Dating glacial sediments

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The Snowball Earth hypothesis proposes that during one or more of Earth's icehouse climates, Earth's surface became entirely or nearly entirely frozen, sometime earlier than Mya million years ago during the Cryogenian period. Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record. Opponents of the hypothesis contest the implications of the geological evidence for global glaciation and the geophysical feasibility of an ice - or slush -covered ocean [3] [4] and emphasize the difficulty of escaping an all-frozen condition. A number of unanswered questions remain, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open or seasonally open water. The snowball-Earth episodes are proposed to have occurred before the sudden radiation of multicellular bioforms known as the Cambrian explosion.
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Numerical dating of glacial deposits is important for understanding Quaternary glacial evolution. Optically stimulated luminescence OSL dating is one of the techniques widely used on such sediments. Initially, four glacial stages were identified based on field observations of moraine distribution and geomorpho-stratigraphic relations. A total of 39 OSL samples were then collected from glaciofluvial sand layers or lenses and from till. Contrary to previous experience suggesting that coarse grains are usually better bleached than fine grains prior to deposition, our results show that estimated OSL ages for fine grains are generally younger than those for medium grains. This suggests that the two fractions may have come from different sources and thus have different bleaching histories, and that fine-grained quartz may be more suitable for OSL dating of these materials. Applying the minimum age model to data from medium-grained quartz yields ages close to those obtained from fine-grained quartz, suggesting that both can be used for dating glacial advances.
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Dating glacial sediments

Optically stimulated luminescence OSL dating provides an age estimate for the last time sediment was exposed to light. In glacial environments solar resetting of the luminescence signal prior to deposition is not assured and can lead to significant age overestimates. Sediments derived from glacial settings also commonly have other deleterious properties such as weak quartz luminescence signals, feldspar contamination and high levels of electron thermal transfer. Samples for quartz OSL dating were collected from carefully selected shallow-water and well-sorted facies of glaciofluvial and glaciolacustrine sediments to select sediments most likely to have been exposed to light prior to deposition. OSL dating is preferred over other dating methods in the study areas because evidence for multiple pre-LGM glaciations is commonly preserved as buried and over-run packages of diamicton, outwash and lacustrine sediment, excluding application of surface-exposure techniques.
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Dating glacial and associated sediments is essential to provide a temporal framework for accurate reconstructions of past climatic conditions and for helping to determine the nature and magnitude of glaciation for landscape evolution studies. There are few widely applicable, accurate and precise methods available to date Quaternary landforms and sediments, despite the numerous numerical dating methods that are currently available. Further-more, there are few methods that can be utilized for the whole of the late Quaternary c.
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