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A New Look from Familiar Environments: Early Carbonate Precipitates and Biofilms in Hot Springs, Salt Ponds, and Reefs

Corley, Margaret E.1; Puckett, Mary K.1; Kirkland, Brenda L.1; McNeal, Karen S.1; Folk, Robert L.2; Lynch, Francis L.1
1 Department of Geosciences, Mississippi State University, Mississippi State, MS.
2 Department of Geological Sciences, University of Texas at Austin, Austin, TX.

TEM, FESEM, ESEM and confocal microscopy show that carbonate precipitation occurs within biofilms from hot springs near Viterbo, Italy (four samples); biofilms from Fowl Cay Reef, Abaco, Bahamas (seven samples, 15 to 65 ft deep); and gelatinous sediment/biofilm from Salt Pond, San Salvador, Bahamas (five samples, top 5 cm to 30 cm deep). In each of these environments, precipitation occurs within biofilms and organic matter provides an initiation point for inorganic crystal precipitation.

TEM of sections stained with osmium, uranyl acetate, and lead citrate reveal organic cores with nanometer-scale (nannobacteria?) spherical textures in aragonite fuzzy dumbbells from one hot spring biofilm sample and one 8-24 micron thick layer from the salt pond. In both environments, TEM sections show that aragonite crystals nucleate on organic matter, which forms a core structure within the dumbbells and botryoids. Most of this organic matter appears amorphous, however, in some of the hot spring samples, nannobacteria may be present. In one dumbbell from the hot springs, growth bands (possibly daily) of dark organic matter are visible on the outer margin of the dumbbell. In the single layer from the salt pond where we found dumbbells, they are oriented perpendicular to each other. The cyanophyte Spirulina is present in association with dumbbells from both environments.

In dehydrated SEM samples from each of these environments, organic matter is ubiquitous. In the salt pond and reef samples organic matter is significantly more abundant than sediment and precipitate is rare. In the hot spring samples, high magnification SEM reveals nanometer-scale textures in the cores of fuzzy dumbbells, yet aragonite needles on the outer margin are euhedral with smooth surfaces.

Confocal microscopy of samples from these three environments reveals striking similarities. In each setting, precipitates, such as peloidal cement, nucleate within biofilm or in close association with organic matter. Although magnification is not as high as SEM or TEM, organic matter is visible in close association with precipitates, revealed by differences in fluorescence. Samples collected for confocal microscopy were viewed un-preserved, fixed in 2% gluteraldehyde and rinsed, or dehydrated, sectioned, and stained. Each procedure revealed similar results. Confocal microscopy and TEM represent a novel approach in the search for the relationship between organic matter and carbonate cement.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009