyzed shells dated to the Neolithic phase, between 5700 and 7600 cal yr BP, indicate that assuming constant oxygen isotopes of seawater, SSTs remained consistently warm, between 20°C and 22°C, that is, 2°C-4°C warmer than today. Results point to warmer conditions during the Neolithic, supporting the hypothesis that the rise of a food production mode of life in NE Morocco could have in part been triggered by warming conditions following the colder 8.2 event. [ABSTRACT FROM AUTHOR]

Fifty new heat flow measurements from the Chukchi Borderland (CBL) and adjacent regions of the central Arctic Ocean are reported, where heat flow measurements have been very limited. The average heat flow of the CBL (65 mW/m2) is significantly higher than that of the entire Amerasia Basin (53 mW/m2). In the CBL, the heat flow of the Chukchi Plateau (CP), Northwind Basin (NWB), and Northwind Ridge (NWR) are 64 mW/m2, 69 mW/m2, and 60 mW/m2, respectively. With the aid of nearby seismic profiles, the elevated regional heat flow in the CP and NWR was attributed to the enhanced radiogenic heating of the thick crust (25–29 km), while the most prominent heat flow in the relatively thin crust of the NWB is likely due to the residual heat of a major tectonic extension event. Further numerical heat conduction modeling suggests that the initial rifting of the Amerasia Basin during the Jurassic–Early Cretaceous could not induce such high heat flow in the NWB. Together with recent interpretations of seismic profiles, the elevated heat flow in the NWB could be explained by the residual heat of the Late Cretaceous–early Cenozoic extension. This extension event was contemporaneous with the seafloor spreading in the Makarov Basin and had also been associated with the onset of seafloor spreading in the Eurasia Basin. Plain Language Summary: Heat flux through the seafloor elucidates the thermal evolution of the oceanic lithosphere, and environmental change factors, such as hydrothermal circulations, bottom‐water temperature perturbations, and gas hydrate stability. Seafloor heat flux is often determined using temperatures measured with a series of sensors mounted on the outside of gravity‐driven corers and thermal conductivities measured on the recovered sediment cores. Partly due to sea ice, heat flux measurement in the Arctic Ocean is quite difficult. Here, we present 50 new heat flow sites in Chukchi Borderland and adjacent regions in the central Arctic Ocean. We show that the heat flux in Chukchi Borderland is significantly higher than the average in the Amerasia Basin. Although the heterogeneity in radiogenic heat content and thickness of the crust could explain part of the high heat flux, the highest regional heat flux in Chukchi Borderland is best explained by a warm lithosphere that experienced tectonic extension during Late Cretaceous‐early Cenozoic. This extension event may share the same driving force of the subsequent opening of the Eurasia Basin. Key Points: 50 new heat flow sites on the Chukchi Borderland and adjacent regions are reported, where heat flow measurements have been very limitedHeat flow in the Chukchi Borderland is significantly higher than the average in the Amerasia Basin, likely due to high radiogenic heatingNorthwind Basin of the Chukchi Borderland has the highest heat flow, which could be ascribed to the Late Cretaceous–early Cenozoic extension [ABSTRACT FROM AUTHOR]

Carbonate clumped isotope thermometry is based on the ordering of 13C and 18O in the carbonate lattice and is based on the relative abundance of 13C18O16O in CO2 produced through acid digestion of carbonate minerals. The major advantage of this technique is its non‐dependency on the δ18O value of water from which the carbonate precipitated. Ghosh et al. (2007, https://doi.org/10.1016/j.gca.2007.03.015) previously published calibration data for fish otoliths referenced to heated gases and used the Gonfiantini 17O parameter set in their data evaluation. Herein, we present a new clumped isotope (Δ47) calibration for aragonitic fish otoliths in the absolute reference frame using the International Union of Pure and Applied Chemistry 17O correction. Our revised calibration equation for otolith is: Δ47CDES=0.0364±0.005×(106)T2+0.2619±0.0657(R2=0.9,p−value<0.001). To test the accuracy of this calibration, we apply it to otoliths of modern Lutjanus lutjanus from the Bay of Bengal. The estimated average temperature (22.3°C ± 4.2°C) for the Bay of Bengal and δ18OV‐SMOW composition of waters of −1.7‰ (±0.5) are consistent with the onsite observations. We also apply the new calibration to well‐preserved otoliths of "genus Ambassidarum" sp. and "genus Gobiidarum" sp. from lower Miocene (Burdigalian) sediments of the Quilon Formation, India to quantify coastal water conditions. Estimated average environmental water temperatures in their habitats were 12.9°C ± 1.7°C, and the average δ18OV‐SMOW of ambient waters calculated yielded a value between −3.5‰ and −2.6‰ (V‐SMOW) (mean: −2.9‰ ± 0.4) and −4.4‰, respectively. These results indicate δ18O values reflect the kinetic effects impacting the δ18O of fish otoliths independently of Δ47, although we cannot fully preclude diagenesis. Key Points: A revised clumped isotope calibration equation for otoliths was established based on the International Union of Pure and Applied Chemistry parameter setThe new calibration equation is validated by quantifying the environmental water δ18O of modern watersNew calibration equation allows for the reconstruction of lower Miocene coastal conditions in southwest India utilizing fossil otoliths [ABSTRACT FROM AUTHOR]

The Δ 47 (paleo)thermometer has opened a new avenue to determine carbonate formation temperatures independent of the oxygen isotopic composition of the fluid from which the carbonate crystallized. A major limitation of this thermometer is related to kinetic effects if homogeneous isotopic equilibrium is not attained during carbonate precipitation. Dual clumped isotope thermometry – the high-precision analysis of Δ 48 along with Δ 47 in CO 2 evolved from phosphoric acid digestion of carbonates – makes it possible to resolve temperature from the kinetic information recorded in an individual carbonate phase. Therefore, it provides a new opportunity to identify (bio)mineralization pathways and to determine carbonate formation temperatures devoid of a kinetic bias, based solely on isotopic analysis of a single carbonate phase. Identification of the nature and extent of kinetic effects as well as the reconstruction of accurate formation temperatures requires knowledge of the position of equilibrium in Δ 47 vs Δ 48 space. Here, we present Δ 47 and Δ 48 data of carbonates that were previously considered as having crystallized closest to equilibrium in a temperature range of 8 to 1100 °C. Across this range, the temperature dependences of Δ 47 and Δ 48 are best expressed by the following fourth order polynomials of 1/T: Δ 47 (CDES 90) (‰) = 1.038 (−5.897 1/T − 3.521 103 1/T2 + 2.391 107 1/T3 − 3.541 109 1/T4) + 0.1856 Δ 48 (CDES 90) (‰) = 1.028 (6.002 1/T − 1.299 104 1/T2 + 8.996 106 1/T3 − 7.423 108 1/T4) + 0.1245 with CDES 90 representing the Carbon Dioxide Equilibrium Scale at a reaction temperature of 90 °C. In its entire temperature range, our Δ 47 (CDES 90) - T - relationship agrees within 2 ppm with two previous Δ 47 (I-CDES) - T - relationships reported by Jautzy et al. (2020) and Anderson et al. (2021). Accuracy of our proposed Δ 47 (CDES 90) − Δ 48 (CDES 90) equilibrium relationship is independently confirmed by additional dual clumped isotope data of experimental and geothermal carbonates which precipitated from potentially equilibrated dissolved inorganic carbon pools at a temperature range of 25–100 °C. Furthermore, we reprocessed original dual clumped isotope data of natural carbonates (Bajnai et al., 2020) and compared their composition to the position of equilibrium in Δ 47 vs Δ 48 space. These results corroborate preliminary evidence that the hydration/hydroxylation reactions became rate-limiting during the calcification of a speleothem-like sample, a warm water coral, a cold water coral and a brachiopod, finally evoking significant departures of carbonate- Δ 47 and - Δ 48 from dual clumped isotope equilibrium. An anti-clumped Δ 48 value of −419 (±16) ppm (95% confidence interval level) is obtained for a technical calcite that was precipitated by the injection of CO 2 into a Ca(OH) 2 -saturated solution. Its negative Δ 48 value largely arises from a combinatorial effect, i.e. the carbonate oxygen derives from two sources with different bulk isotopic compositions. Besides the identification of the nature and the extent of (bio)mineralization kinetics and the reconstruction of carbonate formation temperatures unbiased by kinetics, dual clumped isotope analysis, therefore, allows tracing the isotopic heterogeneity of oxygen pools contributing to carbonate formation. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) reconstructions based on isoprenoid glycerol dialkyl glycerol tetraether (isoGDGT) distributions from the Eocene southwest (SW) Pacific Ocean are unequivocally warmer than can be reconciled with state-of-the-art fully coupled climate models. However, the SST signal preserved in sedimentary archives can be affected by contributions of additional isoGDGT sources. Methods now exist to identify and possibly correct for overprinting effects on the isoGDGT distribution in marine sediments. Here, we use the current proxy insights to (re-)assess the reliability of the isoGDGT-based SST signal in 69 newly analyzed and 242 reanalyzed sediments at Ocean Drilling Program (ODP) Site 1172 (East Tasman Plateau, Australia) following state-of-the-art chromatographic techniques. We compare our results with paleoenvironmental and paleoclimatologic reconstructions based on dinoflagellate cysts. The resulting ∼ 130 kyr resolution Maastrichtian–Oligocene SST record based on the TetraEther indeX of tetraethers with 86 carbon atoms (TEX 86) confirms previous conclusions of anomalous warmth in the early Eocene SW Pacific and remarkably cool conditions during the mid-Paleocene. Dinocyst diversity and assemblages show a strong response to the local SST evolution, supporting the robustness of the TEX 86 record. Soil-derived branched GDGTs stored in the same sediments are used to reconstruct mean annual air temperature (MAAT) of the nearby land using the Methylation index of Branched Tetraethers with 5-methyl bonds (MBT' 5me) proxy. MAAT is consistently lower than SST during the early Eocene, independent of the calibration chosen. General trends in SST and MAAT are similar, except for (1) an enigmatic absence of MAAT rise during the Paleocene–Eocene Thermal Maximum and Middle Eocene Climatic Optimum, and (2) a subdued middle–late Eocene MAAT cooling relative to SST. Both dinocysts and GDGT signals suggest a mid-shelf depositional environment with strong river runoff during the Paleocene–early Eocene progressively becoming more marine thereafter. This trend reflects gradual subsidence and more pronounced wet/dry seasons in the northward-drifting Australian hinterland, which may also explain the subdued middle Eocene MAAT cooling relative to that of SST. The overall correlation between dinocyst assemblages, marine biodiversity and SST changes suggests that temperature exerted a strong influence on the surface-water ecosystem. Finally, we find support for a potential temperature control on compositional changes of branched glycerol monoalkyl glycerol tetraethers (brGMGTs) in marine sediments. It is encouraging that a critical evaluation of the GDGT signals confirms that most of the generated data are reliable. However, this also implies that the high TEX 86 -based SSTs for the Eocene SW Pacific and the systematic offset between absolute TEX 86 -based SST and MBT' 5me -based MAAT estimates remain without definitive explanation. [ABSTRACT FROM AUTHOR]

Branched glycerol dialkyl glycerol tetraethers (BrGDGTs) are a suite of orphan bacterial membrane lipids commonly used as paleo-environmental proxies for mean annual air temperature (MAT) and pH. Recent calibrations between the Methylation of Branched Tetraethers index (MBT′ 5ME) and MAT, based on modern surface soils (including peats), show a considerable amount of scatter, especially in mid- and high latitude soils, suggesting that brGDGT signals are influenced by additional environmental and/or biological controls at these sites. Here we test the impact of soil chemical gradients and bacterial community changes (16S rDNA sequence-based) on brGDGT distributions at two grasslands sites (Ossenkampen [NL], ForHot [IS]), and one agricultural site (Craibstone [UK]). In addition to the variation in soil chemistry, the ForHot site experiences belowground warming. Of the studied edaphic parameters, soil pH is the primary factor that explains simultaneous changes in both the bacterial community composition and the brGDGT distribution. Variations in the MBT′ 5ME at two sites without soil warming indeed correlate strongly to soil pH (r = 0.9–1.0, pH = 4.5–7.3), whereas pH explains part of the variation in the MBT′ 5ME at the site with soil warming (mean soil temperature ranging between 5 and 14 °C). At all sites, soil pH is positively related with the same brGDGTs (Ib, IIb, IIIb, IIIc, IIa′, IIb′, IIc′, IIIa′, IIb′, IIIc′) and influences the ratio between main brGDGT compounds Ia, IIa and IIIa, impacting the MBT′ 5ME values. This change in brGDGT distributions coincides with a change in the composition of the bacterial community at all sites. The bacterial clades that vary at the three experimental sites (specifically Acidobacteria subgroups 1, 2, 3, 6, 22) have previously been shown to also respond to soil pH on a global scale. As soil pH changes on geological timescales, the impact of changing pH on the MBT′ 5ME paleothermometer should be considered when performing paleoclimate studies. [ABSTRACT FROM AUTHOR]

• BrGMGTs are produced within the water column and surface sediments of Lake Chala. • Aquatic production of brGMGTs responds to seasonal mixing and oxycline depth. • BrGMGT production is greatest in the lake's anoxic bottom sediments. • Sedimentary production complicates the use of brGMGTs as an air temperature proxy. Branched glycerol monoalkyl glycerol tetraethers (brGMGTs) are membrane-spanning lipids which were initially identified in marine settings and have more recently been found in peats and lake sediments. In the latter settings, their abundance relative to that of the branched glycerol dialkyl glycerol tetraethers (brGDGTs) appears to increase under warmer climate conditions. Furthermore, their relative distribution in bottom sediments from an altitudinal transect of East African lakes has been linked to local mean annual air temperature (MAAT). To shed light on their sources and identify potential environmental factors driving their production in lakes, we investigated their occurrence and distribution in the water column, surficial lake-bottom sediments, and catchment soils of Lake Chala, a permanently stratified (meromictic) tropical crater lake in East Africa. Data from the water column comprise both suspended particulate matter (SPM) collected monthly at eight depth intervals between 0 and 80 m for 17 consecutive months, and settling particles collected monthly at a depth of 35 m over a period of 53 months. In catchment soils, brGMGTs are either relatively scarce or absent. In the water column, their occurrence is dominated by four of the seven known brGMGTs and mostly limited to the anoxic zone. Whereas the spatial distribution and concentration of brGMGTs in SPM were strongly influenced by the seasonal cycle of strong water-column stratification alternating with deep mixing, this trend is not consistent on the multi-year timescale of the settling-particles record. Moreover, their relative distribution in both SPM and settling particles did not respond to temperature variation during the studied interval. In contrast to the water column, surficial lake-bottom sediments contain the full suite of known brGMGTs in a distinct and spatially uniform fractional abundance, strongly suggesting that the majority of brGMGT production takes place there. BrGMGT-inferred MAAT for the Chala area reasonably estimates measured MAAT and conforms to the East African lake brGMGT calibration. Our results have important implications for the use of brGMGTs as proxy for MAAT, in that outside of the equatorial zone the presumably greater offset between bottom-water temperature and MAAT may create a bias towards cold-season air temperature. Regardless, the more singular source of brGMGTs in lake sediments is an advantage for paleotemperature reconstruction compared to brGDGTs, which several studies now indicate to have a mixed aquatic and terrestrial source of which the relative contribution cannot easily be disentangled or assumed to have been constant through time. [ABSTRACT FROM AUTHOR]

During the Early Cretaceous, significant tectonic regime transformation occurred in East China. Meanwhile, the Jehol Biota, which contains world‐famous feathered dinosaurs, flourished. However, few studies have been conducted to investigate the influence of tectonic regime transformation on regional topography and climates and the possible climatic causes of the unique characteristics of these dinosaurs. Here, we address these issues by applying clumped isotope paleothermometry to Early Cretaceous carbonates in NE China. In the Sihetun area, the mean annual paleotemperature derived from paleosol carbonates was 5.9 ± 1.7°C, and the paleoelevation was 2.8–4.1 km during the Early Cretaceous. The topographic evolution was closely related to the subduction of the paleo‐Pacific plate beneath East Asia during this period. Our results suggest a high altitude and cold habitat with frozen winters for the Jehol Biota in the Sihetun area, which implies possible climate‐Keywords:influenced evolution of the feathered characteristic of the dinosaurs. Plain Language Summary: During the Early Cretaceous, the Jehol Biota, especially the unique feathered dinosaurs, flourished in western Liaoning Province, China. As insulation devices, the feathers helped these nonavian dinosaurs resist the cold climate, although conventional opinions have suggested that the Early Cretaceous was a typical "greenhouse climate" world. Based on the clumped isotopes of the paleosol carbonates in the Sihetun area, our study estimates the paleotemperature and paleoaltitude of the living habitat of these feathered dinosaurs during the Early Cretaceous. We suggest that the feathered dinosaurs in NE China lived in a high‐altitude habitat with frozen winters and volcanic eruptions, which implies possible climate‐influenced evolution of the feathered characteristic of the dinosaurs. We also suggest that topographic change‐induced cooling was caused by significant tectonic regime transformation in East China. Key Points: We analyze carbonate clumped isotopes to estimate the Early Cretaceous elevation of the habitat of feathered dinosaurs in NE ChinaThe feathered dinosaurs lived in a high‐altitude habitat with freezing conditions in winter in NE China during the Early CretaceousTopographic changes were caused by the eastward subduction of the paleo‐Pacific plate [ABSTRACT FROM AUTHOR]

Massive dome-shaped coral Porites are the predominant choice for paleoclimate studies due to their consistent and reliable growth. When growing close to sea level, they become limited in their vertical growth and form so-called 'microatolls'. Microatolls have not yet been extensively explored for paleoclimate reconstruction. Here, we investigate how reliable modern Porites microatolls are against empirical sea-surface temperature using Sr/Ca, δ18O, Li/Mg and Sr U paleothermometry methods on samples from the Society Islands, French Polynesia. Our results show Sr/Ca ratios have the lowest Standard Error of the Inverse Prediction (SEIP) at 0.415 °C (N = 41) with a calibration of Sr/Ca (mmol mol−1) = −0.082 ± 0.006 SST (°C) + 11.256 ± 0.170 and with high reproducibility across multiple corals. The reproducibility of δ18O was less good, with SEIP increasing to 0.829 °C (N = 41). Considering methods directly from the literature, Li/Mg ratio empirically corrected for Sr/Ca had the best balance between bias and precision where no local calibration could be available. This study independently evaluates and confirms the suitability of Porites microatolls from well-flushed environments for paleoclimate studies. Fossil dome-shaped Porites grow anywhere between near-surface and roughly 20 m depths which inherently incorporates uncertainty into any sea surface temperature reconstruction. This uncertainty is significantly reduced for microatolls due to their well-constrained bathymetry. The study represents a fundamental step in paleoclimate research targeting consistently near the water-air interface bringing reliability and, especially when combined with their ability to reconstruct past sea-level changes, microatolls have the potential to be central for future paleoenvironmental studies. • Investigation into the potential of Porites microatolls for SST reconstruction. • Comparison between recent and more conventional coral paleoclimatology methods. • Application of Sr U and Li/Mg paleothermometer. • Accuracy and reproducibility of Sr/Ca proved to be the most suitable proxy for SST reconstruction. [ABSTRACT FROM AUTHOR]

Carbonate clumped-isotope (Δ 47) paleothermometry offers a novel means of reconstructing the paleoburial temperature of carbonate rocks. However, the application of this thermometer is challenging when applied to thermal history reconstruction in deeply buried reservoirs, due to potential modifications to the Δ 47 value of carbonate mineral by C–O bond solid-state reordering. Although four reordering models have been proposed based on high-temperature laboratory experiments, the operation of Δ 47 reordering under geological conditions is still not fully understood. The Δ 47 and homogenization temperature (T h) of primary fluid inclusions trapped within the dolomite cements from the Central Sichuan Basin, China were measured. The Δ 47 evolution of the dolomite samples were predicted with the four published reordering models under the constraints of independent thermal history reconstruction and T h -derived initial Δ 47 values. The results of this study demonstrate that T h of fluid inclusion can be a valid indication of precipitation temperature of dolomite cements even when the samples experience peak burial temperature (T max) up to ∼200–240 °C. It also indicate current reordering models have overall equivalent ability for Δ 47 prediction although there are discernible differences between sample types across different models. Both T max and model-determined threshold behavior of Δ 47 reordering control the reordering pathway for model prediction, and the threshold behaviors of our dolomite samples were explored under geological conditions. In this framework, the threshold temperature (∼190 °C) for detectable dolomite reordering may be substantially higher than previous model predictions (120–150 °C). Moreover, the temperature at which the Δ 47 temperature fully equilibrates with the ambient temperature during sequential burial could exceed 240–250 °C. These threshold behaviors of bond reordering in dolomite under geological conditions are different from previous model calculations. Therefore, T max and threshold temperatures of Δ 47 reordering for different models should be considered when reconstructing thermal history based clumped-isotope solid-state reordering modeling. Our findings have significant implications for understanding the dolomite clumped-isotope reordering process and reconstructing the thermal history of deeply buried carbonate rocks. • Homogenization temperature of fluid inclusion is used to indicate initial Δ 47 value. • Independently thermal history is used to constrain bond reordering kinetic modeling. • Reordering models are evaluated under geological conditions. • New threshold temperatures for Δ 47 reordering in dolomite are proposed. [ABSTRACT FROM AUTHOR]

A new inductively coupled plasma atomic emission spectrometry (ICP‐AES) method is presented for rapid and routine analysis of Sr/Ca molar ratios in seawater, with a long‐term precision of < 0.2%. It is an adaptation of a method widely employed for the analysis of coral aragonite Sr/Ca ratios in marine paleothermometry studies, which are based on the assumption that the seawater Sr/Ca ratio is constant in space and time. While prior studies have shown variations of up to 1% with depth, smaller variations at the ocean surface are generally accounted for via empirical, species‐specific calibrations of coral Sr/Ca vs. temperature. We found Sr/Ca variations in some coastal waters to be even larger, with distinct periodicity, complicating this approach. Although the high precision necessary for measurements of seawater Sr/Ca has previously relied on advanced mass spectrometry, long analysis times, and expensive isotopic spikes, our method uses more accessible instrumentation and is both time‐ and cost‐saving. The intricate composition of seawater, relative to coral aragonite solutions, requires an intensity ratio calibration technique combined with rigorous normalization to a suitable seawater standard. Key aspects of our method are discussed, including the choice of wavelengths, instrument parameters, accuracy, precision, and matrix effects. Special attention is given to the need for a certified seawater Sr/Ca reference standard, which does not presently exist. Analytical validation is provided by concurrent sharp gradients in Sr/Ca and δ18O, coinciding with the Florida landfall of hurricane Irma, as recorded at near‐daily resolution in a continuous seawater sample collected with an osmotic pump. [ABSTRACT FROM AUTHOR]

• δ13C-δ18O-Δ 47 characterize methane-derived bivalve shells and cements from an Estuary. • Seep shells show full isotopic equilibrium due to slow precipitation. • Cements show unique Δ 47 and δ18O positive offsets from marine signals. • Cements define a new differential disequilibrium domain for natural carbonates. • This isotope study refines the basis for interpreting ancient seep carbonates. Recent studies using the CO 2 carbonate clumped isotope (13C-18O-16O, i.e., Δ 47) paleothermometer and bulk isotopes (δ13C, δ18O) have brought new insights into the prevailing conditions during carbonate formation around marine methane seeps. These studies mostly revealed δ18O or paired δ18O and Δ 47 disequilibria between precipitating minerals, water and DIC species. Here, we have sampled bivalves and cements from two modern, slow-release methane seeps located in the St. Lawrence Estuary. The bivalve shells have marine isotopic signals, whereas the cements show wide Δ 47 (up to +0.05‰) and minor δ18O (+0.8‰) positive offsets relative to marine equilibrium, with a narrow δ13C range (−33.5 to −31.1‰). The observed isotopic trends show that, unlike in previous studies, the aragonite shells precipitated at full isotopic equilibrium, whilst the cements suggest differential disequilibrium precipitation. We propose that the suite of results were generated by distinct Δ 47 and δ18O pathways, as supported by DIC-water isotopic exchange numerical experiments. The observed trends typify slow rates of methane oxidation and carbonate precipitation from residual gas with high-δ18O and low-Δ 47 values after methane-derived CO 2 diffusion. Overall, we suggest that the shells represent potential archives of seep conditions, and that the seep cements define a new Δ 47 and δ18O domain widening the known spectrum of marine conditions producing natural carbonates. Our results, compiled with literature data, help refine the basis for interpreting carbonate precipitation mechanisms in ancient seeps or other geological settings. [ABSTRACT FROM AUTHOR]

RI‐OH′ and RI‐OH (ring index of hydroxylated tetraethers) are two novel organic paleothermometers which could either complement or replace more established paleothermometric proxies, such as UK′37 (C37 ketone unsaturation ratio) and TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms). Despite a few promising attempts, the paleothermometric potential of RI‐OH′ and RI‐OH is not fully constrained. Here we present new high‐resolution temperature records over the 160‐45 ka BP (before present = year 1950 CE) period using four organic proxies (RI‐OH′, RI‐OH, TEX86, and UK′37) from three deep sea sediment cores located in a north‐south transect along the Iberian Margin. We analyzed all organic proxies from a single set of lipid extracts to optimize proxy‐proxy comparisons and phase relationship studies. RI‐OH′ responds to Dansgaard‐Oeschger and Heinrich events, better resembles UK′37 than TEX86, and better records the influence of (sub)polar waters during Heinrich events than does RI‐OH. While RI‐OH′ gives realistic sea surface temperatures and latitudinal gradients coherent with those from independent paleothermometers, a more extensive RI‐OH′‐temperature calibration for the North Atlantic is clearly needed. However, the absence of a significant warm bias in RI‐OH′‐based temperatures compared to a shallow sea site suggests that endemic, deep‐dwelling archeal communities affect TEX86 but not RI‐OH′ in the Iberian Margin. TEX86 leads RI‐OH′ and UK′37 during four Heinrich‐like events, potentially due to background fluxes from deep waters for nonhydroxylated tetraethers summed with primary productivity dependent fluxes from surface waters for all investigated lipid classes. Relationships with Greenland temperatures further support RI‐OH′‐based paleothermometry. Key Points: RI‐OH′ not only responds to Dansgaard‐Oeschger and Heinrich events but also better resembles UK′37 than TEX86RI‐OH′ gives realistic sea surface temperatures and latitudinal gradients consistent with those from independent paleothermometersRelationships with Greenland temperatures further support RI‐OH′‐based paleothermometry [ABSTRACT FROM AUTHOR]

The spatial coverage and temporal resolution of the Early Paleozoic paleoclimate record are limited, primarily due to the paucity of well-preserved skeletal material commonly used for oxygenisotope paleothermometry. Bulk-rock δ18O datasets can provide broader coverage and higher resolution, but are prone to burial alteration. We assess the diagenetic character of two thick Cambro-Ordovician carbonate platforms with minimal to moderate burial by pairing clumped and bulk isotope analyses of micritic carbonates. Despite resetting of the clumped-isotope thermometer at both sites, our samples indicate relatively little change to their bulk δ18O due to low fluid exchange. Consequently, both sequences preserve temporal trends in δ18O. Motivated by this result, we compile a global suite of bulk rock δ18O data, stacking overlapping regional records to minimize diagenetic influences on overall trends. We find good agreement of bulk rock δ18O with brachiopod and conodont δ18O trends through time. Given evidence that the δ18O value of seawater has not evolved substantially through the Phanerozoic, we interpret this record as primarily reflecting changes in tropical, nearshore seawater temperatures and only moderately modified by diagenesis. Focusing on the samples with the most enriched, and thus likely leastaltered, δ18O values, we reconstruct Late Cambrian warming, Early Ordovician extreme warmth, and cooling around the Early-Middle Ordovician boundary. Our record is consistent with models linking the Great Ordovician Biodiversification Event to cooling of previously very warm tropical oceans. In addition, our high-temporal-resolution record suggests previously unresolved transient warming and climate instability potentially associated with Late Ordovician tectonic events. [ABSTRACT FROM AUTHOR]

The Gulf of Aqaba is home to three giant clam species with differing ecological niches and levels of photosymbiotic activity. Giant clams grow a two‐layered shell where the outer layer is precipitated in close association with photosymbiont‐bearing siphonal mantle, and the inner layer is grown in association with the light‐starved inner mantle. We collected 39 shells of the three species (the cosmopolitan Tridacna maxima and T. squamosa, as well as the rare endemic T. squamosina) and measured carbon and oxygen isotope ratios from inner and outer shell layers, to test for differences among species and between the layers of their shells. T. squamosina records higher temperatures of shell formation as determined by oxygen isotope paleothermometry, consistent with its status as an obligately shallow‐dwelling species. However, the known negative fractionation imparted on tissue carbon isotopes by photosymbiotic algae did not produce measurable offsets in the carbonate δ13C values of the more symbiotic T. squamosina and T. maxima compared to the more heterotrophic T. squamosa. Across all species, outer shell layers recorded mean growth temperatures 1.8°C higher than corresponding inner layers, which we propose is a function of the high insolation, low albedo microenvironment of the outer mantle, and potentially the activity of the symbionts themselves. Population‐wide isotopic sampling of reef‐dwelling bivalve shells can help constrain the ecological niches of rare taxa and help reconstruct their internal physiology. Plain Language Summary: Giant clams are large bivalves which live on reefs and, like corals, also contain algae living as symbionts within their tissue. These clams record temperature via the ratio of two isotopes of oxygen (the heavier 18O and lighter 16O) in their shell, and it has been proposed that their carbon isotope ratios are influenced by their symbionts. The three species of giant clam from around the Gulf of Aqaba (Northern Red Sea): Tridacna squamosina, T. squamosa, and T. maxima, are thought to live in different habitats and to have different amounts of photosynthesis. To test this, we collected several dozen shells to compare the oxygen and carbon isotopes of each species. We found that the rare T. squamosina records the highest average growth temperatures, which supports previous observations that it lives only in shallow water. Giant clams also have two layers in their shells, and we compared the isotopes of the layers. We found that the Sun‐drenched outer shell layer records higher temperatures than the inner one, which we propose is due to solar heating of the algae‐filled outermost tissue. Symbiosis does not appear to change the carbon isotope ratios between the layers or between species with different amounts of photosynthesis. More work is needed to investigate how ecology and physiology influence the isotopes between bivalve species and within their shells. Key Points: The outer shell layer of Tridacna is depleted in 18O compared to the inner layer, corresponding to a 1.7°C higher reconstructed temperatureThere is no significant difference in δ13C value between species, but δ18O is lower in the obligately shallow‐dwelling Tridacna squamosinaWhile carbon isotopes did not track different degrees of photosymbiosis, oxygen isotopes can be used to constrain ecological niche partitioning for rare species [ABSTRACT FROM AUTHOR]

Key features of late Neogene climate remain uncertain due to conflicting records derived from different sea surface temperature (SST) proxies. To understand scenarios in which proxy‐derived temperature estimates can be used interchangeably or are instead measuring different aspects of the same system, it is necessary to explore both the consistencies and differences between specific paleothermometers. Here, we report orbital‐scale climate records from ODP Site 846 in the eastern equatorial Pacific (EEP) for the interval from ~5–6 Ma using alkenone and archaeal lipid paleothermometers. Results from both proxies are similar in their secular trends and magnitude of long‐term temperature change, and spectral analysis demonstrates that the records are coherent and in‐phase in both the obliquity and precession bands. However, we find that the temperatures reconstructed by TEX86 are consistently offset toward colder values by ~2 °C relative to Uk′37‐derived temperatures in global calibrations, or by ~0.8 °C in Bayesian‐based calibrations. All combinations of calibrations also yield approximately twice the amplitude of orbital‐scale variation in TEX86 relative to Uk′37‐derived temperature fluctuations. Both temperature proxies are negatively correlated with sedimentary alkenone concentrations, which we use as an indicator of increased export productivity. Removing this productivity contribution from TEX86 results in an adjusted TEX86 record with temperature sensitivity identical to Uk′37. In future applications, this signal may be decoupled using additional sedimentary indicators of paleoproductivity, which likely will be most important for upwelling zone environments. There remain other nonexplained factors that contribute to differences between TEX86 and Uk′37 that warrant additional investigation. Plain Language Summary: The remains of organisms found in ocean sediments contain information about the climate at the time the organisms were alive. Some of these remains enable the reconstruction of past temperature and are used as "paleothermometers." Many different types of paleothermometers exist; however, none work across all time periods or geographic locations. Thus, data from different paleothermometers are commonly used interchangeably to study Earth's past climate. However, little previous work has been done to study the limitations of this approach. Two of the most widely used paleothermometers are based on lipids of marine microorganisms. Here, we generated high‐resolution temperature reconstructions using both techniques. This allowed us to compare the timing, direction, and magnitude of the reconstructed temperature changes over a time interval of nearly 1 million years. Encouragingly, both paleothermometers show the same general patterns through time, although they yield modestly different results. These differences are greater during cold climate conditions and appear to be in part due to the influence of the amount of local biological productivity on one of the paleothermometers. We propose some ways that the contribution of factors other than temperature, including not only productivity but also community composition, might be explored in the future. Key Points: Orbital‐resolution Uk′37 and TEX86 paleotemperature records are coherent and in‐phase in the late Neogene eastern equatorial PacificRegardless of calibration choice, archaeal lipid records yield colder temperatures with larger‐amplitude variations than alkenone recordsTemperature and productivity explain half the variance between Uk′37 and TEX86 records; other unconstrained factors warrant exploration [ABSTRACT FROM AUTHOR]

To establish whether ongoing climate change is outside the range of natural variability and a result of anthropogenic inputs, it is essential to reconstruct past oceanic and atmospheric temperatures for comparison with the modern world. Reconstructing past temperatures is a complex endeavor that employs indirect proxy indicators. Over the past two decades, promising paleothermometers have been developed that use isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs) from the membrane lipids of archaea preserved in marine sediments. These proxies are based on the observed relationship between lipid structure and temperature. As with all proxy indicators, observed relationships are often complex. Here, we focus on the application of isoGDGT paleotemperature proxies in the polar oceans, critical components of the global climate system. We discuss the application of and caveats regarding these archaeal membrane lipid-derived proxies and make recommendations to improve isoGDGT-derived polar ocean temperature reconstructions. We also review initial successes using hydroxylated (OH) isoGDGTs proxies in cold Arctic and Southern Ocean regions and recommend that multi-proxy approaches, including both hydroxylated and non-hydroxylated isoGDGTs, be used to contribute to the robustness of paleotemperature reconstructions. [ABSTRACT FROM AUTHOR]

A series of papers published shortly after the Integrated Ocean Drilling Program Arctic Coring Expedition (ACEX, 2004) on Lomonosov Ridge indicated remarkably high early Eocene sea surface temperatures (SSTs; ca. 23 to 27 ∘ C) and land air temperatures (ca. 17 to 25 ∘ C) based on the distribution of isoprenoid and branched glycerol dialkyl glycerol tetraether (isoGDGT and brGDGT) lipids, respectively. Here, we revisit these results using recent analytical developments – which have led to improved temperature calibrations and the discovery of new temperature-sensitive glycerol monoalkyl glycerol tetraethers (GMGTs) – and currently available proxy constraints. The isoGDGT assemblages support temperature as the dominant variable controlling TEX 86 values for most samples. However, contributions of isoGDGTs from land, which we characterize in detail, complicate TEX 86 paleothermometry in the late Paleocene and part of the interval between the Paleocene–Eocene Thermal Maximum (PETM; ∼ 56 Ma) and the Eocene Thermal Maximum 2 (ETM2; ∼ 54 Ma). Background early Eocene SSTs generally exceeded 20 ∘ C, with peak warmth during the PETM (∼ 26 ∘ C) and ETM2 (∼ 27 ∘ C). We find abundant branched GMGTs, likely dominantly marine in origin, and their distribution responds to environmental change. Further modern work is required to test to what extent temperature and other environmental factors determine their distribution. Published Arctic vegetation reconstructions indicate coldest-month mean continental air temperatures of 6–13 ∘ C, which reinforces the question of whether TEX 86 -derived SSTs in the Paleogene Arctic are skewed towards the summer season. The exact meaning of TEX 86 in the Paleogene Arctic thus remains a fundamental issue, and it is one that limits our assessment of the performance of fully coupled climate models under greenhouse conditions. [ABSTRACT FROM AUTHOR]

Thermoluminescence (TL) of feldspar is investigated for its potential to extract temperature histories experienced by rocks exposed at Earth's surface. TL signals from feldspar observed in the laboratory arise from the release of trapped electrons from a continuous distribution of trapping energies that have a range of thermal stabilities. The distribution of trapping energies, or thermal stabilities, is such that the lifetime of trapped electrons at room temperature ranges from less than a year to several billion years. Shorter lifetimes are associated with low-temperature TL signals, or peaks, and longer lifetimes are associated with high temperature TL signals. Here we show that trapping energies associated with shorter lifetimes, or lower-temperature TL signals (i.e. between 200 and 250 ∘C), are sensitive to temperature fluctuations occurring at Earth's surface over geological timescales. Furthermore, we show that it is possible to reconstruct past surface temperature histories in terrestrial settings by exploiting the continuous distribution of trapping energies. The potential of this method is first tested through theoretical experiments, in which a periodic temperature history is applied to a kinetic model that encapsulates the kinetic characteristics of TL thermometry. We then use a Bayesian approach to invert TL measurements into temperature histories of rocks, assuming that past temperature variations follow climate variations observed in the δ18 O records. Finally, we test the approach on two samples collected at the Mer de Glace (Mont Blanc massif, European Alps) and find similar temperature histories for both samples. Our results show that the TL of feldspar may be used as a paleothermometer. [ABSTRACT FROM AUTHOR]

The clumped isotope (Δ 47) proxy is a promising geochemical tool to reconstruct past ocean temperatures far back in time and in unknown settings, due to its unique thermodynamic basis that renders it independent from other environmental factors like seawater composition. Although previously hampered by large sample-size requirements, recent methodological advances have made the paleoceanographic application of Δ 47 on small (<5 mg) foraminifer samples possible. Previous studies show a reasonable match between Δ 47 calibrations based on synthetic carbonate and various species of planktonic foraminifers. However, studies performed before recent methodological advances were based on relatively few species and data treatment that is now outdated. To overcome these limitations and elucidate species-specific effects, we analyzed 14 species of planktonic foraminifers in sediment surface samples from 13 sites, covering a growth temperature range of ∼0–28 °C. We selected mixed layer-dwelling and deep-dwelling species from a wide range of ocean settings to evaluate the feasibility of temperature reconstructions for different water depths. Various techniques to estimate foraminifer calcification temperatures were tested in order to assess their effects on the calibration and to find the most suitable approach. Results from this study generally confirm previous findings that there are no species-specific effects on the Δ 47 -temperature relationship in planktonic foraminifers, with one possible exception. Various morphotypes of Globigerinoides ruber were found to often deviate from the general trend determined for planktonic foraminifers. Our data are in excellent agreement with a recent foraminifer calibration study that was performed with a different analytical setup, as well as with a calibration based exclusively on benthic foraminifers. A combined, methodologically homogenized dataset also reveals very good agreement with an inorganic calibration based on travertines. Our findings highlight the potential of the Δ 47 paleothermometer to be applied to recent and extinct species alike to study surface ocean temperatures as well as thermocline variability for a multitude of settings and time scales. [ABSTRACT FROM AUTHOR]

Carbonate clumped isotope thermometry has emerged as a useful tool for the reconstruction of precipitation temperatures for minerals from surface and subsurface environments. Few studies have examined the impact of variable solution chemistry and mineral chemistry on clumping. Therefore, we used theoretical calculations to predict the impact of the presence of cations in a solution on the equilibrium clumped, oxygen (18O/16O and 17O/16O, θ and κ), and carbon isotopic (13C/12C) compositions of dissolved inorganic carbon species (DIC), and the impact of cation type and stoichiometry on carbonate mineral composition within the calcite-dolomite-magnesite solid solution. We examined the impact of the presence of Mg2+, K+, Na+, and Ca2+ on equilibrium Δ 63 values of carbonate ion groups in DIC species and the composite DIC pool using the B3LYP/6-311++G(2d, 2p) model. We developed new cluster models for dolomite, magnesite, and high-magnesium calcite (Ca 0.83 Mg 0.17)CO 3 and (Ca 0.67 Mg 0.33)CO 3) and examined the effect of extraneous cations substituted into the crystal lattices of the carbonate minerals. We find the presence of added cations in the DIC pool increases equilibrium Δ 63 values slightly, depending upon their solution concentrations. For the cations examined, the identity of the added cation does not seem to be significant. Cumulative effects may account for small-scale differences (<0.02‰) but are unlikely to be substantial unless cation/anion association and ion pairing are important in the solution that carbonates are growing from. We also find that at a given temperature, stoichiometric dolomite and magnesite Δ 63 values are similar to calcite and less than aragonite. Surprisingly, substitution of an extraneous cation into a carbonate mineral (e.g., as observed in high-magnesium calcite) generally lowered Δ 63 by a slight amount, which suggests that high and low-magnesian calcites may have resolvable differences in equilibrium clumped isotope signatures, potentially providing an explanation for observations of different Δ 47 values in slow-growing minerals. These results imply the cationic composition of the solution may be a minor factor that should be considered for crystals that rapidly grow from solution, and that cationic substitution into carbonate mineral lattices will influence equilibrium compositions of carbonate minerals. Solution chemistry effects on mineral composition are likely to be observed at high growth rates. In contrast, equilibrium isotope compositions arising from cationic substitution in minerals, such as differences between high-Mg calcite and low-Mg calcite, or high-Mg calcite and dolomite, would likely be observed at slow growth rates. [ABSTRACT FROM AUTHOR]

When the Notre-Dame de Paris Cathedral burnt, the oak frame was almost completely destroyed. The temperatures reached were determined by an originalmethod of Raman paleothermometry applied to charcoals collected after the fire. The Raman spectra of the charcoals evolve monotonically and irreversibly with the carbonization temperature and can therefore be used as "fossil thermocouples". A calibration curve was constructed from the carbonization between 500 and 1300 ±C of unburnt oak pieces from the cathedral. Thus, the maximum charcoal formation temperature during the cathedral fire can be determined. The Raman analysis of charcoals sampled in situ serves to construct the first cartography of the carbonization temperatures. The temperatures reach values above 1200 ±C in the crossing. These experimental data are the basis for a discussion of the consequences of the fire regarding possible lead dissemination and thermal alterations of limestone. [ABSTRACT FROM AUTHOR]

Accurate temperature records for the deep geological past are a vital component of paleoclimate research. Distributional changes of branched glycerol dialkyl glycerol tetraether (brGDGT) lipids in geological archives including paleosoils are a promising indicators to infer past continental air temperatures. However, the 'orphan' status of the brGDGTs, the potential effect of temperature-independent parameters on their relative distribution, and the uneven geographical distribution of the soils used for calibration contribute to the high uncertainty of brGDGT-based transfer functions (root mean squared error, RMSE: ±5 °C). Here, we expand the soil dataset from the previous calibration(s) with new and published soil data. We use Bayesian statistics to calibrate the relationship of the 5-methyl brGDGTs (MBT′ 5Me) and mean annual air temperature (MAAT). The addition of soils from warm (>28 °C) environments from India substantially increases the upper limit of the Bayesian calibration (BayMBT) from 25 to 29 °C, aiding in the generation of temperature records for past greenhouse climates, such as the Eocene. The BayMBT model also effectively minimizes the structured MAAT residuals prevalent in previous calibrations, therefore giving the opportunity to explore confounding factors on the calibration. We formulate a set of alternative calibration models to test the effect of specific environmental parameters and show that soils at mid-latitudes with temperature seasonalities >20 °C are not well described by the BayMBT model. We find that the MBT′ 5Me index is best correlated to the average temperature of all months >0 °C, called the BayMBT 0 model. This finding supports the hypothesis that brGDGT production ceases or slows down in the winter months. However, a persistent feature of the BayMBT model and previous calibrations is the significant scatter at mid-latitudes, which is speculatively linked with a possible increase in diversity of microbial brGDGT-producing communities in these locations. [ABSTRACT FROM AUTHOR]