Denitrification in the Eastern Arabian Sea: evaluation of the role of continental margins using Ra isotopes

Abstract

The ²²⁸Ra/²²⁶Ra activity ratios (henceforth denoted as [228/226] have been measured both at the surface and at 300 m depth as a function of distance from the Western Continental margins of India to the open Arabian Sea (between 15°N and 21°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 ocean) and meridional (towards the equator) directions. These range between 1.3×10⁶ and 3.1×10⁶ cm² 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 open Arabian Sea is computed using the K_x value. The required gradient is computed to be 4μmol dm⁻³ 100 km⁻¹, which is within the error associated with DOC measurements.