(Submitted Abstract to the 2003 Geological Society of America Meeting, Seattle, Washington)
Microbial influence on sulfur speciation in Lower Kane Cave, WY
* Mabin, Katrina, Bennett,
Philip, Stern, Libby A.,
and Engel, Annette S.
University of Texas at Austin, Department of Geological
Sciences, Austin, TX 78712 United States
A distinctive microbial community is being studied
within Lower Kane Cave (LKC) of the Madison Limestone, near Lovell, WY, where
the cave forming process is principally sulfuric acid replacement of limestone
with gypsum. The aquatic microbial mat includes a consortium of both S-reducing
and S-oxidizing bacterial communities, which cycle sulfur along the reach of the
cave stream. Multiple techniques are being employed to characterize the
speciation and distribution of sulfur within LKC in order to identify the
individual metabolic pathways, and to what degree sulfur chemistry within the
cave is controlled by microbial processes. Aqueous sulfide levels were
determined immediately in the field using colorimetric methods and
volatilization was directly measured by field GC. Dissolved sulfide levels
generally decrease with distance from the stream source, ranging from 0.85 to
0.03 ppm. Volatilization increases over the microbial mats however due to local
sulfide production by sulfate reducing bacteria. Cave water, sediment and
microbial biomass were sampled from the cave and characterized for major element
and sulfur chemistry. Laboratory HPLC determination of transient aqueous sulfoxy
species was done to characterize intermediate species, and low concentrations of
thiosulfate and trace polythionates were detected. Sediment samples were
analyzed for total sulfur and operational sulfur fractions, including acid
volatile sulfur (AVS), total reducible sulfur, pyrite and elemental S. Elemental
analysis was used to determine the distribution of total S within sediment and
biomass to identify potential sulfur storage within the system. Total S ranges
from 0.35% dry weight in sediment to 51% dry weight in mats. Operational sulfur
fractions were isolated using a modified Johnson-Nishita method, and AVS
fractions range up to 0.2% (wt/wt). The presence of microbial mats appears to
enhance volatilization of sulfur gases by mechanisms as yet unknown. Correlation
of S distribution and speciation with the current microbial communities and
stream morphology within LKC indicates influences from both biotic and abiotic
processes. The results from this study, however, suggest that microbial
consumption dominates over abiotic auto-oxidation and volatilization of
dissolved sulfides, while the anaerobic community provides an additional source
of reduced S.
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