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.

Marine organic matter in the remote environment of the Cape Verde islands – an introduction and overview to the MarParCloud campaign

“…The region is bounded by a highly productive eastern boundary upwelling system (EBUS) along the African coast, by the Cape Verde Frontal Zone (CVFZ) on its western side and by zonal current bands towards the Equator (Stramma et al, 2005). Upper water masses towards the archipelago are dominated by North Atlantic Central Water (NACW) masses with enhanced salinity, whereas the South Atlantic Central Water (SACW) mass is the dominating upper-layer water mass in the EBUS region (Pastor et al, 2008). Filaments and eddies generated in the EBUS region propagate westwards into the open ocean and usually dissipate before reaching the archipelago.…”

Responses of ocean biogeochemistry to atmospheric supply of lithogenic and pyrogenic iron-containing aerosols

“…The possible reasons for the overestimation are explained below. (3) As well as biological uptake, scavenging onto living phytoplankton cells could also play a role in removing DFe from the surface waters (Hudson & Morel 1989;Pagnone et al 2019), particularly in regions with high biological production. The decline of DFe is shown to correlate well with the high surface fluorescence at c. 15°N (Rijkenberg et al 2014).…”

Slowly Sinking Particles Underlie Dissolved Iron Transport Across the Pacific Ocean

“…Observational data suggest that Fe is sourced from continental shelf sediments and supplied via lithogenic particles (Pérez‐Tribouillier et al., 2020). Model studies have demonstrated that lithogenic particles can also have a significant impact on both the source and sink of DFe (Beghoura et al., 2019; Pagnone et al., 2019; Ye & Völker, 2017). Contrastingly, observational data obtained from a study in the Sea of Okhotsk have shown that Fe is transported longer distances in association with organic matter and that lithogenic particles with a higher specific gravity are lost in shorter distances (Shigemitsu et al., 2013).…”