Geochemical investigations of modern lacustrine microbialites of laguna Negra, Catamarca Province, Argentina.

BEELER, S.; GOMEZ, F.J.; BRADLEY, A.

Bloomington

Simposio; Midwest Geobiology Symposium 2015; 2015

Department of Geological Sciences, Indiana University

Microbialites are a conspicuous component of much of the sedimentary rock record, and provide some of the earliest evidence of life on Earth. Thus, these structures provide great potential to enhance our understanding of Earth history. However, our ability to interpret the information preserved in microbialites is complicated by our lack of understanding of the processes that drive microbialite formation and growth. Modern environments where microbialites are actively forming offer the ability to study the development and growth of microbialites in situ as an analog for their formation in ancient environments. One such environment is Laguna Negra, a high alpine hypersaline lake located in the Puna region of northwestern Argentina, which contains a zone of microbialite formation at its southeastern margin. Laguna Negra has thus far been only minimally studied and many questions remain about the processes driving microbialite formation at the lake. The microbialites at Laguna Negra exhibit several distinct morphologies (oncoid, stromatolite, and laminar crust) transitioning laterally across the lake edge. The presence of this diverse set of morphologies within a relatively small spatial scale make Laguna Negra a compelling location to study the processes that drive microbialite growth and their impact on fossil form. Deciphering the geochemical differences both between and within each microbialite morphology will help to elucidate the mechanisms controlling their formation. For this purpose, a representative example of each of the morphological endmembers observed at Laguna Negra was analyzed for variability in elemental abundances. Elemental mapping was completed via electron-probe microanalysis (EPMA) to qualtitatively analyze differences in elemental abundances within the microbialites. Spot analyses were then performed on points of interest identified from the elemental maps in order to provide quantitative values for comparison. Variability in elemental abundance was observed between individual microbialite layers and the morphological endmembers. Differences were observed in magnesium and silicon abundances, which is potentially indicative of mineralogical differences. The abundance of minor elements manganese, strontium, and aluminum are also variable, which may be indicative of geochemical changes in the water where the microbialites precipitated. Overall, the observed changes in elemental abundances imply that both spatial and temporal geochemical changes are preserved inthe microbialites. Future work will focus on more detailed analyses of mineralogy and both physical and organic geochemistry to better constrain the sources and meaning of the variability in elemental abundances observed in this study.