Abstract
The chemistry of the calcite test of foraminifera is often relating to the environmental conditions under which they lived. These so-called proxy-relations are regularly used as a tool to reconstruct past climates. Accuracy of these proxies is, however, often limited due to uncertainties or absence of proxy-parameter calibrations. This thesis
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focuses on the calibration, validation and application of new and existing foraminiferal carbonate based paleo-climate proxies. Several processes, such as bioturbation, hamper the accuracy of foraminiferal-based proxies. Bioturbation mixes sediments of different ages, thereby smoothing short-term events in the sedimentary record. Bioturbation can be quantified and corrected by using δ18O frequency distributions of individually measured foraminifera. Subsequent interpretation with simple spreadsheet models allows unmixing of temperature records affected by bioturbation, enhancing the accuracy of paleo-climate reconstructions. Analyzing δ18O on single foraminifera from one sample furthermore allows reconstruction of the seasonal temperature contrast. This novel proxy was calibrated and validated in the Mediterranean Sea and Atlantic Ocean, where the range in δ18Oand Mg/Caderived temperature estimated from singlespecimen analysis resembled the seasonal range in temperature at the sea surface (0–50 m). The accuracy of the foraminiferal Mg/Ca-thermometer is limited by the additional impact of inter-individual variability, seawater Mg/Ca and salinity. Inter-individual variability is mainly controlled by biological imperfections in the foraminiferal calcification mechanism. The magnitude of this uncertainty on the accuracy of foraminiferal Mg/Ca-thermometer depends on the sensitivity of the used species-specific calibrations and the amount of individuals measured. Seawater Mg/Ca values are relatively constant over shorter time scales, but fluctuate appreciably over longer time scales, impacting Mg/Ca-temperature reconstructions. However, Mg/Casw can now be reconstructed using the temperature-dependent offset in magnesium incorporation (sensitivity) between porcelaneous and hyaline foraminifera, thereby improving the accuracy of foraminiferal Mg/Ca as a proxy for temperature. A third parameter impacting foraminiferal Mg/Ca values is salinity. The incorporation of magnesium increases with salinity as a result of the increased activity of free magnesium-ions with respect to that of calcium-ions. The Na/Ca values of benthic foraminifer Ammonia tepida, cultured over a range of salinities provide a robust and independent tool to accurately reconstruct seawater salinity, thereby correcting Mg/Ca-temperatures for any effect of salinity. The combined results of the developed proxies based on foraminiferal calcite were applied to a sapropel (S5) from the Eastern Mediterranean. The newly developed Na/Ca-salinity proxy provided the first independent evidence for a major freshening of surface waters during sapropel formation. The effect of salinity on foraminiferal Mg/Ca-temperatures was subsequently quantified and corrected for using these reconstructed salinities. Corrected temperatures and salinity combined with foraminiferal δ18O showed that the Mediterranean basin changed from an evaporation basin to a basin with less evaporation and a more “Atlantic signal” during sapropel formation. This overall change in basin hydrology might be more important than the impact of the increased Nile outflow as a cause for sapropel formation.
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