Sedimentary oxygen consumption and microdistribution at sites across the Arabian Sea oxygen minimum zone (Pakistan margin)

Biogeochemical processes and fluxes occurring across the sediment–water interface on continental margins impacted by oxygen minimum zones (OMZs) are important to bioelement cycles, ocean inventories, and productivity. The nature and magnitude of these processes depend heavily on spatial and temporal variability in dissolved O2 concentrations in bottom waters and porewaters.In 2003, four research cruises to the Indus margin of the Arabian Sea (Pakistan) were undertaken to survey the benthic biogeochemical processes in the resident OMZ before and after the southwest monsoon. Sediment O2 microdistribution and consumption rates were measured at five stations along a depth transect (140–1850 m) across the OMZ, during the spring intermonsoon and the late-to-post southwest monsoon periods, using in situ benthic research platforms (landers). Lander O2 electrode data show that the intermonsoon and late-to-postmonsoon bottom-water O2 levels had little to no variation (300 m, no change; 940 m, 1.7–2.8 μM; 1200 m, 10.2–12.6 μM; and 1850 m, 82–80 μM). In contrast, at the shallowest station (140 m), a large fluctuation occurred between the intermonsoon (O2=44.5 μM) and the late-to-postmonsoon (O2=1 μM), due to monsoon-forced shoaling of the upper OMZ boundary. Oxygen did not penetrate into the sediments at the 300-m site during either sampling season. During the intermonsoon season at the 140-m site, O2 penetrated to a depth of ∼3 mm, but no measurable O2 penetration occurred after the monsoon. At the 940 and 1200-m sites, O2 penetration into the sediments was small (ca. 1–2 mm at both sites) and did not measurably change between the two sampling seasons. In contrast, at the 1850-m site, O2 penetration decreased after the monsoon (18–12 mm). Calculated late-to-postmonsoon O2 consumption rates were generally similar to or lower than intermonsoon values (0 vs. 2.22 mmol m−2 d−1 at 140 m, 0.37 vs. 0.31 mmol m−2 d−1 at 1200 m, and 0.73 vs. 1.01 mmol m−2 d−1 at 1850 m). The relatively small seasonal signal suggests that organic matter delivered during the monsoon period may have already been largely remineralized by the late-to-postmonsoon sampling period. Modelling of porewater O2 profiles indicates that subsurface O2 consumption associated oxidation of reduced inorganic species makes a significant contribution to total O2 consumption at some sites. Similarly, differences in O2 consumption rates determined by porewater profile modelling and whole-core incubations at some sites indicate significant contributions associated with bioturbation and bioirrigation.