David Ernst

• Marine chemical sedimentary rocks, like banded iron formations (BIFs), ferromanganese (Fe-Mn) crusts and nodules, marine carbonates or cherts, are of great scientific interest because they can preserve primary information on the physico-chemical conditions of ambient seawater. Especially for research on the Precambrian, marine chemical sedimentary rocks are invaluable archives as they are the only remaining access point to the geochemical conditions of the Archaean and Palaeoproterozoic marine environment. This PhD thesis investigates the geochemical partner couples of gallium, aluminium (Ga-Al), germanium, and silicon (Ge-Si). Those two couples show mostly coherent geochemical behaviour in igneous and clastic sedimentary processes. However, in (low- low-temperature) aqueous environments, both partners decouple from each other. This thesis aims to investigate the behaviour of Ga-Al and Ge-Si during the precipitation of Fe (oxyhydr)oxides in the natural environment and to elaborate on whether characteristic distributions of Ga/Al and Ge/Si ratios in marine chemical sedimentary rocks can be applied as geochemical proxies.