Processes affecting dissolved iron across the Subtropical North Atlantic: a model study

Abstract

Trace metal measurements in recent years have revealed a complex distribution of dissolved iron (dFe) in the ocean that models still struggle to reproduce. The GEOTRACES section GA03 across the subtropical North Atlantic was chosen to study the driving processes involved in the Fe cycle in the region. Here, field observations found elevated dFe near the surface under the Saharan dust plume, a strong dFe minimum below the mixed layer depth, a maximum at the oxygen minimum zone near the African shelf, a hydrothermal maximum near the Mid Atlantic Ridge and lower dFe values in the deep eastern basin than in the west. We show that several of these features can be understood and be reproduced in models when they take into account scavenging on dust particles and phytoplankton, a variable ligand concentration and a hydrothermal dFe source. By doing so in a sequence of parameterisation changes, we are able to relate physical and biological processes, as well as internal and external dFe sources to observed features of the dFe distribution. In agreement with the observations, the additional scavenging on dust generates lower dFe concentrations in the deep eastern basin while the new ligand distribution results in a dFe maximum in the intermediate waters in the east basin and moderates the deep dFe gradient between the eastern and western basins.