A CRITICAL-EVALUATION OF DEPOSITIONAL PARAMETERS CONTROLLING THE VARIABILITY OF ORGANIC-CARBON IN ARABIAN SEA SEDIMENTS

Organic carbon distribution in the surficial sediments of the Arabian Sea to some extent mirrors the surface productivity, being both high in the peripheral portions and low in the central region, suggesting that the productivity is a primary source of organic matter. However, a critical examination reveals that the organic enrichment on the slope of the Indian margin is two to four fold higher (max. 16.71%) than on the slope of the Arabian Peninsula (max. 7.54%) while the productivity in the former region is three to four times lower than the latter. This observation suggests that the influence of productivity on organic enrichment is of secondary importance. High concentrations (>4%) of organic carbon exactly coincide with the oxygen minimum zone (150-1500 m water depth) evidently reflecting the crucial role played by the anoxic bottom waters in the preservation of organic carbon. Incidently, the organic-rich band along the western Indian slope is wide, long and highly concentrated as compared to that of the slope of the Arabian Peninsula and no organic enrichment is found on other continental slopes of the Arabian Sea although an equally intense oxygen minima impinge on the floor of the slope. Such a variability of organic carbon on the slopes is attributed to various geological parameters such as texture of sediments, rates of fine terrigenous sediment deposition, shelf width, slope gradient, bottom currents and adsorption capacity of individual clay minerals. All these observations suggest that the productivity is not an ultimate control for the organic enrichment of bottom sediments but it is the bottom water anoxia in conjunction with various depositional parameters which determine the "degree of preservation".