Seismic investigations of the Greenland ice sheet at EGIG points

The compressional and the shear wave velocities in the Greenland ice sheet are derived from seismic records of the EGIG 1959. Further the variation of velocities in the firn and the dependance of Poisson's ratio from depth are determined. At Station Centrale, two P-waves are recorded from underground layers. Their velocities show that the ice basement consists of crystalline rocks. The P-wave velocities derived from reflections agree well with those obtained by refraction shooting. From this agreement results that the ice is ± homogenous and ± isotropic for Pwaves. The elastic constants for isotropic ice are calculated. Finally the temperature dependence of the velocities is discussed.

Sedimentation and sediment redistribution on the Cocos Ridge from two seismic acquisition cruises, NEMO-3 and MV1014

We use digital seismic reflection profiles within a 1° * 1° survey area on the Cocos Ridge (COCOS6N) to study the extent and timing of sedimentation and sediment redistribution on the Cocos Ridge. The survey was performed to understand how sediment focusing might affect paleoceanographic flux measurements in a region known for significant downslope transport. COCOS6N contains ODP Site 1241 to ground truth the seismic stratigraphy, and there is a seamount ridge along the base of the ridge that forms a basin (North Flank Basin) to trap sediments transported downslope. Using the Site 1241 seismic stratigraphy and densities extrapolated from wireline logging, we document mass accumulation rates (MARs) since 11.2 Ma. The average sediment thickness at COCOS6N is 196 m, ranging from outcropping basalt at the ridge crest to ~ 400 m at North Flank Basin depocenters. Despite significant sediment transport, the average sedimentation over the entire area is well correlated to sediment fluxes at Site 1241. A low mass accumulation rate (MAR) interval is associated with the 'Miocene carbonate crash' interval even though COCOS6N was at the equator at that time and relatively shallow. Highest MAR occurs within the late Miocene-early Pliocene biogenic bloom interval. Lowest average MAR is in the Pleistocene, as plate tectonic motions caused COCOS6N to leave the equatorial productivity zone. The Pliocene and Pleistocene also exhibit higher loss of sediment from the ridge crest and transport to North Flank Basin. Higher tidal energy on the ridge caused by tectonic movement toward the margin increased sediment focusing in the younger section.

Sediment wave layer summary in ODP Site 172-1062

Abyssal mud waves (or fine-grained sediment waves) are often cited as evidence for deep current activity because subbottom profiles show that the wave form has migrated with time. The migration history of a fine-grained sediment wave on the Blake-Bahama Outer Ridge (ODP Site 1062) has been studied through the analysis of multiple ODP holes spaced across the wave. Additional information about wave migration patterns comes from 3.5-kHz records and watergun seismic profiles. These data suggest that wave migration has varied during the last not, vert, similar ~10 Myr, although the only sediments sampled are younger than 4.8 Ma. Seismic profiles suggest wave migration was initiated about 8-10 Ma, and wave migration was pronounced from about 5 Ma to about 1 Ma (with an episode of wave reorganization about 4.5 Ma). Analysis of ODP cores suggests that migration rates have been somewhat lower and more variable during the last 1 Myr. Intervals of no wave migration are observed for several time intervals and appear to characterize deglaciations, especially during the last 500 kyr. Comparisons between seismic profiles and the core record show that most of the seismic horizons correlate closely with time horizons, and thus that the seismic profiles give a reasonable representation of sediment wave migration. Models suggest that wave migration is more pronounced during periods of higher bottom current flow and less pronounced during periods of lower current flow. Thus the migration record is consistent with generally higher bottom flow speeds at this site prior to 1 Ma and lower bottom flow speeds after 1 Ma. The Mid-Pleistocene Transition from a dominant climatic periodicity of 40 kyr to a dominant climatic periodicity of 100 kyr starts at about this time, suggesting an overall reduction in bottom flow speed at this site coincident with changing climate patterns. These changes in flow speed could be related to changes in the depth of the Western Boundary Undercurrent as well as to changes in the speed of thermohaline circulation.