Free-drifting icebergs as sources of iron to the Weddell Sea

In recent years glaciers on the Antarctic Peninsula have retreated, resulting in loss of mass from ice shelves and increased supply of icebergs to the Southern Ocean. Free-drifting icebergs may serve as an important source of Fe to surrounding waters. We measured concentrations of dissolved Fe and Fe content of suspended particulate material (via flow injection-chemiluminescence and ICP-MS, respectively) in the waters surrounding several icebergs during cruises to the Scotia and Weddell Seas in June 2008 and March 2009. Surface dissolved Fe (DFe) concentrations varied from 0.58 to 2.92 nM and were elevated up to 60% at some stations <1 km from the nearest iceberg. The highest surface DFe concentrations were associated with low salinity waters, regardless of distance to the iceberg. Depth profiles revealed surface enrichment at most stations >10 km from the icebergs, indicating general Fe enrichment in the lower-salinity surface layer. However surface DFe within 1 km of the iceberg was similar to that measured at depths below the draft of the iceberg, a feature that may result from upwelling of circumpolar deep water caused by basal melting at the face of the iceberg. Iron concentrations in ice collected following calving events were highly variable (4-600 nM) but were elevated above concentrations in seawater. Particulate Fe (normalized to particulate P) in suspended particulate material was up to 20-fold higher within 0.6 km of two large tabular icebergs, but no enrichment was observed at two smaller icebergs. Iron:phosphorus ratios of suspended particulate material were also higher in deeper (40 m) waters compared to shallow (ca. 15 m) waters at the iceberg face. Iron release appeared to be episodic, producing a spatially heterogeneous environment around icebergs. This source term may increase in coming years as Antarctic ice shelves degrade further, potentially impacting the ecology and biogeochemistry of low-Fe waters of the Southern Ocean.