Pyrite sulfur isotopic composition (δ34Spy) is a crucial proxy for reconstructing ancient Ediacaran marine environments. However, recent in situ isotopic analyses of sedimentary pyrite have revealed distinct δ34Spy signatures among different pyrite morphologies, indicating that secular changes in bulk δ34Spy may reflect variations in proportions of different pyrite morphologies rather than environmental signals. Up to now, intragrain isotopic patterns within individual pyrite grains have not yet been extensively investigated for Ediacaran samples. The absence of this specific data set has hindered our ability to understand current complexities of bulk δ34Spy in reconstruction paleoenvironment.
In this study, Yongliang Hu, Wei Wang, and other co-authors elucidated δ34Spy patterns by conducting scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), Raman spectroscopy, and nanoscale secondary ion mass spectrometry (NanoSIMS) to examine the crystal texture, element atomic ratios (S/Fe), mineral composition, and in situ isotopic composition of individual pyrite grains from Ediacaran drill-core samples. The results highlight significant microenvironmental heterogeneity and dynamic sulfur pool mixing on rapid short-term timescale during pyrite growth.
Key findings of this study include:
(1) The observed pyrite grains exhibit significant variations in in situ δ34Spy values across μm-scale regions. Targeted euhedral/subhedral pyrite crystals generally show uniform mineral texture, although some grains show varying degrees of dissolution edges and surface cavities. These pyrite grains are found in banded aggregations, positioned parallel or subparallel to bedding planes or scattered around lens-shaped pyrite, surrounded by authigenic clay and calcite. This distribution suggests they formed in sediment pores. In situ isotopic analysis reveals significant intragrain δ34S heterogeneity, with differences reaching up to 69.3‰ on a micrometer scale.
(2) This heterogenous intragrain δ34S pattern of pyrite may be related to the formation model of pyrite grains. Targeted euhedral/subhedral pyrite grains formed rapidly, originating from numerous nucleation sites simultaneously. The sulfur in pyrite could originate from several sources, including diffusive sulfate ions from the upper part of the sulfate reduction zone (SRZ), sulfide diffusion from the water column or shallow sediments, which produces more 34S-depleted isotopic signals, or from deeper sediments where sulfate-driven anaerobic oxidation of methane generates 34S-enriched isotopes. Shallow sediment depth and low sedimentation rates create a stable pore-water microenvironment, resulting in consistent pyrite formation with uniform δ34S values. In contrast, deeper sediment depths and higher sedimentation rates lead to highly positive and divergent δ34S values. This variability highlights dynamic environmental conditions and complex mixing processes of sulfur pools over rapid geological timescale during pyrite growth.
(3) Differences in elemental or mineral composition have minimal impact on the sulfur isotope heterogeneity of pyrite grains. The δ34Spy values show a slight positive correlation with the S/Fe ratios of the pyrite, implying that lower δ34Spy values generally coincide with lower pyrite S/Fe ratios. Raman spectroscopy indicates the possible presence of pyrrhotite minerals within the pyrite grains. However, their influence on the generation of in situ δ34S heterogeneity within the grains appears to be less pronounced due to their small isotopic fractionation during mineral conversion.
Recently, this study has been published on line in Marine and Petroleum Geology. The publication issues are as follows:
Yongliang Hu, Wei Wang*, Xianye Zhao, Chengguo Guan, Chuanming Zhou, Chenran Song, Hongyi Shi, Yunpeng Sun, Zhe Chen, Xunlai Yuan, 2025. Extreme sulfur isotope heterogeneity in individual Ediacaran pyrite grains revealed by NanoSIMS analysis. Marine and Petroleum Geology, 171, 1−14. https://doi.org/10.1016/j.marpetgeo.2024.107201.
Fig. 1 Microscopic features and mineralogy of pyrite in samples LTS01-02
Fig. 2 Pitting locations, histograms and box-and-whisker diagrams of in situδ34Spy measurements on targeted pyrite grains
Fig. 3 Growth patterns for the pyrite grains in the Lantian drill-core samples
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