Influence of a [CO 2 (aq)] dependent biological C‐isotope fractionation on glacial 13 C/ 12 C ratios in the ocean

Planktonic foraminiferal shells buried in deep ocean sediments record lower δ 13 C 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 p CO 2 values during glacial times would raise the surface ocean δ 13 C 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 δ 13 C values in the sea surface due to a strengthening of the biological carbon pump is counteracted by processes which drive the δ 13 C 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 [CO 2 (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 δ 13 C values in surface water dissolved inorganic carbon by about 0.3‰. The additional effects of the glacially elevated CO 3 2− concentration (0.25–0.50‰) combined with the 0.35‰ lowering of δ 13 C 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 δ 13 C drop of 0.6–0.85‰. Hence a small glacial decrease of the planktonic foraminifera δ 13 C of the order of 0.25‰ instead of an increase is predicted.