Lithogeochemical and sulfide trace-element systematics across the Permian–Triassic boundary, Perth Basin, Western Australia: constraints on the shallow marine environment during the end-Permian mass extinction

Abstract

Abstract Sedimentary pyrite trace-element composition is an established proxy for determining paleo-ocean geochemistry and atmospheric oxygen concentrations through deep time. However, its applicability over shorter time-scales (i.e. <20 Ma) is not well known. To test this, we targeted fine-grained pyrite in the Hovea Member of the Kockatea Shale (Perth Basin, Western Australia), which encompasses the late Permian inertinitic interval and the end-Permian to Early Triassic sapropel, and spans approximately 10 million years. The end-Permian mass extinction (EPME) was the largest extinction event in Earth history, and its greatest effect is documented in the marine environment. Samples were collected from two oil exploration wells—Redback-2 and Hovea-3—spaced ∼20 km apart. In the two boreholes, a change in depositional facies (i.e. between the inertinite and sapropel) occurs below the Permian–Triassic boundary and records the transition from a marginal marine to a shelf environment. This transition is highlighted by several lithogeochemical indicators (e.g. negative shift δ13C values and Corg reduction; increases in Ca, Fe and P), which are themselves tied to fundamental changes in modal mineralogy between the two zones. Importantly, the sapropel also records a major increase in iron sulfide burial over that in the inertinite. LA-ICPMS analyses of pyrite demonstrate that trace-element abundance is highest in samples below the facies transition, and in places reaches a few percent, particularly of Ni (4 wt%), Co (1.5 wt%) and As (2.8 wt%). Moreover, these and other trace elements decrease by an order of magnitude in concert with the negative shift in δ13C values in the sapropel zone. Various whole-rock based paleosalinity indicator ratios (e.g. B/Ga) indicate that the areas of the Perth Basin intersected by Redback-2 and Hovea-3 were not fully connected to the open ocean at the time of the EPME, which leads us to conclude that the very high trace-element values in the sedimentary sulfides are reflective of regional environmental shifts rather than a global signal. Nonetheless, a geochemical contribution from a distant igneous province, such as the Siberian Traps Large Igneous Province, cannot be ruled out. Our work underscores the strength of sedimentary pyrite as a robust paleoenvironmental proxy in the marine environment and highlights the need for further investigation of pyrite trace-element profiles across the mass extinction interval in other sedimentary sequences around the globe. KEY POINTS LA-ICPMS-based geochemistry of sedimentary pyrite from the Hovea Member of the Kockatea Shale is considered within a lithochemostratigraphic context. The overall interpretation of the results involves a change in depositional setting from the marginal in the late Permian brackish waters to shelfal marine and loss of oxygen in the Early Triassic Perth Basin.