The Siberian Traps and the End-Permian mass extinction: A critical review

The association between the Siberian Traps, the largest continental flood basalt province, and the largest-known mass extinction event at the end of the Permian period, has been strengthened by recently- published high-precision [superscript 40]Ar/[superscript 39]Ar dates from widespread localities across the Siberian province[1]. We argue that the impact of the volcanism was amplified by the prevailing late Permian environmental conditions—in particular, the hothouse climate, with sluggish oceanic circulation, that was leading to widespread oceanic anoxia. Volcanism released large masses of sulphate aerosols and carbon dioxide, the former triggering short-duration volcanic winters, the latter leading to long-term warming. Whilst the mass of CO[subscript 2] released from individual eruptions was small compared with the total mass of carbon in the atmosphere-ocean system, the long ‘mean lifetime’ of atmospheric CO[subscript 2], compared with the eruption flux and duration, meant that significant accumulation could occur over periods of 10[superscript 5] years. Compromise of the carbon sequestration systems (by curtailment of photosynthesis, destruction of biomass, and warming and acidification of the oceans) probably led to rapid atmospheric CO[subscript 2] build-up, warming, and shallow-water anoxia, leading ultimately to mass extinction

Discovery of two new super-eruptions from the Yellowstone hotspot: Is Yellowstone hotspot waning?

Super-eruptions are amongst the most extreme events to affect the Earth’s surface, but too few examples are known to assess their global role in crustal processes and environmental impact. We demonstrate a robust approach to recognise them at one of the best-preserved intraplate large igneous provinces, leading to the discovery of two new super-eruptions. Each generated huge and unusually hot pyroclastic density currents that sterilised extensive tracts of Idaho and Nevada, USA. The ~8.99 Ma McMullen Creek eruption was magnitude 8.6, larger than the last two major eruptions at Yellowstone. It exceeds 1,700 km3, covering ≥12,000 km2. The ~8.72 Ma Grey’s Landing eruption was even larger, at magnitude of 8.8 and volume of ³2,800 km3. It covers ≥23,000 km2 and is the largest and hottest documented eruption from the Yellowstone hotspot. The discoveries show the effectiveness of distinguishing and tracing vast deposit sheets by combining trace-element chemistry and mineral compositions with field and paleomagnetic characterisation. This approach should lead to more discoveries and size estimates, here and at other provinces. It has increased the number of known super-eruptions from Yellowstone hotspot, shows that the temporal framework of the magmatic province needs revision, and suggests the hotspot may be waning.</p

Magnetic anisotropy in rhyolitic ignimbrite, Snake River Plain: implications for using remanant magnetism of volcanic rocks for correlation, paleomagnetic studies and geological reconstructions

Individual ignimbrite cooling units in southern Idaho display significant variation of magnetic remanence directions and other magnetic properties. This complicates paleomagnetic correlation. The ignimbrites are intensely welded and exhibit mylonite-like flow banding produced by rheomorphic ductile shear during emplacement, prior to cooling below magnetic blocking temperatures. Glassy vitrophyric lithologies commonly have discrepantly shallow remanence directions rotated closer to the orientation of the subhorizontal shear fabric when compared to the microcrystalline center of the same cooling unit. To investigate this problem, we conducted a detailed paleomagnetic and rock magnetic study of a vertical profile through a single ignimbrite cooling unit and its underlying baked soil. The results demonstrate that large anisotropy of thermal remanent magnetization correlates with large (up to 38°) deflections of the stable remanence direction. Anisotropy of magnetic susceptibility revealed no strong anisotropy. A strong lineation and deflection of the remanence declination suggest that rheomorphic shear above magnetic blocking temperatures is the dominant mechanism controlling the formation of the magnetic fabric, with compaction contributing to a lesser extent. Nucleation and growth of anisotropic fine-grained magnetite in volcanic glass at high temperatures after, and perhaps also during, emplacement is indicated by systematic variation of magnetic properties from the quickly chilled ignimbrite base to the interior. These properties include remanence directions, anisotropy, coercivity, susceptibility, strength of natural remanent magnetization, and dominant unblocking temperature. The microcrystalline ignimbrite center has a magnetic direction that is the same as the underlying baked soil and, therefore, is a more reliable recorder of the paleofield direction than the glassy margins of highly welded ignimbrites