Frederick A. Bowles

Orinoco and Amazon River Sediment Input to the Eastern Caribbean Basin

Abstract Carbonate and mineralogic analyses of piston cores from Aves Ridge, eastern Caribbean Basin, reveal climatically modulated relationship among Quaternary carbonate deposition and terrigenous sediment influxes from the Orinoco and Amazon Rivers. Low carbonate content and dark sediment color, extending into the Pleistocene, suggest considerable dilution of pelagic components by terrigenous sediment on the western flank of Aves Ridge, which is characterized by a southwest-thickening sedimentary wedge. Contrary to typical trends observed in the more distal areas of the central basin west of Aves Ridge, carbonate content is lowest during interglacial climates and highest during glacial climates. A high clay-mineral input correlates with interglacial climate; however, the Orinoco tracer mineral pyrophyllite is low during interglacials and high during glacials. The observed trends suggest that Amazon sediment influx to the eastern Caribbean is highest during interglacials, or times of high sea level. The Amazon influx is promoted by deposition on the shelf and westward coastal transport of sediment during high sea level, and reduced by shelf-edge deposition and loss to the Amazon Canyon during low sea level. The Orinoco influx is similarly modulated, but it undergoes a relative (and possibly absolute) increase during low sea level because of proximity to the depositional site. The terrigenous dilution produces the reversal of carbonate trends. However, total sediment accumulation rates on southernmost Aves Ridge remain higher during glacials, possibly as a result of increased carbonate production caused by upwelling.

A depauperate benthic assemblage from the nutrient-poor sediments of the Puerto Rico Trench

Abstract The Puerto Rico Trench contains a low-density-biomass assemblage of macrofauna and meiofauna. The depauperate character of this assemblage is primarily the result of the low percentage and refractory nature of organic matter in the sediments. Recent turbidite flows may have contributed to the paucity of fauna. Sediments from this portion of the Puerto Rico Trench are the result of pelagic sedimentation interspersed with thin laminae that probably originated as turbidity flows from the northern and eastern, or deep-sea, walls of the trench. These recently deposited pelagic clays are low in available organic matter. Bioturbation and other mechanical and geochemical processes have not compacted these sediments into a stable, habitable substrate. The “oligotrophic” assemblages found in the Puerto Rico Trench contrast with “eutrophic” hadal assemblages which inhabit coarser-grained, organic-rich sediments. These types of sediments usually originate as turbidity flows from the walls of island-arc trenches proximal to shallow-water shelves and provide a substrate of higher shear strength and greater nutritive value.

Acoustic stratigraphy, structure, and depositional history of the Nicobar Fan, eastern Indian Ocean

Abstract The Nicobar Fan is a topographically isolated segment of the Bengal Deep-Sea Fan. Seismic reflection data indicate that the Nicobar Fan may have begun forming in late Miocene to early Pliocene time, but considerably later than the Bengal Fan. Since then, the original basement relief has been gradually inundated and buried by fan sediments, although numerous basement peaks still pierce the surface of the fan. Sediment thicknesses in excess of 1.6 km occur in the northern part of the fan. The fan surface is marked by turbidity-current channels and numerous faults which, in some cases, may have acted as channels. The faults are apparently related to tensional stresses associated with a descending lithosphere and to intraplate seismic activity. Relatively few channels are encountered on the fan, and they are noticeably absent on the northern part. Most of the sediment filling the northern Java Trench consists of depressed fan sediments, although deposition from local sources is evident. Plate motion has gradually isolated the fan from its major source of sediment to the extent that a buried “fossil” fan surface exists in the southeastern portion of the survey area, and much, if not all, of the remaining Nicobar Fan is now receiving only pelagic sediments.

Sediment properties, flow characteristics, and depositional environment of submarine mudflows, Bear Island Fan

Abstract Recent gravity flows on the Bear Island Fan consist of fine-grained sediments that presently exhibit high densities (1.8–2.0 g/cm 3 ), low water contents (30–40% dry weight), and high (estimated) yield strengths (1100–1350 Pa). Rheological analyses, however, reveal exceptionally low yield strengths (3.5 Pa and lower) for laboratory slurries ( 3 ) made with the mudflow sediments. The low yield strengths are inconsistent with previous interpretations that the mudflows were spawned from glacial sediments that were rapidly deposited as till deltas on the upper slope of the fan, or that they were emplaced as low-density, low-cohesion, fluidized mud. Abundant glacial flour in the mudflow samples is indicative of subglacial meltwater discharge. This, in turn, suggests that the sediment originated from turbid plumes that settled into a low-density, high-water-content mud deposit in an open-marine, tidewater glacier environment. Such an environment may have existed during the mid/late Weichselian when an embayment in the Barents shelf ice-sheet left much of the inner shelf ice-free. Subsequent advance of the ice front closed the embayment, compressing the mud. Loss of water during compression increased the bulk density and yield strength of the mud. The modified embayment mud was gradually pushed off the shelf, generating mudflows on the fan. Morphologic evidence suggests that some mudflows may have hydroplaned, indicating that flow speeds varied above and below a critical speed (ca. 5 m/s) marking the onset of hydroplaning.

Geological/geophysical observations and inferred bottom-current flow: South flank Iceland—Faeroe Ridge

Abstract The surface morphology of the Iceland—Faeroe Ridge and Hatton Drift, as shown by seismic reflection (3.5 kHz) and bottom photographic data, is predominantly shaped by vigorous bottom currents but also influenced, in the case of the ridge, by a generally smooth acoustic basement of probable basaltic composition. Redistribution by the currents has sculpted the sediment on the south flank of the ridge into lens and wedge-like drift deposits having remarkably smooth surfaces and uniform internal stratification. In other areas, notably over large areas of the ridge crest, acoustic basement appears to be exposed. Estimated current velocities range from a few centimeters per second on off-ridge areas up to and perhaps greater than 30 cm s −1 on the ridge itself. Three distinct current flows can be differentiated: an intense mid-flank flow representing the outflow of Norwegian Sea Deep Water from the Faeroe Channel; a moderately strong flow on the upper flank produced by water passing over the ridge crest; and a weak to moderate flow on the lower flank which is a continuation of the north-flowing Hatton Drift current. Discontinuous, channel-like features on the eastern part of the ridge flank are attributed to the erosive action of the currents as they accelerate around topographic highs. In contrast, the extreme western end of the ridge and Icelandic slope is cut by several turbidity current channels which transport basaltic debris from Iceland to the Maury Fan. Turbidite deposits account for the characteristic prolonged echo type exhibited in the 3.5-kHz profiles across the upper fan; the south flank and Hatton Drift contourites produce sharp bottom echoes with continuous, parallel subbottom reflectors, whereas sharp bottom echoes with no subbottom reflectors typify the ridge crest and outcrop areas. Sediment thicknesses on the ridge range from virtually zero on the crest to usually 0.3 s on the lower flank. Near Iceland and over the Maury Fan the sediment accumulation may exceed 1.0 s.

Identification of potential sites for deep-ocean waste isolation with a geographic site-selection model

Abstract Identification of optimal sites for the isolation of waste on the abyssal seafloor was performed with two approaches: by the traditional method of map overlays of relevant attributes, and by a specially developed, automated Site-Selection Model (SSM). Five initial, Surrogate Sites, identified with the map-overlay approach, were then compared with the more rigorously produced scores from the SSM. The SSM, a process for optimization of site locations, accepts subjective, expert-based judgments and transforms them into a quantitative, reproducible, and documented product. The SSM is adaptable to any siting scenario. Forty-one factors relevant to the isolation scenario, including 21 weightable factors having a total of 123 scorable categories, have been entered into the SSM. Factors are grouped under project definition, unique environments, anthropogenic, geologic, biologic, weather, oceanographic and distance criteria. The factor scores are linked to a georeferenced database array of all factors, corresponding to 1°×1° latitude–longitude squares. The SSM includes a total of 2241 one-degree squares within 1000 n.m. of the U.S. coasts, including the western North Atlantic, the Gulf of Mexico, and the eastern North Pacific. Under a carefully weighted and scored scenario of isolation, the most favorable sites identified with the SSM are on the Hatteras and Nares Abyssal Plains in the Atlantic. High-scoring sites are also located in the Pacific abyssal hills province between the Murray and Molokai Fracture Zones. Acceptable 1° squares in the Gulf of Mexico are few and of lower quality, with the optimum location on the northern Sigsbee Abyssal Plain. Two of the five Surrogate Site locations, on the Hatteras and Sigsbee Abyssal Plains, correspond to the best SSM sites in each ocean area. Two Pacific and a second Atlantic Surrogate Site are located in low-scoring regions or excluded by the SSM. Site-selection results from the SSM, although robust, are an initial attempt to quantify the site-selection process. The SSM database exposes a significant lack of high-quality information for many areally mappable attributes on the abyssal seafloor, particularly bottom-current speed and measures of biologic productivity and flux. Terminologies and classifications of some measures, such as sediment types, suffer from parochialism and vary by ocean. Considerable research is needed even for a broad understanding of the environmental measures required to make sound societal decisions about use of the abyssal seafloor for disposal or other purposes.

Stratigraphy and sedimentation of the archipelagic apron and adjoining area southeast of Bermuda

Abstract Seismic reflection records collected east and south of the Bermuda Pedestal by USNS “Kane” and USNS “Wilkes” show that the acoustic basement morphology throughout most of the area is typical of rugged ocean crust; only around the base of the pedestal and within a lobe to the southeast does basement appear noticeably smoother. Structural trends, revealed by bathymetric and isopach maps, tend to parallel the mid-ocean ridge. The sediments above basement vary in thickness up to 1.5 sec. A belt of thick sediment (1.0 sec or greater) around the base of the pedestal indicates a structural moat and also reflects the input of erosional debris from the pedestal in addition to the normal pelagic sedimentation. Locally, the sediments around Bermuda are separated into upper and lower sections by a zone of acoustically laminated sediments. The zone correlates with a sequence of upper Eocene to upper Oligocene volcanoclastic turbidites recovered at DSDP site 386 and is interpreted as an early archipelagic apron and plain which has since been deformed by basement movements and differential compaction. Older reflective horizons also occur locally to the south-southeast of Bermuda and may correlate with lower Eocene siliceous and carbonate turbidites at site 386. The distribution pattern of these sediments suggests a source (or sources) other than Bermuda, possibly the continental margin of North America. A second major erosional and depositional phase, apparently resulting from the Pleistocene sea-level fluctuations, is represented by ponded, reflective sediments which form the sea floor at the base of the pedestal. Depositional asymmetry indicative of bottom-current activity is evident throughout the survey area; a vigorous abyssal circulation appears to have been active since before the upper Eocene. No evidence was found indicating the existence of a major seamount structure predating the present Bermuda Pedestal, although a small, incipient “bulge” is not excluded.

Diminished amplitude reflections (fadeout) in Bermuda Rise stratification

Abstract Single and multichannel deep-tow seismic records collected on the southwestern Bermuda Rise reveal areas within the acoustic layering where the intensity of reflectors fade or even disappear over lateral distances of 20–60 m. The fadeouts are conspicuously present in a highly stratified interval consisting of lower to middle Eocene siliceous turbidites and thin, cherty layers interbedded with hemipelagic/pelagic clays and oozes. Acoustic scattering at rough interfaces, gas pockets, and changes in geoacoustic properties do not readily account for the diminished amplitudes of the reflections. Detailed examination of the wave coda from selected fadeout areas reveal changes in reflector geometry due to faulting of the sediments. Strong reflections within the stratified sequence are thought to result from the thin, high impedance chert beds that occur within the turbidite layers. Secondary reflections (multiples) generated by the hard, chert interfaces are thought to interact constructively with primary reflections to generate strong interference reflections, but also destructively to cause fadeouts where the faulting has changed the layer configuration. Alternatively, the fadeouts may result from the dissolution of chert beds due to the upward migration of silica-deficient pore fluids along faults and fractures in response to excess pore pressures.