Ni isotope cycling in sediments of highly productive upwelling systems
Sweere, T.C.; Archer, C.; Fleischmann, S.; Scholz, F.; Kraal, P.; Vance, D. (2025). Ni isotope cycling in sediments of highly productive upwelling systems. Earth Planet. Sci. Lett. 668: 119549. https://dx.doi.org/10.1016/j.epsl.2025.119549
In: Earth and Planetary Science Letters. Elsevier: Amsterdam. ISSN 0012-821X; e-ISSN 1385-013X, more
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| Authors | | Top |
- Sweere, T.C.
- Archer, C.
- Fleischmann, S.
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- Scholz, F.
- Kraal, P., more
- Vance, D.
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| Abstract |
Nickel is a bio-essential micronutrient in the ocean and the element holds significant potential for the reconstruction of paleoenvironmental conditions. Nickel isotopes provide information on processes controlling internal oceanic cycling and the oceanic mass balance. These insights provide the basis for inferences from the sedimentary record. Recent studies have shown that diagenetic Ni cycling can cause isotope fractionation and may affect the global oceanic dissolved pool, but direct data to constrain such processes in detail remain relatively scarce. Here, we present a large Ni isotope dataset for sediments and pore waters at eight different locations along the Namibian and Peruvian margins, some of the most productive and oxygen-depleted regions in the open ocean. These different stations represent a wide range of depositional redox conditions, allowing evaluation of Ni behaviour in different environments and better constraints on sedimentary Ni cycling and the potential of Ni as a paleo-environmental proxy. The sedimentary data presented here reinforce findings from previous studies that have suggested that excess Ni burial in organic-rich, strongly reducing sediments underlying highly productive surface waters is unfractionated from the deep ocean isotope composition (1.33 ± 0.07 ‰), with an average δ60Niexcess of 1.37 ± 0.16 ‰ (1 SD, n = 74) for Ni-enriched samples (fexcess >0.8). This observation suggests that sedimentary records from organic-rich continental margin upwelling settings can serve as an archive for past deep-ocean δ60Ni values. The pore water data provide additional constraints on Ni isotope cycling in different redox conditions and show that the preferential precipitation of isotopically light Ni leaves sulphidic pore waters enriched in isotopically heavy Ni, with an average δ60Ni value of 2.04 ± 0.41. Pore-water Ni concentrations are higher than those in the overlying bottom water, implying a diffusive benthic source in these environments, with an average local flux of -28 μmol m-2 yr-1. Where outputs from the oceanic dissolved pool are defined in terms of the characteristics of deeper sediments, this benthic flux should be viewed as recycled, not as a new net input in the oceanic mass balance. With other recent studies, our data highlight benthic Ni fluxes as a feature of highly productive environments despite sulphidic pore waters. However, the observed benthic fluxes in these studies are relatively small compared to sedimentary Ni accumulation rates, suggesting Ni burial is relatively efficient, with 92 ± 7 % (1SD, n = 11) of deposited Ni being retained in the sediment.
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