Planktonic foraminiferal shells buried in deep ocean sediments record lower delta(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 parts per thousand and 0.9 parts per thousand is observed, whereas the mean change in low and middle latitudes is only +/-0.15 parts per thousand. However, a stronger biological carbon pump sufficient to explain the 80 ppmv lower atmospheric pCO(2) values during glacial times would raise the surface ocean delta(13)C values of dissolved inorganic carbon by about 1.0 parts per thousand. Here the results of a three-dimensional ocean circulation model study are presented which demonstrate that the increase of delta(13)C values in the sea surface due to a strengthening of the biological carbon pump is counteracted by processes which drive the delta(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 [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 delta(13)C values in surface water dissolved inorganic carbon by about 0.3 parts per thousand. The additional effects of the glacially 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 whole 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.
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