Age of the Isua Supracrustal Sequence of West Greenland: A plausible repository for early life

Abstract

The-3.7-3.8 Gyr-old Isua supracrustal (i.e. sedimentary and volcanic) rocks of West Greenland preserve the oldest known fragmentary record of terrestrial surface environments. Evidence for biological activity at that time is still inconclusive, largely because of the intense overprint of metamorphism and deformation. Nevertheless, the overall depositional environment of the Isua rocks was similar to that of the oldest known (-3.45 Gyr) sedimentary rocks in Western Australia, where putative microbial fossils have been identified [1]. After a brief review of published dates for the Isua supracrustals, some of which are regarded as problematical, we present a new samarium-neodymium (Sm-Nd) whole-rock regression age of 3772 ± 33 Myr for selected rock types which were apparently resistant to resetting by later metamorphism. This age value with its associated error probably encompasses the deposition of most of the Isua supracrustals. It postdates termination of well-preserved and reliably dated major lunar impacts by some 30-40 Myr. Contemporaneous impacts are widely postulated to have affected earth and inhibited life. A metamorphosed graphitic shale from Isua yields a Sm-Nd age of-3.1-3.2 Gyr, postdating deposition by some 0.6 Gyr. 13CPC ratios in Isua rocks have been interpreted to indicate biologically mediated isotope fractionation [2]. Demonstration of a strong metamorphic overprint implies that the effects (if any) of thermally and biologically mediated carbon isotope fractionation must be clearly identified and distinguished.