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Growing pressure on sustainable water resource allocation in the context of global development and rapid environmental change demands rigorous knowledge of how regional water cycles change through time. One of the most attractive and widely utilized approaches for gaining this knowledge is the analysis of lake carbonate stable isotopic compositions. However, endogenic carbonate archives are sensitive to a variety of natural processes and conditions leaving isotopic datasets largely underdetermined. As a consequence, isotopic researchers are often required to assume values for multiple parameters, including temperature of carbonate formation or lake water δ 18 O, in order to interpret changes in hydrologic conditions. Here, we review and analyze a global compilation of 57 lacustrine dual carbon and oxygen stable isotope records with a topical focus on the effects of shifting hydrologic balance on endogenic carbonate isotopic compositions. Through integration of multiple large datasets we show that lake carbonate δ 18 O values and the lake waters from which they are derived are often shifted by >+10‰ relative to source waters discharging into the lake. The global pattern of δ 18 O and δ 13 C covariation observed in >70% of the records studied and in several evaporation experiments demonstrates that isotopic fractionations associated with lake water evaporation cause the heavy carbon and oxygen isotope enrichments observed in most lakes and lake carbonate records. Modeled endogenic calcite compositions in isotopic equilibrium with lake source waters further demonstrate that evaporation effects can be extreme even in lake records where δ 18 O and δ 13 C covariation is absent. Aridisol pedogenic carbonates show similar isotopic responses to evaporation, and the relevance of evaporative modification to paleoclimatic and paleotopographic research using endogenic carbonate proxies are discussed. Recent advances in stable isotope research techniques present unprecedented opportunities to overcome the underdetermined nature of stable isotopic data through integration of multiple isotopic proxies, including dual element 13 C-excess values and clumped isotope temperature estimates. We demonstrate the utility of applying these multi-proxy approaches to the interpretation of paleohydroclimatic conditions in ancient lake systems. Understanding past, present, and future hydroclimatic systems is a global imperative. Significant progress should be expected as these modern research techniques become more widely applied and integrated with traditional stable isotopic proxies. [ABSTRACT FROM AUTHOR]

There is increasing evidence that nitrifying Thaumarchaeota in the deep ocean waters may contribute to the sedimentary composition of isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs), impacting TEX86 paleothermometry. We investigated the potential effect of deep-water dwelling Thaumarchaeota in the warm and saline Mediterranean Outflow Water (MOW) on the distribution of isoGDGTs by analysing suspended particulate matter (SPM) and surface sediments collected along five land-ocean transects along the southern Portuguese continental margin. To this end, we directly compared for the first time the composition of intact polar lipid (IPL)-derived isoGDGTs of SPM with the diversity, abundance, and activity of Thaumarchaeota based on the genetic analysis of the genes coding for the archaeal ammonia monooxygenase (amoA) and the geranylgeranylglyceryl phosphate (GGGP) synthase involved in the isoGDGT biosynthetic pathway. Our results revealed a strong positive relationship between water depth and TEX86H values for both SPM and surface sediments. The increasing TEX86H trends for both core lipid (CL) and IPL-derived fractions were accompanied by increasing fractional abundances of GDGT-2 and crenarchaeol regio-isomer and decreasing fractional abundances of GDGT-1 and GDGT-3 with increasing water depth. Phylogenetic analyses based on the archaeal amoA and the GGGP synthase proteins showed that Thaumarchaeota populations detected at 1 m and 50 m water depth were different from those detected in 200 m and 1000 m water depth, which had an increased contribution of so-called 'deep water' Thaumarchaeota. The differences in the fractional abundances of isoGDGTs with water depth were compatible with the increasing contribution of 'deep water' Thaumarchaeota harboring a different GGGP synthase enzyme which has been suggested to relate to changes in the relative proportion of synthesized isoGDGTs. Accordingly, it appears that the sedimentary distribution of CL isoGDGTs used in TEX86H along the Portuguese margin is primarily influenced by water depth due to the increasing contribution of the deep-water population of Thaumarchaeota residing in the MOW. Our study also reveals that the effect of deep water Thaumarchaeotal communities on sedimentary isoGDGT distributions should be considered globally. [ABSTRACT FROM AUTHOR]

In this study we have compared the oxygen isotopic composition of two genera of benthic foraminifera (Uvigerina and Cibicidoides) from core-top samples with modern oxygen isotopic composition of seawater (delta18O). Based on a new relationship between delta18O and salinity for the mid-latitude western South Atlantic, we estimated the isotopic composition of equilibrium calcite (delta18Oeq) using two different equations: (1) O'Neil et al. (1969), modified by McCorkle et al. (1997) and (2) Kim & O'Neil (1997). When using (1), the small difference between delta18Oeq and delta18O of Uvigerina suggests that this genus precipitates its shell close to equilibrium with ambient seawater. The delta18O Cibicidoides data are 0.82 ‰ lower than the predicted (equilibrium) oxygen isotopic composition. Conversely, using (2) the Cibicidoides delta18O data show excellent agreement with the oxygen isotopic composition predicted from delta18O and water temperature while Uvigerina delta18O data are 0.69 ‰ higher than predicted oxygen isotope equilibrium values. Based on the evidences presented here and on the results from previous studies we suggest using the genus Cibicidoides and applying Kim & O'Neil's (1997) equation for down-core paleotemperature investigations. In the absence of enough Cibicidoides specimens we suggest using Uvigerina delta18O data and applying a correction factor of -0.69 ‰.Neste estudo, compara-se a composição de isótopos de oxigênio de dois gêneros de foraminíferos bentônicos (Uvigerina e Cibicidoides) de amostras de topo de testemunho com a composição isotópica moderna da água do mar (delta18O). Baseados em uma nova relação entre delta18O e salinidade para a latitude média do Atlântico Sul ocidental, estimou-se a composição isotópica da calcita em equilíbrio (delta18Oeq) a partir de duas equações diferentes: (1) O'Neil et al. (1969), modificada por McCorkle et al. (1997) e (2) Kim & O'Neil (1997). Utilizando (1), a pequena diferença entre delta18Oeq e delta18O de Uvigerina sugere que este gênero precipita as suas testas próximo ao equilíbrio com a água. Já os dados de delta18O de Cibicidoides são 0,82 ‰ menores que a composição isotópica prevista. Ao contrário, utilizando (2) os dados de delta18O de Cibicidoides mostram uma concordância excelente com a composição isotópica esperada, enquanto que os dados de delta18O de Uvigerina são 0,69 ‰ maiores que os valores de equilíbrio previstos. A partir das evidências apresentadas neste trabalho e em estudos prévios sugerimos a utilização do gênero Cibicidoides e a aplicação da equação de Kim & O'Neil (1997) para pesquisas de paleotemperatura. Na ausência de suficientes espécimens de Cibicidoides sugerimos a utilização de delta18O de Uvigerina aplicando um fator de correção de -0,69 ‰.

Greenhouse warming is a predicted consequence of the Chicxulub impact, but supporting data are sparse. This shortcoming compromises understanding of the impact ’ s effects, and it has persisted due to an absence of sections that both contain suitable material for traditional carbonate- or organic-based paleothermometry and are complete and expanded enough to resolve changes on short time scales. We address the problem by analyzing the oxygen isotopic composition of fish debris, phosphatic microfossils that are relatively resistant to diagenetic alteration, from the Global Stratotype Section and Point for the Cretaceous/Paleogene boundary at El Kef, Tunisia. We report an ~1 per mil decrease in oxygen isotopic values (~5¡C warming) beginning at the boundary and spanning ~300 centimeters of section (~100,000 years). The pattern found matches expectations for impact-initiated greenhouse warming. [ABSTRACT FROM AUTHOR]

The equilibrium clumped isotope composition of carbonate minerals is independent of the composition of the aqueous solution. However, many carbonate minerals grow at rates that place them in a non-equilibrium regime with respect to carbon and oxygen isotopes with unknown consequences for clumped isotopes. We develop a process-based model that allows one to calculate the oxygen, carbon, and clumped isotope composition of calcite as a function of temperature, crystal growth rate, and solution pH. In the model, carbon and oxygen isotope fractionation occurs through the mass-dependent attachment/detachment kinetics of the isotopologues of HCO − 3 and CO 2− 3 to and from the calcite surface, which in turn, influence the clumped isotope composition of calcite. At experimental and biogenic growth rates, the mineral is expected to inherit a clumped isotopic composition that is similar to that of the DIC pool, which helps to explain (1) why different organisms share the same clumped isotope versus temperature calibration curves, (2) why many inorganic calibration curves are slightly different from one another, and (3) why foraminifera, coccoliths, and deep sea corals can have near-equilibrium clumped isotope compositions but far-from-equilibrium carbon and oxygen isotope compositions. Some aspects of the model can be generalized to other mineral systems and should serve as a useful reference in future efforts to quantify kinetic clumped isotope effects. [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]

Sedimentary organic matter derives from biological precursor material which undergoes systematic, irreversible changes upon burial which mainly reflect increasing diagenetic temperatures, although other factors have also an influence. Several parameters have been established in the last decades to determine the palaeotemperature history of sedimentary rocks based on geochemical or petrographical methods. Organic matter is the most temperature-sensitive solid constituent in sedimentary rocks and vitrinite reflectance (VR), miospore and conodont colour alteration are among the most widely used optical maturity parameters. Despite tremendous interest in estimating maturity parameters in the oil generation zone as well as for high grade diagenesis and metamorphism and despite decades of research only a few of these methods have been well established and compared to each other by now. The focus of this review is on some new aspects with respect to organic matter maturation and optical palaeotemperature parameters, especially for high grade diagenesis and anchimetamorphism. Such a discussion might be a prerequisite for a better understanding of palaeotemperature assessments in different sedimentological and/or geotectonic settings and useful for different fields in applied sciences. Furthermore, suggestions for further research are discussed. [ABSTRACT FROM AUTHOR]

The TEX 86 paleothermometer was applied for reconstructing sea surface temperature (SST) from glycerol dialkyl glycerol tetraethers (GDGTs) in marine sediments. It has become clear that GDGT-producing archaea live throughout the water column, with maximum concentration usually well below the surface mixed layer of the ocean. Why the TEX 86 parameter correlates well with SST remains poorly understood. Here we evaluate the fidelity of the TEX 86 thermometer using surface sediments and suspended particles from the Yellow Sea (YS), a shallow marginal sea between China and the Korean peninsula. The highest concentration of GDGTs in the water column at a site (A02) from the middle of the YS occurred in the bottom layer, at 70 m. This contrasts with phytoplankton lipids, which were most abundant near the surface. Consistent with the maximum abundance of GDGTs in bottom water, TEX 86 values in surface sediments correlated better with mean annual bottom water temperature (BWT, R 2 0.81) than with mean annual SST ( R 2 0.74). Moreover, TEX H 86 temperature derived from a global core top calibration gave values 0.1–9.4 °C colder than mean annual SST but much closer to mean annual BWT (2.7–4 °C). Lastly, TEX 86 and U 37 K ′ paleotemperature values displayed distinctly different trends over the last 12 kyr for sediments from the shelf between the YS and the East China Sea (ECS), consistent with the notion that TEX 86 and U 37 K ′ reflect different temperature signals. This preponderance of evidence supports the use of TEX 86 as a proxy for BWT in the YS. Therefore, we propose a local calibration of TEX L 86 for reconstructing mean annual BWT (TEX L 86 = 0.03 BWT-0.94; R 2 0.86, n = 22, P < 0.0001). The combination of alkenone ( U 37 K ′ ) derived SST with TEX L 86 BWT yielded a quantitative reconstruction of the vertical thermal gradient in the YS, and an insight into understanding the impact of the Kuroshio Current and East Asian monsoon on the YS. [ABSTRACT FROM AUTHOR]

The Wonoka–Shuram Anomaly represents the largest negative carbon isotope excursion recognized in the geologic record and is associated with the emergence and diversification of metazoan life ca. 580 million years ago (Ma). The origin of the anomaly is highly debated, with interpretations ranging from primary to diagenetic, each having unique and potentially transformative implications for early life. Here, we apply carbonate clumped isotope thermometry to three sections expressing the anomaly in order to constrain mineral formation temperatures and thus directly calculate water oxygen isotope compositions (δ 18 O w ) with which carbonate minerals equilibrated. With δ 18 O w known, it is possible to address previous hypotheses for the origin of the anomaly. In each section, precipitation temperatures correlate positively with reconstructed δ 18 O w . Previous hypotheses, based on the covariance of δ 18 O carb vs. δ 13 C carb (uncorrected for temperature effects), suggested a meteoric diagenetic origin for the anomaly. However, reconstructed δ 18 O w values do not covary with carbon isotope compositions (δ 13 C carb ) within anomaly facies. Rather, the oxygen isotope and temperature data are consistent with carbonate recrystallization and equilibration under increasingly rock-buffered conditions. Based on simple modeling and comparison to modern formation fluids, recrystallization may have occurred in an environment far removed from the initial depositional or early diagenetic regime. In addition, although clumped isotope temperatures vary significantly and reach elevated values consistent with burial diagenesis, it is unclear to what degree, if at all, carbon isotope values were reset during recrystallization. Ultimately, these new data indicate that Wonoka–Shuram-aged carbonates experienced equilibration with fluids under increasingly closed-system conditions. The clumped isotope data do not provide a means to distinguish previous hypotheses outright, but provide additional context for the evaluation of geochemical signatures within these ancient carbonate rocks. [ABSTRACT FROM AUTHOR]

The temperature of ancient oceans is an important constraint for understanding the climate history of our planet. The classical oxygen isotope paleothermometry on fossil shells, while very proficient when applied to the younger (Cenozoic) portion of the geologic record, is believed to yield only unreliable results for the Phanerozoic “deep time”, either because the empirically well documented secular trend to more negative δ 18 O values with increasing age was generated by post-depositional recrystallization processes or, if primary, implies ecologically unpalatable hot early oceans. Here we present a compilation of δ 18 O measurements for 58,532 low-Mg calcite marine shells that cover almost the entire Phanerozoic eon, argue that the secular decline of about − 6‰ is primary, propose that it reflects the changing oxygen isotopic composition of sea water, and define a new baseline trend for δ 18 O of paleo-sea water; the latter providing a new template for calculation of ambient habitat temperatures of fossil specimens. The resulting pattern for fossil taxa (foraminifera, brachiopods, belemnites and bivalves) mimics their modern counterparts in temperature ranges and modes. This conceptual framework enables application of actualistic concepts to ambient habitat temperatures of fossils and provides us with a long overdue tool for interpretation of “deep time” geologic history. [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]

A new data set of temperature-dependent clay mineral parameters and vitrinite reflectance of the Tuscan successions in the Northern Apennines (Italy) displays decreasing levels of thermal maturity from hinterland to foreland, and abrupt changes parallel to the strike of the chain which are structurally controlled by northeast-southwest--trending faults (e.g., Marecchia valley lineament). To the southeast of the Marecchia valley lineament, paleothermal indicators show deep diagenetic conditions in the hinterland and early diagenetic conditions in the foreland (Ro% ranges from 0.80% to 0.30%; illite content in mixed-layer illite-smectite [I-S] from 86% to 38%). To the northwest of the Marecchia valley lineament, in the hinterland, Ro% is up to 0.95% and mixed-layer I-S have an illite content of ~87%-88%, both gradually decreasing toward the northeast (to Ro% of 0.33%, and illite in I-S of 50%). Thermal models allowed us to constrain the geometry of the Miocene thrust wedge with special regard to the original thickness and distribution of its allochthonous uppermost structural unit (Ligurian unit) across northeast-southwest--trending tectonic lineaments. The thickness of the Ligurian unit ranges from 1 to 1.5 km to the south of the Marecchia valley lineament, to 3 km to the north-northwest. This tectonic lineament affected wedge geometry, amounts of tectonic transport, and thickness of the uppermost structural unit, and, possibly, Neogene--Quaternary levels of exhumation. [ABSTRACT FROM AUTHOR]

Sedimentary cherts are unusually abundant in early Archean (pre-3.0 Ga) sequences, suggesting a silica cycle that was profoundly different than the modern system. Previously applied for the purpose of paleothermometry, Si isotopes in ancient cherts can offer broader insight into mass fluxes and mechanisms associated with silica concentration, precipitation, diagenesis, and metamorphism. Early Archean cherts contain a rich suite of sedimentological and petrographic textures that document a history of silica deposition, cementation, silicification, and recrystallization. To add a new layer of insight into the chemistry of early cherts, we have used wavelength-dispersive spectroscopy and then secondary ion mass spectrometry (SIMS) to produce elemental and Si and O isotope ratio data from banded black-and-white cherts from the Onverwacht Group of the Barberton Greenstone Belt, South Africa. This geochemical data is then interpreted in the framework of depositional and diagenetic timing of silica precipitation provided by geological observations. SIMS allows the comparison of Si and O isotope ratios of distinct silica phases, including black carbonaceous chert beds and bands (many including well-defined sedimentary grains), white relatively pure chert bands including primary silica granules, early cavity-filling cements, and later quartz-filled veins. Including all chert types and textures analyzed, the δ 30 Si dataset spans a range from −4.78‰ to +3.74‰, with overall mean 0.20‰, median 0.51‰, and standard deviation 1.30‰ ( n = 1087). Most samples have broadly similar δ 30 Si distributions, but systematic texture-specific δ 30 Si differences are observed between white chert bands (mean +0.60‰, n = 750), which contain textures that represent primary and earliest diagenetic silica phases, and later cavity-filling cements (mean −1.41‰, n = 198). We observed variations at a ∼100 μm scale indicating a lack of Si isotope homogenization at this scale during diagenesis and metamorphism, although fractionations during diagenetic phase transformations may have affected certain textures. We interpret these systematic variations to reflect fractionation during silica precipitation as well as isotopically distinct fluids from which later phases originated. SIMS δ 18 O values fall in a range from 16.39‰ to 23.39‰ ( n = 381), similar to previously published data from bulk gas source mass spectrometry of Onverwacht cherts. We observed only limited examples of texture-related variation in δ 18 O and did not observe correlation of δ 18 O with δ 30 Si trends. This is consistent with hypotheses that Si isotope ratios are more resistant to alteration under conditions of rock-buffered diagenesis (Marin-Carbonne et al., 2011). Our results indicate that low temperature processes fractionated silicon isotopes in early Archean marine basins, a behavior that probably precludes the application of chert δ 30 Si as a robust paleothermometer. The values we observe for facies that sedimentological and petrographic observations indicate formed as primary and earliest diagenetic silica precipitates from seawater are more 30 Si-rich than that expected for bulk silicate Earth. This is consistent with the hypothesis that the silicon isotope budget is balanced by the coeval deposition of 30 Si-enriched cherts and 30 Si-depleted iron formation lithologies. Precipitation of authigenic clay minerals in both terrestrial and marine settings may have also comprised a large 30 Si-depleted sink, with the corollary of an important non-carbonate alkalinity sink consuming cations released by silicate weathering. [ABSTRACT FROM AUTHOR]

Fossil long-chain alkenones have been used for several decades to reconstruct past ocean surface water temperatures and gained recent interest as a paleotemperature proxy for continental lake settings. However, factors besides temperature can affect alkenone distributions in haptophyte algae, and alkenone compositions can differ between haptophyte species. Alkenone-biosynthesizing haptophyte algae are genetically much more diverse in lakes than in the marine realm, and species-level variations in alkenone compositions could have implications for alkenone paleothermometry. Here, we performed a paired analysis of alkenone distributions and haptophyte species compositions using ancient DNA in up to 270 ka-old sediments of Lake Van in Turkey to reveal a possible species-effect on fossil alkenone distributions and paleotemperature estimates. The same predominant haptophyte in Lake Van today prevailed also since the last ∼100 ka. However, a calibration of alkenone paleotemperature especially in the oldest analyzed intervals is complicated due to a more complex haptophyte species composition predominated by a haptophyte (LVHap_6), which is phylogenetically different from sequences recovered from currently existing lakes including Lake Van and from haptophyte species existing in culture. The predominance of LVHap_6 coincided with the presence of alkenone MeC38:3 and relatively high MeC37:3/4 (2.4) and MeC38:4/5 ratios (3.0). Uk37 index values in the sediment core over the last 270 ka reflect relative changes in past temperature and are additionally linked to haptophyte species composition. A sustained period of high salinity, as indicated by pore-water salinity measurements, could potentially have triggered the succession of haptophytes as sources of alkenones in Lake Van. [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]

The oxygen isotope composition of carbonate minerals varies with temperature as well as other environmental variables. For carbonates that precipitate slowly, under conditions that approach thermodynamic equilibrium, the temperature-dependence of 18 O uptake is the dominant signal and the measured 18 O content can be used as a paleotemperature proxy. In the more common case where carbonate minerals grow in a regime where they are not in isotopic equilibrium with their host solution, their oxygen isotope compositions are a convolution of multiple environmental variables. Here we present results from calcite growth experiments demonstrating the occurrence of large ( > 2 ‰ ) non-equilibrium oxygen isotope effects under conditions relevant to biogenic calcite growth and many natural inorganic systems. We show that these non-equilibrium effects vary systematically with pH and crystal growth rate. An isotopic ion-by-ion crystal growth model quantifies the competing roles of temperature, pH, and growth rate, and provides a general description of calcite–water oxygen isotope fractionation under non-equilibrium conditions. The crystal growth model results show that (1) there are both equilibrium and kinetic contributions to calcite oxygen isotopes at biogenic growth rates, (2) calcite does not directly inherit the oxygen isotope composition of DIC even at fast growth rates, (3) there is a kinetically controlled variation of about 1‰ per pH unit between pH = 7.7 and 9.3 at constant growth rate for inorganic calcite as well as biogenic calcite, and (4) extreme light isotope enrichments in calcite in alkaline environments are likely due to disequilibrium among DIC species in aqueous solution. The model can be extended to 13 C uptake into carbonates as well as clumped isotopes but additional data are needed to constrain the kinetic fractionation factors for carbon isotopes. The experimental and model results constitute an important step in separating the relative influence of inorganic and biologic processes on isotopic fractionation and may aid the development of new paleoproxies based on non-equilibrium effects. [ABSTRACT FROM AUTHOR]

The simultaneous production and diffusion of cosmogenic noble gases offers the potential to constrain past temperatures on Earth and other planetary surfaces. Knowledge of both the production rate and diffusion kinetics of cosmogenic nuclide pairs is required to utilize this open-system behavior for paleothermometry. Here, we investigate the diffusion kinetics of spallogenic 3 He and 21 Ne in quartz through a series of step-degassing experiments on individual, proton-irradiated quartz grains. Quartz often, but not always, exhibits two stages of linear Arrhenius behavior, with He and Ne exhibiting similar release patterns. This two-stage behavior does not appear to correlate with heating-induced structural changes or anisotropy, nor is it an artifact of proton irradiation. The behavior may instead be associated with a sample-specific property such as radiation damage, mineral inclusions, fluid inclusions, or structural defects. We interpret these two Arrhenius arrays to represent multiple diffusion domain (MDD)-type behavior in quartz, as two-domain models closely reproduce the experimental data. However, we are currently unable to link this behavior with a clear physical mechanism; a different, more mechanistic model may be more appropriate in future studies. For both He and Ne, modeled Arrhenius diffusion parameters (activation energy, E a , and pre-exponential factor, D 0 ) display a range of values in the quartz samples analyzed. For 3 He, E a ranges from 73.0 to 99.8 kJ/mol and D 0 from 5.9 × 10 0 to 1.0 × 10 4 cm 2 s −1 for the initial, low-temperature linear Arrhenius arrays; when observed, a second array at higher temperatures corresponds to E a ranging from 85.2 to 106.4 kJ/mol and D 0 from 1.7 × 10 −1 to 3.5 × 10 0 cm 2 s −1 . For 21 Ne, E a ranges from 95.7 to 153.8 kJ/mol and D 0 from 6.6 × 10 −1 to 3.2 × 10 3 cm 2 s −1 for the initial, low-temperature array; linearity at high temperatures is not well constrained, likely because the α- to β-quartz transition occurs during the relevant temperature range. When extrapolated to Earth surface temperatures and geologically relevant timescales, these results suggest that 1 mm-radius quartz grains lose significant amounts of cosmogenic 3 He by diffusion at sub-zero temperatures from the low-retentivity domain over >10 3 yr timescales and from the high-retentivity domain over >10 4 yr, whereas quantitative retention of cosmogenic 21 Ne occurs over >10 6 yr at temperatures ⩽40 °C in most cases. While these results are generally consistent with previously reported studies, they also reveal that sample-specific diffusion parameters are required for quantitative application of cosmogenic noble gas paleothermometry. The cosmogenic 3 He abundance in one quartz sample with a simple Holocene exposure history and the stepwise degassing pattern of cosmogenic 3 He and 21 Ne from another quartz sample with a ∼1.2 Ma exposure history agree well with diffusion experiments on proton-irradiated aliquots of the same samples. For the sample with a simple Holocene exposure history, a production and diffusion model incorporating sample-specific diffusion parameters and the measured 3 He abundance predicts an effective diffusion temperature consistent with the effective modern temperature at the sample location. This internal consistency demonstrates that the empirically determined, sample-specific diffusion kinetics apply to cosmogenic 3 He and 21 Ne in quartz in natural settings over geologic timescales. [ABSTRACT FROM AUTHOR]

To shed more light on the applicability of glycerol dialkyl glycerol tetraether (GDGT) based proxies for reconstructing environmental change in shallow marginal seas, we analyzed the GDGT composition of surface sediments from the east coastal sea of China (ECSC). The GDGT distribution in 58 surface sediment samples from the ECSC shows that the isoprenoid GDGTs ( i GDGTs) are mainly from autochthonous planktonic Thaumarchaeota and that the relative abundance of the acyclic GDGT (GDGT-0) in the ECSC increases with latitude, whereas that of cyclic GDGTs decreases. The fractional abundances of i GDGTs correlated significantly with annual and seasonal sea surface temperature (SST), especially with summer SST. Based on this, we propose a new calibration of the TEX 86 H proxy to reconstruct past SST in the ECSC. The new calibration shows a higher calibration coefficient ( R 2 0.84, p < 0.001), leading to a more robust reconstruction of temperature in China marginal seas. The 90 yr SST record derived with the new calibration for a sediment core from the ECSC is consistent with climate data from NOAA, demonstrating the applicability of the new calibration for summer SST reconstruction in this shallow marginal sea [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]

We present the first universal calibration of the clumped isotope thermometer for calcites of various mineralizing types. These are an eggshell of an ostrich, a tropical bivalve, a brachiopod shell, cold seep carbonate, and three foraminifera samples that grew between 9 and 38 °C. CaCO 3 was digested at 90 °C using a common acid bath. Considering a difference in phosphoric acid fractionation factors between reaction at 25 and 90 °C of 0.069‰ (Guo et al., 2009), the function between growth temperature T and the excess of 13 C– 18 O bonds in the evolved CO 2 is expressed by a linear regression between 1/ T 2 and absolute Δ 47 ( R 2 = 0.9915): Δ 47 = 0.0327 ( ± 0.0026 ) × 10 6 / T 2 + 0.3030 ( ± 0.0308 ) ( with Δ 47 in ‰ and T in K ) . Both the slope and intercept of our regression line deviate significantly from the first experimental calibration based on synthetic calcites digested at 25 °C (Ghosh et al., 2006a) and from several other studies having confirmed this pioneering calibration (i.e., Came et al., 2007; Tripati et al., 2010; Thiagarajan et al., 2011; Grauel et al., 2012; Saenger et al., 2012; Zaarur et al., 2013). However, our relationship between temperature and absolute Δ 47 values is indistinguishable from that determined by Henkes et al. (2013) if the same difference in phosphoric acid fractionation factors between 25 and 90 °C is applied to both datasets. Our study and that of Henkes et al. (2013) have in common that data were primarily projected onto the absolute scale proposed by Dennis et al. (2011) – a reference frame that allows comparison of clumped isotope data measured in different laboratories. Furthermore, at any T, our regression line lies within 0.006‰ of the theoretical calcite calibration of Guo et al. (2009). The observation that both empirical calibrations are indistinguishable from each other implies that clumped isotope data can be directly compared between laboratories and referenced to a unique temperature calibration if (1) the phosphoric acid digestion temperature is set to 90 °C, and (2) raw data are primarily projected to the absolute scale. [ABSTRACT FROM AUTHOR]

Faulting mechanics is examined by combining geochemical and mechanical analyses from a key exposure of a major fault zone crossing the Argentera crystalline massif (French-Italian Alps). The analyses are carried out on co-genetic and syn-tectonic quartz and chlorite extension veins and shear veins associated with late stage deformation in the fault zone. Paleothermometry based on chlorite compositions gives formation temperatures of 300 °C ± 20 °C, that is near the base of the seismogenic zone. δ18O values of quartz and δD and δ18O values of chlorite indicate that veins crystallized from a metamorphic fluid. In parallel, a mechanical analysis based on the Mohr-Coulomb theory shows that the pore fluid factor λv required to simultaneously reactivate the studied fault and to open the extension veins was close to a lithostatic value (λv ~ 1). Comparisons with the 2003-2004 Ubaye seismic swarm, having occurred in the continuity of the studied fault zone, suggest that the base of the seismogenic zone may act as a limit separating an upper permeable reservoir saturated with meteoric waters under hydrostatic to supra-hydrostatic pressures from a lower low-permeability reservoir containing metamorphic waters under lithostatic pressure. This study suggests that overpressured fluids can be released upwards in the brittle crust by shear-enhanced permeability and can trigger earthquakes. [ABSTRACT FROM AUTHOR]

Abstract: Subaqueous calcite in Devils Hole, Nevada, was growing continuously from slightly super-saturated groundwater, providing a 570 kyr-long paleoclimate record. Due to its very slow growth it has been assumed to have grown under conditions of isotopic equilibrium. However, its Holocene value is 1.5‰ higher than predicted by laboratory-precipitation-based oxygen isotope thermometer calibrations. The suggestion that Devils Hole calcite anchors the isotope thermometer to more 18O-enriched values has stirred a debate as to which paleothermometer calibration is relevant for paleoclimate and casts doubt on the validity of -based paleotemperatures. We used clumped isotopes to test the assumptions of the Devils Hole alternative 18O-thermometer. Carbonate clumped isotopes are a temperature proxy that measures the abundance of 13C 18O bonds in CaCO3. This proxy is independent of the water composition and therefore gives independent estimates of temperatures when calcite forms at thermodynamic and isotopic equilibrium. We find that Devils Hole water paleotemperatures were constant at between 27 and 180 ka, similarly to the modern groundwater temperature of 32.8–34.3 °C. The proximity of the Devils Hole clumped isotope data to values expected based on modern groundwater temperatures supports the notion that Devils Hole calcite grew under equilibrium conditions. Therefore, the commonly used laboratory-based -temperature calibrations should be reconsidered. The constant water temperature over the glacial–interglacial cycles indicates that the long Devils Hole record reflects only variations in the groundwater values and as such, represents a valuable archive of paleoclimate and isotope paleohydrology. [Copyright &y& Elsevier]

Abstract: We present a theoretical basis for reconstructing paleotemperatures from the open-system behavior of cosmogenic noble gases produced in minerals at Earth's surface. Experimentally-determined diffusion kinetics predicts diffusive loss of cosmogenic 3He and 21Ne from common minerals like quartz and feldspars at ambient temperatures; incomplete retention has also been observed empirically in field studies. We show that the theory of simultaneous production and diffusion that applies to radiogenic noble gases in minerals—the basis of thermochronology—can also be applied to cosmogenic noble gases to reconstruct past surface temperatures on Earth. We use published diffusion kinetics and production rates for 3He in quartz and 21Ne in orthoclase to demonstrate the resolving power of cosmogenic noble gas paleothermometry with respect to exposure duration, temperature, and diffusion domain size. Calculations indicate that, when paired with a quantitatively retained cosmogenic nuclide such as 21Ne or 10Be, observations of cosmogenic 3He in quartz can constrain temperatures during surface exposure in polar and high altitude environments. Likewise, 21Ne retention in feldspars is sensitive to temperatures at lower latitudes and elevations, expanding the potential geographic applicability of this technique to most latitudes. As an example, we present paired measurements of 3He and 10Be in quartz from a suite of Antarctic sandstone erratics to test whether the abundances of cosmogenic 3He agree with what is predicted from first principles and laboratory-determined diffusion kinetics. We find that the amounts of cosmogenic 3He present in these samples are consistent with the known mean annual temperature (MAT) for this region of Antarctica between −25 and −30 °C. These results demonstrate the method's ability to record paleotemperatures through geologic time. [Copyright &y& Elsevier]

We measured TEX86 indices in surface sediments and sediment cores from marginal seas and open ocean sites in the North Pacific to test the accuracy of TEX86 paleothermometry in the region. Application of the global core top TEX 86 L and TEX 86 H calibrations in the Sea of Okhotsk and northern North Pacific sub-polar region results in over- and under-estimations of TEX86 derived SSTs in surface sediments in deep-ocean and coastal areas, respectively. Further challenges apparent in applying TEX 86 L and TEX 86 H in late Quaternary sediment cores from the Okhotsk Sea (MR06-04 PC7) and northwestern Pacific (MR00-K03 PC1) are unrealistically warm Holocene mean annual reconstructed temperatures (TEX 86 L) and excessively cold Glacial temperatures (TEX 86 H) , compared with other high-latitude records and climate models. Application of a calibration based on a sub-set of deep (<1000m) core-top sites appears to improve temperature reconstructions in the Holocene, but still yields underestimation of SSTs (mean annual=−5°C for TEX 86 H) in the Glacial period in the Okhotsk Sea core MR06-04 PC7. However, a regional comparison of Okhotsk Sea and sub-polar surface sediment TEX 86 L values with observational sea-water temperatures at different seasons and water depths reveals that an optimized positive correlation is obtained with summer sub-surface temperatures (August, 20m depth). The unique hydrographic setting of the Okhotsk Sea region likely exerts a strong control on the ecology of GDGT producers (assumed to be primarily be marine Thaumarchaeota), resulting in highly season and depth specific export production and a distinct regional gradient in surface sediment TEX86 values. When our new regional TEX 86 L calibration is applied to the sediment cores, TEX 86 L and alkenone U 37 K ′ temperature estimates overlap in the Holocene and TEX 86 L produces plausible LGM to Holocene temperature reconstructions. Our study provides a region specific calibration and confirms the importance of acquiring local calibration equations, to optimize TEX86 paleothermometry for specific regions and ocean basins. [ABSTRACT FROM AUTHOR]

Abstract: The major glaciation at the end of the Ordovician is associated with the 2nd largest mass extinction event of the Phanerozoic. Growth of Late Ordovician ice sheets requires a dramatic cooling from the ‘greenhouse’ conditions that prevailed for most of the Ordovician, but when and how fast this cooling occurred are controversial issues. The controversy is due in large part to a lack of good geochemical constraints on the temperature history of the Katian (453–445.2Ma). To address this uncertainty, we measured phosphate δ18O values from 3 conodont species collected from sections in the midcontinent region of the United States that span an ~5.7m.yr. long interval covering most of the Katian. Results reveal a statistically significant offset in δ18O values between some taxa and show up to 2‰ differences among samples. However, there are no apparent long-term trends within or between sections; rather, values fluctuate around a δ18O mean of ~19‰ VSMOW. Our study provides the longest, relatively high resolution, species specific conodont record generated for this interval, and we found no evidence supporting progressive cooling during the Katian. [Copyright &y& Elsevier]

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]

Abstract: Spatially extensive paleoelevation records of the Altiplano plateau are critical to determining the geodynamic mechanisms that formed and support high elevations over a broad area. Prior stable isotope data reveal a climate history for the northern Bolivian Altiplano that has been interpreted to show rapid surface uplift of between . This study applies clumped isotope paleothermometry to paleosol carbonates formed at both a low-elevation site and temporally overlapping high-elevation sites in the southern Altiplano/Eastern Cordillera during the middle to late Miocene. Surface paleotemperature decreased by 14 °C in the southern Altiplano/Eastern Cordillera relative to stable low-elevation paleotemperatures, implying surface elevation increase of between 16 and 13 Ma and an additional between 13 and 9 Ma. Both the large magnitude of surface temperature decrease and earlier onset ( ) in the south as compared to the north suggest rapid elevation increase by piecemeal removal of lower lithosphere beneath the plateau and possible northward lower crustal flow. [Copyright &y& Elsevier]

The Cretaceous-Paleogene (K-Pg) mass extinction has been attributed to the impact of a large bolide at the end of the Cretaceous Period, although other potential causes have also been proposed, most notably climate change caused by Deccan Traps (India) flood volcanism. Reconstructing paleoclimate, particularly in terrestrial settings, has been hindered by a lack of reliable proxies. The recent development of carbonate clumped isotope paleothermometry has contributed to temperature reconstructions using geochemical proxies in terrestrial settings. We employ this method, along with new stratigraphic constraints, in the Hell Creek (Cretaceous) and overlying Fort Union (Paleogene) Formations (Montana, USA) to examine changes in temperature leading to and across the K-Pg boundary. We demonstrate that well-preserved ca. 66 Ma aragonitic bivalves serve as suitable paleoclimate archives. Although there are limitations in the stratigraphic availability of fossil bivalves for clumped isotope analysis, we record an apparent 8 °C decrease in summer temperatures over the last 300 k.y. of the Cretaceous that corresponds broadly with patterns observed in other paleotemperature proxies. This observed decrease plausibly could be explained by an absolute temperature decrease or by other environmental stresses on the organisms, but in either case suggests changing living conditions over the interval. Previously documented declines in vertebrate and invertebrate biodiversity occur over the same stratigraphic interval at this study location. These results are consistent with published models of the end- Cretaceous mass extinction in which destabilized ecosystems become more susceptible to an abrupt event like a bolide impact. [ABSTRACT FROM AUTHOR]

Highlights: [•] Clay mineralogy of the Chaotian reference section for the late Permian/Early Triassic. [•] Palaeothermicity of the Sichuan Basin using clay minerals of K-bentonites and T max data. [•] Discussion on the origin of the Wangpo Bed considered as a reworked bentonite. [Copyright &y& Elsevier]

During the Late Cretaceous, central Nevada may have been a high elevation plateau, the Nevadaplano; some geodynamic models of the western US require thickened crust and high elevations during the Mesozoic to drive the subsequent tectonic events of the Cenozoic while other models do not. To test the hypothesis of high elevations during the late Mesozoic, we used carbonate clumped isotope thermometry to determine the temperature contrast between Late Cretaceous to Paleocene carbonates atop the putative plateau in Nevada versus carbonates from relatively low paleoelevation central Utah site. Lacustrine carbonates from the Nevada site preserve summer temperatures ∼13 °C cooler than summer temperatures from paleosol carbonates from the Utah site, after correcting for ∼1.2 °C of secular climatic cooling between the times of carbonate deposition at the two sites. This ∼13 °C temperature difference implies an elevation difference between the two sites of ; including uncertainties from age estimation and climate change broadens this estimate to . Our findings support crustal thickness estimates and Cenozoic tectonic models that imply thickened crust and high elevation in Nevada during the Mesozoic. [ABSTRACT FROM AUTHOR]

The Mg/Ca ratio of the planktonic foraminifera Globigerinoides ruber (white) has become a widely used proxy for reconstructing sea surface temperature (SST), as numerous studies have shown that temperature is the primary control on Mg/Ca ratios in foraminiferal calcite. However, a recent study of core-top sediments across an Atlantic meridional transect suggests that salinity might have a stronger control on foraminiferal Mg/Ca ratios than previously thought. By analyzing Mg/Ca ratios and values in G. ruber (white), Arbuszewski et al. (2010) found a 27% increase in G. ruber Mg/Ca ratios per 1 salinity unit increase for seawater salinities above 35.5. Here, we use shell weight analyses and SEM images from a subset of the core-tops used in the Arbuszewski et al. (2010) study across a narrow depth range (3197–3733 m) to show that G. ruber shells from the equatorial region are highly dissolved compared to those from the subtropical North and South Atlantic gyres, significantly impacting their Mg/Ca-SSTs. Shell weights from the higher-productivity equatorial region of the Atlantic are on average 20% and 15% lower than those from the oligotrophic North and South Atlantic gyres, respectively. Given the large preservation gradient along the transect studied by Arbuszewski et al. (2010), application of a single dissolution-corrected Mg/Ca:SST calibration equation (Dekens et al., 2002) on cores from the subtropical gyres and the equatorial region is not appropriate. When regional differences in preservation are considered, as well as realistic habitat depths and calcification seasons for G. ruber in temperate latitudes, we find a strong correlation between observational SSTs and calculated G. ruber Mg/Ca-SSTs in core-top samples spanning 43°N to 25°S in the Atlantic. In addition, our re-calibrated Mg/Ca-SSTs are more strongly correlated with isotopic calcification temperatures across the transect than originally reported by Arbuszewski et al. (2010). This study provides evidence that regional differences in preservation, rather than salinity, significantly affects G. ruber Mg/Ca-SSTs in the Atlantic, and supports culturing experiments that found salinity only has a minor influence on foraminiferal Mg/Ca ratios. [ABSTRACT FROM AUTHOR]

The present work provides a combined structural and paleothermal (fluid inclusions, illite content in mixed layers illite–smectite, Kübler index and vitrinite reflectance) investigation of the Alpine cleavage domain affecting the western South Pyrenean Zone (Central Pyrenees). Integration of structural and paleothermal data allows us (i) to determine the position of the cleavage front in the sedimentary sequence and (ii) to define mechanisms and temperature conditions for cleavage development. Cleavage in the eastern part of the study area is approximately limited to the forelimb of the Gavarnie hanging wall anticline, but to the West it extends progressively southwards, affecting younger turbiditic units. At regional scale, the cleavage front is oblique to the strike of bedding and the overall trend of macrostructures. Pressure-solution is the main mechanism for cleavage development, and a strong thermal control on the position of the cleavage front is inferred. The transition between sedimentary and tectonic, compressional fabric is marked by vitrinite reflectance values of 1.12–1.14%, Kübler index values between 0.71 and 0.74°Δ2θ and the transition between R1 and R3 mixed layers illite–smectite. A lower thermal boundary of 140–160°C is suggested for incipient cleavage development in the shaly beds of the turbiditic sequence, as well as an upper boundary constrained by a maximum measured value of about 215°C. [ABSTRACT FROM AUTHOR]

Recent studies using secondary ion mass spectrometry revealed microscale heterogeneity of Sr/Ca and $\updelta^{18}$O in shallow-water coral skeletons, i.e., Sr/Ca and $\updelta^{18}$O differ significantly between two basic microfeatures of the skeleton: the center of calcification (COC) and surrounding fibrous skeleton (SFS). The COC, in contrast with the SFS, consists of highly irregular crystals intermingled with significant amount of organic matter; therefore, analyzing the SFS only would probably be favourable for paleotemperature reconstruction. Conventional Sr/Ca and $\updelta^{18}$O paleothermometers are, however, based on the analysis of the mixture of the COC and SFS, and thus may be significantly affected by the above-mentioned heterogeneity. In this study, I have evaluated the heterogeneity-induced effects on the conventional paleothermometers of Porites skeletons using published Sr/Ca, $\updelta^{18}$O and volume-fraction data of the COC and SFS and published observations of seasonal variability of bulk skeletal density. Results indicate that the effects may yield significant or serious errors in paleotemperature reconstruction. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we review the TEX86 palaeothermometer for sea surface temperature (SST) and evaluate its application to the Palaeogene, with a focus on the principal ecological, physical or chemical processes that can bias glycerol dialkyl glycerol tetraether (GDGT) distributions. Recent investigations of Palaeogene sediments have revealed temperature offsets between two different GDGT-based approaches, TEX86 L and TEX86 H, with the former agreeing with SST estimates derived from inorganic proxies (Hollis et al., 2012). These are surprising observations because the two GDGT approaches, although based on two distinct groups of compounds, apparently agree at SSTs>15°C in modern oceans. Here we reassess the relationship between raw GDGT distributions and the ratios used to construct TEX86 H and TEX86 in both the modern core-top dataset and a new compilation of Palaeogene data. We show that the offset between TEX86 H and TEX86 L (ΔH–L) is a function of the GDGT-2/GDGT-3 ratio ([2]/[3] ratio), and that this can be used to separate low- and high-latitude GDGT distributions in the modern core-top dataset: a range of [2]/[3] ratios and ΔH–L values occur in polar regions, whereas [2]/[3] ratios are high and ΔH–L values are small at temperatures >15°C. However, in the Palaeogene dataset, we observe a wide range of [2]/[3] ratios, even for SST estimates above 15°C. Crucially, we find that water depth is a better discriminator of ΔH–L values and [2]/[3] ratios than SST in the combined modern and Palaeogene dataset: ΔH–L values are low (<3.0°C) and [2]/[3] ratios are high (>5.0) where water depth is >1000m. Modern water column studies show that the [2]/[3] ratios in suspended particulate matter (SPM) increase with depth, suggesting that high [2]/[3] ratios reflect a contribution from Archaea living in the deeper water column. This suggests that export dynamics influence GDGT-derived SST estimates. We argue for new approaches to SST reconstruction: 1) continued use of core-top calibrations, in which export dynamics have been implicitly incorporated into the current core-top calibration datasets, but with the influence of water depth taken into account; and 2) use of SPM or mesocosm-based calibrations, with water depth and palaeo-export dynamics independently assessed. [Copyright &y& Elsevier]

RATIONALE The burgeoning field of 'clumped isotope' paleothermometry, which has broad applications in geosciences, depends almost entirely on measurements made on two types of mass spectrometer with the same ion source design. Demonstration that these measurements can be carried out on a retrofitted mass spectrometer with a different type of ion source is important to the growing community of geoscientists considering employing this technique. METHODS We summarize the sample preparation techniques, mass spectrometry, and data processing involved in establishing an absolute reference frame for the measurement of Δ47 in CO2 from carbonate minerals using a prototype 5 kV mass spectrometer. The prototype differs from previously marketed mass spectrometers only in the presence of six extra Faraday cups designed specifically to measure ion beams of masses 44-49. We employ pressure baseline corrections on the small mass 47 signal. RESULTS The system was fully capable of establishing absolute reference frames for clumped isotope carbonate paleothermometry, despite alternative use of the system for other types of measurements (carbonate δ18O and δ13C values, water δ18O and δ2H values via equilibration, and δ13C and δ15N values of sedimentary organic material) in different reference frames. Repeated measurements of CO2 from carbonate standards yielded external precisions comparable with those from other laboratories and adequate for paleothermometry to a precision of near or below ±5 °C. The accuracy was demonstrated by reasonable temperatures being obtained for different types of repeatedly measured carbonates. Measurements of the pressure baseline with every sample increased analysis times by 20 min, but decreased the stabilization time of the reference frame by days to weeks, while improving precision. CONCLUSIONS Simple modifications to existing instrumentation will allow more laboratories to perform clumped isotope measurements. The prototype mass spectrometer used herein provides precision comparable with all published data. We show that, where negatively charged species are associated with large ion beams, making pressure baseline corrections with every measurement is increasingly important no matter what instrument is used. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Given the importance of high-latitude areas in the ocean–climate system, there is need for a paleothermometer that is reliable at low temperatures. Here we assess the applicability of the Mg/Ca-temperature proxy in colder waters (5–10 °C) by comparing for the first time the seasonal Mg/Ca and δ 18O cycles of N. pachyderma (s) and G. bulloides using a sediment trap time-series from the northern North Atlantic. While both species show indistinguishable seasonal δ 18O patterns that clearly track the near surface temperature cycle, their Mg/Ca are very different. G. bulloides Mg/Ca is high (2.0–3.1 mmol/mol), but varies in concert with the seasonal temperature cycle. The Mg/Ca of N. pachyderma (s), on the other hand, is low (1.1–1.5 mmol/mol) and shows only a very weak seasonal cycle. The δ 18O patterns indicate that both species calcify in the same depth zone. Consequently, depth habitat differences cannot explain the contrasting Mg/Ca patterns. The elevated Mg/Ca in pristine G. bulloides might be due to the presence of high Mg phases that are not preserved in fossil shells. The contrasting absence of a seasonal trend in the Mg/Ca of N. pachyderma (s) confirms other studies where calcification temperatures were less well constrained. The reason for this absence is not fully known, but may include species-specific vital effects. The very different seasonal patterns of both speciesʼ Mg/Ca underscore the importance of parameters other than temperature in controlling planktonic foraminiferal Mg/Ca. Our results therefore lend further caution in the interpretation of Mg/Ca-temperature reconstructions from high northern latitudes. [Copyright &y& Elsevier]

Abstract: Paleotemperature reconstructions rely on knowledge of the equilibrium separation of oxygen isotopes between aqueous solution and calcium carbonate. Although oxygen isotope separation is expected on theoretical grounds, the temperature-dependence remains uncertain because other factors, such as slow exchange of isotopes between dissolved CO2-species and water, can obscure the temperature signal. This is problematic for crystal growth experiments on laboratory timescales and for interpreting the oxygen isotope composition of crystals formed in natural settings. We present results from experiments in which inorganic calcite is precipitated in the presence of dissolved bovine carbonic anhydrase (CA). The presence of dissolved CA accelerates oxygen isotope equilibration between the dissolved carbon species CO2, H2CO3, , and water, thereby eliminating this source of isotopic disequilibrium during calcite growth. The experimental results allow us to isolate, for the first time, kinetic oxygen isotope effects occurring at the calcite–water interface. We present a framework of ion-by-ion growth of calcite that reconciles our new measurements with measurements of natural cave calcites that are the best candidate for having precipitated under near-equilibrium conditions. Our findings suggest that isotopic equilibrium between calcite and water is unlikely to have been established in laboratory experiments or in many natural settings. The use of CA in carbonate precipitation experiments offers new opportunities to refine oxygen isotope-based geothermometers and to interrogate environmental variables other than temperature that influence calcite growth rates. [Copyright &y& Elsevier]

Abstract: Thrust sheets accumulate internal strain before they start moving along discrete fault planes. However, there are no previous studies evaluating the time difference between initiation of strain and fault displacement. In this paper we use observations from the Eastern Cordillera of Colombia to evaluate this interval. We utilize multiple thermochronometers and paleothermometers to refine the timing of deformation. Based on these new data we build time-temperature path estimates that together with geometric outcrop-based structural analysis and fluid inclusions allow us to assign relative timing to features associated with strain, such as cleavage, veins and certain types of fractures, and compare that with the timing of thrusting. We find that cleavage was only formed close to maximum paleotemperatures, almost coeval with the onset of thrust-induced denudation by the Late Oligocene. The corresponding structural level of fold-related veins suggest that they were formed later but still when the country rocks were at temperatures higher than 160 °C, mostly during the Early Miocene and still coexisted with the latest stages of cleavage formation. Our data show that the main period of strain hardening was short (probably a few million years) and occurred before first-order basement thrusting was dominant, but was associated with second-order folding. [Copyright &y& Elsevier]

The shells of marine mollusks are widely used archives of past climate and ocean chemistry. Whilst the measurement of mollusk δ18O to develop records of past climate change is a commonly used approach, it has proven challenging to develop reliable independent paleothermometers that can be used to deconvolve the contributions of temperature and fluid composition on molluscan oxygen isotope compositions. Here we investigate the temperature dependence of 13C-18O bond abundance, denoted by the measured parameter Δ47, in shell carbonates of bivalve mollusks and assess its potential to be a useful paleothermometer.We report measurements on cultured specimens spanning a range in water temperatures of 5 to 25 °C, and field collected specimens spanning a range of -1 to 29 °C. In addition we investigate the potential influence of carbonate saturation state on bivalve stable isotope compositions by making measurements on both calcitic and aragonitic specimens that have been cultured in seawater that is either supersaturated or undersaturated with respect to aragonite. We find a robust relationship between Δ47 and growth temperature. We also find that the slope of a linear regression through all the Δ47 data for bivalves plotted against seawater temperature is significantly shallower than previously published inorganic and biogenic carbonate calibration studies produced in our laboratory and go on to discuss the possible sources of this difference. We find that changing seawater saturation state does not have significant effect on the Δ47 of bivalve shell carbonate in two taxa that we examined, and we do not observe significant differences between Δ47-temperature relationships between calcitic and aragonitic taxa. [ABSTRACT FROM AUTHOR]

A high-resolution multiproxy record, including pollen, foraminifera, and alkenone paleothermometry, obtained from a single core (DG9603) from the Okinawa Trough, East China Sea (ECS), provided unambiguous evidence for asynchronous climate change between the land and ocean over the past 40 ka. On land, the deglacial stage was characterized by rapid warming, as reflected by paleovegetation, and it began ca. 15 kaBP, consistent with the timing of the last deglacial warming in Greenland. However, sea surface temperature estimates from foraminifera and alkenone paleothermometry increased around 20-19 kaBP, as in the Western Pacific Warm Pool (WPWP). Sea surface temperatures in the Okinawa Trough were influenced mainly by heat transport from the tropical western Pacific Ocean by the Kuroshio Current, but the epicontinental vegetation of the ECS was influenced by atmospheric circulation linked to the northern high-latitude climate. Asynchronous terrestrial and marine signals of the last deglacial warming in East Asia were thus clearly related to ocean currents and atmospheric circulation. We argue that (i) early warming seawater of the WPWP, driven by low-latitude insolation and trade winds, moved northward via the Kuroshio Current and triggered marine warming along the ECS around 20-19 kaBP similar to that in the WPWP, and (ii) an almost complete shutdown of the Atlantic Meridional Overturning Circulation ca. 18-15 kaBP was associated with cold Heinrich stadial-1 and delayed terrestrial warming during the last deglacial warming until ca. 15 kaBP at northern high latitudes, and hence in East Asia. Terrestrial deglacial warming therefore lagged behind marine changes by ca. 3-4 ka. [ABSTRACT FROM AUTHOR]

The warmest global temperatures of the past 85 million years occurred during a prolonged greenhouse episode known as the Early Eocene Climatic Optimum (52-50 Ma). The Early Eocene Climatic Optimum terminated with a long-term cooling trend that culminated in continental-scale glaciation of Antarctica from 34 Ma onward. Whereas early studies attributed the Eocene transition from greenhouse to icehouse climates to the tectonic opening of Southern Ocean gateways, more recent investigations invoked a dominant role of declining atmospheric greenhouse gas concentrations (e.g., CO2). However, the scarcity of field data has prevented empirical evaluation of these hypotheses. We present marine microfossil and organic geochemical records spanning the early-to-middle Eocene transition from the Wilkes Land Margin, East Antarctica. Dinoflagellate biogeography and sea surface temperature paleothermometry reveal that the earliest throughflow of a westbound Antarctic Counter Current began ~49-50 Ma through a southern opening of the Tasmanian Gateway. This early opening occurs in conjunction with the simultaneous onset of regional surface water and continental cooling (2-4 °C), evidenced by biomarker- and pollen-based paleothermometry. We interpret that the westbound flowing current flow across the Tasmanian Gateway resulted in cooling of Antarctic surface waters and coasts, which was conveyed to global intermediate waters through invigorated deep convection in southern high latitudes. Although atmospheric CO2 forcing alone would provide a more uniform middle Eocene cooling, the opening of the Tasmanian Gateway better explains Southern Ocean surface water and global deep ocean cooling in the apparent absence of (sub-) equatorial cooling. [ABSTRACT FROM AUTHOR]

Archaea are abundant in marine and terrestrial aquatic environments, sediments, and soils. They inhabit at least an 85°C temperature range from the polar ocean to hydrothermal springs. Many Archaea produce membrane lipids called glycerol dialkyl glycerol tetraethers (GDGTs). Experiments on pure and enrichment cultures as well as an empirical correlation for marine sediments (the TEX86 index) together show positive relationships between temperature and the number of cyclopentane or cyclohexane rings in GDGTs. The resulting TEX86 paleotemperature proxy has been applied across a wide range of geologic history and depositional settings. The exact relationship between TEX86 and temperature, however, remains poorly understood. Environmental systems and cultures have different temperature dependencies, and the ecological niche(s) of aquatic Archaea are still a subject of active investigation. Here we review some of the remaining questions about GDGT paleotemperature proxies. Better answers to these challenging problems will help refine future paleoceanographic applications. [ABSTRACT FROM AUTHOR]

Abstract: Using a new and rapid NanoSIMS-based method, we quantified the sensitivity of skeletal Sr/Ca in coral to the aragonite saturation state of seawater (ΩSW). Skeletal Sr/Ca is a common proxy for temperature while ΩSW is a parameter that varied in the past ocean and is predicted to change with continued ocean acidification. Five adult branches of the surface coral Stylophora pistillata were grown at different ΩSW from 2.7 to 4.9 (pH of 7.9–8.5) but at a constant temperature of 25°C. Despite a large range of growth parameters and a twofold range in calcification rates, the average skeletal Sr/Ca of coral exposed to each condition are within 1.2% of each other (2σ std. dev. of the 5 means). Furthermore, the average skeletal Sr/Ca measured in this study agrees with the results of two previous coral culture experiments conducted at the same temperature but where ΩSW was not controlled. These results suggest that aragonite saturation has little or no influence on Sr/Ca paleothermometry over the range of ΩSW investigated. Combined with existing data for low ΩSW conditions, our results were used to elucidate the mechanisms controlling calcifying fluid acid–base chemistry during coral biomineralization. Assuming that coral drive precipitation through alkalinity pumping, our data suggest that this pumping occurs until the calcifying fluid reaches a target pH. Below a threshold ΩSW bounded by 1<ΩSW <2.4, however, coral do not pump enough alkalinity to reach the target pH and instead pump a maximal but finite amount of alkalinity. In this low ΩSW regime, calcifying fluid pH is expected to decrease with ΩSW. The interplay between these two alkalinity pumping regimes and external seawater composition explain the full range of observed Sr/Ca sensitivity to ΩSW and suggest that surface coral may become increasingly sensitive to ocean acidification below a threshold ΩSW bounded by 1<ΩSW <2.4. [Copyright &y& Elsevier]

Understanding how global temperature changes with increasing atmospheric greenhouse gas concentrations, or climate sensitivity, is of central importance to climate change research. Climate models provide sensitivity estimates that may not fully incorporate slow, long-term feedbacks such as those involving ice sheets and vegetation. Geological studies, on the other hand, can provide estimates that integrate long- and short-term climate feedbacks to radiative forcing. Because high latitudes are thought to be most sensitive to greenhouse gas forcing owing to, for example, ice-albedo feedbacks, we focus on the tropical Pacific Ocean to derive a minimum value for long-term climate sensitivity. Using Mg/Ca paleothermometry from the planktonic foraminifera Globigerinoides ruber from the past 500 k.y. at Ocean Drilling Pro- gram (ODP) Site 871 in the western Pacific warm pool, we estimate the tropical Pacific climate sensitivity parameter (λ) to be 0.94-1.06 °C (W m-2)-1, higher than that predicted by model simulations of the Last Glacial Maximum or by models of doubled greenhouse gas concentration forcing. This result suggests that models may not yet adequately represent the long- term feedbacks related to ocean circulation, vegetation and associated dust, or the cryosphere, and/or may underestimate the effects of tropical clouds or other short-term feedback processes. [ABSTRACT FROM AUTHOR]

The Arctic region is subject to a great amplitude of climate variability and is currently undergoing large-scale changes due in part to anthropogenic global warming. Accurate projections of future change depend on anticipating the response of the Arctic climate system to forcing, and understanding how the response to human forcing will interact with natural climate variations. The Svalbard Archipelago occupies an important location for studying patterns and causes of Arctic climate variability; however, available paleoclimate records from Svalbard are of restricted use due to limitations of existing climate proxies. Here we present a sub-decadal- to multidecadal-scale record of summer temperature for the past 1800 yr from lake sediments of Kongressvatnet on West Spitsbergen, Svalbard, based on the fi rst instrumental calibration of the alkenone paleothermometer. The age model for the High Arctic lake sediments is based on 210Pb, plutonium activity, and the fi rst application of tephrochronology to lake sediments in this region. We fi nd that the summer warmth of the past 50 yr recorded in both the instrumental and alkenone records was unmatched in West Spitsbergen in the course of the past 1800 yr, including during the Medieval Climate Anomaly, and that summers during the Little Ice Age (LIA) of the 18th and 19th centuries on Svalbard were not particularly cold, even though glaciers occupied their maximum Holocene extent. Our results suggest that increased wintertime precipitation, rather than cold temperatures, was responsible for LIA glaciations on Svalbard and that increased heat transport into the Arctic via the West Spitsbergen Current began ca. A.D. 1600. [ABSTRACT FROM AUTHOR]