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Project Progress

The presence of organic biomarkers in Late Archean sediments is as exciting as it is provocative. Past work has characterized biomarkers that document inputs from cyanobacteria and a variety of consumer organisms. The work also found evidence of eukaryote inputs, through the presence of sterol-derived structures. These findings prompted Sheridan, Freeman and Brenchley (2002) to reevaluate the timing of divergences recorded in molecular phylogenies of the prokaryotes. The biomarker results serve as a time marker for estimating minimum divergence events based on 16s rRNA sequences for prokaryotes, from which we found early and rapid diversification of bacteria, especially in groups that carry out anaerobic respiration. We proposed that this diversification is linked to the rise of oxygen in the environment.

Our current efforts have confirmed the diversity of compounds found in earlier biomarker studies, and we have expanded the approach to evaluate signatures from a range of sedimentary units, and to explore relationships between the extractable components and the insoluble material (kerogen) that is intimately associated with the mineral matrix. To date, our work has documented ancient biomarker compounds from a wide array of microorganisms, including cyanobacteria, green-sulfur bacteria and methane-oxidizing bacteria. In addition, compounds present are also consistent with inputs from archea, but they are not, strictly speaking, diagnostic. One of the primary concerns in this approach is that the freely extracted materials are in fact derived from organisms that lived at the time and place of deposition, and do not reflect contamination from ancient migrating fluids. To address this issue, we are evaluating the structure and isotopic character of kerogen and compounds released by pyrolysis in hydrogen. This research constitutes the PhD project of Jen Eigenbrode, who is assisted by an undergraduate researcher (Theresa Diehl) and collaborators in labs around the world.

The isotopic record of life during this period has sparked debate regarding its origin, with most models involving methane. Pavlov, Kasting, Eigenbrode and Freeman (2001) recently proposed a novel approach to this problem, through the formation of a methane haze.