Processes controlling forms of phosphorus in surficial sediments from the eastern Arabian Sea impinged by varying bottom water oxygenation conditions
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The surficial sediments from the upper continental slope of the eastern Arabian Sea, impinged by the oxygen minimum zone (OMZ, 150-1200 m water depth), show varying concentrations of the biogenic element phosphorus (P, 0.1-0.2%) in the northern and southern areas even though total organic carbon concentrations are relatively constant (TOC, 2-5%). To understand this discordance, 17 surface sediment samples from shelf, slope and deep sea of the eastern Arabian Sea were investigated using a five-step sequential extraction scheme to delineate the process responsible for P enrichment in OMZ. High fractions of organic phosphorus (P sub(org) 10-26%), biogenic phosphorus (P sub(bio) 36-48%), relatively low molar C sub(org)/P sub(org) ratios (322-447), and C sub(org)/P sub(reactive) ratios close to Redfield Ratio in OMZ sediments of the SE Arabian Sea suggest accumulation under high surface production and lo residence time of labile forms of P due to high sedimentation rates. Despite higher productivity in surface waters, low fractions of P sub(org) (8-13%; less than deep-sea sediments of the study area 12-13%), P sub(bio) (25-33%), relatively high molar C sub(org)/P sub(org) ratios (341-508), and C sub(org)/P sub(reactive) ratios less than Redfield Ratio in OMZ sediments from the NE Arabian Sea may indicate a higher degree of regeneration and diagenetic transformation of labile forms of P to other phases. Authigenic phosphorus (P sub(aut)) fraction varies by a factor of 2-8 in sediments from the OMZ when compared to well-oxygenated deep-sea sediments of the study area. While the P sub(total) remains constant, significant P transformation seems to occur in NE Arabian Sea, which is suggested by high P sub(aut) fraction (approx. 50%) compared to low P sub(aut) fraction (10-39%) in the SE Arabian Sea sediments. Supply rates of phosphorus, variable rates of P dissolution under varying dissolved oxygen contents in the bottom waters, and early diagenetic transformation of P within the sediments seem to influence P geochemistry in sediments overlain by the OMZ in the eastern Arabian Sea.
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