Research

Since the 1950s, various approaches for the isotopic and chemical analyses of foraminifera shells were developed and refined, and millions of paleoclimate data points were generated that helped to shape our understanding of the Earth's past climate. Of particular importance is a detailed understating of past 'hothouse climates', which can be regarded as past climate analogues as they provide the opportunity to assess the sensitivity of the Earth's climate system to projected future increases in atmospheric greenhouse gas concentrations. Global circulation models simulating the Earth's future climate development, including changes in global and regional precipitation/evaporation patterns and the rate and amount of future sea level rise, are tested and calibrated on these past climate analogues, thus, the acquisition of robust paleoclimate data is a task of high sociological importance.

In particular the oxygen isotope (δ¹⁸O) values and Mg/Ca ratios of foraminifera calcite are two widely used geochemical proxies for paleoceanographic reconstructions, yet the fidelity of these records is frequently questioned on the grounds that foraminifera shells found in deep-sea sediments are susceptible to postdepositional diagenesis.

One approach to minimizing the effects of diagenesis is to use geochemical data derived from exceptionally well preserved "glassy" (translucent) planktic foraminiferal shells recovered from clay-rich marine sediments; these shells are prized as the 'gold standard' for reconstructing past ocean temperatures. Unfortunately, sample locations preserving glassy planktic foraminifera in the sedimentary record are extremely sparse and largely limited to nearshore environments that may have been influenced by low δ¹⁸O waters from continental sources. The vast majority (>90%) of fossil foraminifera shells available for paleoclimate studies is less-than-ideally preserved, thus, the acquisition of robust paleoclimate data from moderately preserved sample material is one of the biggest challenges in paleoclimatology.

It was found that typically only certain domains of the shells are significantly altered, often in form of coatings or inorganic calcite reprecipitates on the outer and/or inner chamber wall, whereas minute domains encapsulated within the chamber wall may still largely represent the original isotopic and chemical composition from the time the shell was formed in the water column millions of years ago. However, until recently, it was not possible to exclude the unwanted material from analysis due to the extremely small dimensions of these domains (typically only >20 microns). Conventionally, whole foraminifera shells are analyzed, and the resulting compositional data are therefore a mixture of the original biogenic and the secondary diagenetic calcite. While certain corrections for the effects of diagenetic overprints can be applied, it is not always possible to precicely quantify their extent.

These challenges can be circumvented by the use of new or improved analytical approaches that only analyze the better-preserved domains within foraminifera shells. My research group is focusing on new sample screening techniques combined with in situ analytical approaches by Secondary Ion Mass Spectrometry (SIMS), Electron Probe Microanalyzer (EPMA), and Laser-Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICPMS) to deduce more robust paleoclimate data from foraminifera shells that are only moderately preserved but are representative of >90% of the samples recovered from marine drill cores.