ROBBINS, L. L.¹, I. KALAKAUSKIS¹, and G. WHITTLE^2
1University of South Florida, Tampa, FL
²Minerals Management Services, New Orleans, LA

Abstract: An Investigation of the Crystallographic and Elemental Signatures of Microbially- Precipitated Carbonates

The influences of marine microbes on precipitation of carbonate sediments and cements were studied in order to provide a baseline for the identification of microbial sediments and fabrics within the geologic record. Sediments, water, and microbial samples were obtained in March, 1997, from 5 locations around Lee Stocking Island (Exumas, Bahamas). Field samples and laboratory experiments will provide a gauge by which microbial sediments can be identified.

Thin sections were prepared each sample location and petrographically analyzed to determine sediment mineralogy and cement fabric. Microbes from samples were identified using gram staining, light, epiflourescence, and electron microscopy and cultured to provide a stock of microbes from each locality for laboratory experiments. Microbial communities were then used to induce calcium carbonate precipitation in the laboratory through in-vivo and in-vitro experimentation. In order to evaluate organic control on mineral morphology, in vitro crystallization experiments utilized matrix protein extracts from the microbial communities and induced precipitation in calcium carbonate supersaturated environments. Elemental analyses were also performed on sediments and laboratory precipitates.

Data from analysis of thin sections, microbial induction, and protein crystallization experiments were compared for morphologic differences and similarities. Cement fabrics observed in thin sections included whisker fibers, micritic envelopes, fibers, blades and cryptocrystalline fabrics. Crystals formed during laboratory experiments included micrite, blades, modified rhombs, and few acicular fibers. Elemental data indicated concentrations of trace elements within the crystals, including Cu, Ni, Mg, and Zn. These data provide a baseline to establish criterion for microbially produced sediments.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas