Porewater and solid phase speciation geochemistry of phosphorus in the Western Bay of Bengal: An assessment of depositional pathways

Abstract

In this study, we used a combination of porewater phosphate and solid-phase P speciation to examine the present-day benthic biogeochemical cycling and pathways of phosphorus (P) deposition at four sites in the western Bay of Bengal (BoB). We concentrate on two regions, with 2 sediment cores from each region. One is the Palar basin where ancient phosphorites and phosphatic grains were discovered before, and the other in the Cauvery basin, where no phosphories are known to occur. Both of these regions fall in the depths of the oxygen minimum zone (OMZ). Our results suggest that the influx of lithogenic material, or lack of it, largely regulates the sedimentary cycling of P in BoB. Both locations lack active phosphogenesis due to substantial inputs of non-reactive P relative to reactive P from river runoff and its quick burial by the ballasting effect, which appears to prevent P from transforming into an authigenic phase. Despite the oxygen-depleted conditions, low bacterial respiration limits the microbial reduction of iron oxyhydroxide and makes the redox cycling of iron a weak player in the sedimentary cycling of P in the studied cores. The burial efficiency of P is relatively high in the Cauvery basin cores owing to the high sediment accumulation rate and suggests that most of the sedimentary P is undergoing burial with a minimum regeneration. Lack of/Very low recent sedimentation in the Palar basin is possibly due to the reported migration of river course towards the south during the mid-to Late Holocene.