IMPLICATIONS OF GEOCHEMICAL SIGNATURES OF MICROBIALITES IN A MIOCENE LAKE SYSTEM

Deeper knowledge of microbialites can be used to understand the paleoecology of Earth systems and other planets with the conditions to support life. In addition, with recent petroleum discoveries in rift basin lakes with microbial carbonate units, outcrop analogs for these reservoirs are rare and necessary for both exploration and production. The Lovell Wash Member (LWM) of the Horse Spring Formation gives us a unique view of microbialite morphologies, textures and geochemistry because they are lightly altered, well exposed, and can be traced laterally. Furthermore, these strata have a wide range of both stromatolitic and thrombolitic morphologies. For this project, we sampled Miocene microbialites from the LVM near the Lake Mead area of Nevada, U.S. We took sub-samples from two sets of seven microbialites from the same, relatively thin stratigraphic interval; one set was stromatolitic, the other thrombolitic. We analyzed these samples for total and organic carbon, δ¹³C, and δ¹⁸O. We compared these geochemical results to microbialite macroscopic and microscopic textures. This led to the conclusion that some morphologies tend to house more organic carbon than others. We observed that different microbialite morphologies have varying, and sometimes unique, stable isotopic signatures, with δ¹⁸O varying over 10‰ and δ¹³C over 8‰. We compared these values to a vertical stratigraphic section of the LWM. With respect to δ¹³C and δ¹⁸O there is comparable isotopic variation between microbialite morphologies in this one stratigraphic interval as there is in the total vertical stratigraphic section of the LWM. This has important implications for the interpretation of stable isotopic values as paleoecological proxies in lacustrine settings. Isotopic fractionation by microbial communities can play as large a role as paleoclimate or paleohydrological changes in lake water chemistry. Assuming that water chemistry was relatively constant for the White Basin lake during this narrow stratigraphic time slice, we infer that clustering of isotopic values by microbial morphology and texture indicate that microbes play a role in the isotopic variation detected in the rock record and this role may be substantial.