Rapid marine oxygen fluctuations linked to climate-driven weathering during the Late Ordovician Mass Extinction

Abstract The Late Ordovician Mass Extinction (LOME) comprised two pulses of biodiversity loss under contrasting climatic conditions, yet how these transitions reshaped marine redox conditions remains enigmatic. Here we present paired carbonate cerium isotope (δ142Ce) and neodymium isotope (δ146Nd) records from an Upper Ordovician section in South China. Two pronounced negative δ142Ce excursions at the Katian-Hirnantian boundary (onset of LOME-1) and middle Hirnantian (LOME-2) indicate two episodes of benthic deoxygenation extending above the storm wave base, separated by a brief oxic interval associated with proliferation of the Hirnantia fauna. Beyond the prevailing view that links LOME-1 primarily to climatic cooling and rapid sea-level fall, anoxia shoaled onto shallow carbonate platforms also imposed acute ecological stress on shallow-marine communities. In parallel, the δ146Nd records a climate-driven reorganization of continental weathering, as Katian warming favored congruent weathering, early Hirnantian glaciation enhanced incongruent weathering, and postglacial warming promoted a return toward congruent weathering. These weathering transitions coincided with LOME-1 and LOME-2. Earth-system experiments further suggest that climate-driven shifts in weathering regime modulated riverine nutrient delivery and primary productivity, thereby contributing to repeated shallow-marine deoxygenation. Together, our results support that rapid climate oscillations destabilize shallow-marine oxygenation through changes in continental weathering and nutrient supply.