The Apex Basalt in the Marble Bar greenstone belt of the Pilbara Craton, Western Australia, has long been a focus for the investigation of early Earth environments and potential microbial life. At 3.46 billion years old, it is particularly notable for the presence of carbonaceous microstructures in hydrothermal black chert veins at the Chinaman Creek locality. Interpreted in the 1980s by Schopf as representing species of fossilised prokaryotes, the origin and nature of these ‘microfossils’ has been highly debated, particularly by the department’s late Martin Brasier. Recent morphological and geochemical analyses by Brasier, David Wacey and others, have conclusively shown the structures to be mineral artefacts arising from fluid flow within an active hydrothermal system.
Whilst the hydrothermal chert veins have been studied in great detail, relatively little is known about the overlying sedimentary cherts that are interbedded with volcanic rocks of the Apex Basalt. These units, often internally laminated and preserving a range of microscopic structures, may provide evidence of sedimentary environments (and their associated biotas) reflecting quiet, shallow marine periods in an otherwise volcanic ‘Apex time’.
Keyron-Hickman Lewis (St Edmund Hall, 2011) undertook a 4th year undergraduate Master’s project to document and assess the origins of the carbonaceous microstructures present in the lowermost of the stratiform chert units (informally known as the ‘Apex chert’. Keyron tells us “I had no hesitation in asking Martin Brasier to be my Master’s supervisor – his lectures and tutorials on the relationship between the ancient Earth and its biosphere were inspiring. He set me this very challenging, though immensely intellectually rewarding, task – it is rather like a geological detective piece, since in such complex, silicified rocks, the clues hesitate to be easily forthcoming! I hope it reflects well the multi-strategy approach he advocated for these types of studies”. Using confocal laser scanning microscopy, Keyron was able to identify differences between the carbon found within clotted grey-black cherts and carbon in microgranular cherts; the former provide no evidence to support biological origin of the carbon, but the latter contain laminated structures which pass a number of the morphological criteria for biogenicity. He then undertook geochemical mapping of the microgranular chert using laser Raman micro-spectroscopy and NanoSIMS. This confirmed the age of the carbon, and revealed an interesting finding. Correlations of biologically significant elements are clear in these microstructures, and the morphology of filament-like laminae adjacent to the microgranular cherts are directly comparable to a sub-type of microbially induced sedimentary structure (MISS).
Whilst conscious not to re-open the ‘Apex Chert’ controversy, Keyron’s work suggests the dominantly volcanic Archaean environment may yet yield evidence of biological activity, and thus the beginnings of life on this planet. Keyron explains: “The findings of our paper shed new light on an ancient lithology: although the stratiform unit is but a few metres from the controversial hydrothermal chert vein, it portrays a wholly different environment of deposition, and a selection of microstructures having the promise of biogenicity. I hope that our work will encourage further evaluative appraisals into such structures, both in the Apex chert and in other ancient rocks, for it is only through geologically informed investigations that we can best appreciate the scenes and the players on this Archaean stage”.
The 4th year Masters project represents 50% of the final year of the MEarthSci at Oxford, and often leads to further academic study. Students become immersed in research groups and spend much of their time working with postdocs and Faculty on projects with real academic rigour. Publication in academic journals is not unusual, as seen by Luke Jones’ recent inclusion in a paper on the ocean’s carbon cycle.
Paper: Carbonaceous microstructures from sedimentary laminated chert within the 3.46 Ga Apex Basalt, Chinaman Creek locality, Pilbara, Western Australia, by Keyron Hickman-Lewis, Russell J. Garwood, Martin D. Brasier, Tomasz Goral, Haibo Jiang, Nicola McLoughlin and David Wacey www.sciencedirect.com/science/article/pii/S0301926816300262