Dissolved Al distribution, particulate Al fluxes and coupling to atmospheric Al and dust deposition in the Arabian Sea

Abstract

Abstract Concentrations of dissolved aluminium (Al) were measured in the Arabian Sea during three different seasons. Atmospheric Al concentrations and particulate Al fluxes at 100 m depth were measured during the northeast monsoon. Particulate Al fluxes in the deep Arabian Sea were determined from a 16 months sediment trap mooring. Surface water Al concentrations were higher in the more oligotrophic southern Arabian Sea (10–15 nM) than in the north-western Arabian Sea (3–9 nM). Seasonal variability was observed in the central and northern Arabian Sea with elevated concentrations during spring/summer. Mixed-layer residence times of 0.4–4 years for dissolved Al estimated with respect to both atmospheric input and particle flux exports indicate rapid removal from surface layers. Scavenging removal exerts major control on Al cycling. There are indications of a sediment source for dissolved Al. Atmospheric Al concentration during the northeast monsoon were 10–30 nmol m−3 and up to 81 nmol m−3 during dust pulses. The Al content in aerosol particles was variable with an average of 950 μmol g−1 (or 2.6% by weight), which is about threefold lower than in average continental crust (8.04%). Using available aerosol Al concentrations combined with satellite data, the annual atmospheric deposition rate of Al is estimated to ∼16000 μmol m−2 yr−1. Depending on the Al content in aerosol material (2.6–8.04%), this converts to a dust deposition rate of 5.4–16.7 g m−2 yr−1 that agrees with sediment trap results and sediment accumulation data. A simple model based on dissolved Al concentrations in surface waters is used to estimate dust deposition rates. We demonstrate that a relatively short residence time of ⩽3 yr and an aerosol Al content of only 2.6% provide improved constraints under steady-state conditions. The strong seasonality in particulate Al flux in the deep central Arabian Sea was dominated by the southwest monsoon sedimentation signal; about 46% of the annual Al flux is provided within only ca. 2 1 2 months. The estimated total atmospheric Al input and the observed deep-water flux of particulate Al of about 14000 μmol m−2 yr−1 agree closely. This suggests that the atmosphere is the major Al source to the Arabian Sea.