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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]

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]

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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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

Arctic shrubification is an observable consequence of climate change, already resulting in ecological shifts and global‐scale climate feedbacks including changes in land surface albedo and enhanced evapotranspiration. However, the rate at which shrubs can colonize previously glaciated terrain in a warming world is largely unknown. Reconstructions of past vegetation dynamics in conjunction with climate records can provide critical insights into shrubification rates and controls on plant migration, but paleoenvironmental reconstructions based on pollen may be biased by the influx of exotic pollen to tundra settings. Here, we reconstruct past plant communities using sedimentary ancient DNA (sedaDNA), which has a more local source area than pollen. We additionally reconstruct past temperature variability using bacterial cell membrane lipids (branched glycerol dialkyl glycerol tetraethers) and an aquatic productivity indicator (biogenic silica) to evaluate the relative timing of postglacial ecological and climate changes at a lake on southern Baffin Island, Arctic Canada. The sedaDNA record tightly constrains the colonization of dwarf birch (Betula, a thermophilous shrub) to 5.9 ± 0.1 ka, ~3 ka after local deglaciation as determined by cosmogenic 10Be moraine dating and >2 ka later than Betula pollen is recorded in nearby lake sediment. We then assess the paleovegetation history within the context of summer temperature and find that paleotemperatures were highest prior to 6.3 ka, followed by cooling in the centuries preceding Betula establishment. Together, these molecular proxies reveal that Betula colonization lagged peak summer temperatures, suggesting that inefficient dispersal, rather than climate, may have limited Arctic shrub migration in this region. In addition, these data suggest that pollen‐based climate reconstructions from high latitudes, which rely heavily on the presence and abundance of pollen from thermophilous taxa like Betula, can be compromised by both exotic pollen fluxes and vegetation migration lags. [ABSTRACT FROM AUTHOR]

Coral skeletons are the most commonly used high‐resolution temperature proxy in the tropical oceans, providing paleoclimate reconstructions dating back centuries to millennia. However, physiological differences in skeletal formation modes together with artifacts arising from coral biomineralization (vital effects) can confound the temperature dependence of single element‐to‐calcium ratios. In efforts to reduce vital effects and isolate temperature, new approaches have been developed based on Sr‐U and Li/Mg, which combine Sr/Ca and U/Ca and Li/Ca and Mg/Ca, respectively. Here we examine the systematics of Sr/Ca, Sr‐U, and Li/Mg paleothermometry in 33 colonies of branching (Acropora, Pocillopora, and Stylophora) and foliose (Turbinaria) genera. To address the calibration of these morphologically complex calcifiers, we conducted repeat field trips every 1 to 3 months and collected the most recent (~1 month) uppermost growth of individual colonies over ~18‐ to 24‐month periods. This enables seasonally resolved calibration of genera that exhibit rapid extension and slower secondary calcification. Based on this experimental design, we show that all three proxies capture seasonal to annual temperature variations for their respective growth intervals, providing calibrations across an 11 °C range. Species effects on the temperature dependence were largest for Sr/Ca (22.7%) yet minor for Li/Mg (7.2%) and Sr‐U (6.3%). Residuals from proxy‐temperature regressions were correlated between Sr/Ca and Li/Mg, indicating similar biological processes may influence Sr/Ca and Li/Mg thermometry. The implications of this study are that Sr‐U and to a lesser extent Li/Mg are applicable to fossil branching coral skeletons identified to genus level without the need for modern‐day calibration. We further show that all three paleothermometers provide complementary temperature constraints, with Li/Mg and the more species‐dependent Sr/Ca showing greater effectiveness at resolving seasonal variability and Sr‐U showing greater reliability at capturing mean annual temperature. Plain Language Summary: In this paper, we report skeletal trace element compositions in seven branching and plating coral species from locations spanning an ~11 °C mean monthly temperature range and latitudes of ~23 to 34 °S along the Western Australian coastline in the southeast Indian Ocean. We compare three coral paleothermometers (Sr/Ca, Li/Mg, and Sr‐U) with respect to performance and species effects. Repeat field trips to collect coral skeletons were undertaken regularly (approximately bimonthly) and skeletons were subsampled to obtain only the most recent ~1 month of new growth, minimizing sampling of secondary skeletal infilling. Using this experimental design, we find that all three paleothermometers record seasonal cycles in temperature, overcoming limitations posed by complex secondary infilling calcification. We find that coral Sr/Ca shows the largest offsets between species (22.7%) with residuals between Sr/Ca and Li/Mg proxy‐temperature regressions being correlated, indicating that similar biological processes influence both Sr/Ca and Li/Mg thermometry. Coral Sr‐U and to a lesser extent Li/Mg exhibit significantly reduced species offsets and appear largely unaffected by the vital effects that influence Sr/Ca. These results support the applicability of coral paleothermometry, especially Sr‐U, for deriving mean annual temperature. We thus provide an approach for reconstructing paleotemperatures using trace elements in common branching fossil coral skeletons without the need for modern calibrations. Key Points: Geochemical temperature proxy calibrations are obtained for branching and foliose corals by repeat (~ bimonthly) collection of new skeletal growthLi/Mg and Sr/Ca were most effective for resolving seasonal temperature while Sr‐U was most effective for capturing mean annual temperatureA multiproxy approach is recommended where Sr‐U is applied to estimate mean annual temperature and Sr/Ca and Li/Mg are used to resolve subannual temperature changes [ABSTRACT FROM AUTHOR]

The oxygen isotopic composition of planktic foraminiferal calcite (δ18Oc) is one of the most prevalent proxies used in the paleoceanographic community. The relationship between δ18Oc, temperature, and seawater oxygen isotopic composition (δ18Ow) is firmly rooted in thermodynamics, and experimental constraints are commonly used for sea surface temperature (SST) reconstructions. However, in marine sedimentary applications, additional sources of uncertainty emerge, and these uncertainty constraints have not as of yet been included in global calibration models. Here, we compile a global data set of over 2,600 marine sediment core top samples for five planktic species: Globigerinoides ruber, Trilobatus sacculifer, Globigerina bulloides, Neogloboquadrina incompta, and Neogloboquadrina pachyderma. We developed a suite of Bayesian regression models to calibrate the relationship between δ18Oc and SST. Spanning SSTs from 0.0 to 29.5 °C, our annual model with species pooled together has a mean standard error of approximately 0.54‰. Accounting for seasonality and species‐specific differences improves model validation, reducing the mean standard error to 0.47‰. Example applications spanning the Late Quaternary show good agreement with independent alkenone‐based estimates. Our pooled calibration model may also be used for reconstruction in the deeper geological past, using modern planktic foraminifera as an analog for non‐extant species. Our core top‐based models provide a robust assessment of uncertainty in the δ18Oc paleothermometer that can be used in statistical assessments of interproxy and model‐proxy comparisons. The suite of models is publicly available as the Open Source software library bayfox, for Python, R, and MATLAB/Octave. Key Points: We develop Bayesian calibration models for planktic δ18OcAccounting for seasonal abundance and species‐specific sensitivities improves inferenceOur models produce realistic SST reconstructions for both recent and "deep‐time" δ18Oc data [ABSTRACT FROM AUTHOR]

During the Eocene‐Oligocene Transition (EOT; 34–33.5 Ma), Antarctic ice sheets relatively rapidly expanded, leading to the first continent‐scale glaciation of the Cenozoic. Declining atmospheric CO2 concentrations and associated feedbacks have been invoked as underlying mechanisms, but the role of the quasi‐coeval opening of Southern Ocean gateways (Tasman Gateway and Drake Passage) and resulting changes in ocean circulation is as yet poorly understood. Definitive field evidence from EOT sedimentary successions from the Antarctic margin and the Southern Ocean is lacking, also because the few available sequences are often incomplete and poorly dated, hampering detailed paleoceanographic and paleoclimatic analysis. Here we use organic dinoflagellate cysts (dinocysts) to date and correlate critical Southern Ocean EOT successions. We demonstrate that widespread winnowed glauconite‐rich lithological units were deposited ubiquitously and simultaneously in relatively shallow‐marine environments at various Southern Ocean localities, starting in the late Eocene (~35.7 Ma). Based on organic biomarker paleothermometry and quantitative dinocyst distribution patterns, we analyze Southern Ocean paleoceanographic change across the EOT. We obtain strong indications for invigorated surface and bottom water circulation at sites affected by polar westward‐flowing wind‐driven currents, including a westward‐flowing Antarctic Countercurrent, starting at about 35.7 Ma. The mechanism for this oceanographic invigoration remains poorly understood. The circum‐Antarctic expression of the phenomenon suggests that, rather than triggered by tectonic deepening of the Tasman Gateway, progressive pre‐EOT atmospheric cooling played an important role. At localities affected by the Antarctic Countercurrent, sea surface productivity increased and simultaneously circum‐Antarctic surface waters cooled. We surmise that combined, these processes contributed to preconditioning the Antarctic continent for glaciation. Plain Language Summary: The ice sheets of Antarctica are geologically a relatively recent phenomenon. Only by the end of the Eocene Epoch (±34 million years ago), major ice sheets began to develop, likely related to declining greenhouse gas concentrations. We still do not understand what the role—if any—of the tectonic openings of key land bridges (i.e., the present‐day ocean conduits between Antarctica and Tasmania and the southern tip of South America) was in cooling the Antarctic continent and stimulating it to become glaciated. In this study we use organic marine microfossils to date and correlate several marginal marine sediment successions, dispersed throughout the Southern Ocean. We then show that the sediment composition at these sites changed abruptly throughout the Southern Ocean by about 35.7 million years ago, roughly two million years before the ice sheets rapidly expanded. We interpret this change in sediment composition to reflect enhanced surface ocean circulation. We furthermore analyzed chemical fossils to derive changes in past sea‐water temperatures. By combining these data with counts of the marine organic microfossil species, we reconstructed past environmental change across the periods prior, during and after the growth of the Antarctic ice sheets. The results indicate that from about 35.7 million years ago onward, enhanced surface ocean circulation led to sediment winnowing, higher biological productivity in‐ and cooling of the surface waters around Antarctica. Irrespective of deepening of the Tasman Conduit, progressive intensification of ocean currents, probably as a result of stronger atmospheric circulation need to be considered in understanding the conditions that allowed rapid Antarctic ice sheet to expansion. Key Points: Late Eocene accelerated deepening of the Tasman Gateway led to invigorated surface and bottom water circulation in the Southern OceanBiomarker paleothermometry and quantitative dinocyst distribution patterns coevally demonstrate cooling and enhanced productivityInvigoration of a wind‐driven Antarctic counter current had profound effects and aided preconditioning Antarctica for glacial expansion [ABSTRACT FROM AUTHOR]

RI‐OH (ring index of hydroxylated tetraethers) has recently been proposed to reconstruct paleotemperatures in middle‐ to low‐latitude marginal seas. However, RI‐OH has barely been tested in marginal seas under substantial terrigenous inputs. Here we analyze tetraether lipids in two adjacent marine cores from the Gulf of Lions. We then test for the first time the RI‐OH paleothermometer from 160 to 9 ka BP in the western Mediterranean Sea. While terrigenous inputs prevent TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms) from behaving as a paleothermometer, RI‐OH is generally consistent with other paleothermometric proxies. RI‐OH also responds systematically and coherently to glacial‐interglacial transitions as well as to abrupt climatic events. The average difference between RI‐OH temperatures and November–May UK′37 (C37 ketone unsaturation ratio) temperatures is −2.0 °C with a standard error of 0.4 °C based on 249 RI‐OH‐UK′37 comparisons. This systematic difference suggests that hydroxylated tetraethers and alkenones record different temperatures, for instance, winter and/or subsurface temperatures for RI‐OH. Another source of bias could be linked to the available RI‐OH‐temperature calibration, which clearly needs more work at the global and regional scales, notably for semienclosed basins such as the Mediterranean Sea. Nevertheless, our RI‐OH‐based interglacial‐glacial anomalies are of 10 °C, a value within the high end of anomalies from previously published temperature records in the western Mediterranean Sea (from 3 to 13 °C). The RI‐OH‐based temperature anomalies also confirm the regional differences and seasonal contrasts in interglacial‐glacial anomalies produced by models. Key Points: RI‐OH consistently records glacial‐interglacial transitions and abrupt climatic events despite substantial terrigenous inputsRI‐OH could reflect winter and/or subsurface temperatures rather than annual sea surface temperatures in the Gulf of LionsRI‐OH‐based interglacial‐glacial anomalies are high but plausible compared to reported anomalies in the western Mediterranean Sea [ABSTRACT FROM AUTHOR]

The unsaturation ratios of alkenones have been applied widely to reconstruct pelagic sea surface temperatures (SSTs). However, applications to costal settings have been hampered by the effects of salinity and nutrient dynamics. Here we present a 4.5‐year‐long record of alkenones in water column particulate organic matter from sites in Narragansett Bay (RI, USA) spanning wide ranges in salinity and nutrients, and a 300‐year alkenone record from two Narragansett Bay sediment cores. Particulate organic matter results suggest that there are two distinct alkenone producing populations. One is a widespread population that blooms in summer (July–September) and displays alkenones typical of Group III producers—the basis for the widely applied marine Uk'37 temperature index. The other population is confined to brackish waters in the Providence River during spring salinity minima and displays alkenone profiles consistent with Group I production—the first detection of Group I in a brackish nonlacustrine environment. The sedimentary alkenone profiles consist of only Group III alkenone distributions, allowing for the reconstruction of a 300‐year‐long record of Narragansett Bay SST. Comparison of the most recent 100 years of this reconstruction with local and regional instrumental SST records shows excellent agreement, confirming a regional pattern of SST rise in the coastal northeastern United States that exceeds global or hemispheric rates. Group III alkenone production in coastal settings may be common and may open the way to high resolution local and regional SST reconstruction. Key Points: A 4.5‐year‐long particulate organic matter alkenone record is compared with measured temperature and salinity in Narragansett BaySurface water alkenone profiles suggest widespread Group III production and spatiotemporally restricted Group I productionWe use sediment Uk'37 ratios to develop a 300‐year‐long record of sea surface temperature that is consistent with local and regional records [ABSTRACT FROM AUTHOR]

The response of the Humboldt Current System to future global warming is uncertain. Here we reconstruct alkenone‐derived near‐surface temperatures from multiple cores along the Peruvian coast to infer the driving mechanisms of upwelling changes for the last 20 kyr. Our records show a deglacial warming consistent with Antarctic ice‐core temperatures and a Mid‐Holocene cooling, which, in combination with other paleoceanographic records, suggest a strengthening of upwelling conditions. This cooling, during the globally warm Mid‐Holocene, is consistent with an intensification of the Walker Circulation and the South Eastern Pacific Subtropical Anticyclone, indicative of La Niña‐like conditions in the Tropical Pacific. Surprisingly, oxygen contents in the subsurface increased and productivity was low during the Mid‐Holocene, which are at odds with La Niña‐like conditions. This suggests that the Humboldt Current System reacts in multiple ways to a warmer world and may even include a reversal in the present day subsurface deoxygenation. Plain Language Summary: The Humboldt Current System (HCS) located off Peru produces almost 10% of the global fish catch. This high productivity is promoted by the upwelling of subsurface nutrient‐rich water driven by the effect of the winds over the ocean surface. However, the HCS is highly sensitive to climate change, and the effect of continued warming on upwelling dynamics and in consequence for fishery productivity is not known. In the present work we reconstruct changes in upwelling during the last 20 thousand years to understand the mechanisms that drove upwelling changes in the past. For this purpose we use organic molecules preserved in marine sediments that are sensitive to water temperatures. We found cool temperatures during the mid‐Holocene (8 to 4 thousand years ago), a period characterized by global warm conditions, that we infer as an increase in upwelling intensity. Surprisingly, the productivity was low, and the oxygen contents in the subsurface waters were high which is at odds with increased upwelling off Peru. These results indicate that the HCS responds in multiple and unexpected ways to global warming, suggesting that any impact of future global warming on the biological productivity in the HCS remains uncertain. Key Points: Near‐surface temperatures were reconstructed in multiple cores along the Peruvian marginOur data show a deglacial warming consistent with Antarctic ice‐core temperatures and a Mid‐Holocene coolingStrong upwelling during the Mid‐Holocene is associated with a weak Oxygen Minimum Zone and low export production [ABSTRACT FROM AUTHOR]

About a decade after its introduction, the field of carbonate clumped isotope thermometry is rapidly expanding because of the large number of possible applications and its potential to solve long‐standing questions in Earth Sciences. Major factors limiting the application of this method are the very high analytical precision required for meaningful interpretations, the relatively complex sample preparation procedures, and the mass spectrometric corrections needed. In this paper we first briefly review the evolution of the analytical and standardization procedures and discuss the major remaining sources of uncertainty. We propose that the use of carbonate standards to project the results to the carbon dioxide equilibrium scale can improve interlaboratory data comparability and help to solve long‐standing discrepancies between laboratories and temperature calibrations. The use of carbonates reduces uncertainties related to gas preparation and cleaning procedures and ensures equal treatment of samples and standards. We present a set of carbonate standards of diverse composition, discuss how they can be used to correct for mass spectrometric biases, and demonstrate that their use significantly improves the comparability among four laboratories. We propose that the use of these standards or of a similar set of carbonate standards will improve the comparability of data across laboratories. Key Points: Standardization and correction procedures for carbonate clumped isotopes are reviewedWe propose a carbonate‐based standardization scheme to improve reproducibilityWe demonstrate improved interlaboratory comparability of clumped isotope measurements [ABSTRACT FROM AUTHOR]

Abstract: Paleomagnetic data can be used to estimate deposit temperatures (Tdep) of pyroclastic density currents (PDCs) by finding the laboratory temperature at which a PDC‐associated thermal remanence is removed. Paleomagnetic paleothermometry assumes that (1) blocking (Tb) and unblocking (Tub) temperatures are equivalent, and (2) the blocking spectrum remains constant through time. The first assumption fails for multidomain (MD) grains, and recent evidence shows that the second is violated in many titanomagnetites, where Tc is a strong function of thermal history. Here we assess the extent to which the standard paleomagnetic method may be biased by a changing Tb spectrum, and we explore a new magnetic technique that instead exploits these changes. Using samples from the 1980 PDCs at Mt. St. Helens, we find that standard methods on oriented lithic clasts provide a Tdep range that overlaps with measured temperatures, but is systematically slightly higher. By contrast, juvenile pumice give Tdep_min estimates that greatly exceed lithic estimates and measured temperatures. We attribute this overestimate to (1) depth‐dependent variations in Tc and Tub resulting from thermally activated crystal‐chemical reordering and (2) MD titanomagnetite where Tub > Tb. Stratigraphic variations in Tc are interpreted in terms of Tdep, giving results mostly consistent with measured temperatures and with the lower end of estimates from lithic clasts. This new method allows us to evaluate temporal and spatial variations in Tdep that would not have been possible using standard paleomagnetic techniques in these lithic‐poor deposits. It also provides information on deposits not accessible by surface temperature probes. [ABSTRACT FROM AUTHOR]

Paleozoic rocks in the Cantabrian Zone, and the Variscan foreland fold-and-thrust belt on the Iberian Peninsula have been affected by a sequence of diagenetic to epizonal thermal events. Late- to Post-Variscan hot fluid circulation caused a large-scale burial dolomitization and ore mineralization, mostly in Cambrian and a Lower to Middle Carboniferous carbonate sucessions. The goal of this study is to analyze and compare the temperatures experienced by the carbonate precursor rocks, as well as the under- and over-lying siliciclastic ambient rocks to gain a better understanding of the thermicity of dolomitization. These temperatures are evaluated based on published paleothermal datasets combined with new data obtained from Rock-Eval pyrolysis and vitrinite reflectance analysis of Carboniferous rocks rich in organic matter. The overall results indicate that reworking of detrital sediments in synorogenic ambient siliciclastics results in an anomalously high thermal maturity recorded by bulk rock techniques such as illite crystallinity and Rock-Eval pyrolysis. In situ VR-derived paleotemperatures recorded by ambient siliciclastic rocks appear to be higher compared to CAI-derived temperatures for carbonate precursor rocks. This variation in thermal maturity is likely related to the analytical techniques used to obtain CAI and VR data, and the empirical equations applied to calculate corresponding paleotemperatures. Conodont fragments were not as sensitive compared to vitrinite, and the color alteration process could have suffered from hydrothermal alteration. A secondary cause might be a different response to mechanical deformation between siliciclastic and carbonate units during the Variscan and post-Variscan geodynamic evolution of the study area. Rigid precursor carbonate units experienced fluid circulation mainly along distinct and spaced fracture zones, creating fracture-related dolomite geobodies and ore mineralization. Soft ambient siliciclastic rocks experienced more diffuse fluid circulation and heat dissipation. The different paleothermometry datasets compiled for the study area indicate that the fluids circulating during Late- to Post-Variscan times, with associated fracture-related dolomitization and ore mineralization in carbonate precursors, are hydrothermal. The highest paleotemperatures were recorded in ambient and precursor rocks in the highly tectonized northern part of the study area, where several thrusts and faults allowed intense fluid circulation. Positive temperature anomalies within the precursor carbonates correlate well with the occurrence of dolomite geobodies and ore mineral deposits. Such anomalies could thus be used as an exploration tool for hydrothermal dolomite bodies in analog sub-surface settings. [ABSTRACT FROM AUTHOR]

Thrusting at shallow depths often precludes analysis by means of structural indicators effective in other geological contexts (e.g., mylonites, sheath folds, shear bands). In this paper, a combination of techniques (including structural analysis, magnetic methods, as anisotropy of magnetic susceptibility and paleomagnetism, and paleothermometry) is used to define thrusting conditions, deformation, and transport directions in the Cameros-Demanda thrust (North Spain). Three outcrops were analyzed along this intraplate, large-scale major structure having 150 km of outcropping length, 30 km of maximum horizontal displacement, and 5 km of vertical throw. Results obtained by means of the different techniques are compared with data derived from cross sections and stratigraphic analysis. Mixed-layer illite-smectite and vitrinite reflectance indicating deep diagenetic conditions and mature stage of hydrocarbon generation suggests shallow depths during deformation, thus confirming that the protolith for most of the fault rocks is the footwall of the main thrust. Kinematic indicators (foliation, S/C structures, and slickenside striations) indicate altogether a dominant NNW movement of the hanging wall in the western zone and NE in the eastern zone of the thrust, thus implying strain partitioning between different branches of the main thrust. The study of AMS in fault rocks (nearly 400 samples of fault gouge, breccia, and microbreccia) indicates that the strike of magnetic foliation is oblique to the transport direction and that the magnetic lineation parallelizes the projection of the transport direction onto the k / k plane in sites with strong shear deformation. Paleomagnetism applied to fault rocks indicates the existence of remagnetizations linked to thrusting, in spite of the shallow depth for deformation, and a strong deformation or scattering of the magnetic remanence vectors in the fault zone. The application of the described techniques and consistency of results indicate that the proposed multidisciplinary approach is useful when dealing with thrusts at shallow crustal levels. [ABSTRACT FROM AUTHOR]

Thermoluminescence (TL) paleothermometry enables direct determination of past rock surface temperatures. For this, it exploits the dependence of electron de-trapping on ambient temperatures over geological timescales. It focuses on trapping energies connected to shorter lifetimes and lower-temperature signals (i.e. TL signals in the ~180-250 °C range of quartz and feldspar) that are typically disregarded during dating applications. The use of feldspar is particularly promising due to the continuous distribution of trapping energies and thus paleothermometers. Recent methodological advances [1] have made it feasible to explore the possibility of worldwide continental rock surface temperature reconstruction. This allows for headway to be made in closing the knowledge gap of continental air temperatures during the Last Glacial Maximum (LGM) and Pleistocene-Holocene transition. Our ultimate objective is to test the applicability of TL paleothermometry by studying sites from Scandinavia to equatorial east Africa, aiming to provide LGM to Holocene surface temperature fluctuations for a meridional gradient of approximately 7000 km length. In addition, we have recently sampled two altitudinal gradients for the Mont Blanc Massif (France) and Rwenzori Mountains (Uganda) to constrain changes in the atmospheric lapse rate since the LGM. Such data are critical in testing climate models and inferring the dynamics in atmospheric circulation across glacial-interglacial cycles. This contribution presents the methodology and associated analytical steps of TL paleothermometry by relaying first results of kinetic parameter determination and dose response characterisation for samples from the Mont Blanc Massif. In addition, we address one of the main methodological challenges of the TL paleothermometry method: the inherent temperature offset between the reconstructed rock surface temperatures and respective past surface air temperatures. To overcome this challenge, reliable offset correction related to site-specific factors such as aspect, elevation and potential incoming solar irradiation is required. We thus conclude our contribution with discussing the potential of the method and future methodological work (e.g., temperature offset correction and definition of saturation levels for various quartz and feldspar signals) required to establish the TL paleothermometry method. [ABSTRACT FROM AUTHOR]

The Paleocene-Eocene Thermal Maximum (PETM) is a remarkable climatic and environmental event that occurred 56 Ma ago and has importance for understanding possible future climate change. The Paleocene-Eocene transition is marked by a rapid temperature rise contemporaneous with a large negative carbon isotope excursion (CIE). Both the temperature and the isotopic excursion are well-documented by terrestrial and marine proxies. The CIE was the result of a massive release of carbon into the atmosphere. However, the carbon source and quantities of CO2 and CH4 greenhouse gases that contributed to global warming are poorly constrained and highly debated. Here we combine an established oxygen isotope paleothermometer with a newly developed triple oxygen isotope paleo-CO2 barometer. We attempt to quantify the source of greenhouse gases released during the Paleocene-Eocene transition by analyzing bioapatite of terrestrial mammals. Our results are consistent with previous estimates of PETM temperature change and suggest that not only CO2 but also massive release of seabed methane was the driver for CIE and PETM. [ABSTRACT FROM AUTHOR]

Isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs), major membrane lipids in some Archaea, have been widely used to examine past archaeal activity and infer ancient paleotemperature in aquatic environments. However, it is still unclear whether soil thaumarchaeotal iGDGTs also record temperature, and how thaumarchaeotal iGDGTs in the different climatic zones respond to temperature. Here we investigated iGDGTs in soils along an altitudinal transect of Mt. Shennongjia in central China, and determined the response of thaumarchaeotal membrane lipids to temperature in soils from a typical subtropical region. The TEX86proxy, reflecting the number of GDGT cyclopentyl moieties, in these soils is significantly correlated with mean annual air temperature (MAT) at a given site, apparently recording a temperature response by Thaumarchaeota that dominate in soils. However, the TEX86values are similar both at Mt. Shennongjia where a warm and humid climate prevails and at Mt. Xiangpi where a cold and dry climate is predominant. This could result from the seasonality of thaumarchaeotal production in soils from cold and (or) dry regions, and a local calibration is required if the TEX86proxy is to be applied to reconstruct paleoaltimetry or paleotemperature in paleosols from cold and (or) dry regions. The TEX86for soils from Mt. Rungwe (Tanzania), Mt. Jianfengling (China) and another climatic transect generally follows the rising trend of TEX86vs. MAT for Mt. Shennongjia, suggesting that the thaumarchaeotal iGDGTs in soils from subtropical and tropical regions respond to temperature similarly among sites. However, the different calibrations of TEX86paleothermometer among sites also reveal factors other than MAT may affect the TEX86values in soils. [ABSTRACT FROM AUTHOR]

Oxygen isotopic ratios of conodont apatite can be a robust proxy for sea surface temperatures and provide important constraints on global climate. The oxygen isotopic composition of seawater is a function of local, regional, and global processes. To determine the relative importance of regional and local influences (and therefore make global inferences), it is necessary to document high resolution δ18O records from a variety of paleogeographic and environmental settings. Available conodont δ18Ophos records for the Ordovician, however, are biased towards North American samples, often come from multiple discontinuous sections, and tend to focus only on specific intervals (e.g., the Katian Stage). To extend the paleogeographic range and test the generality of Ordovician δ18Ophos trends, we measured δ18Ophos values from species-specific conodont assemblages from New South Wales, Australia. Our results from this region show that Early Ordovician δ18Ophos averages are ∼2‰ lower than those in the Late Ordovician and are consistently ∼2.5‰ lower relative to values reported from North America and central Australia. This offset suggests that regional environmental conditions influenced the δ18Ophos record from New South Wales. Despite evidence suggesting a departure from open ocean conditions, our results still show increasing δ18Ophos values through the Early Ordovician in agreement with documented trends from other regions. This similarity suggests that the δ18Ophos increase (documented on separate continental blocks and different environmental settings) reflects global cooling rather than a change in the oxygen isotopic composition of the Ordovician oceans and strengthens the argument that biodiversification and climate change in the first half of the Ordovician were related. [ABSTRACT FROM AUTHOR]

Carbonate clumped isotope paleothermometry is based upon the principle that the formation temperature of carbonates is proportional to the relative abundance of COO in CO produced through carbonate acid digestion. Furthermore, the relative abundance of COO is independent of the δO value of parent water, providing a viable alternative to the classic oxygen isotope carbonate-water paleothermometer. The temperature resolution of this newly developed paleothermometer primarily relies on the analytical techniques required to determine the clumped isotope composition of a carbonate mineral. These analytical techniques involve: (1) CO gas evolution, in which carbonate is reacted with phosphoric acid, yielding CO with a clumped isotope composition proportional to the carbonate formation temperature; (2) CO purification, in which a rigorous purification of acid-liberated CO is achieved by removing contaminants; (3) clumped isotope measurement, in which a customized gas-source isotope ratio mass spectrometer quantifies the raw clumped isotope composition (Δ) of the purified CO gas; and (4) inter-laboratory standardization, in which raw Δ values are normalized with respect to a community-accepted reference frame. This review provides an overview and comparison of the analytical techniques currently utilized in stable isotope laboratories for Δ measurements, specifically discussing carbonate pre-treatment techniques, phosphoric acid digestion systems, isotope and temperature effects during carbonate phosphoric acid digestion, CO purification processes, and the challenges associated with the measurement and standardization of the Δ value. Continued refinement of these analytical techniques will aid in reducing sample size, increasing sample throughput, and improving Δ external precision and associated paleotemperature resolution. Furthermore, these advances may assist in elucidating the source of discrepancies in slope between two groups of reported carbonate clumped isotope thermometer calibrations. [ABSTRACT FROM AUTHOR]

RATIONALE: The measurement of multiply substituted isotopologues of CO2 derived from carbonate has allowed the reconstruction of paleotemperatures from a single phase (CaCO3), circumventing uncertainty inherent in other isotopic paleothermometers. Traditional analytical techniques require relatively large amounts of carbonate (3-8 mg per replicate), which limits the applicability of the clumped isotope proxy to certain geological materials such as marine microfossils, commonly used for paleoclimate reconstructions. METHODS: Clumped isotope ratio measurements of small samples were made on a new, high-efficiency, dual-reservoir sample-preparation inlet system attached to a Thermo-Finnigan MAT 253 mass spectrometer. Sample gas produced on the inlet is introduced from a 10 mL reservoir directly into the source via a capillary. Reference gas fills an identical 10 mL reservoir installed between the reference bellows and the capillary. The gas pressures in the two reservoirs are initially balanced, and are allowed to decrease together over the run. RESULTS: Carbonate samples from 1 mg to 2.6 mg produced Δ47 values equivalent to those from the traditional two-bellows method with identical single-sample precision (1 SE = 0.005-0.015‰) and external standard error (SE = 0.006-0.015‰, n=4-6). The size of sample needed to achieve good precision is controlled by the sensitivity of the mass spectrometer and the size of the fixed reservoirs and adjacent U-trap installed on our inlet. CONCLUSIONS: The high-precision clumped isotope measurements of small aliquots of carbonate obtained in this method allows for the application of this proxy to a wider range of geological sample materials, such as marine microfossils, that until now have been nearly impossible to analyze given sample size limitation. [ABSTRACT FROM AUTHOR]

We present and examine a multi-sensor global compilation of mid-Holocene (MH) sea surface temperatures (SST), based on Mg/Ca and alkenone palaeothermometry and reconstructions obtained using planktonic foraminifera and organic-walled dinoflagellate cyst census counts. We assess the uncertainties originating from using different methodologies and evaluate the potential of MH SST reconstructions as a benchmark for climate-model simulations. The comparison between different analytical approaches (time frame, baseline climate) shows the choice of time window for the MH has a negligible effect on the reconstructed SST pattern, but the choice of baseline climate affects both the magnitude and spatial pattern of the reconstructed SSTs. Comparison of the SST reconstructions made using different sensors shows significant discrepancies at a regional scale, with uncertainties often exceeding the reconstructed SST anomaly. Apparent patterns in SST may largely be a reflection of the use of different sensors in different regions. Overall, the uncertainties associated with the SST reconstructions are generally larger than the MH anomalies. Thus, the SST data currently available cannot serve as a target for benchmarking model simulations. Further evaluations of potential subsurface and/or seasonal artifacts that may contribute to obscure the MH SST reconstructions are urgently needed to provide reliable benchmarks for model evaluations. [ABSTRACT FROM AUTHOR]

The mass extinction at the Cretaceous-Paleogene boundary, ~66 Ma, is thought to be caused by the impact of an asteroid at Chicxulub, present-day Mexico. Although the precise mechanisms that led to this mass extinction remain enigmatic, most postulated scenarios involve a short-lived global cooling, a so-called "impact winter" phase. Here we document a major decline in sea surface temperature during the first months to decades following the impact event, using TEX86 paleothermometry of sediments from the Brazos River section, Texas. We interpret this cold spell to reflect, to our knowledge, the first direct evidence for the effects of the formation of dust and aerosols by the impact and their injection in the stratosphere, blocking incoming solar radiation. This impact winter was likely a major driver of mass extinction because of the resulting global decimation of marine and continental photosynthesis. [ABSTRACT FROM AUTHOR]