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

Abstract

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 determined the influence of this dust event on the concentration of 30 minor 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 contributed 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. Additionally, the obtained results of element concentrations and particles flux show 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) for the minor and TE analyzed in the sediment traps during the dust event were calculated. EF was used to determine how minor and trace element concentrations in sinking particles vary. The values ranged between 0.35 and 421 in both 200 m and 1000 m sediment traps. For most of the analyzed elements, the obtained EF values were higher than 1. On the contrary, V, Y, Zr, Nb, and Ce showed EF ~ 1 while Cr, Ni, Cu, Zn, Sn, and Pb showed EF < 1. Despite the variability in the EF values, vertical fluxes integrated during the dust deposition 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 period, which decrease or increase from 200 m to 1000 m, depending on the element. This suggests that sinking dust particles were acting generally as sinks of the TE. We conclude that, apart from I, a substantial portion of the elements from atmospheric dust input from a single deposition event can reach the mesopelagic layer of the Western Mediterranean basin without increasing the budget of those elements in the water column.