Isotopic Studies of Precambrian Silicified Erosion Surfaces

将对两个元古界碳酸盐单元顶部的古岩溶层位进行测绘、取样和同位素研究，以获得定量的气候温度，并进一步调查前寒武纪陆地表面生物活动的可能性。古岩溶层沿不整合面分布，代表了发生在内陆的风化作用，发生在早期亚稳碳酸盐相的原始沉积、硅化和白云岩化之后很长一段时间。在碳酸盐的风化、溶解和改造过程中形成了新一代的硅质岩和碳酸盐；一个主要目标是清楚地区分和区分这些在陆地条件下形成的次生相和在海洋或沿海条件下形成的早期相。由大气降水形成的次生硅质岩在18O和D中相对于早期阶段应强烈亏损，并应确定平行于大气水线的同位素阵列，这与早期硅质岩的数据形成了强烈对比。这种关系已经为一个地方建立了大约，这将允许在元古界的一个关键时间间隔内建立非常精确的地表温度。以前对这个时代的风化碳酸盐进行的碳同位素分析表明，陆地表面已经有光合作用的有机体。将在一个新的更好的地点对前寒武纪陆地表面有机活动的碳同位素证据进行进一步评估，该地点似乎具有更大和更令人信服的碳同位素变化。

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.