Abrupt global-ocean anoxia during the Late Ordovician-early Silurian detected using uranium isotopes of marine carbonates.

Widespread marine anoxia is hypothesized as the trigger for the second pulse of the Late Ordovician (Hirnantian) mass extinction based on lithologic and geochemical proxies that record local bottom waters or porewaters. We test the anoxia hypothesis using δ238U values of marine limestones as a global seawater redox proxy. The δ238U trends at Anticosti Island, Canada, document an abrupt late Hirnantian ∼0.3‰ negative shift continuing through the early Silurian indicating more reducing seawater conditions. The lack of observed anoxic facies and no covariance among δ238U values and other local redox proxies suggests that the δ238U trends represent a global-ocean redox record. The Hirnantian ocean anoxic event (HOAE) onset is coincident with the extinction pulse indicating its importance in triggering it. Anoxia initiated during high sea levels before peak Hirnantian glaciation, and continued into the subsequent lowstand and early Silurian deglacial eustatic rise, implying that major climatic and eustatic changes had little effect on global-ocean redox conditions. The HOAE occurred during a global δ13C positive excursion, but lasted longer indicating that controls on the C budget were partially decoupled from global-ocean redox trends. U cycle modeling suggests that there was a ∼15% increase in anoxic seafloor area and ∼80% of seawater U was sequestered into anoxic sediments during the HOAE. Unlike other ocean anoxic events (OAE), the HOAE occurred during peak and waning icehouse conditions rather than during greenhouse climates. We interpret that anoxia was driven by global cooling, which reorganized thermohaline circulation, decreased deep-ocean ventilation, enhanced nutrient fluxes, stimulated productivity, which lead to expanded oxygen minimum zones.

Isotopic evidence for long term warmth in the Mesozoic.

Atmospheric CO2 concentrations appear to have been considerably higher than modern levels during much of the Phanerozoic and it has hence been proposed that surface temperatures were also higher. Some studies have, however, suggested that Earth's temperature (estimated from the isotopic composition of fossil shells) may have been independent of variations in atmospheric CO2 (e.g. in the Jurassic and Cretaceous). If large changes in atmospheric CO2 did not produce the expected climate responses in the past, predictions of future climate and the case for reducing current fossil-fuel emissions are potentially undermined. Here we evaluate the dataset upon which the Jurassic and Cretaceous assertions are based and present new temperature data, derived from the isotopic composition of fossil brachiopods. Our results are consistent with a warm climate mode for the Jurassic and Cretaceous and hence support the view that changes in atmospheric CO2 concentrations are linked with changes in global temperatures.