Wet deposition in the remote western and central Mediterranean as a source of trace metals to surface seawater

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.

Trace Metal Fluxes of Cd, Cu, Pb and Zn From the Congo River Into the South Atlantic Ocean Are Supplemented by Atmospheric Inputs

“…Concentrations of most TMs in rainwaters in our study, including Cd, Co, Cu, Mn, Ni, Pb and Zn, were similar to those reported for the central and western Mediterranean Sea (two individual samples), where wet deposition is influenced by high dust loading and anthropogenic emissions (Desboeufs et al., 2022). However, Fe concentrations were lower by a factor of ∼20 compared to the Mediterranean study (Desboeufs et al., 2022). The difference is perhaps related to the enhanced dust loadings in the Mediterranean Sea region, derived from the Sahara and Sahel areas (Baker et al., 2013; Nickovic et al., 2013; Sedwick et al., 2007) and/or low Fe concentrations of pyrogenic aerosol from South Africa (Barkley et al., 2019; Sholkovitz et al., 2012).…”

“…The Congo shelf experienced numerous heavy rain events during our cruise, with precipitation rates of >1 mm hr −1 as confirmed by satellite data (Figure 1 and Figure S1 in Supporting Information ). We thus consider wet deposition as a potentially important source of TMs to surface waters as reported for other ocean regions (Desboeufs et al., 2022; Schlosser et al., 2014). As a result of the episodic and dispersed nature of rain events, it is challenging to decouple rain from river inputs of dTMs.…”

“… Trace metal concentrations in rainwater. (a) Volume‐weighted‐mean dissolved (dTM, orange) and total dissolvable (TdTM, blue) TM concentrations (log scale) in different areas including the North Atlantic (Baker et al., 2013; Shelley et al., 2017) (squares), Mediterranean (Desboeufs et al., 2022) (crosses), South Atlantic (Baker et al., 2013; Chance et al., 2015) (diamonds), ITCZ (Baker et al., 2013) (triangles) and the Congo shelf (this study, circles). (b) Enrichment factor (EF, log scale) and (c) fractional solubility (solubility).…”

Major ions and Nutrients loading by atmospheric wet deposition in Mbeya Urban, Tanzania

“…(Yechieli & Wood, 2002). Atmospheric inputs are signi cant sources of trace metals, dissolved Fe, and Co in surface seawater (Desboeufs et al, 2022). The Carbon and nutrient inputs into soil and surface water are aided by atmospheric deposition (Mladenov et al, 2012;Qiao et al, 2015).…”

Isotopic compositions of copper and zinc in plankton from the Mediterranean Sea (MERITE-HIPPOCAMPE campaign): Tracing trophic transfer and geogenic inputs

“…The Mediterranean Sea is subject to natural and anthropogenic exogenous chemical substances coming from rivers, effluents and atmospheric deposition (Guieu et al, 2002;Heimbürger et al, 2010Heimbürger et al, , 2011Pey et al, 2010;Llamas-Dios et al, 2021;Desboeufs et al, 2022). Important changes have been observed in recent decades in small pelagic fish populations of the northwestern Mediterranean Sea related to various environmental factors (Coll et al, 2019).…”