The controversial Younger Dryas impact hypothesis suggests that at the onset of the Younger Dryas an extraterrestrial impact over North America caused a global catastrophe. The main evidence for this impact-after the other markers proved to be neither reproducible nor consistent with an impact-is the alleged occurrence of several nanodiamond polymorphs, including the proposed presence of lonsdaleite, a shock polymorph of diamond. We examined the Usselo soil horizon at Geldrop-Aalsterhut (The Netherlands), which formed during the Allerød/Early Younger Dryas and would have captured such impact material. Our accelerator mass spectrometry radiocarbon dates of 14 individual charcoal particles are internally consistent and show that wildfires occurred well after the proposed impact. In addition we present evidence for the occurrence of cubic diamond in glass-like carbon. No lonsdaleite was found. The relation of the cubic nanodiamonds to glass-like carbon, which is produced during wildfires, suggests that these nanodiamonds might have formed after, rather than at the onset of, the Younger Dryas. Our analysis thus provides no support for the Younger Dryas impact hypothesis.radiocarbon dating | carbon spherules | wildfire temperature | electron microscopy

Major earthquakes frequently nucleate near the base of the seismogenic zone, close to the brittle-ductile transition. Fault zone rupture at greater depths is inhibited by ductile flow of rock. However, the microphysical mechanisms responsible for the transition from ductile flow to seismogenic brittle/frictional behaviour at shallower depths remain unclear. Here we show that the flow-to-friction transition in experimentally simulated calcite faults is characterized by a transition from dislocation and diffusion creep to dilatant deformation, involving incompletely accommodated grain boundary sliding. With increasing shear rate or decreasing temperature, dislocation and diffusion creep become too slow to accommodate the imposed shear strain rate, leading to intergranular cavitation, weakening, strain localization, and a switch from stable flow to runaway fault rupture. The observed shear instability, triggered by the onset of microscale cavitation, provides a key mechanism for bringing about the brittle-ductile transition and for nucleating earthquakes at the base of the seismogenic zone.

[1] We report isostatic compaction experiments performed on granular quartz under hydrothermal conditions (3-129 mm of initial grain size, 300-600°C, 200 MPa of fluid pressure, and 25-100 MPa of effective pressure). From microstructural evidence, it was determined that, whereas microcracking controlled precompaction at room temperature, pressure solution was the main mechanism during hydrothermal compaction, although a role of microcracking could not be excluded entirely. Our mechanical data, together with theoretical pressure solution rate models, further indicated that pressure solution was controlled by interface kinetics, dissolution being the most likely rate-controlling mechanism. An empirical relation of the form _ e = 10 À7.8 (f/f 0 ) 10.0 s 3.4 d À1 exp (À105,000/RT) was fitted to our data to describe experimental compaction rates. Electron backscatter diffraction (EBSD) analysis performed on one sample showed limited evidence for plastic deformation (Dauphiné twinning and lattice bending) at grain contact points under high stress. Contact microstructures formed during compaction were studied using quartz single crystal discs or a mica plate as reference surfaces. This showed that, in all contacts, microstructures were rough, with a micrometer scale roughness. Many contacts also showed internal microcracking, and it is inferred that microcracking is likely an important mechanism for creating and maintaining rough contacts during quartz pressure solution, at least under the present experimental conditions. Extrapolation of our empirical equation for compaction rates to natural conditions is consistent with previous observations that, during burial and diagenesis of a sandstone, pressure solution starts to operate at depths of about $1.5-2 km, becoming the dominant compaction mechanism at depths greater than $2.5-3 km. The extrapolation gives good agreement with porosity-depth trends reported for natural, arenitic sandstones.

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