Input, composition, and potential impact of terrigenous material from free-drifting icebergs in the Weddell Sea

The hypothesis that Fe fertilization of the Southern Ocean could account for at least part of the glacial-interglacial difference in atmospheric CO₂ has lead to a search for the Fe source. Iron fertilization in the open ocean can occur naturally through increased dust input, cross-shelf export, upwelling of Fe replete deep water, and release of terrigenous Fe from icebergs. However, the relative importance of these mechanisms, in terms of the flux and bioavailability of the Fe, is the source of much interest and debate. An unambiguous tracer of terrigenous material, excess ²²⁴Ra, was used to show that free-drifting icebergs are a significant Fe source to surface waters in the Southern Ocean. Activity of ²²⁴Ra (half-life 3.7 days) was measured in the Weddell Sea in surface waters surrounding three free-drifting icebergs and in Iceberg Alley. Inventories of excess ²²⁴Ra indicate a local terrigenous input 1-3 orders of magnitude greater than estimates of aeolian dust fluxes to the Southern Ocean. Burdens of fine terrigenous material associated with current rates of iceberg ejection are estimated to be on the order of 90×10⁶ tons per year for Antarctica, yielding a total Fe input of ~3×10⁶ tons per year. Chemical and mineralogical composition of the terrigenous material confirms that 0.04-0.4% exists as labile Fe in ferrihydrite that is potentially bioavailable (on the order of 4-40×10⁴ tons of per year). These results suggest that free-drifting icebergs can contribute significantly to atmospheric CO₂ drawdown though Fe fertilization in the Southern Ocean.