Climatic Temperature Record of Cherts

研究人员将研究古燧石的氧和氢组成，以探索地球气候温度历史烙印在这些岩石的同位素数据中的观点。燧石的同位素组成存在较大的长期变化，这可归因于气候温度的变化、海洋同位素组成的变化或后来同位素记录的改变。越来越多的证据表明，海洋的同位素组成不能解释观测到的变化，这就要求我们重新审视这些长期变化是否记录了地球气候温度的历史。来自关键时期和不同地质环境的新样本将被分析，以建立目前有限的数据库，并测试后来的变化是否对观察到的长期变化负责。在过去的10年里，不同的小组为古生物学和沉积学研究收集了现有的前寒武纪燧石样本，将对这些样本进行分析。初步数据表明，前寒武纪-寒武纪边界附近出现了一次大变暖，并将收集和分析位于该边界附近的新样品。氢同位素分析可以为解释提供额外的限制，这些将用于合适的燧石，例如来自古岩溶层的燧石。变质作用的影响和用于选择最小蚀变样品的野外/岩石学标准将通过分析层状和模块化燧石来评估，这些燧石在不同程度上变质到比长期变化研究中取样的等级更高。随后由大气地下水造成的蚀变将通过共存碳酸盐的同位素分析来评估。中新世蒙特雷组的现有数据将得到加强，以证实即使是在深埋期间形成的燧石也会留下具有当地气候特征的同位素特征。这项工作的预期结果是建立一个新的地质时期气温数据库，可以作为过去气候温度的一个指标进行有意义的评估。

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.