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The Trace Element Composition of Size Fractionated Suspended Particulate Matter Samples from the Qatari EEZ of the Arabian Gulf: The Role of Atmospheric Dust

Abstract. We analyzed net-tow samples of natural assemblages of plankton, and associated particulate matter, from the Exclusive Economic Zone (EEZ) of Qatar in the Arabian Gulf. Size fractioned suspended particles were collected using net-tows with mesh sizes of 50 μm (phytoplankton) and 200 μm (zooplankton) to examine the composition of plankton populations. Samples were collected in two different years (October 2012; April and October 2014) from 11 sites to examine temporal and spatial variabilities. We calculated the excess metal concentrations by correcting the bulk composition for inputs from atmospheric dust using aluminum (Al) as a lithogenic tracer and the metal / Al ratios for average Qatari dust. Atmospheric dust in Qatar is depleted in Al and enriched in calcium (Ca) relative to global average Upper Continental Crust (UCC) due to the geology of the outcropping sedimentary rocks and topsoil deposits in the source areas of the dust. To evaluate the fate of this carbonate fraction when dust particles enter seawater is uncertain, we leached a sub-set of dust samples using an acetic acid-hydroxylamine hydrochloride (HAc-HyHCl) procedure that should solubilize CaCO3 minerals and associated elements. We found that Ca was removed and that the metal/aluminum (Me / Al) ratios for most elements increased after leaching because the change in sample mass resulting from the leach was more important than the loss of metals solubilized by the leach. Because surface seawater is supersaturated with respect to CaCO3 and acid soluble Ca is abundant in the particulate matter, we only used unleached dust for the lithogenic correction. The concentrations of some elements in net-tow plankton samples appear to be mostly of lithogenic (dust) origin. These include Al, Fe, Cr, Co, Mn, Ni, Pb and Li. Several elements are mostly biogenic/anthropogenic origin. These include as Cd, Cu, Mo, Zn and Ca. The excess concentrations, relative to average dust, for most elements (except Cd) decreased with distance from shore, which may be due to differences in biology, currents, proximity to the coast or interannual processes.
