Raining lead around 250mya : a smoking gun for an Australian impact origin of the Permian Extinction

Abstract

Recent documentation of extreme atmospheric sulfur and methane contents at the time of the vast Permo-Triassic (P-T) extinction makes it possible to interpret an observation that has lain unnoticed in the geological literature for 40 years. This is the finding of microscopic metallic lead tear drops in the fluvial strata of the early Triassic sandstones that overlie Permian coal beds and other sedimentary deposits in the Sydney basin of Australia. Elemental lead is almost unknown in nature, so its occurrence in these graphite-loaded sandstones is a provocative finding. While climate change and vulcanism could explain the carbon and sulfur anomalies, the only way to account for metallic lead aerodynamic droplets is by massive impact and vaporization of lead mineral-containing formations. Since lead occurs geologically as the sulfide and since lead is an easily reduced element, its occurrence in conjunction with sulfur and carbon count anomalies suggests a bolide impact on carbon-loaded strata in a sulfide mineral-rich region. From these clues, and from stream cross-bedding data, we identify a probable site for the impact, in Bass Strait between Tasmania and Victoria. A gravitational anomaly, similar to that known at the site of the Cretacious-Tertiary (C-T) impact site, conforms with a central uplift. The sudden compression at an Australian impact site would provoke tension fissuring on the opposite side of a shell- structured planet, hence is consistent with the unprecedented flood basalts initiated in Siberia at the P-T boundary. Our interpretation requires that lead microspheres and graphite occur together elsewhere, possibly in Antarctic sandstones.