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Multiple abrupt warming events (“hyperthermals”) punctuated the Early Eocene and were associated with deep-sea temperature increases of 2 to 4 °C, seafloor carbonate dissolution, and negative carbon isotope (δ¹³C) excursions. Whether hyperthermals were associated with changes in the global ocean overturning circulation is important for understanding their driving mechanisms and feedbacks and for gaining insight into the circulation’s sensitivity to climatic warming. Here, we present high-resolution benthic foraminiferal stable isotope records (δ¹³C and δ¹⁸O) throughout the Early Eocene Climate Optimum (~53.26 to 49.14 Ma) from the deep equatorial and North Atlantic. Combined with existing records from the South Atlantic and Pacific, these indicate consistently amplified δ¹³C excursion sizes during hyperthermals in the deep equatorial Atlantic. We compare these observations with results from an intermediate complexity Earth system model to demonstrate that this spatial pattern of δ¹³C excursion size is a predictable consequence of global warming-induced changes in ocean overturning circulation. In our model, transient warming drives the weakening of Southern Ocean-sourced overturning circulation, strengthens Atlantic meridional water mass aging gradients, and amplifies the magnitude of negative δ¹³C excursions in the equatorial to North Atlantic. Based on model-data consistency, we conclude that Eocene hyperthermals coincided with repeated weakening of the global overturning circulation. Not accounting for ocean circulation impacts on δ¹³C excursions will lead to incorrect estimates of the magnitude of carbon release driving hyperthermals. Our finding of weakening overturning in response to past transient climatic warming is consistent with predictions of declining Atlantic Ocean overturning strength in our warm future.

 

Abstract Large Oligocene Antarctic ice sheets co-existed with warm proximal waters offshore Wilkes Land. Here we provide a broader Southern Ocean perspective to such warmth by reconstructing the strength and variability of the Oligocene Australian-Antarctic latitudinal sea surface temperature gradient. Our Oligocene TEX 86 -based sea surface temperature record from offshore southern Australia shows temperate (20–29 °C) conditions throughout, despite northward tectonic drift. A persistent sea surface temperature gradient (~5–10 °C) exists between Australia and Antarctica, which increases during glacial intervals. The sea surface temperature gradient increases from ~26 Ma, due to Antarctic-proximal cooling. Meanwhile, benthic foraminiferal oxygen isotope decline indicates ice loss/deep-sea warming. These contrasting patterns are difficult to explain by greenhouse gas forcing alone. Timing of the sea surface temperature cooling coincides with deepening of Drake Passage and matches results of ocean model experiments that demonstrate that Drake Passage opening cools Antarctic proximal waters. We conclude that Drake Passage deepening cooled Antarctic coasts which enhanced thermal isolation of Antarctica.