T166 Emerging New Methods in Early Earth Studies: Unraveling the Co-Evolution of Earth and Life

Tuesday, 30 October 2007
1:30 PM - 5:30 PM

KEROGEN-BOUND AROMATIC HYDROCARBONS AS POTENTIAL MOLECULAR BIOSIGNATURES FOR OVERMATURE ORGANIC MATTER IN 3.43 GA STRELLEY POOL CHERTS FROM PILBARA CRATON, WESTERN AUSTRALIA

Paper number: T166-5
Program: Technical Sessions
Time: 3:00 PM – 3:15 PM

Authors

Abstract

Hopanes and steranes (and their methylated derivatives) in 2.5-2.7 Ga sediments from the Pilbara Craton currently constitute the earliest convincing molecular biomarker evidence for microbial life in the Archean (Brocks et al., 1999). The overall yields of these polycyclic alkane markers, however, represent only a very small fraction of total organic matter. Detailed molecular and compound-specific isotope analyses of a larger portion of the preserved organic matter will help independently assess the origins of this material. This is now of particular importance given that bulk 13C-depleted isotopic values for Archean kerogens cannot be used to unambiguously discern a biogenic origin from abiogenically-derived organics (McCollom and Seewald, 2006).

Our recent study (Marshall et al., 2007) has shown that genuine aromatic and aliphatic hydrocarbons could be released from overmature kerogens prepared from 3.43 Ga Strelley Pool cherts using catalytic hydropyrolysis (HyPy, Love et al., 2005). Upon HyPy we observed a complex distribution of hydrocarbon products, composed predominantly of aromatics derived from bond cleavage and alkanes released from trapping in the microporous network. Importantly, we demonstrated for the first time a positive correlation between the degree of alkylation of aromatic products (related to thermal maturity) with bulk structural parameters derived from elemental analysis and Raman spectroscopy.

Here we show that the molecular profiles of hydrocarbons generated from HyPy of Strelley Pool kerogens exhibit very similar patterns to a range of mature biogenic Phanerozoic and Proterozoic kerogens. Furthermore, the molecular patterns are distinct from fragments generated from HyPy of the insoluble carbonaceous matter in the Murchison meteorite (Sephton et al., 2005). HyPy constitutes a powerful method for fragmenting kerogen and helping constrain the origins of organic matter found in Earth's oldest sediments.

References Brocks, J.J., Logan, G.A., Buick, R. & Summons, R.E. (1999) Science 285, 1033-1036. Love, G.D., Snape, C.E., Carr, A.D. & Houghton, R.C. (1995) Org. Geochem. 23, 981-986. Marshall, C.P., Love, G.D., Snape, C.E., Hill, A.C., Allwood, A.C., Walter, M.R.,Van Kranendonk, M.J., Bowden, S.A., Sylva, S.P. & Summons, R.E. (2007) Precam. Res. 155, 1-23. McCollom T.M. and Seewald, J.S. (2006) EPSL 243, 74-84. Sephton, M.A., Love, G.D., Meredith, W., Snape, C.E., Sun, C.-G. & Watson, J.S. (2005) Planetary & Space Science 53, 1280-1286.