Southern Ocean Iron Fertilization near Islands: Fe-Isotope Exchange between Reactive Particles and Seawater

• Applicant: Professor Dr. Michael Staubwasser
• Subject Area: Oceanography
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 504850859

Ocean Fe-fertilization by islands is an important process governing marine phytoplankton production and atmospheric CO2 drawdown. The process is commonly observed in the Southern Ocean, where glacial weathering on islands is believed to supply excess bio-reactive particulate Fe (pFe). Glacial weathering is one aspect particularly sensitive to global warming. Diagenetic recycling of Fe and resuspension from the shelf’s sediments is another mechanism of island ocean fertilization that can be observed throughout the global ocean. Our previous work on King George Island – Antarctic Peninsula – has shown that Fe-isotopes allow for a discrimination between the two sources. However, Fe-isotope data in dissolved Fe (dFe) from throughout the Southern Ocean vary by about 2‰ (δ56Fe) between sites. This suggest either locally different combinations of sources, regionally diverse processes of Fe-cycling with highly variable interaction between dFe and pFe phases, a regionally very dynamic biologic Fe-cycle in the water column, or a variable combination of all three aspects. I propose to use Fe-isotopes to trace the two source types of particulate Fe, and their transformation into bio-reactive dFe from the coast of South Georgia into the open ocean high productivity region during an upcoming FS Polarstern cruise (PS-133), and determine the dissolution and exchange processes between dFe and pFe during Fe-fertilization on the basis of dFe-pFe isotope fractionation. I seek to determine underlying process-based Fe-isotope fractionation factors – which are generally poorly constrained in the marine environment – by a series of laboratory and on-board experiments. Exchange experiments comprise mixtures of pure Fe-oxyhydroxide and Mn-oxide mineral phases with artificial seawater, and mixtures of natural marine particles from the island shelf and slope enriched by crossflow-filtration with crossflow-filtered particle-free seawater from upstream of the island. Likewise, the above mineral phases will be mixed with filtered Southern Ocean seawater. All exchange experiments utilize isotope-spiked water, time-controlled sampling over long intervals, and the three-isotope method that allows for an accurate extrapolation of isotope fractionation factors even when the exchange reaction does not run to incompletion. These experimental fractionation factors serve as the basis for identification of molecular-level dFe-pFe exchange reactions inherent to the natural fertilization process. I anticipate to observe and sample the Fe-fertilization process from the littoral to the open ocean during the PS-133 cruise. Additional samples from an earlier campaign to the littoral and coastal sea off King Georg Island, where isotopic compositions of sediment source endmembers are already known, will be analyzed for comparison.

• DFG Programme: Research Grants