Silicate weathering and North Atlantic silica burial during the Paleocene-Eocene Thermal Maximum

Abstract

During the Paleocene-Eocene Thermal Maximum (PETM, ca. 56 Ma), thousands of gigatons of carbon were released into the ocean and atmosphere over several thousand years, offering the opportunity to study the response of ocean biogeochemistry to a carbon cycle perturbation of a similar magnitude to projected anthropogenic CO 2 release. PETM scenarios typically invoke accelerated chemical weathering of terrestrial silicate rocks as a significant negative feedback driving the recovery and termination of the event. However, the implications of this mechanism for the geochemical cycling of silica during the PETM have received little attention. I use “back-of-the-envelope” calculations and a simple two-box geochemical model of the marine silica cycle to demonstrate that the sequestration of thousands of gigatons of carbon by enhanced silicate weathering during the PETM would have dramatically increased the riverine supply of dissolved silica (H 4 SiO 4 ) to the oceans. This would have elevated seawater [H 4 SiO 4 ], encouraging both increased opal (SiO 2 ) production by siliceous organisms and enhanced preservation of SiO 2 in the water column and sediments. Both of these factors would have promoted a prompt (due to the relatively short oceanic residence time of silica) increase in sedimentary opal burial, thus balancing the marine silica budget. Several recently recovered deep-sea sedimentary records from the central North Atlantic demonstrate elevated SiO 2 content across the Paleocene-Eocene boundary, which I argue is the result of enhanced production and/or preservation of SiO 2 in response to elevated [H 4 SiO 4 ] in the North Atlantic, representing the ultimate fate of excess Si weathered from the continents during the PETM.