Ancient hyperthermals are rapid global warming events linked to massive emissions of CO₂. The ocean’s biological carbon pump (BCP) plays a crucial role in climate regulation by facilitating the long-term removal of atmospheric CO₂. However, changes in the BCP during past hyperthermals remain poorly understood. Here we use the vertical carbon isotope (δ¹³C) gradients between surface and mid-waters as a tracer for the BCP and microbial metabolism during the Permian−Triassic, the early Toarcian, and the Palaeocene−Eocene hyperthermal events, to explore their impact on global warming. We detect a two- to three-fold increase in the vertical δ¹³C gradients during the three hyperthermals. Simulations with the Earth system model cGENIE indicate that elevated rates of organic matter remineralization, driven by increased microbial respiration due to warming, are the primary trigger for increased vertical δ¹³C gradients. We found that warming-induced enhancements in microbial metabolism caused a remarkable two- to six-fold reduction in the transfer efficiency of particulate organic carbon from the surface to the deep ocean during the three hyperthermals, potentially leading to diminished organic carbon sequestration in the ocean interior. This implies that the warming-induced enhancement of microbial metabolism may have amplified and exacerbated global warming during ancient hyperthermals via a positive feedback mechanism.

carbon cycle,Biological carbon pump,mass extinction,Global warming