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1. The seawater carbon inventory at the Paleocene–Eocene Thermal Maximum

   https://doi.org/10.1073/pnas.2003197117  

   Haynes, Laura L. ; Hönisch, Bärbel ( September 2020 , Proceedings of the National Academy of Sciences)

   The Paleocene–Eocene Thermal Maximum (PETM) (55.6 Mya) was a geologically rapid carbon-release event that is considered the closest natural analog to anthropogenic CO₂emissions. Recent work has used boron-based proxies in planktic foraminifera to characterize the extent of surface-ocean acidification that occurred during the event. However, seawater acidity alone provides an incomplete constraint on the nature and source of carbon release. Here, we apply previously undescribed culture calibrations for the B/Ca proxy in planktic foraminifera and use them to calculate relative changes in seawater-dissolved inorganic carbon (DIC) concentration, surmising that Pacific surface-ocean DIC increased by$+1,{010}_{-646}^{+\mathrm{1,415}}$µmol/kg during the peak-PETM. Making reasonable assumptions for the pre-PETM oceanic DIC inventory, we provide a fully data-driven estimate of the PETM carbon source. Our reconstruction yields a mean source carbon δ¹³C of −10‰ and a mean increase in the oceanic C inventory of +14,900 petagrams of carbon (PgC), pointing to volcanic CO₂emissions as the main carbon source responsible for PETM warming.

    

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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)

   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. The middle to late Eocene greenhouse climate modelled using the CESM 1.0.5

   https://doi.org/10.5194/cp-16-2573-2020  

   Baatsen, Michiel ; von der Heydt, Anna S. ; Huber, Matthew ; Kliphuis, Michael A. ; Bijl, Peter K. ; Sluijs, Appy ; Dijkstra, Henk A. ( January 2020 , Climate of the Past)

   null (Ed.)

   Abstract. The early and late Eocene have both been the subject of many modelling studies, but few have focused on the middle Eocene. The latter still holds many challenges for climate modellers but is also key to understanding the events leading towards the conditions needed for Antarctic glaciation at the Eocene–Oligocene transition. Here, we present the results of CMIP5-like coupled climate simulations using the Community Earth System Model (CESM) version 1. Using a new detailed 38 Ma geography reconstruction and higher model resolution compared to most previous modelling studies and sufficiently long equilibration times, these simulations will help to further understand the middle to late Eocene climate. At realistic levels of atmospheric greenhouse gases, the model is able to show overall good agreement with proxy records and capture the important aspects of a warm greenhouse climate during the Eocene. With a quadrupling of pre-industrial concentrations of both CO2 and CH4 (i.e. 1120 ppm and ∼2700 ppb, respectively, or 4 × PIC; pre-industrial carbon), sea surface temperatures correspond well to the available late middle Eocene (42–38 Ma; ∼ Bartonian) proxies. Being generally cooler, the simulated climate under 2 × PIC forcing is a good analogue for that of the late Eocene (38–34 Ma; ∼ Priabonian). Terrestrial temperature proxies, although their geographical coverage is sparse, also indicate that the results presented here are in agreement with the available information. Our simulated middle to late Eocene climate has a reduced Equator-to-pole temperature gradient and a more symmetric meridional heat distribution compared to the pre-industrial reference. The collective effects of geography, vegetation, and ice account for a global average 5–7 ∘C difference between pre-industrial and 38 Ma Eocene boundary conditions, with important contributions from cloud and water vapour feedbacks. This helps to explain Eocene warmth in general, without the need for greenhouse gas levels much higher than indicated by proxy estimates (i.e. ∼500–1200 ppm CO2) or low-latitude regions becoming unreasonably warm. High-latitude warmth supports the idea of mostly ice-free polar regions, even at 2 × PIC, with Antarctica experiencing particularly warm summers. An overall wet climate is seen in the simulated Eocene climate, which has a strongly monsoonal character. Equilibrium climate sensitivity is reduced (0.62 ∘C W−1 m2; 3.21 ∘C warming between 38 Ma 2 × PIC and 4 × PIC) compared to that of the present-day climate (0.80 ∘C W−1 m2; 3.17 ∘C per CO2 doubling). While the actual warming is similar, we see mainly a higher radiative forcing from the second PIC doubling. A more detailed analysis of energy fluxes shows that the regional radiative balance is mainly responsible for sustaining a low meridional temperature gradient in the Eocene climate, as well as the polar amplification seen towards even warmer conditions. These model results may be useful to reconsider the drivers of Eocene warmth and the Eocene–Oligocene transition (EOT) but can also be a base for more detailed comparisons to future proxy estimates. 

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4. Enhanced Poleward Flux of Atmospheric Moisture to the Weddell Sea Region (ODP Site 690) During the Paleocene‐Eocene Thermal Maximum

   https://doi.org/10.1029/2019PA003811  

   Kozdon, Reinhard ; Penman, Donald E. ; Kelly, D. C. ; Zachos, J. C. ; Fournelle, John H. ; Valley, J. W. ( June 2020 , Paleoceanography and Paleoclimatology)

   Earth's hydrological cycle was profoundly perturbed by massive carbon emissions during an ancient (56 Ma) global warming event referred to as the Paleocene‐Eocene thermal maximum (PETM). One approach to gaining valuable insight into the response of the hydrological cycle is to construct sea‐surface salinity (SSS) records that can be used to gauge changes in the rates of evaporation and precipitation during the PETM in such climatically sensitive areas as the circum‐Antarctic region. Here, we pair oxygen isotope (δ¹⁸O) and magnesium‐calcium (Mg/Ca) measurements to reconstruct PETM sea‐surface temperatures (SSTs) and δ¹⁸O composition of seawater (δ¹⁸O_sw) at austral Site 690 (Weddell Sea). Several discrepancies emerge between the δ¹⁸O‐ and Mg/Ca‐based SST records, with the latter indicating that the earliest PETM was punctuated by a short‐lived ~4°C increase in local SSTs. Conversion of the δ¹⁸O_swvalues to SSS reveals a ~4 ppt decrease ~50 ka after peak PETM warming at Site 690. This negative SSS (δ¹⁸O_sw) anomaly coincides with a prominent minimum in the planktic foraminifer δ¹⁸O record published for the Site 690 PETM section. Thus, our revised interpretation posits that this δ¹⁸O minimum signals a decrease in surface‐ocean δ¹⁸O_swfostered by a transient increase in mean annual precipitation in the Weddell Sea region. The results of this study corroborate the view that the poleward flux of atmospheric moisture temporarily increased during a distinctive stage of the PETM.

    

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5. A Depth‐Transect of Ocean Deoxygenation During the Paleocene‐Eocene Thermal Maximum: Magnetofossils in Sediment Cores From the Southeast Atlantic

   https://doi.org/10.1029/2022JB024714  

   Xue, Pengfei ; Chang, Liao ; Dickens, Gerald R. ; Thomas, Ellen ( August 2022 , Journal of Geophysical Research: Solid Earth)

   The Paleocene‐Eocene Thermal Maximum (PETM, ∼56 Ma) presents a past analog for future global warming. Previous studies provided evidence for major loss of dissolved oxygen during the PETM, although understanding the degree and distribution of oxygen loss poses challenges. Magnetofossils produced by magnetotactic bacteria are sensitive to redox conditions in sediments and water columns, and have been used to reconstruct paleoredox conditions over a range of geological settings. Here, we present records of well‐preserved magnetofossils from cores along Walvis Ridge in the Southeast Atlantic that span the PETM across a depth transect (∼1,500–3,600 m paleodepth). Hysteresis, isothermal remanent magnetization curves, first‐order reversal curve diagrams, and low‐temperature magnetic measurements document large variations in magnetic properties of magnetofossils, which relate to time and water depth. Abundant magnetofossil grains are present within the studied sediments, and their morphologies change with paleodepth, as shown by transmission electron microscope observations. Magnetofossils from samples within the PETM onset at the deeper sites (∼2,600–3,600 m paleodepth) have lower coercivity values, a higher oxidation degree, and smaller grain sizes than those from shallower sites (∼1,500–1,800 m paleodepth), likely reflecting changes in paleoredox conditions at different paleodepths. We use the magnetofossil records to reconstruct relative changes in dissolved oxygen content at different water depths through the PETM, and suggest that ocean deoxygenation likely expanded downwards in the early stages of the PETM. We thus demonstrate the value of magnetofossil records for paleoenvironmental reconstructions over time and space, particularly for sediments that lack carbonate fossils.

    

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