BATHYMETRY (PART-I), SEDIMENTARY REGIMES (PART-II), AND ABYSSAL WASTE-DISPOSAL POTENTIAL NEAR THE CONTERMINOUS UNITED-STATES

Placing waste on the seafloor, with the intention that it remain in pl ace and isolated from mankind, requires a knowledge of the environment al factors that may be applicable to a specific seafloor area. DBDB5 ( Digital Bathymetric Database gridded at 5' latitude by 5' longitude ce ll dimension) is used here for regional assessments of seafloor depth, slope, and relief at five surrogate abyssal waste sites; two each in the western Atlantic and eastern Pacific, and one in the Gulf of Mexic o. Only Pacific-1 exhibits a 'high' slope (2 degrees) by DBDB5 standar ds, whereas the remaining sites are located on almost level seafloor. Detailed examination of the sites using multibeam-based contour sheets show the area around Atlantic-1 to be a featureless plain. Atlantic-2 and both Pacific sites are surrounded by abyssal hill topography, wit h local slopes ranging from greater than 6 degrees at all sites to abo ve 15 degrees at Pacific-2. Neither Pacific site features a seafloor a s 'flat' as at Atlantic-1 or at the Gulf of Mexico site. Locating wast e sites on sedimented slopes could have serious consequences due to ca tastrophic slope failure and downslope displacement of waste by mass s ediment-transport processes. Neither slumping nor sliding are perceive d as critical processes affecting the surrogate sites because of their locations on negligibly sloping seafloors. However, debris flows and turbidity currents are capable of transporting large volumes of sedime nt for long distances over low gradients and, in the case of turbidity currents, at great speed. Dispersal of loose waste material by these processes is virtually assured, but less likely if the waste is bagged . The turbidity current problem is alleviated (but not eliminated) by locating waste sites on distal portions of abyssal plains. Both Pacifi c sites are surrounded by abyssal hills and, in the case of Pacific-2, far beyond the reach of land-derived turbidity currents, Thin sedimen t cover and low rates of sedimentation have also resulted in highly st able slope (abyssal hill) deposits. Hence, the probability of locally derived, small-volume flows is low at these sites. Existing high sea l evels have also resulted in a worldwide decrease in turbidity current activity relative to glacial times when sea levels were much lower. (C ) 1998 Elsevier Science B.V.