Controls on nickel isotope compositions of modern seawaters and marine sediments

Thesis

In this thesis, Ni isotope fractionation during marine phytoplankton uptake and amorphous Ni-sulfide formation are characterized, to improve the understanding of biogeochemistry of Ni and its isotopes in the ocean, from the surface to the bottom of the ocean. The first direct Ni isotope measurement in phytoplankton cells suggests that marine phytoplankton preferentially take up heavy Ni isotopes from the culturing medium with species-dependent isotope fractionation factors. This finding contrasts to previous hypothesis in the community that biological activity consumes isotopically light Ni in the surface ocean where the enrichment of isotopically heavy Ni is observed. The contradiction inspires a new framework of considering relative binding strength between biological metal acquisition mechanism and ambient ligands, which reconciles findings from culturing and the modern ocean. Coccolithophores show an enhancement of Fe, depletions of Co and Cu, and a decreasing isotopic pattern; with increasing Ni availability in the media, which are not seen in cultures of other species. The distinct Ni/Co efflux proteins for regulating excess cellular Ni shown in coccolithophores may cause their contrasting isotopic pattern. The Ni isotope fractionation during amorphous sulfide formation and the mineral transformation from amorphous to crystalline sulfides; suggest a potential benthic flux of isotopically heavy Ni from sulfide-containing sediments in euxinic ocean settings. This diagenetic remobilization of Ni is supported by the slightly lighter Ni isotope compositions found in nearshore euxinic sediments off Namibian margin; compared to the offshore sediments, which are anoxic but not euxinic according to the porewater chemistry. This thesis addresses important processes in the biogeochemical cycling of Ni and its isotopes in the modern ocean, demonstrating the potential and limitation of its application as a novel geochemical tracer.

Authors: Wang, T-H

• DOI: 10.5287/ora-doydm2zqe
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: sediments; trace metal cycling; Ni isotopes; sulfides; phytoplankton
• Subjects: Chemical oceanography; Stable isotope tracers