Resource Management and Geography - Theses
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ItemSynchrony and precise timing of millennial-scale climate events during the last glacial period based on speleothem palaeoclimate records( 2022)The climate of the last glacial period (115,000 – 11,500 years ago) is characterised by a series of abrupt climate changes, termed millennial-scale climate events, that are one of the best examples of naturally occurring abrupt climate change. These events are best recorded in Greenland ice cores, as well as marine and terrestrial records from a wide range of climate zones. However, many unknowns remain regarding the events, including whether they occurred synchronously between different climate regions, the precise timing of the events and the underlying cause. The objective of this thesis is to advance our understanding of millennial-scale climate events through the use of speleothem palaeoclimate records. Speleothems, cave deposits such as stalagmites, can provide high-resolution and precisely dated records of millennial-scale climate events from a range of climate regions. This thesis is approached through two complimentary projects. The aim of the first project is to assess if millennial-scale climate events occurred synchronously between different regions. Sixty-three published, precisely dated speleothem records were compiled, and the timing of the onset of interstadial conditions (the warming component of the events in Greenland) was compared between records. It is found that the events occurred synchronously between Greenland, Europe and the Mediterranean, and the Asian and South American Summer Monsoon regions. This finding supports the long held, but previously untested, assumption of synchrony in the timing of millennial-scale climate events, and suggests that the events were rapidly propagated between these regions through atmospheric teleconnections. A key output of this project is the development of the SIOC19 data set of precise radiometric constraints for the timing of the onset of interstadial events based on the compiled speleothem records. This data set enables the accuracy of non-radiometric chronologies to be evaluated. The second project of this thesis investigates the timing and expression of millennial and sub-millennial-scale climate events during the early last glacial period. This time period is less well represented by existing speleothem records, and exhibits a diversity of sub-millennial-scale climate variability which is not well studied outside of Greenland. Five new speleothem palaeoclimate records are assembled from Saint-Marcel and Orgnac Caves in Ardeche, south-east France, that collectively span the period from 127 to 87 ka. The replicated d18O and d13C time series provide a comprehensive record of millennial and sub-millennial scale climate variability, which mirrors events observed in the Greenland ice-core record, suggesting a strong climate coupling between south-east France and Greenland. Interstadial periods are interpreted to be associated with an abrupt increase in soil and vegetation activity, reflecting an increase in temperature, and an increase in the dominance of precipitation from the Atlantic. Variation in Mg/Ca, Sr/Ca and (234U/238U)0 is also observed across the main stadial-interstadial transitions, the interpretation of which is explored. A decoupling between temperature and precipitation changes over some sub-millennial events is found to occur, suggesting the events are distinctly different to the main millennial-scale climate events. A stacked chronology is produced, based on 130 individual uranium-thorium ages, that provides a well constrained chronology of the early last glacial period. Comparison is made between the timing of the onset of interstadial events in Greenland, the Ardeche speleothems, and in the SIOC19 data set. Overall, this thesis provides key insights into the synchronicity, precise timing and regional expression of millennial-scale climate events, that is useful for validating climate model output and testing hypotheses on the underlying cause of the events