Fracture-filling calcite from the deep continental crust – an archive for microbial lipid biomarkers

A significant fraction of Earth’s prokaryotic biomass is produced in the continental deep biosphere, but the exact size, diversity and evolution of this important habitat is still incompletely understood. In several studies, we targeted ancient microbial activity in the deep subsurface by analyzing inter alia the lipid biomarker content of fracture calcites. We detected fatty acids (mainly straight-chained and branched C_12:0 to C_20:0) of biological origin in calcites from various fracture systems of the continental crust (Fennoscandian and Greenlandic shields), including a broad depth interval (down to 1000 m) and different precipitation ages (back to 410 Ma; e.g., Drake et al., 2015, 2021, 2023, 2024). In locations where calcite co-occurred with pyrite, different branched C_15:0 and C_17:0 acids were found (e.g., Drake et al., 2018, 2021, 2023, 2024), which represent biomarkers of sulfate reducers. In addition, α,ω-diacids of unknown source were sometimes present. Based on our results, it seems that fatty acids are well preserved in facture calcites from the deep continental subsurface over long geological time scales. Such enhanced preservation likely derives from the physicochemical properties of the carboxyl groups of the fatty acids, matching those of Ca²⁺-ions on the calcite surface (e.g., Suess, 1970; Zullig and Morse, 1988). In addition, fatty acids and diacids may also serve as an initial nucleation site for calcite precipitation in the fractures, thereby being entrapped in the mineral lattice. Fracture calcite from the deep continental subsurface may therefore constitute an important archive for lipid biomarkers of ancient subsurface microorganisms.