Abstract
Abstract. This study reports the only recent characterization of two contrasted wet deposition events collected during the PEACETIME (ProcEss studies at the Air–sEa Interface after dust deposition in the MEditerranean Sea) cruise in the open Mediterranean Sea (Med Sea) and their impact on trace metal (TM) marine stocks. Rain samples were analysed for Al, 12 TMs (Co, Cd, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Ti, V and Zn) and nutrient (N, P, dissolved organic carbon) concentrations. The first rain sample collected in the Ionian Sea (Rain ION) was a typical regional background wet deposition event, whereas the second rain sample collected in the Algerian Basin (Rain FAST) was a Saharan dust wet deposition event. Even in the remote Med Sea, all background TM inputs presented an anthropogenic signature, except for Fe, Mn and Ti. The concentrations of TMs in the two rain samples were significantly lower compared to concentrations in rains collected at coastal sites reported in the literature, due to the decrease in anthropogenic emissions during the preceding decades. The atmospheric TM inputs were mainly dissolved forms, even in dusty Rain FAST. The TM stocks in the mixed layer (ML, 0–20 m) at the FAST station before and after the event showed that the atmospheric inputs were a significant supply of particulate TMs and dissolved Fe and Co for surface seawater. Even if the wet deposition delivers TMs mainly in soluble form, the post-deposition aerosol dissolution could to be a key additional pathway in the supply of dissolved TMs. At the scale of the western and central Mediterranean, the atmospheric inputs were of the same order of magnitude as ML stocks for dissolved Fe, Co and Zn, highlighting the role of the atmosphere in their biogeochemical cycles in the stratified Med Sea. In case of intense dust-rich wet deposition events, the role of atmospheric inputs as an external source was extended to dissolved Co, Fe, Mn, Pb and Zn. Our results suggest that the wet deposition constitutes only a source of some of dissolved TMs for Med Sea surface waters. The contribution of dry deposition to the atmospheric TM inputs needs to be investigated.
Highlights
Atmospheric deposition of continental aerosol has long been recognized to influence trace element concentrations in remote oceanic surface waters (Buat-Ménard and Chesselet, 1979; Hardy, 1982; Buat-Ménard, 1983)
The marine trace metal (TM) concentrations measured during the cruise being typical of Mediterranean surface seawater concentrations, we can conclude that wet deposition events are an external supply of dissolved Fe, Co and Zn for the stratified Mediterranean Sea (Med Sea) and of all dissolved TMs during intense wet dust deposition. This study provides both the dynamic properties and chemical characterization of two rain events collected in the open Med Sea concurrent with TM marine stocks in surface seawater
Our results are the only recent report of TM concentrations, enrichment factors (EFs) and fractional solubility values in rain samples collected in the remote Med Sea
Summary
Atmospheric deposition of continental aerosol has long been recognized to influence trace element concentrations in remote oceanic surface waters (Buat-Ménard and Chesselet, 1979; Hardy, 1982; Buat-Ménard, 1983). Sciare et al, 2003; Kanakidou et al, 2011) In addition to this anthropogenic background, the Mediterranean Basin is subject to seasonal contributions of particles from biomass fires in summer (Guieu et al, 2005) and to intense sporadic Saharan dust inputs (e.g. Loÿe-Pilot and Martin, 1996; Vincent et al, 2016). Many TMs, including Mn, Co, Ni (Mackey et al, 2012), Cu (Annett et al, 2008) and Zn (Morel et al, 1991), play physiological roles for phytoplanktonic organisms These TMs are present in very low concentrations in oligotrophic systems such as the Med Sea, possibly limiting (or co-limiting) phytoplankton growth (PinedoGonzàles et al, 2015) and pointing to the importance of dust deposition as a source of TMs for planktonic communities. Atmospheric deposition of European aerosols was identified to have a negative effect on chlorophyll concentrations (Gallisai et al, 2014) by providing Cu at toxic levels (Jordi et al, 2012)
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