Ni and Mo enrichment mechanisms in framboidal pyrite during methane-release events (Baiyun Sag, South China Sea)

Framboidal pyrite, a common form of authigenic pyrite in marine sediments, forms through the co-precipitation of equant, equidimensional microcrystals that can effectively sequester trace elements. Nevertheless, the relationships among key attributes of framboids (i.e., framboid size, microcrystal dimensions, and number of microcrystals) and the detailed mechanisms of trace−element enrichment within framboids are not yet well understood. To address this gap, we present a dataset encompassing framboid key attributes, sulfur isotopes, and trace−element (Ni, Mo) concentrations from two gas−hydrate−bearing drillsites (GMGS4−SC−W02B and GMGS4−SC−W03B) in the Shenhu area, Pearl River Mouth Basin, South China Sea. In this methane−seep−influenced setting, framboid size is primarily controlled by microcrystal diameter and secondarily by microcrystal abundance. Nickel enrichment in framboidal pyrite follows a two−stage mechanism. Initially, Ni is taken up into FeS precursors before being incorporated into pyrite via isomorphous substitution on {111} microcrystal surfaces during framboid growth. This results in a strong positive correlation with microcrystal size (R² = 0.73, p < 0.001) rather than with their number (R 2 = 0.18, p = 0.02). In contrast, Mo content shows positive correlations with both framboid size and microcrystal abundance (R² = 0.66, p < 0.01 and R² = 0.42, p = 0.01, respectively), consistent with its incorporation as nano−inclusions or nanoparticles within the pyrite lattice. The enrichment of both elements initiates from the strong adsorption capacity of iron monosulfide precursors. In methane release environments, enhanced anaerobic oxidation of methane facilitates the conversion of molybdate to particle-reactive thiomolybdates, which are effectively captured by growing pyrite framboids. Nickel, on the other hand, benefits from the increase in microcrystalline surface area during growth, where it replaces exposed Fe 2+ through isomorphic substitution, leading to enrichment on microcrystal surfaces. This study advances the mechanistic understanding of trace−element incorporation in framboidal pyrite, and these findings strengthen the reliability of nickel and molybdenum as robust proxies for reconstructing paleoenvironmental conditions and paleo-methane release activity.