Melting ice shelves and iron-controlled primary productivity in the Amundsen Sea, West Antarctica

Antarctic shelf regions provide iron (Fe) to otherwise Fe-limited Southern Ocean ecosystems. Variations in ecosystem productivity among the various Antarctic shelf regions appear to be related to glacial meltwater input.  Observations in the Amundsen Sea during the ASPIRE program of 2010-11 show that the strong outflow from the cavity under the Dotson Ice Shelf, at 150-400m depth, contains about 2% meltwater, and high dissolved and particulate Fe concentrations. The distribution of Fe in the adjacent Amundsen Sea Polynya (ASP) suggests that the injection of Fe into the upper water column from the cavities of the Dotson and neighboring ice shelves contributes to the high productivity of the ASP. However, observations and modeling suggest that Fe availability limits rates of bio-production during at least part of the bloom season.

 Recent high-resolution modeling (validated by field observations) concluded that carbon flux in the polynya depends on Fe delivered by Dotson cavity outflow, but also on a flux of comparable magnitude from the coastal current. This current delivers material from other ice shelf cavities located to the east, but with different seasonal timing from cavity input.  ASPIRE measurements suggest that dissolved Fe carried into the Dotson cavity from shelf sediment sources may account for the dissolved Fe content of the Dotson outflow. There is thus no requirement for significant soluble Fe from glacial melting within the cavity.  However, the very high suspended particle content of the outflow suggests that sediment resuspension within the cavity or continental subglacial melt injections at the grounding line may also be sources of Fe at this location.

The role of glacial melting may be in physically driving the overturning circulation within the cavity (the “meltwater pump”) that allows Fe from sedimentary sources to be injected into the upper water column, where it can fuel intense blooms. Distinguishing between mechanisms controlling Fe flux is critical to evaluating links between ongoing climate change and Antarctic shelf productivity. The very recent IPCC Special Report tells us that Antarctic ice melted three times faster in the decade ending 2016 than in the previous decade. However, more melting does not necessarily mean more productivity in the ASP; numerical modeling of future scenarios suggests that changing sea ice seasonality will play a countering role.