Project: Collaborative Research: Manganese Cycling and Coupling Across Redox Boundaries within Stratified Basins of the Baltic Sea

NSF Award Abstract:
The trace element manganese (Mn) is distributed widely throughout the global ocean where it cycles among three dominant oxidation states. Manganese in the higher oxidation states is highly reactive and thereby influences the cycling of nearly all other elemental cycles, including those of oxygen and nitrogen. The intermediate Mn species has only recently become recognized as an abundant component of the Mn pool, presenting now a previously unrecognized factor that may control the chemistry of the ocean. The Baltic Sea contains high Mn concentrations and preliminary investigations have pointed to the presence of an operationally defined "reactive" form of Mn but the composition and consequence of this Mn pool are unknown. This research will explore the cycling of Mn within the Baltic Sea enabled by an established collaboration with the Leibniz Institute for Baltic Sea Research in Warnemunde, Germany. By coupling field measurements and targeted shipboard incubations, this study will shed light on the processes controlling the Mn cycle and its link to the oxygen, iodine, and nitrogen cycles. This project will educate several undergraduate and graduate students and promote scientific exchange between research groups within the United States and Germany. Further, outreach efforts associated with this research will continue an existing collaboration between the PIs and the Boston Green Academy in South Boston to introduce high school students to chemical oceanography, and in particular biogeochemistry.

Manganese (Mn) is intricately linked to nearly all elemental cycles, and yet we know little about the processes governing its redox cycling within natural systems. Over the past decade a number of key scientific discoveries have provided greater insight into the diversity of processes and mechanisms involved in Mn redox cycling and introduced Mn(III) ligand complexes as important components of the dissolved Mn pool. The Baltic Sea is one of the most well studied stratified marine systems and reactive Mn has been implicated as a key factor in the formation and maintenance of suboxic zones. Thus, the goal of this research is to explore the cycling and elemental coupling of Mn within stratified basins of the Baltic Sea. The PIs predict that reactive Mn, as Mn(III) ligand complexes and Mn oxide particles, is a primary control on the redox landscape of stratified marine waters, particularly at redox boundaries and within the suboxic zone. The PIs propose fieldwork in a local permanently stratified brackish pond to refine experimental procedures followed by two cruises to suboxic basins in the Baltic Sea enabled by an established collaboration with the Leibniz Institute for Baltic Sea Research in Warnemunde. Field measurements will be obtained using a combination of in situ sensors and ship/lab-based instrumentation at several Baltic Sea sites to define the distribution of Mn species and the rates of Mn redox transformations spanning the redoxcline along with a suite of chemical information. Further, a matrix of shipboard incubations will be conducted to constrain the underlying (a)biotic processes responsible for the observed Mn profiles. Specifically, across oxygen and Mn gradients spanning the redoxcline, the PIs will interrogate the link between the Mn cycle and iodine and nitrogen species, which will ultimately help constrain current gaps in the mass balance of these elements in Baltic Sea models.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.