UC Santa Cruz

This dissertation reconstructs the response of marine carbonate chemistry to rapid carbon injection during the Paleocene-Eocene Thermal Maximum (PETM). Chapter 1 uses boron-based proxies to reconstruct ocean acidification during the PETM for the first time, concluding that surface seawater pH declined by ~0.3 units at the onset of the event, and remained acidified for at least 70 thousand years before recovering in step with temperature and the carbon isotopic signature of the PETM. Chapter 2 describes a new sedimentary record of the PETM from the deep North Atlantic which provides the first evidence for a hypothesized "overshoot" of carbonate saturation in the aftermath of the PETM in response to long-term weathering feedbacks on climate. Stable isotope records from that section demonstrate that this overshoot occurred during the recovery of the PETM, ~70 thousand years after its onset, which provides novel constraints on the evolution of the carbonate compensation depth over the event. Finally, Chapter 3 uses these new (and previous) records to constrain carbon cycle model simulations of the PETM. Consistent runs require both a large initial release as well as a protracted release over tens of thousands of years (possibly representing a slow positive feedback to warming) and the removal of isotopically light carbon to accelerate the PETM recovery, likely representing organic carbon burial. No consistent scenarios feature under-saturated conditions in the surface ocean during the PETM, consistent with the lack of calcifier extinctions during the event. Comparison of the most consistent PETM scenarios with forecasts of anthropogenic carbon emissions demonstrate that carbonate chemistry change during the PETM was less severe and far more gradual than what might be expected in coming centuries.