The response of the Earth system to greenhouse-gas-driven warming is of critical importance for the future trajectory of our planetary environment. Hyperthermal events - past climate transients with global-scale warming significantly above background climate variability - can provide insights into the nature and magnitude of these responses. The largest hyperthermal of the Cenozoic was the Paleocene-Eocene Thermal Maximum (PETM ∼ 56 Ma). Here we present new high-resolution bulk sediment stable isotope and major element data for the classic PETM section at Zumaia, Spain. With these data we provide a new detailed stratigraphic correlation to other key deep-ocean and terrestrial PETM reference sections. With this new correlation and age model we are able to demonstrate that detrital sediment accumulation rates within the Zumaia continental margin section increased more than 4-fold during the PETM, representing a radical change in regional hydrology that drove dramatic increases in terrestrial-to-marine sediment flux. Most remarkable is that detrital accumulation rates remain high throughout the body of the PETM, and even reach peak values during the recovery phase of the characteristic PETM carbon isotope excursion (CIE). Using a series of Earth system model inversions, driven by the new Zumaia carbon isotope record, we demonstrate that the silicate weathering feedback alone is insufficient to recover the PETM CIE, and that active organic carbon burial is required to match the observed dynamics of the CIE. Further, we demonstrate that the period of maximum organic carbon sequestration coincides with the peak in detrital accumulation rates observed at Zumaia. Based on these results, we hypothesise that orbital-scale variations in subtropical hydro-climates, and their subsequent impact on sediment dynamics, may contribute to the rapid climate and CIE recovery from peak-PETM conditions.

Microorganisms in marine subsurface sediments substantially contribute to global biomass. Sediments warmer than 40°C account for roughly half the marine sediment volume, but the processes mediated by microbial populations in these hard-to-access environments are poorly understood. We investigated microbial life in up to 1.2-kilometer-deep and up to 120°C hot sediments in the Nankai Trough subduction zone. Above 45°C, concentrations of vegetative cells drop two orders of magnitude and endospores become more than 6000 times more abundant than vegetative cells. Methane is biologically produced and oxidized until sediments reach 80° to 85°C. In 100° to 120°C sediments, isotopic evidence and increased cell concentrations demonstrate the activity of acetate-degrading hyperthermophiles. Above 45°C, populated zones alternate with zones up to 192 meters thick where microbes were undetectable.