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about 2%« over the mean 8 C of the deep water.

Deep water masses today form at high latitudes, At their sites of formation, the main deep water masses, the North Atlantic deep water (NADW) and the Antarctic bottom water (AABW) are depleted in nutrients and enriched in hC relative to mean ocean water After leaving the surface layers of the ocean, <5UC of deep w ater masses changes only through two processes: remineralization of organic carbon and mixing between water masses with different ¿13C values. Because remineralization of organic carbon consumes oxygen at depth and releases COj depleted in 1 there is a close relationship between the oxygen dissolved in deep waters and their *C. As the oxidation of organic matter regenerates nutrients (phosphate, nitrate), there is also a close relationship between deep water nutrient content and their Kroopnick (1984) demonstrated that the distribution of S'^C in deep water delineates the general distribution of w ater masses and that modern gradients in the deep ocean follow the net flow of the deep waters from the Atlantic to the Pacific (Figure 14.4a).

Some species of benthic foraminifera (genus ttbiddes) closely record the modern &[}C distribution in the world ocean, providing a good proxy for the reconstruction of the past <SLiC gradients in the ocean (Duplessy et al., 1984; Duplessy and Shackleton, 1985). It is assumed that this was true also in the past. However, in addition to effects of deep water circulation, temporal changes in benthic foraminiferal 51JC occur because of global changes in the distribution of carbon between the ocean and various reservoirs such as continental biosphere or organic carbon in shelf sediments (Boyle and Keigwin, 1982; Broecker and Peng, 1982; Shackleton, 1977). Any individual record of benthic foraminiferal SnC is therefore a complicated combination of global effects, circulation effects, local productivity effects, and changes in mixing ratios between the principal deep water masses (Curry et al., 1988), m

«MODERN OCEAN

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