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.
Original language | English (US) |
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Pages (from-to) | 130-151 |
Number of pages | 22 |
Journal | Deep-Sea Research Part II: Topical Studies in Oceanography |
Volume | 116 |
DOIs | |
State | Published - Jun 1 2015 |
Keywords
- Anopore
- Chemical oceanography
- Colloids
- Cross flow filtration
- GEOTRACES
- Iron
- Iron isotopes
- North Atlantic Ocean
- Trace metals
- Ultrafiltration
ASJC Scopus subject areas
- Oceanography