International audienc

The cause of the end-Cretaceous mass extinction is vigorously debated, owing to the occurrence of a very large bolide impact and flood basalt volcanism near the boundary. Disentangling their relative importance is complicated by uncertainty regarding kill mechanisms and the relative timing of volcanogenic outgassing, impact, and extinction. We used carbon cycle modeling and paleotemperature records to constrain the timing of volcanogenic outgassing. We found support for major outgassing beginning and ending distinctly before the impact, with only the impact coinciding with mass extinction and biologically amplified carbon cycle change. Our models show that these extinction-related carbon cycle changes would have allowed the ocean to absorb massive amounts of carbon dioxide, thus limiting the global warming otherwise expected from postextinction volcanism.

The Sudbury Igneous Complex (SIC) is interpreted as the solidified impact melt body of the 1.850-g.y.-old Sudbury impact structure. First results of cratering and thermal modeling for this ~250-km sized multi-ring structure are presented. The numerical calculations were done for the vertical impact of a stony (granite) body (cylindrical projectile, 12.5 km in diameter and height) impacting at a granite target with a velocity of 20 km s -1 . These simulations yield estimates of the transient cavity dimensions and the temperature field below the impact structure just after the modification stage. One-dimensional heat transfer modeling sets constraints for the thermal history of the impact melt. Cooling of the melt sheet, the present SIC, from the initial temperature of 2,000°K to the liquidus at 1,450°K lasted several 100 k.y, and below the solidus at 1,270°K, about 300 k.y. to 2 m.y., depending on the initial melt thickness H(SIC). The cooling sequence was modeled for H(SIC) of 2.5, 4, and 6 km. Given this long duration of cooling, postimpact tectonic processes during the Penokean orogeny may well have deformed the melt sheet prior to its final solidification. Prolonged cooling as well as this large-scale deformation may explain the present structural position and the composition of the Offset Dikes, consisting of differentiated impact melt.Ivanov, B. A.

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Abstract-Planetary surfaces are subjected to meteorite bombardment and crater formation. Rocks forming these surfaces are often porous and contain fluids. To understand the role of both parameters on impact cratering, we conducted laboratory experiments with dry and wet sandstone blocks impacted by centimeter-sized steel spheres. We utilized a 40 m two-stage light-gas gun to achieve impact velocities of up to 5.4 km s )1 . Cratering efficiency, ejection velocities, and spall volume are enhanced if the pore space of the sandstone is filled with water. In addition, the crater morphologies differ substantially from wet to dry targets, i.e., craters in wet targets are larger, but shallower. We report on the effects of pore water on the excavation flow field and the degree of target damage. We suggest that vaporization of water upon pressure release significantly contributes to the impact process.