Seminar

During the Paleocene-Eocene Thermal Maximum (PETM), thousands of gigatons of carbon were released into the ocean and atmosphere over thousands of years, giving us the opportunity to elucidate the response of long-term carbon cycle processes to a CO2 release comparable in mass to ongoing anthropogenic emissions. In particular, we can critically assess the long-held assumption that the rate of CO2 consumption by terrestrial silicate weathering responds to climate, thus forming a long-term stabilizing feedback that drives the recovery of carbon cycle and climate system from perturbation. The products of silicate weathering (dissolved inorganic carbon, alkalinity, and dissolved silica) are delivered to the oceans, where they are biomineralized by calcareous and siliceous organisms and eventually buried in sediments. The occurrence of enhanced silicate weathering during the PETM must therefore leave behind evidence in the sedimentary record in the form of elevated carbonate and silica burial during the event and its recovery. I will present new records from IODP Expedition 342 in the North Atlantic that show a clear increase in carbonate and silica content at the P-E boundary. Site U1403 (at ~4.5 km paleo-water depth, the deepest record of the PETM recovered to date) features a transition from carbonate-barren Upper Paleocene clay to carbonate-bearing Lower Eocene sediments, representing the first direct evidence for an “overshoot” in the calcite compensation depth (CCD), a phenomenon predicted by carbon cycle model simulations of the event. Stable isotope records across this transition allow us to correlate the onset of carbonate sedimentation to the early stages of the PETM recovery (~70 thousand years after the event’s onset), coincident with an increase in carbonate accumulation at shallower sites globally. Sites U1403, U1408, and U1409 each contain anomalous silica-rich facies (siliceous limestones and claystones, porcellainite, and chert) at the P-E boundary, apparently representing the burial of excess silica weathered off the continents during the PETM. Together, these sedimentary records represent clear evidence for the operation of the silicate weathering feedback during the PETM, and provide novel constraints on its magnitude and timing.