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Searching for the earliest signs of cellular life on Earth

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The rover Curiosity is currently exploring for signs of life in ancient lakes and stream beds on Mars. Back here on Earth, molecular evidence is being taken to suggest that major innovations in the biosphere, notably the origins of life, of cyanobacteria, and green algae, could each have taken place within ‘terrestrial’ settings (e.g., springs, lakes, rivers, soils). Rather less plausibly, Ediacara–type biotas have recently been heralded as terrestrial lichens. So what is the potential for developing a respectable fossil record from non-marine rocks of Proterozoic to Archaean age?

Critical investigation into the early terrestrial fossil record is long overdue, not least because such deposits are highly sensitive environmental indicators that may hold clues to the early phosphorus, carbon and oxygen cycles. Precambrian habitats of this kind seem also to have been highly conducive to early diagenesis of the sediment surface. Hence early river, lake and shoreline deposits could contain a wealth of information about the evolution of early cellular life. But decoding this record requires careful and critical contextual analysis using high resolution techniques, of the kind now becoming available.

This talk will review collaborative research at Oxford into remarkable cellular preservation within very early terrestrial and shoreline settings, using techniques including FIB TEM and nanoSIMS (UWA, Bergen and Boston with D. Wacey, M. Kilburn, N. McLoughlin, P. Strother), and synchrotron (DLS Harwell with R. Garwood, and PSI , Villingen with B. Schirmeister and P. Donoghue). These techniques are being used by us to explore 3D morpho- and biogeochemical mapping of early assemblages found within 1000 Ma ‘open’, phosphogenic lake ecosystems, as well as 1200 Ma ‘closed’, sulfidic lake ecosystems of Scotland (e.g., Strother et al. 2011); 1900 Ma microbes from shoreline conglomerates of the Gunflint chert in Canada (e.g., Wacey et al. 2012); c.2900 Ma fluvial conglomerates of Witwatersrand in South Africa (with Lawrence Robb); the earliest (~3460-3430 Ma) preserved land and beach deposits (Strelley Pool chert, Wacey et al. 2011); and the controversial but fascinating Apex chert (e.g., Brasier 2002, 2012).

Brasier, M.D., Green, O.R., Jephcoat, A.P., Kleppe, A.K., Van Kranendonk, M.J., Lindsay, J.F., Steele, A. & Grassineau, N.V. 2002. Questioning the evidence for Earth’s oldest fossils. Nature 416, 76-81

Brasier, M.D., Green, O.R., Lindsay, J.F., McLoughlin, N., Stoakes, C.A., Brasier, A.T. and Wacey, D. (2012). Geology and putative microfossil assemblage of the c. 3460 Ma ‘Apex chert’¸ Chinaman Creek, Western Australia – A Field and Petrographic Guide. Geological Survey of Western Australia, 60pp.

Strother, P.K., Battison, L., Brasier, M.D. and Wellman, C.H. (2011). Earth’s earliest non-marine eukaryotes. Nature, 473, 505–509.

Wacey, D., Kilburn, M., Saunders, M., Cliff, J. and Brasier, M.D. (2011). Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia. Nature Geoscience , 4, 698-702.

Wacey, D., Menon, S., Green, L., Gerstmann, D., Kong, C., McLoughlin, N., Sanders, M. and Brasier, M.D. (2012). Taphonomy of very ancient microfossils from the ∼3400 Ma Strelley Pool Formation and ∼1900 Ma Gunflint Formation: New insights using a focused ion beam. Precambrian Research, 220/221, 234-250.

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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