| Reconstructions of the carbon-isotopic evolution of seawater serve a dual purpose. On the one hand, these records provide a correlation tool, whereby trends in secular variation in δ13C can be matched between separated sections. On the other hand, when combined with other geochemical, biological, sedimentological, and chronological information, they are the basis for interpreting perturbations to the earth's surface environment. In the Neoproterozoic, where the variations in δ13C are extreme and other correlations tools (e.g. biostratigraphy) are sparse, the carbon-isotopic record is particularly important. A combined sequence stratigraphic and isotopic investigation in the Ombaatjie Formation (Otavi Group) in northern Namibia has revealed that a large (>10‰) decline in seawater δ 13C presaged the Marinoan-aged Ghaub glaciation. This is an important finding because the large δ13C excursions characteristic of the Neoproterozoic had been considered to be strictly a post-glacial phenomenon, occurring in the unusual cap carbonates that commonly drape the glacial deposits. Geological considerations suggest that the decline was gradual (>0.5 m.y.) and preceded the onset of continental glaciation. Assuming a link between the global carbon cycle and climate, these clues impose critical constraints on what mechanisms may have been responsible for the anomaly and the initiation of global cooling. At the same time, this pre-glacial δ13C anomaly can be used for correlation. Presumably the anomaly shows up in successions around the word, including northeastern Svalbard and northern Norway. Correlations between Namibia, Svalbard, and Norway (all of which host a pair of diamictites) based on the anomaly, as well as cap carbonate characteristics, have important implications. First, both diamictites in Svalbard are Marinoan in age; analogous Marinoan diamictite pairs are found in several other successions. Second, the upper diamictite in Norway must be post-Marinoan, indicating that there were three Neoproterozoic ice ages. This late (Varanger) glaciation appears to be related to the most extreme Neoproterozoic negative δ13 C anomaly, but is not overlain by a cap carbonate. Third, the Svalbard and Namibia records can be combined to produce a high-resolution composite δ 13C record that spans over half of the Neoproterozoic. |
