Large shifts in the isotopic composition of seawater sulphate across the Permo–Triassic boundary in northern Italy

Abstract

Carbonate-associated sulphate (CAS) extracted from a Permo–Triassic succession at Siusi in northern Italy is shown to preserve a true seawater-sulphate isotope record. Two periods of increasing δ 34S and δ 18O in CAS provide evidence for increased oceanic anoxia in the Late Permian and the Early Triassic. These two anoxic episodes are separated by an event characterised by the addition of isotopically light sulphur and oxygen to the oceanic sulphate pool. Simple mass balance calculations suggest that this sulphate originates from the reoxidation of bacterially derived H 2S during oceanic mixing, rather than a volcanogenic source. A dramatic fall in CAS-δ 18O directly above the P– T boundary, not accompanied by a large change in CAS-δ 34S, records an oceanic deoxygenation event probably caused by the release of methane from gas hydrates, subsequently recorded in the carbonate-carbon isotope record. The decline of Early Triassic oceanic anoxia is not recorded by a fall in CAS-δ 34S, but is preserved by declining CAS-δ 18O. This is because of an increase in the flux of reactive iron to the oceans during the Early Triassic anoxic episode, triggered by the demise of land plants. This permanently removes a greater proportion of light sulphur from the oceanic sulphate reservoir as pyrite, and means that the heavy residual sulphate-sulphur isotope signature of Griesbachian anoxic seawater is preserved as a geochemical ‘fossil’ until the beginning of the Middle Triassic.