Distribution and cycling of dissolved aluminium in the Arabian Sea and the Western equatorial Indian ocean

Abstract

Dissolved aluminium (dAl) distribution has been studied over the full vertical water column profiles along the GEOTRACES-India (GI) transect, GI-05, in the Arabian Sea (AS) and the Western Equatorial Indian Ocean (W-Eq.IO) during the fall inter-monsoon period. Surface dAl distribution in the AS demonstrates an east-west gradient, i.e. elevated dAl (6.5-21.6 nM) close to the Indian coastal region and low dAl (1.5-3.6 nM) along the western boundary of the AS. Rapid surface dAl removal due to relatively high biological productivity and a decrease in atmospheric dust deposition during the fall inter-monsoon result in low surface dAl levels in the western AS. Mass balance for surface dAl variation in the AS reveals very short scavenging removal timescales (0.01-0.47 yr) between the mid-summer and fall inter-monsoon period. Further, dAl input/dilution due to surface water advection is found to play an important role in controlling the surface dAl variation, varying between 190 and 300% of the dust-supported dAl input in different regions of the AS. These results have important implications for the use of surface dAl as a proxy of dust deposition in the AS. In the W-Eq.IO, a relative increase observed in the surface dAl concentrations, compared to the western AS and the central equatorial region, suggests a local dAl input, presumably, due to the dust influx from the Somali coast. Probable mechanism for this could be dust input from Somalia to the coastal western equatorial region and subsequent advection of the dAl enriched coastal waters to the offshore sampling sites, facilitated by mesoscale eddies. The intrusions of the high salinity water masses (the Arabian Sea High Salinity Water and the Persian Gulf Water) in the thermocline depths (~75-300 m) are observed to carry the dAl-rich signal of their formation regions to the open AS. This dAl enrichment in thermocline waters is, however, mostly restricted to the northern and north-western AS during the study period. Correlated dissolved Al and Fe maxima observed in the deep water over the Murray Ridge in the northern AS suggest Al and Fe release from reactive clay minerals (e.g., illite), found abundantly in the sediments deposited over the ridge. Further, levated dAl levels were also seen near the Laxmi-Panikkar-Palitana ridge system (~10.0 nM) and the Carlsberg Ridge (~4.5 nM) in the AS and the W-Eq.IO, respectively.