Microbial metabolism amplified warming in three Phanerozoic hyperthermal events

The ocean's biological carbon pump (BCP) plays a crucial role in climate regulation by facilitating long-term removal of atmospheric CO₂. However, BCP changes during past hyperthermals remain poorly understood. Here we use the vertical carbon isotope (δ¹³C) gradients between surface and mid-waters to trace the BCP changes during the Permian-Triassic, early Toarcian, and Palaeocene-Eocene hyperthermals, detecting a two- to three-fold increase in the vertical δ¹³C gradients. We found that enhanced organic matter remineralization, driven by increased microbial respiration under warming, are the primary trigger for increased vertical δ¹³C gradients. Warming-induced enhancements in microbial metabolism caused a 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 reducing carbon sequestration in the deeper ocean interior. This implies that the warming-induced enhancement of microbial metabolism may have amplified global warming during ancient hyperthermals via a positive feedback mechanism.