(ESH) The Most Severe Glaciations in Earth History: By Presaged Greenhouse Conditions

9510339 Hoffman 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与典型的暖水碳酸盐密切互层，这意味着非常突然的气候振荡。 冰川沉积之前的碳酸盐 富含13 C（13 C +6%PDB），表明有机碳埋藏的比例极高，而那些直接跟随冰川沉积物的是高度贫化的（13 C-4%PDB）。 这意味着大气中CO2的减少是冰川作用的原因，而pO2的增加是后生动物最初辐射的必然结果。 相反，国家的大陆出现，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。 要求提供资金调查新元古代全球变化的两个核心问题：（1）冰川沉积物是否代表全球同步事件的穿时产物或快速大陆漂移？ (2)观察到的变化 13 C碳酸盐反映了海洋表面沃茨的纯粹长期变化，还是部分反映了不同水团的空间效应？ 拟议的研究将由P.I.在正在进行的实地项目上进行。安东尼R。Prave（纽约城市大学）关于纳米比亚达马拉造山带的层序地层学、构造地质学和区域构造的论文，被认为是冈瓦纳大陆新元古代构造重组的关键。 正在进行的项目已经确定了可能适合于U-Pb锆石年代学的火山层位，这些火山层位位于三个独立的冰川形成层之内或之间。 单颗粒U-Pb锆石分析将在马萨诸塞州理工学院在Samuel A.鲍林最近的年代地层学工作极大地改进了寒武纪早期的时间尺度。 目前正在进行的项目已经在建立纳米比亚北方冰川沉积物之上和之下碳酸盐岩的层序地层框架和区域相变化方面取得了很大进展。 碳同位素变化的取样和解释将以这项工作和后续成岩研究为基础。 同位素和成岩作用研究将由哈佛大学的Alan J.考夫曼进行，他发表了大量关于新元古代同位素化学地层学的文章，包括在纳米比亚的勘测研究。 请求提供资金，以支持铀铅和碳同位素分析，并支付考夫曼的薪金和前往实地的旅费。 在纳米比亚的外地行动费用和对其他人员的支助费用将由其他地方承担。

9510339 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 degrees of the equator. The glaciogenic sediments in many regions are intimately intercalated with typical warm-water carbonates, implying very abrupt climatic oscillations. Carbonates preceding the glacial deposits are extraordinarily enriched in 13 C (13C+6% PDB), signifying extreme rates of proportional organic carbon burial, whereas those directly following the glacial deposits are high depleted ( 13C-4% PDB). This implicates a drawdown of atmospheric CO2 as the cause of glaciation and a corollary increase in pO2 as enabling the initial radiation of metazoans. In contrast to the state of continental emergence that presaged Cenozoic and late Paleozoic glacial epochs, stratigraphic and radiogenic isotope 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. Funding is requested to investigate two outstanding questions central to Neoproterozoic global change: (1) Do the glacial deposits represent globally synchronous events of diachronous products or rapid continental drift? (2) Do the observed shifts in 13Ccarbonate reflect purely secular variations in ocean surface waters or in part the spatial effects of differing water masses? The proposed study will be piggy-backed on ongoing field projects by the P.I. and by Anthony R. Prave (City University of New York) on the sequence stratigraphy, structural geology and regional tectonics of the Damara orogen in Namibia, considered a keysto ne for the Neoproterozoic tectonic reorganization of Gondwanaland. The ongoing projects have identified volcanic horizons potentially suitable for U-Pb zircon chronology within or bracketing three separate glaciogenic formations. Single-grain U-Pb zircon analyses will be done by thermal ionization mass spectrometry at the Massachusetts Institute of Technology under the direction of Samuel A. Bowring, whose recent chronostratigraphic work has dramatically refined the early Cambrian time scale. The ongoing projects have already gone far towards establishing the sequence stratigraphic framework and regional facies variations for carbonates above and below the glacial deposits in northern Namibia. Sampling and interpretation of carbon isotopic variations will be grounded on this work and on follow-up diagentic studies. The isotopic and diagenetic studies will be undertaken by Alan J. Kaufman at Harvard University, who has published extensively on Neoproterozoic isotopic chemostratigraphy , including reconnaissance studies in Namibia. Funding is requested to support the U-Pb and C isotopic analyses, and salary and travel to the field for Kaufman. The cost of field operations in Namibia and support for other personnel will be borne elsewhere.