Analysis of a major Aeolian dust input event and its impact on element fluxes and inventories at the DYFAMED site (Northwestern Mediterranean)

Description

Continental dust can be suspended and transported by the wind, reaching seawater masses far away from its source. The deposition of the aerosols on the ocean surface can alter the abundance of chemical species in the water column and contribute to element inventories in seafloor sediments. A major Saharan dust intrusion into the Western Mediterranean Sea was recorded at the DYFAMED site (Ligurian Sea) in 20th February 2004. We determine the influence of this dust event on the concentration of 30 major and trace level elements (TE) in sinking particles collected by sediment traps deployed at 200 m and 1000 m depth, and how a dust flux event like this contribute to the exchange of TE, including Fe, with the water column during major dust events. With coupled sediment traps and aerosol samples, we assessed the short-term implications of dust events in the water column. The event produced a flux of fast (> 111 m d-1) and slow (< 20 m d-1) sinking dust particles, detected during 3 weeks at 200 m and 4 weeks at 1000 m depth. This study demonstrates that a single dust deposition event can produce a sinking flux equivalent to annual deposition rates of elements relevant to biogeochemical cycles and/or pollution studies: (>60% for Cr and Cu, >70% for Al, >80% for Ni and Zn, >90% for V and Mn, >100% for Fe and Pb). The corresponding Enrichment Factors (EF) in the sediment traps during the atmospheric dust as reference were calculated, which in the TE analysed in the traps ranged between 0.35 and 421 in both 200 m and 1000 m sediment traps. For most of the TE, EF >1. A few exceptions were V, Y, Zr, Nb, and Ce (EF ~ 1) and Cr, Ni, Cu, Zn, Sn, and Pb (EF<1). Despite the variability in the EF values, vertical fluxes integrated during the dust event increased from 200 m to 1000 m, except for I, which decreased. This contrasts strongly with the element fluxes integrated for the complete sampling season, which decrease or increase from 200 m to 1000 m, depending on the element. This suggests that sinking dust particles are acting generally as sinks of the TE. We conclude that, apart from I, a substantial portion of the atmospheric dust input from one of these deposition events can reach the mesopelagic layer of the Western Mediterranean basin without increasing the budget of the dissolved TE.

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