Sedimentary and mineral dust sources of dissolved iron to the World Ocean

J. K. Moore (1) , O. Braucher (1)

1 ESS - Department of Earth System Science [Irvine]

A worldwide database of dissolved iron observations is used to improve simulations of the marine iron cycle within a global-scale, Biogeochemical Elemental Cycling (BEC) ocean model. Modifications to the model include: 1) an improved particle scavenging parameterization based on the sinking mass flux of particulate organic material, biogenic silica, calcium carbonate, and mineral dust particles; 2) desorption of dissolved iron from sinking particles; and 3) an improved sedimentary source for dissolved iron. Most scavenged iron (90%) is put on sinking particles to remineralize deeper in the water column. The model-observation mismatches are greatly reduced both in surface waters and in the deeper ocean. Inclusion of desorption has little effect on surface water iron concentrations where adsorption/scavenging is strongly dominant, but significantly increases simulated iron concentrations in the deep ocean. Our results suggest that there must be substantial removal of dissolved iron from subsurface waters (where iron concentrations are <0.6 nM in most regions) to match observed distributions. Aggregation and removal on sinking particles of Fe bound to organic colloids is a likely mechanism.

The improved BEC model is used to address the relative contributions of mineral dust and marine sediments in driving ocean productivity and observed dissolved iron distributions. The sedimentary iron source from the continental margins has a strong impact on open ocean iron concentrations, particularly in the North Pacific. Plumes of elevated dissolved iron concentrations develop at depth in the Southern Ocean, extending from source regions in the SW Atlantic and around New Zealand. The lower particle flux and weaker scavenging in this region allows the continental iron source to be advected far from source areas. Both the margin sediment and mineral dust Fe sources significantly impact global scale primary production, export production, and nitrogen fixation, with inputs from dust deposition having a modestly stronger impact. Ocean biogeochemical models need to include the sedimentary source for dissolved iron, or they will overestimate the impact of dust deposition variations on the marine carbon cycle.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Interfaces continentales, environnement; Océan, Atmosphère; Sciences de la Terre

J. K. Moore, O. Braucher. Sedimentary and mineral dust sources of dissolved iron to the World Ocean. Biogeosciences Discussions, 2007, 4 (2), pp.1279-1327. ⟨hal-00297887⟩