The marine carbonate system is an essential component of the global carbon cycle. Due to the CO₂ exchange at the air-sea interface the oceans play an important role by regulating the amount of atmospheric carbon dioxide and are thus closely coupled to climate variability. Anthropogenic intervention alters the natural climate variability and raises the question of how environmental conditions might change in the future. To validate climate models, past climate reconstructions present the best opportunity. Useful information about the geological history of the marine carbonate system is preserved in ocean sediments. So-called "proxies" are used to reconstruct past ocean conditions. They are measurable descriptors that are closely correlated to desired but unobservable target parameters. For instance, the stable isotopic composition and trace element ratios within organic and inorganic compounds of microfossils are recorders for certain parameters describing the marine carbonate system such as temperature, alkalinity and pH. Foraminifera (unicellular organisms secreting a carbonate shell) are among the most important recorders of past environmental conditions. However, the recordability of the target parameter is often biased by a dependency of the proxy on additional variables. Species specificity, various vital effects (controlled by abiotic factors like light intensity, nutrient availability, CO₂ concentration) and susceptibility to diagenesis complicate their interpretation. Hence, proxy relationships are not as robust as one would like them to be. Culture experiments and field studies in concert with numerical modelling enable us to identify the mechanisms that control the primary relationship. This knowledge is required to develop robust proxy relationships for high quality paleoclimatic reconstructions. Next to their carbon isotopic composition, planktonic foraminifera shells are investigated with regard to oxygen and boron isotopes and trace element/calcium ratios like Ba/Ca, U/Ca, Mg/Ca and Sr/Ca. Another objective is to investigate the stable carbon isotopic composition of biomarkers such as alkenones.