Reassessing sedimentary sulfur isotope records: Local glacial-sedimentary processes, not global anoxia, drove the Hirnantian δ34Spy excursion

Abstract

The Hirnantian Stage (ca. 445.21–443.07 Ma) of the Late Ordovician records a pronounced positive excursion in pyrite sulfur isotopes (δ³⁴S_py) concurrent with Gondwanan glaciation and the Late Ordovician Mass Extinction (LOME). This δ³⁴S_py excursion is conventionally attributed to increased pyrite burial in anoxic seafloor, linking marine anoxia to the LOME. This hypothesis, however, lacks verification from global sedimentary sulfur content data and fundamentally conflicts with the stability of carbonate-associated sulfate sulfur isotopes (δ³⁴S_CAS: ∼ 25‰), which indicates minimal changes in global pyrite burial flux, thus hindering our understanding of the underlying mechanisms of the Hirnantian δ³⁴S_py excursion and its paleoenvironmental implications. Here, I integrate geochemical data (δ³⁴S_py, Δ³³S_py, δ³⁴S_CAS, total sulfur (TS), total organic carbon (TOC), and Fe-speciation) from 13 globally distributed successions with well-constrained graptolite biozones to decipher controls on Hirnantian δ³⁴S_py shift and their linkage to marine sulfur cycling. The results reveal a continuous decline in pyrite burial from the early to middle Hirnantian, coincident with Hirnantian glacial advance and the δ³⁴S_py excursion, contradicting hypotheses of enhanced pyrite burial. Notably, the Hirnantian δ³⁴S_py enrichment correlates with lithofacies changes (from organic-rich shales to carbonaceous mudstone/limestones), increased sedimentation rates, and oceanic oxygenation during glacial-eustatic sea-level fall, rather than with marine anoxia. I interpret the observed S-isotopic variations as reflecting a transition of microbial sulfate reduction in open-system (pre-Hirnantian) to closed-system, sulfate-limited conditions during the Hirnantian. I propose this marked shift was driven by glaciation-induced sea-level fall, which enhanced shallow marine oxygenation and increased sedimentation rates. Mechanistically, global shallowing of depositional settings and intensified ocean ventilation would deepen the microbial sulfate reduction zone within sediments. When coupled with higher sedimentation rates, these processes promoted a Rayleigh distillation behavior, enriching δ³⁴S_py values. Numerical modeling of multiple S-isotopic covariations (δ³⁴S_py-Δ³³S_py) further corroborate the dominant influence of local processes on the S-isotopic trends observed before and across the Hirnantian. These findings demonstrate the Hirnantian positive δ³⁴S_py excursion primarily reflects local sedimentological responses to glaciation, rather than globally enhanced pyrite burial and oceanic anoxia. This necessitates a reevaluation of the late Ordovician sulfur cycling and careful assessment of the global paleoenvironmental significance of sedimentary δ³⁴S_py records over geological time.