Partitioning of dissolved iron and iron isotopes into soluble and colloidal phases along the GA03 GEOTRACES North Atlantic Transect

Abstract

The size partitioning of dissolved Fe (dFe<0.2µm) into soluble (sFe<0.02µm) and colloidal (0.02µm<cFe<0.2µm) species was investigated at 18 stations along the GEOTRACES GA03 North Atlantic Transect. Upper ocean dFe size partitioning was highly variable with depth: 79±6% of aerosol-derived surface dFe was maintained in the colloidal size fraction, while cFe disappeared completely at the deep chlorophyll maximum, presumably a result of preferential cFe biological uptake and/or scavenging. In the intermediate and deep ocean, however, dFe was evenly partitioned ~50:50% into sFe and cFe phases, which we hypothesize results from a “steady state” of dFe exchange reactions during and following remineralization including ligand exchange, sorption/desorption, and aggregation/disaggregation. There were only two exceptions to this constant partitioning in the intermediate/deep ocean. First, cFe dominated (82–96%) at and downstream of the Mid-Atlantic Ridge hydrothermal system. Also, along Line W between Woods Hole and Bermuda the dFe partitioning favored ~60–80% cFe, with the excess cFe likely resulting from inorganic cFe inputs from the margin. Thus, in the North Atlantic Ocean we propose a new model of dFe size partitioning where a “steady state” of dFe exchange reactions during and following remineralization re-partitions intermediate and deep ocean dFe into constantly fractionated sFe and cFe pools, while in the upper ocean, downstream of the Mid-Atlantic Ridge, and along Line W, sFe and cFe appear to cycle more independently, since either not enough time has passed to reach a new dFe exchange steady state or one of the dFe phases is non-labile to dFe exchange. This surface-subsurface decoupling model of North Atlantic dFe size partitioning is supported by Fe isotope ratio analyses of the sFe and dFe size fractions, which recorded isotopically heavy sFe (δ56Fe of +1.3‰ to +1.5‰) relative to dFe (δ56Fe~+0.5‰) in the surface ocean where sFe and cFe cycle independently, and identical sFe and dFe isotope ratios below 250m where sFe and cFe are constantly partitioned and exchanged.