Turning point in Pangean environmental history at the Permian/Trias sic (P/Tr) boundary

Profound short-lived events about the Permian/Triassic (P/Tr) boundary, determined here as 250 Ma, include (1) magmatic events: rapid eruption of the vast flood basalt of the Siberian Traps and the peak of convergent magmatism in the Gondwanaland province along the Panthalassan margin, represented in Australia by the Dundee/Emmaville Volcanics and in South America by the Choiyoi province; (2)paleoclimatic change: following the last ice-rafting dropstone in eastern Australia, an abrupt change from coal measures to redbeds in the Gondwanaland province; (3) biota extinctions: indicated by a minimum standing diversity of marine invertebrates and by the replacement of the Glossopteris flora by the Dicroidium flora; (4) a sea-level minimum; (5) seawater isotope changes: a minimum ⁸⁷Sr/⁸⁶Sr, a precipitous drop in δ¹³C, and a broad minimum in δ³⁴S; (6) magnetic polarity shift: change from the Permo-Carboniferous Reversed Polarity Superchron to the Permo-Triassic Mixed Superchron. All but the demonstrably older magnetic polarity change are possibly synchronous, because any apparent differences in age fall within the uncertainties of measuring radiometric ages and of calibrating the biostratigraphic and radiometric timescales. Eastern Australia, which represents the Panthalassan margin of the Gondwanaland province, has a record in this stratigraphic order of the following abrupt changes at the P/Tr boundary: (3) sediment containing redbeds with Dicroidium replaces (2) the last Glossopteris in coal measures with thick tuff from eruption of the Emmaville Volcanics, and (1) the last ice-rafted dropstones. This chain of events is consistent with the following global model: (1) An abrupt change -255 Ma in the heat regime at the core/mantle boundary generated a change of magnetic chron and released a mantle plume that erupted at the surface -250 Ma as the Siberian Traps, at the same time as the peak of convergent volcanism along the Gondwanaland margin of Panthalassa. (2) The volcanics vented CO₂ that effected rapid warming by the greenhouse effect; (3) the resulting biotal extinctions and a decreased biomass caused lower organic productivity. (4) Oxidation of surface sediment, including progenitors of coal, led to a drop in seawater δ¹³C and, in much of Gondwanaland and China, a gap in the preservation of coal. (Seawater δ¹³C rose and the preservation of coal was resumed 20 m.y. later at 230 Ma in the Late Triassic.) (5) Warm surface water at high latitudes slowed bottom ocean-water circulation so that the ocean floor became anoxic.