Many environmental parameters (e.g. temperature, salinity, pH) influence the isotopic and elemental composition of shells formed by marine calcifiers. Consequently the composition of the shells can be used to reconstruct environmental conditions. Chemical or isotopic parameters that reflect environmental conditions are called "proxies". E.g. oxygen isotopes and strontium concentration of corals both are proxies for water temperature. However, no proxy is influenced by only one single parameter. Other environmental factors may additionally influence the proxy. E.g. salinity influences the Mg/Ca ratio of foraminifera, which is primarily a proxy for temperature. Proxies in shells are further influenced by biological processes and by kinetic factors related to the growth rate of the shells. For fossil shells diagenetic alteration of the original composition may play an important role and can completely destroy the original environmental information recorded by the proxy.
The reasearch unit Marine Geosystems studies proxies in modern and fossil biogenic and inorganic minerals. By using shells and mineral precipitates grown in the laboratory under strictly controlled conditions we explore how different environmental and biological parameters influence the proxy incorporation. The results are compared with studies on shells and skeletons grown in natural settings where the environmental parameters during shell growth have been recorded. Comparing fossils with different degrees of diagenetic alteration we can estimate the robustness of proxies under conditions established in sedimentary burial.
In the research unit Marine Geosystems we study and develop new potential proxies like stable strontium, calcium and magnesium isotopes and carry out research in order to understand the mechanisms and processes generating and deteriorating proxy records. In the trilateral DFG project TRION the suitability of stable strontium isotopes as a proxy in coral skeletons is studied in cooperation with Israelian and Palestinian partners. Influences of diagenetic alteration on proxies are studied in the framework of the DFG Research Unit CHARON. Proxies for changes in the acidity and carbonate saturation state of the ocean are studied in framework of the joint BMBF project BIOACID II.