Integrative studies of post-glacial cap carbonates in Namibia and Canada

新元古代“雪球地球”假说(Kirschvink，1992；Hoffman等人，1998；Kirschvink等人，2000；Hoffman&Amp；Schrag，2002)不仅试图解释古地磁数据显示热带地区广泛的冰川作用，而且还试图解释其他观察结果，包括广泛存在的冰期后“冰帽碳酸盐”(FairChild，1994；Grotzinger&Amp；Knoll，1995；Kennedy，1996；James et al.，2001；Hoffman&Amp；Schrag，2002)，它们是岩石学上独特的白云岩和/或石灰岩的连续层，直接覆盖全球冰川海洋层序而没有明显的间断。在新元古代地质学中，没有什么问题比盖状碳酸盐的起源更具争议性，也没有比雪球地球概念更核心的问题了。盖状碳酸盐显示了一套独特的神秘的沉积构造和岩石学类型，这些构造和岩石学类型在几乎每个古大陆的大陆边缘和盆地中重复出现。我们建议在纳米比亚北部和加拿大西北部的麦肯齐山脉这两个地区对后马里诺世的沉积序列进行全面的调查。拟议的工作将继续保罗·霍夫曼和丹·施拉格之间富有成效的合作。经过多年的研究，纳米比亚北部奥塔维碳酸盐台地的地层学和化学地层学的发展在区域规模上是相对众所周知的(Hedberg，1979；Kaufman等人，1991；Hoffmann&Amp；Prave，1996；Hoffman等人，1998；Halverson等人，2002；Hoffman，2002)。最近发现了一条从下坡到陆架断裂的连续断面，我们建议在那里详细地绘制和记录前、同时期和后冰期序列。我们还将探索沿这一断面的马里诺海帽碳酸盐中一系列独特的沉积结构，这些构造对冰川消融期间热带风暴的强度具有潜在的影响。我们还建议进行一系列的地球化学分析，以更好地描述冰川后海水的化学和同位素分层和演化。碳和氧同位素记录是从纳米比亚的Maieberg碳酸盐岩层序获得的，但其他替代记录(例如硫酸盐中的硫和氧同位素以及氧化还原敏感元素)仅从一到两个部分获得。在这里请求的支持下，我们希望在纳米比亚复制新的代理记录，并使用已有的样本集从加拿大西北部获得类似和互补的记录。此外，我们建议通过扩大之前的框模拟的范围以包括硫酸盐的硫和氧同位素来推进我们的地球化学模拟工作。尤其令人感兴趣的是位于盖层中白云石和方解石单元之间的连续的富含重晶石的层位。智力价值：与假想的马里诺雪球地球有关的独特的冰后冰盖碳酸盐将在其结构保存最好的地区(加拿大西北部)和其古环境分带最暴露的地区(纳米比亚北部)进行沉积学、岩石学、地球化学学和同位素特征的表征。这些观察结果将为评估关于盖状碳酸盐起源的相互竞争的理论及其在低纬元古代冰川这一更大问题上的意义提供可靠的基础。广泛的影响：由Paul Hoffman和Dan Schrag指导的一名博士生将得到拟议研究的支持，一些针对本科生的实地研究项目将在纳米比亚进行。长期以来，我们一直有一个最小的网站，人们可以从这个网站上下载我们在Snowball Earth上发表和未发表的文章，以及一套教学幻灯片。我们可以做得更多：关于雪球地球的文献每年都在翻一番，它吸引了生物学家、天文学家、气象学家、海洋学家、地球化学家、气候动态学家、冰川学家、大气科学家和各个年龄段的学生的兴趣。我们建议聘请一位兼职的前研究地质学家(Robert S.Hildebrand，1980年博士)，他拥有与加拿大地质调查局发展起来的经验和技能，使他非常适合研究、创建和维护一个有吸引力的、全面的和有用的网站，解决与雪球地球假说有关的问题。

ABSTRACTThe Neoproterozoic "snowball Earth" hypothesis (Kirschvink, 1992; Hoffman et al., 1998; Kirschvink et al., 2000; Hoffman & Schrag, 2002) attempts to explain not only paloemagnetic data showing extensive glaciation in the tropics, but other observations including the widespread occurrence of post-glacial "cap carbonates" (Fairchild, 1994; Grotzinger & Knoll, 1995; Kennedy, 1996; James et al., 2001; Hoffman & Schrag, 2002), which are continuous layers of lithologically-distinctive dolomite and/or limestone that directly overlie glacial marine sequences globally without significant hiatus. No issue in Neoproterozoic geology is more contentious than the origin of cap carbonates, nor more central to the snowball Earth concept. Cap carbonates display a unique panoply of enigmatic sedimentary structures and petrographic types, which recur in broadly the same stratigraphic sequence on continental margins and basins on virtually every paleocontinent.We propose a comprehensive investigation of the post-Marinoan depositional sequence in two regions.northern Namibia and the Mackenzie Mountains of northwest Canada. The proposed work will continue the productive collaboration between Paul Hoffman and Dan Schrag. After years of study, the stratigraphic and chemostratigraphic development of the Otavi carbonate platform in northern Namibia is relatively well known on the regional scale (Hedberg, 1979; Kaufman et al., 1991; Hoffmann & Prave, 1996; Hoffman et al., 1998; Halverson et al., 2002; Hoffman, 2002). A continuous transect from the lower slope to the shelf break was recently identified and we propose to map out and log the pre-, syn- and post-glacial sequences there in detail. We will also explore a series of distinctive sedimentary structures in Marinoan cap carbonates along this transect, which have potential implications for the intensity of tropical storms during the deglaciation. We also propose to perform a series of geochemical analyses to better characterize the chemical and isotopic stratification and evolution of seawater after the glaciations. Carbon and oxygen isotope records have been obtained from the Maieberg capcarbonate sequence in Namibia, but other proxy records (e.g., sulfur and oxygen isotopes in sulfate, and redox-sensitive elements) have been obtained from one or two sections only. With the support requested here, we wish to replicate the new proxy records in Namibia, and to obtain similar and complementary records from northwest Canada, using sample sets already in hand.In addition, we propose to advance our geochemical modeling efforts by extending the scope of our previous box modeling to include sulfur and oxygen isotopes of sulfate. Of particular interest is a continuous barite-rich horizon that occurs between dolomite and calcite units in the cap sequence.Intellectual merit: The unique post-glacial cap carbonate associated with the hypothesized Marinoan snowball Earth will be characterized sedimentologically, petrographically, geochemically and isotopically in the regions where it is texturally best preserved (northwest Canada), and where its paleoenvironmental zonation is best exposed (northern Namibia). These observations will form a reliable basis for evaluating competing theories for the origin of cap carbonates, and their significance in the larger problem of lowlatitude Proterozoic glaciation.Broader impacts: One PhD student supervised by Paul Hoffman and Dan Schrag will be supported by the proposed research, and a number of fieldbased research projects for undergraduates will be undertaken in Namibia. We have long had a minimal website, from which one can download our published and unpublished articles on snowball Earth, as well as a set of teaching slides. We could do much more: the literature on snowball Earth is doubling annually and it attracts interest from biologists, planetologists, meteorologists, oceanographers, geochemists, climate dynamicists, glaciologists, atmospheric scientists, and students of all ages. We propose to hire part-time a former research geologist (Robert S. Hildebrand, PhD 1980) who has experience and skills developed with the Geological Survey of Canada that make him well suited for the task of researching, creating, and maintaining an attractive, comprehensive and useful website for issues relating to the snowball Earth hypothesis.