ORGANIC-MATTER IN GREAT-SALT-LAKE OOIDS (UTAH, USA) - FIRST APPROACH TO A FORMATION VIA ORGANIC MATRICES

The aragonitic ooids of the Great Salt Lake (Utah USA) are a product o f an organomineralization process. The comparative study of ooids, mic robialitic ooid chips, and microbialite crusts demonstrates the crucia l role of proteic and glucidic substances in the formation of ooids. T he ooids contain strikingly high amounts of intracrystalline organic m atter (up to 528 mu g/g carbonate), which consists of about 23% protei c and 77% glucidic material. High values of acidic amino acids (12 mol e-% asp and 45 mole-% glu) characterize the proteic phase, whereas mon osaccharides are enriched in fucose, glucose, and mannose. Fluorescenc e microscopy of histochemically treated ooids reveals distinct dissolu tion/reprecipitation features probably controlled by the acidity of th e organic matter involved. Direct influence of the few attached bacter ia is negligible, so that our model of ooid formation refers to the fr ee organic substances. We propose, that the free organic substances of ooid surfaces first inhibit carbonate precipitation due to their very strong acidity. Increasing neutralization of the free carboxylic and sulfate-groups finally results in a retarded precipitation of aragonit e fibers and micrite. Therefore, the ooid fabric is most likely contro lled by small-scale biochemical gradients and not by clay minerals, as previously assumed. GC-MS analyses of the hydrocarbons extracted from the ooids reveal high concentrations of n-alkanes derived from terrig enous sources as well as long-chain monomethylalkanes. The latter comp ounds are typical constituents of insect waxes and most likely origina te from the salt fly Ephydra living in the area. In comparison to the hydrocarbon pattern of the microbialitic ooid chips, only small amount s of biomarkers deriving from heterotrophic bacteria and cyanobacteria were observed in the ooids.