Evaluating the biosignature potential of nitrogen concentrations in graphite and associated K-silicates

The oldest remnants of life on Earth from various localities in the Isua supracrustal belt Greenland date back to >3.7 billion years ago (Ga). They are form graphite, whose biogenicity is controversial. Previous studies used presence and isotopic composition nitrogen graphite along argue both for against biogenicity. To determine if chemistry can indeed serve as a biosignature, we investigated hydrothermal deposit south-east (1.87–1.82 Ga). We found indications that molar C/N ratios may be similar those biogenic Archean rock record, meaning content itself perhaps not diagnostic ancient life, requiring caution future studies. However, also revealed unusually low N concentrations associated silicates, despite wide range K up 5 wt%. Using thermodynamic model speciation paired with previously published partition coefficients ammonium K-silicates, show abiotic process explain these N-concentrations around 1 μg/g potassic silicates. Higher >10 μg/g, such graphitic metapelites supcracrustal belt, would, however, require an ammonium-rich fluid. Such fluid most easily derived breakdown biomass within sediments prior graphitization. therefore conclude silicates indirect biosignature. Our approach supports previous inferences at least 3.7 Ga.