Belemnite palaeo-proxies and dating of Mesozoic carbonates

• Main Author: Li, Q.
• Published: University College London (University of London) 2011
• Subjects: 550
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.565503

This PhD project contains two related parts, both linked by a study of belemnites. The first part is to test the validity and quality of belemnite palaeo-environmental proxies, and the second to investigate the feasibility of U-Pb dating of Mesozoic carbonates. In the first part, a wide range of well-preserved Mesozoic belemnites was analyzed for bulk stable isotope and trace element compositions, so as to access species-specific effects on compositions and to evaluate belemnite palaeo-proxies (notably δ18O, δ13C, Mg/Ca and Sr/Ca) as palaeo-temperature indicators. Then, high-resolution profiles of trace elements (Mg, Ca, Sr, Na, etc.) were obtained from those best-preserved specimens by electron microprobe and laser ablation-ICP-MS, and compared with the bulk analysis for a better understanding of intro-rostral variations. Results of four different belemnite polulations with differing ages and from differing stratigraphic levels show that Mg/Ca ratios in bulk analysis neither correlate with δ18O within one single species nor within one belemnite population. Mg/Ca is therefore not considered as a palaeo-temperature indicator for belemnites. Variations in Mg profiles along belemnite cross-sections are then interpreted to likely reflect the changes of ontogeny and growth rate. Part two is to apply U-Pb dating method to Mesozoic belemnites and other carbonates. An innovative approach of dating carbonates in deep time has been developed, using both laser-ablation MC-ICP-MS and isotope dilution measurements; and ages with the highest achieveable precision (±1%) and accuracy for Mesozoic calcite cements from three separate ammonites have been produced. Part I contains chapters 1 to 8. Chapter 1 gives a brief account of belemnites (e.g. the morphology, biogeography, palaeo-proxies and palaeo-ecology) and problems that need addressing in belemnite studies; and chapters 2-8 then focus on the geochemistry (isotopic ratios and elemental compositions) of belemnites from four different Mesozoic stratigraphic levels of Europe (Pliensbachian of Yorkshire, Toarcian of Dorset, Callovian of Oxfordshire and Valangigian of Vergol, SE Fance). Part II only contains chapter 9 which presents the U-Pb dating work of a range of Mesozoic carbonate samples.