DENITRIFICATION IN THE EASTERN ARABIAN-SEA - EVALUATION OF THE ROLE OF CONTINENTAL MARGINS USING RA ISOTOPES

The Ra-228/Ra-226 activity ratios (henceforth denoted as [228/226] hav e been measured both at the surface and at 300 m depth as a function o f distance from the Western Continental margins of India to the open A rabian Sea (between 15 degrees N and 21 degrees N). A two-dimensional eddy diffusion model is used to fit the measured [228/226] profiles to deduce horizontal eddy diffusivities in both zonal (towards open ocea n) and meridional (towards the equator) directions. These range betwee n 1.3x10(6) and 3.1x10(6) cm(2) s(-1). Assuming that the organic-rich and deeply anoxic continental margin sediments at 300 m depth can be a source of organic C to the core of the denitrification layer and that the deficit flux in the carbon budget can be covered by this supply, the required gradient dC/ dZ in DOC content of waters from coast to op en Arabian Sea is computed using the K-x value. The required gradient is computed to be 4 mu mol dm(-3) 100 km(-1), which is within the erro r associated with DOC measurements.