The 79°N Glacier cavity modulates subglacial iron export to the NE Greenland Shelf

Abstract

Approximately half of the freshwater discharged from the Greenland and Antarctic Ice Sheets enters the ocean subsurface as a result of basal ice melt, or runoff draining via the grounding line of a deep ice shelf or marine-terminating glacier. Around Antarctica and parts of northern Greenland, this freshwater then experiences prolonged residence times in large cavities beneath floating ice tongues. Due to the inaccessibility of these cavities, it is unclear how they moderate the freshwater associated supply of nutrients such as iron (Fe) to the ocean. Here, we show that subglacial dissolved Fe export from Nioghalvfjerdsbrae (the ‘79°N Glacier’) is decoupled from particulate inputs including freshwater Fe supply, likely due to the prolonged ~162-day residence time of Atlantic water beneath Greenland’s largest floating ice-tongue. Our findings indicate that the overturning rate and particle-dissolved phase exchanges in ice cavities exert a dominant control on subglacial nutrient supply to shelf regions.

Trace Element (Fe, Co, Ni and Cu) Dynamics Across the Salinity Gradient in Arctic and Antarctic Glacier Fjords

“…In contrast to dFe, summertime TdFe declined steadily along Nuup Kangerlua in all transects, from peak concentrations of 1.3-14.8 µM in the inner fjord to 100-300 nM in the fjord mouth. This is similar to observations elsewhere of rapid TdFe loss with distance from glacial outflows compared to dFe (Kanna et al, 2020) and consistent with either sedimentation and/or a gradual shift towards reduced particle lability with sedimentation likely accounting for most of the decline (Krisch et al, 2021). Whilst fewer transects are available for Ameralik (only May and August 2019), similar conclusions can be drawn.…”

“…A further implication of the Si* trend observed for Arctic sites (Figure 4), is that the field surveys herein are likely capturing a large part of the regional scale over which glacial outflow directly influences macronutrient stoichiometry of outflowing nearsurface waters. Si* is slightly negative in the relatively high macronutrient saline inflowing waters, which have been characterised previously at two of the sites discussed herein (−3.5 ± 1.8 µM for Nuup Kangerlua, Meire et al, 2017; and −5.0 ± 0.3 µM for 79NG, Krisch et al, 2021). This is similar to shelf surface waters elsewhere around Greenland (e.g.…”

“…This continuous seawater supply was sampled inside a laminar flow hood. On RV Polarstern during GEOTRACES cruise PS100/ GN05 in Fram Strait, near-surface samples (<10 m depth) were collected from Go-Flo samplers (Ocean Test Equipment) mounted on an epoxy-coated, trace metal clean rosette (Seabird, see Krisch et al, 2021). In other fjords, sampling from small rigid inflatable vessels was conducted from surface waters using acid cleaned 1 L high-density polyethylene bottles deployed upwind of the boat, which were rinsed once and then filled and sub-sampled.…”

“…To verify the accuracy of trace element measurements reference materials were analysed alongside samples (Supplementary Table S1). Coastal concentrations for other sections of the Greenland shelf were obtained from cruises GEOVIDE (stations 53, 56,60, 61 and 63;Tonnard et al, 2020), D354 (profiles 10-14; Achterberg et al, 2018;Achterberg et al, 2020) and PS100 (east of 15°W, as per Figure 1, Krisch et al, 2021) with sample handling, analysis and quality control following GEOTRACES protocols (Cutter and Bruland, 2012).…”

“…Yet dCu is also less clearly related to Si*, implying that any glacially-associated sources from these elements are too minor to be evident, or that Si* and trace metal sources are spatially de-coupled. Sedimentary exchange, melting icebergs, and discharge from land-or marine-glaciers are all likely to exert differing degrees of influence on the distribution of all elements considered herein (Wehrmann et al, 2013;Hawkings et al, 2020;Krisch et al, 2021).…”