Porewater nutrient concentrations and benthic nutrient fluxes across the Pakistan margin OMZ

Porewater concentrations and benthic fluxes of phosphate, silicate, ammonia, nitrate and nitrite were measured at five sites spanning the Pakistan margin oxygen minimum zone (OMZ), in order to characterise the biogeochemical processes occurring, and to assess whether oxygen concentration and a seasonal pulse of organic matter are controlling factors. Typical concentrations of 1–70 μM, 50–250 μM, 0–270 μM, <5 μM and 0–4 μM for PO43−, H4SiO4, NH4+, NO3− and NO2−, respectively were obtained. Evidence was found for the occurrence of intense denitrification, sorption of PO43− onto iron and manganese oxyhydroxides and possibly fluoroapatite precipitation at depth (>30 cm) in the sediment. These processes are all redox-sensitive, and their intensities varied across the margin, suggesting that oxygen concentration exerts a strong influence over nutrient concentrations and cycling. Variation in nutrient concentrations and fluxes before and after the summer monsoon was limited to an oxygen-driven change to the PO43− profile at one site, indicating that either nutrient profiles do not generally alter on seasonal timescales, or that any impact of the monsoon had subsided before the post-monsoon sampling period. Porewater profile modelling tended to underestimate the magnitude of fluxes, but was in general agreement with the directions of measured fluxes, and in situ and shipboard flux measurements also generally agreed. Phosphate and H4SiO4 concentrations and benthic fluxes on the Pakistan margin were similar to those reported at abyssal sites from around the world, while NH4+ and NO3− concentrations and fluxes were comparable to shallower, more productive and/or hypoxic marine settings.