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The decline in background extinction rates of marine animals through geologic time is an established but unexplained feature of the Phanerozoic fossil record. There is also growing consensus that the ocean and atmosphere did not become oxygenated to near-modern levels until the mid-Paleozoic, coinciding with the onset of generally lower extinction rates. Physiological theory provides us with a possible causal link between these two observations-predicting that the synergistic impacts of oxygen and temperature on aerobic respiration would have made marine animals more vulnerable to ocean warming events during periods of limited surface oxygenation. Here, we evaluate the hypothesis that changes in surface oxygenation exerted a first-order control on extinction rates through the Phanerozoic using a combined Earth system and ecophysiological modeling approach. We find that although continental configuration, the efficiency of the biological carbon pump in the ocean, and initial climate state all impact the magnitude of modeled biodiversity loss across simulated warming events, atmospheric oxygen is the dominant predictor of extinction vulnerability, with metabolic habitat viability and global ecophysiotype extinction exhibiting inflection points around 40% of present atmospheric oxygen. Given this is the broad upper limit for estimates of early Paleozoic oxygen levels, our results are consistent with the relative frequency of high-magnitude extinction events (particularly those not included in the canonical big five mass extinctions) early in the Phanerozoic being a direct consequence of limited early Paleozoic oxygenation and temperature-dependent hypoxia responses.

The Eocene (56–34 million years ago) is characterized by declining sea surface temperatures (SSTs) in the low latitudes (∼4°C) and high southern latitudes (∼8–11°C), in accord with decreasing CO2 estimates. However, in the mid‐to‐high northern latitudes there is no evidence for surface water cooling, suggesting thermal decoupling between northern and southern hemispheres and additional non‐CO2 controls. To explore this further, we present a multi‐proxy (Mg/Ca, δ18O, TEX86) SST record from Bass River in the western North Atlantic. Our compiled multi‐proxy SST record confirms a net decline in SSTs (∼4°C) between the early Eocene Climatic Optimum (53.3–49.1 Ma) and mid‐Eocene (∼44–41 Ma), supporting declining atmospheric CO2 as the primary mechanism of Eocene cooling. However, from the mid‐Eocene onwards, east‐west North Atlantic temperature gradients exhibit different trends, which we attribute to incursion of warmer waters into the eastern North Atlantic and inception of Northern Component Water across the early‐middle Eocene transition. Plain Language Summary: Over the past 541 million years, the Earth has oscillated between warm (greenhouse) and cold (icehouse) climates. The most recent transition between a greenhouse and icehouse climate state occurred during the Eocene (56–34 million years ago). This transition shows a gradual cooling, previously suggested to be driven by a decline in atmospheric carbon dioxide (CO2). However, we know little about this transition in the North Atlantic Ocean. Previous studies show limited cooling of surface waters in this region. This suggests that changes in North Atlantic temperatures are not driven by CO2. To understand how sea surface temperature changes in the western North Atlantic, we analyzed the chemistry of microscopic marine fossils in sediments. Our results show a 4°C decline in temperature from the early (∼53 Ma) to the middle Eocene (∼42 Ma). This matches computer simulations of Eocene climate and confirms CO2 was responsible for the transition. The lack of cooling observed in previous work is probably due to the development of an ancient water mass known as Northern Component Water (observed today as North Atlantic Deep Water) and changes in how the Eocene ocean transported heat. Key Points: Long‐term (∼4°C) decline in North Atlantic sea surface temperatures (SSTs) between the early (∼53–49 Ma) and middle (∼44–41 Ma) EoceneThis indicates that CO2 was likely responsible for the onset of long‐term Eocene coolingHowever, east‐west temperature gradients in the North Atlantic are decoupled, possibly due to additional non‐CO2 forcing mechanisms [ABSTRACT FROM AUTHOR]

The Eocene (56–34 million years ago) is characterized by declining sea surface temperatures (SSTs) in the low latitudes (∼4°C) and high southern latitudes (∼8–11°C), in accord with decreasing CO2 estimates. However, in the mid‐to‐high northern latitudes there is no evidence for surface water cooling, suggesting thermal decoupling between northern and southern hemispheres and additional non‐CO2 controls. To explore this further, we present a multi‐proxy (Mg/Ca, δ18O, TEX86) SST record from Bass River in the western North Atlantic. Our compiled multi‐proxy SST record confirms a net decline in SSTs (∼4°C) between the early Eocene Climatic Optimum (53.3–49.1 Ma) and mid‐Eocene (∼44–41 Ma), supporting declining atmospheric CO2 as the primary mechanism of Eocene cooling. However, from the mid‐Eocene onwards, east‐west North Atlantic temperature gradients exhibit different trends, which we attribute to incursion of warmer waters into the eastern North Atlantic and inception of Northern Component Water across the early‐middle Eocene transition. Plain Language Summary: Over the past 541 million years, the Earth has oscillated between warm (greenhouse) and cold (icehouse) climates. The most recent transition between a greenhouse and icehouse climate state occurred during the Eocene (56–34 million years ago). This transition shows a gradual cooling, previously suggested to be driven by a decline in atmospheric carbon dioxide (CO2). However, we know little about this transition in the North Atlantic Ocean. Previous studies show limited cooling of surface waters in this region. This suggests that changes in North Atlantic temperatures are not driven by CO2. To understand how sea surface temperature changes in the western North Atlantic, we analyzed the chemistry of microscopic marine fossils in sediments. Our results show a 4°C decline in temperature from the early (∼53 Ma) to the middle Eocene (∼42 Ma). This matches computer simulations of Eocene climate and confirms CO2 was responsible for the transition. The lack of cooling observed in previous work is probably due to the development of an ancient water mass known as Northern Component Water (observed today as North Atlantic Deep Water) and changes in how the Eocene ocean transported heat. Key Points: Long‐term (∼4°C) decline in North Atlantic sea surface temperatures (SSTs) between the early (∼53–49 Ma) and middle (∼44–41 Ma) EoceneThis indicates that CO2 was likely responsible for the onset of long‐term Eocene coolingHowever, east‐west temperature gradients in the North Atlantic are decoupled, possibly due to additional non‐CO2 forcing mechanisms [ABSTRACT FROM AUTHOR]