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 manganes e were investigated in the Arabian Sea (Northwestern Indian Ocean) during t he 1995 Spring Intermonsoon period (March-April; US JGOFS Process Cruise 2; TN045). The region is characterized by an intense, basin-wide oxygen minim um zone (OMZ) that strongly influences the manganese distribution. In the O MZ? two distinct dissolved Mn (d-Mn) maxima were observed, at depths of 200 -300 m and 600 m, respectively. The latter peak displayed concentration max ima of approximately 6 nanomolar and was largely confined to stations north of 19 degrees N latitude (Stations N2-N7). This mid-depth maximum was asso ciated with the low oxygen core of the OMZ ([O-2] < similar to 2 mu M), and appears to be maintained by a southward horizontal advective-diffusive flu x of dissolved manganese from highly reducing Pakistan margin sediments, ra ther than input from the Oman Margin as previously suggested by Saager et a l. (1989, Geochimica ct Cosmochimica Acta, 53, 2259-2267). This signal was largely absent at stations along the southern transect, likely due to oxida tive scavenging of d-Mn to the suspended particulate phase. Mid-depth parti culate Mn maxima at some southern stations (Stations S4-S11) appear to be r emnants 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 similar to 3 to as high as 8 nm at most stations east of about 62 degrees E longit ude. The signal was everywhere correlated with the secondary nitrite maximu m, at stations within the main denitrification zone delineated by Naqvi (19 91). Nepheloid layers, presumably bacterial, also were associated with this depth interval. Particulate Mn profiles displayed corresponding concentrat ion minima and low Mn/Al and reactive/refractory Mn ratios for this same de pth interval; suggesting reductivedissolution of Mn-oxyhydroxides. These ob servations imply that in situ microbially mediated processes may be the pre dominant source of d-Mn in the upper OMZ, while horizontal advection is mor e important deeper in the water column. (C) 2000 Elsevier Science Ltd. All rights reserved.