Restricted Inter-ocean Exchange and Attenuated Biological Export Caused Enhanced Carbonate Preservation in the PETM Ocean

Abstract:

Carbon dioxide (CO₂) release during the Paleocene-Eocene Thermal Maximum (PETM, 55.8 Myr BP) acidified the oceans, causing a decrease in calcium carbonate (CaCO₃) preservation. During the subsequent recovery from this acidification, the sediment CaCO₃ content came to exceed pre-PETM values, known as over-deepening or over-shooting. Past studies claim to explain these trends, but have failed to reproduce quantitatively the time series of CaCO₃ preservation. We employ a simple biogeochemical model to recreate the CaCO₃ records preserved at Walvis Ridge of the Atlantic Ocean. Replication of the observed changes, both shallowing and the subsequent over-deepening, requires two conditions not previously considered: (1) limited deep-water exchange between the Indo-Atlantic and Pacific oceans and (2) a ~50% reduction in the export of CaCO₃ to the deep sea during acidification. Contrary to past theories that attributed over-deepening to increased riverine alkalinity input, we find that over-deepening is an emergent property, generated at constant riverine input when attenuation of CaCO₃ export causes an unbalanced alkalinity input to the deep oceans (alkalinization) and the development of deep super-saturation. Restoration of CaCO₃ export, particularly in the super-saturated deep Indo-Atlantic ocean, later in the PETM leads to greater accumulation of carbonates, ergo over-shooting, which returns the ocean to pre-PETM conditions over a time scale greater than 200 kyr. While this feedback between carbonate export and the riverine input has not previously been considered, it appears to constitute an important modification of the classic carbonate compensation concept used to explain oceanic response to acidification.