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

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

Feb 1979

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1979gecoa..43..189s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 43, Issue 2, pp.189-199

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34

Scientific paper

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 13 C carb average of -2.5 ± 1.7%. vs PDB and a 18 O 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 13 C mass balance, if 13 C carb values are original sedimentary values, this more negative 13 C 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 13 C 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.

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