HR: 0800h
AN: OS21A-1208    [Abstracts]
TI: Precambrian Limestones: Products of Microbial Diagenesis?
AU: * Gammon, P R
EM: p\_r\_gammon@yahoo.com.au
AF: University of Ottawa, Dept. of Earth Sciences, Ottawa, On K1N 6N5 Canada
AU: Arnott, R W
EM: warnott@uottawa.ca
AF: University of Ottawa, Dept. of Earth Sciences, Ottawa, On K1N 6N5 Canada
AU: McKirdy, D M
EM: david.mckirdy@adelaide.edu.au
AF: University of Adelaide, School of Earth and Evnironmental Sciences, Adelaide, SA 5005 Australia
AB: The anomalous abundance and texture of Precambrian sedimentary dolomites have not precluded their C-isotopic compositions being interpreted in terms of paleoclimatology and chemostratigraphy (e.g. Snowball Earth Hypothesis; Hoffman et al. 1998, Science, v. 281, p. 1342-1346). Neoproterozoic dolomitic and calcitic limestones from the Australian Adelaidean, are, as elsewhere, comprised of interlocking, 5-50 $\mu$m dolomite and calcite microspar, as opposed to particulate, $<$5 $\mu$m, modern micrite. The microspar crystals are commonly zoned, with a 3-4 $\mu$m core having $<$100 ppm of Mn$^{2+}$ plus Fe$^{2+}$, surrounded by outwardly increasing levels of these reduced cations (to $>$6 wt%). Timing criteria demonstrate that these crystals represent an initial micrite precipitated from oxidized fluids (i.e. Neoproterozoic seawater), which was subsequently overgrown by a microspar cement precipitated from early diagenetic pore fluids with low Eh due to ongoing microbial decomposition of organic matter. Dolomitisation is thermodynamically advantaged over calcification by sulphate reduction and high carbonate alkalinity. Lacking both a substantive foodweb and bioturbative substrate ventilation, the Precambrian was a time when the relative flux of labile organic matter entering the substrate was maximized. Such an increased flux will have driven, through microbial consumption, more rapid anoxia and higher carbonate alkalinity. Thus the abundance of Precambrian sedimentary dolomites, and their microspar texture, reflects the importance of substrate-based microbial processes in the pre-metazoan carbon cycle. "Reduced" microspar overgrowths contribute $>$75% of virtually all analyzed Adelaidean calcitic and dolomitic limestones. Since their petrography is representative of Neoproterozoic limestones in general, we suggest that the existing Neoproterozoic C-isotope database most likely records early diagenetic microbial processes, not seawater carbon biogeochemistry as currently interpreted. We further suggest that the wide range in global Neoproterozoic carbonate C-isotope data (+10 to -8 \permil) is entirely consistent with the compositions of modern early diagenetic carbonates, rather than extreme secular oscillations in the C-isotopic signature of seawater.
DE: 4806 Carbon cycling
DE: 4815 Ecosystems, structure and dynamics
DE: 3675 Sedimentary petrology
DE: 4267 Paleoceanography
DE: 1050 Marine geochemistry (4835, 4850)
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting