NI ISOTOPE FRACTIONATION VIA ADSORPTION ONTO FERROMANGANESE OXYHYDROXIDES

Nickel isotopes have recently been used as a new tool for investigating the geochemical cycle of this biologically important trace element. Understanding the processes that control the Ni isotopic composition of seawater could provide insight into the evolution of marine life over Earth’s history. Ferromanganese oxides likely play an important role in controlling the isotopic compositions of sources and sinks of Ni in the oceans. These minerals can adsorb/desorb Ni during rock weathering and affect the chemistry of surface and ground waters that deliver most of the Ni to the marine system. Ferromanganese crusts and nodules on the seafloor are also one of the major repositories for dissolved Ni in the oceans.

Here we present laboratory experiments designed to investigate the fractionation of Ni isotopes during sorption of Ni onto manganese oxyhydroxide (birnessite) and iron oxyhydroxide (ferrihydrite). Batch experiments were run at controlled pH with varying proportions of dissolved Ni and oxide particles (one mineral at a time). After mixing, each experiment was filtered to separate the sorbed Ni from the remaining dissolved Ni, and the isotope ratios were measured in each fraction.

For both substrates, Ni sorbed to particles was isotopically lighter than the remaining dissolved Ni. Nickel sorbed to birnessite exhibited the larger isotope fractionation effect, with a δ^60/58Ni value ~1.4‰ lighter than Ni in solution. The δ^60/58Ni value of Ni sorbed onto ferrihydrite was ~0.3‰ lighter than that of dissolved Ni. Adsorption of light Ni isotopes to ferromanganese crusts likely contributes to the heavy isotope composition of rivers compared to average continental rocks (Cameron and Vance 2013). Our experiments suggest that Ni isotopes might also fractionate during the removal of Ni from seawater into ferromanganese crusts and nodules. Additional oxide adsorption experiments using Ni dissolved in synthetic seawater are being performed to determine whether or not a similar fractionation effect is expected in saline water.

Cameron and Vance (2013) Mineralogical Magazine 77, 811