Persistent global marine euxinia in the early Silurian.

Area of Science:

• Paleoceanography
• Geochemistry
• Mass Extinction Events

Background:

• The Late Ordovician mass extinction's second pulse (~444 Ma) is linked to marine anoxia extending into the Silurian.
• Previous studies on the Hirnantian ocean anoxic event primarily focused on its onset, often using carbonate δ238U.
• Quantifying uncertainty in metal isotope mass balance models for past anoxic events remains challenging.

Purpose of the Study:

• To probabilistically evaluate coupled metal isotopes and sedimentary archives for improved constraints on Hirnantian-Rhuddanian anoxia.
• To quantify the uncertainty in metal isotope mass balance approaches for reconstructing past ocean conditions.
• To extend the known duration of the Hirnantian-Rhuddanian ocean anoxic event.

Main Methods:

• Analysis of iron speciation, metal concentrations, δ98Mo, and δ238U in Rhuddanian black shales from the Murzuq Basin, Libya.
• Evaluation of newly generated data alongside published carbonate δ238U data.
• Application of a coupled stochastic mass balance model to assess metal isotope and sedimentary sink data.

Main Results:

• Combined statistical analysis of metal isotopes and sedimentary sinks provided uncertainty-bounded constraints on Hirnantian-Rhuddanian euxinia.
• The study suggests anoxia persisted for over 3 million years, significantly longer than many previously studied Mesozoic ocean anoxic events.
• Probabilistic evaluation enhanced the quantification of uncertainty in metal isotope mass balance reconstructions.

Conclusions:

• The Hirnantian-Rhuddanian ocean anoxic event was prolonged, lasting over 3 million years.
• Coupled metal isotope and sedimentary data, analyzed with stochastic mass balance models, offer robust, uncertainty-bounded paleoceanographic reconstructions.
• This research provides a more comprehensive understanding of the duration and intensity of Late Ordovician marine anoxia and its potential link to the mass extinction.