Collaborative Research: Geochemistry and Paleontology of Precabrian Exposure Surfaces

野外工作、岩石学和同位素工作相结合，在前寒武纪碳酸盐岩地层(800米。年。贝克泉白云石，加利福尼亚州1200英里。年。AZ)，在前寒武纪侵蚀事件中被硅化。R Horodyski在这些层位的硅质岩样本中发现了微型化石，这是居住在前寒武纪陆地表面的第一个生物化石记录。伴生碳酸盐的碳同位素测量表明，陆地表面广泛地占据了陆地光合作用群落。因此，硅质岩中的微体化石可能不是孤立的例子，而是这个广泛群落的组成部分。将尝试建立前寒武纪陆地环境的第一个古生物记录。野外、同位素和岩石学研究将确定古岩溶层的古环境条件，并将详细确定前寒武纪风化过程中形成的硅质相，而不是原始海相碳酸盐岩早期成岩过程中形成的硅质相。在厚厚的岩石学切片中，将检查具有强烈结构和地球化学特征的水下建造的硅质岩，以寻找微化石的存在。将评估微结构的生物起源能力，并努力为任何发现的微化石建立生物亲缘关系。将特别注意将与暴露表面有关的微化石与海底硅质岩中的微化石进行比较，以确定发现的与暴露表面有关的微化石确实代表了生活在陆地环境中的微生物。

A combination of field, petrographic, and isotopic work has identified 2 horizons in Precambrian carbonate strata (800 m.yr. Beck Spring Dolomite, CA. and 1200 m.yr. Mescal Limestone, AZ) which were silicified during Precambrian erosion events. Microfossils have been discovered in chert samples from these horizons by R Horodyski and represent the first fossil record of organisms that inhabited the Precambrian land surface. Carbon isotope measurements in the associated carbonate reveal that the land surface was extensively occupied by a terrestrial photosynthesizing community. The microfossils in the chert are therefore probably not isolated examples but are elements of this extensive community. An attempt will be made to establish the first paleontologic record for the Precambrian terrestrial environment. Field, isotopic, and petrographic work will establish the paleoenvironmental conditions of the paleokarst horizons and will establish, in detail, which silica phases formed during Precambrian weathering as opposed to those which formed during early diagenesis of the original marine carbonates. Cherts with strong textural and geochemical characteristics indicative of subaerial formation will be examined in thick petrographic sections for the presence of microfossils. The biogenicity of microstructures will be evaluated, and an effort will be made to establish biologic affinities for any microfossils found. Particular attention will be given to comparing microfossils associated with the exposure surfaces with those in the underlying marine cherts to establish that the microfossils found associated with the exposure surfaces truly represent microorganisms that were living in a terrestrial setting.