Resource Management and Geography - Theses
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ItemGeochemical investigations of Corchia speleothems: implications for past climate change( 2016)This PhD thesis addresses three research questions using speleothems from Corchia Cave (Italy). An outline of the major findings follows. The first question concerns glacial-interglacial cycles. The greatest obstacle in resolving the question of which orbital parameter (or combination thereof) forces Earth’s climate to switch between glacial and interglacial states is a lack of radiometrically dated records of glacial terminations. In this study, three stalagmites (CC8, CC119 and CC122) were precisely and accurately dated using the U-Pb dating method, and a composite age-depth model developed. The speleothem chronology was then transposed to a series of deep-sea sediment records, allowing three glacial terminations during the Middle Pleistocene Transition (MPT) to be radiometrically dated. The results reveal that Terminations XII and X are separated by two obliquity cycles, and that Marine Isotope Stages 25 to 22 are not part of a 100-kyr cycle as previously thought. Both terminations started at the same phase of obliquity but show an inconsistent relationship with precession-driven maxima in summer insolation intensity. This implicates obliquity as the major forcing parameter, while the role of precession is less conclusive. The second question concerns the integrity of speleothems for radiometric dating. Speleothems are considered as one of the most suitable materials for U-series dating. High-resolution U-Th dating of Corchia Cave stalagmite CC26, however, revealed a number of age outliers, which was unexpected due to the otherwise pristine appearance of the speleothem. The aim was to explore the potential causes for these outliers. By using a multi-aliquot approach, analytical problems were eliminated. Micro-CT analyses revealed a limited number of micro-voids, and it is proposed that these were pathways for small (localised), but significant (in terms of dating implications) U loss. Uranium-loss modeling constrains the possible timing of the diagenetic processes. These results show that such “invisible” diagenesis has implications for the accuracy of U-Th based chronologies for speleothems and suggests a need for a more careful approach, particularly in studies which are based on low-resolution dating. The third and final question relates to the drivers of stable carbon isotope (δ13C) changes in Corchia Cave speleothems at the onset of interglacials. The ‘dead carbon proportion’ (DCP) record from stalagmite CC26 was used to develop a more robust interpretation of the factors driving carbon isotope variation. High DCP (44.8 to 68.8%) was interpreted as a result of two processes: closed-system dissolution and sulphuric-acid dissolution. A ~parallel decrease in δ13C and DCP during the Holocene provides support for the previous interpretation of δ13C as reflecting long-term post-glacial soil recovery. Additionally, short-term changes in DCP are compared to the Mean Anomaly Index based on the trace element and stable isotope record from the same specimen (Regattieri et al., 2014), which reveals that DCP is also sensitive to millennial-scale changes in recharge hydrology.