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The Paleocene–Eocene Thermal Maximum (PETM) is associated with climatic change and biological turnover. It shares features with the Oceanic Anoxic Events (OAEs) of the Mesozoic, such as transient global warming and biogeochemical perturbations. However, the PETM experienced a more muted expansion of marine anoxia compared to the Mesozoic OAEs (especially OAE2), with geographically limited evidence for photic zone euxinia (PZE). We explore the extent and drivers of marine deoxygenation during the PETM using biomarkers for water column euxinia and anoxia as well as an intermediate complexity Earth system model (cGEnIE). These reveal that the water column in the North‐East Peri‐Tethys became anoxic, with euxinic conditions reaching the photic zone (PZE) during the PETM. Our model shows that euxinia developed due to a global increase in the ocean nutrient inventory with concomitant oxygen consumption, similar to findings for OAE2. The particularly strong regional response in the NE Peri‐Tethys appears to arise from a combination of global CO2‐weathering forcing, regionally restricted circulation and upwelling of sulphidic thermocline waters. Unlike OAE2, anoxia and PZE do not become widespread in our PETM simulations, consistent with new and existing geochemical and biological proxy data. This globally muted response could result from reduced biogeochemical feedbacks to climate forcing relative to the mid‐Cretaceous climate. Our observations suggest that similar mechanisms operated in response to disparate Cenozoic (PETM) and Mesozoic (OAEs) transient global warming events, while also highlighting that background conditions are crucial in modulating the sensitivity of Earth's system to them. Plain Language Summary: Ancient global warming events can help us understand the impact(s) of current global warming. A response to many past global warming events has been the expansion of oxygen deficient waters in the ocean. This is also true for the Paleocene‐Eocene Thermal Maximum that occurred about 56 million years ago. Here we show that this decrease in oxygen was so dramatic in the Peri‐Tethys (a precursor to the Mediterranean Sea of today) that it extended into the photic zone and stimulated the growth of unusual bacteria that require both light and hydrogen sulfide. This did not appear to be a direct result of warming but rather an indirect result ‐ warming caused the delivery of nutrients that stimulated the production of organic matter, which consumed oxygen in the underlying waters when it sank and degraded. This has potential analogs throughout Earth history and potentially for the future, but our work also shows that the magnitude of this response will be governed by other factors, such as the restriction of circulation, biogeochemical feedbacks and location of upwelling zones. Key Points: Photic zone euxinia (PZE) occurred in the NE Peri‐Tethys during the Paleocene–Eocene Thermal MaximumPhotic zone euxinia was likely stimulated by an increase in the nutrient inventory rather than as a direct response to ocean warmingThe restriction of PZE to specific regions is governed by geography [ABSTRACT FROM AUTHOR]

The 2‐methylhopanes (2‐MeHops) are molecular fossils of 2‐methylbacteriohopanepolyols (2‐MeBHPs) and among the oldest biomarkers on Earth. However, these biomarkers' specific sources are currently unexplained, including whether they reflect an expansion of marine cyanobacteria. Here, we study the occurrence of 2‐MeBHPs and the genes involved in their synthesis in modern bacteria and explore the occurrence of 2‐MeHops in the geological record. We find that the gene responsible for 2‐MeBHP synthesis (hpnP) is widespread in cyano‐ and ⍺‐proteobacteria, but absent or very limited in other classes/phyla of bacteria. This result is consistent with the dominance of 2‐MeBHP in cyano‐ and ⍺‐proteobacterial cultures. The review of their geological occurrence indicates that 2‐MeHops are found from the Paleoproterozoic onwards, although some Precambrian samples might be biased by drilling contamination. During the Phanerozoic, high 2‐MeHops' relative abundances (index >15%) are associated with climatic and biogeochemical perturbations such as the Permo/Triassic boundary and the Oceanic Anoxic Events. We analyzed the modern habitat of all hpnP‐containing bacteria and find that the only one species coming from an undisputed open marine habitat is an ⍺‐proteobacterium acting upon the marine nitrogen cycle. Although organisms can change their habitat in response to environmental stress and evolutionary pressure, we speculate that the high sedimentary 2‐MeHops' occurrence observed during the Phanerozoic reflect ⍺‐proteobacteria expansion and marine N‐cycle perturbations in response to climatic and environmental change. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) is used to infer past changes in the state of the climate system. Here we use a combination of newly generated and published organic paleothermometer records, together with novel high‐resolution benthic foraminiferal δ18O stratigraphy, from four sites in the midlatitude North Atlantic (41–58°N) to reconstruct the long‐term evolution of the latitudinal SST gradient during the Pliocene and early Pleistocene (4.0 to 2.4 Myr), the last time atmospheric CO2 reached concentrations above 400 ppmv. We demonstrate that the latitudinal SST gradient in the North Atlantic nearly collapsed twice during this period. We conclude that the latitudinal SST gradient in the midlatitude North Atlantic has two end‐members: a maximum as existing at present and a minimum that existed during certain periods of the (late) Pliocene. Our results suggest that the 400‐ppmv Pliocene world was more dynamic than currently thought. Key Points: New SST records spanning Plio/Pleistocene from the North AtlanticReveal episodic collapse latitudinal SST gradient North AtlanticWe speculate that collapse is related to changes in Gateways [ABSTRACT FROM AUTHOR]

The extent and persistence of anoxia in the South Atlantic Ocean during its early opening phase in the Early Cretaceous is not well constrained, hindering a holistic understanding of the processes and mechanisms that drive past changes in water column redox conditions, as well as the impacts of such changes on marine ecosystems. Here we provide high-resolution geochemical records from Deep Sea Drilling Project Site 364 that document variations in redox conditions, chemocline depth, marine productivity, and marine ecosystem dynamics in the northern South Atlantic during the Aptian. We show that many of these parameters varied across discrete sedimentary cycles expressed in the Deep Sea Drilling Project 364 succession. Our data indicate that during the initial stages of basin development, anoxic and euxinic conditions were prevalent and occasionally extended into the upper water column. However, strong cyclicity in sedimentological and geochemical parameters imply that the anoxia/euxinia was not a persistent state. We argue that the water column redox conditions during the Aptian were driven by changes in the hydrological cycle, induced by variations in astronomical forcing. We suggest that the episodically amplified hydrological cycle not only enhanced nutrient availability and marine productivity, but might also have caused density-driven upper ocean stratification. The presence of black shales of similar age in other ocean basins suggests that this mechanism is broadly important for the formation of Early Cretaceous organic-rich successions. [ABSTRACT FROM AUTHOR]