Millennial Scale Impacts of Marine Biogenic Calcification Changes on Ocean Carbon Cycling

Ocean acidification resulting from increasing anthropogenic carbon dioxide (CO2) emissions are likely to impact calcification rates in pelagic organisms which may, in turn, lead to changes in the strength of the ocean carbon sink. However, the responses of pelagic calcifying organisms to acidification driven changes vary widely between species. This leads to a degree of uncertainty in predicting the future fate of anthropogenic CO2 and the resulting climate change. Here, I address this uncertainty by introducing a dependence of calcium carbonate (CaCO3) production on calcite saturation state (ΩCaCO3) in the University of Victoria Earth System Climate Model, an intermediate complexity coupled carbon-climate model. In a series of model simulations, I examine the changes in global ocean carbon cycling following both “business-as-usual” and “mitigation” CO2 emissions scenarios. By the year 3500, global CaCO3 production rates will have decreased by between 0.003 Pg C y-1 and 0.264 Pg C y-1 relative to the standard model configuration depending on the sensitivity of calcification rates to ΩCaCO3. This, in turn, would result in an atmospheric CO2 drawdown of 0.630 – 59.8 Pg C, a weakening of the vertical ocean alkalinity and DIC gradients, and a change to sediment and ocean carbon pools of 1.00 – 42.0 Pg C and 0.800 to 70.0 Pg C respectively. These results suggest that the response of pelagic calcifying organisms to anthropogenically-driven changes in ocean ΩCaCO3 can have an important influence on marine biological carbon cycling, leading to changes in carbon partitioning between the ocean and atmosphere on millennial timescales