Organic biosignatures through the geological record: challenges and progress

Abstract

This thesis investigates the reliability of organic biosignatures in geological materials that span most of Earth’s geological record. The aim is to determine the syngeneity and reliability of organic matter (OM) and the best techniques to use on geological materials with different compositions and thermal histories.

Active to recently fossilised (<14 ka) hot spring sinter samples from El Tatio, Chile, Taupo Volcanic Zone (TVZ), New Zealand, and Yellowstone National Park (YNP) were examined as these are key astrobiological targets due to their excellent preservation potential.

El Tatio and TVZ samples were analysed using gas chromatography-mass spectrometry (GC-MS). TVZ samples returned aromatic hydrocarbon ratios, low isoprenoid/n­-alkane ratios, and smooth n-alkane distributions, that indicate OM had reached the oil window despite lack of burial. Surface samples from three localities in YNP record a similar complex history. Raman spectrometry and GC-MS results (hopane ratios and low isoprenoid/n-alkane ratios) are again indicative of the oil window. Thus, some of the preserved OM in hot springs was transported into the samples from hot subsurface reservoirs.

GC-MS analyses of Jurassic (~178 – 151 Ma) sinter from Argentina show that the proximal sinter apron contains a low abundance of compounds with mixed thermal maturities, whereas the cooler distal apron contains thermally immature OM and a wide variety of hydrocarbons. These results indicate that the distal apron of fossil sinters is an attractive target for organic biosignature research.

The reliability of OM in ancient rocks was tested on samples of three distinct fossil types from an overmature c. 2.4 Ga microbialite reef in the Turee Creek Group, Western Australia using in situ micro-Fourier transform infrared spectrometry (micro-FTIR) and flash pyrolysis GC-MS. Micro-FTIR revealed differences in branching and chain length of the hydrocarbon moiety, but anthropogenic contaminants were also detected.

This thesis highlights the need for a whole system approach when considering the origin of OM in geological materials. The veracity of results depends on a thorough understanding of both the environmental context and geological history, and on bespoke analytical approaches to increase confidence in biosignature detection.