Dissolved trace element concentration and speciation in the South Pacific Ocean (research cruise SO289)

The South Pacific is one of the most remote and least explored ocean regions on our planet. It is also the largest ocean region with extremely low productivity: its crystal clear, blue waters indicate a very low abundance of phytoplankton. These microscopic plants live on the ocean surface and depend on nutrients such as nitrates and trace elements like iron and cobalt to grow. Therefore, their low abundance in the South Pacific is thought to be due to a low supply of these elements. Low productivity also limits the uptake of atmospheric carbon dioxide in this part of the ocean. Hydrothermal vents on the seafloor, however, provide nutrients to the deep water – and the question is whether these nutrients also reach the surface of this ocean desert. 

Travel route and station plan of the SO289 expedition from Chile to New Caledonia.

To study these questions, expedition SO289 (February- April 2022) with the German research vessel SONNE traversed the South Pacific at 32.5°S from Valparaiso (Chile) to Noumea (New Caledonia), a distance of 6500 nautical miles. The seven-week cruise and the project are funded by the German Federal Ministry of Education and Research (BMBF) and are part of the international GEOTRACES program. The project is led by GEOMAR Helmholtz Centre for Ocean Research Kiel (Prof. Eric Achterberg, Prof. Martin Frank) in cooperation with Jacobs University (Prof. Andrea Koschinsky). For Jacobs University, postdoc Sandra Pöhle, PhD student Nico Fröhberg, Humboldt Fellow Ignacio Pedre, and Earth and Environmental Sciences student Can Gürses, who does his BSc thesis based on the research cruise, were onboard during SO289.

Cruise preparation under pandemic constraints: Andrea Koschinsky (bottom center) and the cruise team (from top left to bottom right: Nico Fröhberg, Ignacio Pedre, Can Gürses, Sandra Pöhle) meet online to discuss the final cruise preparations shortly before the cruise team is starting their quarantine.

Onboard, GEOTRACES-approved clean sampling and analysis methods for trace metals are used. Based on the large number of water samples taken as depth profiles along the cruise transect, the sources, sinks, and distributions of trace elements in the water column along this zonal section in the South Pacific are determined in detail. Sources of trace elements include Chilean and Australian dust, rivers such as the Maipo in Chile, sediments on the continental shelf, and hydrothermal vents on the seafloor of the East Pacific Rise and the Kermadec Arc. 

For the Jacobs samples, we aim to distinguish between the truly dissolved, colloidal and particulate fraction for particle reactive trace elements (Ti, Zr, Hf, Nb), ligand analysis (Fe and Cu), and the oxyanion group (Mo, V, U, W). Therefore, a sequential filtration with different pore sizes and ultrafiltration are applied on-board using e.g. a laminar flow hood for the filtration set-up to ensure a trace-metal clan environment during sample processing. In the home laboratories, analyses of trace elements from all preserved samples are done using ICP-MS (including SeaFAST preconcentration); voltammetric techniques are applied for titrating metal-binding ligands and to determine the redox speciation of Cr (Cr(III) – Cr(VI)). Vanadium speciation (V(III) – V(V)) is done using column separation and ICP-MS measurements.

Cruise blog: https://www.oceanblogs.org/geotraces/

The JUB team onboard RV SONNE during expedition SO289 from Chile to New Caledonia
The JUB team onboard RV SONNE during expedition SO289 from Chile to New Caledonia
Nico Fröhberg and Can Gürses prepare the trace metal clean rosette water sampler prior to the next deployment during cruise SO289 (photo taken by Eduard Fadeev).
Lab-work onboard RV SONNE during the RV SONNE expedition SO289.
Ultrafiltration of water samples onboard RV SONNE expedition SO289.