Project: RUI: Where does all the Mo go? Evaluating the role of iron sulfides in Mo accumulation in marine sediments

Overview:

Molybdenum accumulation in ocean sediments is widely used as a proxy for conditions during sediment

deposition to draw conclusions about paleoclimates and modern dead-zone formation. Although Mo is an important tracer, conflicting data exists as to how Mo accumulates and whether iron sulfides are the ultimate host in sediments. We seek to clarify the relative importance in marine sediments of: (1) sorption of Mo to iron sulfides, (2) retention of Mo during the formation of pyrite and/or exclusion of Mo upon growth of crystalline pyrite domains, and (3) possible formation of a distinct FeMoS product. Our recently developed, environmentally relevant procedure has enabled foundational adsorption experiments with tetrathiomolybdate. The results from this work established that mackinawite and greigite readily sorb tetrathiomolybdate suggesting that Mo removal from sulfidic waters occurs with the initial formation of metastable iron sulfides. Subsequent experiments showed the retention of Mo during the transformation of these precursor phases to the thermodynamically stable form of pyrite. With this groundwork, we are now prepared to conclusively determine the role that iron sulfides play in the accumulation of Mo in marine sediments. This clarification on the importance of iron sulfides for Mo accumulation and the concomitant factors that enhance Mo preservation in the solid phase will increase the value of Mo as a paleotracer for past sulfidic conditions.

Intellectual Merit:

There is compelling yet conflicting data for how Mo accumulates in ocean sediments and it is important

to reconcile this contradiction with a detailed understanding of the process. We hypothesize that this inconsistent data for sorption of Mo to iron sulfides exists alongside data for accumulation around iron sulfide grains because the nature of Mo accumulation varies over time as the iron sulfide transforms from metastable mackinawite and greigite to pyrite. Our proposal consists of three objectives to evaluate this hypothesis. 1) We will use the environmentally relevant, high yield process we have developed to synthesize metastable iron sulfides and evaluate Mo adsorption to these species under various conditions that will elucidate the roles of important factors including Mo thiolation, iron sulfide surface chemistry, and redox behavior. Adsorption experiments will be coupled with careful characterization of the resulting solid phases. 2) We will monitor the evolution of Mo during the transformation of these materials to pyrite and determine how various initial conditions alter the retention or incorporation of Mo during this process. Crucially, we will evaluate the redox processes of Mo, Fe, and S during the transformation. 3) We will develop a model that traces the fate of Mo through these processes and reconciles the disparate literature evidence.

Broader Impacts:

Resolving the question of how Mo accumulates in sulfidic ocean sediments and the role that iron sulfides and their phase transformations play, we will provide a strong foundation for work that employs Mo as a paleotracer to better understand ancient ocean conditions or the implications of climate change on ocean chemistry. Through this work, we will also strengthen undergraduate training in geochemistry through early exposure to and involvement in this project. The PIs will use the proposed research to establish a structured progression of student experiences, guiding them from pre-matriculation through post-graduate planning. First, a cohort-mentor model will support STEM-interested students pre-matriculation and will continue through their first year of introductory science courses at F&M. Our Summer Research Cohort will join our group following their first year at F&M to work with our Summer Research Students, who will typically be rising juniors or seniors at F&M. The Research Students will be engaged in every aspect of this project while also acting as peer mentors for the Research Cohort students—together the Research Students and Cohort will create an efficient and effective research team.