Isotopic Studies of Precambrian Silicified Erosion Surfaces

为了获得定量的气候温度和进一步研究前寒武纪陆地表面生物活动的可能性，将沿2个元古代碳酸盐岩单元的古岩溶层位进行测绘、采样和同位素研究。古岩溶层沿不整合面发育，代表了早期亚稳碳酸盐相原始沉积、硅化和白云化后较长时间内发生的内陆风化作用。在碳酸盐的风化、溶蚀和蚀变过程中，形成了新一代的燧石和碳酸盐；一个主要目标是明确区分和区分这些在陆地条件下形成的次级阶段与在海洋或海岸条件下形成的早期阶段。相对于早期阶段，由大气水形成的次生燧石在18O年和18d年应该被强烈地耗尽，并且应该定义一个平行于大气水线的同位素阵列，与早期燧石的数据形成强烈对比。这种关系已经在一个地区近似地建立起来了，它将使我们能够在元古代的一个关键时间间隔内大大精确地建立地表温度。先前对这个时代风化碳酸盐的碳同位素分析表明，陆地表面已经被光合作用的生物所覆盖。前寒武纪陆地表面有机活动的碳同位素证据将在一个新的、更好的地点得到进一步评估，该地点似乎有更大、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.