Evidence for Early Life in ∼3.5 Billion-Year-Old Pillow Lavas

Recently discovered biosignatures in the formerly glassy rims of ∼3.5 billion-year-old pillow lavas from the Barberton Greenstone Belt (BGB) in South Africa suggest they were colonized by microbes early in Earth's history. These subaqueous volcanic rocks represent a new geological setting in the search for early life on Earth. This is not entirely surprising since microbial alteration of basaltic glass in pillow lavas and volcaniclastic rocks has been well documented from recent oceanic crust and well-preserved ophiolites. The BGB magmatic sequence contains exceptionally well-preserved mafic to ultramafic pillow lavas, sheet flows, and intrusions interpreted to represent 3.48 to 3.22 billion-year-old oceanic crust and island arc assemblages. We observed micron-sized tubular structures mineralized by titanite in the formerly glassy rims of the BGB pillow lavas. Based on their similarity to textures observed in recent glassy pillow basalts we interpret these structures to represent ancient traces of microbial activity formed during biogenic etching of the originally glassy pillow rims as microbes colonized the glass surface. Petrographic observations coupled with overlapping metamorphic and magmatic dates indicate this process occurred soon after eruption of the pillow lavas. Subsequent greenschist facies seafloor hydrothermal alteration caused the structures to be mineralized by titanite; a process also observed in ophiolitic pillow lavas of much younger age. X-ray mapping reveals the presence of carbon along the margins of the tubular structures interpreted as residual organic material. Disseminated carbonates within the microbially-altered BGB pillow rims have low carbon isotope values consistent with microbial oxidation of organic matter. In contrast, disseminated carbonate in the crystalline pillow interiors have carbon isotope values bracketed between Archean marine carbonate and mantle carbon dioxide. It remains to be seen how deep into the Archean oceanic crust these microbes penetrated. In modern oceanic crust their highest activity occurs near 300m subsurface at temperatures around 70 degrees Celsius. If, as some suggest, the Archean ocean was relatively hot, then the depth distribution of the microborings should be biased to pillows shallower in the crust than in the modern seafloor. Based on the observed petrographic and geochemical features we propose the glassy rims of the BGB pillow lavas hosted microbial life that left behind biomarkers ∼3.5 billion years ago. Remnants of Archean oceanic crust may thus be one of the most promising places to search for vestiges of early life on Earth.