Discussion
We have presented geochemical, geochronological, biological and petrological evidence that genetically links the Bedout structure to end-Permian impact deposits worldwide (Fig. 1). The recognition of an impact breccia in the Bedout High emphasizes the difficulties in interpreting old impact structures that are subtle in their expression and do not retain the pristine characteristics of younger, well-preserved craters such as Chicxulub (12). Available drill core only has sampled the upper portion (~22 m of intact core in Bedout-1 and 391 m of cuttings Lagrange-1) of the impact melt breccia and contains mostly highly shocked materials. The shock pressures recorded in the Bedout core were sufficient to produce maskelynite (22) (35-45 GPa) and silica glass (>45-65 GPa), are too high to preserve PDFs in quartz (<35 GPa), but are sufficiently high to form stishovite (15-40 GPa) and perhaps hexagonal diamond (70-140 GPa) (45). Thus, other samples from the Bedout High may produce additional evidence of shock (stishovite, coesite and diamond), assuming suitable target rocks were present. Similarly, future analyses may isolate pristine mineral grains for radiometric dating, and thus better constrain the end-Permian age and its hypothesized association with the P/Tr boundary. Additional geophysical data, and perhaps coring, are needed to better determine the size of the structure.

The evidence for yet another impact event coincident (within the age uncertainty) with severe flood basalt volcanism raises the question of the relationship of such catastrophes to each other and mass extinction events (46). There has been increasing speculation that large bolide impacts have been responsible for processes such as continental flood basalt eruptions and mantle plumes (47, 48). Present models suggest that impacts may induce a volcanic outburst if the bolide strikes a pre-existing hot spot. However, the probability of such an event occurring is extremely remote (49, 50). In the case of Chicxulub and now Bedout, the crater locations are opposite (rather than exactly antipodal; Fig. 1) to the position of the volcanic province (i.e. Deccan and Siberia respectively). Indeed, Melosh (49) has calculated that the amount of kinetic energy needed to create the volume of the Deccan traps (~500,000 km3) would require some 5 x 1023 J or 2x the amount of kinetic energy generated by the Chicxulub impactor (10 km at 20 km/sec).

While it seems clear that an impact may not be the direct cause of the volume of flood basalts, it may still act as a “trigger” of the event. At both Siberia and Deccan, Ar-Ar dating has shown that volcanic rocks with mantle plume affinities pre-date the main pulse of the Deccan and Siberian traps (51-52). Thus, the impact(s) and subsequent energy release might enhance the catastrophic eruption of a pre-existing mantle plume. New models may need to be considered to properly assess, identify, and confirm extraterrestrial impact events and to further understand the impact process and its relationship to severe volcanism and mass extinctions in the geologic record.