Some perspectives on isotope biosignatures for early life

The idea of using stable isotope compositions of light elements, particularly of carbon, as a sign of biological activities (biosignatures), both present and past, dates back to the dawn of stable isotope geochemistry in the 1940s. In the wake of the discovery of large variations in 13C/12C ratios among various carbon-bearing materials including plants and fossil fuels, the contentious debate ensued between Kalervo Rankama and Harmon Craig in the early 1950s whether the origin of graphitic carbon in ancient rocks (biogenic vs. abiogenic) can be uniquely identified by its isotopic compositions. During the last half-century, great progress has been made in understanding biogeochemical processes in modern and ancient terrestrial environments. Rapid developments in the last decade of novel analytical techniques and the birth of new geoscience fields such as geomicrobiology and astrobiology has prompted resurgence in the application not only of conventional light stable isotopes, but also of those of metal and other intermediate elements. However, recent debates on the origin of graphitic carbons from Early Archean rocks from Australia and Greenland are very much reminiscent of the Rankama–Craig debate half a century ago. Here, an attempt is made to review briefly the history of isotope biosignatures and to critically assess current criteria for early life. A number of possible abiotic pathways exist that lead to the synthesis of various organic and reduced carbon compounds (alkanes, PAH, amino acids, lipids, graphitic carbon, etc.) depleted in 13C due to large kinetic isotope effects (up to ca. − 60‰). It also seems very likely that the mantle contains indigenous carbons (graphite, diamond, dissolved carbons, carbide) with low δ13C values (− 20‰ to − 30‰), which have previously been considered recycled sedimentary organic carbons or surface contamination. This analysis prompts us to reassess our current and future strategies for identifying early and extraterrestrial life using isotope biosignatures.