The concentrations of seawater solutes (e.g. magnesium, calcium, potassium, carbonate, sulfate) are determined by processes that are related to weathering of rocks, climate, volcanism and plate tectonics, sedimentation and biological activities. The magnitudes of these processes and the related solute fluxes have varied throughout Earth's history. Consequently seawater composition has changed with time. Changes in solute fluxes are often reflected in the isotopic composition of the solutes. Such isotope shifts can reflect imbalances between input and output fluxes, shifting solute sources, or changes in the isotope fractionation of the related processes. Isotopic composition and element ratios are recorded in fossil shells and other mineral deposits. These fossil archives can be used to reconstruct isotope trends of the oceans through time. For this purpose fractionation processes during rock formation, weathering, transport, biomineralization, and diagenesis have to be considered.

The research unit Marine Geosystems studies fossil archives from marine sediments and mineral deposits in the ocean crust to reconstruct how the seawater composition has varied during Earth history. We use modern analogues of fossil shells, culturing and inorganic precipitation experiments to study how different processes and environmental parameters can influence the isotopic composition of biogenic and inorganic minerals. The results are applied to fossil archives to reconstruct compositional trends of isotopes and elements in seawater. Modelling the related element and isotope fluxes assists in the interpretation of the reconstructed trends in terms of the responsible processes (e.g volcanic activity, climate change, sediment deposition).

In research unit Marine Geosystems we develop the application of new, so-called "non-traditional" isotope systems like calcium, magnesium, and stable strontium isotopes, to reconstruct ion flux histories of the oceans and the related processes. In the trilateral project TRION, a cooperation between Palestinian, Israelian, and German scientists, coordinated by the research unit Marine Geosystems we study the cycling and archiving of stable strontium isotopes in the oceans and the related processes.