Imaging element distributions within small marine calcifiers : a NanoSIMS perspective

Climate change is one of the major challenges of our time. Human induced increase in atmospheric carbon dioxide will, without the effort to decrease carbon emissions world-wide, likely lead to a global mean surface temperature rise exceeding 1.5 °C by the end of the 21st century compared to pre-industrial times. In order to mitigate climate change, we rely on model simulations of the Earth System, which are based on observations of past climate. Indirect observations can be made by making use of proxies: chemical or isotopic signatures of old materials we can measure today, of which a relationship with environmental parameters has been established. Many proxies are based on inorganic chemical signatures of fossil shells of marine calcifiers, such as foraminifera and coccolithophores. However, all estimates based on proxies are burdened with uncertainties, not in the least because interactions between multiple environmental factors and biological processes, the so-called vital effects, influence the proxy signal. Understanding of the fundamental controls of biomineralization and thereby the underlying mechanisms of proxies, thus the how and why proxies actually work, is presently of large interest in the scientific community. In this thesis, nano-scale secondary ion mass spectrometry (NanoSIMS) is the analytical technique of choice to investigate the submicron-scale distribution of minor and trace elements in small marine calcifiers. NanoSIMS provides several important advantages over bulk analytical techniques: (1) high spatial resolution, which is sufficient to resolve small samples, such as coccoliths, in considerable detail; (2) high mass resolution, which is sufficient to gain data largely free of isobaric interferences; and (3) the capability to collect data as images, which enables one to selectively choose uncontaminated sample areas for investigation. As this technique is expensive in both time and money, it is used in this thesis not to establish analytical routines to construct proxy records, but rather to investigate, with a sub-micrometer resolution, the distribution of minor and trace elements in marine carbonates to enhance our understanding of paleoceanographic proxies based on coccoliths and foraminiferal calcite. Overall, this thesis emphasizes the importance of micro-scale measurements for gaining insights into the mechanisms involved in marine calcification and the concomitant incorporation of minor and trace elements into marine carbonates. Getting a handle on the underlying mechanisms of calcification improves the reliability of paleoceanographic proxies and will thus potentially help to advance our understanding of the climate system as a whole. Not only were pilot studies conducted whether additional, not yet investigated elements such as Na in coccoliths, or Cl in foraminifera, may potentially serve as new paleo proxies, this thesis also highlights the necessity in scrutinizing analytical procedures with highly-sensitive instruments such as the NanoSIMS.