SPREADING OF WATER MASSES AND REGENERATION OF SILICA AND RA-226 IN THE INDIAN-OCEAN

The magnitudes of silica and Ra-226 inputs to water (through particle regeneration, in situ, and from sediments) and the validity of observe d Si and Ra-226 as tracers of water masses and advective processes wer e examined in the Indian Ocean using the GEOSECS data. The regenerated quantities of these two parameters were calculated as the difference between the observed and the expected concentrations; the latter were estimated from a three end-member mixing model employing potential tem perature and salinity as conservative tracers. Here we present results on the quantitative spreading of the Antarctic Bottom Water (AABW); t he Modified North Atlantic Deep Water (MNADW, also known as the Circum polar Water) and the North Indian Deep Water (NIDW)-both these were re presented together as High Salinity Deep Waters (HSDW); the Antarctic Intermediate Water (AAIW); the North Indian Intermediate Water (NIIW) and the Central Indian Water (CIW). Our results concur with recent res ults in the literature. Briefly, the northward flow of the AABW is une ven; the MNADW core layer is found to be closer to the Antarctic that spreads to the north, and AAIW is largely restricted to the Indian Oce an south of 10 degrees S. Our results also reveal that: roughly 10% mo re AABW enters the Bay of Bengal than the Arabian Sea; there is greate r possibility for deep waters to enter the Central Indian Basin from t he Bay of Bengal; CIW occupies a larger part of the Bay of Bengal than of the Arabian Sea; and 10% of the NIIW reaches 30 degrees S in the w estern Indian Ocean.The regenerations of Si and Ra-226 are mainly from the underlying sediments rather than through the dissolution of parti cles in the water column. The sediments in the northern parts seem to supply Ra-226 and Si to the rest of the Indian Ocean. At 10 degrees S there is a subsurface (similar to 600 m) maximum in regenerated Si, wh ich is possibly connected to the advection of particles by Indonesian waters. The maxima in regenerated Ra-226 and Si contribute about 50% a nd 30%, respectively, to the observed abundances, suggesting that the observed Si is a more useful tracer of water masses and mixing process es than Ra-226. Linear relationships were found between regenerated Si and Ra-226, but departures noticed for Ra-226 in the eastern Indian O cean may be attributed to its release from particles transported by th e Indian rivers. Diverse regimes with respect to the extent of sources and dissolution of opal were noted in the Indian Ocean: high diatom a bundance but low Si regeneration in the Antarctic, high diatom abundan ce and high regenerated Si in the Arabian Sea, and low diatom abundanc e but high regenerated Si in the Bay of Bengal.