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1. Long- and short-term coupling of sea surface temperature and atmospheric CO ₂ during the late Paleocene and early Eocene

   https://doi.org/10.1073/pnas.2318779121  

   Harper, Dustin T; Hönisch, Bärbel; Bowen, Gabriel J; Zeebe, Richard E; Haynes, Laura L; Penman, Donald E; Zachos, James C (September 2024, Proceedings of the National Academy of Sciences)

   The late Paleocene and early Eocene (LPEE) are characterized by long-term (million years, Myr) global warming and by transient, abrupt (kiloyears, kyr) warming events, termed hyperthermals. Although both have been attributed to greenhouse (CO₂) forcing, the longer-term trend in climate was likely influenced by additional forcing factors (i.e., tectonics) and the extent to which warming was driven by atmospheric CO₂remains unclear. Here, we use a suite of new and existing observations from planktic foraminifera collected at Pacific Ocean Drilling Program Sites 1209 and 1210 and inversion of a multiproxy Bayesian hierarchical model to quantify sea surface temperature (SST) and atmospheric CO₂over a 6-Myr interval. Our reconstructions span the initiation of long-term LPEE warming (~58 Ma), and the two largest Paleogene hyperthermals, the Paleocene–Eocene Thermal Maximum (PETM, ~56 Ma) and Eocene Thermal Maximum 2 (ETM-2, ~54 Ma). Our results show strong coupling between CO₂and temperature over the long- (LPEE) and short-term (PETM and ETM-2) but differing Pacific climate sensitivities over the two timescales. Combined CO₂and carbon isotope trends imply the carbon source driving CO₂increase was likely methanogenic, organic, or mixed for the PETM and organic for ETM-2, whereas a source with higher δ¹³C values (e.g., volcanic degassing) is associated with the long-term LPEE. Reconstructed emissions for the PETM (5,800 Gt C) and ETM-2 (3,800 Gt C) are comparable in mass to future emission scenarios, reinforcing the value of these events as analogs of anthropogenic change. 

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2. Shelf Ecosystems Along the U.S. Atlantic Coastal Plain Prior to and During the Paleocene‐Eocene Thermal Maximum: Insights Into the Stratigraphic Architecture

   https://doi.org/10.1029/2022PA004475  

   Doubrawa, Monika; Stassen, Peter; Robinson, Marci M.; Babila, Tali L.; Zachos, James C.; Speijer, Robert P. (October 2022, Paleoceanography and Paleoclimatology)

   Abstract The Paleocene‐Eocene Thermal Maximum (PETM) is the most pronounced global warming event of the early Paleogene related to atmospheric CO₂increases. It is characterized by negative δ¹⁸O and δ¹³C excursions recorded in sedimentary archives and a transient disruption of the marine biosphere. Sites from the U.S. Atlantic Coastal Plain show an additional small, but distinct δ¹³C excursion below the onset of the PETM, coined the “pre‐onset excursion” (POE), mimicking the PETM‐forced environmental perturbations. This study focuses on the South Dover Bridge core in Maryland, where the Paleocene‐Eocene transition is stratigraphically constrained by calcareous nannoplankton and stable isotope data, and in which the POE is well‐expressed. The site was situated in a middle neritic marine shelf setting near a major outflow of the paleo‐Potomac River system. We generated high‐resolution benthic foraminiferal assemblage, stable isotope, trace‐metal, grain‐size and clay mineralogy data. The resulting stratigraphic subdivision of this Paleocene‐Eocene transition is placed within a depth transect across the paleoshelf, highlighting that the PETM sequence is relatively expanded. The geochemical records provide detailed insights into the paleoenvironment, developing from a well‐oxygenated water column in latest Paleocene to a PETM‐ecosystem under severe biotic stress‐conditions, with shifts in food supply and temperature, and under dysoxic bottom waters in a more river‐dominated setting. Environmental changes started in the latest Paleocene and culminated atthe onset of the PETM, hinting to an intensifying trigger rather than to an instantaneous event at the Paleocene‐Eocene boundary toppling the global system. 

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3. Paleosol‐Based Reconstruction Indicates Decoupling of Mean Annual Precipitation and Precipitation Intensity During the Paleocene‐Eocene Thermal Maximum in the Uinta Basin, Utah

   https://doi.org/10.1029/2024PA004966  

   Slawson, J; Plink‐Bjorklund, B P; Beverly, E J; Bachtadse, V (April 2025, Paleoceanography and Paleoclimatology)

   Abstract The Earth is transitioning to a state unprecedented in human history. This transition poses a challenge for predicting the future, as climate models require testing and calibration with real‐world data from high greenhouse gas climates. Despite significant progress in climate modeling, changes in the precipitation remain highly uncertain. The Paleocene‐Eocene Thermal Maximum (PETM) was the warmest period of the Cenozoic Era, and thus serves as an analog for a hydrological cycle altered by extreme greenhouse gas warming. Here, we use paleosol‐based geochemical proxies to quantify changes in mean annual precipitation (MAP) during the PETM in the Uinta Basin, Utah. We find no change in MAP during this warming event. However, paleosol mass balance results track increased translocation of carbonates, increased clay illuviation, and increased accumulation of redox‐sensitive elements. These results, along with shifts in fluvial stratigraphy, provide evidence for increased intensity and intermittency of extreme precipitation events that may be related to changes in the transport direction, seasonality, and moisture transport capability of the North American Monsoon. Surprisingly, changes in fluvial stratigraphy, clay illuviuation, and redoximorphy continued for 10⁵–10⁶ years after the PETM, suggesting persistent changes in precipitation intensity despite a decrease in global temperature. These findings provide further support for an intensification of the hydrological cycle during and after the PETM, provide evidence for a decoupling between mean and extreme precipitation, and indicate the importance of multi‐proxy, regional studies for understanding the complexities of climate change. 

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4. Synchronous Marine and Terrestrial Carbon Cycle Perturbation in the High Arctic During the PETM

   https://doi.org/10.1029/2020PA003942  

   Cui, Ying; Diefendorf, Aaron_F; Kump, Lee_R; Jiang, Shijun; Freeman, Katherine_H (April 2021, Paleoceanography and Paleoclimatology)

   Abstract The Paleocene‐Eocene Thermal Maximum (PETM; 56 Ma) is considered to be one of the best analogs for future climate change. The carbon isotope composition (δ¹³C) ofn‐alkanes derived from leaf waxes of terrestrial plants and marine algae can provide important insights into the carbon cycle perturbation during the PETM. Here, we present new organic geochemical data and compound‐specific δ¹³C data from sediments recovered from an early Cenozoic basin‐margin succession from Spitsbergen. These samples represent one of the most expanded PETM sites and provide new insights into the high Arctic response to the PETM. Our results reveal a synchronous ∼−6.5‰ carbon isotope excursion (CIE) in short‐chainn‐alkanes (nC₁₉; marine algae/bacteria) with a ∼−5‰ CIE in long‐chainn‐alkanes (nC₂₉andnC₃₁; plant waxes) during the peak of the PETM. Although δ¹³C_n‐alkanesvalues were potentially affected via a modest thermal effect (1‰–2‰), the relative changes in the δ¹³C_n‐alkanesremain robust. A simple carbon cycle modeling suggests peak carbon emission rate could be ∼3 times faster than previously suggested using δ¹³C_TOCrecords. The CIE magnitude of both δ¹³C_n‐C19and δ¹³C_n‐C29can be explained by the elevated influence of¹³C‐depleted respired CO₂in the water column and increased water availability on land, elevatedpCO₂in the atmosphere, and changes in vegetation type during the PETM. The synchronous decline in δ¹³C of both leaf waxes and marine algae/bacteria argues against a significant contribution to the sedimentary organic carbon pool from the weathering delivery of fossiln‐alkanes in the Arctic region. 

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5. Staying Grounded: Establishing Baseline Critical Zone Conditions in Mississippi Prior to the Onset of Paleocene-Eocene Hyperthermals

   Rathbun, S.; Platt, B.F. (January 2021, AGU Fall Meeting)

   Paleocene-Eocene hyperthermals are viewed as some of the best ancient analogs for projected future anthropogenic climate change. In order to fully evaluate the magnitude of these climactic perturbations, however, a more complete understanding of prevailing background conditions is necessary. The Mississippi Embayment, a major southwest-dipping sedimentary basin in the Gulf of Mexico coastal region of North America, contains an extensive record of Paleocene strata deposited prior to the onset of the Paleocene Carbon Isotope Maximum (PCIM), a gradual warming trend upon which the Paleocene-Eocene Thermal Maximum (PETM) was superimposed. In order to evaluate pre-PCIM paleoclimate, we focus on paleosols in the Upper Paleocene Naheola Formation. A continuous section of the Naheola is available in archival core collected by Mississippi Minerals Resources Institute from Tippah County, Mississippi, USA. We performed a suite of initial core description methods, including logging of visual observations (e.g., grain size and Munsell colors), gamma density, magnetic susceptibility, smear slide analysis, and scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDS). Results indicate a > 8-m-thick interval of 5 stacked paleosols associated with 4 lignite seams. The paleosols range in thickness from 0.6 m to 1.9 m, while the lignite seams range in thickness from 0.3 m to 1.3 m. Paleosols are characterized by low chroma matrix colors, mottling, and abundant carbonized roots. The thickest paleosols each exhibit an interval that coarsens and then fines upward; these are likely composite paleosols. Applying SEM-EDS results from all paleosols to the chemical index of alteration minus potash (CIA-K) yields preliminary mean annual precipitation estimates between 1200 and 1300 mm. The oldest paleosol contains abundant kaolinite and future stable isotope analysis will be used to reconstruct paleotemperature. Ongoing work will evaluate the relative influence of each of the five soil-forming factors on Naheola paleosol development and reexamine Paleocene- Eocene hyperthermals within the context of our results. Future work will include pollen analysis to improve chronostratigraphic control and evaluate paleoecological response to the Paleocene- Eocene climate change. 

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