Abyssal Sediments Research Articles

Clay mineral distribution in surface sediments of the South Atlantic: sources, transport, and relation to oceanography

Surface samples, mostly from abyssal sediments of the South Atlantic, from parts of the equatorial Atlantic, and of the Antarctic Ocean, were investigated for clay content and clay mineral composition. Maps of relative clay mineral content were compiled, which improve previous maps by showing more details, especially at high latitudes. Large-scale relations regarding the origin and transport paths of detrital clay are revealed. High smectite concentrations are observed in abyssal regions, primarily derived from southernmost South America and from minor sources in Southwest Africa. Near submarine volcanoes of the Antarctic Ocean (South Sandwich, Bouvet Island) smectite contents exhibit distinct maxima, which is ascribed to the weathering of altered basalts and volcanic glasses. The illite distribution can be subdivided into five major zones including two maxima revealing both South African and Antarctic sources. A particularly high amount of Mg- and Fe-rich illites are observed close to East Antarctica. They are derived from biotite-bearing crystalline rocks and transported to the west by the East Antarctic Coastal Current. Chiorite and well-crystallized dioctaedral illite are typical minerals enriched within the Subantarctic and Polarfrontal-Zone but of minor importance off East Antarctica. Kaolinite dominates the clay mineral assemblage at low latitudes, where the continental source rocks (West Africa, Brazil) are mainly affected by intensive chemical weathering. Surprisingly, a slight increase of kaolinite is observed in the Enderby Basin and near the Filchner-Ronne Ice shelf. The investigated area can be subdivided into ten, large-scale clay facies zones with characteristic possible source regions and transport paths. Clay mineral assemblages of the largest part of the South Atlantic, especially of the western basins are dominated by chlorite and illite derived from the Antarctic Peninsula and southernmost South America and supported by advection within the Circumantarctic Deep Water flow. In contrast, the East Antarctic provinces are relatively small. Assemblages of the eastern basins north of 30°S are strongly influenced by African sources, controlled by weathering regimes on land and by a complex interaction of wind, river and deep ocean transport. The strong gradient in clay mineral composition at the Brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins.

The abyssal bounty fan and lower Bounty Channel: evolution of a rifted-margin sedimentary system

The Bounty Channel and Fan system provides the basis for a model for deep-sea channel and fan development in a rifted continental margin setting. The sedimentary system results from an interplay between tectonics (fan location; sediment source), turbidity currents (sediment supply), geostrophic currents (sediment reworking and distribution) and climate (sea level, and hence sediment supply and type). Today, sediment is shed from the collisional Southern Alps, part of the Pacific/Indo-Australian plate margin, and passes east across the adjacent shelf and into the Otago Fan complex at the head of the Bounty Trough. Paths of sediment supply, and locations of sediment deposition, are controlled by the bathymetry of the Bounty Trough, with axial slopes as high as 37 m/km (2°) towards the trough head, diminishing to around 3.5 m/km (0.2°) along the trough axis. The Bounty Fan is located 800 km further east, where the Bounty Channel debouches onto abyssal oceanic crust at the mouth of the Bounty Trough. The Bounty Fan comprises a basement controlled fan-channel complex with high leveed banks exhibiting fields of mud waves, and a northward-elongated middle fan. Channel-axis gradients diminish from 6 m/km (0.35°) or more on the upper fan to less than 1 m/km (<0.06°) on the lower fan. Parts of the left bank levee and almost the entire middle fan are being eroded and re-entrained within a Deep Western Boundary Current (DWBC), which passes along the eastern New Zealand margin at depths below 2000 m. The DWBC is the prime source of deep, cold water flow into the Pacific Ocean, with a volume of ca. 20 Sv and velocities up to 4 cm/s or greater. The mouth of the Bounty Channel, at a depth of 4950 m at the south end of the middle fan, acts as a point source for an abyssal sediment drift entrained northward under the DWBC at depths below 4300 m. The Bounty Fan probably originated in the early to middle Neogene, but has mostly been built during the last 3 Myr (Plio-Pleistocene), predominantly as climate-controlled sedimentary couplets of terrigenous, micaceous mud (acoustically reflective; glacial) and biopelagic ooze (acoustically transparent; interglacial), deposited under the pervasive influence of the DWBC.

Rapid bioturbation in equatorial Pacific sediments: evidence from excess 234Th measurements

In order to evaluate bioturbation in equatorial Pacific sediments, profiles of excess 234Th were measured in cores collected during November and December 1992, at six abyssal sites along the JGOFS Equatorial Pacific 140°W transect, from 5°S to 9°N. Excess 234Th (half-life=24.1 days) was detected at all sites. Mean inventories ranged from a low of 0.25 dpm cm −2 at 9°N, to a high of 2.6 dpm cm −2 at 2°N. Eddy-diffusive models of the excess 234Th profiles suggest that, on 100-day timescales, particle mixing is slow at 5°S and 2°S (eddy diffusion coefficient, D b , < 0.4 cm 2 year −1), faster at 0° and 2°N ( D b =0.34–4.8 cm 2 year −1), and fastest at 5°N ( D b =4.7−11 cm 2 year −1). At 9°N, 234Th is found almost entirely in the upper 5 mm of the seabed, and D b values are <0.1 cm 2 year −1. D b values generally increase as POC flux increases, although the linear relationship ( r 2=0.334, p=0.365) displays considerable scatter. However, if the only sites included in the regression are those where 234Th profiles were measured in at least two core sets (i.e. 0°, 2°N, 5°N and 9°N), the linear regression improves substantially ( r 2=0.754, p=0.0012). These results suggest that measurements on a single core may not fully characterize the sediment mixing regime in a particular region, and highlight the need for multiple samples from an individual site. Thorium-234- D b values at 5°S, 2°S and 9°N are in rough agreement with published D b values calculated from the profiles of intermediate half-life radioisotopes, such as 210Pb and 137Cs, in open-ocean abyssal sediments. However, at 0°, 2°N and 5°N, 234Th- D b values are as much as an order of magnitude greater than published 210Pb- D b values. This phenomenon has been observed in other marine environments and may result from age-dependent mixing, non-steady-state mixing or a combination of the two processes. Particle mixing in cores collected at 5°S and 5°N appears to be best described as a largely eddy diffusive process. However, in cores collected at 2°S, 0°, 2°N and 9°N, non-local mixing processes appear to affect 15–30% of the excess 234Th inventory. At these sites, non-local mixing appears to transport recently-deposited material at least 3–4 cm below the sediment-water interface within 100 days of its arrival at the seafloor. The combination of rapid biodiffusive mixing and non-local transport allows recently-deposited, reactive biogenic debris to be rapidly mixed into the seabed. This may affect a wide range of diagenetic processes, including oxygen consumption, dissolution of biogenic debris and organic carbon remineralization. Models of early diagenesis in deep-sea sediments may need to incorporate both non-diffusive transport terms and rapid biodiffusive mixing of recently-deposited particles.

Sedimentary evolution models of Lower Triassic deep-water carbonate rocks of west Qinling Mts.

Based on the study of Lower Triassic deep-water carbonate rock sequence of west Qinling Mts . deep-water carbonate rock sequence is divided into lower fine-grain euxinic deposits of a carbonate gentle slope type and upper bathyal and abyssal sediments of carbonate steep slope type. The upper member is emphatically analysed and synthesized into five fades associations. They comprise four fining- and thinning-upward megacycles, each of them representing a sedimentary column which accumulated after a tensional fault subsidence event, which recorded a whole rifting process of west Qinling ocean trough during Lower Triassic.

Early diagenesis of lipid biomarker compounds in North Atlantic sediments

Near‐surface sediments (0–45 mm depth interval) from the Iceland Basin (59°N, 21°W, 3070‐m water depth) and Biscay Abyssal Plain (48°N, 17°W, 4105‐m water depth) were sectioned at millimeter‐scale resolution to assess alterations in key lipid biomarkers during early diagenesis. Inventories (µg cm−2 in the topmost 45 mm) of biomarkers from presumed terrestrial sources (&gt;C20 linear n‐alkanes, n‐alkanoic acids, n‐alkanols) and from phytoplankton sources (phytosterols, long‐chain alkenones and alkenoates) were 1.5–4 times more abundant in the Iceland Basin core than in the Biscay Abyssal Plain core, indicating greater sedimentary inputs and/or preservation at the Iceland Basin site. Biomarker concentrations varied significantly over millimeter‐scale depth intervals. Pronounced subsurface concentration maxima were present at both sites. At the Biscay Abyssal Plain site, steep concentration gradients in the uppermost few mm of the sediment column indicate extensive diagenetic losses at or near the sediment/water interface. Strong covariance was seen in the concentrations of various compounds within each class (e.g., long‐chain n‐alkanes, n‐alkanols, n‐alkanoic acids and long‐chain alkenones and alkyl alkenoates), indicating that individual compounds within each class are associated with the same organic matrix and have similar degradation and mixing rates. Thus biomarker indices for assessment of land‐derived inputs (CPI, C29/C31, ACL) and sea surface temperature (IPT) remain largely unaltered by early diagenetic processes in oxic, abyssal sediments in spite of extensive bioturbation and degradative losses within the uppermost few centimeters of the sediment column.

Grain size distribution and depositional processes of the mineral component of abyssal sediments: Lessons from the North Pacific

Grain size analyses of the mineral component of abyssal Pacific surface sediments show two distinctly different size distributions that can be associated with sediments that are dominantly eolian and dominantly hemipelagic, respectively. In the range of 1 to 30 µm, eolian grains show a distinct size mode at about 2 µm (9Φ) and little material coarser than about 16 µm (6Φ). Hemipelagic grains are characterized by a much flatter size distribution that has a broad size mode near 2 µm (9Φ) and important amounts of material coarser than 16 µm (6Φ). This ability to distinguish between hemipelagic and eolian grains will obviate some long‐standing concerns in the interpretation of deep‐sea sediments. Examples of the utility of this technique are given for the North Pacific and South Atlantic.

Micro‐ and Nanoforaminifera from Abyssal Northeast Atlantic Sediments: A Preliminary Report

AbstractRose Bengal stained benthic foraminifera which pass through a 63 μm mesh (microforaminifera and nanoforaminifera) have been extracted by handsorting the fine sieve residues (&gt; 45 μm, 31 μm, 28 μm, 20 μm, 15 μm) of abyssal sediment samples. The samples were collected using a multiple corer in four areas of the northeast Atlantic between 31° N and 59° N. The abundance of these minute foraminifera varied from 2 specimens per 1 cm2 (Madeira Abyssal Plain) to &gt; 110 per 1 cm2 (BIOTRANS area). They include a variety of taxa, the most common being certain rotaliid species, hormosinaceans and other multilocular agglutinated forms, the unilocular agglutinated genus Lagenammina, soft‐bodied agglutinated sphaeres and flasks (saccamminids and psammosphaerids) and allogromiids. Some specimens are &lt; 63 μm in maximum dimension but others belonging to elongate taxa are longer.Two samples taken 10 cm apart on the Porcupine Abyssal Plain suggest that minute foraminifera may be patchily distributed on a small scale. One sample, which was overlain by substantial amounts of phytodetritus, contained &gt; 100 stained specimens (&gt; 30 per 1 cm2) while the other, in which much less phytodetritus was present, yielded only 10 specimens (2.9 per 1 cm2). This observation suggests that some micro‐ and nanoforaminifera may flourish in the presence of decaying organic matter, perhaps consuming the associated bacteria. The presence of phytodetritus may also explain why two of our samples from the Madeira Abyssal Plain (MAP) contained an order of magnitude more stained tiny foraminifera than two other MAP samples in which phytodetritus was absent.

Abyssal sediment waves in the Gulf of Mexico: An enigma

A sediment wave field in the western Gulf of Mexico is similar to many other fields that are located on continental rises and related to thermohaline contour currents, especially western boundary undercurrents. Such currents should not occur in the Gulf, because it is silled. At present, the only postulated source of deep currents in the Gulf is the Loop Current (in the eastern half), eddies detached from it, and Rossby waves but no thermohaline currents. If any of the postulated processes cause the mud waves, it is a new mode of formation. If they do not, much more needs to be learned about deep Gulf circulation. These questions are important, because the sediment waves represent paleoceanographic conditions that have persisted for over 5 million years.

Diffusive and total oxygen uptake of deep-sea sediments in the eastern South Atlantic Ocean: in situ and laboratory measurements

Total O 2 uptake rates were measured by the benthic flux chamber lander ELINOR, and O 2 microprofiles were measured by the profiling lander PROFILUR in the eastern South Atlantic. Diffusive O 2 fluxes through the diffusive boundary layer and the depth distribution of O 2 consumption rates within the sediment were calculated from the obtained microprofiles. The depth integrated O 2 consumption rate agreed closely with the diffusive O 2 uptake at all stations. Total O 2 uptake was 1.2–4.2 times the diffusive O 2 uptake, and the difference correlated with the abundance of macrofauna in the sediment. Diffusive O 2 uptake and O 2-penetration depths correlated with the organic content of the sediments and exhibited an inverse correlation with water depth. Total and diffusive rates of in situ O 2 uptake were higher than previously published data for shelf and abyssal sediments in the Atlantic, but were comparable to rates from upwelling areas in the eastern Pacific. Laboratory measurements on recovered sediment cores showed lower O 2 penetration depths and higher diffusive uptake rates than in situ measurements. The differences increased with increasing water depth. We primarily ascribe this compression of O 2 profiles to a transiently increased temperature during recovery and enhanced microbial activity in decompressed sediment cores. Total O 2 uptake rates measured in the laboratory on macrofauna-rich stations were, in contrast, lower than those measured in situ because of underrepresentation and disturbance of the macrofauna.

Resistivity structure of the underconsolidated sediments of the Cascadia Basin

SUMMARY The Cascadia Basin is located on the abyssal plain of the Juan de Fuca Plate, off the west coast of Vancouver Island. The sediments of the Cascadia Basin are very thick, over 2 km, and are underconsolidated because of a high rate of sedimentation. Seismic results suggest that the sediment acoustic velocities are low, and increase slowly and monotonically with depth from the sea-floor. A vertical electric dipole (VED) survey was conducted to obtain the resistivity structure of the upper kilometre of sediments, to determine the physical properties of a thick sequence of abyssal basin sediments, and for comparison with seismic results. While models with multiple layers can be found which fit the data, the‘best-fitting’models share a common feature—decreasing resistivity with depth, which is suggestive of a gradient in the resistivity. The feature is consistent with the results of other geological and geophysical studies. In particular, the seismic results do not indicate any significant contrast in the physical properties, i.e. layering, in the upper 500 m. We extend the VED theory to the case where resistivity varies continuously with depth, and obtain a gradient model that fits the Cascadia Basin data marginally better than simple layered models. The model resistivity is combined with heat-flow measurements for the determination of physical properties versus depth; the physical property models, the porosity, for example, are not consistent with the physical properties derived from seismic survey results. The mismatch is diagnostic of the underconsolidation, and is primarily due to the effects of the high rate of sedimentation on the seismic velocity and on the heat flow, which in turn influence the physical properties.

Sedimentary history of the Tethyan basin in the Tibetan Himalayas

After an epicontinental phase, the sedimentary rocks in the Tibetan Himalayas document a complete Wilson cycle of the Neo-Tethyan (Tethys Ill) evolution between the Gondwana supercontinent and its northward drifting margin (Lhasa block) from the Late Permian to the Eocene. During the Triassic rift stage, the basin was filled with a huge, clastic-dominated sediment wedge with up to > 5 000 m of flysch in the northern zone. Widespread deltaic clastics and shallow-water carbonates of late Norian to earliest Jurassic age in the southern zone mark, in conjunction with decreasing tectonic subsidence, the transition to the drift stage. Some 4 500 m of Jurassic and Early Cretaceous shallow-water carbonates and siliciclastics accumulated on the Tethyan Indian passive margin. Deepening-upward sequences with condensed beds at their tops alternate with repeated progradational packages of shelf sediments. Extensive abyssal sediments with basaltic volcanics in the northern deep-water zone reflect continued ocean spreading and thermal subsidence. Paleomagnetic data, gained separately for the northern Indian plate and the Lhasa block, indicate that the Neo-Tethys reached its maximum width about 110 Ma ago with a spreading rate of 4.8 cm/year, before it commenced to close again. During the remnant basin stage in the Late Cretaceous and Paleogene, a shallowing-upward megasequence, capped by a carbonate platform, developed in the southern inner shelf realm. In the northern slope/basin plain zone, turbidites and chaotic sediments, derived from both the acretionary wedge and the steepening slope of the passive margin, accumulated. The depositional center of the remnant basin shifted southward as a result of flexural subsidence and southward overthrusting. The sediments from the Triassic to the Paleogene are tentatively subdivided into five mega-sequences, which are controlled mainly by regional tectonics. Climatic influence (e.g., carbonate deposition), due to northward plate motion, is partially subdued by terrigenous input and/or increased water depth. During the Oligocene and Miocene, crustal shortening led to rapid uplift and the deposition of fluvial molasse in limited basins.

時間指標テフラ阿蘇３によって示唆される最終間氷期直後５ｄ期の海面低下

We have discovered a distal tephra with similar petrographic and chemical properties to the Aso-3 pyroclastic flow deposits of the Aso caldera origin at several localities of Honshu, Japan, and adjacent seas. It is concluded to be a fallout ash associated with the Aso-3 pyroclastic flow, The tephra, named Aso-3, is a cry tal vitric fine-grained ash, consisting of dacitic pumiceous and bubble-walled glass shards, and orthopyroxene and crynopyroxene as mafic phertocrysts. It is identified with the aid of combined petrographic and stratigraphic parameters : relatively high index of glass (n = 1.5131.519), low index of orthopyroxene (γ =1.7021.705) and dacitic glass composition (rich in alkali, K2O> Na2O, rich in TiO2, FeO* and CaO low abundance in light rare earth elements and characteristic low degree of angle in chondritenormalized pattern).This tephra was dated by several radiometric methods : 103±4.2 (FT), 110±3 (TL) and 123±6 (K-Ar) in ka. The stratigraphic position of this tephra is above the last interglacial culmination in Kyushu and Honshu, and below such widespread time-marker tephras as Toya, Ata, On-Pml, SK, etc. In addition, the tephra occurs in the marine isotope stage (MIS) 5d in the abyssal sediment in the northwest Pacific.The occurrence of this tephra in the Quaternary sequence in northern coastal areas of Miyazaki, south Kyushu, clearly indicates that the sea level was more than c. 70 m lower than the preceding high stand of sea level of the MIS 5e. At the coastal cliff of Kawaminami, the Aso-3 tephra occurs 21 m above the present sea level in the basal part of the valley-filling deposits (Tohriyamahama formation) which followed the prominent marine terrace, Sanzaibaru of the MIS 5e. Former shoreline of this stage was uplifted to the elevation of c. 90 m. This tephra was also recognized in the brackish to fresh water layer sandwiched between marine sediments, called Shibikawa formation in Oga Peninsula, northern Honshu. It clearly suggests significant lowering of sea level at the time of Aso-3 eruption as well.

Characterization of fine-grained turbidite deposits from the South China Sea sediment cores

Two depositional processes controlled the muddy sediments in the South China Sea Basin. Bathyal sediments depositional rate was 7.66 cm/ka in the northern continental slope of the Basin where turbidity current was almost nonexistent. In the northern margin of the Basin, abyssal sediment depositional rate was 5.05 cm/ka and turbidity current occurrence averaged 0.22 per 1000 years. Turbidite was found in the middle of the Basin. Over half of the muddy sediments in the deep sea basin were deposited by turbidity currents, and had typically graded bedding, and contents of organic matter, calcareous material and micropaleontologic species inconsistent with the environment.

Abyssal sediment drifts, erosion and history of bottom water flow in the Tasman Sea southwest of New Zealand

Deep‐sea sediments in the southeastern Tasman Sea underwent extensive bottom current erosion and transport during the late Cenozoic. Analysis of seismic facies shows that a northward flow caused profound erosion in narrow scoured deeps and over wide areas in the southeastern Tasman Sea southwest of New Zealand. At 47°S the flow diverted north‐northwest into the central Tasman Sea and a large lobate laminated sediment drift was constructed at the turn. The flow directions are established from mega‐bedform geometries in and on the drift and from the asymmetry of sediment remnants in the regions of profound erosion. Current activity and erosion began late in the Eocene or early in the Oligocene, probably due to both oceanographic and geographic changes at that time. Oceanographic data show that flows with similar erosional strength do not operate today. To explain the severity of past erosion by northward flows at this very eastern location in the basin, it is proposed that at times in the late Cenozoic, nor...

Microbial potential in deep-sea sediments

In spite of high pressures and low temperatures in abyssal sediments of the North Atlantic Ocean, bacterial activity is evident and highest in the top 10 cm. At these locations the input of degradable organic material to the deep-sea bottom is low. Oxygen, therefore, remains the dominant oxidant in surface sediments. Although alternative electron acceptors like nitrate, oxidized manganese and sulfate are present in large amounts, they are not utilized in this natural habitat. In sediment cores which were collected from the site for laboratory perturbation studies, it was possible to stimulate microbially mediated processes which are dormant in situ. When the oxygen supply was cut off, nitrate and manganese reduction occurred. Denitrification was the major process observed in the upper anoxic layers, while nitrate-ammonification and manganese reduction occurred in deeper sediment strata (4–8 cm). This is evidence for the presence of a variety of different bacteria and of an anaerobic heterotrophic potential. Most of the activity is located in the top 10 cm of these sediments. The shift to anaerobiosis initiates microbial activities through which metals are converted into their mobile species at the lowered redox potential. Evaluation of the suitability of the deep sea as a repository for waste materials will have to account for the large dormant potential of microbial activities and the consequences of their release by changing the environmental conditions at the sea floor.

Oceanic pelagic prey of benthopelagic fish in the benthic boundary layer of a marginal oceanic region

The availability of oceanic plankton and micronekton to the benthopelagic fish assemblages on the slopes of the Rockall Trough, a marginal region of the northeast Atlantic deep-sea environment, is examined. The vertical distribution of pelagic species predated by the fish was determined in the off-slope water column, as were the depths at which they are predated by the fish in the benthic boundary layer of the slope. The benthopelagic fish predated pelagic species a t depths on thta slope corresponding to the daytime depths of the pelagic prey. The occurrence of these pelagic species a t the benthic boundary layer is primarily through truncation of their pelagic vertical distributions rather than horizontal impingement, although this does occur: thls applies not only to epiand mesopelagic but also to the bathypelagic species which can even reach abyssal sediments. Die1 vertical migration of the pelagic species did not make them available to slope fishes in shallower depths presumably because vertical migration is strictly upwards within the pelagic water column and not u p the contours of the slope sediment.

Abyssal sediment erosion in the Central Indian Basin: Evidence from radiochemical and radiolarian studies

Radiochemical analyses of three spade cores collected around 78°E between 10°S and 12°S from the abyssal depths in the Central Indian Basin yield an average accumulation rate of 2 mm/ka. The ratios of 230Th flux in the sediments to its production rate in the overlying water column ( Fa Fp ) are extremely low (∼0.2). This fact, and the absence of Collosphaera invaginata (first appearance datum, 150–200 ka), an index radiolarian species of Neogene Radiolarian Zone 1, indicate intense erosion and lateral transport of younger sediments from this region. The radiochemical and radiolarian biostratigraphic evidence for nearly 175 ka of erosion of the chronological record of the sediments in this region has been attributed to the effect of turbulent Antarctic Bottom Water entering the Central Indian Basin through the northern saddles of the Ninety East Ridge.

Correlation between OCR and Depth for Deep‐Sea Sediments

An analysis of the apparent overconsolidation properties of deep-sea abyssal plain sediments taken from holes DSDP 576A and MPG-1 is presented. Apparent overconsolidation is believed to be caused by interparticle bonding, or cementation. The cementation can be caused by any one or a combination of the following: silica, potassium, calcium carbonate, ferric oxide, manganese, or biological excretions. Plotting OCR versus depth is shown to result in a “spurious correlation” because both depth and OCR have the same independent variable in common, namely depth (\Iz\N). An alternative method for presenting consolidation information is to plot preconsolidation stress versus depth, particularly when the depth is small. Using this procedure plots of overburden and preconsolidation stresses versus depth indicates that for hole 576A an overconsolidated condition exists down to a depth of 35 m. Below this depth an apparent condition of underconsolidation exists. Hole MPG-1 shows an over consolidated condition exists to the maximum depth investigated of 3.5 m.

Evolution of Pliocene to Recent abyssal sediment waves on Bounty Channel levees, New Zealand

Levees bordering Bounty Channel 900 km east of New Zealand accommodate a 400 m-thick sequence (maximum) of sediment waves that have formed since Pliocene times. These bedforms, with amplitudes of 2–17 m and wavelengths of 0.6–6 km occur in 4100–4900 m of water and were formed by turbidity currents, as indicated by their restriction to levee backslopes, the frequent occurrence of turbidites in cores and the preferential but not exclusive development of waves on the left-bank levee in accord with the Southern Hemisphere coriolis deflection. The wave field was instigated in the Late Pliocene when glacially lowered sea level allowed rivers draining the Southern Alps of South Island to discharge directly into Bounty Channel and its attendant canyons. The field grew vertically through the coalescence of small waves into larger bedforms that continually migrated across and up levee backslopes at an average rate of 5.6m/100 yrs. Wave growth decreased into the Late Pleistocene probably in response to progressive containment of turbidity currents as the relief of Bounty Channel increased to 200 m or more. The glacial periods of wave growth were interrupted by interglacial interludes of quiescence when the field was draped mainly by pelagic calcareous ooze.

Sound velocity and related physical properties of fine-grained abyssal sediments from the Brazil Basin (South Atlantic Ocean)

Empirical relationships between compressional wave (sound) velocity and selected physical properties have been determined for fine-grained abyssal sediments from the east Brazilian continental margin and Pernambuco abyssal plain of the Brazil Basin (NW South Atlantic Ocean). Porosity and wet bulk density were found to be the best sedimentological indices to sound velocity. In agreement with numerous other studies, other sediment properties were lesser, but still very good, indices. In order of decreasing statistical significance, these properties include mean grain size and percent clay. Averaged values of sediment physical properties of the Brazil Basin were compared with the published results of Hamilton and Bachman [J. Acoust. Soc. Am. 72, 1891–1904 (1982)] and show good agreement. The observed differences between the two studies were insignificant and attributed to slight mineralogical differences that can be expected in light of the differing sedimentation processes of the Pacific and Atlantic Oceans. Velocities of the Brazil Basin study, converted to in situ conditions from laboratory measurements, were compared with those previously reported by Hamilton and Bachman, which were converted to in situ conditions using bottom water velocities of this study and their velocity ratios. A plot of the velocities showed good agreement, but those of the present study were about 20 m/s lower. This discrepancy is probably not mineralogical in origin, but instead results from a systematic difference in the sediment velocimeters used in the two studies. The exact cause of the discrepancy is not known, and a detailed system evaluation of the velocimeters is recommended.