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

Planktonic foraminiferal shells buried in deep ocean sediments record lower delta(13)C values of surface water dissolved inorganic carbon during glaci al times than during Holocene, In the Southern Ocean and at high northern l atitudes, a drop of between 0.3 parts per thousand and 0.9 parts per thousa nd is observed, whereas the mean change in low and middle latitudes is only +/-0.15 parts per thousand. However, a stronger biological carbon pump suf ficient to explain the 80 ppmv lower atmospheric pCO(2) values during glaci al times would raise the surface ocean delta(13)C values of dissolved inorg anic carbon by about 1.0 parts per thousand. Here the results of a three-di mensional ocean circulation model study are presented which demonstrate tha t the increase of delta(13)C values in the sea surface due to a strengtheni ng of the biological carbon pump is counteracted by processes which drive t he delta(13)C values in the opposite direction, This was found by performin g 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 biolo gical carbon isotope fractionation between Glacial and Holocene is responsi ble for a lowering of delta(13)C values in surface water dissolved inorgani c carbon by about 0.3 parts per thousand. The additional effects of the gla cially elevated CO32- concentration (0.25-0.50 parts per thousand) combined with the 0.35 parts per thousand lowering of delta(13)C values for the who le ocean due to a transfer of terrestrial organic carbon from the biosphere to the ocean-atmosphere reservoir also contribute to a further delta(13)C drop of 0.6-0.85 parts per thousand. Hence a small glacial decrease of the planktonic foraminifera delta(13)C of the order of 0.25 parts per thousand instead of an increase is predicted.