Chemical oceanography

Abstract

Extremes in atmospheric forcing, asymmetrically distributed over the northern Indian Ocean, bring about exceptionally large hydrographical changes and produce a wide variety of ecosystems or biogeochemical provinces. While these conditions generally have little effect on the cycles of major elements, except carbon and calcium, they greatly modify transformations of minor constituents. A hydrochemical discontinuity at approx. 10 degrees S latitude separates the low oxygen - high nutrients waters of the northern Indian Ocean from the southern waters having opposite characteristics. In the northwestern sector (the Arabian Sea), which experiences intense upwelling along both its eastern and western boundaries during the southwest monsoon, and open-ocean convective mixing down to 100-120 m during the northeast monsoon, the widespread enrichment of surface waters with nutrients sustains very high levels of primary production (PP). In comparison PP in the northeastern sector (the Bay of Bengal) is lower due to strong thermohaline stratification resulting from enormous river runoff. Both regions experience severe depletion of oxygen at mesopelagic depths. However, while the lowest oxygen concentrations in the Bay of Bengal are just above the threshold value (less than 0.02 ml/l) required for the onset of vigorous denitrification (microbial reduction of nitrate to molecular nitrogen), this process occurs within a large body of intermediate waters in the Arabian Sea. Denitrification brings about a loss of bio-utilizable combined nitrogen and leaves a large impact on stable nitrogen isotope abundance (accumulation of nitrogen-15 in nitrate) because of which past changes in the intensity of this process can be reconstructed from the sedimentary record. The unusual biogeochemical conditions affect the cycling of climatically important gases viz. carbon dioxide (CO sub(2)), nitrous oxide (N sub(2)O) and methane. Surface waters are normally supersaturated with CO sub(2) in the Arabian Sea and undersaturated in some parts of the Bay of Bengal with the region serving as a modest net source of CO sub(2) for the atmosphere. The Arabian Sea is a globally significant source of atmospheric N sub(2)O because of the sensitivity of N sub(2)O cycling to oxygen concentration in the low range. Geochemistry of trace metals such as iron, manganese and cerium that undergo valency changes in oxygen-depleted environments is greatly impacted in the suboxic Arabian Sea, but elsewhere in the region the distributions are similar to those encountered elsewhere in the ocean. The nutrient-like metals (e.g. Cd) show expected distribution pattern (i.e. their deep water concentrations are intermediate of those in the Atlantic and Pacific oceans).