δ¹³C of marine organic matter and ocean pH

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The stable carbon isotope ratio (δ¹³C) of organic matter from the deep sea sediments shows an increase of 1 to 2‰ 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 (1) change in the relative mixing proportions of the marine (δ¹³C = -20‰) and terrestrial (δ¹³C = -26‰) organic matter; (2) reduction in the pCO₂ 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 et al., 1990; Krishnamurthy, 1990). These would result in an increased use of dissolved bicarbonate (Hayes, 1993)-enriched in ¹³C by 9‰ relative to dissolved CO₂, thereby enriching the glacial organic matter in ¹³C 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 et al., 1995).