An Integrated Sedimentological and Multi-proxy Geochemical Investigation of the Neoproterozoic Climatic Paradox
Zusammenfassung der Projektergebnisse
The Ghaub glacial interval shows evidence of climate conditions oscillating between dry-cool and glacial, as well as interglacial. Sea-level fluctuated repeatedly. The diamictites in the area of investigation are mainly periglacial sheddings oh the slope, dropstones at the top indicate the presence of melting glaciers in the vicinity at least at that time. Tectonism was active throughout the period of interest, affecting and overprinting the glacial record. The glacial sea-level drop did, however, not expose large parts of the shallow-water carbonate platform, where microbial dolomites developed during glacial periods, which probably contained clathrates. Degassing due to post-glacial (or interglacial?) warming caused the formation of gas-escape tubes, which only occur in the areas without diamictites. Even before glaciations the climate was probably cold. Strong Ca oversaturation before and after the glaciation (not directly linked to post-glacial weathering) caused unusual sediment fabrics and minerals (glendonite after icaite, cap dolomites! aragonite rays). Different layers of glendonite as well as changes in palynologie content indicate sea-level fluctuations in the cap dolomite time and also changes in ocean temperature which was repeatedly near freezing, in total contrast to the super-greenhouse assumed in snowball Earth hypothesis. Carbon isotope data are in line with the sedimentologie interpretations, indicating a strong relief in preglacial time with distinctly differing water masses, and a return to uniform water bodies with the strong postglacial sea level rise. The reason for the distinct and long lasting negative shift of carbon isotopes starting within the cap dolomites and lasting approximately until the end of Maieberg T2 member (= cap carbonate sequence) remains unsolved. Recent interpretations of paired carbon Isotopic data (Sansjofre et al., 2011) postulate Pco2 values possibly as low as the current value. Sulfate concentration of the Neoproterozoic seawater was much lower than today. It was (still) too low to reflect a globally homogenous signature. Highly variable and strongly positive δ34S values suggest the influence of bacterial sulfate reduction. But this appears to reflect local/regional perturbations more than secular shifts lin the global isotopic composition of oceanic sulfate. In view of the scientific objective of this study, the global significance of reported short-term fluctuations in δ34S In the Neoproterozoic post-glacial ocean appears questionable. Sr isotopes indicate strong diagenetic influence on the core investigated. Only the Maieberg cap carbonate sequence gives values close to the Marinoan/Elatina ocean waters. This is the same postglacial interval that shows also uniformly negative carbon-isotope values in the different sections. Unexpected was the widespread presence of microbial laminites with degassing fabrics ("tubes"), which occur only apart from diamictites and lack distinct erosional features. The laminites might be pre- or synglacial, degassing is apparently late- to postglacial (or interglacial). The fabrics indicate, that the sea-level fall during glaciations was either not very distinct, or very slow, counterbalanced by subsidence. The snowball Earth model implies a worldwide halt of marine organic productivity, a distinct sea-level drop and a widespread exposure of carbonate platforms, covered by ice. Our observations are in strong contrast to this hypothesis. Glaciation seems more local, large marine and terrestriasi areas not covered by ice must have existed.
Projektbezogene Publikationen (Auswahl)
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2006. Marinoan glacial and postglacial successions at the southern rifted margin of the Congo craton (Neoproterozoic, northern Namibia): fades, palaeogeography and hydrocarbon perspective. GSL, Conference "Global Infracambrian Hydrocarbon Systems and the Emerging Potential in North Africa", London, Nov. 2006
Bechstädt, T., Spence, G., Jäger, H. & Werner, G.
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2007. Neoproterozoic Ice Patch Earth: the paradox of low latitude sea level glaciation coeval with regions of open ocean, B5RG, Dec. 2007, Birmingham, UK
Spence, G.H., Jäger, H. & Bechstädt, T.
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2008. Facies, geochemistry and palynology ofa Neoproterozoic (Cryogenian) postglacial cap dolomite at the southern rifted margin ofthe Congo craton (northern Namibia). 26th Regional meeting of the IAS, September 2008, Bochum, Germany
Bechstädt, T., Jäger, H., Rittersbacher, A. & Spence, G.
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2008. Neoproterozoic ice patch earth: the paradox of low latitude sea level glaciation coeval with regions of open ocean. EUG annual conference, April 2008, Vienna, Austria
Spence, G.H., Jäger, H., Bechstädt, T. & Wipf, M.
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2009. Late Cryogenian (Neoproterozoic) glacial and post-glacial successions at the southern margin of the Congo Craton, northern Namibia: facies, palaeogeography and hydrocarbon perspective. In: Craig, J., Thurow, J., Thusu, B., Whitham, A., Abutarruma, Y. (eds), Global Neoproterozoic Petroleum Systems: The Emerging Potential in North Africa. Geological Society, London, Special Publications, 326, 255-287
Bechstädt, T., Jäger, H., Spence, G. & Werner, G.
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2011. A review of the Chemical Index of Alteration (CIA) and its application to the study of Neoproterozoic glacial deposits and climate transitions. In: Arnaud, E., Halverson, G. P. & Shields-Zhou, G. (eds), The Geological Record of Neoproterozoic Glaciations. Geological Society, London, Memoirs, 36, 81-92
Bahlburg, H. & Dobrzinski, N.
