Ni Sorption to Birnessite Drives a Surprisingly Large Fractionation

With current knowledge of sources and sinks for marine Ni, as compiled recently by Gall et al. [1], it is not possible to construct a balanced steady-state mass balance model for Ni in the global oceans. The recent discovery of considerable Ni stable isotope variations among seawater and the various inputs and outputs implies that Ni isotopes may be able to aid construction of a valid mass balance model, since the isotope systematics provide an additional dimension of constraints that must be balanced to close the budget. However, much work remains to be done to measure and especially to understand the controls on Ni isotope distribution among marine reservoirs.

Ferromanganese sediments represent a major sink for Ni, and most of these rocks are variably enriched in heavier isotopes of Ni relative to seawater [1]. In an effort to understand why that is the case, we undertook experiments to quantify Ni isotope fractionation during sorption to birnessite, the dominant Mn phase in ferromanganese crusts, at low and high ionic strength. At low ionic strength we observed lighter isotopes of Ni preferentially sorbed, with an average magnitude of Δ^60/58Ni_{sorbed-dissolved} = -1.38 ±0.16 ‰. The isotopic compositions of dissolved and sorbed Ni define parallel, linear trends as a function of fraction of Ni sorbed, indicating a reversible, equilibrium isotope effect. In simplified, synthetic seawater, the observed fractionation are even larger, up to Δ^60/58Ni_{sorbed-dissolved}= -4‰. The large magnitudes of fractionation observed in this system are surprising, but the most puzzling aspect of these results is that, in our experiments, lighter isotopes are preferentially sorbed, while in the natural system Ni in ferromanganese crusts is quite heavy relative to seawater. Likely reasons for this discrepancy will be discussed, as well as possible causes of this surprisingly large metal isotope effect.

[1] Gall et al. (2013, EPSL 375, 148)