Dissolved and particulate trace metal micronutrients under the McMurdo Sound seasonal sea ice: basal sea ice communities as a capacitor for iron.

Abigail E Noble,Dawn M Moran,Andrew E Allen,Mak A Saito

doi:10.3389/fchem.2013.00025

Abstract

Dissolved and particulate metal concentrations are reported from three sites beneath and at the base of the McMurdo Sound seasonal sea ice in the Ross Sea of Antarctica. This dataset provided insight into Co and Mn biogeochemistry, supporting a previous hypothesis for water column mixing occurring faster than scavenging. Three observations support this: first, Mn-containing particles with Mn/Al ratios in excess of the sediment were present in the water column, implying the presence of bacterial Mn-oxidation processes. Second, dissolved and labile Co were uniform with depth beneath the sea ice after the winter season. Third, dissolved Co:PO3−4 ratios were consistent with previously observed Ross Sea stoichiometry, implying that over-winter scavenging was slow relative to mixing. Abundant dissolved Fe and Mn were consistent with a winter reserve concept, and particulate Al, Fe, Mn, and Co covaried, implying that these metals behaved similarly. Elevated particulate metals were observed in proximity to the nearby Islands, with particulate Fe/Al ratios similar to that of nearby sediment, consistent with a sediment resuspension source. Dissolved and particulate metals were elevated at the shallowest depths (particularly Fe) with elevated particulate P/Al and Fe/Al ratios in excess of sediments, demonstrating a sea ice biomass source. The sea ice biomass was extremely dense (chl a >9500 μg/L) and contained high abundances of particulate metals with elevated metal/Al ratios. A hypothesis for seasonal accumulation of bioactive metals at the base of the McMurdo Sound sea ice by the basal algal community is presented, analogous to a capacitor that accumulates iron during the spring and early summer. The release and transport of particulate metals accumulated at the base of the sea ice by sloughing is discussed as a potentially important mechanism in providing iron nutrition during polynya phytoplankton bloom formation and could be examined in future oceanographic expeditions.

Highlights

The HNLC region of the Ross Sea is an important sink for atmospheric CO2, with high pulses of primary productivity and subsequent export occurring during the spring and summer blooms (Arrigo et al, 2008)
We have previously suggested that the absence of a water column scavenging signal in the Ross Sea may be due to mixing on the shelf at a rate that is faster than removal by scavenging and/or alternatively that there is insufficient water column co-oxidation of cobalt by manganese oxidizing bacteria to impose a discernible scavenging signal on the dissolved cobalt vertical structure (Saito et al, 2010)
TOTAL DISSOLVED AND LABILE COBALT DISTRIBUTIONS We observed two overriding features in the cobalt dataset that were consistent with our previous observations in the Ross Sea: (1) non-saturating concentrations of cobalt-binding ligands throughout the water column, and (2) no evidence of cobalt scavenging with depth, which is typically seen for this hybrid-type element in other oceanic regimes (Martin et al, 1989; Saito et al, 2004, 2010; Noble et al, 2008, 2012; Bown et al, 2011; Pohl et al, 2011; Biller and Bruland, 2012; Figure 2)

Summary

Introduction

The HNLC region of the Ross Sea is an important sink for atmospheric CO2, with high pulses of primary productivity and subsequent export occurring during the spring and summer blooms (Arrigo et al, 2008). The Ross Sea influences the chemical composition of the deep ocean when these waters sink and contribute to Antarctic Bottom Water formation (Orsi and Weiderwohl, 2009) It is a relatively well-mixed shallow sea with an average 500 m depth and gyre-like water movement that drives across the shelf (Smith et al, 2007; Orsi and Weiderwohl, 2009). McMurdo Sound is characterized by an annual sea ice sheet that forms in the winter, melts in the late spring/summer, and feeds into the Ross Sea. Springtime hydrography data suggest that the shallow coastal waters there are well-mixed down to the bottom during the winter (Dinniman et al, 2003; Smith et al, 2007; Figure 1)

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