Evaporative stress during the PETM in Wyoming: applying triple oxygen isotopes
As an NSF EAR Postdoctoral Fellow at the University of Michigan, I am investigating how continental water cycles in the North American western interior responded to the Paleocene–Eocene Thermal Maximum (PETM), a rapid global warming event ~56 million years ago marked by massive carbon release, rising temperatures, and major ecological disruption.
Using the emerging triple oxygen isotope proxy, I analyze fossil mammal teeth and paleosol carbonates to detect signals of evaporative stress and changes in water balance across the PETM. This approach provides one of the first applications of triple oxygen isotopes as an aridity proxy in deep time, allowing me to test competing hypotheses about whether the PETM drove transient drying in the western interior.
By combining these new isotopic records with stratigraphy, paleontology, and other climate proxies, I aim to disentangle the roles of temperature vs. water availability in driving ecological change. Preliminary results will be presented at the GSA 2025 Annual Meeting in San Antonio—come by to see where this work is headed!
Studying a paleosol sample at Polecat Bench, Wyoming.
Reconstructed water isotope values from tooth enamel of different vertebrates in the Cloverly Formation. Differences among crocodyliforms and turtles reflect ecological partitioning and variation in water use. For details, see Allen et al. (2025), Frontiers in Earth Science, doi:10.3389/feart.2025.1497416.
Paleoenvironment and Paleoecology of a mid-Cretaceous Vertebrate Fauna from the non-marine Cloverly Formation
In a recent paper in Frontiers in Earth Science (Allen et al., 2025), I led a study that used stable oxygen isotopes from more than 100 fossil specimens—crocodylomorphs, turtles, dinosaurs, and fishes—to reconstruct paleotemperature, hydrologic conditions, and patterns of water use among vertebrates of the Cloverly Formation in Montana.
These reconstructions provided the first quantitative estimates of Cloverly temperatures: warm-season surface air temperatures of ~24 °C . Beyond climate, the data revealed ecological differences—crocodylomorphs and turtles partitioned water resources in distinct ways, reflecting contrasts in physiology and habitat use. Some groups showed unexpected isotopic signals, suggesting behaviors such as terrestrial foraging or use of evaporatively enriched water bodies.
These findings place the Cloverly biota in climatic context, provide insights into greenhouse latitudinal climate gradients, and show how isotope geochemistry links climate and paleobiology to explain the structure of ancient ecosystems.
High-resolution Chronostratigraphy of the Cloverly Formation
I am finalizing a study that integrates high-precision U–Pb zircon geochronology with new carbon isotope records to build the most refined age model yet for the Cloverly Formation. Our results place the main fossil-bearing mudstones in the mid-Albian (~110–104 Ma) and reveal partial diachroneity between sections. While visually striking isotope excursions appear in some horizons, quantitative tests (dynamic time warping) show that they likely do not provide reliable regional or global correlation.
These findings underscore the potential of ashfall zircon geochronology in nonmarine deposits while cautioning against overinterpretation of carbon isotope signals in floodplain systems. The manuscript is in final preparation for submission to Geosciences (MDPI).
Carbon isotope records from two Cloverly Formation sections, plotted against age. These chemostratigraphic data, combined with geochronology, refine the temporal framework for comparing fossil and climate records across the mid-Cretaceous.
Tracking Mid-Cretaceous Climate Change in the Cloverly Formation
Visiting the Cloverly Formation type section while collecting samples for detailed paleoclimate reconstructions. Like the younger Willwood Formation, the Cloverly is marked by striking candy-striped badlands composed of ancient floodplain paleosols.
Terrestrial climate records from the mid-Cretaceous are far fewer than marine records, yet they are essential for understanding how greenhouse climates operated on land and how ecosystems responded. By pairing a refined chronostratigraphic framework for the Cloverly Formation with a multiproxy approach, I reconstructed past temperature, precipitation, and water cycle dynamics using clumped isotopes of carbonates, paleosol weathering indices, and oxygen isotopes of carbonates.
These proxies reveal that the Cloverly climate was highly dynamic: precipitation rates shifted dramatically, from less than 400 mm per year (~16 inches) to more than 1600 mm per year (~63 inches), while temperatures remained relatively stable and consistent with estimates from vertebrate fossils. This pattern suggests that fluctuations in the hydrologic cycle were driven more by changes in atmospheric moisture transport than by temperature itself.
By documenting variability in rainfall and water balance within an overall greenhouse climate, this study provides new perspective on how mid-Cretaceous climates differed from today and offers a framework for placing evolutionary and biogeographic patterns into their proper environmental context.
Vertebrate diversity across time and space in the mid-Cretaceous of North America
As part of my broader work on mid-Cretaceous ecosystems in the Western Interior Basin, I have been developing a quantitative framework for comparing vertebrate faunas across the Cloverly, Cedar Mountain, and Antlers formations. This project uses standardized occurrence data and shareholder quorum subsampling (SQS) to reduce sampling bias and evaluate faunal similarity across time and space. Preliminary results suggest that Aptian–Albian faunas were substantially more similar to one another than to those of either the preceding Morrison Formation or the younger Cenomanian Mussentuchit fauna. These results highlight the distinctiveness of mid-Cretaceous ecosystems and their importance for understanding evolutionary transitions in North America. I presented this work at the 2022 Geological Society of America meeting, and while it is currently on the back burner, I am collaborating with colleagues to finalize the analyses and bring the study to publication.
Skull of Deinonychus antirrhopus, an iconic theropod dinosaur found in Albian (mid-Cretaceous) deposits from Montana to Texas.
Rare specimen of Eumedlicottia burckhardti from the Permian Delaware Mountain Group of Sibley Ranch, west Texas.
Middle Permian Ammonoids of West Texas: Systematics, Evolution, and Paleobiology
My master’s research focused on newly documented Permian ammonoid localities at Sibley’s Last Chance Ranch in the Delaware Mountains of west Texas. This work combined traditional paleontological description with the first application of modern morphometric and quantitative methods to Middle Permian ammonoids from the region, providing a framework for analyzing sutural complexity and shell morphology in Cyclolobidae. I described several previously unreported faunas, confirmed type localities for key taxa such as Newellites richardsoni, and introduced new tools for capturing shape variation and assessing morphological disparity. These methods yielded insights into evolutionary patterns and ontogenetic trajectories, with broader implications for refining the taxonomy and biostratigraphy of Guadalupian ammonoids. This research is now being revived and expanded in collaboration with colleagues, with manuscripts currently in preparation.
Reconstructing middle-late Paleocene Paleoenvironmental Change in Wyoming
In collaboration with Dr. Anna Wisniewski, I am investigating environmental change across the middle to late Paleocene of the Bighorn Basin, a critical but understudied interval leading up to the PETM and early Eocene. Our project combines geochemical and sedimentological proxies to unravel climatic and depositional influences on terrestrial ecosystems. We are analyzing fossil tooth enamel, pedogenic carbonates, gar scales, and paleosols to reconstruct hydrology and surface conditions, while Anna is improving fossil sampling across this interval. Together, we are developing a detailed stratigraphic framework to better understand pre-PETM environmental dynamics and provide context for mammalian turnover during this climatically dynamic time.
Collecting paleosol samples from the Paleocene Fort Union Formation at Polecat Bench, Wyoming.
Conceptual illustration of how different processes shift speleothem triple oxygen isotope compositions in oxygen-18 versus triple-oxygen space. Cave kinetic effects, rainfall distillation, mineralization temperature, and evaporation or infiltration seasonality each follow characteristic slopes, shown relative to a reference endmember (red star). Figure by Tyler Huth.
Using Triple Oxygen Isotopes to Detect and Characterize Cave Kinetic Effects in Speleothem Paleoclimate Records
Speleothem oxygen isotope records are powerful but often obscured by cave-internal processes. In collaboration with Dr. Annabel Wolf and others, I am testing whether triple oxygen isotope measurements can identify and quantify these kinetic overprints. Using an ideal set of altered speleothem samples, we are evaluating whether combining triple and conventional oxygen isotopes distinguishes equilibrium from non-equilibrium effects. Preliminary results show patterns consistent with model predictions, suggesting this approach can improve the reliability of speleothem-based paleoclimate reconstructions.
Future Directions
A central goal of my future research is to leverage triple oxygen isotope geochemistry to resolve the dynamics of water balance in both modern and ancient systems. One key priority is to characterize Δ′17O in meteoric waters across climatic gradients, including precipitation, soil waters, aquifers, and cave drips, in order to establish how triple oxygen signals are distributed in the hydrologic cycle and to improve interpretations of both modern and fossil archives.
Building on this foundation, I plan to calibrate gar scales as a new archive for terrestrial hydroclimate, using modern specimens to test Δ′17O as a proxy for humidity and water balance, and applying clumped isotope (Δ47) thermometry of fossil gars to reconstruct freshwater temperatures during past greenhouse intervals such as the PETM and mid-Cretaceous. Beyond gars, I will broaden this effort to other vertebrate and pedogenic archives, constructing spatiotemporal isotope records to constrain latitudinal moisture gradients and hydroclimate sensitivity.
Finally, my exploration geology experience, combined with paleoclimate and geochemical expertise, positions me to investigate the origin and distribution of lithium brines. By linking basin history, hydroclimate, and geochemical signals, I aim to improve strategies for locating and extracting high-concentration lithium brine deposits, contributing to sustainable resource development in the context of the energy transition.
Together, these projects will expand the terrestrial paleoclimate toolkit, bridge modern and ancient hydroclimate research, and inform both fundamental science and applied challenges related to water security and resource exploration.