Carbon isotope geochemistry of the 3.7 × 10 9-yr-old Isua sediments, West Greenland: implications for the Archaean carbon and oxygen cycles

Abstract

One hundred and twenty-four carbonate samples from the meta-sedimentary sequence of the 3.7 × 10 9 yr old Isua supracrustal belt (W-Greenland) have yielded a δ 13C carb average of −2.5 ± 1.7%. vs PDB and a δ 18O carb average of +13.0 ± 2.5%. vs SMOW. The oxygen mean comes fairly close to the averages of other early Precambrian carbonates. The carbon average, however, is some 2%. more negative than those of younger marine carbonates. In terms of a simple terrestrial 13C mass balance, if δ 13C carb values are original sedimentary values, this more negative δ 13C average would imply a considerably smaller C org C carb ratio in the sedimentary shell during Isua times, and would thus support the concept of a gradual buildup of a sedimentary reservoir of organic carbon during the early history of the Earth. Since, however, the Isua supracrustal rocks have experienced amphibolite-grade metamorphism, which in other areas has been shown to lower δ 13C carb values, it is most likely that the original values of these rocks were approx 0%.. This indicates that C orx and C carb were present in the ancient carbon reservoir in about ‘modern’ proportions. Unless this early stabilization of the terrestrial carbon cycle in terms of a constant partitioning of carbon between the reduced and oxidized species is shown to have been caused by some inorganic geochemical process, a considerably earlier start of chemical evolution and spontaneous generation of life must be considered than is presently accepted.