Nickel and its isotope in response to differential manganese minerals accumulation in Northwest Pacific sediments

Nickel (Ni) is a biologically essential element in marine systems, yet its oceanic sources and sinks remain incompletely quantified. Micro manganese nodules (MMNs) and particles (MMPs) in marine sediment are major authigenic phase that scavenge critical metals though adsorption or incorporation. The accumulation of these minerals depends strongly on the seawater conditions and diagenetic processes within the sediments. Despite their significance, the influence of different manganese phase on the cycling of polymetallic elements-particularly Ni and its isotopes-has received little attention. This study analyzed MMPs bearing sediments, MMNs, and MMN bearing sediments from the Northwest Pacific Ocean to characterize their geochemical and Ni isotopic signatures. In core JL190, MMNs display Mn/Fe ratio and trace element patterns typical of suboxic-oxic diagenesis. Across all three cores, Ni concentrations track Mn accumulation closely, as shown by strong Mn/Al–Ni/Al correlations. However, core TS01-B10 from the Mariana Trench, which contains diatom-rich clay, yields weaker correlations than the two Philippine Sea cores. Despite these differences, bulk sediment δ⁶⁰Ni values remain light (0.01–0.79 ‰, median 0.27 ‰), consistent with most pelagic sediments. In core B10, declining δ⁶⁰Ni alongside rising Mn/Al ratios imply preferential adsorption of lighter Ni isotopes. In contrast, JL189 show simultaneous increase in Mn/Al and δ⁶⁰Ni, suggesting porewater exchange and isotope fractionation on existing Mn oxides. The heavier Ni isotopic signature in diagenetic MMNs relative to bulk sediments likely reflects prolonged porewater interaction, preserving the porwater's isotopic signature. These results provide the first δ⁶⁰Ni data for MMNs and elucidate their role as Ni carriers. These findings highlight the importance of authigenic Mn oxides in influencing the benthic flux of heavier Ni isotopes, which contributes to balancing the oceanic Ni budget.