Deep Abyssal Plain Research Articles

Habitat and benthic fauna of the Wallaby-Cuvier escarpment, SE Indian ocean

The Wallaby-Cuvier Escarpment is a 700 km long, NW-SE trending linear geological feature that marks the southern boundary of the Cuvier Plateau. The Plateau rises from the adjacent 5500 m deep abyssal plain to a topographical high of 3000 m, approximately 450 km off the coast of Western Australian. In 2021, a 50 km long segment of the escarpment, covering an area of 1700 km2, was mapped by a high-resolution full-ocean depth multibeam echosounder. The habitat of the study area was explored during two video surveys undertaken using the crewed submersible Limiting Factor. Using Benthic Terrain Modeler (BTM), we classified the seabed into five geomorphological classes; flat plains, depressions, broad slopes, steep slopes, and crest. Video footage was used to classify habitat types and to record benthic megafauna occurrences which revealed highly heterogeneous and rapidly changing habitats. Six habitats were proposed based on the nature of geomorphology, slope, and substrate textures. A total of 202 organisms comprising 52 morphotaxa were scored during 198 min of survey that included at least eight phyla and 29 families. Despite a high number of morphotaxa, the total abundance of organisms is considered low which we attribute to oligotrophic surface waters. However, the distribution of organisms and their feeding strategies were well-aligned with habitat type. This study documents faunal distribution and habitat diversity of a rarely explored type of deep-sea geomorphological feature, and in a largely unexplored area and depth of the Indian Ocean.

Ambient sound levels, directionality, and transport velocity around a seamount measured by a deep-drifting acoustic vector sensor

In September 2022 an opto-acoustic drifter was deployed several times in the vicinity of the Kelvin seamount (38°49’N, 64°2’W) in the North Atlantic Ocean, which rises to a depth of 1500 m from a 5 km deep abyssal plain. The “SQUALL-E” drifter incorporates stereo video cameras, vertical (1.75 m aperture) and tetrahedral hydrophone arrays, and a 2-D Geospectrum M35 acoustic vector sensor. One overnight deployment descended to 250 m depth, and a another to 400 m depth, which resulted in highly different drift rates. The M35 sensor demonstrated the ability to estimate the directionality of ambient sounds between 1 and nearly 30 kHz. Sperm whales, delphinids, various vessels, and other discrete and distributed sources were detected. For both deployments we present distributions of ambient sound levels, azimuthal and elevation angles, and transport velocity. We discuss how these distributions evolved throughout the deployments. [Work sponsored by ONR TFO program.]

Characteristics of sediment resuspension on a deep abyssal plain in the Eastern Tropical Pacific Ocean

We examined the biogenic and lithogenic particle composition and radiocarbon content of sinking particulate organic carbon to investigate sediment resuspension and its contribution to sinking particles on the deep abyssal plain of the Eastern Tropical Pacific Ocean. Samples were collected using sediment traps from August 2011 to July 2012 at depths of 4500 and 4950 m (50 m above the seafloor); above and within the benthic nepheloid layer (BNL), respectively. Biogenic particles derived from export production were the major source of sinking particles and their flux showed a unimodal temporal distribution, with larger values between February and April. At a depth of 4500 m, the lithogenic material flux was slightly greater than, or similar to, the flux of atmospheric dust deposition. In comparison, lithogenic material and excess Mn consistently showed a greater contribution to resuspended particles at 4950 m. The lithogenic material flux was proportional to the biogenic flux. These observations imply that resuspended particles exist at a background concentration in the BNL throughout the year, and are scavenged by sinking biogenic particles, especially during the high flux period.

Into the deep: A coarse-grained carbonate turbidite valley and canyon in ultra-deep carbonate setting

New high-resolution multibeam mapping images detail the southern part of Exuma Sound (Southeastern Bahamas), and its unchartered transition area to the deep abyssal plain of the Western North Atlantic, bounded by the Bahama Escarpment extending between San Salvador Island and Samana Cay. The transition area is locally referred to as Exuma Plateau. The newly established map reveals the detailed and complex morphology of a giant valley draining a long-lived carbonate platform from its upper slope down to the abyssal plain. This giant valley extends parallel to the slope of Long Island, Conception Island, and Rum Cay. It starts with a perched system flowing on top of a lower Cretaceous drowned main carbonate platform. The valley shows low sinuosity and is characterized by several bends and flow constrictions related to the presence of the small relict isolated platforms that kept alive longer than the main platform before drowning and merging tributaries. Turbidite levees on either side of the valley witness the pathway of multiple gravity flows, generated by upper slope over steepening around Exuma Sound through carbonate offbank transport, some of them locally >15°, and resulting slumping. In addition, additional periplatform sediments are transported to the main valley through numerous secondary slope gullies and several kilometre-long tributaries, draining the upper slopes of cays and islands surrounding Exuma Plateau. Some of them form knickpoints indicating surincision of the main Exuma Valley which is consistent with an important lateral supply of the main Exuma Valley. Prior to reaching the abyssal plain, the main valley abruptly evolves into a deep canyon, 5 km in width at its origin and as much as 10 km wide when it meets the abyssal plain, through two major knickpoints named “chutes” with outsized height exceeding several hundred of meters in height. Both chutes are associated with plunge pools, as deep as 200-m. In the deepest pools, the flows generate a hydraulic jump and resulting sediment accumulation. When the canyon opens to the San Salvador abyssal plain, the narrow, deep, and strong flows release significant volume of coarse-grained calcareous sediments in numerous turbidite layers interbedded with fine mixed siliciclastic and carbonate sediments transported by the Western Boundary Undercurrent (WBUC) along the Bahama Escarpment. Carbonate gravity flows exiting the canyon decelerate at the abyssal plain level and construct a several-kilometre-wide coarse-grained deep-sea turbidite system with well-developed lobe-shape levees, partially modified by the flow of strong contour-currents along the Bahama Escarpment.

Vertical stratification in the distribution of demersal fishes along the walls of the La Jolla and Scripps submarine canyons, California, USA

The geographic distributions of many coastal marine fish assemblages are strongly driven by habitat features, particularly among demersal fishes that live along the seafloor. Ecologists have long recognized the importance of characterizing fish habitat associations, especially where spatial management is under consideration. However, little is known about fish distributions and habitat suitability in unique demersal habitats such as submarine canyons. The active continental margin of the California coast is cut by eight submarine canyons, several of which extend from the shore to the deep abyssal plain. We sampled the demersal fish assemblages in two of those canyons: (1) the Scripps submarine canyon in the San-Diego-Scripps State Marine Conservation Area (SMCA) and (2) the La Jolla canyon in the Matlahuayl State Marine Reserve (SMR) to gain insight into both the distributions and habitat associations of demersal fishes in canyons. A remotely operated vehicle was used to conduct 21 vertically oriented transects along the canyon walls in depths ranging from 20 to 300m. Species composition was assessed in three depth-stratified zones (100m per zone) along the canyon walls. Species richness, abundance, and attributes of the surrounding canyon habitat structure (slope and benthic terrain ruggedness) were quantified. Three distinct assemblage groupings were identified, which comprised 35 species of demersal fishes from 17 families. Among all factors analyzed in this study, depth, slope, and ruggedness were strong explanatory variables of patterns of species richness and abundance; however, the relationship between depth and assemblage structure was non-linear. The greatest number of species was observed in the mid depth-stratified zone. These trends suggest that variation in canyon dynamics across depth strata may facilitate distinct assemblage groupings of demersal fishes, which can in turn be used to better manage these unique habitats.

Late Quaternary paleoenvironmental changes revealed by multi-proxy records from the Chukchi Abyssal Plain, western Arctic Ocean

Late Quaternary paleoenvironmental changes in the western Arctic Ocean are revealed by multi-proxy records of core 03M03 from the Chukchi Abyssal Plain (CAP). Proxy parameters include lithology, grain size fractions, and mineralogy and petrology of ice-rafted detritus (IRD), element contents, biogenic components, δ18O, δ13C and Mg/Ca of planktonic foraminifera Neogloboquadrina pachyderma (sin.) (Nps). Seven IRD (>250μm) peaks are interpreted as marking detrital input by rafting sea ice or icebergs during MIS 3 interstadials and early MIS 1. High MnO, CaO and MgO contents and high Ca/Al and Mg/Al ratios during MIS 3 and MIS 1 correspond to increases in ice-rafted detrital carbonates and the synchronous declines in siliciclastic elements (e.g., Al2O3, Fe2O3). Therefore, these warmer periods were characterized by a high detrital carbonate input entrained in icebergs from the Canadian Arctic Archipelago coeval with an increased input of Mn through rivers and/or coastal erosion. Relatively stable contents of siliciclastic elements and their ratios in the grayish sediment units are interpreted from turbid surface water plumes or nepheloid flows delivered by meltwater and/or brine rejection from ice-sheet margins at the Arctic Ocean periphery. Relatively stable clay- and silt-sized fractions were attributed mainly to sea ice entrainment over glacial–interglacial cycles. High foraminiferal abundances in the brown units during MIS 3 and 1 are related to enhanced calcareous plankton productivity under more open water conditions and/or the incremental input of Atlantic water masses. Relatively high TOC and opal contents in the grayish units of MIS 3 appear to have accumulated by lateral transport of organic matter from the Chukchi shelf to the deep abyssal plain. Lower contents of biogenic material in the brown units probably result from increased dilution by rapid IRD deposition, and from early diagenetic degradation. Depletions in Nps-δ18O and -δ13C concurrent with high foraminiferal abundances and IRD peaks within the brown units of MIS 3 and 1 are indicative of meltwater pulses, as previously documented across the Arctic Ocean. However, several Nps-δ18O and -δ13C depletions between the brown units B3 and B2 in MIS 3 could have resulted from enhanced sea ice formation. The Nps-Mg/Ca has the potential to record paleotemperature changes in the Arctic region, but still awaits a better calibration of the Nps-Mg/Ca–temperature relationship with results from core-tops, sediment traps, and plankton tows.

The MARCONI reflection seismic data: A view into the eastern part of the Bay of Biscay

Seismic data acquisition for the MARCONI project took place in 2003 in the eastern half of the Bay of Biscay. The overall goal of the multichannel experiment was to collect seismic data to elucidate the structure and evolution of the southern margin of the Bay where an along-the-margin transition between oceanic subduction in the west and continental convergence in the east occurs, and its relationship to the Pyrenean realm. We describe the multichannel experiment and present an interpretation of the abyssal plain sector of the profiles. Altogether, eleven regional seismic reflection profiles were collected with a total length of 1800 km, between the longitudes 6° and 2°W and latitudes 43.5° and 46°N. The seismic profiles show first images of the thick sedimentary pile that covers the eastern abyssal plain of the Bay. The interpretation includes three sedimentary sequences separated by unconformities that correspond to the pre-, syn- and post-Alpine sequences. The post-orogenic deposits lay unconformably over the previous compressional structures in the eastern part of the Bay, floored by transitional crust. These compressional features are evident in the presence of a young accretionary prism sealed at its first stages of development. The overall structure and shape is similar to those of active subduction prisms worldwide, but evidencing differences in width and sediment wedge thickness from west to east along the margin. Buoyancy of the transitional crust and resistance to subduction led to stronger plate coupling towards the east, where oceanic crust is absent, and may be the main cause for these morphological changes along the east–west traverse of the wedge. The MARCONI multichannel deep seismic dataset provides the first estimation of the thickness of the sediment load on the 5000 m deep abyssal plain of Bay of Biscay, where no direct data were available.

Post-Eocene seismic stratigraphy of the deep ocean basin adjacent to the southeast African continental margin: a record of geostrophic bottom current systems

A high-resolution seismic-reflection survey of the Transkei Basin and Natal Valley permits the first recognition of three major reflectors that mark basin-wide unconformities across the continental rise and deep abyssal plain off the southeast African continental margin. Reflector O marks a change in acoustic reflectivity, coincident with a change in sedimentary bedforms from generally parallel bedding below to large-scale lenticular and clinoform shapes above. Reflector O probably marks the onset of cold, abyssal current circulation around the Eocene–Oligocene boundary. The overlying O sequence records deposition of a contourite drift (Oribi Drift) by northeast flowing abyssal currents at ∼4000 m water depths along the continental rise of the northeastern Agulhas Fracture Zone. This water depth is shallower than present-day Antarctic Bottom Water (AABW). The M reflector unconformity (possibly lower Middle Miocene) marks seafloor erosion in 4500 m water depth in the Transkei Basin and the cessation of drift construction along the continental rise. Above reflector M in the abyssal plain, a contourite drift (M-Drift) records deposition from an east-flowing bottom current in a location similar to, but slightly shallower than present-day AABW. The stagnation of bottom current activity in the northern Natal Valley and/or a rapid influx of sediment accumulation is marked by M sequence turbidite sediments (the Mzimkulu apron) deposited against and burying the Oribi Drift on the continental rise. Reworking of M sequence sediment along the continental rise to form low mounds (M 4) and sediment waves in the northern Natal Valley indicate that a shallow, bottom current flowed at depths of 3800 to 3600 m. The coeval current-molding of the slope and abyssal plain indicates a two-layered structure of the bottom water may have commenced in the Miocene. Reflector P is the most pronounced unconformity in the deep abyssal plain, where it truncates M Sequence reflectors, and marks the base the Agulhas Drift which stands approximately 200 m above the surrounding seafloor. The P Sequence sedimentation is estimated to have begun in the Pliocene prior to or concurrent with an expansion of Southern and Northern polar ice-caps. Major slumping of the continental slope in the Natal Valley also began at this time, probably triggered by a combination of onland neotectonic activity and erosion of the base of the slope by vigorous bottom currents (possibly North Atlantic Deep Water, NADW).

Characteristics of warm rings in the African sector of the Antarctic Circumpolar Current

Both models and observations of the Southern Ocean circulation suggest an important role for mesoscale eddies in the meridional transport of heat and salt and in the vertical transport of momentum. However, in contrast with western boundary currents, there are few observations of individual mesoscale features originating at the Antarctic Circumpolar Current (ACC). We used hydrographic data from a quasi-synoptic survey, Geosat altimetry data and historical hydrographic data to estimate some characteristics of warm rings in the sector 20–40°E. A sequence of maps of sea surface height anomalies derived from the Geosat data shows anticyclonic rings with a typical diameter of 150–200 km. The rings are shed from the meanders of the ACC at the Antarctic Polar Front (APF), which evolve over the deep abyssal plain immediately behind the mid-ocean ridge. Their subsequent southward movement is confined to a rather narrow (300–400 km) corridor between 26 and 32°E. The rings drift southward with an average speed of about 3 cm s −1. After reaching 55–57°S they tend to stagnate and decay. According to altimetry data, the lifetime of the rings is about one year. Formation of five warm rings was detected during the two-year period November 1986 to October 1988. Since the rings do not recoalesce again with the ACC, their translation results in net southward heat and salt transport. We also describe the hydrographic structure and southward translation of an individual ring found in February 1988 near 28°E, 56°S. Estimates of heat and salt anomalies for this ring are 6 × 10 19 J and 13 × 10 11 kg relative to the Warm and 13 × 10 19 J and 26 × 10 11 kg relative to the Cold regime of the Weddell Gyre, i.e. an order of magnitude larger than corresponding estimates for the previously described Drake Passage cold rings. Mesoscale variability patterns suggest a similar hydrographic regime in a few other sectors of the Southern Ocean. However, available hydrographic data do not allow definite assessment about the overall importance of warm rings for the meridional transports of heat and salt south of the APF.

Mesozoic neo‐tethyan pre‐orogenic deep marine sediments along the Indus–Yarlung Suture, Himalaya

Shelf, forereef and basin margin (slope) olistoliths (Exotic blocks of limestone) of Permian–Jurassic age are tectonically juxtaposed within the Triassic to Eocene age pre‐orogenic, deep abyssal plain turbidites of the Lamayuru. The pre‐collision tectonic setting and depositional environment of the limestone olistoliths can be reconstructed from within the neighbouring Zanskar range. The disorganized Ophiolitic Melange Zone, an association of different tectonic rock slivers of Jurassic–Eocene age, is tectonically underlain by the overthrusted Lamayuru Formation and tectonically overlain by the Nindam Formation. Tectonic slivers of Late Jurassic–Early Cretaceous age red radiolarian cherts represent a characteristic lithotectonic unit of the Ophiolitic Melange Zone, those occurring near the contact zone with the Lamayuru Formation, were deposited within the neo‐Tethyan deep‐ocean floor of the Indian passive margin below the carbonate compensation depth. These tectonic slivers accumulated along the northern margin of the Indus–Yarlung Suture Zone of the Ladakh Indian Himalaya during subduction accretion associated with the initial convergence of the Indian plate beneath the Eurasian plate.

Design and operation of a deep seismic survey in the Tasman Sea Basin

Deep crustal reflections, appearing at two-way times of 6–20 s, are characterised by a low signal-to-noise ratio and a low-frequency content (~ 5–30 Hz). Optimising the visibility of these reflections requires that: 1.(1) the source energy be concentrated in this band;2.(2) the receiver be as long as practically possible;3.(3) the shot spacing be as snail as possible.An airgun array simulation program was used to investigate the effect of operating a 10-gun array at different depths. It predicted that the source energy could be enhanced at low frequencies by running the gun array at depths of up to 25 m, which would improve the signal-to-noise ratio for deep reflectors by a factor of 2 relative to the same array at 7.5 m depth at the expense of some loss of temporal resolution.The receiver length was limited to 4.4 km by the number of cable sections available and the maximum tension allowable in the cable. It was configured as 88 channels of 50 m length. To reduce the shot spacing, minimise the tow noise and reduce tension in the cable, the ship speed was reduced to 4 knots. A cable depth of 20 m was chosen to reduce sea state noise and keep the receiver ghost notch frequency outside the frequency band of interest.A variety of operational problems was encountered. Fish bites holed 50% of the total cable inventory and cable sections were damaged when they sank below their maximum rated depth after being holed. Shot-generated noise was the limiting factor in deep water. Despite these problems, useful data were obtained over the Lord Howe Rise, Dampier Ridge, Gippsland Basin margin and the deep abyssal plain and the survey demonstrated the use of an airgun array tuned for maximum energy.

Oil and Gas Potential of the Maritime Boundary Region in the Central Gulf of Mexico

The U.S. Geological Survey conducted a detailed study in the first half of 1981 of the oil and gas resource potential in the Maritime Boundary region of the central Gulf of Mexico. The Maritime Boundary region encompasses a part of the Gulf where jurisdiction over natural resources by adjacent coastal countries has not yet been established. The region of investigation is divided into six assessment areas on the basis of their generally distinct geologic characteristics. Individual assessment areas comprise a total area of approximately 58,940 mi2 (152,660 km2) and contain a total estimated sediment volume of 188,140 mi3 (784,170 km3). Water depths within the overall study region range from a minimum of 98 ft (30 m) on the contin ntal shelf off the Rio Grande to a maximum of about 12,270 ft (3,740 m) in the deep abyssal plain of the west-central Gulf; more than 75% of the study region is in water depth exceeding 10,000 ft (3,048 m). The study focused on factors critical to the generation, migration, and entrapment of hydrocarbons, such as: source beds and thermal maturation, reservoir rocks, structural and stratigraphic traps, and seals and timing of hydrocarbon migration relative to formation of traps. The oil and gas potential was analyzed using all publicly available geophysical data recorded in the region supplemented by a limited amount of geologic information synthesized from reports on drill holes and bottom cores within and adjacent to the region of study. A variety of evidence suggests that favorable conditions exist for the occurrence of crude oil and natural gas resources in the Maritime Boundary region. Estimates of the in-place resources (recovery factors were not applied) of crude oil and natural gas range from 2.24 to 21.99 billion bbl and from 5.48 to 44.4 tcf, respectively. The individual assessment areas appear to have the most attractive petroleum potential in the following order: Perdido Foldbelt, Sigsbee Knolls, Abyssal Gulf Basin, Campeche Escarpment, Rio Grande Margin, and Sigsbee Escarpment. Estimates of the amount of petroleum resources that might be ultimately recoverable were not calculated because not enough is known at present about reservoir properties, economics, and the technology needed to develop the deep-water areas. However, exploitation of these deep-water areas. However, exploitation of these deep-water resources is expected to be technically feasible within the next 20 to 30 years.

Eocene Benthonic Foraminiferal Biostratigraphy and Paleobathymetry of Orphan Knoll (Labrador Sea)

A benthonic foraminiferal fauna of about 80 taxa is recorded from the Eocene of Orphan Knoll (Labrador Sea). The dominant supraspecific taxa are Cibicidoides (eight species), Anomalinoides (six species), Pleurostomella and Bulimina (five species each), Stilostomella (four species), and Chrysalogonium and Ellipsodimorphina (three species each). The numerically dominant specific taxa include Nuttallides truempyi (Nuttall), Oridorsalis ecuadorensis (Galloway and Morrey), Gyroidinoides girardanus (Reuss), Buliminella grata Parker and BermOdez, Stilostomella curvatura Cushman, S. aculeata Cushman and Renz, Cibicidoides hercegovinensis (de Witt Puyt), and Bulimina orphanensis, n. sp., described herein. The Stilostomella-Pleurostomella-Nuttallides assemblage suggests deposition at lower bathyal to abyssal depths (1000-2000 m.) and supports an earlier interpretation by geologists of DSDP Leg 12 that Orphan Knoll subsided rapidly to its present depth (1800 m.) during Paleocene time. INTRODUCTION The present paper is a contribution to a joint project between the Woods Hole Oceanographic Institution and the Soci6t6 Nationale des P6troles d'Aquitaine devoted to a better understanding of the stratigraphy and paleoecology of Cenozoic benthonic foraminifera, primarily in the circum-Atlantic and Mediterranean (Tethyan) regions. It may be viewed as a companion paper to recent studies on the Paleogene fauna of the North Atlantic: Late Paleocene and Early Eocene (Berggren, 1974) and Late Paleogene (Late Eocene and Oligocene) of HattonRockall Plateau (Berggren and Aubert, 1976). In this paper we report on the Eocene benthonic foraminiferal faunas of Orphan Knoll (Labrador Sea). Preliminary studies on these faunas were made by Berggren and Van Hinte (in Laughton, Berggren, et al., 1972, chap. 3) and Berggren (1972). GEOGRAPHIC SETTING Orphan Knoll is an isolated submarine feature at the foot of the eastern Canadian Continental Rise and is bounded on the northeast by the 4000-m.-deep abyssal plain of the Labrador Sea. It is situated about 550 km. northeast of Newfoundland and 350 km. north of Flemish Cap (text-figure 1). The Knoll is about 75 km. in width and extends in a northeast-southeast direction between latitudes 49? 45' and 51? 30'N. Deep-sea drilling by the Glomar Challenger in 1970 revealed that it is a small continental remanent separated from its parent block during the early stages of sea-floor spreading and continental drift either by differential sinking or lateral movement. The Knoll is separated into two parts (text-figure 2): a larger, southern part rises to depths of less than 1800 m., whereas the smaller northern extension rises to depths slightly less than 2400 m. The northeast margin is quite linear, has steep slopes of 30 to 40 degrees, and falls abruptly to the abyssal plain at 4000 m.; it is separated from the shelves of the Labrador and Flemish Cap by water 2800 m. to 3400 m. deep. The western margins of the Knoll are broadly semicircular in outline, with a canyon-like feature incised into the southern flank (text-figure 1). Although essentially flat-topped, seismic profiles reveal a series of pronounced bathymetric highs which interrupt the smooth surface. These have been interpreted as forming a series of continuous ridges lying subparallel to the linear northeast margin of the main southern part of the Knoll, and they rise in some places to about 500 m. below micropaleontology, vol. 22, no. 3, pp. 327-346, pls. 1-4, july, 1976 327 This content downloaded from 157.55.39.211 on Mon, 08 Aug 2016 06:07:06 UTC All use subject to http://about.jstor.org/terms

Queensland plateau and Coral Sea Basin: structural and time-stratigraphic patterns

The Western Coral Sea region contains one major and three minor marginal plateaux, partly surrounding a deep abyssal plain. The Coral Sea Abyssal Plain is underlain by about 1km sediment and oceanic crust generated by an early Eocene seafloor spreading phase. The large Queensland Plateau consists of subsided continental crust with basement rocks of Palaeozoic age which are tectonically part of the onshore Tasman Geosyncline. Graben, or continental features occur beneath the Queensland Trough, and beneath the plateau/basin margin. Here 1-3 kms of probable Upper Cretaceous rift valley sediments overlie basement. Subsidence of the plateaux followed seafloor spreading in the basin and marked the onset of post-breakup sedimentation. Lows in the plateau basement are infilled by Eocene sediments, while changing ocean current patterns gave rise to an Oligocene depositional break. Residual plateau highs along old Palaeozoic trends subsided in the early Miocene and are locally capped by modern coral reefs.