Implications of analyzing isotope chemostratigraphy variations in laminar and clotted
layers of stromatolitic extremophiles, Storrs Lake, San Salvador, Bahamas. NATHAN
LANDRUM and FRANK R. ETTENSOHN, Department of Earth & Environmental
Sciences, and MARC F. ETTENSOHN, Biotechnology Program, University of
Kentucky, Lexington, KY 40506.
Characterizing extremophiles, organisms which thrive in extreme environmental conditions, is becoming
increasingly important as science tries to grapple with the origin of life and the possibility of extraterrestrial
life. Bahamian inland lakes are commonly characterized by hyper- or hyposaline, “extreme” environmental
conditions perfect for such organisms. One such lake, Storrs Lake on the Bahamian island of San Salvador,
harbors easily accessible bacterial, archaean, and eukaryotic extremophile assemblages in the form of
thrombolites and stromatolites. Storrs Lake is a hypersaline, alkaline lake which varies drastically in both
pH and salinity. The salinity variations are controlled by inflow from a marine karstic system, rain, and
evaporation. The stromatolites in Storrs Lake are carbonate structures precipitated in finely laminated
layers to well-cemented thrombolitic clots with inner voids. Previous work has suggested that variations in
salinity may control this change in morphology. δO18 and δC13 isotope analyses have been used as
indicators of paleoclimate conditions such as temperature and salinity. The heavier O18 isotope is more
indicative of higher temperatures and salinities, C13 can also be indicative of salinity as well as bicarbonate
precipitation. We examined δO18 and δC13 stable-isotope ratios from a single, Storrs Lake stromatolite head
to see if a correlation between isotope ratios and morphology exists; such correlation would suggest an
environmental control on stromatolite morphology.
In our project, a stromatolite head was collected, sawed in half, and photographed. Patterns of
laminated versus thrombolitic layers were noted, individual layers were classified as thrombolitic clots or
laminae, and samples were taken accordingly for isotope analyses. The samples were tested with the intent
of discovering whether the pattern of thrombolitic and/or laminar layers could be used as a proxy for larger
scale climatic and environmental factors. δC13 values showed a 7.5% variation, both above and below the
standard for precipitated carbonates. δO18 values remained above the carbonate standard, as would be
expected for a hypersaline environment. Both δO18 and δC13 values were, on average, lower in the
thrombolitic layers and individual clots. There appeared also to be alternating periods with higher and
lower levels of heavy isotopes relative to the age and depth of the samples. The overall isotopic patterns
were indistinct, however, several samples displaying results not correlative with morphology. As previous
studies have suggested, the wide variation seen in our data may indicate that isotopic values are more
reflective of micro-environmental changes within individual stromatolites than they are of macroenvironmental changes within the lake. Hence, our data are not encouraging for the use of isotope
chemostratigraphy or stromatolite morphology as proxies for macro-environmental conditions.