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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. Enhanced ocean heat storage efficiency during the last deglaciation

   https://doi.org/10.1126/sciadv.adp5156  

   Zhu, Chenyu; Sanchez, Saray; Liu, Zhengyu; Clark, Peter U; He, Chengfei; Wan, Lingfeng; Lu, Jiuyou; Zhu, Chenguang; Li, Lingwei; Zhang, Shaoqing; et al (September 2024, Science Advances)

   Proxy reconstructions suggest that increasing global mean sea surface temperature (GMSST) during the last deglaciation was accompanied by a comparable or greater increase in global mean ocean temperature (GMOT), corresponding to a large heat storage efficiency (HSE; ∆GMOT/∆GMSST). An increased GMOT is commonly attributed to surface warming at sites of deepwater formation, but winter sea ice covered much of these source areas during the last deglaciation, which would imply an HSE much less than 1. Here, we use climate model simulations and proxy-based reconstructions of ocean temperature changes to show that an increased deglacial HSE is achieved by warming of intermediate-depth waters forced by mid-latitude surface warming in response to greenhouse gas and ice sheet forcing as well as by reduced Atlantic meridional overturning circulation associated with meltwater forcing. These results, which highlight the role of surface warming and oceanic circulation changes, have implications for our understanding of long-term ocean heat storage change. 

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3. Hot Tropical Temperatures During the Paleocene‐Eocene Thermal Maximum Revealed by Paired In Situ δ ¹³ C and Mg/Ca Measurements on Individual Planktic Foraminifer Shells

   https://doi.org/10.1029/2023PA004834  

   Kozdon, Reinhard; Kelly, D Clay (August 2024, Paleoceanography and Paleoclimatology)

   The Paleocene‐Eocene thermal maximum (PETM, 56 Ma) is an ancient global warming event closely coupled to the release of massive amounts of d13C‐depleted carbon into the ocean‐atmosphere system, making it an informative analogue for future climate change. However, uncertainty still exists regarding tropical sea‐surface temperatures (SSTs) in open ocean settings during the PETM. Here, we present the first paired d13C:Mg/Ca record derived in situ from relatively well‐preserved subdomains inside individual planktic foraminifer shells taken from a PETM record recovered in the central Pacific Ocean at Ocean Drilling Program Site 865. The d13C signature of each individual shell was used to confirm calcification during the PETM, thereby reducing the unwanted effects of sediment mixing that secondarily smooth paleoclimate signals constructed with fossil planktic foraminifer shells. This method of “isotopic screening” reveals that shells calcified during the PETM have elevated Mg/Ca ratios reflecting exceptionally warm tropical SSTs (∼33–34°C). The increase in Mg/Ca ratios suggests ∼6°C of warming, which is more congruent with SST estimates derived from organic biomarkers in PETM records at other tropical sites. These extremely warm SSTs exceed the maximum temperature tolerances of modern planktic foraminifers. Important corollaries to the findings of this study are (a) the global signature of PETM warmth was uniformly distributed across different latitudes, (b) our Mg/Ca‐based SST record may not capture peak PETM warming at tropical Site 865 due to the thermally‐induced ecological exclusion of planktic foraminifers, and (c) the record of such transitory ecological exclusion has been obfuscated by post‐depositional sediment mixing at Site 865. 

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4. Assessing environmental change associated with early Eocene hyperthermals in the Atlantic Coastal Plain, USA

   https://doi.org/10.5194/cp-19-1677-2023  

   Rush, William; Self-Trail, Jean; Zhang, Yang; Sluijs, Appy; Brinkhuis, Henk; Zachos, James; Ogg, James G.; Robinson, Marci (January 2023, Climate of the Past)

   Abstract. Eocene transient global warming events (hyperthermals) can provide insight into a future warmer world. While much research has focused on the Paleocene–Eocene Thermal Maximum (PETM), hyperthermals of a smaller magnitude can be used to characterize climatic responses over different magnitudes of forcing. This study identifies two events, namely the Eocene Thermal Maximum 2 (ETM2 and H2), in shallow marine sediments of the Eocene-aged Salisbury Embayment of Maryland, based on magnetostratigraphy, calcareous nannofossil, and dinocyst biostratigraphy, as well as the recognition of negative stable carbon isotope excursions (CIEs) in biogenic calcite. We assess local environmental change in the Salisbury Embayment, utilizing clay mineralogy, marine palynology, δ18O of biogenic calcite, and biomarker paleothermometry (TEX86). Paleotemperature proxies show broad agreement between surface water and bottom water temperature changes. However, the timing of the warming does not correspond to the CIE of the ETM2 as expected from other records, and the highest values are observed during H2, suggesting factors in addition to pCO2 forcing have influenced temperature changes in the region. The ETM2 interval exhibits a shift in clay mineralogy from smectite-dominated facies to illite-rich facies, suggesting hydroclimatic changes but with a rather dampened weathering response relative to that of the PETM in the same region. Organic walled dinoflagellate cyst assemblages show large fluctuations throughout the studied section, none of which seem systematically related to CIE warming. These observations are contrary to the typical tight correspondence between climate change and assemblages across the PETM, regionally and globally, and ETM2 in the Arctic Ocean. The data do indicate very warm and (seasonally) stratified conditions, likely salinity-driven, across H2. The absence of evidence for strong perturbations in local hydrology and nutrient supply during ETM2 and H2, compared to the PETM, is consistent with the less extreme forcing and the warmer pre-event baseline, as well as the non-linear response in hydroclimates to greenhouse forcing. 

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5. Polar amplification of orbital-scale climate variability in the early Eocene greenhouse world

   https://doi.org/10.5194/cp-20-1303-2024  

   Fokkema, Chris D; Agterhuis, Tobias; Gerritsma, Danielle; de_Goeij, Myrthe; Liu, Xiaoqing; de_Regt, Pauline; Rice, Addison; Vennema, Laurens; Agnini, Claudia; Bijl, Peter K; et al (January 2024, Climate of the Past)

   Abstract. Climate variability is typically amplified towards polar regions. The underlying causes, notably albedo and humidity changes, are challenging to accurately quantify with observations or models, thus hampering projections of future polar amplification. Polar amplification reconstructions from the ice-free early Eocene (∼56–48 Ma) can exclude ice albedo effects, but the required tropical temperature records for resolving timescales shorter than multi-million years are lacking. Here, we reconstruct early Eocene tropical sea surface temperature variability by presenting an up to ∼4 kyr resolution biomarker-based temperature record from Ocean Drilling Program (ODP) Site 959, located in the tropical Atlantic Ocean. This record shows warming across multiple orbitally paced carbon cycle perturbations, coeval with high-latitude-derived deep-ocean bottom waters, showing that these events represent transient global warming events (hyperthermals). This implies that orbital forcing caused global temperature variability through carbon cycle feedbacks. Importantly, deep-ocean temperature variability was amplified by a factor of 1.7–2.3 compared to the tropical surface ocean, corroborating available long-term estimates. This implies that fast atmospheric feedback processes controlled meridional temperature gradients on multi-million year, as well as orbital, timescales during the early Eocene. Our combined records have several other implications. First, our amplification factor is somewhat larger than the same metric in fully coupled simulations of the early Eocene (1.1–1.3), suggesting that models slightly underestimate the non-ice-related – notably hydrological – feedbacks that cause polar amplification of climate change. Second, even outside the hyperthermals, we find synchronous eccentricity-forced temperature variability in the tropics and deep ocean that represent global mean sea surface temperature variability of up to 0.7 °C, which requires significant variability in atmospheric pCO2. We hypothesize that the responsible carbon cycle feedbacks that are independent of ice, snow, and frost-related processes might play an important role in Phanerozoic orbital-scale climate variability throughout geological time, including Pleistocene glacial–interglacial climate variability. 

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