HR: 0800h
AN: B31A-0962    [Abstracts]
TI: Microbe-Mineral Interactions Along Biogeochemical Gradients in Bahamian Stromatolites: Key to Lithification and Preservation
AU: * Andres, M S
EM: mandres@rsmas.miami.edu
AF: RSMAS-MGG Univ. of Miami, 4600 Rickenbacker Csway, Miami, FL 33149 United States
AU: Sumner, D Y
EM: sumner@geology.ucdavis.edu
AF: Geology Dept. Univ. of California Davis, 1 Shields Ave, Davis, CA 95616 United States
AU: Visscher, P T
EM: visscher@uconnvm.uconn.edu
AF: Dept. of Marine Sciences Univ. of Connecticut, 1084 Shennecossett Rd, Groton, CT 06340 United States
AU: Swart, P K
EM: pswart@rsmas.miami.edu
AF: RSMAS-MGG Univ. of Miami, 4600 Rickenbacker Csway, Miami, FL 33149 United States
AU: Reid, R P
EM: preid@rsmas.miami.edu
AF: RSMAS-MGG Univ. of Miami, 4600 Rickenbacker Csway, Miami, FL 33149 United States
AB: Understanding on how modern stromatolites form and lithify is critical to properly interpreting the origins of ancient stromatolites and the early evolution of life. Lithification in Bahamian stromatolites is tied to specific, 20-60-micron thick horizons characterized by laterally continuous sheets of microcrystalline carbonate (aragonite). Microbial processes associated with these horizons are 1) photosynthetic production by cyanobacteria and 2) heterotrophic respiration by bacteria as well as the production of extrapolymeric substances (EPS). The aim of this study is to better understand the coupling of microstructure and microbial processes. The competing influences of photosynthetic CO₂ uptake, sulfate reduction, and degradation of Ca-binding EPS influence both carbonate saturation states and the isotopic composition of dissolved inorganic carbon (DIC). In Bahamian stromatolites, photosynthesis and sulfate reduction are associated with specific microbial mat types creating distinctive chemical gradients that can be preserved in authigenic carbonate. Aragonite that precipitated within stromatolites is > 1 per mill depleted in ¹³C relative to aragonite precipitated in equilibrium with local seawater. These data suggest that more aragonite precipitates when and where respiration, rather than photosynthesis, influences local DIC, which is consistent with sulfate reduction promoting carbonate precipitation and calcium release during decay of exopolymeric substances. Biogeochemical gradients vary on a temporal and spatial scale as indicated by in-situ pH measurements across a the living mat. Highest pH correlates to maximum photosynthesis signal in the early afternoon while the lowest pH to that of maximum respiration just before sunrise. Corresponding carbon isotope analysis of authigenic carbonate precipitate will determine when microscale biological activity is captured in the mineral phase and potentially preserved.
DE: 0419 Biomineralization
DE: 0448 Geomicrobiology
DE: 0463 Microbe/mineral interactions
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Biogeosciences [B]
MN: Fall Meeting 2005