Influence of a [CO2(aq)] dependent biological C‐isotope fractionation on glacial 13C/12C ratios in the ocean

Abstract

Planktonic foraminiferal shells buried in deep ocean sediments record lower δ13C values of surface water dissolved inorganic carbon during glacial times than during Holocene. In the Southern Ocean and at high northern latitudes, a drop of between 0.3‰ and 0.9‰ is observed, whereas the mean change in low and middle latitudes is only ±0.15‰. However, a stronger biological carbon pump sufficient to explain the 80 ppmv lower atmospheric pCO2 values during glacial times would raise the surface ocean δ13C values of dissolved inorganic carbon by about 1.0‰. Here the results of a three‐dimensional ocean circulation model study are presented which demonstrate that the increase of δ13C values in the sea surface due to a strengthening of the biological carbon pump is counteracted by processes which drive the δ13C values in the opposite direction. This was found by performing simulations employing the three‐dimensional Hamburg Model of the Oceanic Carbon Cycle (HAMOCC) combined with a [CO2(aq)] dependent parameterization of the biological carbon isotope fractionation. The difference in the biological carbon isotope fractionation between Glacial and Holocene is responsible for a lowering of δ13C values in surface water dissolved inorganic carbon by about 0.3‰. The additional effects of the glacially elevated CO32− concentration (0.25–0.50‰) combined with the 0.35‰ lowering of δ13C values for the whole ocean due to a transfer of terrestrial organic carbon from the biosphere to the ocean‐atmosphere reservoir also contribute to a further δ13C drop of 0.6–0.85‰. Hence a small glacial decrease of the planktonic foraminifera δ13C of the order of 0.25‰ instead of an increase is predicted.