Resolving controls on the marine nickel budget

Northern Arizona University, Flagstaff AZ

Investigators

Abstract

Nickel (Ni) is a crucial trace metal in several enzymes involved in the carbon, nitrogen, and oxygen cycles. Its concentration in seawater has been demonstrated to have remained roughly constant for many millions of years, and yet scientists have not yet been able to satisfactorily “balance the budget” of nickel in the modern oceans. That is, the known sources of nickel to seawater are not balanced by the known removal processes. This project will examine the mechanisms by which nickel is removed to two major types of marine sediments, in order to answer key questions about the modern nickel budget. Beyond this specific contribution, the work will aid those who wish to reconstruct the ancient history of Ni cycling in the oceans. Dramatic shifts in the Ni budget have played crucial roles in the co-evolution of Earth and life, such as before the Great Oxidation Event 2.4 billion years ago and just before the Permian-Triassic mass extinction. In addition, recent studies have implied that most of the Ni inputs from rivers to estuaries and oceans today is from human activities. Results from the proposed work may help predict the fate of this new Ni once it enters the oceans. Two Ph.D. students and two undergraduates will be supported by this project, both directly in the research and through involvement in department-wide activities aimed at increasing diversity, equity, and inclusion (DEI) in geosciences. The students will invite some of the nation’s top geoscientist-activists to visit the Northern Arizona University campus, to speak about their research and their innovative contributions to DEI, and to appear as role models for members of the university community. At present, published models of the Ni cycle either deviate from steady state in the extreme, which is implausible, or rest necessarily on speculative assumptions about Ni behavior in certain types of marine sediment. A few recent studies aimed to use Ni stable isotopes as an additional constraint for a balanced budget: just as source and sink fluxes should balance, the sum of flux-weighted isotopic compositions of sources to the ocean should equal the sum of flux-weighted isotopic compositions of sinks. Thus far, inadequate knowledge regarding two critical sedimentary outputs precludes a satisfactory solution: (1) Mn-rich, authigenic clay-rich sediments and (2) carbonates. In order to resolve apparent contradictions in the recent literature about these two outputs and to enable more accurate models of the Ni budget, the team will conduct systematic experiments to quantify Ni isotope fractionation during sorption of Ni on Mn oxyhydroxide (birnessite) and on smectite. To determine the Ni isotopic composition of pelagic sediment below the zone of Mn-reducing diagenesis and to learn whether Mn oxyhydroxide/smectite ratios control Ni isotope compositions in pelagic sediments, they will analyze selected samples from equatorial Pacific sediment cores (RV Sonne Expedition SO240). The research team will also collect the first Ni isotope data for well-characterized, young carbonate sediments, to assess fractionation during primary precipitation from seawater and during early diagenesis. 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.