δ13C of marine organic matter and ocean pH

The stable carbon isotope ratio (delta(13)C) organic matter from the deep sea sediments shows an increase of 1 to 2 parts per thousand during glacial periods relative to inter glacial periods (Muller et al., 1983; Fontugne and Duplessy, 1986; Sackett, 1986; Sarkar et al., 1993). This has been variously explained as due to (I)change in the relative mixing proportions of the marine (delta(13)C = -20 parts per thousand) and terrestrial (delta(13)C = -26 parts per thousand) organic matter; (2) reduction in the pCO(2) of the surface oceans accompanying that in the atmosphere (Rau et al., 1991) and (3) change in the oxic/anoxic conditions in the deep sea environment induced by changes in the surface ocean productivity. While these interpretations may have some merit, we suggest an alternative possibility, viz., a reduced availability of dissolved CO2 in the surface ocean for photosynthesis during glacial times due to (a) a reduction in the atmospheric CO2 concentration (Barnola et al., 1987) and (b) enhanced rates of photosynthesis due to a more vigorous atmospheric circulation in some regions (e.g., Pacific, Pedersen, 1983) or (c) reduced rates of air-sea exchange of CO2 due to the failure of monsoons in the northern Indian Ocean (Duplessy, 1982; Prell, 1984; Sarkar er al., 1990; Krishnamurthy, 1990). These would result in an increased use of dissolved bicarbonate (Hayes, 1993)-enriched in C-13 by 9 parts per thousand relative to dissolved CO2, thereby enriching the glacial organic matter in C-13 and causing an increase in oceanic pH. Using data reported for the northern Indian Ocean, we calculate such pH changes to be in the range of 0.01 to 0.13, consistent with recent estimates based on boron isotope analysis (Sanyal er al., 1995).