Use of high-frequency (3.5-12 kHz) echograms in the study of near-bottom sedimentation processes in the deep-sea: A review

During the past 15 years in creasing numbers of studies have demonstrated that continuous echograms recorded on high-frequency (3.5–12 kHz) precision depth recorders aboard surface ships provide a valuable tool for the study of near-bottom sedimentation processes on the deep-sea floor. By constructing an echo character map of the various types of bottom echoes recorded in a region and combining this information with additional data (cores, bottom photographs, nephelometer, hydrographic and temperature), it is often possible to determine the types and regional influence of the various sedimentation processes (e.g., contour currents, turbidity currents, slumps and debris flows) which have shaped that region. The types and distributions of bottom echoes from most of the basins of the Atlantic as well as for several regions of the western Indian Ocean and the western Pacific have now been classified and mapped on a regional scale. Each of these regions returns several discrete types of echoes; most types are similar from basin to basin as well as ocean to ocean. In this paper a few examples of previous echo character studies are reviewed in order to illustrate how high-frequency echograms can be utilized to study near-bottom processes. (1) Regional echo-character maps often reveal the distribution and abundance of sediments deposited by down-slope processes such as turbidity currents. Three echo types which are widespread throughout most basins of the world show a qualitative correlation with the relative abundance of coarse, bedded turbidites (silt, sand, and gravel) in the upper few meters of the sea floor: (a) distinct echoes with continuous sub-bottoms are recorded from regions with little or no coarse sediment; (b) regions with moderate amounts of silt/sand return semi-prolonged echoes with intermittent or discontinuous sub-bottoms; and (c) regions with large amounts of silt/sand return very prolonged echoes with no sub-bottoms. (2) A wide variety of hyperbolic bottom echoes are recorded from regions of the sea floor where sedimentation is controlled by deep thermohaline flow (contour currents). These hyperbolic echoes are returned by erosional/depositonal bed forms such as non-migrating sediment waves and erosional furrows. Such regions of hyperbolic echoes are often clearly distributed beneath strong near-bottom thermohaline currents such as the Antarctic Bottom Water. Thus the distributions of these types of echoes often show the regional influence of contour currents. In addition to the hyperbolic echoes, a variety of undulating, quasi-hyperbolic echoes with shifting, non-conformable or truncated sub-bottoms are also often recorded. These bedforms appear to be migrating sediment waves and are generally observed in regions affected by contour currents. In some cases, however, it appears that turbidity currents also form such migrating waves. (3) Echograms have also been of great value in revealing the presence and distribution of mass-wasting processes such as slumps, slides and debris flows. Such deposits generally appear as convex, lense-shaped transparent layers which return a fuzzy, prolonged bottom echo. Often undisturbed, well-stratified sediments are visible below the deposits as are scarps and displaced blocks at the head of the slump complex. Echograms have revealed slump/debris-flow complexes ranging from 30,000 km2 in size. High-frequency echograms are thus an extremely valuable tool for studying the marine geology, sediment types, and near-bottom sedimentation processes active on the sea floor. Although widely spaced data lines in many regions can only define various sedimentary processes and bedforms on a broad regional scale, such data often indicate the best locations for concentrating more detailed and quantitative studies using closely spaced surveys and sophisticated instrument packages (e.g., Deep-tow, GLORIA, submersibles, etc.). Often only the detailed studies such as these can reveal the true shapes, origins, and hydrodynamic setting of various bedforms which return specific echo types on surface-ship echograms. In addition, most echocharacter studies to date have been qualitative. In the future more sophisticated, quantitative studies will be necessary in order to fully interpret and utilize the information contained in surface-ship echograms.