| Summary: | To ensure we can prepare for and mitigate ongoing climate change, a thorough understanding of the climate system as well as its past behaviour is necessary. This requires not only that we reconstruct the severity but also the timing of changing climate parameters. Since parameters like paleotemperature cannot be measured directly, a so-called proxy is needed. A very commonly used proxy is the relationship between Mg/Ca in formainiferal shells to sea water temperature. Foraminifera are single celled organisms, some of which build carbonate shells in which the amount of Mg incorporated correlates to temperature. To confidently apply such proxies, it is crucial to understand the fundamental process involved. Although used for several decades now, there are still many open questions when it comes to Mg/Ca in foraminifera. In this thesis I show that a large amount on the natural variability observed in foraminiferal Mg/Ca can be attributed to variability in the timing of calcification. If a foraminifer starts biomineralization in the evening, the average Mg/Ca of that carbonate will be significantly higher than if it had been precipitated starting in the morning. This highlights that while Mg/Ca as a proxy for temperature works well, we still lack understanding of processes involved in Mg incorporation. Through culture experiments I was able to show that Mg is fractionated in two steps during biomineralization. First when Mg is transported from seawater to the site of calcification through transmembrane transport and a second step occurs during precipitation in the calcifying fluid. Together these steps describe the overall fractionation between Mg isotopes in seawater and those in the foraminiferal calcite. Using samples collected from the Mediterranean Sea, I expanded the Mg/Ca-T calibration for the planktonic species Globigerinoides ruber towards its lower temperature limits. Though I also showed that the reconstrution of small scale changes is very difficult because planktonic foraminifera experience a large ...
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