2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 4
Presentation Time: 8:45 AM


COHEN, Phoebe, MIT NASA Astrobiology Team, Massachusetts Institute of Technology, E25-631, 45 Carleton Street, Cambridge, MA 02139, SCHOPF, J. William, Department of Earth & Space Sciences, Molecular Biology Institute, and Institute of Geophysics & Planetary Physics, Univ of California, CSEOL - Geology Building, Los Angeles, CA 90095-1567, BUTTERFIELD, Nicholas J., Department of Earth Sciences, University of Cambridge, Downing Street, Cambridgeshire, Cambridge, CB2 3EQ, United Kingdom, KUDRYAVTSEV, Anatoliy B., Center for the Study of Evolution and the Origin of Life, Univ of California, Los Angeles, CSEOL - Geology Building, 595 Charles Young Circle Drive East, Los Angeles, CA 90095-1567 and MACDONALD, Francis A., Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, pcohen@complex-life.org

Diverse microfossils have previously been described from early diagenetic cherts in the pre-Sturtian Lower Tindir Group of the Yukon Territory. Although these unique ‘scale-like' microfossils had been interpreted as mineralized forms, their actual composition and structure remained enigmatic due to the limitations of traditional light microscopy. Here we describe additional fossil specimens from Lower Tindir cherts as well as newly discovered specimens hosted in coeval limestones that show much greater morphological detail and allow for direct microchemical analysis. We applied scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to macerated specimens and confocal laser scanning microscopy (CLSM) and confocal Raman spectroscopy to chert-hosted specimens of multiple morphotypes. SEM and CLSM results reveal intricate detail on the microstructure and morphology of the fossils. EDS and Raman analysis show that the fossils fall into two categories: most fossils are composed of apatite and kerogen, while a few are composed of silica and kerogen. SEM-imaged microstructure suggests that the fossils were originally kerogenous and/or kerogenous and siliceous, with secondary replacement by apatite. Replacement is most widespread in specimens macerated from limestones, with only certain morphotypes retaining their original siliceous composition. Chert-hosted specimens show a broader range of compositions that are similarly taxon-specific. We hypothesize that this variation is due to differing temporal scales of preservation: sedimentological evidence shows that chert nodules were lithified earlier than the surrounding limestone matrix, thus providing a shorter window of time for replacement to occur in chert-hosted specimens than in carbonate-hosted specimens. Alternatively, the apatite could be primary, implying two distinct modes of biomineralization present in the samples. Although their taxonomic affinity remains elusive, new specimens and improved analysis of these Tindir microfossils suggest that they are the oldest direct evidence of eukaryotic biomineralization in the fossil record.