Abstract
ABSTRACT Glaciers overrun organic matter (OM) during periods of advance, making this overrun OM available as a metabolic substrate for subglacial microbes. The biogeochemical fate of this overrun OM remains poorly understood, ultimately limiting our understanding of subglacial biogeochemical cycling, particularly for cold-based glaciers. This study presents evidence for the biogeochemical transformation of algal mat material that was overrun by a cold-based glacier (Suess Glacier, Taylor Valley, Antarctica) during its advance 4840–3570 years BP. We use a suite of stable isotope analyses to show that active nitrogen cycling has depleted N-isotope values to amongst the lowest reported in Taylor Valley (-15.59 ‰) from an initial value of ~-1.88 ‰, while potentially depleting C-isotope values by 2.46 ‰. While this study examines biogeochemical conditions beneath a single glacier, all glaciers export meltwater during the melt season that may host algae and other OM in proglacial streams and lakes that may be overridden during glacier advance. As such, subglacial nitrogen cycling detected here may represent processes that occur in cold zones beneath glaciers generally.
Highlights
Once considered to be biologically sterile, glacial and subglacial environments are recognized as microbially active and biogeochemically relevant ecosystems (Anesio and Laybourn-Parry 2012; Fountain and Tranter 2008; Hodson et al 2008; Stibal, Sabacka, and Zarsky 2012)
The soil data points surrounding the endolith-derived organic matter (OM) are generally from samples collected higher above Taylor Valley lakes and, presumably, are derived from OM remaining from previous high-lake stands (Burkins et al 2000)
The Lake Chad algae has a δ13C of −10.29‰ ±0.05 (n = 2) and a δ15N of −1.88‰ ±0.02 (n = 2), suggesting that it is of shallow lacustrine or surface origin, which is in accordance with the environment from which it was sampled
Summary
Once considered to be biologically sterile, glacial and subglacial environments are recognized as microbially active and biogeochemically relevant ecosystems (Anesio and Laybourn-Parry 2012; Fountain and Tranter 2008; Hodson et al 2008; Stibal, Sabacka, and Zarsky 2012). The biogeochemical fate of this legacy OM in glacial environments remains poorly constrained, limiting our understanding of the global significance of glacial environments in global biogeochemical cycling. This is true of cold-based glaciers (those that are frozen to the glacier bed), tidewater glaciers, and ice sheets where subglacial meltwater cannot necessarily be used to monitor biogeochemical processes occurring subglacially. Subglacial environments beneath large ice sheets represent an unknown in our understanding of the global nutrient cycling, where the potential exists for biogeochemically altered substrates to produce potentially important metabolic byproducts such as methane, carbon dioxide, or NOx species. We use a suite of isotopic analyses to show that subglacial heterotrophy affects nitrogen cycling using an ancient organic substratum
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