2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 256-12
Presentation Time: 4:00 PM

CARBON ISOTOPIC COMPOSITION OF ORGANIC FOSSILS ACROSS ISOTOPIC EXTREMES IN CRYOGENIAN CARBONATES


MATYS, Emily D.1, FREEMAN, Katherine H.2, MACDONALD, Francis A.3, BIRD, Laurence2, MORAN, James J.4 and BOSAK, Tanja5, (1)Department of Earth, Atmospheric, and Planetary Sciences, MIT, Cambridge, MA 01239, (2)Department of Geosciences, Pennsylvania State University, 235 Deike Building, University Park, PA 16802, (3)Department of Earth and Planetary Sciences, Harvard University, 2, Cambridge, MA 02138, (4)Pacific Northwest National Laboratory, Richland, WA 99354, (5)Department of Earth, Atmospheric, and Planetary Sciences, MIT, Cambridge, MA 02139, ematys@mit.edu

Neoproterozoic carbonate strata record large perturbations to many biogeochemical cycles. Studies of these strata rely primarily on the bulk isotopic records of organic and inorganic carbon. We characterize fossil-specific C isotopic composition of organic-walled fossils from carbonate strata from the Cryogenian Taishir member of the Tsagaan Olom Formation in Mongolia. These strata span a range of carbonate δ13C values, from +7 to -6 per mil.

The morphology of fossil tests suggests a planktonic lifestyle of their precursor organisms. Thus, their isotopic composition should reflect that of the surface waters during the deposition of the Taishir member. Our preliminary results, obtained from the analysis of 8 samples of 15 individual tests by LA-IRMS and 8 samples of 75-125 individual tests by Nano-EA indicate that the carbon isotopic composition of fossil tests do not co-vary with the C isotopic composition of the host carbonate or bulk organic carbon. This implies the presence of isotopically heterogeneous organic matter within the carbonates characterized by large negative δ 13C values. The isotopic heterogeneity will be tested by SIMS analysis of organic-rich material in thin sections and LA-IRMS analyses of bulk organic residues from fossiliferous samples.