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

新元古代“雪球地球”假说（Kirschvink，1992;霍夫曼等人，1998; Kirschvink等人，2000年;霍夫曼施拉格，2002年）试图解释不仅古地磁数据显示在热带地区广泛的冰川作用，但其他观察，包括广泛出现的冰后期“帽碳酸盐”（费尔柴尔德，1994年;格罗青格诺尔，1995年;肯尼迪，1996年;詹姆斯等人，2001年;霍夫曼施拉格，2002年），这是连续层的岩性独特的白云岩和/或石灰岩，直接覆盖在全球冰川海洋序列没有显着的间隙。在新元古代地质学中，没有什么问题比碳酸盐岩帽的起源更有争议，也没有什么问题比雪球地球概念更重要。帽碳酸盐岩显示了一个独特的全套神秘的沉积结构和岩相类型，它重现在大致相同的地层序列的大陆边缘和盆地上几乎每一个paleocontino.We提出了一个全面的调查后马里诺沉积序列在两个地区。北方纳米比亚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的富有成效的合作。经过多年的研究，纳米比亚北方奥塔维碳酸盐台地的地层和化学地层发展在区域范围内相对较为人所知（Hedberg，1979年;考夫曼等人，1991; Hoffmann Prave，1996;霍夫曼等人，1998; Halverson等人，2002;霍夫曼，2002）。最近确定了一个连续的断面从较低的斜坡到陆架坡折，我们建议绘制和记录的前，同冰期和冰期后的序列有详细的。我们还将探讨一系列独特的沉积结构，在马里诺帽碳酸盐沿着这一断面，这对热带风暴的强度在冰川消退的潜在影响。我们还建议进行一系列的地球化学分析，以更好地表征化学和同位素分层和冰川后的海水演化。碳和氧同位素记录已从纳米比亚的Maieberg碳酸盐岩序列中获得，但其他代用记录（例如，硫酸盐中的硫和氧同位素以及氧化还原敏感元素）仅从一个或两个剖面中获得。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，并从加拿大西北部获得类似的和互补的记录，使用样品集已经在手。此外，我们建议通过扩展我们以前的箱建模的范围，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在盖层序列中白云石和方解石单元之间出现的连续富重晶石层。与假设的马里诺雪球地球有关的独特的冰后期帽碳酸盐将在其结构保存最好的地区进行沉积学、岩相学、地球化学和同位素特征分析（加拿大西北部），以及其古环境地带性最好的暴露（北方纳米比亚）。这些意见将形成一个可靠的基础，用于评估竞争的理论帽碳酸盐岩的起源，其意义在低纬度元古宙glaciation.Broader影响的更大的问题：一个博士生由保罗霍夫曼和丹施拉格监督将支持拟议的研究，和一些基于实地的研究项目，本科生将在纳米比亚进行。我们一直有一个最小的网站，从那里可以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：有关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。我们建议雇用一名兼职的前研究地质学家（罗伯特S。Hildebrand博士，1980年），他拥有加拿大地质调查局开发的经验和技能，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，用于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.