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The Permian-Triassic boundary records the most severe mass extinctions in Earth's history. Siberian flood volcanism, the most profuse known such subaerial event, produced 2 million to 3 million cubic kilometers of volcanic ejecta in approximately 1 million years or less. Analysis of (40)Ar/(39)Ar data from two tuffs in southern China yielded a date of 250.0 +/- 0.2 million years ago for the Permian-Triassic boundary, which is comparable to the inception of main stage Siberian flood volcanism at 250.0 +/- 0.3 million years ago. Volcanogenic sulfate aerosols and the dynamic effects of the Siberian plume likely contributed to environmental extrema that led to the mass extinctions.

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The Siberian Traps represent one of the most voluminous flood basalt provinces on Earth. Laser-heating (40)Ar/(39)Ar data indicate that the bulk of these basalts was erupted over an extremely short time interval (900,000 +/- 800,000 years) beginning at about 248 million years ago at mean eruption rates of greater than 1.3 cubic kilometers per year. Such rates are consistent with a mantle plume origin. Magmatism was not associated with significant lithospheric rifting; thus, mantle decompression resulting from rifting was probably not the primary cause of widespread melting. Inception of Siberian Traps volcanism coincided (within uncertainty) with a profound faunal mass extinction at the Permo-Triassic boundary 249 +/- 4 million years ago; these data thus leave open the question of a genetic relation between the two events.

[1] To investigate arsenic mobility in the Bengal Basin groundwater, we sampled water wells and sediments throughout the region. There are strong correlations among high levels of dissolved arsenic and iron, ammonia, and methane, especially in samples from a single site (Laxmipur). No linkage is seen between As and agricultural tracers such as phosphate. The association of As and Fe occurs because arsenic strongly adsorbs onto FeOOH particles in river water. They flocculate with other fine-grained particles at the freshwater/saltwater transition zone. Subsequent bacterially mediated reduction of FeOOH in the clay releases the adsorbed arsenic. Weathering of As-bearing mica plays a significant role in the As budget. The ''correlated'' presence of As, CH 4 , and NH 4 in water supply wells is the result of diffusion out of organic-rich clay into the more permeable zones. Arsenic is mainly released from recent sediments at <50 m depth deposited in the GBR floodplain as sea level rose throughout the Holocene.