Abstract
Earth’s crust contains a substantial proportion of global biomass, hosting microbial life up to several kilometers depth. Yet, knowledge of the evolution and extent of life in this environment remains elusive and patchy. Here we present isotopic, molecular and morphological signatures for deep ancient life in vein mineral specimens from mines distributed across the Precambrian Fennoscandian shield. Stable carbon isotopic signatures of calcite indicate microbial methanogenesis. In addition, sulfur isotope variability in pyrite, supported by stable carbon isotopic signatures of methyl-branched fatty acids, suggest subsequent bacterial sulfate reduction. Carbonate geochronology constrains the timing of these processes to the Cenozoic. We suggest that signatures of an ancient deep biosphere and long-term microbial activity are present throughout this shield. We suggest that microbes may have been active in the continental igneous crust over geological timescales, and that subsurface investigations may be valuable in the search for extra-terrestrial life.
Highlights
Earth’s crust contains a substantial proportion of global biomass, hosting microbial life up to several kilometers depth
Euhedral calcite in the cavities and veins occurs in several generations as shown by zonation detected in crystal cross-sections (Fig. 2b, f) and secondary ion mass spectrometry (SIMS) analytical transects (Fig. 2c, g, summary in Supplementary Data 1, full data in Supplementary Data 2–4)
The oldest calcite phase is of blocky habit and is intergrown with bitumen, and is related to bitumen emplacement in the fracture system (Figs. 2a and 3a, b)
Summary
Earth’s crust contains a substantial proportion of global biomass, hosting microbial life up to several kilometers depth. Studies that address deep ancient life in the igneous continental crust by combining either isotopic, morphological, or molecular biosignatures with geochronology only exist from a few Fennoscandian shield sites (Fig. 1, yellow symbols). In the Fennoscandian shield, the morphological evidence of ancient subsurface microbial life include chitin-bearing partly preserved and partly mineralized fungal hyphae[31], and putative prokaryotic cells preserved within calcite veins[17] It still remains unclear whether the biosignatures from the few sites described so far are anomalous or represent the bulk of the habitable continental igneous crust over geological timescales. The molecular composition of solid bitumen and biomarkers from the mineral coatings are determined, including the stable carbon isotope composition of specific organic compounds
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