Elements and Isotopes in Foraminifera: Magnesium Uptake, Biomineralization and Proxy Application

Abstract

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 range of environmental parameters throughout their life cycle due to lateral transport via ocean currents. Large scale features such as circulation patterns can be inferred from foraminifera though, in a down core study I analysed redox sensitive elements in benthic and planktonic foraminifera from the Alboran Sea and found that from 9 to 5 kyr BP, while the eastern Mediterranean experienced low oxygen conditions, benthic foraminifera picked up mobilized Mn that was transported through deep water currents from the eastern basin to the western basin. This indicates that at that time the circulation pattern still resembled the anti-estuarine circulation we see today.

While having a long fossil record, culture experiments indicate that some foraminifera might thrive under future climate conditions, too. Counterintuitively, large benthic foraminifera showed thicker shell walls and a larger number of chambers added under elevated atmospheric CO2 levels. Only when the concentration of CO2 increases too much do the negative effects of the lowered seawater pH outbalance the positive effects of increased availability of dissolved carbon species – building blocks needed to form carbonate – a net negative effect and a reduction in growth occurs.