The activities of 228Ra and 226Ra in the seawaters of the eastern Indian Ocean and its adjacent seas were measured during the 1996/1997 Piscis Austrinus Expedition using R/V Hakuho-Maru. The surface Ra data are in good agreement with those in literature and indicate that the continental shelves of the southeast Asian Seas are strong sources for 228Ra and, to much lesser extent, for 226Ra which are largely transported by the surface currents into the Indian Ocean. The vertical profiles of 228Ra obtained at six stations from the different oceanic regimes along the cruise track generally showed an almost exponential decrease from the surface to the middepths of 1200--1500 m. Coupling the vertical profiles of 228Ra and nitrate and using the vertical one-dimensional model developed by Ku et al. [1995], we estimated 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 nitrate 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¿ and 40¿S, north of the polar front, dissolved Si is depleted relative to nitrate in the upper water column as compared to their ~1:1 molar ratio demanded by diatoms. In such high-nitrate but low-silicate regions, 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 glacial period, supply of atmospheric dust containing silicon and iron to the surface ocean was much more intense than that during the interglacial period, and hence, diatom production might have been enhanced in the Si-depleted regions. Diatoms would utilize nitrate readily and inhibit growth of coccoliths, resulting in increases in alkalinity and pH in the surface water. This species change of phytoplankton may account for the lowered atmospheric CO2 level during the glacial period without causing a large-scale depletion of dissolved oxygen in the deep sea. The records of opal and carbonate contents in the Antarctic sediment cores studied by Charles et al. [1991] seem to be consistent with this silicon-induced alkalinity pump hypothesis. ¿ 2001 American Geophysical Union