In Search of Archean Biomarkers: Re-analysis of 2.7 Ga Metasediments from the Abitibi Greenstone Belt, Ontario, Canada.

Abstract:

Biomarkers in Archean sediments are now generally considered contaminants, either incorporated into the sediments after deposition or introduced during coring, sample collection, or laboratory analysis. Not all previously studied Archean formations have yet been re-analyzed using techniques that take a more stringent approach to account for contamination. Here we re-analyze 2.676 - 2.703 billion-year-old (Ga) lower greenschist metasediments collected from the Abitibi Greenstone Belt in Ontario Canada to determine if previously observed biomarkers are actually syndepositional or artifacts.

One method to assure the syngeneity of biomarkers with their host rock is to study hydrocarbons trapped within carbon-rich inclusions of specific mineral phases. We analyzed observed carbon-rich inclusions within the Abitibi metasediments to either verify or nullify the results of Ventura et al. 2007, which found evidence for archaeal, bacterial, and eukaryotic life present during primary deposition, and possible archaeal and bacterial life present during post deposition subsurface hydrothermal activity. 16 samples collected from 8 locations within the Tisdale and Porcupine Groups in Timmins, ON, Canada, were made into 75 - 100 μm thick slides, photographed, and petrographically analyzed for carbon-rich inclusions. Carbon-rich inclusions up to 10 - 20 μm in diameter were positively identified within 2.703 Ga greywackes. The 10 - 20 μm inclusions occur within the interstices of occluded quartz grains and within secondary mineral phases such as large (100 - 300 μm) ankerite grains which formed during the precipitation of minerals from hydrothermal fluid at 2.670 ± .007 Ga. The next step in analyzing these inclusions is to use full spectrum-Laser Desorption Postionization-Mass Spectrometry (fs-LDPI-MS), which has the capability of analyzing samples to the 2 μm scale to determine the hydrocarbon composition of the carbon-rich inclusions.