| dc.description.abstract | Arabian Sea contains one of the world's three large oxygen deficient zones (ODZ). Within the ODZ oxygen concentration is vanishingly small between about 200 and 800 m depth and in this depth interval denitrifications is the major mode or organic matter oxidation. This makes the Arabian Sea a globally important sink in the marine combined nitrogen cycle. Within the ODZ nitrate concentration profiles typically show a local minimum near 250 m and nitrate is depleted relative to phosphate. Stoichiometrically based estimates of the amount of nitrate removed from the water column, or the nitrate deficit, suggest maximum values of approx. 12 Mu M. However, the amount of excess nitrogen gas, which is presumably a result of denitrification,h as been estimated to be almost twice this value. Although the reason for this discrepancy between the nitrate deficit and the quantity of excess nitrogen gas remains unresolved, possible causes include non-Redfield stoichiometry for denitrification, contributions due to nitrogen fixation either in the Arabian Sea or in the ODZ source waters, anammox, or sedimentary denitrification. Nitrate deficit based estimates of overall denitrification rate are about 40 Tg N a sup(-1), but if the larger excess nitrogen gas measurements are correct they suggest the rate could be correspondingly higher. Nitrate is enriched in the heavy N isotope within the denitrification zone due to denitrification. During upwelling this enriched N is brought into the euphotic zone where it is take up by phytoplankton. When remains of the phytoplankton sink and become incorporated into the sediment the resulting sediments are also enriched in the heavy isotope relative to other areas of the ocean. However, during glacial periods the N in the sediment was not enriched suggesting better ventilation of the ODZ waters and a lack of denitrification. Nitrous oxide, an important green-house gas and an intermediate in both denitrification and nitrification typically displays local maxima at the upper and lower boundaries of the ODZ, while it is found in very low concentrations within the ODZ. Low values in the ODZ are the results denitrification and this is evidenced by enrichment in the heavy isotopes of both N and O. Maxima at the boundaries of the ODZ may result from either nitrification or denitrification and the N is depleted in the heavy isotopes, while the O is enriched | |
