Isotopic and elemental proxies in mollusc and brachiopod calcite:diagenesis, vital effects and climatic trends

• Main Author: Ullmann, Clemens Vinzenz
• Format: Book
• Language: English
• Published: Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen 2013
• Subjects: New Zealand; Pacific; Crassostrea gigas; Pacific oyster
• Online Access: https://curis.ku.dk/portal/da/publications/isotopic-and-elemental-proxies-in-mollusc-and-brachiopod-calcite(fa1603e3-c68f-4a72-bcb9-735b3ce39adf).html; https://soeg.kb.dk/permalink/45KBDK_KGL/fbp0ps/alma99122936014105763

This thesis summarizes the findings of studies conducted at the University of Copenhagen from September 2010 to September 2013, aiming at a better understanding of the geochemical signatures in marine, biogenic calcite. Throughout the history of the Earth, the climatic conditions and the chemical composition of the oceans have changed. These changes of environmental conditions and seawater composition are primarily caused by plate tectonic processes, the cyclic variations of the Earth’s orbital parameters and living organisms, actively causing, and passively adapting to shifting conditions in their habitats. With the rapid, global spreading of shell-creating organisms about 540 million years ago, an archive was created that contains information about past marine environmental conditions. This information is stored as specific concentrations and isotopic ratios of the chemical elements in the shells. Additional to environmental parameters, the composition of the shell is controlled by biological processes (i.e. vital effects), and post-depositional alteration. The chemical fingerprints of all the parameters influencing the shell composition need to be understood and ideally quantified in order to allow for a meaningful interpretation of isotopic and elemental ratios in shell materials. Without this knowledge the correct interpretation of the shell composition in terms of past environments is impossible. Calcite shells of brachiopods, bivalves and belemnites were analyzed here for shell ultra-structure and geochemical proxies, mainly using Optical Microscopy, Scanning Electron Microscopy (SEM), Gas Source Isotope Ratio Mass Spectrometry (GS IRMS), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and Thermal Ionization Mass Spectrometry (TIMS). Combined results of these methods were used to calibrate isotopic (C, O, Ca) and element proxies (Mg/Ca, Sr/Ca, Mn/Ca) in the Giant Pacific Oyster (Crassostrea gigas) as a modern analogue for fossil ostreoids. For the Late Jurassic of New Zealand (Kawhia Harbour) and the Late Triassic of New Caledonia, trends of post depositional alteration were studied. C and O isotope trends from high southern palaeo-latitudes as well as Sr/Ca ratios of seawater were reconstructed using Permian to Jurassic fossils from New Zealand and New Caledonia. A study on Sr/Ca ratios in ostreoids and belemnites was conducted to provide the first high resolution Sr/Ca curve of the European Early and Middle Jurassic (~201-163 Ma). Finally, a study on belemnite rostra - spanning the early Toarcian (Early Jurassic, ~183 Ma) - was conducted. For the first time, organic carbon isotope values from belemnite rostra are reported and geochemical data are discussed in the light of belemnite evolution and adaptation to rapid environmental change.