Seasonal variations in the distribution of Fe and Al in the surface waters of the Arabian Sea

Abstract

Concentrations of dissolved Al and Fe in the surface mixed layer were measured during five cruises of the 1995 US JGOFS Arabian Sea Process Study, Concentrations of both Al and Fe were relatively uniform between January and April, the NE Monsoon and the Spring Intermonsoon period, ranging from 2 to 11 nM Al (mean 5.3 nM) and 0.5 to 2.4 nM Fe (mean 1.0 nM). In July/August, after the onset of the SW Monsoon, surface water Al and Fe concentrations increased significantly (Al range 4.5–20.1 nM; mean=10 nM, Fe range 0.57–2.4 nM; mean=1.3 nM), particularly in the NE part of the Arabian Sea, as the result of the input and partial dissolution of eolian dust. Using the enrichment of Al in the surface waters, we estimate this is the equivalent to the deposition of 2.2–7.4 g m −2 dust, which is comparable to values previously estimated for this region. Approximately one month later (August/September), surface water concentrations of both Al and Fe were found to have decreased significantly (mean Al 7.4 nM, mean Fe 0.90 nM) particularly in the same NE region, as the result of export of particulate material from the euphotic zone. Fe supply to the surface waters is also affected by upwelling of sub-surface waters in the coastal region of the Arabian Sea during the SW Monsoon. Despite the proximity of high concentrations of Fe in the shallow sub-oxic layer, freshly upwelled water is not drawn from this layer and the NO 3/Fe ratio in the initially upwelled water is below the value at which Fe limitation is through to occur. Continued deposition of eolian Fe into the upwelled water as it advects offshore provides the Fe required to raise this ratio above the Fe limitation value.