Iron Supply by Subglacial Discharge Into a Fjord Near the Front of a Marine‐Terminating Glacier in Northwestern Greenland

Abstract

Glacial fjords in Greenland show high productivity owing to the runoff of meltwater from the glaciers. Macronutrient dynamics (of nitrate, phosphate, and silicate) associated with subglacial discharge plumes in front of marine-terminating glaciers are widely cited as important drivers of summer phytoplankton blooms in the fjords. However, the dynamics of iron (Fe), an essential micronutrient for primary production, remain largely unstudied in glacial fjords. To investigate the role of subglacial discharge plumes in Fe supply processes in glacial fjords, a comprehensive survey of Bowdoin Fjord, adjacent to the marine-terminating Bowdoin Glacier in northwestern Greenland, was conducted. The subglacial discharge of Fe-rich meltwater induces a buoyancy-driven upwelling plume in front of the glacier that entrains nutrient-rich Arctic-and Atlantic-origin waters. The plume water potentially carried 4.5-8.7 × 10 7 g day −1 of total dissolvable Fe out of Bowdoin Fjord in summer. The concentration of dissolved Fe (dFe) in the plume water (~15.6 nmol kg −1) was 4 times higher than that in the water in the outer part of the fjord (~3.8 nmol kg −1). The dFe:nitrate + nitrite ratio (mmol mol −1) in the plume water varied between 0.58 and 3.2, several orders of magnitude higher than the phytoplankton cellular Fe:nitrate ratio estimated using the hypothetical Fe:C ratio and observed particulate organic carbon:nitrate ratio of the fjord. Hence, the plume water is replete with Fe with respect to phytoplankton demands. Subglacial discharge drives the upwelling of Fe and macronutrients toward the euphotic zone, which is vital for the generation of summer phytoplankton growth in glacial fjords.

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

“…However, the absence of a correlation between dFe and either LpFe or TdFe in the 79NG dataset remains even if surface water masses are excluded. Dissolved Fe and total dissolvable Fe data from other, smaller Arctic glacier catchments including Godthåbsfjord 41 , Kongsfjorden (Svalbard) 21 , Bowdoin Fjord 43 and the Gulf of Alaska 16 , 61 corroborate our observations near the 79NG of intense localized particulate Fe enrichment and decoupling of dFe and LpFe concentrations. This suggests that the distribution of dFe downstream of the 79NG is not unusual compared to other Arctic glacier fjords despite the presence of an ice tongue.…”

“…Surface dFe concentrations close to the 79NG (1.5 nM at 5 m, S1) and neighboring glacier termini (2.5 nM at 10 m, S11) were comparable to, or lower than, concentrations downstream of other Arctic glacier catchments at similar salinities (e.g., 4–7 nM, Kongsfjorden 21 ; <3–6 nM, Godthåbsfjord 41 ; ~4 nM, Copper River estuary 42 ; ~4 nM, Inglefield Bredning 43 ). Whilst a pronounced peak in surface dFe was observed beyond the NE Greenland shelf break in Fram Strait (S7, Fig.…”

“…It should be noted however that the extent to which dFe concentrations change along freshwater water courses and during mixing with saline waters is poorly constrained. The extent of dFe removal on timescales of hours to days may vary both between catchments and temporally with estimates ranging from 76% to 99% dFe loss 16 , 21 , 43 . Fluxes derived only from measured freshwater concentrations are thereby subject to uncertainty of at least an order of magnitude.…”

“…We suggest a key reason for similarities in dFe concentrations between these diverse field sites is the prolonged residence time of water in ice cavities that allows thermodynamic equilibrium to be approached between dFe, LpFe and ligand concentrations. Higher dFe concentrations on the order of 10–100 nM are invariably associated with runoff corresponding to much more recent time periods since freshwater entered the marine environment 9 , 21 , 43 and thus could be described as under kinetic control. In a few case studies the timeframe of lateral dFe export has been quantified.…”

Glaciers and Nutrients in the Canadian Arctic Archipelago Marine System

“…However, such enrichments from glacial sources were hypothesized based on recently reported distributions of dissolved Mn and Fe in the broader CAA region (Colombo et al, 2020). Others have also observed increases in Fe availability as a result of direct glacial input in glacially impacted systems in Greenland (Hawkings et al, 2014;Hawkings, Benning, et al, 2018;Kanna et al, 2020) and Antarctica (Annett et al, 2017;Henkel et al, 2018).…”