Impacts of dust deposition on dissolved trace metal concentrations (Mn, Al and Fe) during a mesocosm experiment

[摘要] The deposition of atmospheric dust is the primary process supplying traceelements abundant in crustal rocks (e.g. Al, Mn and Fe) to the surface ocean.Upon deposition, the residence time in surface waters for each of theseelements differs according to their chemical speciation and biologicalutilization. Presently, however, the chemical and physical processesoccurring after atmospheric deposition are poorly constrained, principallybecause of the difficulty in following natural dust events in situ. In thepresent work we examined the temporal changes in the biogeochemistry ofcrustal metals (in particular Al, Mn and Fe) after an artificial dustdeposition event. The experiment was contained inside trace metal cleanmesocosms (0–12.5 m depths) deployed in the surface waters of thenorthwestern Mediterranean, close to the coast of Corsica within the frame ofthe DUNE project (a DUst experiment in a low Nutrient, low chlorophyllEcosystem). Two consecutive artificial dust deposition events, eachmimicking a wet deposition of 10 g m⁻² of dust, were performed duringthe course of this DUNE-2 experiment. The changes in dissolvedmanganese (Mn), iron (Fe) and aluminum (Al) concentrations were followedimmediately after the seeding with dust and over the following week. The Mn,Fe and Al inventories and loss or dissolution rates were determined. Theevolution of the inventories after the two consecutive additions of dustshowed distinct behaviors for dissolved Mn, Al and Fe. Even though the mixingconditions differed from one seeding to the other, Mn and Al showed clearincreases directly after both seedings due to dissolution processes. Threedays after the dust additions, Al concentrations decreased as a consequenceof scavenging on sinking particles. Al appeared to be highly affected by theconcentrations of biogenic particles, with an order of magnitude differencein its loss rates related to the increase of biomass after the addition ofdust. In the case of dissolved Fe, it appears that the first dust additionresulted in a decrease as it was scavenged by sinking dust particles, whereasthe second seeding induced dissolution of Fe from the dust particles due tothe excess Fe binding ligand concentrations present at that time. Thisdifference, which might be related to a change in Fe binding ligandconcentration in the mesocosms, highlights the complex processes that controlthe solubility of Fe. Based on the inventories at the mesocosm scale, theestimations of the fractional solubility of metals from dust particles inseawater were 1.44 ± 0.19% and 0.91 ± 0.83% for Al and41 ± 9% and 27 ± 19% for Mn for the first and the second dustaddition. These values are in good agreement with laboratory-based estimates.For Fe no fractional solubility was obtained after the first seeding, but0.12 ± 0.03% was estimated after the second seeding. Overall, thetrace metal dataset presented here makes a significant contribution toenhancing our knowledge on the processes influencing trace metalrelease from Saharan dust and the subsequent processes of bio-uptake andscavenging in a low nutrient, low chlorophyll area.

[学科分类] 地球化学与岩石