Radium 228 based nitrate fluxes in the eastern Indian Ocean and the South China Sea and a silicon-induced "alkalinity pump" hypothesis

The activities of Ra-228 and Ra-226 in the seawaters of the eastern Indian Ocean and its adjacent seas were measured during the 1996/1997 "Piscis Aust rinus" Expedition using R/V Hakuho-Maru. The surface Ra data are in good ag reement with those in literature and indicate that the continental shelves of the southeast Asian Seas are strong sources for Ra-228 and, to much less er extent, for Ra-226 which are largely transported by the surface currents into the Indian Ocean. The vertical profiles of Ra-228 obtained at six sta tions from the different oceanic regimes along the cruise track generally s howed an almost exponential decrease from the surface to the middepths of 1 200-1500 in. Coupling the vertical profiles of Ra-228 and nitrate and using the vertical one-dimensional model developed by Ku et al. [1995], we estim ated the nitrate fluxes into the euphotic zone ranging from 1.5 mmol N m(-2 ) d(-1) (or 0.55 Mol m(-2) yr(-1)) in Andaman Sea to 4.0 mmol N m(-2) d(-1) (1.5 Mol m(-2) yr(-1)) in the Southern Ocean. In steady state, these nitra te fluxes correspond to the new production of 11-26 mmol C m(-2) d(-1) (or 3.9-9.6 Mol C m(-2) yr-1). In the sub-Antarctic region between 30 degrees a nd 40 degreesS, north of the polar front, dissolved Si is depleted relative to nitrate in the upper water column as compared to their similar to1:1 mo lar ratio demanded by diatoms. In such high-nitrate but low-silicate region s, growth of diatoms is likely to be limited by available silicate. It also favors calcareous coccoliths to grow by utilization of nitrate, as deduced from a decrease in alkalinity in the surface waters. During the last glaci al period, supply of atmospheric dust containing silicon and iron to the su rface ocean was much more intense than that during the interglacial period, and hence, diatom production might have been enhanced in the Si-depleted r egions. Diatoms would utilize nitrate readily and inhibit growth of coccoli ths, resulting in increases in alkalinity and pH in the surface water. This species change of phytoplankton may account for the lowered atmospheric CO 2 level during the glacial period without causing a large-scale depletion o f dissolved oxygen in the deep sea. The records of opal and carbonate conte nts in the Antarctic sediment cores studied by Charles et al. [1991] seem t o be consistent with this silicon-induced alkalinity pump hypothesis.