Nickel Isotope Systematics in the Talvivaara Paleoproterozoic Black Shale Deposit Reveal Mineralogy-Controlled Fractionation with a Preserved Biogenic Signal

Anna Neubeck , Kirsti Loukola-Ruskeeniemi, Vyllinniskii Cameron, Delia Rösel, Sophie Charlotte Roud, Jonathan Lewis, John Manrique Carrheño, Daniel Buczko, Madeleine Bohlin, Pierre-Antoine Tranchet, Bjarne Almqvist

Nickel isotope systematics in sediment-hosted sulfide deposits are now more commonly used to infer redox and diagenetic processes, yet their potential to record biological signals and their preservation through diagenesis and metamorphism remains poorly constrained. Here we present micro-scale coupled δ60Ni, δ34S, δ13C, REE pattern, and paleoproductivity proxy data from the Paleoproterozoic Talvivaara Ni-Zn-Cu-Co deposit, a ~2.1-1.9 Ga black shale associated with the Shunga Event at the end of the Great Oxidation Event. Our data potentially reveals how biological signals are selectively preserved or overprinted by mineralogical reorganization. The deposit is hosted by graphite-sulfide schists representing metamorphosed black shales deposited in a restricted, anoxic to euxinic basin and subsequently altered to upper-greenschist- to amphibolite-facies conditions. Through most of the drill core, Ni isotope systematics are governed by the pyrite-pyrrhotite-pentlandite reaction series where pyrite consistently hosts isotopically heavier Ni than coexisting pyrrhotite. This reflects mineralogy-controlled fractionation during diagenetic sulfidization and recrystallization. The heaviest δ60Ni values in pentlandite-bearing intervals occur at depth, marking late-stage metamorphic Ni redistribution, with comparably heavy values also recorded in shallow pyrite-dominated domains through a distinct mineralogical control. In contrast, a discrete interval at 118.68-152.27 m depth preserves a coherent biogenic overprint, where coupled shifts toward light δ60Ni (-0.43 to -0.03‰), strongly negative δ34S (-10 to -13‰), elevated organic carbon contents, HREE-dominated REE patterns, strong P-Ni coupling, and anomalous paleoproductivity proxies collectively record intensified microbial sulfate reduction and organic matter degradation. While the multi-proxy convergence supports a biogenic interpretation, the δ60Ni values alone overlap with abiogenic ranges, and the possible contribution of metal-dependent anaerobic oxidation of methane to Fe-S-Ni systematics in the deeper intervals cannot be excluded. Magnetic susceptibility independently confirms that this excursion reflects early diagenetic sulfide precipitation rather than metamorphic recrystallization or hydrothermal overprint. To our knowledge, this is the first direct micro-scale evidence for a preserved biogenic δ60Ni signature in Paleoproterozoic black shales. These results demonstrate that bulk analyses systematically average over mineralogically distinct Ni pools, destroying transient biological signals that are recoverable only through micro-scale, mineralogically resolved multi-proxy analysis.