Sandra Pöhle

• This PhD-study focuses on the distribution of the 9 different metals titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), vanadium (V), molybdenum (Mo), and tungsten (W) from subgroups IV, V, and VI of the periodic table of elements in seawater. Within this work an online-preconcentration method has been developed using the SeaFAST-system for Ti, Zr, Hf, Nb, Ta, W. The developed methods have been applied to 194 seawater samples (16 stations) located in the (sub)tropical Atlantic (GEOTRACES research cruise M81/1). Results on dissolved Zr and Nb concentrations reveal deviations from the typical depth patterns of a particle-reactive metal. The concentration of Zr increased with depth, whereas the distribution of Nb was predominantly conservative except off West Africa. A sequential filtration approach showed that dissolved Zr is present in the size fraction < 0.015 µm and hence organic ligands may complex and stabilize Zr in the dissolved phase leading to the observed increase with depth. Depth profiles of Mo and W reveal deviations from the commonly assumed conservative depth distribution. In the area influenced by Saharan dust deposition, Mo and W seem to sorb onto deposited mineral particles whereas dust seems to be source for V. Riverine particulate matter seem to release V and Mo in the Amazon estuary while mixing effects with freshwater might explain the observed higher W concentrations in this area. As a base for future studies on Cr as a paleoproxy for ancient seawater Cr speciation under varying oxygen conditions has been investigated in the Pacific Ocean (research cruise GP13) and in the Atlantic Ocean (M81/1) applying a voltammetric method. Dissolved oxygen did not significantly affect the distribution of Cr redox species in both investigated areas. Cr(III) consisted almost quantitatively of Cr(III_unreactive) in the South Pacific and the Atlantic samples and hence photoreduction processes and organic complexation seem to