Carbonate weathering, CO2 redistribution, and Neogene CCD and pCO2 evolution

Abstract

The carbonate compensation depth (CCD), δ13C of marine carbonate, atmospheric pCO2 and major ion composition of seawater provide constraints on how geological carbon cycle processes evolved over the Neogene. I use simple models and the LOSCAR ocean carbon system model to assess what changes in carbon fluxes to the ocean are necessary to explain observations since the early Miocene. The calculations consider estimates of early Miocene seawater temperatures and ion composition and a range of possible pCO2. Changes in shelf-basin partition could explain up to ≈45% the observed CCD deepening. Increased carbonate flux (likely range 28±12%) to the oceans is necessary to explain the rest. Despite changes in pCO2 from early Miocene values of 450–900 ppm to a pre-anthropogenic value of 280 ppm, the size of the total ocean-atmosphere carbon reservoir shows only moderate or no net change, implying that weathering and/or organic carbon burial result in little net CO2 consumption. Decreasing Ca++ and increasing deepwater carbonate saturation over the Neogene require a large increase in deepwater CO=3 and leads to decreasing DIC/TALK which is the main driver for falling pCO2. The primary driver of pCO2 reduction is redistribution of CO2 from the atmosphere to the oceans, not net removal of CO2 from excess silicate weathering or organic carbon burial. The main impact of tectonic perturbation of the carbon cycle during the Neogene is to enhance carbonate weathering while only weakly affecting the net balance of degassing vs. silicate weathering or kerogen oxidation vs. organic carbon burial.