Iron fractionation in pack and fast ice in East Antarctica: Temporal decoupling between the release of dissolved and particulate iron during spring melt

Iron is a fundamental nutrient limiting phytoplankton growth in vast regions of the Southern Ocean. Sea ice, which covers ∼80% of the Southern Ocean (south of 60°S) during maximum extent, can concentrate iron up to two orders of magnitude higher than in the underlying sea water. The fractionation of iron between the particulate and dissolved fractions depends on the location and type of sea-ice formation and can impact on the bioavailability of this important trace element. This study is the first to document iron fractionation and concentration in both pack and fast ice during a single research study. Sampling was from within the 110–130°E sector of Antarctica. We observed markedly higher concentrations of particulate iron at our fast-ice site (0.96–214 nM) relative to several pack-ice sites (0.87–77.7 nM). A high particulate-to-dissolved iron ratio was observed at the fast-ice site (285:1) relative to the highest observed in pack ice (23:1). This suggests a decoupling between the sources and/or sinks of the dissolved and particulate fractions. Preferential release of dissolved iron (and not particulate iron) into brines at all sites sampled with the sack hole method (and therefore indicative of brine drainage) indicates the diffuse nature of the dissolved fraction. Furthermore, this indicates that there may be a temporal decoupling between the release of the dissolved and the particulate fractions into the water column as sea ice becomes more permeable during the seasonal melt. Implications for phytoplankton production in Antarctic sea ice are discussed.