John E. Damuth

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.

Echo character of the western equatorial Atlantic floor and its relationship to the dispersal and distribution of terrigenous sediments

Abstract The floor of the western equatorial Atlantic Ocean can be divided into several distinct provinces based on detailed characteristics of the bottom echos recorded with short-ping ( Indistinct echos can be further sub-divided into (A) continuous prolonged echos; and (B) hyperbolic echos. Each class of echos contains two or more unique echo types. The regional distributions of the various echo types recorded from the continental rise, Amazon Cone, and abyssal plains reveal much information about sedimentary processes. In the western equatorial Atlantic, hyperbolic echos are recorded only from small, isolated portions of the continental rise. This contrasts with the continental rise of the western North Atlantic where previous investigators have shown that hyperbolic echos parallel bathymetric contours along the entire rise and thus reflect shaping of the rise by geostrophic contour currents (Heezen et al., 1966; Hollister, 1967). The fact that regions of hyperbolic echos show little or no relationship to bathymetric contours of the continental rise of the western equatorial Atlantic suggests that contour currents have been unimportant in shaping the rise in this region. The three most widespread echo types recorded from the continental rise, Amazon Cone, and abyssal plains reveal much information about terrigenous sediment dispersal and deposition in the western equatorial Atlantic. Comparison of the thicknesses and frequencies of coarse (silt- to gravel-size), bedded, terrigenous sediment in piston cores with the echo type recorded at each coring site shows a correlation between echo type and the relative amount of coarse, bedded sediment within the upper few meters of the sea floor. The regional distributions of these three echo types indicate that dispersal of coarse terrigenous sediment has been downslope across the continental rise and Amazon Cone to the abyssal plains via gravity-controlled sediment flows. The Amazon River is the major sediment source and most coarse sediment is deposited on the lower Amazon Cone and proximal portions of the Demerara abyssal plain.

Amazon Cone: Morphology, Sediments, Age, and Growth Pattern

The morphology, sediment distribution, and growth pattern of the Amazon cone are similar to those of other deep-sea fans; its sediment, at least during the late Quaternary Period, was deposited in response to glacial-interglacial cycles, and its age of formation is estimated to be middle to late Miocene. Sedimentation on the Amazon cone, at least during Quaternary time, has been climatically controlled. During high sea-level stands, terrigenous sediment is trapped on the inner continental shelf, and only pelagic sediment is deposited on the cone. During low sea-level stands, the Amazon River discharges terrigenous sediment into the Amazon Submarine Canyon, from where it is easily transported to the cone by gravity-controlled sediment flows. Wisconsin sedimentation rates on the cone were in excess of 30 cm/10 3 yr. Average sedimentation rates for the Pleistocene Epoch, based on the extrapolated age (2.2 m.y.) of a prominent acoustic reflector within the cone, range from 50 to 115 cm/10 3 yr. The Amazon cone began to form about 8 to 15 m.y. B.P. and is thus about one-tenth the age of the Equatorial Atlantic.

Equatorial Atlantic Deep-Sea Arkosic Sands and Ice-Age Aridity in Tropical South America

Arkosic sands of latest Wisconsin age from deep-sea piston cores taken in the Guiana Basin off northeast South America between lat 20° N. and lat 10° S. support previous conclusions that an arid to semiarid climate dominated large portions of equatorial South America during the Pleistocene glacial phases in complete contrast to the present-day and Pleistocene interglacial humid tropical climate. Thirty-nine cores showed stratigraphic relations that characterize the transition from Wisconsin to Holocene. Sand beds from 23 of the cores were of latest Wisconsin age and contained 25-60 percent feldspar. In contrast, a sample of Holocene sand taken from the continental shelf northeast of the mouth of the Amazon River contained only 17—20 percent feldspar. The Quaternary history of Brazil appears to have been climatically controlled by a repeated displacement of the South Atlantic high-pressure cell by some 1500 km—northward during the glacial phases and southward during the interglacial phases. It was further influenced during glacial phases by a lowering of the snow line in the Andes by about 1000 m. In consequence, the glacial phase climates of the lower Amazon Basin were marked by cold, dry, southerly winds while the South Atlantic trade winds were deflected off the northeast coast. A semiarid to and climate ensued, coupled with a eustatic drop of sea level that caused degradation of the principal river valleys near the coast. In this way unweathered feldspars, chlorite, and other relatively coarse elastics were transported into the tropical Atlantic in contrast to the usual lateritic clays (gibbsite and kaolimte) of interglacial stages.

Echo character of the Norwegian—Greenland Sea: Relationship to Quaternary sedimentation☆

Abstract Ten discrete types of bottom echoes are recorded and mapped in the Norwegian—Greenland Sea using 3.5-kHz echograms; eight of these are similar to echo types mapped previously in the equatorial Atlantic. The two other types (IA-2 and IC) have not been previously observed and apparently are recorded from bedforms created by deposition and/or erosion by glaciers or ice shelves. A qualitative correlation is observed between the relative abundance of coarse (silt, sand, gravel), bedded sediment in piston cores and three types of echoes. Distinct echoes with continuous subbottoms (IB) are recorded from regions containing little or no coarse sediment; regions of semiprolonged echoes with intermittent zones of subbottoms (IIA) contain low to moderate amounts of coarse sediment; and very prolonged echoes with no subbottoms are returned from regions with high concentrations of coarse sediment. The areal distribution of these three echo types indicate that only low to moderate amounts of coarse, bedded terrigenous sediment occur within the upper few meters of most of the Norwegian—Greenland Sea floor. The types and extremely limited areal distributions of hyperbolic echoes suggest that large-scale contour-current activity has not been an important sedimentary process on a regional basis. This conclusion is further supported by bottom photographs and piston cores which generally show tranquil bottom conditions at present, and an absence of contourite deposition during the Late Quaternary. The only regions where strong current activity is observed are along the top and southern flank of the Faeroe—Iceland Ridge and in the Denmark Strait where the Norwegian Sea Overflow Water flows out of the Norwegian Basin and sinks into the Atlantic Basin. The echo character, as well as most piston cores, show that deposition of terrigenous sediment throughout the Norwegian—Greenland Sea has been mainly by downslope processes (turbidity currents, slumps and related mass flows) and by ice-rafting. Glacial/interglacial climate fluctuations apparently control the influx of terrigenous sediment. During glacial phases the combination of lowered sea level, total ice cover of the Norwegian—Greenland and Barents seas, and widespread continental glacier build-up permitted large amounts of terrigenous sediment to be transported to, and deposited in, the deep basins. In contrast, during interglacials (such as the present), the absence of sea-ice cover and continental glaciers, plus the high sea level which inundated the continental shelves, greatly reduced the quantity of terrigenous sediment reaching the deep basin.

Echo character of the East Brazilian continental margin and its relationship to sedimentary processes

Abstract The nature and regional distributions of various types of bottom echoes recorded on 3.5-kHz echograms from the East Brazilian continental margin (8–30°S) provide valuable information about sedimentary processes which have been active on a regional scale. The ten types of echoes observed fall into two major classes: distinct and indistinct. Indistinct echoes have two sub-classes; prolonged and hyperbolic. A qualitative correlation is observed between three types of distinct and indistinct-prolonged echoes and the relative abundance of coarse, bedded sediment (silt, sand, gravel) in piston cores. Regions returning distinct echoes with continuous parallel sub-bottoms contain little or no coarse sediment; regions returning indistinct very prolonged echoes with no sub-bottoms contain very high concentrations of coarse sediment; and regions returning indistinct semiprolonged echoes with intermittent sub-bottoms contain moderate or intermediate amounts of coarse sediment. Thus the regional distributions of these three echo types reflect the dispersal of coarse terrigenous sediment throughout the region. High concentrations of coarse sediment are restricted to relatively small areas which are generally proximal to large deep-sea channels, whereas very low concentrations occur in distal regions such as the lowermost continental rise and adjacent abyssal plain. Moderate concentrations of coarse sediment occur throughout most of the continental rise. Five of the six types of hyperbolic echoes observed are reflected from erosional/depositional bed forms. Although some of these bed forms (especially on the upper continental rise) have probably been produced by gravity-controlled mass flows (turbidity currents, slumps, etc.) the fact that the most extensive and widespread regions of hyperbolic echoes occur in distal regions beneath the present axis of flow of the Antarctic Bottom Water suggests that most of these bed forms are the result of sediment reworking by the contour-following bottom currents of this water mass.

Late Quaternary sedimentation in the western equatorial Atlantic

The late Quaternary terrigenous sediments of the continental margin and abyssal plains of the western equatorial Atlantic consist predominantly of hemipelagic silty clay rich in organic detritus with interbeds of redeposited silts and sands; deposition was cyclic and was controlled by glacial-interglacial climatic fluctuations. The redeposited beds are graded, as much as several metres in thickness, and were episodically deposited by turbidity currents and related mass flows. Although the terrigenous component of the hemipelagic sediments was also transported to abyssal depths via gravity-controlled bottom flows, the pelagic component (foraminifera) of these sediments indicates that accumulation occurred relatively slowly (5 to >30 cm/10 3 yr) and continuously for periods of as much as 100,000 yr. Thus some near-bottom process (other than simple downslope flow), such as redistribution and deposition by contour-following bottom currents, apparently was responsible for deposition of the hemipelagic sediment; however, evidence for such contour-current deposition is sparse. The hemipelagic and redeposited sediments were continuously deposited during relatively long intervals (∼90,000 yr) of low sea level that accompanied the Wisconsin and previous glacial phases. During these intervals the continental shelf was emergent, and river sediments were discharged directly into submarine canyons from where they could easily be transported downslope to the continental margin and abyssal plains. In contrast, during the relatively short intervals (5,000 to 20,000 yr) of high sea level that accompanied the Holocene, beginning of the Last Interglaciation (125,000 to 115,000 yr B.P.), and beginning of previous interglacial phases, the locus of river sedimentation migrated landward across the continental shelf. The low gradient and great width of the shelf plus strong longshore currents prevented seaward movement of sediment beyond the innermost shelf. Thus terrigenous sedimentation was briefly, but completely, halted throughout the entire basin and only pelagic sediments were deposited.

Quantitative characteristics of sinuous distributary channels on the Amazon Deep-Sea Fan

Morphometric analysis of meandering channels on the middle and lower Amazon Deep-Sea Fan demonstrates that these channels have definite similarities with meandering subaerial rivers. The relationships between meander wavelength and both channel width and radius of meander curvature for fan channels are similar to those observed for large rivers; however, channel width, depth, and cross-sectional area decrease down a fan channel. Channel slope or gradient, measured along the channel axis, decreases smoothly down fan even though the fan slope (valley slope) which the channel traverses decreases irregularly down fan. Channel sinuosities range from about 1.05 to 2.6 on the fan, and sinuosity along a single channel, especially on the middle fan, appears to increase or decrease locally to compensate for varying fan surface slope (valley slope) to maintain a smoothly decreasing channel slope. This dynamic relationship between valley slope and channel sinuosity suggests that the sinuosities of the Amazon Fan channels have changed (that is, the channels have meandered) to obtain the optimum channel slopes, and the optimum channel slope decreases down fan. It is not possible, however, to determine whether that meandering occurred early in the development of the channel/levee system or throughout its evolution. Down-channel changes in fan and channel slope and maximum flow thickness (combined with variations in flow density) may produce systematic changes in flow characteristics and channel facies down fan.

Distributary channel meandering and bifurcation patterns on the Amazon deep-sea fan as revealed by long-range side-scan sonar (GLORIA)

We mapped the distributary channel system of the Amazon deep-sea fan using the GLORIA long-range side-scan sonar. Individual channels were continuously traced for distances of up to 150 km. Channel bifurcation, although observed in only a few places, results in many cases from breaching of channel levees on the outsides of meander loops. Whether both channels remain active after branching or the original channel is abandoned by avulsion generally cannot be determined. The most striking channel characteristic is high sinuosity that results in extensive, intricate, often recurving meanders. Cutoffs and abandoned meander loops (oxbows) are observed in a few places. These meandering channels are comparable in size and appearance to those of mature fluvial systems on land, such as on the lower Mississippi River. The formation, maintenance, and modification of such extensive, well-developed meander systems would seem to require large volumes of continuous turbidity flow through the channels for relatively long time periods. This may challenge the traditional concept that channel formation and modification are accomplished by intermittent or sporadic turbidity-current events.

Migrating sediment waves created by turbidity currents in the northern South China Basin

A large (∼ 25,000 km 2 ) field of migrating sediment waves on the gently dipping seaward wall of the Manila Trench in the northern South China Basin parallels the trench floor for 450 km and extends up the wall to 900 m above the trench floor. Wavelengths range from 200 m to 5 km, and amplitudes range from 5 to 50 m. The internal structure of the waves generally suggests upslope migration. These waves have characteristics common to migrating waves that were deposited by thermohaline (contour) currents at other locations throughout the world. However, the regional setting and sediments of these South China Sea waves, together with the apparent absence of contour-current activity within the South China Basin indicate that the waves probably formed by turbidity currents or related down-slope flows that moved southward along the Manila Trench floor. These sediment waves thus demonstrate that downslope, gravity-induced flows can create fields of migrating waves that are of regional extent and are morphologically identical to sediment waves deposited by thermohaline-induced contour currents.