Inputs of glacially derived dissolved and colloidal iron to the coastal ocean and implications for primary productivity

Abstract

Glacial meltwaters draining shield bedrock under the Greenland Ice Sheet (GIS) contain <0.4 μm “total dissolved” Fe (TDFe) with an average flow weighted concentration of ∼53 nM. The concentrations of <0.03 and 0.03–0.4 μm Fe vary over the ablation period, with weighted means for each of these fractions being respectively 22.4 nM and 30.8 nM. These concentrations are lower estimates as an adjacent larger glacier (a more representative source of glacial meltwater) had higher dissolved Fe concentrations, and reactions of meltwaters with proglacial sediments could also enhance dissolved Fe concentrations. This source of TDFe is additional to the reactive (oxyhydr)oxide phases identified by Raiswell et al. (2006) that are also introduced to adjacent polar seas from glaciers. The Fe concentrations in the shield bedrock underlying the GIS are lower than those of other crustal rocks (4.0% cf. 6.2%), but we argue that these Fe concentrations are not limiting on the total dissolved Fe concentrations we measure. The biogeochemical weathering processes operating on the subglacial debris and suspended sediment in our catchment are likely to be similar to those in other glaciated catchments. Therefore the meltwater Fe concentrations reported here can be used to give a first estimate of global fluxes of meltwater dissolved Fe to coastal polar waters. A lower estimate of the global flux of TDFe from glacial meltwaters is ∼75 × 106 moles Fe/a. This glacial meltwater input of Fe to adjacent polar waters will be greatest around Greenland where there are highest annual meltwater discharges. However, the greatest impact of this source of glacial meltwater Fe is anticipated to be in Antarctic high nutrient low chlorophyll (HNLC) waters where phytoplankton productivity is typically limited by availability of Fe. For Antarctic waters the estimated meltwater Fe (TDFe) input is about 10% of that suggested to come from sea ice melting, but glacial inputs continue throughout the austral summer ablation period after sea ice melt is complete.

Enhanced Southern Ocean marine productivity due to fertilization by giant icebergs

“…Recently, concern over the stability of the West Antarctic Ice Sheet has arisen 4,23 , with implications for more ice discharge in the future and thus carbon drawdown through fertilization. Note that even an increase in regional sediment-rich ice sheet meltwater into coastal waters can lead to enhanced fertilization 10,12,24 , although that associated with giant iceberg melting may be even greater (Figure 3). The future may therefore see an increase in Southern Ocean carbon sequestration through this iceberg fertilization mechanism, acting as a secondary negative feedback on climate change.…”

Iron biogeochemistry across marine systems – progress from the past decade

“…2.2). Moreover, Statham et al (2008) recently addressed glacier meltwater input of iron and colloidal matter from the Greenland Ice Sheet. In the context of the expected changes for Fe biogeochemistry discussed here, their study illustrates how atmospheric warming can act on various levels, evidentially affecting iron biogeochemistry in the sea.…”

“…Estimates of these sources' magnitudes are poorly constraint. Recent studies have highlighted the importance of these sources Lannuzel et al, 2007;Statham et al, 2008;Aguilar-Islas et al, 2008;Raiswell et al, 2008Raiswell et al, , 2006Smith et al, 2007;Croot et al, 2004). Iron accumulates in sea ice with concentrations one to two orders of magnitude higher than the underlying seawater.…”

The Iron Biogeochemical Cycle Past and Present

“…Wadham et al (2012) ). Statham et al (2007) have analysed meltwater from two Greenland glaciers using rigorous trace metal clean procedures and sequential filtration. The weighted means of the iron concentrations in the <0.03 µm (assumed to be aqueous) and 0.03-0.4 µm (assumed to be colloidal/nanoparticulate) filtered size fractions were 22.4 and 30.8 nM respectively.…”