Isotopic Studies of Precambrian Silicified Erosion Surfaces

前寒武纪硅化侵蚀面的同位素研究

• 批准号: 8904347
• 负责人: L. Paul Knauth
• 金额: $ 8万
• 依托单位: Arizona State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1989
• 资助国家: 美国
• 起止时间: 1989-07-01 至 1991-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=8904347&HistoricalAwards=false
• 关键词: Isotopic; Studies; Precambrian; Silicified; Erosion

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.

2元古界顶面古岩溶层位沿着 将对碳酸盐岩单元进行测绘、取样和同位素分析， 调查，以获得定量的气候 温度和进一步调查的可能性， 前寒武纪地表的生物活动。 的 古岩溶层位沿着不整合面分布， 发生在内陆的风化作用， 沉积，硅化，早期的硅化， 亚稳碳酸盐相 新一代的燧石和 碳酸盐在风化、溶解过程中沉淀， 碳酸盐的蚀变;一个主要目标是明确 区分和分离这些次级阶段， 在陆地条件下， 在海洋或沿海条件下。 大气沃茨形成的次生硅质岩 相对于早期阶段，18 O和D强烈亏损 应该能定义出一个平行于流星的同位素阵列 水位线与早期的数据形成强烈对比 燧石 这种关系，已经大致 为一个地方建立，将允许大大改善表面 在一个关键的时间间隔内建立温度， 元古代的 碳酸盐岩风化物碳同位素分析 这个时代表明陆地表面已经 由进行光合作用的生物组成。 碳同位素 前寒武纪地表有机活动的证据 将在一个新的更好的地方进行进一步评估， 似乎有更大更有说服力的碳同位素 变化.