GEOL Thesis Defense - Katherine HickokTuesday, July 11, 2017 - 10:00am
Serpentinization and Synthesis: Searching for abiotic and biotic non-volatile organic molecules in the subsurface of the Atlantis Massif
High concentrations of hydrogen created during serpentinization can promote the formation of abiotic organic carbon molecules such as methane, formate, and short chain hydrocarbons and, in laboratory experiments, larger molecules containing up to 32 carbon atoms. Subsurface archaeal and bacterial communities can use these reduced compounds for metabolic energy. International Ocean Discovery Project Expedition 357 drilled 17 boreholes into the Atlantis Massif with the goals of investigating carbon cycling and the presence of life in a zone of active serpentinization. The expedition recovered multiple lithologies including gabbros, basalts, carbonate sands, and serpentinites. A subset of contrasting lithologies were analyzed for n-alkane and fatty acid content to determine if non-volatile organic molecules are produced abiotically in serpentinizing environments and to identify ‘hot spots’ of microbial life in the subsurface. Given the high potential for contamination during drilling, a suite of materials used in sample collection and processing were also analyzed to characterize their signatures.
Biologically-derived lipid biomarkers could not be identified in any of the samples, indicating any biological communities present in the subsurface of the Massif were in abundances below our ability to detect them. An n-alkane series ranging from C18 to C30 with δ13C isotopic values of -30.9‰ to -28.8‰ were present in various lithologically diverse samples. The distribution of these compounds was similar to those observed in previous grab samples from the same region, and to compounds formed abiotically in laboratory experiments. For the current set of samples, multiple lines of evidence point to the rock saw used to remove core exteriors during sample processing as the source of the n-alkanes. This result highlights the importance of careful prevention and characterization of contamination to allow for more accurate interpretations of complex and dynamic subsurface processes. Many of the other sample-handling procedures designed to reduce surface contamination were determined to be effective and should be implemented in future projects. The definitive detection and identification of abiotic and biological lipids in the subsurface of an actively serpentinizing system would be a significant step towards understanding the evolution of pre-biotic chemistry and life in extreme environments, but future reports of these compounds must occur in conjunction with thorough contamination assessments.