Particulate iron dynamics during FeCycle in subantarctic waters southeast of New Zealand
Résumé
The FeCycle experiment provided an SF6 labeled mesoscale patch of high-nitrate low-chlorophyll (HNLC) water in austral summer 2003. These labeled waters enabled a comparison of the inventory of particulate iron (PFe) in the 45-m-deep surface mixed layer with the concurrent downward export flux of PFe at depths of 80 and 120 m. The partitioning of PFe between four size fractions (0.2-2, 2-5, 5-20, and >20 μm) was assessed, and PFe was mainly found in the >20-μm size fraction throughout FeCycle. Estimates of the relative contribution of the biogenic and lithogenic components to PFe were based on an Al:Fe molar ratio (0.18) derived following analysis of dust/soil from the nearest source of aerosol Fe: the semi-arid regions of Australia. The lithogenic component dominated each of the four PFe size fractions, with medians ranging from 68 to 97% of PFe during the 10-day experiment. The Fe:C ratios for mixed-layer particles were ∼40 μmol/mol. PFe export was ∼300 nmol m−2 d−1 at 80 m depth representing a daily loss of ∼1% from the mixed-layer PFe inventory. There were pronounced increases in the Fe:C particulate ratios with depth, with a five-fold increase from the surface mixed layer to 80 m depth, consistent with scavenging of the remineralized Fe by sinking particles and concurrent solubilization and loss of particulate organic carbon. Significantly, the lithogenic fraction of the sinking PFe intercepted at both 80 m and 120 m was >40%; that is, there was an approximately twofold decrease in the proportion of lithogenic iron exported relative to that in the mixed-layer lithogenic iron inventory. This indicates that the transformation of lithogenic to biogenic PFe takes place in the mixed layer, prior to particles settling to depth. Moreover, the magnitude of lithogenic Fe supply from dust deposition into the waters southeast of New Zealand is comparable to that of the export of PFe from the mixed layer, suggesting that a large proportion of the deposited dust eventually exits the surface mixed layer as biogenic PFe in this HNLC region.