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Quantitative reconstructions of hydrological change during ancient greenhouse warming events provide valuable insight into warmer-than-modern hydrological cycles but are limited by paleoclimate proxy uncertainties. We present sea surface temperature (SST) records and seawater oxygen isotope (δ 18 O sw ) estimates for the Middle Eocene Climatic Optimum (MECO), using coupled carbonate clumped isotope (Δ 47 ) and oxygen isotope (δ 18 O c ) data of well-preserved planktonic foraminifera from the North Atlantic Newfoundland Drifts. These indicate a transient ~3°C warming across the MECO, with absolute temperatures generally in accordance with trace element (Mg/Ca)–based SSTs but lower than biomarker-based SSTs for the same interval. We find a transient ~0.5‰ shift toward higher δ 18 O sw , which implies increased salinity in the North Atlantic subtropical gyre and potentially a poleward expansion of its northern boundary in response to greenhouse warming. These observations provide constraints on dynamic ocean response to warming events, which are consistent with theory and model simulations predicting an enhanced hydrological cycle under global warming.

Clumped isotope thermometry can independently constrain the formation temperatures of carbonates, but a lack of precisely temperature-controlled calibration samples limits its application on aragonites. To address this issue, we present clumped isotope compositions of aragonitic bivalve shells grown under highly controlled temperatures (1–18°C), which we combine with clumped isotope data from natural and synthetic aragonites from a wide range of temperatures (1–850°C). We observe no discernible offset in clumped isotope values between aragonitic foraminifera, mollusks, and abiogenic aragonites or between aragonites and calcites, eliminating the need for a mineral-specific calibration or acid fractionation factor. However, due to non-linear behavior of the clumped isotope thermometer, including high-temperature (>100°C) datapoints in linear clumped isotope calibrations causes them to underestimate temperatures of cold (1–18°C) carbonates by 2.7 ± 2.0°C (95% confidence level). Therefore, clumped isotope-based paleoclimate reconstructions should be calibrated using samples with well constrained formation temperatures close to those of the samples.

Clumped isotope thermometry can independently constrain the formation temperatures of carbonates, but a lack of precisely temperature-controlled calibration samples limits its application on aragonites. To address this issue, we present clumped isotope compositions of aragonitic bivalve shells grown under highly controlled temperatures (1‒18°C), which we combine with clumped isotope data from natural and synthetic aragonites from a wide range of temperatures (1‒850°C). We observe no discernible offset in clumped isotope values between aragonitic foraminifera, mollusks, and abiogenic aragonites or between aragonites and calcites, eliminating the need for a mineral-specific calibration or acid fractionation factor. However, due to non-linear behavior of the clumped isotope thermometer, including high-temperature (>100°C) datapoints in linear clumped isotope calibrations causes them to underestimate temperatures of cold (1‒18°C) carbonates by 2.7 ± 2.0°C (95% confidence level). Therefore, clumped isotope-based paleoclimate reconstructions should be calibrated using samples with well constrained formation temperatures close to those of the samples.

Supplementary files belonging to the paper titled "Temperature dependence of clumped isotopes (∆47) in aragonite" submitted to Geophysical Research Letters. Key points Precise control on carbonate formation temperatures enables more accurate clumped isotope-temperature calibrations Isotopic ordering and acid fractionation in aragonite have a similar temperature dependence as in calcite, enabling combined calibrations The Δ47-relation in carbonates is non-linear, so linear calibrations require temperature-specific calibration data Abstract Clumped isotope thermometry can independently constrain the formation temperatures of carbonates, but a lack of precisely temperature-controlled calibration samples limits its application on aragonites. To address this issue, we present clumped isotope compositions of aragonitic bivalve shells grown under highly controlled temperatures (1‒18°C), which we combine with clumped isotope data from natural and synthetic aragonites precipitated in a wide range of temperatures (1‒850°C). We observe no discernible offset in clumped isotope values between aragonitic foraminifera, mollusks, and abiogenic aragonites or between aragonites and calcites, eliminating the need for a mineral-specific calibration or acid fractionation factor. However, due to non-linear behavior of the clumped isotope thermometer, including high-temperature (>100°C) datapoints in linear clumped isotope calibrations causes them to underestimate temperatures of cold (1‒18°C) carbonates by 2.7 ± 2.0°C (95% confidence level). Therefore, clumped isotope-based paleoclimate reconstructions should be calibrated using samples with well constrained formation temperatures close to those of the samples. &nbsp

The origin of widespread dolomite deposits in Paleozoic strata is a longstanding open question. Here we use a suite of geochemical methods to investigate a prominent Paleozoic dolomite occurrence in SW China. The upper Ordovician Wufeng Formation of the upper Yangtze Block (SW China) covers the time interval just before the Ordovician-Silurian Mass Extinction event, consisting of wide-spread black shale with an interlayer of bioclastic limestone, the Guanyinqiao Bed. In the western part of the Yangtze Block, in Shizhu county however, the Guanyinqiao Bed contains a particular organic-rich dolomite interlayer of which the formation mechanism is unclear. To distinguish between potential primary dolomite precipitation and late stage diagenetic/magmatic alteration, we studied this dolomite interlayer with petrological and geochemical techniques including microscopy, REE content and carbonate clumped isotope thermometry. The petrological observations show micritic dolomite with euhedral-subhedral shape and relatively low ordering (0.33 to 0.39). Conventional stable isotope analysis revealed negative carbon (δ13C of −3.17 ± 0.04‰ to −2.48 ± 0.03‰) and oxygen isotope compositions (δ18O of −11.28 ± 0.18‰ to −10.99 ± 0.20‰), and the organic carbon content ranged from 3.69% to 4.33%. The averaged clumped isotope temperature estimate revealed hot dolomite formation conditions of 87−10+11 °C, in a presumable fluid source with isotopic composition of δ18Ow = −1.8−1.5+1.5 ‰. The elevated formation temperature estimates, the slightly depleted δ18O of dolomite (~−11.14‰) with respect to contemporary deposited limestone (~−10‰) and the δ18Ow which is similar to the assumed late Ordovician seawater points at diagenetic alteration of primary limestone at a burial depth of approximately 3000 m. The analyzed dolomite samples with low contents of Fe, Zn and small Eu anomaly show further evidence for a diagenetic fluid source and exclude magmatic/hydrothermal alteration. We suggest that at this burial depth the temperature pressure conditions triggered significant Mg release from the surrounding clay to the diagenetic fluid, providing ideal conditions to transform the primary limestone into diagenetic dolomite.

Seasonal variability in sea surface temperatures plays a fundamental role in climate dynamics and species distribution. Seasonal bias can also severely compromise the accuracy of mean annual temperature reconstructions. It is therefore essential to better understand seasonal variability in climates of the past. Many reconstructions of climate in deep time neglect this issue and rely on controversial assumptions, such as estimates of sea water oxygen isotope composition. Here we present absolute seasonal temperature reconstructions based on clumped isotope measurements in bivalve shells which, critically, do not rely on these assumptions. We reconstruct highly precise monthly sea surface temperatures at around 50 °N latitude from individual oyster and rudist shells of the Campanian greenhouse period about 78 million years ago, when the seasonal range at 50 °N comprised 15 to 27 °C. In agreement with fully coupled climate model simulations, we find that greenhouse climates outside the tropics were warmer and more seasonal than previously thought. We conclude that seasonal bias and assumptions about seawater composition can distort temperature reconstructions and our understanding of past greenhouse climates.

Supplementary data belonging to the manuscript entitled: "First absolute seasonal temperature estimates for greenhouse climate from clumped isotopes in bivalve shells" by de Winter et al.

Utrecht University, The Netherlands (1); Unviersity of Bergen, Norway (2) The Eocene–Oligocene Transition (EOT, ~34 Ma), is marked by the rapid development of semi-permanent Antarctic ice-sheet1. Foraminiferal stable oxygen isotopes (δ18O) as well as Mg/Ca and other indicators (e.g. ice-rafted debris) indicate the development of permanent glaciation that potentially coincides with ~2.5 °Cdeep-sea cooling2. However, due to the nature of the δ18O proxy, uncertainties in the Mg/Ca concentrations of the palaeo-seawater, and calibration extrapolation/saturation to/at higher temperatures for organic proxies, it remains unclear how sea surface temperature (SST) changed across the EOT. In this study, we apply clumped-isotope palaeothermometry to well-preserved planktic foraminifera from the drift sediments of IODP Site 1411, Newfoundland, across four intervals bracketing the EOT. Initial findings indicate minor cooling across the interval, with absolute temperatures that are significantly lower than those reconstructed using other proxies3, a discrepancy that warrants further research. 1: Coxall, H. K., Pearson, P. N. (2007). The Eocene-Oligocene transition. Deep Time Perspectives on Climate Change: Marrying the Signal From Computer Models and Biological Proxies, p. 351-387. 2: Lear, C. H., Bailey, T. R., Pearson, P. N., Coxall, H. K. Rosenthal, Y. (2008) Cooling and ice growth across the Eocene-Oligocene transition. Geology 36, p. 251–254. 3: Liu, Z., He, Y., Jiang, Y., Wang, H., Liu, W., Bohaty, S. M., Wilson, P. A. (2018). Transient temperature asymmetry between hemispheres in the Palaeogene Atlantic Ocean. Nature Geoscience, 11(9), p. 656.

The clumped isotope composition of carbonates can be directly related to their formation temperature in the environment, but a robust temperature calibration for siderite is still lacking, and thus limits the applicability of this tool. Here, we present a new calibration for the clumped isotope composition of siderites precipitated with two different techniques between 8.5 and 62 °C: Δ 47 = 0.0428 ± 0.002 ∗ 10 6 T 2 + 0.0683 ± 0.022 ± 95 % C L , T i n K e l v i n R 2 = 0.92 The slope of our calibration obtained at an acid digestion temperature of 115 °C is statistically indistinguishable from recently published calcite and dolomite clumped isotope calibrations. The intercept differs from calcite at phosphoric acid digestion temperatures of 70 and 115 °C due to a difference in phosphoric acid fractionation, highlighting the necessity of a siderite specific calibration. We performed the first survey of the clumped isotope compositions of siderites from different environmental settings at ambient temperatures including two permanently waterlogged swamp sites as modern analogues for pedogenic siderite formation in the geologic record. We conclude that pedogenic siderites form at temperatures between the Mean Annual Air Temperature (MAAT) and the summer air temperature highlighting the strong potential of siderites for paleoclimate reconstructions in continental environments.

The reliable determination of past seawater temperature is fundamental to paleoclimate studies. We test the robustness of two paleotemperature proxies by combining Mg/Ca and clumped isotopes (Δ47) on the same specimens of core top planktonic foraminifera. The strength of this approach is that Mg/Ca and Δ47 are measured on the same specimens of foraminifera, thereby providing two independent estimates of temperature. This replication constitutes a rigorous test of individual methods with the advantage that the same approach can be applied to fossil specimens. Aliquots for Mg/Ca and clumped analyses are treated in the same manner following a modified cleaning procedure of foraminifera for trace element and isotopic analyses. We analysed eight species of planktonic foraminifera from coretop samples over a wide range of temperatures from 2 to 29°C. We provide a new clumped isotope temperature calibrations using subaqueous cave carbonates, which is consistent with recent studies. Tandem Mg/Ca–Δ47 results follow an exponential curve as predicted by temperature calibration equations. Observed deviations from the predicted Mg/Ca-Δ47 relationship are attributed to the effects of Fe-Mn oxide coatings, contamination, or dissolution of foraminiferal tests. This coupled approach provides a high degree of confidence in temperature estimates when Mg/Ca and Δ47 yield concordant results, and can be used to infer the past δ18O of seawater (δ18Osw) for paleoclimate studies., Geochimica et Cosmochimica Acta, 236, ISSN:0016-7037, ISSN:1872-9533