Coupled nickel and molybdenum isotopes quantify the evolution of the ocean system since the Late Cretaceous

Nickel isotopes (δ 60 Ni) are an emerging tool for understanding ocean biogeochemical evolution, given improved modern Ni budget constraints. Here, we provide constraints on past ocean Ni isotope compositions from organic-rich sediments from Tarfaya Basin core SN°4. The data constrain Late Cretaceous ocean δ 60 Ni to ∼+0.8 to +1.0 per mil (‰), lower than the modern value of +1.34‰. Mass balance modeling suggests two drivers of this difference: changes in riverine input isotope composition during the Cenozoic and a more reducing Late Cretaceous ocean. Because Ni and Mo isotopes respond differently to changes in ocean inputs and redox-sensitive sinks, coupled Ni-Mo models help to deconvolve the relative importance of the two processes. The results show that the areal fraction of the euxinic sink was higher than today, reaching ∼2% during OAE2 (Oceanic Anoxic Event 2), and that the importance of the deep ocean oxic sink relative to the euxinic sink increased during the Cenozoic, potentially driven by changes in the biological pump and ocean circulation.