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1. Timing of Pedogenic Carbonate Formation in Fine‐Grained Soils: Decoupled T(Δ ₄₇ ) and δ ¹⁸ O _w Seasonal Bias

   https://doi.org/10.1029/2024PA005071  

   Havranek, Rachel; Snell, Kathryn; Brookins, Sarah; Davidheiser‐Kroll, Brett (February 2025, Paleoceanography and Paleoclimatology)

   Abstract Historically, clumped isotope thermometry (T(∆₄₇)) of soil carbonates has been interpreted to represent a warm‐season soil temperature based dominantly on coarse‐grained soils. Additionally, T(∆₄₇) allows the calculation of the oxygen isotope composition of soil water (δ¹⁸O_w) in the past using the temperature‐dependent fractionation factor between soil water and pedogenic carbonate, but previous work has not measured δ¹⁸O_wvalues with which to compare to these archives. Here, we present clumped isotope thermometry of modern soil carbonates from three soils in Colorado and Nebraska, USA, that have a fine‐to‐medium grain size, contain clay, and are representative of many carbonate‐bearing paleosols preserved in the rock record. At two of the three sites, Briggsdale, CO and Seibert, CO, T(∆₄₇) overlaps with mean annual soil temperature (MAST), and the calculated δ¹⁸O_woverlaps within uncertainty with measured δ¹⁸O_wat carbonate bearing depths. At the third site, in Oglala National Grassland, NE, mean T(∆₄₇) is 8–11°C warmer than MAST, and the calculated δ¹⁸O_whas a significantly higher isotope value than any observations of δ¹⁸O_w. At all three sites, even in the fall season, δ¹⁸O_wvalues at carbonate bearing depths overlap with spring rainfall δ¹⁸O_w, and there is little to no evaporative enrichment of δ²H_wand δ¹⁸O_wvalues. These data challenge long‐held assumptions that all pedogenic carbonate records a warm‐season bias, and that δ¹⁸O_wat carbonate‐bearing depths is affected by evaporative enrichment. 

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2. Revisiting the Early Late Cretaceous Equable Climate Problem Through a Model‐Data Perspective

   https://doi.org/10.1029/2024PA005002  

   Sarr, A‐C; Poulsen, C_J; Do, E_L (April 2025, Paleoceanography and Paleoclimatology)

   Abstract Early Late Cretaceous (∼90–100 Ma) Sea surface temperatures (SST) records suggest extremely warm Southern Hemisphere high latitudes and a meridional gradient as low as 5°C, attributed to elevated atmospheric CO₂. Climate models have been unable to reproduce such extreme warmth, questioning model performance and/or the validity of SSTs reconstructions. Indeed, the latter partly rely on the measurement of oxygen isotopic composition of marine organisms (δ¹⁸O_c), a proxy that requires knowledge of the δ¹⁸O of past seawater (δ¹⁸O_sw). Here we use the water isotope‐enabled Community Earth System Model (iCESM1.2) to investigate how paleogeography and pCO₂affect δ¹⁸O_swdistribution and our understanding of Cenomanian‐Turonian SSTs. Our simulations suggest that the semi‐isolation of southern South Atlantic‐Indian Ocean resulted in locally very negative δ¹⁸O_swexplaining low δ¹⁸O_cmeasured on planktonic foraminifera. Accounting for this δ¹⁸O_swspatio‐temporal variability increases the estimated meridional temperature gradient by 5°C and narrows the gap between model and proxy‐based reconstructions. 

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3. A Combined Model for Kinetic Clumped Isotope Effects in the CaCO ₃ ‐DIC‐H ₂ O System

   https://doi.org/10.1029/2021GC010200  

   Watkins, James_M; Devriendt, Laurent_S (August 2022, Geochemistry, Geophysics, Geosystems)

   Abstract Most Earth surface carbonates precipitate out of isotopic equilibrium with their host solution, complicating the use of stable isotopes in paleoenvironment reconstructions. Disequilibrium can arise from exchange reactions in the DIC‐H₂O system as well as during crystal growth reactions in the DIC‐CaCO₃system. Existing models account for kinetic isotope effects (KIEs) in these systems separately but the models have yet to be combined in a general framework. Here, an open‐system box model is developed for describing disequilibrium carbon, oxygen, and clumped (Δ₄₇, Δ₄₈, and Δ₄₉) isotope effects in the CaCO₃‐DIC‐H₂O system. The model is used to simulate calcite precipitation experiments in which the fluxes and isotopic compositions of CO₂and CaCO₃were constrained. Using a literature compilation of equilibrium and kinetic fractionation factors, modeledδ¹⁸O and Δ₄₇values of calcite are in good agreement with the experimental data covering a wide range in crystal growth rate and solution pH. This relatively straightforward example provides a foundation for adapting the model to other situations involving CO₂absorption (e.g., corals, foraminifera, and high‐pH travertines) or degassing (e.g., speleothems, low‐pH travertines, and cryogenic carbonates) and/or mixing with other dissolved inorganic carbon sources. 

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4. Record of Neotectonics and Deep Crustal Fluid Circulation Along the Santa Fe Fault Zone in Travertine Deposits of the Lucero Uplift, New Mexico, USA

   https://doi.org/10.1029/2020GC009454  

   Garcia, V. H.; Ma, L.; Ricketts, J. W.; Dosseto, A. (April 2021, Geochemistry, Geophysics, Geosystems)

   Abstract Travertine deposits preserve an invaluable record of both ancient and modern fluid flow. The goal of this study is to reconstruct spatial and temporal patterns in travertine deposition associated with tectonic and climatic controls along the Lucero Uplift in New Mexico, USA. Uranium‐series ages of travertine deposits in the Lucero Uplift range from 0.94 ± 0.01 to 592 ± 110 ka, indicating that travertine formation has been episodically active since at least ∼600 ka. We find minimal evidence to attribute glacial and interglacial cycles to travertine formation in the Lucero Uplift. δ¹³C values in travertine deposits range from 2‰ to 9‰ (Vienna Pee Dee Belemnite), δ¹⁸O values range from 21‰ to 25‰ (Vienna Standard Mean Ocean Water). Positive correlation between travertine δ¹³C and δ¹⁸O values indicate travertine formation is closely associated with various degrees of CO₂degassing.⁸⁷Sr/⁸⁶Sr values in travertine deposits range from 0.714 to 0.717 and (²³⁴U/²³⁸U)_ivalues exhibit a remarkably wide range from 3.6 to 9.3, indicative of fluid‐rock interaction during deep crustal circulation in more radiogenic basement rocks. Reconstructed δ¹³C, δ¹⁸O, and (²³⁴U/²³⁸U)_ivalues in the inferred deep fluid sources showed systematic variations with travertine formation ages, while⁸⁷Sr/⁸⁶Sr values remain relatively constant. Based on dating of undeformed travertine deposits, which overlie tilted Santa Fe Group units, and high (²³⁴U/²³⁸U)_iwe infer that the Santa Fe fault has not produced a ground‐rupturing earthquake within the last 490 ± 52 to 592 ± 110 ka (2σ). Our study suggest that travertine formation is driven by fluid flow facilitated by tectonic and mantle structures. 

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5. A new MIS 5 to MIS 2 speleothem record from Sandkraal Cave on the South African Cape south coast

   https://doi.org/10.1017/qua.2024.3  

   Braun, Kerstin; Bar-Matthews, Miryam; Ayalon, Avner; Matthews, Alan; Zilberman, Tami; Zolotova, Natalya; Cowling, Richard M.; Karkanas, Panagiotis; Cawthra, Hayley C.; Fisher, Erich C.; et al (March 2024, Quaternary Research)

   Abstract We present new stable oxygen and carbon isotope composite records (δ¹⁸O, δ¹³C) of speleothems from Sandkraal Cave 1 (SK1) on the South African south coast for the time interval between 104 and 18 ka (with a hiatus between 48 and 41 ka). Statistical comparisons using kernel-based correlation analyses and semblance analyses based on continuous wavelet transforms inform the relationships of the new speleothem records to other proxies and their changes through time. Between 105 and ~70 ka, changes of speleothem δ¹⁸O values at SK1 are likely related to rainfall seasonality. Variations of δ¹³C values are associated with changes of vegetation density, prior carbonate precipitation (PCP), CO₂degassing in the cave, and possibly variations of the abundance of C₃and C₄grasses in the vegetation. The relationships of δ¹⁸O with other proxies shift between ~70 and 48 ka (Marine Isotope Stages 4–3) so that both stable isotope records now reflect CO₂degassing, evaporation, and PCP. Similar relationships also continue after the hiatus for the deposition phase between 42 and 18 ka. Our findings support modeling results suggesting drier conditions in the study area when the Southern Hemisphere westerlies are shifted north and the paleo–Agulhas Plain is exposed. 

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