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 (< 0.5 nmol kg−1) cause us to question the residence time for this dust-derived subsurface reservoir and hence its role as a supply mechanism for the surface ocean, stressing the need for further studies. Finally, these contrasting responses indicate that dAl is a poor tracer of dFe input in the Mediterranean Sea.
Highlights
Iron (Fe) supply to the surface ocean sets the productivity of major phytoplankton groups (Moore et al, 2013)
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 Dissolved Al (dAl) in Mediterranean waters
Subsurface patches of high dissolved Fe (dFe) concentrations previously observed in the eastern Mediterranean basin, and attributed to hydrothermal activity and mud-volcanoes (Gerringa et al, 2017), were not observed along our cruise track
Summary
Iron (Fe) supply to the surface ocean sets the productivity of major phytoplankton groups (Moore et al, 2013). Poorly constrained controls of Fe solubility partly explain the substantial inter-model difference in the atmospheric dFe input flux to the global ocean (∼ 1–30 Gmol yr−1; Tagliabue et al, 2016) and hinder accurate predictions of the impact of dust on ocean productivity. Our understanding of the role of dust in marine biogeochemical cycles remains limited, partly resulting from the difficulty in quantifying atmospheric dust fluxes to the surface ocean at short timescales. In the absence of direct assessments of atmospheric inputs, marine concentrations of tracers such as aluminum (Al) have been widely used to constrain these fluxes (e.g., Measures and Brown, 1996; Han et al, 2008; Anderson et al, 2016; Menzel Barraqueta et al, 2019). The fact that the distribution of Al can itself be controlled by biological activity (e.g., Mackenzie et al, 1978; Moran and Moore, 1988a; Li et al, 2013; Middag et al, 2015; Rolison et al, 2015) causes us to question its quality as a tracer
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