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SBE Résumé de rapport

Project ID: 42409
Financé au titre de: FP6-MOBILITY
Pays: United Kingdom

Final Activity Report Summary - SBE (Snowball Earth: Equatorial Glaciations in the Neoproterozoic)

The Neoproterozoic era (~1000-542 Million years ago) was one of the most significant periods in Earth's History. It witnessed the break-up of the supercontinent Rodinia, repeated periods of low latitude glaciation and periods when the Earth's magnetic poles may have wandered far from the geographic poles, to be followed by the Cambrian expansion in the diversity of animal life. The Neoproterozoic geological record shows that at least three times in the Earth's history, there were extreme and catastrophic variations in global climate resulting in global glaciations which reached equatorial latitudes, and may have provided the trigger for the Cambrian explosion of multicellular life forms. The presence of glaciomarine rocks, ie rocks which were deposited from sea level ice-sheets, have been identified in numerous palaeo-continents, with evidence for at least three major glaciations termed the 700-750Ma Sturtian event, the ~635Ma Marinoan event, and the youngest Gaskiers event which occurred approximately 580Ma. The corresponding glacial deposits are often immediately overlying warm water cap carbonate rocks.

This unusual stratigraphic juxtaposition of cold and warm water sedimentary rocks, and the geographically widespread distribution of glacial rocks of similar age led to the first ideas of global glaciation. With the advent of reliable palaeomagnetic data, it is clear that in the Neoproterozoic, glacial rocks were deposited in equatorial latitudes. This led to the development of the different models to explain low latitude glaciation, the most popular of which is the Snowball Earth hypothesis. In this model the Earth went into repeated states of "deep-freeze" for millions of years with complete global freezing and temperatures plummeting to -50 degrees C which in turn would cause the breakdown of the hydrological cycle and an end of photosynthetic life.

The second model is the High Obliquity model whereby the Earth's rotation axis was angled at 54 degrees or more to that of its orbital axis around the Sun. However, restoring obliquity to modern values would require impact with a Mars sized body and the model has clear implications for the stability of our planet on astrophysical levels.

The third model is the Zipper-Rift model which discounts all palaeomagnetic data and states that most of the glacial rocks reported in the literature have been mis-identified and are linked to mass flow deposits as the result of supercontinent rifting. The implications of all these models are immense and potentially catastrophic, yet are based on an extremely small palaeomagnetic dataset and poorly constrained sedimentary record. The overarching objective of this research project was to better constrain Proterozoic palaeogeography and depositional environment of key Neoproterozoic successions.

Three particular aspects have been addressed. Firstly, palaeogeography and the position of the continents prior to and during glaciation is pivotal to all models. A number of palaeomagnetic studies have been carried out, allowing us to constrain the position of many cratons during key intervals in the Neoproterozoic, and enabling construction of more accurate palaeogeographic models. Secondly, age constraints for the different glacial deposits are extremely poor. Only two have been well dated, yet the Snowball Earth hypothesis requires all to be globally synchronous. Before any meaningful inter or intra cratonic correlation can be made, reliable age data are vital. Using U-Pb Bd analyses we have obtained crucial age constraints for the oldest Neoproterozoic glacial unit in the western Congo.

Thirdly, a much better understanding of smaller scale palaeogeographic setting and tectonic relationships between the cratons is required to understand the sedimentary systems, ice-sheet flows, continental margins and plate interactions detailed geochemical and isotopic studies of the sedimentary basis are required. Whole rock major and trace element analyses, combined detailed sedimentary provenance analyses and isotope geochemistry have been carried out on a number of key sequences.


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