Biogeochemical evolution of ponded meltwater in a High Arctic subglacial tunnel

Abstract

Abstract. Subglacial environments comprise ∼10 % of Earth's land surface, host active microbial ecosystems, and are important components of global biogeochemical cycles. However, the broadly inaccessible nature of subglacial systems has left them vastly understudied, and research to date has been limited to laboratory experiments or field measurements using basal ice or subglacial water accessed through boreholes or from the glacier margin. In this study, we extend our understanding of subglacial biogeochemistry and microbiology to include observations of a slushy pond of water that occupied a remnant meltwater channel beneath a polythermal glacier in the Canadian High Arctic over winter. The hydraulics and geochemistry of the system suggest that the pond water originated as late-season, ice-marginal runoff with less than ∼15 % solute contribution from subglacial sources. Over the 8 months of persistent sub-zero regional temperatures, the pond gradually froze, cryo-concentrating solutes in the residual water by up to 7 times. Despite cryo-concentration and the likely influx of some subglacial solute, the pond was depleted in only the most labile and biogeochemically relevant compounds, including ammonium, phosphate, and dissolved organic matter, including a potentially labile tyrosine-like component. DNA amplicon sequencing revealed decreasing microbial diversity with distance into the meltwater channel. The pond at the terminus of the channel hosted a microbial community inherited from late-season meltwater, which was dominated by only six taxa related to known psychrophilic and psychrotolerant heterotrophs that have high metabolic diversity and broad habitat ranges. Collectively, our findings suggest that generalist microbes from the extraglacial or supraglacial environments can become established in subglacial aquatic systems and deplete reservoirs of nutrients and dissolved organic carbon over a period of months. These findings extend our understanding of the microbial and biogeochemical evolution of subglacial aquatic ecosystems and the extent of their habitability.

Understanding microbial sourcing in Greenland subglacial runoff

“…Early-season meltwaters more likely comprise a mix of groundwater, basal meltwater and stored surface melt from the previous season retained beneath the ice following drainage system shut down, that would have undergone biogeochemical transformation overwinter (Kellerman et al, 2020;Dubnick et al, 2023). These are https://doi.org/10.5194/egusphere-2024-817 Preprint.…”

“…Therefore, once the subglacial environment has been extensively flushed and drainage system established and stabilised, we can expect a higher proportion of exported microbial assemblages to be derived from glacial surfaces, as well as to consist of more stable and widespread (sub)glacial populations (e.g. putative generalists; Dubnick et al, 2023). This is reflected here by a relative increase during these periods in OTUs related to bacterial populations commonly found on glacial surfaces and/or reported in (sub)glacial systems more broadly A very similar trend in beta-diversity progression linked to relative increased surface-related microbial populations following melt-season progression has been reported in a another, smaller, GrIS catchment in South Greenland (Dubnick et al, 2017).…”

“…OTUs 1 (Rhodoferax), 2 (Polaromonas), 4 (Microbacteriaceae), 5 (Luteolibacter), 25 (Pedobacter); Fig.4, TableS5; e.g Zhang et al (2012),Andrews et al (2018),Kohler et al (2020),. Christner (2022), Rathore et al (2022),Bradley et al (2023), Dubnick et al (2023).…”