SO298 GEOTRACES-GP11: Biogeochemistry of trace metals in the equatorial Pacific
On 14 April 2023, GEOTRACES cruise SO298 (which is a cooperation project between Prof. Eric Achterberg’s team from GEOMAR Kiel and our team of Constructor University) set sail from Guayaquil, Ecuador to Townsville, Australia. The majority of our transect is along the equator on the 0º latitude line, where we are sampling seawater for trace metal analyses, in one of the least-studied regions of the planet. In the Eastern Pacific, we expect the ocean chemistry to be influenced by a high-upwelling region, producing a high-productivity, high-nutrient, oxygen-depleted environment governed primarily by iron limitation. Also characteristic of the Eastern Pacific region are plumes from hydrothermal vents along the East Pacific Rise, which serve as a possible source of trace metals. Moving westward, we will encounter the more oligotrophic environment of the open ocean, governed primarily by nitrate limitation. Finally, in the western Pacific, continental shelves from islands and input from Asian rivers can be expected to influence ocean chemistry as a source of nutrients, trace metals and sediments. In addition, we will investigate proposed hydrothermal vent fields near Papua New Guinea. Throughout the cruise, a strong eastward equatorial undercurrent transports seawater, beneath a westward surface current, resulting in a very dynamic environment of various water masses.
Our team from Constructor University (Natasha, Polina and Adrienne) will collect samples for a variety of parameters including high field-strength elements (HFSE), oxyanions, metal-binding organic ligands (Cu, Fe, Ni) and Cr and V redox speciation in different size fractions across different depths and ocean regimes. Size fractionation can influence trace metal biogeochemistry and availability; for example, a high concentration of colloids can cause removal of metals from the dissolved fraction due to colloidal flocculation. To analyse concentrations at different size fractions, we perform sequential filtration of unfiltered seawater to collect samples in the 0.8, 0.2 and 0.015 µm size fractions. We also save the particle-containing filters for each size, for trace metal analysis. To collect an even smaller size requires ultrafiltration, which allows us to collect a ≤10 kDa (~ 4.4 nm) fraction at selected stations.
Adrienne: My main interest on this cruise is the interaction between trace metals and organic ligands (molecules that bind to metals such as Cu, Fe, Zn, Ni and more). During my PhD, I studied copper-ligand interactions in the Amazon and Pará River estuary. Cu and Fe in seawater are highly organically complexed >99%, which aids in their stabilization and solubilization. Ligands also have an effect on bioavailability and toxicity; for Cu, organic ligand complexation mitigates toxicity by lowering the concentration of free Cu2+ ions. For Fe, some ligands such as siderophores, serve to enhance Fe bioavailability in otherwise Fe-limited regions. By using voltammetric techniques such as competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV), we can measure the binding strength and concentration of these metal-binding ligands in seawater.
Natasha: The focus of my research will be the redox speciation of Cr throughout the water column and within various size fractions, along our transect. Cr is mostly present as Cr6+ in seawater, however, higher than expected concentrations of Cr3+ have been detected in waters influenced by hydrothermal plumes. The Equatorial Pacific transect is a great opportunity to sample multiple different environments, giving us the ability to further investigate the redox speciation of Cr and how it is affected by environmental factors such as hydrothermal plumes and oxygen minimum zones. Samples are collected on board and frozen for further analysis on land, using voltammetry. This data will add to data obtained from previous cruises in the Atlantic and Southern Pacific oceans, giving insight to sources and processes affecting the distribution and redox speciation of Cr.
Polina: Participation in this expedition and collecting samples are significant parts of my PhD research project. During my research, I plan to analyze high field strength elements (HFSE) and tungsten (W) concentrations in ocean water samples. HFSE is a group of elements that includes zirconium (Zr), hafnium (Hf), niobium (Nb), tantalum (Ta), and molybdenum (Mo). These elements have garnered much interest in the field of geochemistry due to their crucial roles in various geological and environmental processes, as well as their potential as paleoproxies for water masses. However, due to their extremely low natural concentrations and high chemical activity, accurately analyzing these trace elements can be challenging. The sources, sinks, and fluxes of these elements are not well understood, and the mechanisms controlling their distribution in seawater are still being studied. I hope that my research will help close this knowledge gap and contribute to the international GEOTRACES program by providing valuable insights about HFSE in ocean water, with a specific focus on the South Pacific Ocean (with samples from the previous cruise SO289) and the Central Pacific Ocean. These regions are currently data-sparse, and I believe that my research can shed light on the distribution and behavior of HFSE in these areas.
Cruise blog: https://www.oceanblogs.org/so298/