Isotopic Studies of Precambrian Silicified Erosion Surfaces

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Paleokarst horizons along the topes of 2 Proterozoic carbonate units will be mapped, sampled, and isotopically investigated in order to obtain quantitative climatic temperatures and to further investigate the possibility of biological activity on the Precambrian land surface. The paleokarst horizons occur along unconformities and represent weathering that occurred well inland and long after the original deposition, silicification, and dolomitization of early, metastable carbonate phases. New generations of chert and carbonate precipitated during the weathering, solution, and alteration of the carbonate; a major goal is to clearly distinguish and separate these secondary phases which formed under terrestrial conditions from the earlier phases which formed under marine or coastal conditions. The secondary cherts that formed from meteoric waters should be strongly depleted in 18O and D relative to the earlier phases and should define an isotopic array parallel to the meteoric water line that contrasts strongly with data for the earlier cherts. This relationship, which has already been approximately established for one locality, will allow greatly refined surface temperatures to be established for one key time interval in the Proterozoic. Previous carbon isotope analyses of weathered carbonates of this age have suggested that the land surface already was populated by photosynthesizing organisms. Carbon isotope evidence for organic activity on the Precambrian land surface will be further evaluated at a new and better locality which appears to have even larger and more convincing carbon isotopic variations.