Search

Ediacaran fossils document the early evolution of complex megascopic life, contemporaneous with geochemical evidence for widespread marine anoxia. These data suggest early animals experienced frequent hypoxia. Research has thus focused on the concentration of molecular oxygen (O 2 ) required by early animals, while also considering the impacts of climate. One model, the Cold Cradle hypothesis, proposed the Ediacaran biota originated in cold, shallow-water environments owing to increased O 2 solubility. First, we demonstrate using principles of gas exchange that temperature does have a critical role in governing the bioavailability of O 2 -but in cooler water the supply of O 2 is actually lower. Second, the fossil record suggests the Ediacara biota initially occur approximately 571 Ma in deep-water facies, before appearing in shelf environments approximately 555 Ma. We propose an ecophysiological underpinning for this pattern. By combining oceanographic data with new respirometry experiments we show that in the shallow mixed layer where seasonal temperatures fluctuate widely, thermal and partial pressure ( pO 2 ) effects are highly synergistic. The result is that temperature change away from species-specific optima impairs tolerance to low pO 2 . We hypothesize that deep and particularly stenothermal (narrow temperature range) environments in the Ediacaran ocean were a physiological refuge from the synergistic effects of temperature and low pO 2 .

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]

Ediacaran fossils document the early evolution of complex megascopic life, contemporaneous with geochemical evidence for widespread marine anoxia. These data suggest early animals experienced frequent hypoxia. Research has thus focused on the concentration of molecular oxygen (O2) required by early animals, while also considering the impacts of climate. One model, the Cold Cradle hypothesis, proposed the Ediacaran biota originated in cold, shallow-water environments owing to increased O2 solubility. First, we demonstrate using principles of gas exchange that temperature does have a critical role in governing the bioavailability of O2-but in cooler water the supply of O2 is actually lower. Second, the fossil record suggests the Ediacara biota initially occur approximately 571 Ma in deepwater facies, before appearing in shelf environments approximately 555 Ma. We propose an ecophysiological underpinning for this pattern. By combining oceanographic data with new respirometry experiments we show that in the shallow mixed layer where seasonal temperatures fluctuate widely, thermal and partial pressure (pO2) effects are highly synergistic. The result is that temperature change away from species-specific optima impairs tolerance to low pO2. We hypothesize that deep and particularly stenothermal (narrow temperature range) environments in the Ediacaran ocean were a physiological refuge from the synergistic effects of temperature and low pO2. [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]