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Title: Advances in U-Pb speleothem chronology using laser ablation MC-ICPMS techniques : applications in quantifying long-term rates of landscape change
Author: Smith, Christopher James Martin
ISNI:       0000 0004 5992 6588
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2015
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The development of U-Pb dating techniques for speleothem deposits has enabled the scientific community to exceed the upper limit of the U-Th chronometer (-600 ka), creating new opportunities for innovative research into palaeoenvironmental change over 106-107 year timescales. However, despite the great potential of the technique, its application in research has been limited, with just 21 papers published over the last 17 years. This paucity of research can be attributed to the technical challenges associated with measuring small quantities of radiogenic Pb and limited UlPb ratio spread measurable by low-spatial resolution techniques such as 10-TIMS and MC-ICPMS. In this thesis I investigate the use of in-situ U/Pb ratio analysis by LA MC-ICPMS, to take advantage of the increased spatial resolution compared to conventional low-spatial resolution techniques in order to better resolve UlPb ratio heterogeneity in speleothem calcite. To test the feasibility, accuracy and precision of U-Pb speleothem dating by LA MC-ICPMS, four speleothem samples, previously dated by low-spatial resolution U-Pb methods, were reanalysed using a 193 nm ArF excimer laser ablation system, coupled to a Nu Plasma HR MCICPMS. Following the initial feasibility study, rates of Quatemary landscape evolution were investigated at two field sites: the Ogof Draenen cave system, Brecon Beacons National Park, South Wales, UK; and the Mulu caves of the Gunung Mulu National Park, Malaysian Borneo. In the feasibility study (Chapter 3), U-Pb dating by LA MC-ICPMS of samples of known age, ranging from -0.25-2.50 Ma, were shown to be concordant with conventional low-spatial resolution U-Pb dating results, yielding typical age uncertainties of 5-6% per isochron. Using age-depth modelling techniques, age uncertainties of as little as 2-3% were achieved. Under normal conditions, U/Pb ratio data for a 40-point isochron could obtained in -1 hour by LA MC-ICPMS, with sample consumption rates of as little as -0.026 mg of calcite per isochron. U-Pb dating of speleothem samples from Ogof Draenen (Chapter 4) was used to quantify past rates of fluvial incision in the Usk Valley during the Quaternary, yielding maximum incision rates of between 0.194-0.231 m ka-1 over the last -1250 ka. Scarp retreat rates along western flank of the Usk Valley for the same period were highly variable and localised, with rates of as little as 0.06 m ka-1 at Cwm y Nant up to 0.542 m ka-1 at Cwm Llanwenarth. U-Pb dating of speleothems from the Mulu caves (Chapter 5) presented a greater challenge, with many samples displaying anomalous UlPb ratios, indicating the presence of unsupported common Pb. Despite this, U-Pb dating of flowstone WR-09-18-03 by LA MC-ICPMS yielded a basal age of 1181.3 +278.0/-256.1 ka, equating to a mean incision rate of 0.216 +0.060/-0.41 m ka-1. This result shows a good level of agreement with the mean rate of 0.190 +0.030/-0.040g m ka-1 obtained by Farrant et al. (1995) for palaeomagnetic sediment dating. In summary, U-Pb dating of speleothems by LA MC-ICPMS offers several key advantages over conventional low-spatial techniques, including a significant increase in measurable U/Pb ratio spread for individual isochrons and a major reduction in sample preparation and analysis time, as well as sample consumption rates. Ultimately, this technique represents a new and valuable alternative to conventional low-spatial resolution U-Pb dating methods.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available