Climatic Temperature Record of Cherts

点击翻译按钮获取中文摘要

9417874 Knauth The oxygen and hydrogen composition of ancient cherts will be investigated to explore the idea that the earth's climatic temperature history is imprinted in isotopic data for these rocks. A large secular variation exists in the isotopic composition of cherts, and this has been attributed to either climatic temperature changes, variations in isotopic composition of the oceans, or later alteration of the isotopic record. Mounting evidence that the isotopic composition of the oceans cannot account for the observed variations warrants a new look at whether the secular variations are recording the earth's climatic temperature history. New samples from key time periods and different geologic settings will be analyzed to build up the currently limited data base and to test whether later alteration is responsible for the observed secular variations. Samples from existing collection of Precambrian cherts made by various groups over the last 10 years for paleontological and sedimentological investigations will be analyzed. Preliminary data suggest a major warming around the Precambrian-Cambrian boundary, and new samples from localities bracketing this boundary will be collected and analyzed. Hydrogen isotope analyses can provide additional constraints on interpretations, and these will be used on suitable cherts such as those from paleokarst horizons. The effects of metamorphism and the field/petrographic criteria used to select least-altered samples will be evaluated by analyzing bedded and modular cherts that have been metamorphosed in vary degrees to higher grades than would ever be sampled for secular variation studies. Later alteration by meteoric ground waters will be evaluated by isotopic analyses of coexisting carbonates. Existing data on the Miocene Monterey Formation will be augmented to confirm that even chert which form during deep burial leave isotopic signatures characteristic of the local climate. The anticipated result of this work is a new data base for che rts over geologic time which can be meaningfully evaluated as an indicator of past climatic temperatures.