ORGANOMINERALIZATION OF MICROBIALITES FROM STORR'S LAKE, SAN SALVADOR ISLAND, BAHAMAS: CALCIUM, CARBON, AND OXYGEN STABLE ISOTOPE ANALYSIS

Studying the isotopic composition and mineralogy of modern microbialites provides us with the key to interpreting the formation processes and environments of ancient microbialites. Growing in the hypersaline and turbid Storr's Lake (SL) on San Salvador Island in the Bahamas are microbialites with low levels of photosynthesis and high levels of sulfate reduction-in contrast to many of their modern counterparts. Living planktonic, motile microorganisms and suspended algal and bacterial debris create the high turbidity of the shallow lake (<2m) and rapidly attenuate sunlight in the water column. Microbialites were collected at both deep (depth >60cm) and shallow (depth <60cm) parts of SL in January 2016. SL had a conductivity 1.2 × seawater, pH ~8.4, and little variation in major cations. The water was more supersaturated with respect to calcite, aragonite, and dolomite than seawater. Carbon (C) isotopes of SL microbialite samples show both photosynthesis (evidenced by high C isotope values) and extracellular organic matter degradation (evidenced by low C isotope values in other samples) controlling organomineralization. This agrees with previous data which found both phototropic and heterotrophic bacterial taxa. Oxygen (O) isotopes are relatively consistent between microbialite sublayers and show an evaporative signature for SL.

Microbe metabolisms interact with the larger environment of SL to precipitate carbonate within microenvironments of the microbial mats. Both high-Mg calcite (HMC) and aragonite are found within some of the microbialites leading to the hypothesis that the organomineralization process involves a step where HMC transforms to aragonite. The mineralogy of shallow and deep water samples will be compared to test the hypothesis that the transition from HMC to aragonite is only found in SL microbialites growing in water deeper than 60cm (at time of collection). New calcium (Ca) stable isotope analyses from the TIMS using a ⁴²Ca-⁴³Ca double spike will be presented to provide evidence to support (or refute) this hypothesis since we expect differences in mineralogy to be the primary control on Ca isotope signatures. These new results will be compared to other modern and ancient carbonate systems to better understand organomineralization processes and how they fractionate Ca isotopes.