Stable isotope evidence for microbial sulphate reduction at the bed of a polythermal high Arctic glacier
Introduction
Recent work has established that glacier beds, although formerly thought to be abiological [1], [2], provide suitable environments for microbial activity [3], [4]. This has resulted in revision of subglacial geochemical weathering mechanisms [5], since microbial activity may drive certain sectors of the bed towards or into anoxia [6]. Significant populations of sulphate-reducing bacteria have been found in subglacial meltwaters from several glaciers [4]. Although this is consistent with there being sulphate reduction in subglacial environments, no geochemical evidence or measurements of this process have been presented to date. Here we report isotopic evidence for microbial sulphate reduction at the bed of a polythermal-based high Arctic glacier. The concentration of SO42− relative to HCO3−, measured by the sulphate mass fraction (SMF; Eq. 1), is calculated for all meltwaters and used to support inferences made on the basis of isotope data [2]:
High SMF values (>0.4) are typically observed in oxic subglacial environments, where carbonate hydrolysis, carbonate dissolution and sulphide oxidation are the dominant reactions [5]. Lower values (<0.4) may be encountered in environments where sulphide minerals are in lesser abundance, e.g. supraglacial streams, or where anoxic conditions prevail and there is sulphate reduction [7], [5]. Finally, we explore possible carbon sources for the sulphate-reducing bacteria and assess whether they have the capacity to utilise bedrock organic matter (kerogen) as an energy source. If this is the case, the sulphate produced from sulphide oxidation in more oxic parts of the hydroglacial system may be reduced in the more anoxic elements, and subglacial environments may act as isolated refugia for life on timescales of >106 yr.
Section snippets
Field Site
Finsterwalderbreen (77°28′N, 15°18′E) is located on the southern side of Van Keulenfjorden, Spitzbergen (Fig. 1A). The active glacier is ∼35 km2 in area, ∼11 km in length and is polythermal [7]. The lithology is mainly sedimentary, with the upper catchment and headwalls consisting of Precambrian carbonates, phyllite and quartzite, Permian sandstones, dolomite and limestones and the rest of the catchment comprising Triassic to Cretaceous siltstones, sandstones and shales [8]. About 70% of the
Methodology
Sampling was conducted during two weeks in the summer of 1997 (2–14 July; days of year 184–195) and a short interval in late winter, 1999 (21–27 April; days of year 111–117). Meltwater samples were collected from the west IMC in summer, and from the SGU in both summer and in winter (Fig. 1B). IMC and SGU samples were collected twice daily (c. 10.00 h and 16.00 h) during summer 1997, in order to sample at approximate times of minimum and maximum flow. Stage in the IMC was also monitored at these
Results
A summary of all solid and aqueous isotope analyses is given in Table 1, Table 2 and Appendix A, Appendix B.
The SMF of IMC and summer/winter SGU waters is plotted against Cl− in Fig. 2A. The latter is derived entirely from snow and icemelt. Snowmelt typically has enhanced concentrations of Cl− relative to icemelt. The Cl− concentration can, therefore, be used to indicate the principle source of water in an IMC and SGU. Summer SGU and IMC samples have an SMF of ∼0.3–0.4, while winter SGU values
Hydrochemistry
Data presented in Fig. 2A confirm the existence of two contrasting systems draining the glacier via the IMC and SGU [7]. Relatively high Cl− concentrations in the SGU are consistent with the partial sourcing of this outflow by concentrated snowmelt within the upper part of the catchment. An SMF of <0.4 (Fig. 2A) and relatively high SO42− concentrations [7] in this closed system indicate sulphide oxidation in a subglacial environment that may be also undergoing sulphate reduction [7]. During
Conclusions
These data show that elements of both the glacier bed and margins become anoxic due to microbial activity given suitable conditions, here a ready supply of sulphides and kerogen from shale and hydroglacial flowpaths that prohibit free exchange with the atmosphere. Isotopic evidence of sulphate reduction in subglacial and ice-marginal meltwaters suggests that bacteria are able to exploit these conditions of increasing anoxia. The extent of sulphate reduction is greatest at the glacier bed. These
Acknowledgements
This work was supported by NERC Grant No. GR9/2550. We would like to thank Mike Gardiner and Richard Hodgkins for assistance in the field, Jenny Mills for conducting the major ion analysis and Rob Newton for assistance with isotope analyses. Thanks also to Jonathan Tooby and Drew Ellis for their help with preparing the figures.[BW]
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2020, Journal of HydrologyCitation Excerpt :Sulfide oxidation can be sustained under anoxic conditions in deeper flows of glacial systems with possible effects on isotope composition of dissolved SO42− (e.g. Bottrell and Tranter, 2002; Wadham et al., 2007; Hindshaw et al., 2016). However, previous studies mainly focused on measuring the isotope compositions of SO42− in localized areas, usually small glacial catchments and their proglacial zones (Bottrell and Tranter, 2002; Wadham et al., 2004, 2007; Szynkiewicz et al., 2013; Ansari, 2016). Conversely, regional comparisons between isotopic compositions and deeper groundwater flows are lacking.