Thermochemical oil formation and the search for a subsurface biosphere hydrocarbon fingerprint in hydrothermal vent sediments at Guaymas Basin, Gulf of California

Abstract

For this study a biomarker survey and multi-molecular chemometric analysis was performed on petroleum-rich sediments from the Cathedral Hill hydrothermal vent site in Guaymas Basin, Gulf of California. The objective of this study was to monitor the progression of petrogenesis, biological activity, and migration of hydrocarbons along a natural geothermal gradient (spanning 0–155 °C) intersected by a push core transect. Biodegradation increases down-core to variable degrees across the transect, while decoupled ratios of bacterial-sourced lipids such as hopene and hopane mark the transition zone of subsurface bacterial habitability. Various proxies show maturity increases with depth and proximity to the vent-center. These parameters, along with chemometric models, subtracted chromatograms, and hydrocarbon diversity measurements reveals that hydrocarbon generation likely begins where vent temperatures surpass 110 °C. This results in even shallower, 8-10 centimeters below seafloor, generation from what is predicted by kinetic models. The results suggest the vent temperatures may vary over time or that other applied geochemical factors (e.g. vent chemistry) may influence catagenesis. Overall, the results indicate marked spatial diversity and complexity in this sedimented hydrothermal site.