Mineralogical controls on the Ni isotope composition in Marine Sediments

Nickel is an essential micronutrient and required for phytoplankton to photosynthesise. Its biological importance and role in many different enzyme systems is encouraging development of Ni and its isotopes in marine archives as tracers for key biogeochemical processes. The utility of Ni isotopes as a biogeochemical tracer however is predicated on a robust understanding of the balance between Ni sources and sinks in the ocean. The dominant sink for Ni from seawater is removal to dispersed Fe-Mn (oxyhydr)oxide phases in oxic sediments, however its size and isotopic composition is still poorly constrained. Ni isotope measurements of Mn-rich sediments encompass the entire known range of Ni isotope compositions in terrestrial samples, which indicates that the processes that preserve Ni and its isotopes are also poorly constrained and thus limit our potential to interpret Ni-isotope archives.

This study provides insight on the fundamental processes that control the preservation of Ni and Ni isotopes in sediments. Using wet chemical methods, I show that the aqueous speciation of Ni in solution and the bonding mechanism of Ni to the mineral effect the binding strength of the metal to birnessite and its isotopic fractionation during sorption to the mineral. I also experimentally simulate marine oxic and suboxic diagenesis, whereby birnessite undergoes mineralogical transformations and show that during these processes Ni is released to solution and is isotopically fractionated. From these experiments, I present a model framework for preservation of Ni and Ni isotopes in Mn-rich sediments, where progressive Ni incorporation during mineral aging, diagenetic mineralogical transformations, and re-adsorption of Ni back to the mineral surface control Ni isotope compositions of marine sediments and seawater.