McMahon, and three co-authors, reported in 2018 that life on Earth extends to several kilometres below the land surface and seafloor. This deep biosphere is second only to plants in its total biomass, is metabolically active and diverse, and is likely to have played critical roles over geological time in the evolution of microbial diversity, diagenetic processes and biogeochemical cycles. However, these roles are obscured by a paucity of fossil and geochemical evidence.
In this paper, the authors applied the recently developed uranium-isotope proxy for biological uranium reduction to reduction spheroids in continental rocks (red beds). Although these common paleo-redox features have previously been suggested to reflect deep bacterial activity, unequivocal evidence for biogenicity has been lacking.
The authors analyses revealed that the uranium present in reduction spheroids is isotopically heavy, which is most parsimoniously explained as a signal of ancient bacterial uranium reduction, revealing a compelling record of Earth’s deep biosphere. This deep biosphere conventionally extends from ~metres depth to several kilometres4,38,39. There is clear evidence that many—perhaps most—reduction spheroids form at the deeper end of this range. In brief:
first, halos are commonly spherical, whereas shallow non-nodular features would be flattened by compaction; second, radiometric ages of authigenic minerals concentrated within some spheroids are >107 years younger than the host rock16,40; third, some spheroids occur in hematite-stained igneous basement, hundreds of metres below the uppermost basement14; fourth, at many localities, the distribution of spheroids was clearly influenced by
pre-existing faults, fractures, cataclastic zones and cleavages younger than the host rock14,41,42. The authors' findings present a clear signal of bacterial uranium reduction in spheroids demonstrably formed at multi-km depth, including one locality (Dingwall) where they appear to be related to the early-stage biodegradation of hydrocarbons, and a weaker but consistent signal at all other localities.
The authors concluded that reduction spheroids represent an important and widespread archive of the deep continental biosphere, present through much of Earth’s geological record. This finding lends weight to the suggestion that reduction spheroids be targeted for analysis and sample return were they to be discovered on Mars22.
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