Reduction spheroids preserve a uranium isotope record of the ancient deep continental biosphere

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. Here we apply the recently developed
uranium-isotope proxy for biological uranium reduction to reduction spheroids in continental
rocks (red beds). Although these common palaeo-redox features have previously been
suggested to reflect deep bacterial activity, unequivocal evidence for biogenicity has been
lacking. Our analyses reveal 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.