Diffraction and Spectroscopy of Marine Calcite Biominerals: Insights into Structure and Geochemistry
Anthropogenic climate change is one of the biggest existential threats to the natural world and the place of humans within it. The key to understanding and shaping the future climate lies in exploring records of climate change through Earth's history. Some of our best records of climate over the past 65 million years are based on chemical information stored in the calcite biominerals produced by marine organisms. Organisms such as foraminifera and coccolithophores provide an unparalleled 'proxy’ archive of past marine conditions. The way in which these organisms record these proxy archives is largely unknown, and is a fundamental question in applying and interpreting palaeoclimate proxies. In this thesis, I apply diffraction and spectroscopic methods to study the structure and chemistry of calcite biominerals, to assess how these structures acquire and record these proxy signals. Understanding these processes is vital to testing the fidelity of palaeoproxies and the interpretations that can be drawn. In Chapter 2 I develop a toolbox for the analysis of electron backscatter diffraction (EBSD) data collected on calcite biominerals. The toolbox is presented in a tutorial format on a section of the foraminifera Orbulina universa. In Chapter 3 I apply this toolbox to a range of foraminifera to explore the micron (µm)-scale differences in the structure and biomineralisation of these calcitic organisms. I explore the structural differences between biologically controlled and biologically induced calcite formation, and present a model for multi-scale hierarchical templating in calcite biomineralisation of foraminifera. In Chapter 4 I investigate the chemical coordination and sub-µm scale distribution of Na in the benthic foraminifera Amphistegina lessonii using scanning transmission X-ray microscopy (STXM). Na in the test of A. lessonii is incorporated in a spatially heterogeneous manner, showing banding in a mode comparable to that seen previously for B and Mg. The near edge X-ray absorption fine structure (NEXAFS) spectra at the Na-edge do not appear consistent with Na directly substituting for Ca within the calcite test of A. lessonii. Observations may suggest that Na is incorporated in two distinct chemical environments, with both an ordered and disordered coordination. I discuss the results in the context of the Na/Ca salinity proxy. In Chapter 5 I apply synchrotron ptychographic imaging to the planktic foraminifera Neogloboquadrina pachyderma to investigate the µm-scale distribution of rare earth elements (REE) found in association with foraminiferal material. I focus on neodymium (Nd), the isotope ratios of which measured on post-depositional Fe-Mn oxide coatings precipitated on planktic foraminifera and are used as a proxy to reconstruct water sources and ocean circulation on million-year to millennial time-scales. I present the first direct µm-scale distribution of Nd in foraminifera, which has previously been impossible due to the low concentrations found in association with foraminifera. I discuss the distribution of Nd in the context of ocean circulation and palaeoclimate reconstructions, and explore the expansion of this technique to other elements. In Appendix A I present a hypothesis for distinguishing between two existing competing models of biomineralisation in coccolithophores: seawater vacuolisation (SWV) and trans-membrane transport (TMT). I use synchrotron STXM in a preliminary investigation on how trace elements are incorporated during coccolithogenesis. I discuss the expected outcomes of successful measurements and how these would lead to distinguishing between SWV and TMT. Throughout the thesis results are discussed in the context of biomineralisation processes which underpin the incorporation and preservation of palaeoproxy records. Developing palaeoproxies is of vital importance to understanding our climate, both past and future.
foraminifera, proxy, calcite, STXM, NEXAFS, ptychography
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