Ichnofossils in Volcanic Glass from Palaeoproterozoic Hydrothermal Vents Were Burrowed by Microorganisms Probably Seeking Phosphate

Ichnofossils in basaltic glass are putative microscopic trace fossils occurring on modern seafloor and throughout Earth history. While their biological origin remains debated, it is unknown why microorganisms thrive in volcanic glass. Here we show ichnofossils associated with biosignatures from sedimentary interbeds of pillow basalt in the ~1,870 million years old Flaherty Formation, Belcher Group, Canada. These rocks are associated with meter-size, pinnacle-shaped structures of silicified basalt, pahoehoe basalt with paleo-caliche, and isolated pods of gossan indicating shallow marine hydrothermal venting. Trails of spheroidal ichnofossils composed of titanite and organic matter surround abundant nanoscopic-size apatite and lepidocrocite, suggesting they caused rock dissolution to obtain phosphate. Stable isotopes provide complementary biosignatures for possible chemolithotrophy. While there is evidence for abiotic carbon-based reactions, the biological origin of spheroidal ichnofossils is supported by 13C-depleted organic matter and calcite, suggesting they were burrowed by microorganisms likely seeking phosphate in volcanic glass near hydrothermal vents.