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

摘要新元古代“雪球地球”假说（Kirschvink, 1992; Hoffman et al., 1998; Kirschvink et al., 2000; Hoffman & Schrag, 2002）不仅试图解释显示热带地区广泛冰川作用的古磁数据，还试图解释其他观测结果，包括冰期后广泛出现的“帽状碳酸盐岩”(Fairchild, 1994; Grotzinger & Knoll, 1995; Kennedy, 1996; James et al., 2001；Hoffman & Schrag, 2002)，它们是岩性独特的白云岩和/或石灰岩的连续层，直接覆盖在全球的冰川海洋序列上，没有明显的间隙。在新元古代地质学中，没有什么问题比盖层碳酸盐岩的起源更有争议，也没有什么问题比雪球地球的概念更重要。盖层碳酸盐岩显示出一种独特的、神秘的沉积构造和岩相类型，它们在几乎每个古大陆的大陆边缘和盆地中大致相同的地层层序中反复出现。我们建议对两个地区的后马里诺期沉积层序进行综合研究。纳米比亚北部和加拿大西北部的麦肯齐山脉。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，在区域尺度上，对纳米比亚北部奥塔维碳酸盐岩台地的地层学和化学地层学的发展已经比较了解（Hedberg, 1979; Kaufman et al., 1991; Hoffmann & Prave, 1996; Hoffman et al., 1998; Halverson et al., 2002; Hoffman, 2002）。最近发现了从下斜坡到陆架断裂的连续样带，我们建议详细绘制和记录那里的前、同期和后冰期序列。我们还将沿着这条样带探索马里诺盖碳酸盐岩中一系列独特的沉积结构，这对消冰期间热带风暴的强度有潜在的影响。我们还建议进行一系列的地球化学分析，以更好地表征冰期后海水的化学和同位素分层和演化。从纳米比亚的Maieberg碳酸盐岩层序中获得了碳和氧同位素记录，但其他代用记录（如硫酸盐中的硫和氧同位素，以及氧化还原敏感元素）仅从一两个剖面中获得。在这里所要求的支持下，我们希望在纳米比亚复制新的代理记录，并利用现有的样本集从加拿大西北部获得类似和补充的记录。此外，我们建议通过扩展我们之前的盒模型的范围，包括硫酸盐的硫和氧同位素，来推进我们的地球化学建模工作。特别令人感兴趣的是盖层序中白云岩和方解石单元之间的连续的富含重晶石的层位。知识价值：独特的冰期后碳酸盐岩盖层与假设的马里诺雪球地球相关联，将在其结构保存最好的地区（加拿大西北部）和古环境带暴露最好的地区（纳米比亚北部）进行沉积学、岩石学、地球化学和同位素学表征。这些观测结果将为评估盖层碳酸盐岩起源的相互竞争的理论，以及它们在低纬度元古代冰川作用这一更大问题中的意义，提供可靠的基础。更广泛的影响：由Paul Hoffman和Dan Schrag指导的一名博士生将得到拟议研究的支持，并将在纳米比亚开展一些基于本科生的实地研究项目。我们一直有一个小型的网站，人们可以从那里下载我们在雪球地球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翻一番，它吸引了生物学家、行星学家、气象学家、海洋学家、地球化学家、气候动力学家、冰川学家、大气科学家和各个年龄段的学生的兴趣。我们建议聘请一位前研究地质学家（Robert S. Hildebrand， 1980年博士），他在加拿大地质调查局拥有丰富的经验和技能，非常适合研究、创建和维护一个有吸引力、全面和有用的网站，以研究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.