Size fractionation of iron, manganese and aluminium in Antarctic fast ice reveals a lithogenic origin and low iron solubility

Melting sea ice represents a large seasonal source of iron (Fe) for planktonic growth in the marginal ice zone, but no data currently show how accessible this Fe is for biological uptake. We investigated the size fractionation in East Antarctic fast ice of Fe, manganese (Mn) and aluminium (Al) in the soluble (< 100 kDa), colloidal (100 kDa–0.2 μm), dissolved (< 0.2 μm), very small (0.2–0.4 μm), small (0.4–2 μm), medium (2–10 μm) and large (> 10 μm) particulate fractions during a time-series carried out in late spring/early summer 2009. Concentrations of all metals in fast ice were 2 to 3 orders of magnitude more concentrated than in ice-free polar waters, across all sizes combined. Dissolved Fe, Mn and Al were coupled in fast ice, and decreased with time, indicating some loss due to spring melting and/or biological uptake. Fractional solubilities (FS = dissolved-to-total metal ratio) demonstrate that particles dominated the total metal pool (97% in the case of Fe, 83% for Al and 57% for Mn). The low FS-Fe values also suggest that Fe is far less bio-available in fast ice than in Antarctic pack ice and surface waters, with soluble and colloidal Fe respectively representing only < 1% and 2% of the total Fe pool. Element-to-element molar ratios suggest that Fe mostly originated from lithogenic sources. Nearly 80% of Fe released from melting fast ice sank to the seafloor in less than 3 days, therefore leaving 20% of Fe available in the water column for biological uptake. Our results emphasise that the Fe released from sea ice into the water column is critical to stimulate new primary production in the marginal ice zone.