Paired Pyrite and Organic Matter Sulfur Isotopes Reveal Evolving Fe–S Co-Limitation during Organic Matter-Driven Sulfur Cycling in a Late Ediacaran Low-Sulfate Restricted Basin

Abstract Pyrite sulfur isotopes (δ34Spy) and pyrite sulfur content (Spy) integrate microbial, reservoir, iron, and burial controls, complicating their interpretation in ancient low-sulfate basins. We combine new paired bulk δ34Spy–δ34Sorg and Spy measurements and solid-bitumen reflectance data with Fe-speciation and backscattered-electron (BSE) data previously reported for late Ediacaran (∼560 Ma) Doushantuo Member IV in the restricted Chengkou sub-basin. Published petrographic observations show common framboidal pyrite and localized overgrowths of unresolved timing. Equivalent vitrinite reflectance values of 3.48–3.73% indicate thermal overmaturity, but the preserved textures and coherent stratigraphic profiles support broad retention of a depositional and/or early diagenetic pyrite-sulfur signal. In the middle interval, total organic carbon (TOC) covaries positively with δ34Spy and inversely with Spy, while heavier δ34Spy accompanies lower Mo/TOC. The TOC−δ34Spy relationship may reflect both rate-dependent changes in microbial sulfate reduction (MSR) fractionation and progressive drawdown of a poorly replenished sulfate reservoir. FeT/Al and FeHR/Al do not increase systematically toward heavier δ34Spy, whereas nonpyritic FeHR/Al increases and Fepy/FeHR decreases, showing that a smaller fraction of the operationally defined FeHR inventory was converted to pyrite. Concurrently, δ34Spy approaches δ34Sorg and Δ34Sorg–py narrows. In this independently established restricted, low-sulfate setting, the combined records are consistent with an evolving Fe–S co-limitation regime in which sulfate-derived sulfide became increasingly limiting relative to reactive Fe during progressive reservoir drawdown.