Particulate iron delivery to the water column of the Amundsen Sea, Antarctica

Abstract

The Amundsen Sea, West Antarctica, is home to the most productive polynyas of the Southern Ocean, where summer primary production can reach 3gCm−2d−1. The remoteness of this region has meant that systematic studies of biogeochemistry in the Amundsen Sea polynyas have been limited, despite their importance to overall Antarctic shelf productivity and proximity to the fastest thinning glaciers on the continent. Particulate iron inputs to the productive shelf waters of the Amundsen Sea may be important to the overall bioavailability of Fe in this region of natural Fe fertilization. Here we discuss findings from the US–Swedish 2007–08 expedition aboard the I/B Oden, during which 12 stations were sampled for particulate trace metal analyses at depths of 8–800m in the eastern and central polynyas as well as in sea ice covered waters, both on the Amundsen continental shelf and in deep waters north of the shelf break. Suspended particulate samples were collected in two size fractions, 0.45–5μm and >5μm. Particulate Fe concentrations ranged from as low as 10pmolL−1 in open Antarctic Circumpolar Current (ACC) waters off the continental shelf to >100,000pmolL−1 near the Crosson Ice Shelf, and were dominated by particles>5μm at all stations. The relative concentrations of total particulate Fe, Al, Mn and P show the near-ubiquitous influence of crustal particles in the water column at stations on the Amundsen continental shelf. However, many samples had Fe/Al and Mn/Al ratios substantially in excess of mean crustal ratios, especially in the small size fraction (0.45–5μm), suggesting that more labile Fe oxyhydroxides and authigenic MnO2 phases, resulting from sediment resuspension, are also present at relatively high concentrations. In contrast, Fe/P ratios indicate that Fe associated with biogenic particles rarely accounts for more than 20% of total particulate Fe, even in offshore stations. A detailed examination of particulate elemental composition and spatial distribution in the context of water mass temperature and salinity gradients suggests that particle delivery processes associated with melting ice shelves and sediment resuspension dominate the particulate Fe sources to the Amundsen Sea water column.