Unravelling the Seasonal Cycle of Dissolved Iron on the Antarctic Continental Margins: Fall-Winter Observations from the Ross Sea Polynyas

Peter Sedwick, Old Dominion University, Norfolk, VA, United States, Stephen F Ackley, University of Texas at San Antonio, NASA Center for Advanced Measurements in Extreme Environments (CAMEE), San Antonio, TX, United States, Michael S Dinniman, Old Dominion University, Center for Coastal Physical Oceanography, Norfolk, VA, United States, Brice Loose, University of Rhode Island, Narragansett, RI, United States, Bettina M Sohst, Old Dominion University, Department of Ocean, Earth and Atmospheric Sciences, Norfolk, VA, United States and Sharon Elisabeth Stammerjohn, University of Colorado Boulder, Boulder, CO, United States
The Ross Sea continental shelf is among the most productive areas in the Southern Ocean, as well as a major source of Antarctic Bottom Water and a regional sink for CO2. Here the magnitude of annual primary production is thought to be limited by the supply of dissolved iron (DFe), which is delivered to surface waters by melting sea ice and glacial ice, intrusion and upwelling of Circumpolar Deep Water, and vertical resupply of iron-rich waters from the benthos. The vertical resupply is presumed to be most prevalent during the winter months, when katabatic winds drive sea ice formation and convective overturn in coastal polynyas, although the impact of these processes on the water-column DFe distribution has not been previously documented. As part of the Polynyas and Ice Production Evolution in the Ross Sea (PIPERS) project, we collected hydrographic data and water-column samples for iron analysis in the Terra Nova Bay (TNB) and Ross Ice Shelf (RIS) polynyas during April-June 2017 (austral fall-winter). We observed intense katabatic wind events in the TNB polynya, where the surface mixed layer varied from ~250 m to ~600 m depth over lateral distances of less than 10 km. Here vertical mixing was just starting to excavate the dense, iron-rich High Salinity Shelf Waters that fill the deeper TNB basin, and provide a potential source of DFe to both Ross Sea surface waters and the abyssal Southern Ocean. Our TNB data also show evidence of lateral DFe inputs in the upper 500 m of the water column. Stations occupied in the RIS polynya, where wind conditions were less extreme, revealed surface mixed layers of less than 300 m depth, and a DFe distribution similar to previous, late-summer observations. These results suggest that convective mixing progresses slowly and episodically during the winter months, although it should be noted that our cruise coincided with a year in which the seasonal advance of sea ice was delayed by 1-2 months, relative to long-term average conditions for the Ross Sea. Thus, additional winter observations will be needed to fully elucidate the seasonal dynamics of iron on the Ross Sea shelf.