Hatteras Abyssal Plain Research Articles

On the cycling of 231Pa and 230Th in benthic nepheloid layers

The naturally-occurring radionuclides protactinium-231 (231Pa) and thorium-230 (230Th) are produced at approximately uniform rates in the ocean and thought to be removed from the water column through a reversible exchange with settling particles. Recent measurements along the U.S. GEOTRACES North Atlantic transect (GA03) revealed two features which are at odds with current understanding about 231Pa and 230Th cycling in the ocean: (i) a sharp decrease in dissolved 231Pa (231Pad) and 230Th (230Thd) activities with depth below 2000–4000 m and (ii) very high particulate 231Pa (231Pap) and 230Th (230Thp) activities near the bottom, at a number of stations between the New England continental shelf and Bermuda. Concomitant measurements of light attenuation from beam transmissometry showed that both features occur in benthic nepheloid layers (BNLs), which suggests that these features may stem, at least partly, from the presence of resuspended sediment in the deep water column.Here we explore the behaviour of 231Pa and 230Th in BNLs by using (i) radionuclide, optical, and hydrographic data from the western segment of GA03 (west of Bermuda) and (ii) a simplified model of particle and radionuclide cycling that includes a lateral particle source. First, the BNLs observed at GA03 stations are characterized from measurements of the beam attenuation coefficient converted to particle concentrations. At all stations, particle concentrations below the clear water minimum were the highest in the bottom mixed layer, whose thickness ranged from 95 to 320 m, and decreased generally with height above the bottom. The thickness of strong BNLs varied from 482 to 1358 m and the vertical integral of particle concentration in excess to that at the clear water minimum varied from 1×104 to 2×106 mg m−2, among different stations. Second, the particle-radionuclide model is fitted to data from stations GT11-04 (New England continental rise) and GT11-08 (Hatteras abyssal plain), where samples for radionuclide analyses were collected in the BNL. The model can reproduce simultaneously the increase of particle concentration with depth, the low 231Pad and 230Thd in the BNLs, and the high 231Pap and 230Thp near the bottom. According to the model, at heights less than about 300 m above the seafloor, the dissolved phase was set primarily by a balance between adsorption and desorption, with vertical turbulent mixing playing a secondary role, whilst the particulate phase behaved largely as a non-reactive constituent supplied laterally and transported vertically by particle settling and turbulent mixing. Sensitivity tests with the model suggest that lateral particle sources near continental slopes and similar reliefs can produce significant biases both in the 230Th normalization method and in the interpretation of sediment 231Pa/230Th records. Our findings yield insights into the influence of sediment resuspension and transport on 231Pa and 230Th in the deep ocean and highlight the need for considering these processes in paleoceanographic applications.

Deep-sea parasitic nematodes of the genus Trophomera Rubtsov et Platonova, 1974 (Benthimermithidae) from the Equatorial Atlantic, with the descriptions of two new species

Nematode females of the genus Trophomera (Benthimermithidae) from the collection of the Smithsonian’s National Museum of Natural History (Washington, DC, USA) were examined. Nematodes were collected in different parts of the Western Atlantic (Hatteras Abyssal Plain, Brazil Basin, and Argentina Basin) from depths of 467–5,223 m. Two new species are described. Body length of T. americana sp. n. is 3,250–4,470 μm; posterior end conical with rounded tip; cephalic setae about 3–4 μm long; trophosome consisting of several longitudinal rows of large cells; ovaries reflected; mature eggs 35 μm in diameter. Body length of T. longiovaris sp. n. is 7,870–15,400 μm; posterior end conical with rounded tip; cephalic sensilla 7 μm long; mouth opening vestigial, present as very narrow apical pore; pharynx devoid of internal lumen and muscular envelope; midgut represents a trophosome without internal lumen; trophosomal cells arranged in 3–4 longitudinal rows; rectum and anus vestigial; female reproductive system didelphic, amphidelphic, very long, occupying about 0.8 total body length; ovaries telogonic, outstretched; oviducts very long, repeatedly folded across body axis; proximal parts of oviducts being than distal ones, uterus distinctly formed. New finds of two known species, T. arnauidi and T. marionensis, are also recorded and described.

Faunal zonation of large epibenthic invertebrates off North Carolina revisited

The dominant populations of large epibenthic megafauna off North Carolina were mapped in the 1960s from the upper continental slope out to the Hatteras Abyssal Plain using multi-shot sea-floor photography. The present re-analysis of the original data using contemporary computer-based methods and geo-referenced mapping (GIS) reveal that the overall patterns inferred initially can be substantiated. Individual species occurred in narrow depth bands that hugged the topography along the entire sampling area, but multi-species assemblages emerge with the modern, more formal quantitative methods.

Desmosomatidae and Nannoniscidae (Crustacea, Isopoda, Asellota) from bathyal and abyssal depths off North Carolina and their evolution

The deep-sea asellote isopod (Crustacea) species, belonging to the families Desmosomatidae and Nannoniscidae, were studied from R/V Eastward collections of Duke University from three study sites off North Carolina. The Desmosomatid isopod Eugerda svavarssonni n. sp. is described from Site Alpha at 620m. Eugerda latipes Hansen, previously known from boreal North Atlantic, is also reported from this upper slope site. Two new desmosomatid isopods, Mirabilicoxa hessleri n. sp. and M. alberti n. sp., and a new nannoniscid isopod Exiliniscus chandravoli n. sp. are also described from site Beta from 2700 to 3700 m. A nannoniscid isopod Leutziniscus jebamoni gen n., n. sp., and a desmosomatid isopod, Prochelator sarsi n. sp., are described from the deepest study site Omega in the Hatteras Abyssal Plain. A new genus, Nannoniscella is erected to accommodate Nannoniscoides bicustatus Siebenaller and Hessler, 1977. This paper also includes discussions on comparative morphology and sexual dimorphism in species of Mirabilicoxa, zoogeography of species of the genus Eugerda and phylogeny with emphasis on the genus Prochelator.

Hydrodynamical dispersion of dredged materials sequestered on the abyssal seafloor

This paper addresses the potential dispersion by hydrodynamical processes of dredged material disposed on the abyssal seafloor off the east coast of the United States. The importance of the large-scale mean currents and of the eddy kinetic energy of the large-scale flow are discussed both for the selection of potential isolation sites and for determining the potential dispersion pattern from selected sites. These processes play a particularly significant role in the Atlantic off the east coast of the United States. There the potential isolation sites are close to a highly energetic region of the abyssal circulation, the North Atlantic Deep Western Boundary Current. The Naval Research Laboratory Layered Ocean Model was used to simulate large-scale ocean circulation in the North Atlantic Subtropical Gyre. Numerical experiments were conducted using monthly climatological forcing. High horizontal resolution, 1/16°, was required to achieve a realistic level of kinetic energy in the deep ocean and a realistic deep mean flow. The prognostic variables came to statistical equilibrium after 50 years of simulation. Then the mean currents and eddy kinetic energy were calculated based on 4 years of simulated data. The patterns and magnitudes of the currents and kinetic energy from the simulation agreed with the observations. The simulated pathways of the Deep Western Boundary Current including bifurcations and reconnections conformed to current concepts of its circulation. The recirculation gyres under the Gulf Stream were simulated, as were the eddies, meanders and high levels of eddy kinetic energy northeast of the New England Seamount chain. The decrease in eddy kinetic energy to the south over the Hatteras Abyssal Plain and the weak currents in the region of 25°N to 30°N along 70°W were also in agreement with observations and verified the results of the site selection model. Tracers were released into the 1° squares that were chosen by the NRL site selection model as the best candidates for isolation of dredged material. Modest quantities of the tracers were dispersed beyond the initial release areas and only trace amounts were mixed out of the bottom layer of the model. The results of this study indicate that if contaminated material was released into the water column from isolation sites in the region chosen by the site selection model it would probably be dispersed over a limited area. However, the dispersion of the tracers was anisotropic and varied spatially among the sites. This underscores the need to conduct both boundary layer-scale and gyre-scale numerical simulations and careful measurement programs at the sites chosen for initial evaluation of deep-ocean isolation of dredged material. The next step is to run mesoscale models that use the results from this study as boundary conditions and the results from plume models of contaminant release to determine the amount of contaminated material that might leak out of the boundary layer into the deep-ocean circulation system.

Abyssal current influence on the southwest Bermuda Rise and surrounding region

Echograms (3.5 kHz) and bottom photographs reveal that the northward flowing Antarctic Bottom Water (AABW) has strongly influenced the modern depositional regime on the southwest Bermuda Rise. The spatial distribution of echo character types, the orientation and nature of current-controlled structures, and limited current meter data show that AABW flows with varying intensities along three primary pathways around and over the southwest Bermuda Rise. The main core of AABW flows clockwise around the eastern and western flanks of the southern Bermuda Rise, roughly parallel to the 5400 m isobath. This current bifurcates at 28°30′N, 69°W where a portion flows northeast over the southwest Bermuda Rise and the remainder continues north along the physiographic boundary between the southwest Bermuda Rise and the Hatteras Abyssal Plain. Secondary ribbons of AABW branch off the main core of AABW during its southerly journey along the southeastern Bermuda Rise, and flow west through fracture zones. Finally, a diffuse, northward flowing AABW sweeps the entire southwest Bermuda Rise. A progression of current-controlled bedforms occurs beneath the main path of the AABW reflecting the spatially varying current velocities and sediment supply. The main core of AABW flows west through the narrow Vema Gap creating erosional furrows along the border between the southwest Bermuda Rise and the Vema Gap. Current velocities greater than 20 cm s −1 are inferred from the bedforms in this region. Farther north along the southwestern edge of the Bermuda Rise, sediment waves become more prevalent. This transition from erosional to more depositional bedforms results from diminished current velocities (5–15 cm s −1) and increased sediment supply. Although some of these bedforms on the southwest Bermuda Rise appear to be relict, their orientation is consistent with current meter data and abyssal current direction inferred from bottom photographs.

Nematode assemblages of deep-sea and shelf break sites in the North Atlantic and Mediterranean Sea

The nematode community structure of several shelf break and deep-sea stations in the North Atlantic and the Mediterranean is compared using data from the literature. Samples from 2 Mediterranean deep-sea canyons have a unique fauna of predatory and scavenging nematodes (e.g. Synonchiella, Halichoanolaimus, Gammanema) which comprise some 20 to 30% of the community, while some of the typical deep-sea genera (Theristus, Acantholaimus) are only marginally present. The nematode generic composition of the other sites reveals a gentle transition from the shelf break (Bay of Biscay; Mediterranean) over the slope (Mediterranean) towards the continental rise, abyssal plain and hadal depths (Bay of Biscay, Puerto Rico Trench, HEBBLE site, Hatteras Abyssal Plain). At all these sites predators and omnivores are less abundant (

The genesis and character of benthic turbid events, Northern Hatteras Abyssal plain

Benthic turbid events, with particle concentrations of up to 1.5 mg l −1, occupy about 20% of a two-year record of near-bottom light attenuation made above the Northern Hatteras Abyssal Plain. Theyare most often associated with deep-sea mesocle eddies, although rare events occur during periods when southward contour flow predominates. In a temporal sense, the presence of deep-sea mesoscale eddies corresponds strongly to periods when cold-core Gulf Stream rings propagate through surface waters of the study area. Additionaly, over half of the benthic turbid events observed are initiated within a day of the passage of atmospheric storms with propagation speeds in excess of 800 km day −1.

Observations and EOF Analysis of Low-Frequency Variability in the Western Part of the Gulf Stream Recirculation

Abstract This study of low-frequency oceanic variability is based on data collected during the Long Term Upper Ocean Study (LOTUS), which was a two year program of (mainly) moored meteorological and oceanographic measurements. The mooring arrays were centered at 34°N, 70°W over the Hatteras Abyssal Plain. With a distance of about 300 km to the mean Gulf Stream axis and the continental slope, LOTUS was the most northern and western long-term mooring site in the Gulf Stream recirculation region to date. The observed low-frequency variability is dominated by zonally elongated motions of the secular time scale (periods <100 days) even great depths (4000 m). In contrast to observations in other parts of the recirculation region, the spectral shapes are strongly depth dependent. The vertical structure of the variability was examined by EOF analysis. Different kinds of EOFs were tested; the best representation of the observed variability was obtained by an EOF representing a undirectional nonrotating (with depth...

Organic carbon turnover time in deep-sea benthos

The ratio of organic carbon supply to the deep-sea floor estimated with sediment traps to total benthic biomass (in terms of organic carbon concentration) was used to calculate “average” turnover or residence times for the benthic community over a broad depth interval in the western North Atlantic. Ratios indicate that the biomass carbon turns over on average time scales of months, with little predictable variation over the depth interval (70m to 5300m). Detrital organic carbon in sediments at the same stations turned over on scales of years to centuries. The shortest detrital carbon turnover was just over 8 years in a sandy continental shelf environment versus 609 years at 5.3km depth on the Hatteras Abyssal Plain. The turnover time estimates calculated with POC fluxes were assessed by comparing POC fluxes with estimates of total community respiration. At each location the POC flux rates were greater than the sediment oxygen demand flux rates (in mg Carbon m −2d −1), and the disparity was much greater in the deep sea than on the continental margin. We suggest that this difference is primarily the result of a decline in the quality of the organic compounds in the rain of sinking POC, rather than physiological responses to cold or pressure. This implies that caution must be used when interpreting the relationships between the total organic carbon fluxes measured with sediment traps at great depths and rates of metabolic processes.

Ecology of deep-sea nematodes from the Puerto Rico trench area and Hatteras Abyssal plain

Nematode assemblages from a bathyal silty-sand and three hadal silty-clays from the Puerto Rico Trench area were compared with that from a silty-clay from the HatterasAbyssal Plain. Fine sediments at each site were largely dominated by species belonging to three major genera: Theristus, Acantholaimus and Halalaimus. At the bathyal sand site the relative abundances of these genera were reduced and those of several others (e.g. Desmodora, Leptolaimus, Camacolaimus, Tricoma, Greeffiella, Setoplectus) elevated. Average body weight of nematodes occurring in the sand was 0.057 μg (dry weight), approximately 20–50% that of the nematodes found in the silty-clays. It is suggested that in the deep sea, as at shallow depths, smaller nematodes are better able to move in heterogeneous, coarser sediments than in more uniform, fine ones. Deposit-feeding species were the dominant feeding type observed in all sediments, but epistrate feeders were most numerous in the bathyal sands, probably the result of increased resource partitioning by worms inhabiting a more heterogeneous sedimentary environment. Species richness in bathyal sands was also high. Normal classification of assemblages found in sediments from the Hatteras Plain, Puerto Rico Trench areas, Venezuela Basin and upper abyssal province off North Carolina suggests that deep-sea nematode families and genera are widely distributed across the Antillean Arc. However, species endemism is high: 67 and 77% of the species identified from the Hatteras Plain and Puerto Rico Trench respectively, occurred only at these sites. Affinities among species from all four sites were very low. The wide distribution of families and genera, and narrow distribution of species, suggests that current radiation of deep-sea nematodes probably occurs at the species level.

Meiobenthos of the hatteras abyssal plain and Puerto Rico trench: abundance, biomass and associations with bacteria and particulate fluxes

An abyssal station on the Hatteras Abyssal Plain (5411 m) and two hadal stations in the Puerto Rico Trench (7460 and 8189 m) were sampled to obtain quantitative information on the abundance and biomass of metazoan meiofauna and, for comparative purposes, bacteria and measurements of particulate flux rates. Average meiofauna abundance (no./10 cm 2 ± 1 S.E. integrated over a sediment depth of 15 cm) was lowest at the 7460 m site (44 ± 10) and highest at the 8189 m (96 ± 15) and Hatteras sites (114 ± 26), the latter two not being statistically significant from each other. Biomasses (μg dry wt/10 cm2 ± 1 S.E., also integrated over a 15 cm depth) at the Hatteras and 7460 and 8189 m Trench sites were 38.4 ± 10.3, 3.8 ± 1.6 and 14.3 ± 5.1, respectively; all differences were statistically significant. High biomass at the Hatteras site was due to large, burrowing harpacticoid copepods that were found to depths of 10–15 cm below the sediment surface, and which were most abundant below the surface (0–2 cm) layer of sediment. Nematodes, the numerically dominant taxon, were most abundant in the upper 6 cm of sediment at all sites, as were copepods at the two Trench sites. Average abundances of bacteria (no. of cells × 10 9 per 10 cm 2 of sediment to a depth of 15 cm), determined by epifluorescence microscopy, were 11.0, 10.5 and 5.6 at the Hatteras, 7460 and 8189 m Trench sites, respectively. Flux rates of organic carbon and total nitrogen (mg m −2 day −1) were about four times higher at the Hatteras site (e.g. 25.8 and 3.8) than at the 7460 m site in the Trench (6.3 and 0.8). An analysis of the meiofaunal and bacterial abundances obtained at the three sites revealed no significant associations between the two organismal groups, though abundances of both groups did decrease significantly and predictably with sediment depth at each station. Associations between particulate flux rates and meiofauna biomass were highly significant, suggesting that flux rates measured in near-bottom traps may be useful indicators of general resources available to the meiobenthos.

Strength and stress‐strain behavior of clays from the nares abyssal plain

Abstract The Nares Abyssal Plain is situated in the Western North Atlantic between 22° and 25° N and 60° and 67° W. This distal plain is connected to the Hatteras Abyssal Plain by the Vema Gap. The study site contains pelagic and hemipelagic sediments with upper meters containing predominantly finegrained turbidites. Large diameter piston cores up to 26 meters in length were obtained within the southern region of the Nares. This study focused on the stress‐strain and strength properties of the pelagic clay units of the sediment column. A significant portion of the sediment column was found to be in a state of under‐consolidation attributed to impeded drainage caused by apparent over consolidation of the surficial zone and significant variations in permeability between pelagic and turbiditic layers. Effective friction angles ranging from 19.9 to 31.6 degrees were generally higher for isotropically as opposed to anisotropically consolidated triaxial series. Additionally, there were correlations between fric...

Areal and lateral changes in a major trailing margin turbidite—The Black Shell Turbidite

The Black Shell Turbidite on the Hatteras Abyssal Plain covers at least 50,000 km2, with a volume over 100 km3. It was initiated by failure on the upper continental slope and was channeled southeast through Hatteras Canyon to the plain. Provenance related shape studies indicate that on the plain the current separated into a sandy Phase which flowed S-SE and a lutitic phase, which traveled E-SE and then veered to the south. A change in the direction of slope caused the sandy phase to be deflected to the SE, where it merged with the lutitic phase on the eastern margin of the plain.

Organic carbon oxidation and preservation in NW Atlantic continental margin sediments

Summary We present distributions of pore-water inorganic metabolites (nitrate, ammonia, manganese and iron), dissolved organic matter (DOM) including dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) and solid-phase organic carbon from the Hatteras Abyssal Plain, the Hatteras Continental Rise and Slope and the northern Bermuda Rise areas of the eastern United States seaboard. Organic carbon oxidation rates in the sediments were calculated from models of the pore-water inorganic metabolites and the solid-phase organic carbon distributions. The pore-water model results were consistently higher than rates determined from the sediment organic carbon distribution. This result may reflect an inappropriate use of bulk sediment mixing coefficients in simple solid-phase models to estimate rates of interfacial diagenetic processes. The organic carbon oxidation rates in the sediments were combined with the organic carbon burial rates to assess the particulate organic flux into the sediments, and these results were compared with the particulate organic rain caught in sediment traps. Trap fluxes were consistently higher than model-estimated fluxes. We cannot make an unequivocal interpretation of the organic carbon mass balance, but we suggest that rates of benthic metabolism estimated from inorganic metabolite and sediment organic carbon distributions underestimate rates of organic carbon cycling because some fraction of the particulate organic carbon rain caught in traps is remineralized as DOM. Pore-water enrichments of DOM, DOC and DON were a factor of 5–10 higher than bottom-water concentrations, suggesting a flux of DOM to bottom waters. DOM fluxes to bottom waters contribute to abyssal metabolism in the benthic boundary layer and not to sediment metabolism.

Radon 222 in the benthic boundary layer

A detailed survey of radon 222 and temperature profiles of the benthic boundary layer in the Hatteras Abyssal Plain shows a strong correlation between the structure of both. The apparent vertical diffusivities estimated from radon 222 are of the order of 50 cm2 s−1 in the mixed layer, and of the order of 1 cm−2 s−1 above it. One profile appears to have been taken in a frontal zone where isotherms dip sharply into the sediments. This is the only profile where there is significant penetration of radon above the mixed layer. Several other profiles suggest that the temperature may take longer than several radon half‐lives to adjust to new mixing and advection regimes. In such cases, one often sees considerable structure in the radon profile within the region where the potential temperature is well mixed.

Quantitative evaluation of bioturbation rates in deep ocean sediments. II. Comparison of rates determined by 210Pb and 239,240Pu

Excess 210Pb was determined in six abyssal cores from the Eastern Atlantic, Hatteras Abyssal Plain, and Puerto Rico Trench. A bioturbation rate for each core was evaluated using the mixing model of Guinasso and Schink. The rates were compared to mixing rates evaluated for the same cores using 239,240Pu. In all cases, the excess 210Pb-derived mixing rates were lower than the plutonium-derived mixing rates. 239,240Pu was also present at greater depths than excess 210Pb. One possible explanation for these observations is the association of plutonium with particles of greater food value which are bioturbated at faster rates.

Preliminary results from the long-term upper-ocean study (LOTUS)

The Long-Term Upper-Ocean Study (LOTUS) is a two-year experiment at 34°N, 70°W, in deep water over the Hatteras abyssal plain, 330 km from major topography and the mean path of the Gulf Stream. The observations began in May 1983. The principal measurements are from an array of three subsurface moorings and one surface mooring carrying meteorological instrumentation as well as near-surface current meters. Some basic meteorological and engineering data are telemetered back via the ARGOS system, but most of the data are self-recorded. Other observations in the program include XBT sections, CTD profiles, and cooperative efforts with other investigators to obtain satellite imagery and other in situ measurements. The main scientific goals of the experiment are: to obtain a description of the local response of the upper ocean to a potpourri of atmospheric forcing in a variety of environmental situations, such as in the presence of Gulf Stream rings or near oceanic fronts, and even to try and describe the statistical response to the fluctuating forcing; to investigate the energy level of the internal wave field over several seasons and during the passage of various forcing and environmental events as mentioned above; and to obtain the complete profile of eddy kinetic energy. We display here selected results from the first year of LOTUS, especially those that do not require the analysis of the complete array of measurements and supporting data. Of particular interest is the eddy energy kinetic energy profile showing a nearly constant energy in the top 500 m; the abyssal eddy energy is also about half the expected level for the site. The high-frequency internal wave field at 200–500 m depths shows a seasonal variation (high in the winter, low in the early fall) that cannot be ascribed to changes in the hydrography; at greater depths the winter maximum is still visible. At each depth the energy varies from a half or a third to 2 or 3 times the mean energy at that depth. Some of the energetic events in the near-inertial field are apparently unconnected to local 3ind events, and may instead be related to the edge of strong mesoscale oceanic currents; perhaps they are being radiated during the process of geostrophic adjustment by the mesoscale features.

An inverse method for reconstructing the density and sound speed profiles of a layered ocean bottom

A standard inverse problem in underwater acoustics is the reconstruction of the ocean subbottom structure (e.g., the density and sound speed profiles) from an aperture- and bandlimited knowledge of the reflection coefficient. In this paper we describe an inverse solution method due to Candel et al. [12] which is based on the scattering of acoustic plane waves by a one-dimensional inhomogeneous medium. As a consequence of applying the forward scattering approximation to a local wave representation of the acoustic field, they obtain an expression for the reflection coefficient in the form of a nonlinear Fourier transform of the logarithmic derivative of the local admittance. Inversion of this integral transform enables the recovery of the admittance profile via the numerical integration of two first-order differential equations which require as reflection data a single impulse response of the medium. Separate recovery of both the density and sound speed profiles requires two impulse responses for two different grazing angles. In this case, four differential equations need to be integrated instead of two. To illustrate the capability of the method, we present numerical reconstructions which are based on synthetic reflection data for a geoacoustic model that represents the acoustic properties of the surficial sediments for a site in the Hatteras Abyssal Plain.

Special issue on oceanic seismic exploration - Introduction to the issue

CEANIC SEISMIC EXPLORATION constitutes an important area of bottom-interacting ocean acoustics. It is concerned with the probing of the ocean bottom and subbottom by means of acousticsignals artificiallygeneratedin the upper layers of the ocean. The signals involved are of low frequencies, ranging typically from nearly zero to a few kilohertz. Due partly to the increased interest and activity in offshore petroleum exploration, oceanic seismic exploration has received considerable impetus in recent years, and is likely to draw increasing attention in the years to come as developing technology enables humankind to begin untapping the immense organic, fluid, and mineral resources lying below our oceans. It is therefore fitting that in the month in which the Institute of Electrical and Electronic Engineers enters the second century of its existence, this journal should devote attention to this important area. In this special issue there are seven papers of particular interest covering various topics in this field. The first paper by D. A. Fisher and G. H. F. Gardner is concerned with the problem of fonvard modeling of the seafloor and basement from seismic reflection data. The common and relative merits of numerical and physical modeling are discussed in the context of two examples. The next two papers, respectively by J. A. DeSanto and D. J. Thomson, are devoted to the issue of inversion of acoustic field data. The paper by DeSanto presents a modal (full-wave) method, based on the Born approximation, for retrieving the ocean sound-speed profile from the values of the transmitted acoustic field measured by an array. On the other hand, the paper by Thomson separately reconstructs the density and sound-speed profiles of the ocean subbottom, by a method for which the theoretical foundations were provided by Candel et al. [ 13 ~ and presents some numerical simulations of this approach for a geoacoustical model of the Hatteras Abyssal Plain deep sea sediments. A closely related topic is discussed in the paper by N. R. Chapman, I. Barrodale, and C. A. Zala, who present a novel approach for obtaining sound speed and sound-speed gradient in surficial seafloor sediments from bottom-reflected seismic signals by an I1-nomz (least absolute value) deconvoltion method. The next two papers are concerned respectively with the analysis and measurement of ocean-acoustic data from towed line arrays. The paper by L. R. LeBlanc presents a method for separating the data measured by a linear acoustic towed array into statistically uncorrelated components and then interpreting the results in terms of angular distribution of energy of the components. The paper by W. Dragoset and K. Lamer demonstrates the data enhancement capabilities of a spstem which uses a recently developed 500-channel digital streamer cable in seismic reflection data collection. Finally, the paper by J. M. Berkson and J. E. Matthews presents an interesting characterization and discussion of the statistical properties of seafloor roughness gleaned from narrow bandwidth echo sounding and seismic reflection profiling. In closing, we would like to thank the authors for having so diligently prepared their contributions for this issue in a very short period of time and to Stan Ehrlich, Editor of this JOURNAL, for his help and advice in all matters concerning this issue.