The Most Severe Glaciations in Earth History: Presaged by Greenhouse Conditions?

9630928霍夫曼 地球历史上最极端的冰川作用发生在元古代末期，当时冰缘在海平面延伸到赤道10度以内。 许多地区的冰川沉积物夹在典型的暖水碳酸盐之间，这意味着非常突然的气候振荡。 冰川沉积之前的碳酸盐异常富集13 C（13 C-4%PDB）。 高度正的13 C值加上低的海水87 Sr/ 86 Sr表明，大气中CO2的下降（可能与高有机生产力和深层缺氧海洋中碳的储存增强有关）是冰川作用的原因。 相反的国家的大陆出现（因此高风化率），预示着新生代和晚古生代冰期，地层和放射性同位素证据表明，新元古代冰川发生在响应大陆洪水。 因此，新元古代冰川作用可能与短命的阿什吉尔（晚奥陶纪）冰川事件有更多的共同点。 这些古老的事件说明了考虑整个地质记录的重要性，它揭示了当前模型无法预测的事件和过程，也没有在新生代代表的1.4%的地球历史中表达出来。 要求重新提供资金，以继续进行综合地层调查。 从第一年的资助非常成功的结果表明，长期变化的13 C值的碳酸盐在前Vendian Otavi组的纳米比亚是区域一致的，并不受大气成岩作用，也不与主要岩相的变化。 在两个主要的层序边界上注意到大的同位素位移，但在其他次要的边界上，13 C值没有明显的变化。 最初选择Otavi群是因为厚碳酸盐岩和冰川杂岩的密切联系。 第二个前Vendian冰川事件-在Otavi组中新认识到的-首先由初步的碳同位素数据表明。 而且，随后的野外工作证实了两个地层上不同的冰川沉积物的存在，这需要对区域地层进行大规模的修订。 这些意见随后证实了U-Pb锆石测年的奥塔维碳酸盐岩内的灰层。 这些早期的研究结果证明了物理层序地层学、同位素化学地层学和U-Pb年代地层学三管齐下方法的有效性。 哈佛大学的Alan J. Kauf man将继续进行碳和锶同位素研究，而马萨诸塞州理工学院将在Samuel A.鲍林 请求提供额外资金，以支持C-和Sr-同位素锆石分析，并为考夫曼和一名实验室技术员提供兼职工资。 霍夫曼在纳米比亚的大部分实地业务费用将由其他地方承担。 ??

9630928 Hoffman The most extreme glaciations in Earth history occurred near the end of the Proterozoic eon, when ice margins extended at sea level to within 10 o of the equator. Glaciogenic sediments in many regions are sandwiched between typical warm-water carbonates, implying very abrupt climatic oscillations. Carbonates preceding the glacial deposits are extraordinarily enriched in 13C 13C-4% PDB). The highly positive 13C values coupled with low seawater 87 Sr/ 86 Sr suggests that a drawdown of atmospheric CO2 (plausibly related to high organic productivity and enhanced storage of carbon on a deep anoxic ocean) was the cause of glaciation. In contrast to the state of continental emergence (and hence high weathering rates) that presaged Cenozoic and late Paleozoic glacial epochs, stratigraphic and radiogenic isotopic evidence suggest that Neoproterozoic glaciations occurred in response to continental inundation. Accordingly, the Neoproterozoic glaciations may have had more in common with the short-lived Ashgill (late Ordovician) glacial event. These ancient occurrences illustrate the importance of considering the entire geological record, which reveals events and processes not predicted by current models, nor expressed in the 1.4% of Earth history represented by the Cenozoic. Renewed funding is requested to continue integrated stratigraphic investigations. Highly successful results from the first year of funding showed that secular variations in 13C values of carbonates in the pre-Vendian Otavi Group of Namibia are regionally consistent, and are not influenced by meteoric diagenesis, nor do they correlate with changes in primary lithofacies. Large isotopic shifts are noted across the two major sequence boundaries, but across other minor boundaries 13C values show no apparent change. The Otavi Group was initially selected because of the close association of thick carbonates and a glacial diamictite. A second pre-Vendian glacial event--newly recognized in the Otavi Group- -was first indicated by preliminary carbon isotope data. And, subsequent field work confirmed the existence of two stratigraphically distinct glacial deposits, which required wholesale revision of the regional stratigraphy. These observations were subsequently corroborated by U-Pb zircon dating of an ash layer within the Otavi carbonates. These early results demonstrate the efficacy ofthe three-pronged approach involving physical sequence stratigraphy, isotope chemostratigraphy, and U-Pb chronostratigraphy. Carbon and strontium isotopic studies will continue to be undertaken by Alan J. Kauf man at Harvard University, while single-grain U-Pb zircon analyses will be continued at the Massachusetts Institute of Technology under the direction of Samuel A. Bowring. Additional funding is requested to support the C- and Sr-isotopic zircon analyses, as well as a part-time salary for Kaufman and a laboratory technician. The cost of most field operations for Hoffman in Namibia will be borne elsewhere. ??