13 Apr 2021

13 Apr 2021

Review status: this preprint is currently under review for the journal BG.

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

Matthieu Bressac1,2, Thibaut Wagener3, Nathalie Leblond4, Antonio Tovar-Sánchez5, Céline Ridame6, Samuel Albani7,8, Sophie Guasco3, Aurélie Dufour3, Stéphanie Jacquet3, François Dulac8, Karine Desboeufs9, and Cécile Guieu1 Matthieu Bressac et al.
  • 1Sorbonne Université, CNRS, Laboratoire d’Océanographie de Villefranche, LOV, Villefranche-sur-mer, 06230, France
  • 2Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, Australia
  • 3Aix Marseille Univ., CNRS, IRD, Université de Toulon, MIO UMR 110, Marseille, 13288, France
  • 4Sorbonne Université, CNRS, Institut de la Mer de Villefranche, IMEV, 06230 Villefranche-sur-Mer, France
  • 5Department of Ecology and Coastal Management, Institute of Marine Sciences of Andalusia (ICMAN-CSIC), 07190 Puerto Real, Spain
  • 6Sorbonne Université, LOCEAN, 4 Place Jussieu – 75252 Paris Cedex 05, France
  • 7Department of Environmental and Earth Sciences, University of Milano–Bicocca, Milan, Italy
  • 8Laboratoire des Sciences du Climat et de l’Environnement (LSCE), UMR 8212 CEA-CNRS-UVSQ, Institut Pierre-Simon Laplace, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
  • 9Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR7583 CNRS, Université de Paris, Université Paris-Est Créteil, Institut Pierre-Simon Laplace, France

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 onto 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 aluminium (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. 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 (up to ~180 m d−1), 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) questions 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.

Matthieu Bressac et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'BG-2021-87 Review', Thomas Holmes, 05 May 2021
  • RC2: 'Comment on bg-2021-87', Anonymous Referee #2, 11 May 2021

Matthieu Bressac et al.

Matthieu Bressac et al.


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Short summary
Phytoplankton growth is limited by the availability of iron in about 50 % of the ocean. Atmospheric deposition of desert dust represents a key source of iron. Here, we present direct observations of dust deposition in the Mediterranean Sea. A key finding is that the input of iron from dust primarily occurred in the deep ocean, while previous studies mainly focused on the surface ocean. This new insight will enable us to better represent controls on global marine productivity in models.