MOLECULAR FOSSILS IN MESOPROTEROZOIC SHALES: BIOMARKER CHARACTERIZATION AND IMPLICATIONS FOR EUKARYOTIC EVOLUTION

Examinations of molecular biomarkers provide evidence of organic matter sources, which can tell us about precursor biota, depositional environments, and the maturity of oil reserves and kerogens. The developing recognition of well-preserved Proterozoic hydrocarbons may, in turn, provide invaluable evidence regarding early biospheric evolution. For instance, eukaryotic steranes are present in the geologic record from ~2700 Ma, but these biomarkers are neither abundant nor diverse within Archean communities, which are dominated by archaebacterial isoprenoids. By ~850 Ma, the Neoproterozoic molecular record is dominated by eubacterial/cyanobacterial hopanes with a significant abundance and diversity of eukaryotic steranes, including those of multicellular eukaryotes (red and green algae), as well as molecular evidence for heterotrophic protists. Little is known, however, about the nature and timing of this transition.

This study compares prokaryotic and eukaryotic biomarkers extracted from organic-rich shales from the ~1300 Ma Dismal Lakes Group (Coppermine Homocline, Canada) and ~1100-900 Ma Atar Group (Taoudenni Basin, Mauritania). Increasing evidence suggests a global biospheric oxygenation event at ~1300-1250 Ma in conjunction with a first-order, positive shift in the marine carbon isotopic record. The appearance of the oldest bedded marine gypsum and sparse fossils of the earliest, unambiguously multicellular eukaryotes support this event. An oxygenation event at ~1300-1250 Ma likely played a significant role, as well, in supporting the more diverse eukaryotic community preserved in the Neoproterozoic molecular record. Examination of molecular compounds from Dismal Lakes and Atar groups shales will therefore (1) contribute new data from a critical geologic interval currently uncharacterized in the biomarker record, and (2) provide valuable insight into the nature and timing of early eukaryotic diversification and the evolution of multicellularity.