An official website of the United States government
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 (δ18Ow) in the past using the temperature‐dependent fractionation factor between soil water and pedogenic carbonate, but previous work has not measured δ18Owvalues 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 δ18Owoverlaps within uncertainty with measured δ18Owat 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 δ18Owhas a significantly higher isotope value than any observations of δ18Ow. At all three sites, even in the fall season, δ18Owvalues at carbonate bearing depths overlap with spring rainfall δ18Ow, and there is little to no evaporative enrichment of δ2Hwand δ18Owvalues. These data challenge long‐held assumptions that all pedogenic carbonate records a warm‐season bias, and that δ18Owat carbonate‐bearing depths is affected by evaporative enrichment.
more »
« less
This content will become publicly available on May 13, 2026
Ecohydrology and paleoenvironment of the Cretaceous (Albian) Cloverly Formation: insights from multi-taxon oxygen isotope analysis of vertebrate phosphates
The Cloverly Formation of Montana and Wyoming preserves abundant nonmarine vertebrate fossils from the mid-Cretaceous, yet its paleoenvironment and faunal niche structure remain poorly understood. We analyzed δ18Οphosphatein over 100 fossil individuals from multiple vertebrate taxa collected from a single microfossil bonebed in Carbon County, Montana.To infer habitat preferences and water-use strategies, we compared δ18Οphosphatevalues within and across taxa. We reconstructed δ18Osurface_waterfrom semi-aquatic reptile values using regressions calibrated with data from modern environments and extant taxa. Using a multi-taxon framework, we estimated warm-season water temperatures from δ18Osurface_waterand δ18Οphosphateof lepisosteid (gar) scales, then converted these to air temperatures using a modern climate transfer function. δ18Οphosphatevalues ranged from 9.5‰ to 23.2‰ (VSMOW) and varied across taxa. Aquatic and semi-aquatic groups exhibited lower values than dinosaurian taxa. Our reconstructed mean δ18Osurface_waterwas −7.9‰ (95% CI: −10.1 to 5.5‰), yielding a warm-season water temperature of 26°C and an air temperature of 24°C. Intertaxon differences reflect niche partitioning and suggest primary isotopic signals are preserved. Unexpectedly high values in Bernissartiid-like neosuchian teeth may indicate greater ecohydrological diversity than previously recognized. Our δ18Osurface_waterestimate aligns with other Aptian-Albian proxies but exceeds model-based predictions, likely due to outdated assumptions underlying the model. The MAWSAT estimate falls within the upper range of model-data assimilation outputs. These results provide new context for ecological structure in the Cloverly fauna and offer the first quantitative temperature estimate for the Formation, helping to define baseline conditions between the Aptian-Albian Cold Snap and the Cretaceous Thermal Maximum.
more »
« less
- Award ID(s):
- 1925942
- PAR ID:
- 10598873
- Publisher / Repository:
- Frontiers
- Date Published:
- Journal Name:
- Frontiers in Earth Science
- Volume:
- 13
- ISSN:
- 2296-6463
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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 (δ18Oc), a proxy that requires knowledge of the δ18O of past seawater (δ18Osw). Here we use the water isotope‐enabled Community Earth System Model (iCESM1.2) to investigate how paleogeography and pCO2affect δ18Oswdistribution 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 δ18Oswexplaining low δ18Ocmeasured on planktonic foraminifera. Accounting for this δ18Oswspatio‐temporal variability increases the estimated meridional temperature gradient by 5°C and narrows the gap between model and proxy‐based reconstructions.more » « less
-
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 (δ18Op) 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 δ18Opvalues (mean 22.0 ± 1.3‰) show no significant changes through the Eocene. Furthermore, the variation in bulk δ18Opvalues often exceeds that in simulated seasonal and regional values. Pelagic and benthic sharks exhibit similar δ18Opvalues 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 δ18Opvalues 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 δ18Opvalues says more about species ecology than the absence of regional or global environmental changes.more » « less
-
Abstract The North American Newark Canyon Formation (NCF; ∼113–98 Ma) presents an opportunity to examine how terrestrial carbonate facies reflect different aspects of paleoclimate during one of the hottest periods of Earth's history. The lower NCF type section preserves heterogeneous palustrine facies and the upper NCF preserves lacustrine deposits. We combined carbonate facies analysis withδ13C,δ18O, and Δ47data sets to assess which carbonate facies preserve stable isotope signals that are most representative of climatic conditions. Palustrine facies record the heterogeneity of the original wetland environment in which they formed. Using the pelmicrite facies that formed in deeper wetlands, we interpret a lower temperature zone (35–40°C) to reflect warm season water temperatures. In contrast, a mottled micrite facies which formed in shallower wetlands records hotter temperatures (36–68°C). These hotter temperatures reflect radiatively heated “bare‐skin” temperatures that occurred in a shallow depositional setting. The lower lacustrine unit has been secondarily altered by hydrothermal fluids while the upper lacustrine unit likely preserves primary temperatures andδ18Owaterof catchment‐integrated precipitation. Resultantly, the palustrine pelmicrite and lacustrine micrite are the facies most likely to reflect ambient climate conditions, and therefore, are the best facies to use for paleoclimate interpretations. Average warm season water temperatures of 41.1 ± 3.6°C and 37.8 ± 2.5°C are preserved by the palustrine pelmicrite (∼113–112 Ma) and lacustrine micrite (∼112–103 Ma), respectively. These data support previous interpretations of the mid‐Cretaceous as a hothouse climate and demonstrate the importance of characterizing facies for identifying the data most representative of past climates.more » « less
-
IntroductionAstarte borealisholds great potential as an archive of seasonal paleoclimate, especially due to its long lifespan (several decades to more than a century) and ubiquitous distribution across high northern latitudes. Furthermore, recent work demonstrates that the isotope geochemistry of the aragonite shell is a faithful proxy of environmental conditions. However, the exceedingly slow growth rates ofA. borealisin some locations (<0.2mm/year) make it difficult to achieve seasonal resolution using standard micromilling techniques for conventional stable isotope analysis. Moreover, oxygen isotope (δ18O) records from species inhabiting brackish environments are notoriously difficult to use as paleoclimate archives because of the simultaneous variation in temperature and δ18Owatervalues. MethodsHere we use secondary ion mass spectrometry (SIMS) to microsample anA. borealisspecimen from the southern Baltic Sea, yielding 451 SIMS δ18Oshellvalues at sub-monthly resolution. ResultsSIMS δ18Oshellvalues exhibit a quasi-sinusoidal pattern with 24 local maxima and minima coinciding with 24 annual growth increments between March 1977 and the month before specimen collection in May 2001. DiscussionAge-modeled SIMS δ18Oshellvalues correlate significantly with bothin situtemperature measured from shipborne CTD casts (r2 = 0.52, p<0.001) and sea surface temperature from the ORAS5-SST global reanalysis product for the Baltic Sea region (r2 = 0.42, p<0.001). We observe the strongest correlation between SIMS δ18Oshellvalues and salinity when both datasets are run through a 36-month LOWESS function (r2 = 0.71, p < 0.001). Similarly, we find that LOWESS-smoothed SIMS δ18Oshellvalues exhibit a moderate correlation with the LOWESS-smoothed North Atlantic Oscillation (NAO) Index (r2 = 0.46, p<0.001). Change point analysis supports that SIMS δ18Oshellvalues capture a well-documented regime shift in the NAO circa 1989. We hypothesize that the correlation between the SIMS δ18Oshelltime series and the NAO is enhanced by the latter’s influence on the regional covariance of water temperature and δ18Owatervalues on interannual and longer timescales in the Baltic Sea. These results showcase the potential for SIMS δ18Oshellvalues inA. borealisshells to provide robust paleoclimate information regarding hydroclimate variability from seasonal to decadal timescales.more » « less
