Lava life: Evidence for fossilized microbial filaments in 2.46 Ga vesicular basalts

The modern oceanic crust, dominated by basaltic lavas and gabbroic sills, is home to a diverse ecosystem of prokaryotes and eukaryotes that are fueled by chemical reactions during seawater alteration of mafic igneous rocks. Despite the abundance and antiquity of submarine basalts, the fossil record of this deep igneous habitat is sparse and controversial. Here we report the presence of filamentous microstructures in drill-core samples (drill depth 610 m) of vesicular submarine basalts intercalated with the 2.46 Ga Kuruman Iron Formation, in Griqualand West, South Africa. A network of sinuous and uniformly thick filaments, which are lined by fine-grained Fe-rich phyllosilicates, is preserved in calcite- and stilpnomelane-cemented vesicles near the top of the flow. Based on their uniform size, regular shape, and similarity to putative microfossils in younger vesicular basalts, the filaments are interpreted to be relicts of cavity-dwelling (cryptoendolithic) microbes that were mineralized by Fe-silicate clays and encased in calcite cement shortly after basalt eruption. Our results suggest that ancient basaltic lavas were rapidly colonized by microbes in a thermally and geochemically active environment that today is occupied by chemoautotrophs and heterotrophs. The findings confirm submarine lavas as a habitat for early life, and extend the antiquity of basalt-hosted cryptoendoliths, hinting at a lengthy fossil record for the deep igneous biosphere.