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

Abstract

The deposition of atmospheric dust is the primary process supplying trace elements 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 these elements differs according to their chemical speciation and biological utilization. Presently, however, the chemical and physical processes occurring after atmospheric deposition are poorly constrained, principally because of the difficulty in following natural dust events in situ. In the present work we examined the temporal changes in the biogeochemistry of crustal metals (in particular Al, Mn and Fe) after an artificial dust deposition event. The experiment was contained inside trace metal clean mesocosms (0–12.5 m depths) deployed in the surface waters of the northwestern Mediterranean, close to the coast of Corsica within the frame of the DUNE project (a DUst experiment in a low Nutrient, low chlorophyll Ecosystem). Two consecutive artificial dust deposition events, each mimicking a wet deposition of 10 g m⁻² of dust, were performed during the course of this DUNE-2 experiment. The changes in dissolved manganese (Mn), iron (Fe) and aluminum (Al) concentrations were followed immediately after the seeding with dust and over the following week. The Mn, Fe and Al inventories and loss or dissolution rates were determined. The evolution of the inventories after the two consecutive additions of dust showed distinct behaviors for dissolved Mn, Al and Fe. Even though the mixing conditions differed from one seeding to the other, Mn and Al showed clear increases directly after both seedings due to dissolution processes. Three days after the dust additions, Al concentrations decreased as a consequence of scavenging on sinking particles. Al appeared to be highly affected by the concentrations of biogenic particles, with an order of magnitude difference in its loss rates related to the increase of biomass after the addition of dust. In the case of dissolved Fe, it appears that the first dust addition resulted in a decrease as it was scavenged by sinking dust particles, whereas the second seeding induced dissolution of Fe from the dust particles due to the excess Fe binding ligand concentrations present at that time. This difference, which might be related to a change in Fe binding ligand concentration in the mesocosms, highlights the complex processes that control the solubility of Fe. Based on the inventories at the mesocosm scale, the estimations of the fractional solubility of metals from dust particles in seawater were 1.44 ± 0.19% and 0.91 ± 0.83% for Al and 41 ± 9% and 27 ± 19% for Mn for the first and the second dust addition. These values are in good agreement with laboratory-based estimates. For Fe no fractional solubility was obtained after the first seeding, but 0.12 ± 0.03% was estimated after the second seeding. Overall, the trace metal dataset presented here makes a significant contribution to enhancing our knowledge on the processes influencing trace metal release from Sahar...

A fine fraction of soil used as an aerosol analogue during the DUNE experiment: sequential solubility in water, decreasing pH step-by-step

“…For the carbonated dust studied in this paper, we do not have enough data to clearly attribute the origin of the dissolved aluminium and iron to a specific solid phase, but a noticeable solubility increase at pH 3 might attribute a part of soluble aluminium and iron to carbonated phases. We can compare the solubility found here with that estimated and calculated in seawater during the DUNE mesocosm experiments, where the same soil fraction was used, but only after processing with synthetic cloud-like water to reproduce condensation-evaporation processes on dust particles during atmospheric transport (Wuttig et al, 2013). For manganese, iron, and aluminium, these authors reported sol- ubilities that are respectively equal to 27-41, 0.12, and 1 %.…”

“…It is important to keep in mind that the calculated solubilities in the mesocosms were marred by a significant uncertainty. The average solubility similarities obtained from unprocessed dust at pH 3 in water (this study) and cloud-processed dust in seawater (Wuttig et al, 2013) indicate that the acidic conditions encountered during the laboratory cloud processing (see Guieu et al, 2010) were recorded in the dust until it was used to seed the mesocosms. This recorded acidity controlled the solubility levels in seawater.…”

Ocean–Atmosphere Interactions of Particles

“…Those results were satisfactorily simulated with a one-dimensional model of the Fe biogeochemical cycle, coupled with a simple ecosystem model (Ye et al 2011). When a second dust wet deposition was simulated, iron dissolution from the dust particles was then evidenced due to the excess Fe binding ligand concentrations produced by the enhanced biological activity (Wuttig et al 2013). When simulating dust wet deposition at least a doubling of Chla (mainly attributed to small phytoplankton (<3 μm); Giovagnetti et al 2013) was observed but the Chla concentrations remained very low (maximum values 0.22 μg L-1) maintaining the oligotrophic status of the tested waters .…”

Quantification of the lithogenic carbon pump following a simulated dust-deposition event in large mesocosms

“…Initial conditions indicate (1) very low dissolved inorganic phosphorus concentrations ([DIP]) (5 ± 3 nM; PulidoVillena et al, 2014), (2) dissolved inorganic nitrogen concentrations ([DIN]) below the detection limit (< 30 nM; Ridame et al, 2013), and (3) dissolved iron concentrations ([DFe]) typical of coastal area (3.3 ± 0.8 nM; Wuttig et al, 2013). Following the first seeding, a decrease in [DFe] due to scavenging by sinking dust (Wuttig et al, 2013) and a transient increase in [DIP] were observed.…”