ORGANIC GEOCHEMISTRY OF THE 3,800 MILLION YEAR OLD METASEDIMENTS FROM ISUA, GREENLAND

The Isua supracrustal formation is comprised of the oldest known terrestrial rocks, dated at 3,800 million years. Although metamorphosed to lower Amphibolite facies, the Isua metasediments may contain evidence of prebiotic activity or indications of the emergence of biological evolution. An analysis of the carbonaceous matter found within the metasediments was made to try to uncover these hidden clues to the conditions which gave rise to life on the Earth.; Field work during the summers of 1977 through 1980 returned hand samples from most metasedimentary outcrops. Research centered on banded ironstones since they may have been formed through biologically mediated reactions. The samples were thoroughly processed and tested to remove all soluble organic contaminants.; A qualitative determination of the graphite crystalline order was made using x-ray diffractometry. Samples ranged from fully-ordered to slightly disordered structures. The H/C atomic ratios were below 0.1. Pyrolysis of the Isua graphitic isolates liberated no discernible organic fragments.; Nitric acid oxidation of the Isua graphite produced primarily mellitic acid. Graphite isolated from a chlorite schist produced, in addition to mellitic acid, ethanedioic, propanedioic, butanedioic, 1,2,3,4-benzenetetracarboxylic, and benzenepentacarboxylic acids. This is viewed as evidence that the graphite was derived from organic matter.; The carbon isotopic composition of twenty-seven graphite isolates and, where possible, associated carbonates were measured. Values for the graphite ranged from -5.8% to -24.2%, PDB. Although it has been suggested that the ('13)C depleted graphite indicates that photosynthetic fractionation occurred 3,800 million years ago (Schidlowski et al, 1979), it is not certain that the measurements reflect syngenetic values. No Isua graphite has been found in this or other studies where the graphite has a lower value than -18% and occur with an abundance greater than 150 ppm. It is conceivable that the ('13)C depleted values are due to post-depositional contaminants. Furthermore, in Isua metasediments where the graphite is greater than 150 ppm, the graphite appears to be at, or approaching, isotopic equilibrium with associated carbonates.; Evidence on the existence of life 3,800 million years ago is inconclusive. Banded ironstones and certain flat-pebble conglomerates are merely suggestive of cyanophytic bacteria. The graphite could be derived from either abiotic or biogenic compounds. Metamorphism has obscured the original nature of the carbon.