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GEOCHEMICAL EXPLORATION
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Journal of Geochemical Exploration 78-79 (2003) 71-74 www.elsevier.colrdlocate/jgeoexp
Abstract
Biogeochemical sulphur cycle in an extreme environment beneath a high arctic glacier, Nunavut, Canada
life
Stephen E. Grasby a'*, Carlton C. Allen b, Teresa G. Longazo c, John T. Lisle d, Dale W. Griffin d, Benoit Beauchamp a aGeological Survey of Canada (Calgary), Natural Resources Canada, 3303 33rd Street NW,, Calgary, AB, Canada, T2L 2A7 bNASA Johnson Space Center, Houston, TX 77058, USA CHernandez Engineering, Houston, TX 77058, USA d United States Geological Survey Center for Coastal and Watershed Research, St. Petersburg, FL 33701, USA
Abstract
Unique springs discharge from the surface of a high arctic glacier, releasing H2S, and depositing native sulphur, gypsum, and calcite. A rare CaCO3 polymorph, vaterite, is also observed. Physical and chemical conditions of the spring water and surrounding environment, as well as mineralogical and isotopic signatures, argue for biologically mediated redox reactions controlling sulfur. Cell counts and DNA analyses, confirm bacteria are present in the spring system. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Sulfur springs; Subglacial; Bacteria; Arctic
1. Introduction
Ten sulfur springs and seeps were observed discharging from the surface o f an approximately 200m-thick glacier, located in a low valley, informally named Borup Fiord pass, in the eastern Krieger Mountains, on northern Ellesmere Island, Nunavut, Canadian High Arctic (Fig. 1). The climate at the site is characterized by extremely cold and dry conditions, with a mean annual air temperature of - 19.7 °C ( - 3 6 . 1 °C in January; +5.4 °C in July) and an annual precipitation o f less than 100 m m (Environment Canada, unpublished data). The springs have
two modes o f occurrence: (1) on the fiat glacial surface they occur as central mounds, comprised o f varying amounts o f native sulphur, gypsum, calcite, and ice, that stand up to 30 cm high, and (2) spring outlets along a steep incised supra-glacial melt-water channel wall have native sulphur that is thinly dispersed over several square meters of the ice surface around the discharge sites. Intermittent odors of H2S are also common in the area. Observations in 3 consecutive years indicate discharge sites are unstable and can completely disappear within 1 year.
2. Results
* Corresponding author. Tel.: +1-403-292-7111; fax: +1-403292-5377. E-mail address."
[email protected](S.E. Grasby).
Water temperatures o f the spring outlets are low ( 1 2 °C), but higher than surface melt water (typically
0375-6742/03/$ - see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0375-6742(03)00026-8
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Abstract
Fig. 1. Regionaland detailedmapsof the studyarea, with geologymodifiedfromThorsteinsson(1974). The Eurekaweatherstationis indicated. around 0.2 °C). The spring waters have pH values of 9.0-9.5, distinctly different from glacial melt water streams and pools without sulphur that have pH values of 4 - 6 . Conductivity values range from 115 to 230 gS, as compared to below 2 gS for melt water elsewhere on the ice. H S - concentrations of up to 0.2 mg/1 were measured. The spring waters are over-saturated with respect to calcite, and under-saturated with respect to gypsum. The stable isotope data for spring waters define a slope of 7.6, similar to the meteoric water line for Eureka (140 km to the SW): 3D=7.43180-9.1 (IAEA/WMO, 1998). The springs have a greater deuterium excess than precipitation at Eureka ( - 20 for the springs as compared to the average of 8.2 for Eureka), however the deuterium excess for the springs is consistent with the range observed for glacial melt waters in the area. Estimated mineral abundance determined by XRD of precipitates varies among samples: sulphur ( 0 99%); calcite (12-75%); gypsum/bassanite (0-99%). At one site, the presence of vaterite, a rare CaCO3 polymorph, was detected. The 334S of native sulfur is fairly consistent among the springs, ranging from 7.7 %0 to 11.4 %o. Gypsum has higher values and generally shows a
wider range, from 18.2 %0 to 27.8 %0. The A334Sgyp_s values vary from 9.4%0 to 19.2%o. For most carbonate samples, the 313C and 3180 values of the precipitates show a fairly narrow range, + 3.2 %0 to +3.9%o and 9.8%0 to 12.9%o, respectively. Two samples have distinctly higher 313C values (5.0%0 and 5.3 %0) and lower 6180 values ( - 6.7 and - 0.8), respectively. Total cell counts for spring water samples ranged from 2 x 104 to 3 x 104 cell/ml. Limited number of sequenced isolates identified bacteria strains that are related to either true psychrophiles or mesophiles that have been shown to inhabit very cold environments, and suggest that a complex community of bacteria live within the glacial system in this study.
3. Discussion
Spring waters at the glacier surface exsolve H2S and deposit gypsum and native sulfur. The simultaneous presence of sulfur in three oxidation states suggests a combination or series of redox reactions. The high 634S values of gypsum precipitates suggests dissolution of anhydrite beds of the Early Pennsylvanian Otto Fiord Formation is the primary S source in
Abstract
the system. Dissolution of anhydrite at depth and subsequent sulfate reduction must be intermediate steps in the formation of native S deposits (e.g., Machel, 1992; Postgate, 1984; Brock and Madigan, 1991). The low temperature conditions and low geothermal gradient in the region (22 °C/km, Taylor and Judge, 1977) suggest bacteria are controlling sulfate reduction. Given that hydrogen sulfide will inorganically oxidize to valence states higher than zero under high pH conditions and environments that are strongly oxidizing, the high pH of the Borup Fiord pass waters (9.0-9.5), and the fact that native sulfur is forming in the open atmosphere (i.e., high pO2) environment, argues for a biologically mediated oxidation of H2S to native sulfur. The ~348 values for the Otto Fiord anhydrites (14.6 %0) fall between values for precipitates of native sulfur (7.7-11.4%o) and gypsum (18.2-27.8 %o) at the spring sites, consistent with a bacterially mediated Rayleigh fractionation, and with other sulfur springs with active bacterial sulfate reduction (e.g., Grasby et al., 2000) as well as with bioepigenetic native sulfur deposits associated with evaporites (e.g., Niec, 1992). Given the above, we ague that bacterially mediated reduction of sulfate to H2S, and subsequent bacterially mediated oxidation of H2S to native sulfur, is the only feasible means to produce the deposits at Borup Fiord Pass. This is confirmed by analysis of the spring waters showing the ubiquitous presence of bacteria. The total cell counts for five springs, ranging from 2 x 104 to 3 x 10 4 cell/ml, are similar to total bacteria counts for melt water in other glaciers (Sharp et al., 1999). Additionally, many sulfur particles are partially enmeshed in carbon-rich webbing, fractions of a micrometer thick (possibly dehydrated EPS biofilm, Fig. 2). The proposed model for microbial sulfate reduction, followed by microbial oxidation of sulfides to native sulfur, requires bacteria with specific metabolic capabilities to operate in a near-freezing environment. Such capabilities have recently been reported. Anaerobic sulfate reducing bacteria occur in the basal ice layer of an Ellesmere glacier and metabolize at temperatures just above freezing (Skidmore et al., 2000). Sulfide oxidizing bacteria isolated from the basal ice layer of a Swiss glacier metabolize at temperatures of 4 °C (Sharp et al., 1999). In addition to S minerals, vaterite was identified at one location (Grasby, in press). Vaterite is an important
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Fig. 2. SEM photos of precipitates from the sulphur springs showing: (a) sulphur particles partially enmeshed in a carbon-rich webbing of dehydrated EPS (biofilm), E-SEM secondary electron image, 10 kV.
mineral in exobiology studies as it has been suggested that the spherical habit can pseudo-morph biogenic carbonate structures (Vecht and Ireland, 2000). The rare occurrence of vaterite at this site is likely to be related to the unusual conditions of an extremely cold climate, supersaturated conditions, alkaline waters, and presence of C a S O 4. This natural occurrence provides insight into environments that this mineral may form.
4. Conclusions
Combining geochemical, stable isotope, and geologic data, we argue that the sulfur springs at Borup Fiord pass are the surface expression of a subglacial circulation system. The site described here indicates that spring systems which harbor microbiological communities can operate in extreme cold environments (average air temperatures of - 1 9 . 7 °C) and can discharge through solid ice. This conclusion supports the idea that life can exist under extreme environmental conditions in isolated geothermal refuges, such as under Snowball Earth conditions and perhaps on Mars or Europa. Discharges of water from such refuges may bring to the surface living microbes, as well as mineralogical and isotopic indications of subsurface life.
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Abstract
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