# Mineralogical controls of the oceanic nickel cycle

## TL;DR

This paper shows how manganese minerals control nickel isotope patterns in marine sediments and seawater, affecting their use as proxies for ancient ocean productivity.

## Contribution

The study identifies Mn mineral aging and transformation as key drivers of Ni isotope variability in oxic marine sediments.

## Key findings

- Nickel isotope variability in Mn-rich sediments is driven by adsorption and structural incorporation into Mn oxides.
- Isotopically heavy Ni is preferentially released during Mn mineral transformation.
- Mn mineral processes significantly influence metal isotope proxies tied to Mn cycling.

## Abstract

Transition metals and their isotopes are promising paleo-productivity proxies, but their utility depends on understanding their cycling between sediment and seawater. Using nickel (Ni) as an example, we show how manganese (Mn) minerals control its isotopic composition in oxic marine sediments. By analysing synthetic and natural samples, and simulating sediment diagenesis, we find that most Ni isotope variability in modern Mn-rich sediments is driven by the relative contribution of two bonding mechanisms – adsorption to and structural incorporation into Mn oxides – which evolve during Mn mineral aging and transformation. We also find that isotopically heavy Ni is preferentially released during transformation. This supports a conceptual model where Mn mineral aging and transformation co-modify sediment and seawater Ni isotopes. Using isotope mass-balance we explore the sensitivity of seawater Ni isotope archives to redox change. We suggest that Mn mineral processes are important for any metal isotope proxy whose cycling is coupled to Mn mineral formation.

Observations of natural samples combined with laboratory experiments reveal that manganese minerals substantially influence nickel cycling in marine sediments and seawater, driven by adsorption and incorporation processes.