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Thilo Bechstädt (Info)
Rainer Zühlke (Info)
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+49 6221 54-8292
+49 6221 54-6055
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+49 6221 54-5503
Thilo Bechstädt (Info)
Rainer Zühlke (Info)
Phone
+49 6221 54-8292
+49 6221 54-6055
Fax
+49 6221 54-5503
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Sedimentological, Isotopic and Palynological Studies, N-Namibia – Evidence for a Neoproterozoic Cold-Water Ice-Patch Earth |
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| Abstract | ||
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| Scientists Thilo Bechstädt, GeoResources and University of Heidelberg Hartmut Jäger, GeoResources and University of Heidelberg Guy Spence, University of Heidelberg (now: University of Manchester) |
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| Abstract The Snowball Earth hypothesis (SEH) suggests two or more separate global glaciations, which reached low latitudes and occurred during the late Neoproterozoic (e.g., Hoffman et al. 1998). The hyptheses is mainly based on the widespread distribution of diamictites (interpreted as glacial deposits) reaching out to equatorial areas, overlain by Cap Carbonates showing negative δ13C excursions, interpreted as synglacial collapse in organic productivity in the oceans caused by the isolation of the global ocean from the atmosphere due to the globally thick ice cover (Hoffman et al. 1998). The escape mechanism from the global glaciation proposed by SEH is the closing down of the carbon cycle leading to an uncontrolled rise in atmospheric CO2 from volcanic outgassing causing a subsequent run-away global warming. However, there is independent sedimentary evidence that areas of open ocean coexisted with low latitude sea-ice (e.g. Embelton and Williams 1986) and for advance and retreat of wet based glaciers (e.g. Allen et al. 2004). Detailed analysis of the Late Neoproterozoic Ghaub and Maieberg Fm (N-Namibia) presents new data to this debate. In many areas the strongly negative δ13C values start already distinctly underneath the diamictites, and continue up-section after the cap carbonate deposition, showing partly even more negative values than in the cap carbonates themselves. This has been shown already by Hoffman et al. (1998), but was not adequately interpreted. In our opinion, this contradicts the predictions of SEH and requires an alternative. Strong lateral and vertical facies changes occur before and in the aftermath of the diamictites. Crystalline basement rocks were found in the latest sheddings only, indicating uplift on the scale of thousands of metres. Tensional activity on the shelf of the southern margin of the Congo craton was repeated, creating different subbbasins before, during and after the Ghaub glaciation. This fits to the Zipper rift model of Eyles & Januszczak (2004). Dropstones in the Maieberg cap carbonate overlying the Ghaub diamictite support an at least partial glacial origin, but many diamictites might represent debris flows in the forefront of glaciers. Glacial cover was not continuous, even locally, as shown by the “tubes” facies (=water or gas escape structures) laterally co-existing with diamictite deposition, overlain by typical Cap Dolomites with partly aragonitic rays. Based on microbial remains, a continuous record of biogenic productivity in the ocean and fluctuations in relative sea-level are seen coeval with dropstones, proving the coexistence of regions of open oceans and low latitude glaciations during the deposition of the Cap Carbonates. An “Ice Patch” Earth model is proposed (Spence et al. 2008) characterized by the advance and retreat of polar glaciers regionally along an irregular latitudinal front, which in places crossed into low latitudes, generating a lower magnitude increase in albedo. Because the negative δ13C excursions are not caused by a global collapse in organic productivity and because they occur in pre- and post-diamictite successions, these excursions might be diachronous and are no sound proof of the correlatability of the diamictite intervals. |
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| Publications Bechstädt, T., Jäger, H., Spence, G. and Werner, G., 2009, Late Cryogenian (Neoproterozoic) glacial and postglacial successions at the southern margin of the Congo Craton, Northern Namibia: Facies, Palaeogeography and hydrocarbon perspective. In: Craig, J. et al., eds., Global Neoproterozoic Petroleum Systems: The Emerging Potential in Northern Africa, Spec. Publ. 326, 255-287, Geological Society London. |
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| Presentations Sediment 2008, IAS SEPM-CES Meeting, Bochum, 2008 EUG General Assembly, Wien, 2008 British Sedimentological Research Group, Annual Meeting, Aberdeen, 2006 Global Infracambrian Hydrocarbon Systems and the Emerging Potential in North Africa, Geological Society Meeting, London, 2006 |
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| References Allen, P.A., Leather, J. & Brasier, M.D., 2004, The Neoproterozoic Fiq glaciation and its aftermath, Huqf Supergroup of Oman: Basin Research, 16 / 4, 507-534. Embelton, B.J.J. & Williams, G.E., 1986, Low paleolatitude of deposition for late Precambrian varvities in South Australia: Implications for paleoclimatology: Earth and Planetary Science Letters, 79, 419-430. Eyles, N., & Januszczak, N., 2004, ‘Zipper-rift’: a tectonic model for Neoproterozoic glaciations during the breakup of Rodinia after 750 Ma: Earth-Science Reviews, 65, 1-73. Hoffman, P.F., Kaufman, A.J., Halverson, G.P. & Schrag, D.P., 1998, A Neoproterozoic Snowball Earth: Science, 281, 1342-1346. Spence, G., Jäger, H., Bechstädt, T., 2008, Neoproterozoic Ice Patch Earth: The paradox of low latitude sea level glaciation coeval with regions of open ocean: EUG General Assembly, Abstracts Volume. |
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| Funding Organizations German Research Fund (DFG) GeoResources, Heidelberg |
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