Basal thermal regime affects the biogeochemistry of subglacial systems

Abstract

Abstract. Ice formed in the subglacial environment can contain some of the highest concentrations of solutes, nutrients, and microbes found in glacier systems, which can be released to downstream freshwater and marine ecosystems and glacier forefields. Despite the potential ecological importance of basal ice, our understanding of its spatial and temporal biogeochemical 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 of meteoric origin. Compared to meteoric 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 was occasionally enriched in dissolved phosphorus and reduced nitrogen (NH4+) 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 meteoric glacier ice. 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 or produced in situ.

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

“…The marine humic-like component H2 has also previously been found in basal ice from Devon Island (Dubnick et al, 2017), Greenland (Kellerman et al, 2020;Pain et al, 2020), and Alaska marine DOM (Walker et al, 2009). 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.…”

“…The elevated ammonium concentrations observed here may also indicate microbial degradation of glacial DOM (Kumar et al, 2016) and/or may be partially sourced from rock weathering in the subglacial environment (Holloway & Dahlgren, 2002). 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).…”

“…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.…”

“…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.…”

“…At the bed, glacial meltwater can become chemically enriched in crustally derived nutrients (e.g., silica, iron, and phosphorus) and carbon Hawkings et al, 2016Hawkings et al, , 2017Hood et al, 2009) before discharging into the marine environment (Kanna et al, 2018). Numerous studies suggest that in situ microbial communities on the glacier surface or at the bed are capable of high rates of biogeochemical/physical weathering and cycling of organic carbon (Dubnick et al, 2017(Dubnick et al, , 2020. In situ microbial nitrogen fixation at the glacier bed is a second important source of glacial nitrate that may be delivered to marine waters (Boyd et al, 2011;Segawa et al, 2014;Telling et al, 2012;Wadham et al, 2016).…”