Rapid and pronounced oceanic deoxygenation fluctuations during the Late Jurassic recorded by thallium isotopes

Abstract The long-term global oxygenation history of ancient oceans provides context for the rapidly decreasing oxygen concentrations in the present-day ocean. Here, we present thallium (Tl) isotope evidence from an epicontinental Laurasian Seaway, deposited during a greenhouse climate between 150 and 149 Ma, that suggests substantial changes in the global burial of manganese (Mn) oxide. This provides evidence for fluctuating global marine (de)oxygenation at a time when there is no evidence for major shifts in the global carbon cycle. Between periods when Tl isotope data suggest similar rates of burial of Mn oxides to the modern, reductions of up to 70% Mn oxide burial are inferred, suggesting major expansions of hypoxic bottom waters. Periods of more extensive global hypoxia are not coincident with regional enrichment of sedimentary organic matter. Fluctuations in deoxygenation occurred on geologically rapid timescales of 10,000 to 100,000 years but are still an order of magnitude lower than those occurring in the modern ocean.