Sedimentary denitrification rates across the Arabian Sea oxygen minimum zone

Abstract

Abstract Sedimentary denitrification was measured at sites across the Pakistan Margin oxygen minimum zone (OMZ) during cruises before and after the 2003 Southwest (SW) Monsoon. Denitrification rates were calculated using nitrate fluxes and N 2 production rates in shipboard whole-core incubations and benthic lander deployments. We observed generally good agreement between the sedimentary denitrification rates obtained from nitrate consumption and N 2 production. Results of these investigations indicate that sedimentary denitrification is a persistent process within the Arabian Sea OMZ and the regions proximal to the permanent OMZ (e.g., 140 and 1200 m). Sedimentary denitrification in the region intensified during the SW Monsoon, as noted by the increase in associated rates between the inter-monsoon (April–May 2003) and post-monsoon (September–October 2003) assessments. Sedimentary denitrification was observed at all stations during the inter-monsoon period; nitrate consumption rates ranged from 0.40 to 3.17 mmol N m −2 d −1 . During the post-monsoon assessment, apparent sedimentary denitrification rates increased in the shallow sites (140 and 300 m), likely in response to a shoaling of the OMZ, but were not definitively observed at the deepest site (1850 m); where observed, post-monsoon sedimentary denitrification rates increased to 0.66–3.78 mmol N m −2 d −1 . These rates compare favorably to sedimentary denitrification rates obtained in other shelf environments and are higher than abyssal sedimentary denitrification rates. By extrapolating these rates to the entire Pakistan Margin OMZ, we estimate a net annual sedimentary denitrification flux of 1.1–10.5 Tg N yr −1 , moderately higher than current estimates of sedimentary denitrification in this region and in line with estimates of regional sedimentary denitrification in other sites.