Subsurface iron accumulation and rapid aluminum removal in the Mediterranean following African dust deposition

Abstract

Abstract. Mineral dust deposition is an important supply mechanism for trace elements in the low-latitude ocean. Our understanding of the controls of such inputs has been mostly built on laboratory and surface ocean studies. The lack of direct observations and the tendency to focus on near-surface waters prevent a comprehensive evaluation of the role of dust in oceanic biogeochemical cycles. In the frame of the PEACETIME project (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea), the responses of the aluminum (Al) and iron (Fe) cycles to two dust wet deposition events over the central and western Mediterranean Sea were investigated at a timescale of hours to days using a comprehensive dataset gathering dissolved and suspended particulate concentrations, along with sinking fluxes. Dissolved Al (dAl) removal was dominant over dAl released from dust. The Fe / Al ratio of suspended and sinking particles revealed that biogenic particles, and in particular diatoms, were key in accumulating and exporting Al relative to Fe. By combining these observations with published Al / Si ratios of diatoms, we show that adsorption onto biogenic particles, rather than active uptake, represents the main sink for dAl in Mediterranean waters. In contrast, systematic dissolved Fe (dFe) accumulation occurred in subsurface waters (∼ 100–1000 m), while dFe input from dust was only transient in the surface mixed layer. The rapid transfer of dust to depth, the Fe-binding ligand pool in excess to dFe in subsurface (while nearly saturated in surface), and low scavenging rates in this particle-poor depth horizon are all important drivers of this subsurface dFe enrichment. At the annual scale, this previously overlooked mechanism may represent an additional pathway of dFe supply for the surface ocean through diapycnal diffusion and vertical mixing. However, low subsurface dFe concentrations observed at the basin scale (

Basin scale distributions of dissolved manganese, nickel, zinc and cadmium in the Mediterranean Sea

“…In the absence of complexation and particulate data, discussion on the underlying processes remains speculative. Nevertheless, dust deposition not only is a source of metals, it also leads to scavenging (Bressac et al, 2021;Gerringa et al, 2017;Sarthou and Jeandel, 2001;Wagener et al, 2010) presumably through the influx of particulates into the water column, and it is thus plausible that the rather homogenous surface concentration of Zn on the order of 1 to 1.5 nM in the upper ~50 m of the water column (Fig. 5; outside the shallow shelf seas) is the equilibrium concentration resulting from the interplay between dissolution and organic complexation on one hand, and scavenging and biological uptake on the other hand.…”

Authigenic Iron Is a Significant Component of Oceanic Labile Particulate Iron Inventories

“…The concentrations of PFe Total and PFe Lith were highly heterogeneous between depths in the upper water column in March, consistent with the short residence times reported previously for these species. During the PEACETIME study in the Mediterranean Sea, PFe Total inventories in the upper 200 m decreased by more than 40% from 1.2 to 0.7 nmol/m 2 over 2–3 days after wet deposition events (Bressac et al., 2021). High variability in March fluxes at 500 m depth have been observed in particle traps in March, with fluxes as much as six times higher than usual in some years (Conte et al., 2019).…”

Response of Dissolved Trace Metals to Dust Storms, Sediment Resuspension, and Flash Floods in Oligotrophic Oceans

“…As such, much of our understanding of the biogeochemical cycling of oceanic TMs depends upon cruise transects, which provide “snapshots” of the water column composition at the moment of sampling. This, coupled with the sporadic nature of atmospheric deposition (Guieu et al., 2010; Mahowald et al., 2009; Ternon et al., 2010) and storm induced sediment resuspension (Gross et al., 1988), makes sampling across short‐term perturbations very challenging and scarce (e.g., Bressac et al., 2021; Ren et al., 2011; Rijkenberg et al., 2008). Consequently, a large knowledge gap exists regarding the impact of short‐term events on the oceanic water column.…”