The Svalbard exemplar of Neoproterozoic glaciation

Lead Research Organisation: Open University
Department Name: Environment, Earth & Ecosystems

Abstract

Although life successfully moderates surface conditions on Earth, some events in Earth History have threatened the viability of most life forms. Arguably the most profound and long-lasting challenge in the last 2 billion years was glaciation on a near-global scale (pan-glaciation), with the best documented event being around 650 to 630 million years ago ('Marinoan' glaciation). One overaching model (Snowball Earth hypothesis) proposes that snow and ice was so widespread that the Earth become much more reflective of solar radiation and cooled to a mean temperature of around -50 degrees Celsius. Glaciation was eventually terminated by the build-up of carbon dioxide emitted from volcanoes, that was not used up by the weathering of rocks, since rocks were buried beneath the extensive snow and ice cover. Although the extremity of the cold and the way in which glaciation terminated have been challenged, there is widespread agreement that glaciation reached tropical latitudes at sea level. New data will significantly constrain future modelling efforts. We have recently made a breakthrough in generating a new suite of chemical data on exceptionally well-preserved carbonate precipitates in saline glacial lakes in the Wilsonbreen Formation rocks of Svalbard, thought to be the same age as glacial deposits found on all the continents and referred to as 'Marinoan'. Firstly we find that in terms of oxygen isotopes, these carbonates are the most evaporative yet discovered and so must have formed in a hyperarid environment. Secondly we use new discoveries about the meaning of the abundances of the isotope 17-O in relation to our measurements of sulphur isotope ratios in order to show that the atmosphere was profoundly different from that which existed during younger glaciations: the simplest explanation for it is that the atmosphere was very high in carbon dioxide. This implies that weathering was indeed inhibited by an extensive ice cover. This study and various previous studies have demonstrated the outstanding importance of the rock exposures in these remote locations to understanding this extraordinary event in Earth history - indeed they are the only place where we can find a chemical sedimentary record that allows us to understand conditions on the Earth surface and in the atmosphere. We propose to make new studies over two summer seasons in this remote field area to enable us to fully describe and archive the field relationships and collect suites of samples that will enable us to understand more clearly the preserved evidence. We will use magnetic properties to reconstruct the palaeolatitude of the glacial deposits and will try to determine the age directly by radiometric methods to see if it is consistent with the 'Marinoan'. Our favoured modern analogue for the Wilsonbreen formation saline glacial lakes are found in the intensely cold McMurdo Dry Valleys of Antarctica. We will test this idea using physical properties of the sediment whilst the chemical properties will be used to constrain how much water is cycled through the atmosphere, how oxidizing the atmosphere was and whether carbon dioxide had already built up in the atmosphere by the time the first glacial lakes formed. Our work will also extend to the apparently warm- and cold-climate marine deposits that are found above and below two glacial units in the Svalbard in order to understand the broader context. Our work includes a number of new approaches as well as applying tried-and-tested modern methods of dating and magnetic analysis in a new area. We expect to emerge with a clear and vivid picture of the nature of the land surface during one of the most extreme cold events in the history of the planet. We will also find out whether this location could be the best place in the world to formally place a 'golden spike' at the base of the Cryogenian geological period. The information will be disseminated and archived in novel ways.

Publications

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Description The Neoproterozoic Era (1000-542 Ma) is a key period in the evolution of the Earth and is characterised by the widespread occurrence of glacigenic sediments associated with global periods of glaciation and by large variations in carbon isotope compositions of seawater. The Neoproterozoic one of the most dramatic periods of change in surface processes on Earth and is also often associated with a large increase of atmospheric oxygen.

The Neoproteorzoic succession of Svalbard have been widely studied and represent some of the best preserved sequences of Neoproteorzoic sediments on Earth. However, correlation of the Svalbard succession with other Neoproterozoic successions is difficult due to the lack of good sedimentary age of deposition.

In this study we have attempted to directly date some glacial lake sediments using U-Pb isotope chronology and to correlate carbonate rocks in the sequence to other Neoproterozoic successions using Sr isotopes.

Key findings are as follows;

U-Pb of detrital zircons ages measured in this study are older than 1000 Ma, although one sample yielded ages of 770 +-30 Ma. The distribution of zircon ages is consistent with other Neoproterozoic successions in the northern hemisphere. However, the U-Pb data obtained in this study do not allow a better constrain of the age of sedimentation.

Strontium isotope studies demonstrate that the sedimentary record in Svalbard is complete and that the

second glaciation in the Svalbard succession is younger than previously thought (that is younger than 640 Ma).

The lack of a carbon isotope excursion in the upper part of the Elderbreen Member indicate that the Trezona carbon isotope excursion might not be global a feature.
Exploitation Route Selection of material for Sr isotope studies is critical to producing useful Sr isotope records in the Neoproterozoic and requires careful characterisation of samples.

Lead isotope dating of sediments in the Neoproterozoic rarely yields depositional ages.
Sectors Environment