Steep oceanic DIC δ13C depth gradient during the Hirnantian Glaciation

The Hirnantian carbon isotopic excursion (HICE) was characterized by a positive δ13C excursion (up to ∼ + 7‰) of oceanic dissolved inorganic carbon (DIC). However its mechanism remains controversial, impeding our understanding of this global event and its environmental significance. Here, we conducted a comprehensive analysis of global carbon-isotope records, based on 65 paleogeographically-widely distributed sections (4612 carbon-isotope data), documenting a steep oceanic DIC δ13C water-depth gradient (e.g., a 4.6‰ offset in the HICE excursion magnitude within Laurentia) during the Hirnantian Stage. We propose a hybrid hypothesis that the HICE records a shift toward more positive values owing to a large glacio-eustatic fall (>100 m) under the herein-identified steep δ13C water-depth gradient, with potential contributions from global-ocean δ13CDIC changes and authigenic carbonates. This study provides the first large-data evidence for a widely density-stratified ocean during the Hirnantian Glaciation, with implications for reconstructions of ocean circulation and the carbon cycle during the Early Paleozoic.