The stable carbon isotope ratio (δ
13C) 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 (δ
13C = -20‰) and terrestrial (δ
13C = -26‰) 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 CO
2 in the surface ocean for photosynthesis during glacial times due to (a) a reduction in the atmospheric CO
2 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 CO
2 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 CO
2, 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).
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