The influence of secondary weathering processes on dissolved nickel isotope compositions under cold climatic conditions - Observations from the Mackenzie Basin

Nickel (Ni) and its stable isotope signature (delta 60Ni) have recently gained momentum as a tracer of nutrient cycling in the modern and past oceans. A robust understanding of Ni isotope cycling in the ocean rests on an accurate understanding of the Ni sources and sinks to and from the oceans. In particular, rivers, which are the dominant Ni source to the oceans, show significant variation in Ni isotopes compared to rocks, thought to reflect variable extents of formation of secondary phases that scavenge light Ni isotopes. The current estimate of the global isotope composition of riverine Ni is based on a few large rivers in warm climates, thus preventing the assessment of a potential climatic control on the Ni isotope flux to the ocean. In this contribution, we investigate the Ni elemental and isotope signatures of river catchments in cold climates, namely the Mackenzie Basin tributaries (Canada) and two rivers, the Nass and Skeena, draining the Western Cordillera. The river solid load in the Mackenzie Basin shows almost no resolvable variation in terms of Ni abundances and Ni isotopes, which are similar to silicate rocks. Furthermore, dissolved Ni and Ni isotopes do not show any relationship with source tracers. This suggests a minimal source control on variations in riverine Ni. This result is particularly intriguing as riverine chemistry in the Mackenzie Basin is often dominated by lithological controls, including rocks other than silicate, such as carbonate and black shale. Instead, the variations in dissolved Ni isotopes are related to the removal of dissolved Ni, reflecting the dominant control by secondary weathering processes. The Ni isotope fingerprint of these secondary weathering processes reflects Ni scavenging into metal oxides, consistent with the literature. The data presented here and literature data show that dissolved Ni and lithium isotopes (a tracer of clay formation) exhibit contrasting patterns between the Mackenzie and the Amazon basins. This suggests that climatic conditions might couple or decouple the formation of metal oxide and clay. In addition, the Ni isotope signatures of the river dissolved load vary with the specific weathering intensity of Ni (the fraction of riverine Ni exported as dissolved species). At high specific weathering intensity of Ni, the Ni isotope signature reflects that of silicate, consistent with the congruent release of Ni from the weathering of silicate. Lower specific weathering intensities of Ni are associated with heavier Ni isotope signatures, suggesting greater scavenging of Ni onto secondary metal oxide. Finally, at the lowest Ni weathering intensity, the dissolved Ni isotope composition is lower again, potentially implying limited metal oxide formation under such conditions. Therefore, and though more data are required to confirm the hypothesis from this initial data, while climate might play a role in the relative rate of Li-and Ni-scavenging phase formation, the specific weathering intensity of Ni may be more significant in controlling dissolved Ni isotope composition.