The 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.
