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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. Aptian-Albian Stable Isotope Paleotemperature Estimates From North American Continental Records

   Suarez, MB; Allen, M; Weldon, A; Forster, C; Suarez, CA (August 2025, International Cretaceous Symposium)

   North America contains abundant Cretaceous continental strata that hosts potential archives of paleoclimate records. While detailed work has been done in Late Cretaceous strata which not only contains archives for stable isotope proxies, but also leaf physiognomic work, less study has occurred in the Early Cretaceous which lack angiosperm-dominated floras. Here we present new paleotemperature estimates from multiple stable isotope proxies including d18O of pedogenic carbonates, clumped isotope paleothermometry, and paired stable isotopes of aquatic and semi-aquatic taxa. The southern-most data originate from the Hensel Formation of South Texas. Clumped isotope paleothermometry of pedogenic carbonates result in temperature estimates between 35°C and 42°C. Clumped isotope paleothermometry of lake and palustrine carbonates from the Cedar Mountain Formation range from 15°C and 31°C for lake carbonates and 20°C to 44°C for palustrine carbonates. Farther north in Wyoming, we utilize the oxygen isotope value of semi aquatic taxa such as crocodilians and turtles to provide a proxy for water and utilize the oxygen isotope value of fish scales and teeth to estimate temperature at 26°C though with significant error. Preliminary clumped isotope paleothermometry provides a temperature of 25°C. The farthest north locality, the Kootenai Formation contains lacustrine, palustrine, and pedogenic carbonates. Utilizing a previous empirically-derived latitudinal gradient of meteoric water, carbonate d18O values were used to estimate a temperature range for the Kootenai between 13°C and 28°C. These values provide a temperature gradient for the Aptian-Albian over the mid latitudes of North America of ~-1°C/degree of latitude. 

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3. MULTI-TAXON OXYGEN ISOTOPE ANALYSIS OF BIOAPATITES FROM THE CLOVERLY FORMATION, WYOMING

   Suarez, MB; Allen, M; Suarez, CA (May 2025, Geological Society of America)

   Understanding of climate parameters and hydrology during past greenhouse conditions is important to forecasting future climate conditions. Oxygen isotopes of vertebrate bioapatites offer an opportunity to sample conditions across ancient landscapes. 112 samples from a variety of ecological niches were sampled from Cloverly Formation (Lower Cretaceous) micro-vertebrate site V1075 reposited at the Sam Nobel Museum at the University of Oklahoma. These samples were analyzed at the Keck Paleoenvironmental and Environmental Stable Isotope lab at the University of Kansas. Phosphate δ18O values range from between 9‰ to 23‰ vs. SMOW and show significant variability based on interpreted ecological niche of the taxa. Utilizing crocodilian and turtle d18Op values, environmental water oxygen isotope values are interpreted to be approximately -8‰ SMOW. These values are about 2-4‰ enriched relative to past estimates based on latitudinal gradients from penecontemporaneous formations from other latitudes. Based on water isotope estimates, fish δ18O values were used to calculate water temperature values of ~26°C which is consistent with estimated temperatures based on latitudinal gradients for the Cretaceous. In addition, preliminary carbonate clumped isotope paleothermometry suggest similar temperatures. These results along with emerging data from similar age deposits are beginning to build an emerging view of climate on the western side of the Western Interior Seaway during the late Early Cretaceous. 

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4. 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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5. Eocene Shark Teeth From Peninsular Antarctica: Windows to Habitat Use and Paleoceanography

   https://doi.org/10.1029/2024PA004965  

   Larocca_Conte, Gabriele; Aleksinski, Adam; Liao, Ashley; Kriwet, Jürgen; Mörs, Thomas; Trayler, Robin B; Ivany, Linda C; Huber, Matthew; Kim, Sora L (November 2024, Paleoceanography and Paleoclimatology)

   Abstract Eocene climate cooling, driven by the fallingpCO₂and tectonic changes in the Southern Ocean, impacted marine ecosystems. Sharks in high‐latitude oceans, sensitive to these changes, offer insights into both environmental shifts and biological responses, yet few paleoecological studies exist. The Middle‐to‐Late Eocene units on Seymour Island, Antarctica, provide a rich, diverse fossil record, including sharks. We analyzed the oxygen isotope composition of phosphate from shark tooth bioapatite (δ¹⁸O_p) and compared our results to co‐occurring bivalves and predictions from an isotope‐enabled global climate model to investigate habitat use and environmental conditions. Bulk δ¹⁸O_pvalues (mean 22.0 ± 1.3‰) show no significant changes through the Eocene. Furthermore, the variation in bulk δ¹⁸O_pvalues often exceeds that in simulated seasonal and regional values. Pelagic and benthic sharks exhibit similar δ¹⁸O_pvalues across units but are offset relative to bivalve and modeled values. Some taxa suggest movements into warmer or more brackish waters (e.g.,Striatolamia,Carcharias) or deeper, colder waters (e.g.,Pristiophorus). Taxa likeRajaandSqualusdisplay no shift, tracking local conditions in Seymour Island. The lack of difference in δ¹⁸O_pvalues between pelagic and benthic sharks in the Late Eocene could suggest a poorly stratified water column, inconsistent with a fully opened Drake Passage. Our findings demonstrate that shark tooth bioapatite tracks the preferred habitat conditions for individual taxa rather than recording environmental conditions where they are found. A lack of secular variation in δ¹⁸O_pvalues says more about species ecology than the absence of regional or global environmental changes. 

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