Basal thermal regime affects the biogeochemistry of subglacial systems

Abstract

<p><strong>Abstract.</strong> Ice formed in the subglacial environment can contain some of the highest concentrations of solutes, nutrients, and microbes found in glacier systems. Upon glacial melt, these materials are released to downstream freshwater and marine ecosystems and glacier forefields. Despite the potential ecological importance of basal ice, our understanding of its biogeochemical characteristics, and their spatial and temporal variability, remains limited. We hypothesize that the basal thermal regime of glaciers is a dominant control on subglacial biogeochemistry because it influences the degree to which glaciers mobilize material from the underlying substrate and controls the nature and extent of biogeochemical activity that occurs at glacier beds. Here, we characterize the solutes, nutrients, and microbes found in the basal regions of a cold-based glacier and three polythermal glaciers and compare them to those found in overlying glacier ice. Compared to its parent glacier ice, basal ice from polythermal glaciers was consistently enriched in major ions, dissolved organic matter (including a specific fraction of humic-like fluorescent material), and microbes, and occasionally enriched in dissolved phosphorus and reduced nitrogen (NH<sub>4</sub><sup>+</sup>) and in a second dissolved component of humic-like fluorescent material. In contrast, the biogeochemistry of basal ice from the cold-based glacier was remarkably similar to that of its parent glacier ice. Although basal ice from the cold-based glacier may have acquired some inorganic and organic nutrients from the underlying substrate, it did not appear to contain significant amounts of either solutes or microbes derived from the glacier bed. These findings suggest that a glacier's basal thermal regime can play an important role in determining the mix of solutes, nutrients, and microbes that are acquired from subglacial substrates and/or produced in situ.</p>

Nutrient and Carbon Export From a Tidewater Glacier to the Coastal Ocean in the Canadian Arctic Archipelago

“…The protein‐like glacier DOM found in meltwater runoff draining Sverdrup Glacier and the humic‐like marine DOM found in the surrounding coastal ocean is consistent with previous findings, indicating that glaciers are microbially based ecosystems capable of supplying comparatively labile DOM to downstream environments (Dubnick et al., 2020; Hood et al., 2009). In the ocean, labile glacial DOM will likely promote secondary productivity, with bacteria and microzooplankton using it as a carbon source.…”

“…A recent study by Dubnick et al. (2020) corroborates this, having found abundant and distinct microbial communities in surface and basal ice at Sverdrup Glacier. The humic‐like component H1 has been found in both marine and terrestrial studies (Coble, 2007; De Souza Sierra et al., 1994; Stedmon et al., 2003) and has been previously observed in basal ice from numerous glaciers on Devon Island (Dubnick et al., 2017).…”

“…Similar to macronutrient concentrations, there was more variability in DOC concentrations in spring glacial samples compared to summer samples, likely representing the larger variety of different sample types collected during the spring season. While summer samples reflect an amalgam of glacier melt from a variety of elevations, spring samples represent distinctly unique glacial sub‐environments, ranging from supraglacial snow and ice to basal ice and waters that have been stored at the bed overwinter (Dubnick et al., 2020). Thus, different geochemistries, deposition regimes, and in situ biological activities could all lead to more variability in the spring samples.…”

“…Sediment‐rich basal ice was collected from the lowest 1 m of ice at the glacier margin and liquid water was collected from within (∼500 m) a marginal channel incised on the eastern side. While these few samples may not be representative of the entire basin, the variety of spring samples gives an appreciation of the variability in the possible nutrient and carbon concentrations across the glacier before the melt season (Dubnick et al., 2020). Bulk ice/snow/water samples were collected aseptically in trace metal clean ProPak® bags (Teledyne ISCO) using an ethanol‐rinsed and flame‐sterilized steel chisel and aluminum ice axe.…”

Trickle and Treat? The Critical Role of Marine‐Terminating Glaciers as Icy Macronutrient Pumps in Polar Regions

“…The authors highlight a potential role for glacial DOC, in contrast to inorganic N, P, and Si. The glacially derived dissolved organic matter (DOM) exhibits fluorophores characteristic of microbially produced highly labile compounds (at least ∼20% more protein‐like fluorescence than marine samples; Dubnick et al., 2020), despite low overall concentrations of DOC (mostly <20 μM), compared to more recalcitrant marine organic matter dominated by humic‐like florescence. Bioavailable glacial DOM is consistent with findings on Arctic land terminating glacier systems (Fellman et al., 2010; Kellerman et al., 2020; Pain et al., 2020), but this is the first demonstration of labile DOM delivery from a marine‐terminating glacier.…”