Abstract. The Paleocene/Eocene thermal maximum (PETM) was a time of rapid global warming in both marine and continental realms that has been attributed to a massive methane (CH4) release from marine gas hydrate reservoirs. Previously proposed mechanisms for this methane release rely on a change in deepwater source region(s) to increase water temperatures rapidly enough to trigger the massive thermal dissociation of gas hydrate reservoirs beneath the seafloor. To establish constraints on thermal dissociation, we model heat flow through the sediment column and show the effect of the temperature change on the gas hydrate stability zone through time. In addition, we provide seismic evidence tied to borehole data for methane release along portions of the U.S. continental slope; the release sites are proximal to a buried Mesozoic reef front. Our model results, release site locations, published isotopic records, and ocean circulation models neither confirm nor refute thermal dissociation as the trigger for the PETM methane release. In the absence of definitive evidence to confirm thermal dissociation, we investigate an altemative hypothesis in which continental slope failure resulted in a catastrophic methane release. Seismic and isotopic evidence indicates that Antarctic source deepwater circulation and seafloor erosion caused slope retreat along the westem margins of the North Atlantic in the late Paleocene. Continued erosion or seismic activity along the oversteepened continental margin may have allowed methane to escape from gas reservoirs trapped between the frozen hydrate-bearing sediments and the underlying buried Mesozoic reef front, precipitating the Paleocene/Eocene boundary methane release. An important implication of this scenario is that the methane release caused (rather than resulted from) the transient temperature increase of the PETM. Neither thermal dissociation nor mechanical disruption of sediments can be identified unequivocally as the triggering mechanism for methane release with existing data. Further documentation with highresolution benthic foraminiferal isotopic records and with seismic profiles tied to borehole data is needed to clarify whether erosion, thermal dissociation, or a combination of these two was the triggering mechanism for the PETM methane release.

No abstract

Elastic constants of selected rocks and minerals are listed, as obtained from measured values of density and of comnressional and rotational wave velocities along three orthogonal directions. Density-velocity correlation and velocity-anisotropy are discussed.

Experimental studies of the propagation of elastic waves on floating ice sheets were made on the ice of Lake Superior and Lake Cayuga. Elastic waves were produced by small explosive charges detonated at various depths in the water, within the ice, and in the air. Seismic detectors consisting of a spread of geophones, a microphone, and a hydrophone recorded the resultant wave motion at varying distances. Shots in the water produced the normal sequence of dispersive flexural waves. For shots in the air the dispersive flexural waves were absent and a train of constant frequency waves was observed, beginning gradually at the approximate time t = r/2va and culminating with the arrival of the air wave at the time t = r/2va (r is the range and va is the speed of sound in air). These waves were interpreted as air‐coupled flexural waves, The frequency of the air‐induced waves is that of flexural waves whose phase velocity equals the speed of sound in air. The generated wave train precedes the air disturbance since the group velocity of flexural waves exceeds the phase velocity, all in accordance with classical theory. The frequency of the air‐coupled vibration is simply related to the ice thickness. The interpretation was supported by subsequent tests which consisted of shooting on shore and recording on the ice, shooting and recording on ice sheets of varying thickness, and recording air‐coupled flexural vibrations with microphones.

No abstract