Processes controlling forms of phosphorus in surficial sediments from the eastern Arabian Sea impinged by varying bottom water oxygenation conditions

Abstract

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.