Atmospheric deposition fluxes over the Atlantic Ocean: a GEOTRACES case study

Christian Schlosser,Bernhard Wenzel,Jessica K. Klar,Geraldine Sarthou,Eric P. Achterberg,Jan-Lukas Menzel Barraqueta,Martha Gledhill,Hélène F. Planquette,Rachel Shelley

doi:10.5194/bg-16-1525-2019

Abstract

Abstract. Atmospheric deposition is an important source of micronutrients to the ocean, but atmospheric deposition fluxes remain poorly constrained in most ocean regions due to the limited number of field observations of wet and dry atmospheric inputs. Here we present the distribution of dissolved aluminium (dAl), as a tracer of atmospheric inputs, in surface waters of the Atlantic Ocean along GEOTRACES sections GA01, GA06, GA08, and GA10. We used the surface mixed-layer concentrations of dAl to calculate atmospheric deposition fluxes using a simple steady state model. We have optimized the Al fractional aerosol solubility, the dAl residence time within the surface mixed layer and the depth of the surface mixed layer for each separate cruise to calculate the atmospheric deposition fluxes. We calculated the lowest deposition fluxes of 0.15±0.1 and 0.27±0.13 g m−2 yr−1 for the South and North Atlantic Ocean (>40∘ S and >40∘ N) respectively, and the highest fluxes of 1.8 and 3.09 g m−2 yr−1 for the south-east Atlantic and tropical Atlantic Ocean, respectively. Overall, our estimations are comparable to atmospheric dust deposition model estimates and reported field-based atmospheric deposition estimates. We note that our estimates diverge from atmospheric dust deposition model flux estimates in regions influenced by riverine Al inputs and in upwelling regions. As dAl is a key trace element in the GEOTRACES programme, the approach presented in this study allows calculations of atmospheric deposition fluxes at high spatial resolution for remote ocean regions.

Highlights

Atmospheric deposition is a major source of micronutrients, especially iron, for the surface ocean (Martin et al, 1991; Moore et al, 2004)
The bottom depth of the surface mixed layer varies due to atmospheric forcing, with turbulent mixing caused by wind stress (Risien and Chelton, 2008), convection caused by heat exchange (Yu and Weller, 2007), in addition to salinity changes due to evaporation and precipitation at the surface (Schanze et al, 2010)
The thickness of the Mixed-layer depth (MLD) is an indication of the amount of water that directly interacts with the atmosphere

Summary

Introduction

Atmospheric deposition is a major source of micronutrients, especially iron, for the surface ocean (Martin et al, 1991; Moore et al, 2004). Aerosol deposition of iron in the tropical and subtropical North Atlantic stimulates N2 fixation (Moore et al, 2009), and in high-latitude waters with high-nitrate– low-chlorophyll conditions it can enhance primary productivity (Baker et al, 2013). Atmospheric deposition is considered to support up to 50 % of global export production (Jickells et al, 2014). Menzel Barraqueta et al.: Atmospheric deposition fluxes over the Atlantic Ocean marine carbon cycle and indirectly influence global climate (Mahowald et al, 2014)

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Results

Conclusion