High temporal resolution observations of spring fast ice melt and seawater iron enrichment in East Antarctica

Abstract

A time series experiment was conducted in late austral spring (November–December 2009) in coastal fast ice, East Antarctica (66°13!07S, 110°39!02E). Iron (Fe) measurements were made in sea ice, snow, brines, and underlying seawater, together with meteorological, physical, and biogeochemical measurements to investigate the processes controlling the release of Fe into the underlying water column. Warming air temperatures were clearly associated with decreasing brine volume fractions. Macronutrient profiles revealed very low (<1 mM) nitrate + nitrite concentrations in the interior of the sea ice, and the brines suggested nitrate + nitrite drawdown exceeded Redfield ratios in comparison to phosphate and silicate. In the basal ice, nitrate + nitrite and silicate were drawn down through time but did not lead to a limiting condition. We found that dissolved Fe tracked the brine volume fraction and was readily transferred from the surface/interior to the underlying water column over time. In contrast, particulate Fe did not show this clear decreasing trend and correlated with particulate organic carbon and chlorophyll a distributions. Over the 28 d of sampling, two distinct mean air temperature warming events were observed (−12.1 to −1.3°C and −6.4 to 0.8°C). This resulted in the release of 419 mmol of TDFe per m2 of sea ice from our coastal fast ice station into the underlying water column during the study period. Assuming an increase of 1 nM Fe is sufficient for Antarctic diatoms to bloom, our study site presented a fertilization potential for 419 m3 of Fe limited surface Southern Ocean seawater with TDFe and 29 m3 with dFe, per m2 of fast ice.