Award: OCE-1924236

The trace element manganese (Mn) is essential for all forms of life and is an important reactant within the environment. The role of Mn in the chemistry and health of the ocean is a function of the species and form of Mn. The oxidation of reduced species of Mn leads to the formation of Mn oxides. Manganese oxides have been coined the scavengers of the sea, due to their high reactivity leading to the accumulation of high concentrations of trace and rare earth elements. These minerals are also strong oxidants of nutrients and carbon, making them an important control on the biogeochemistry of environments spanning the globe. Owing to these properties, Mn oxides are societally important for their role in wastewater treatment and energy storage technologies, and as a critical minerals hosting elements needed for technology (e.g., nickel, lithium).

Despite the importance of Mn oxidation and Mn oxide formation within the environment and for society at large, the processes responsible for Mn oxide formation are poorly understood. As the cycling of Mn is intricately linked to nearly all elemental cycles, a wholistic approach is required to interrogate and identify the governing and underlying processes responsible for Mn oxide formation within the environment. The overall goal of this project was to shed new light on the link between Mn oxide formation and the cycling of other reactive elements predicted to be involved in the oxidation of Mn. We focused our studies on stratified waters typified by a gradient spanning oxygen-rich surface waters to oxygen-deplete bottom waters that are rich in hydrogen sulfide (euxinic). Along this steep oxygen gradient, various other elements are cycled leading to the production of several reactants and oxidants of Mn. Accordingly, this project investigated the formation of Mn oxides in relation to reactive chemical species along oxygen gradients within the two ecosystems, a large coastal estuary (Baltic Sea) and a coastal pond on Cape Cod, MA (Siders Pond).

Over the course of the project, the research team conducted three field campaigns at Siders Pond and two research cruises in the Baltic Sea. Using new methods and in situ sensors, we characterized the concentrations and species of various elements of relevance to Mn cycling, including oxygen, nitrogen, iodine, and sulfur. Our research led to several key discoveries in regards to the processes responsible for Mn oxide formation, including but not limited to: (1) while counterintuitive, rates of Mn oxidation and Mn oxide formation were highest at low oxygen concentrations, (2) microbial nitrogen transformations led to elevated Mn oxide formation, and (3) reaction between Mn oxides and upward diffusing hydrogen sulfide led to a large peak in elemental sulfur formation. Surprisingly, using our new deep-sea sensor, we also discovered that the reactive oxygen species (ROS) superoxide was elevated in waters below the photic (sunlit) zone and where peak carbon remineralization is expected. Superoxide is a highly reactive form of oxygen that reacts with numerous other chemical species, including carbon and metals. ROS, including superoxide, are predicted to form primarily via reactions with light, thus this research highlights new processes responsible for the formation of ROS. Lastly, characterization of metals associated with Mn oxides revealed new insight into the sources and sinks of the toxic metal, thallium, within coastal waters.

Our research team was also fortunate to be on our second cruise within the Baltic Sea when a major Baltic inflow (MBI) event occurred. MBIs occur about once every decade, where oxygen-rich waters from the North Sea flow into the oxygen-depleted basins of the Baltic Sea. We took advantage of this rare opportunity to measure the impact of these inflowing waters on the chemistry of the waters by conducting a transect through the inflow. Due to the continual decline in oxygen within the Baltic Sea there has been a growing interest in geoengineering approaches to add oxygen (reoxygenate) to the Baltic waters. These samples therefore provide a unique dataset to understand how rapid introduction of oxygen to the sulfide- and metal-rich waters will change. Overall, we find significant disruption of the geochemistry of the bottom waters and shoaling of sulfide-rich waters to the surface.

Lastly, this project provided training, education, and experience for 2 postdocs, 7 graduate students, 4 undergraduate students, and 2 research technicians. Research collected as part of this project was part of two PhD dissertations (Haley Gadol, Lina Taenzer). Importantly, this work also enabled the creation of a strong international collaboration between two US institutions (WHOI, Michigan State University), three German institutions (MPI, IOW, U Marburg) and one Danish institution (Aarhus University). This team is continuing to work together and plan for future collaborative research programs.

Last Modified: 04/10/2025
Modified by: Colleen Hansel