Input, particle reactivity, and (bio)geochemical cycling of the transition metals from subgroups IV, V, and VI (Ti-Zr-Hf, V-Nb-Ta, Cr-Mo-W) in the Atlantic
Final Report Abstract
Within the DFG-project we investigated the distribution of these trace metals in the oceanic water column under consideration of different sources, sinks and processes. Prior to the analysis of seawater samples we developed a new method adapting the online preconcentration system SeaFAST with subsequent ICP-MS analysis and succeeded in the determination of dissolved Ti, Zr, Nb, W, V and Mo in seawater. As dissolved Hf and Ta in seawater are present in the ultra-trace concentration range (sub-pmol kg-1) further work would be needed for blank reduction and increasing the preconcentration factor. For Ti, a blank problem made the application of the method to our samples impossible. However, we were able to analyse Zr, Nb, W, V and Mo with good analytical quality. The newly developed online-preconcentration method was applied to 194 Atlantic seawater samples collected at 16 stations during the GEOTRACES cruise GA11. The participation of a PhD-student within a DAAD-funded German-Brazilian sandwich project in a Polarstern cruise provided more data for dissolved Ti, Zr, Mo and V for this project, which we can compare with data from the Meteor M81/1 cruise as both cruise tracks partly covered the same area of investigation. Although Mo and W are mostly considered to be conservative in the oceanic water column we demonstrate slight variations in particular for the area, which is under the influence of Saharan dust input. Tungsten was increased in the vicinity of the Canary Islands. Saharan dust was identified as a source for V and appears to provide sorption sites for Mo and W in the East Atlantic. In the Western Atlantic Basin, concentrations of V, Mo, and W appear to be partly influenced from the input from the Amazon river and sorption and desorption processes from particulate matter. Since the high field strength elements (HFSE: Ti, Zr, Nb) are particle-reactive metals, we expected a decrease in concentration with increasing water depth. However, we detected increasing concentrations for Zr and Nb and present the first Atlantic water depth profile for Nb to our knowledge. We investigated the distribution between the colloidal and truly dissolved phase on Pacific seawater samples collected during the research cruise with RV Sonne applying a two-step filtration approach (0.2 µm and 0.015 µm pore diameter). This approach enabled to assess the role of colloidal metal fractions for the observed increase with depth compared to a possible increase of the truly dissolved fraction by organic complexation in deeper waters. Our results point to an increase of the truly dissolved fraction with depth, which may be related to a release from biogenic opal and stabilization in solution by organic complexation. Moreover, we investigated the distribution of Cr redox species in the Atlantic water column. We detected significant amounts of Cr(III) although inorganic thermodynamic calculations predict Cr(VI) to be the dominant species. Most of the Cr(III) was not reactive Cr, indicating possible organic complexation. Pacific seawater samples collected for comparison during GEOTRACES cruise GP 13 (June 2013) showed similar concentration ranges for Cr, but clear differences with respect to species distribution and their relationship to nutrients, reflecting different biogeochemical controls of Cr distribution in the investigated areas in the Atlantic and Pacific Ocean.
Publications
- (2013): Determination of the Natural Dissolved Concentration of Zirconium in Seawater by Adsorptive Stripping Voltammetry. Electroanalysis, 25 (7), 1628-1634
Schneider, A. B., Paulo C. Nascimento, Denise Bohrer, Leandro M. de Carvalho, Ananda Guarda, Daiane Dias, Andrea Koschinsky
(See online at https://doi.org/10.1002/elan.201300103) - (2015): Determination of Ti, Zr, Nb, V, W and Mo in seawater by a new online-preconcentration method and subsequent ICP-MS analysis. Deep Sea Res. Part I Oceanogr. Res. Pap. 98, 83–93
Poehle, S., Schmidt, K., Koschinsky, A.
(See online at https://doi.org/10.1016/j.dsr.2014.11.014) - (2015): Determination of zirconium and vanadium in natural waters by adsorptive stripping voltammetry in the presence of cupferron, oxalic acid and 1,3-diphenylguanidine. Electroanalysis 27, 1864-1870
Schneider, A.B., Nascimento, P.C., Bohrer, D., de Carvalho, L.M., Guarda, A., Krause, C., Wiethan, B.A., and Koschinsky, A.
(See online at https://doi.org/10.1002/elan.201500140) - (2016): An experimental study on the mixing behavior of the transition metals Ti, Zr, V and Mo in the Elbe, Rhine and Weser estuaries. Estuarine, Coastal and Shelf Science 170, 34-44
Schneider, A. B., Koschinsky, A., Kiprotich J., Pöhle, S., Nascimento P. C.
(See online at https://doi.org/10.1016/j.ecss.2015.12.002) - Depth distribution of Zr and Nb in seawater: The potential role of colloids or organic complexation to explain non-scavenging-type behavior. Marine Chemistry Volume 188, 20 January 2017, Pages 18-32
Poehle, S., Koschinsky, A.
(See online at https://doi.org/10.1016/j.marchem.2016.12.001)
