BIOGEOCHEMICAL INVESTIGATIONS OF ~3.8 GA SUPRACRUSTAL ROCKS FROM SOUTHERN WEST GREENLAND

The oldest known terrestrial sedimentary rocks have been extensively metamorphosed through their history during several thermal and deformational events. Yet, the development and recognition of biosignatures in such rocks is the only direct means we can use to investigate the emergence of the earliest biosphere on Earth and provides a baseline in the search for evidence of extraterrestrial life. Garnet-biotite schists and Fe-rich quartzites probably representing ferruginous pelites and banded iron formations respectively are found throughout the Isua Supracrustal Belt and on the island of Akilia in southern West Greenland. We report new sulfur and nitrogen isotope data for these early Archean “metasediments”. Our results underscore the observation that redox chemistry of the terrestrial atmosphere has co-evolved with the biosphere, from an anoxic system in the Archean to an oxygenated Proterozoic surface environment. Multiple sulfur isotopes (Δ³³S) data collected by mutlicollector SIMS on sulfides in these Archean rocks reveal values from -0.3 to +2.0‰, which indicate mass independent fractionation and imply that the sulfur was cycled through an anoxic atmosphere before incorporation in the Greenland rocks. Furthermore, δ³⁴S data for these sulfides could indicate the presence of microorganisms capable of elemental sulfur reduction, an interpretation supported by molecular phylogenetic arguments, which point to an early emergence of this metabolic pathway. Biotite in schists from Isua contain high concentrations of structural ammonium (NH₄⁺; between 233 and 512 ppm) and is interpreted to have been released from maturing sedimentary organic matter and therefore are diagnostic of a biological origin for the nitrogen. Step-heating laser mass spectrometric analyses of biotite separates from Isua metapelites revealed δ¹⁵N values as low as -2‰ for structural NH₄⁺, a potential signature of ammonium assimilation and/or biological nitrogen fixation at time of deposition. This interpretation is likewise supported by molecular phylogenetic analyses that indicate an early evolution of these nitrogen metabolisms. Our work on sulfur and nitrogen isotope geochemistry hints at the presence of a metabolically diverse early Archean biosphere and represents an analog for early geochemical evolution on Mars.