HR: 11:20h
AN: B21G-05 INVITED     [PDF]
TI: Microbially Influenced Calcite Precipitation in Archean Communities
AU: * Sumner, D Y
EM: sumner@geology.ucdavis.edu
AF: Geology, UC Davis, 1 Shields Ave, Davis, CA 95616 United States
AU: Murphy, M
EM: megmurphy@geology.ucdavis.edu
AF: Geology, UC Davis, 1 Shields Ave, Davis, CA 95616 United States
AU: Shiraki, R
EM: rshiraki@ucdavis.edu
AF: Geology, UC Davis, 1 Shields Ave, Davis, CA 95616 United States
AB: Fenestrate microbialites, which are common in Neoarchean carbonates, are composed of two microbial communities that affected calcite precipitation differently in environments where synsedimentary calcite precipitation was kinetically favorable. Filmy laminae are composed of $>$3$\mu$m thick zones of organic inclusion. They form planar laminae, drape off topographic highs, and display recumbent folds and roll-up structures. Their composition and deformation demonstrate that they were thin, laterally cohesive, extremely flexible biofilms. In contrast, supports are $\sim$200$ \mu$m-wide surfaces that tend to be vertically oriented. They are defined by organic inclusions in calcite and commonly are coated with the first generation of calcite cements; they show evidence for compaction where early precipitation did not occur. Although no microbial analogs for support growth are known, they are also interpreted as microbial communities based on their composition and relationships to filmy laminae. Filmy laminae and supports affected calcite precipitation differently. Petrographic relationships demonstrate that where laminae drape off of supports, initial fibrous calcite nucleated only on the supports and not on the laminated mat. In addition, carbon in calcite associated with the supports in the 2.52 Ga Gamohaan Formation, South Africa, is enriched in $^{13}$C relative to all other calcite in the microbialites. Supports with petrographic evidence for simultaneous calcite precipitation are associated with calcite that is enriched up to 1%o relative to calcite that precipitated in voids or filmy laminae as little as 250 $\mu$m away. Enriched carbon is attributed to microbial fixation of CO$_{2}$ or HCO$_{3}$$^{-}$ during calcite precipitation due to preferential uptake of $^{12}$C species. Calcite in compacted supports, which is interpreted as precipitating after growth of the support, is not enriched in $^{13}$C. To produce measurable carbon isotopic disequilibrium, the microbial communities must have rapidly fixed carbon autotrophically. For this disequilibrium to be preserved in calcite, precipitation must have been contemporaneous with microbial carbon uptake. Thus, isotopic evidence suggests that the support microbial communities fixed CO$_{2}$ or HCO$_{3}$$^{-}$ and metabolically induced calcite precipitation. Some calcite associated with organic inclusions also is enriched in Mg, which may be due to: 1) locally increased concentration of Mg due to microbial processes or binding to organics; 2) release of Mg as a result of organic decay during diagenesis; or 3) local mediation of diagenetic reactions by organics. The preservation of the carbon isotopic signatures suggests that the increased Mg concentrations are most likely due to locally increased concentration of Mg during microbial growth.
DE: 0400 Biogeosciences
DE: 1040 Isotopic composition/chemistry
SC: Biogeosciences [B]
MN: 2003 Fall Meeting