Cooling-induced intensification of ocean anoxia in the mid-Paleozoic.

Mid-Paleozoic oceanic anoxic events (OAEs) have long posed an enigma, with their drivers and dynamics being markedly distinct from the hyperthermal-related events of later eras. Here, we investigate a prominent mid-Silurian OAE, which was associated with the Ireviken Extinction Event and coincided with a cooling climate. We apply Fe speciation, redox-sensitive trace metals, and elemental weathering proxies, alongside sedimentological records and coupled uranium-molybdenum isotope analyses, to deep shelf and basinal sections from the UK. These data demonstrate a gradual spread of anoxia from basinal to shelfal settings, which we postulate was driven by an enhanced nutrient supply delivered via cooling-induced upwelling. Isotope mass balance modeling supports a major increase in the extent of deeper water ferruginous conditions at this time, while euxinia developed on the continental shelf, stressing the shallower water biota. A subsequent transition to ferruginous anoxia occurred on the shelf during the later stages of the event, as climatic conditions recovered and terrestrial chemical weathering rates increased. These changes, occurring when the ocean was poised at a lower redox state under the prevailing, low atmospheric oxygen levels of the mid-Paleozoic, led to OAE dynamics that were markedly different to those of the Mesozoic.