Silicate weathering and North Atlantic chert deposition during the Paleocene-Eocene Thermal Maximum

Abstract:

Decades of study has concluded that during the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma), thousands of gigatons of carbon (as CO₂ and/or methane) were released into the atmosphere over several thousand years. This offers the opportunity to study the response of biogeochemical cycles to a carbon cycle perturbation of a similar order of magnitude to current anthropogenic CO₂ release. Scenarios of the PETM invariably invoke enhanced chemical weathering rates of terrestrial silicate rocks as a long-term carbon sink, thus providing a significant negative feedback that drives the recovery and termination of the event. However, little thought has gone into what effect these proposed elevated silicate weathering rates might have had on the marine biogeochemical cycling of silicon. I use order-of-magnitude calculations and a simple geochemical box model of the marine Si cycle to show that if current estimates for the mass of C released during the PETM as well as the assumption that the majority was neutralized by silicate weathering are both correct, then the supply of dissolved silica to the oceans and subsequently its concentration in seawater must have been significantly elevated during the event and its recovery. Additionally, I describe recently discovered deep-ocean sedimentary sequences from the North Atlantic that (for the first time, to my knowledge) demonstrate elevated silica content across the P-E boundary. I argue that the elevated silica content of these sediments is the result of enhanced production and/or preservation of silica in response to elevated seawater dissolved silica concentrations and speculate as to why elevated silica burial seems to have occurred specifically in the North Atlantic, and not throughout the oceans.