Stable isotope evidence for microbial sulphate reduction at the bed of a polythermal high Arctic glacier

doi:10.1016/S0012-821X(03)00683-6

Earth and Planetary Science Letters

Volume 219, Issues 3–4, 15 March 2004, Pages 341-355

https://doi.org/10.1016/S0012-821X(03)00683-6 Get rights and content

Abstract

Glacier beds may be host to a range of microbial communities, which drive oxic waters towards anoxia along certain hydrological flowpaths. Chemical and isotopic signatures in meltwaters from Finsterwalderbreen, a polythermal glacier on sedimentary bedrock in Svalbard, show clear evidence for anoxia at the glacier bed. Increases in δ³⁴S and δ¹⁸O of sulphate indicate that microbial sulphate reduction has resulted in significant decreases in sulphate concentration. The δ¹³C of the dissolved inorganic carbon (DIC) is isotopically light (δ¹³C=−8‰), which is consistent with the use of bedrock kerogen and/or the necromass of sulphide oxidising bacteria as organic substrates for the sulphate-reducing bacteria. Calculated rates of organic carbon mineralisation correspond to ∼10% of the total annual DIC flux of the glacial meltwaters. This microbial ecosystem is chemoautotrophically based, ultimately being sustained by the kerogen and/or bacterial necromass and sulphides in the bedrock. This work suggests that glacier beds can be refugia for life when climatic and/or atmospheric conditions are otherwise inclement and also supports the contention that microbial life is present in subglacial Lake Vostok.

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]: $SMF = SO^{2−}_{4} (SO^{2−}_{4} + HCO^{−}_{3})$

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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Citation Excerpt :
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Stable isotope evidence for microbial sulphate reduction at the bed of a polythermal high Arctic glacier

Abstract

Introduction

Section snippets

Field Site

Methodology

Results

Hydrochemistry

Conclusions

Acknowledgements

Chem. Geol.

Geochim. Cosmochim. Acta

Fuel

Geochim. Cosmochim. Acta

Chem. Geol.

Fuel

Geochim. Cosmochim. Acta

Chem. Geol.

J. Hydrol.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Chemical models of solute acquisition in glacial meltwaters

J. Glaciol.

A conceptual model of solute acquisition by Alpine glacial meltwaters

J. Glaciol.

Bacterial populations at glacier beds and their relationship to rock weathering and carbon cycling

Geology

Microbial life beneath a high Arctic glacier

Appl. Environ. Microbiol.

Geochemical weathering at the bed of Haut Glacier d’Arolla, Switzerland – a new model

Hydrol. Process.

Sulphide oxidation under partially anoxic conditions at the bed of the Haut Glacier d’Arolla, Switzerland

Hydrol. Process.

The hydrochemistry of meltwaters draining a High Arctic, polythermal-based glacier, south Svalbard: I. The ablation season

Hydrol. Process.