Spatial heterogeneity in salinity and redox dynamics during the Ordovician-Silurian transition: Multi-proxy constraints on the Late Ordovician Mass Extinction mechanisms

The Ordovician-Silurian transition (OST; ∼448–443 Ma) was marked by the Hirnantian glaciation, rapid climatic shifts, and the Late Ordovician Mass Extinction (LOME). While redox changes and ice-sheet dynamics have been widely studied, the role of salinity-pH-redox feedbacks in modulating extinction mechanisms remains poorly constrained. Here, we present a high-resolution multi-proxy dataset (δ11B, Sr/Ba, B/Ga, redox-sensitive trace metals, and iron speciation) from middle- and outer-shelf successions of the Upper Yangtze Sea, South China, to unravel spatial-temporal feedbacks between glacial meltwater, ocean connectivity, and biogeochemical cycles. Our results reveal pronounced salinity stratification in the middle-shelf (brackish to freshwater conditions, B/Ga <4, δ11B < −13 ‰) driven by pulsed meltwater inputs during glacial retreat, which amplified euxinic wedges (Mo > 64 ppm, FePy/FeHR > 0.8) through sulfate limitation and pH-driven boron adsorption. In contrast, the outer shelf maintained stable marine salinity (B/Ga ∼6.2, δ11B ∼ −8 ‰) and suboxic conditions (Mo < 25 ppm, FePy/FeHR < 0.35), acting as refugia for benthic fauna. Crucially, boron isotopes unveil pH-salinity coupling during icehouse collapse−freshwater dilution of the middle-shelf amplified H₂S toxicity by reducing carbonate buffering capacity, while open-marine connectivity stabilized outer-shelf pH. The first LOME pulse was initiated by glacial expansion-driven cooling and habitat contraction, with its severity amplified by pulsed meltwater-induced mid-shelf euxinia, whereas the second pulse was linked to post-glacial transgressive euxinia amplified by sulfate influx. This study establishes paleosalinity as a critical amplifier of climate-biogeochemical feedbacks, demonstrating how spatial ocean connectivity regulated extinction selectivity through salinity stratification. Our findings provide a novel mechanistic framework linking icehouse dynamics to marine ecosystem collapse, with implications for understanding hypoxia expansion in modern warming oceans.