The heterogeneous nature of Fe delivery from melting icebergs

Abstract

The micronutrient iron (Fe) can be transported from marine terminating glaciers to the ocean by icebergs. There are however few observations of iceberg Fe content, and the flux of Fe from icebergs to the offshore surface ocean is poorly constrained. Here we report the dissolved Fe (DFe), total dissolvable Fe (TdFe) and ascorbic acid extractable Fe (FeAsc) sediment content of icebergs from Kongsfjorden, Svalbard. The concentrations of DFe (range 0.63 nM -536 nM, mean 37 nM, median 6.5 nM) and TdFe (range 46 nM -57 µM, mean 3.6 µM, median 144 nM) both demonstrated highly heterogeneous distributions and there was no significant correlation between these two fractions. FeAsc (range 0.0042 to 0.12 wt. %) was low compared to both previous measurements in Kongsfjorden and to current estimates of the global mean. FeAsc content per volume ice did however, as expected, show a significant relationship with sediment loading (which ranged from < 0.1 -234 g L -1 of meltwater). In the Arctic, icebergs lose their sediment load faster than ice volume due to the rapid loss of basal ice after calving. We therefore suggest that the loss of basal ice is a potent mechanism for the reduction of mean TdFe and FeAsc per volume of iceberg. Delivery of TdFe and FeAsc to the ocean is thereby biased towards coastal waters where, in Kongsfjorden, DFe (18 ± 17 nM) and TdFe (mean 8.1 µM, median 3.7 µM) concentrations were already elevated.

Retrieval of Ice Samples Using the Ice Drone

“…in iceberg samples from around the world, which was reported to be around 2.7-17 µM (Raiswell et al, 2016;Hopwood et al, 2017). The more crystalline Fe phases were found to vary globally between 17 and 64 µM (Raiswell et al, 2016), which also fits with our 6 M HCl extractable Fe(III) fraction.…”

“…Icebergs can contain terrigenous sediments (Azetsu-Scott and Syvitski, 1999;Death et al, 2006;Smith et al, 2007Smith et al, , 2013Mugford and Dowdeswell, 2010;Bigg, 2016), with considerable variability in concentration and distribution within a given iceberg, and in size fraction. These sediments are released into the marine environment as icebergs melt and may be an important source of iron, which can enhance marine primary productivity (Raiswell et al, 2008(Raiswell et al, , 2016Lin et al, 2011;Shaw et al, 2011;Bigg, 2016;Duprat et al, 2016;Hopwood et al, 2017Hopwood et al, , 2019Hawkings et al, 2018). Glacial meltwater has also been shown to contain nutrients and silica, which may also enhance primary productivity (Azetsu-Scott and Syvitski, 1999;Meire et al, 2016).…”

“…The relatively small number of samples is understandable when the logistical and safety aspects of sample collection are considered. Iceberg samples have been retrieved using helicopters (Diemand, 1984;Gagnon and Gammon, 1997), and from small boats, either by chipping pieces off with an ice axe (Hawkings et al, 2018) or by collecting floating pieces of glacial ice from the water (Hopwood et al, 2016(Hopwood et al, , 2017Meire et al, 2016). Helicopters have also been used to deploy GPS trackers (Sutherland et al, 2014;Jones and Gudmundsson, 2015;Larsen et al, 2015;Sulak et al, 2017) and wave motion sensors (Lever et al, 1991).…”

Sensitivity of ocean biogeochemistry to the iron supply from the Antarctic Ice Sheet explored with a biogeochemical model

“…This possible biased behavior of our model may result from missing, misrepresented processes or sources that may supply macronutrients in the mixed layer such as the oceanic circulation in ice shelf cavities (Jacobs et al, 2011;Herraiz-Borreguero et al, 2015;White et al, 2019) or the melting of ice shelf and ice sheet (Pritchard et al, 2012;Arrigo et al, 2015Arrigo et al, , 2017Hawkings et al, 2015;Wadham et al, 2016;St-Laurent et al, 2017). Another process that can be advocated is the entrainment of nutrient-rich waters by subglacial discharge plumes induced by basal melting of grounded glaciers, a physical mixing process observed in the western Antarctic Peninsula (Cape et al, 2019) and for Greenland glaciers (Meire et al, 2017;Hopwood et al, 2018;Kanna et al, 2018).…”

Review Article: How does glacier discharge affect marine biogeochemistry and primary production in the Arctic?

“…Variations in the ratio of dissolved NO 3 and Fe in surface waters (< m) adjacent to glaciated regions: whiskers show th and 90 th percentiles; bars median, 25 th and 75 th percentiles; dots all outliers. Data from: Western Antarctic Peninsula (WAP,Annett et al, 2017; Ducklow et al, 2017), the South Greenland shelf(Achterberg et al, 2018; Tonnard et al., 2018),Godthåbsfjord (Hopwood et al, 2016), Kongsfjorden(Hopwood et al, 2017), the…”