Processes controlling the distribution and cycling of manganese in the oxygen minimum zone of the Arabian Sea

Vertical and horizontal distributions of dissolved and particulate manganese were investigated in the Arabian Sea (Northwestern Indian Ocean) during the 1995 Spring Intermonsoon period (March–April; US JGOFS Process Cruise 2; TN045). The region is characterized by an intense, basin-wide oxygen minimum zone (OMZ) that strongly influences the manganese distribution. In the OMZ, two distinct dissolved Mn (d-Mn) maxima were observed, at depths of 200–300 m and 600 m, respectively. The latter peak displayed concentration maxima of approximately 6 nanomolar and was largely confined to stations north of 19°N latitude (Stations N2–N7). This mid-depth maximum was associated with the low oxygen core of the OMZ ([O2]<∼2 μM), and appears to be maintained by a southward horizontal advective–diffusive flux of dissolved manganese from highly reducing Pakistan margin sediments, rather than input from the Oman Margin as previously suggested by Saager et al. (1989, Geochimica et Cosmochimica Acta, 53, 2259–2267). This signal was largely absent at stations along the southern transect, likely due to oxidative scavenging of d-Mn to the suspended particulate phase. Mid-depth particulate Mn maxima at some southern stations (Stations S4–S11) appear to be remnants of this feature. The upper d-Mn maximum (200–300 m depth) was more widely distributed than the 600 m peak, with d-Mn concentrations of ∼3 to as high as 8 nm at most stations east of about 62°E longitude. The signal was everywhere correlated with the secondary nitrite maximum, at stations within the main denitrification zone delineated by Naqvi (1991). Nepheloid layers, presumably bacterial, also were associated with this depth interval. Particulate Mn profiles displayed corresponding concentration minima and low Mn/Al and reactive/refractory Mn ratios for this same depth interval, suggesting reductive dissolution of Mn-oxyhydroxides. These observations imply that in situ microbially mediated processes may be the predominant source of d-Mn in the upper OMZ, while horizontal advection is more important deeper in the water column.