Energy Benthic Boundary Layer Experiment Research Articles

Sediment transport studies—A brief retrospective look

In this paper, we present a short retrospective look at the evolution of acoustical, optical, and related measurements of sediment transport, bottom stress, and bottom bedforms over the past quarter century, using our programs at the Woods Hole Oceanographic Institution as a “representative sample.” Results from the High Energy Benthic Boundary Layer Experiment, the Sediment Transport on Shelves and Slopes experiment, and the Strata Formation on Shelves and Slopes experiment will be shown. Emphasis will be given to both technological and scientific advances. [Work supported by ONR.]

Contourites of the Nova Scotian continental rise and the HEBBLE area

Abstract The Nova Scotian continental rise is swept by a Deep Western Boundary Current system comprising layers of Labrador Sea Water overlying a core of Norwegian Sea Overflow Water at depths of 3100-3900 m, and below about 4600 m a cold stream of southern-source water. Seismic-reflection data show that the rise contains sediments transported downslope in channels and debris lobes, but there is also evidence of current-controlled deposition and erosion in the post-Eocene sequence. The rise is now mantled by Holocene contourites that have accumulated at a rate of c . 6 cm ka -1 and are <1 m thick. Bottom photographs show a zonation in current effects and bedform types, with longitudinal ripples and strong currents prevalent at 4800-5000 m, smaller bedforms and progressively weaker currents up to c . 4000 m, and mostly tranquil seafloor above 4000 m. Bedform scales and orientations also suggest significant short-term (hours to weeks) variability in current velocity but a mean contour-following flow to the southwest at longer time scales (months to years). These structures are not preserved in the sediment because of pervasive bioturbation and the uppermost layers have negligible preservation potential. The sediments display clear current controlled effects in their grain-size structure involving both percentage of (foraminiferal) sand, and size and percentage in the 10-63 µm range, the 'sortable silt'. There is a sand-rich zone at 4800-4900 m and below 5000 m, and a decreasing silt/clay ratio from 5100 m up to 4000 m. Although much of the sedimentary sequence probably has been emplaced by downslope processes, it has been significantly modified by the Deep Western Boundary Current. Particularly strong and variable currents which rework sediments below c . 4800 m probably are engendered by interaction of Gulf Stream eddies with the DWBC. Although strong currents and upstream input from tur-bidites and debris flows might be thought to favour a coarse-grained deposit, the facies at the HEBBLE (High Energy Benthic Boundary Layer Experiment) site is muddy contourite with ≤ 12% sand.

Is the HEBBLE isopod fauna hydrodynamically modified? A second test

Several times per year, the High Energy Benthic Boundary Layer Experiment (HEBBLE) site (4820 m depth, 40°27′N 62°20′W) experiences benthic storms during which near-bottom flows can erode millimeters of sediment. Thistle and Wilson ( Deep-Sea Research, 34 1987, 73–87) predicted that isopods that inhabited the surface of the sediment would be relatively rare at the HEBBLE site compared to those at quiescent deep-sea sites. They tested this prediction by comparing the composition of the HEBBLE isopod fauna to that of a quiescent site and found a significant difference in the predicted direction. Although this result was encouraging, the strength of their inference was limited because only one site from each type of environment had been compared. We performed a second test of Thistle and Wilson's hypothesis by comparing the composition of the isopod fauna from two additional physically quiescent locations (4500 m depth, 14°40′N 125°26′W, and 4800 m depth, 12°57′N 128°19.5′W) to that of the HEBBLE site. Those isopods that are thought to be exposed to the erosion caused by storms occurred in a significantly greater proportion of the samples at the quiescent sites than at the HEBBLE site, a result consistent with Thistle and Wilson's hypothesis.

The fauna of the HEBBLE site: patterns in standing stock and sediment-dynamic effects

At the High Energy Benthic Boundary Layer Experiment (HEBBLE) site at 4820 m depth on the Nova Scotia Rise (40°27′N, 62°20′W), abundances of polychaetes, bivalves, tanaids, and isopods were conspicuously greater than expected for the depth, whereas those of nematodes and harpacticoids, the two dominant meiofaunal taxa, were not. Further, the abundances of polychaetes, bivalves, tanaids, and nematodes were consistent over time and appeared to be unaffected by the periods of strong near-bottom flow that characterized the site. In contrast, isopods and harpacticoid copepods were less abundant in samples taken as a storm abated, apparently because they were eroded during benthic storms. We explore implications of the difference between macrofaunal and meiofaunal abundance for community structure. In addition, because a goal of the HEBBLE project was the parameterization of sediment transport in the deep sea, we summarize ways that the biota affects sediment transport at this high-energy site.

Sediment concentration profiling in HEBBLE using a 1-MHz acoustic backscatter system

As part of the High Energy Benthic Boundary Layer Experiment (HEBBLE), a 1-MHz Acoustic BackScatter System (ABSS) was deployed in 4800 m of water on the Nova Scotian continental rise. The purpose of the instrument was to measure non-intrusively a time series of the vertical concentration profile over the course of one year, from September 1985 to September 1986. In this paper, we discuss the details of the ABSS instrument, the results of the time series data analysis, and the implications of the results on sediment transport modeling in HEBBLE. The systematic variation in the ABSS and light transmissometry calibration with changes of particle size distribution caused by environmental variability is discussed. A comparison of the ABSS estimates of particle concentration with those obtained from light transmissometers seems to indicate that the suspended particle size distribution changed surprisingly little throughout the experiment.

Long-period acoustic and seismic measurements and ocean floor currents

Pressure fluctuations caused by a strong ocean floor current are evident during most of an eighty-day-long record of very-low-frequency acoustic ambient noise measured by an instrument on the seafloor in the western Atlantic in the framework of the HEBBLE (High Energy Benthic Boundary Layer Experiment). The differential pressure gauges on the instrument produce useful measurements over a wide frequency band extending from 0.0005 to 16 Hz. The spectrum of current-induced pressure fluctuations is red with a power-law dependence on frequency with an exponent of -1.5. Turbulence in the ocean floor boundary layer is the source of these pressure fluctuations rather than the effects of flow around the transducers. This record of boundary-layer pressure fluctuations is used to predict the effect of seafloor currents on long-period seismograph measurements from the seafloor and from under the seafloor in boreholes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dedication

During the preparation of this volume, William D. “Bill” Grant, one of our dear friends and fellow Coastal Ocean Dynamics Experiment (CODE) scientists, died of cancer on October 7, 1986. Our shock and disbelief have given way to deep sadness as we come to realize his absence. His intelligence, wit, and friendship will be greatly missed.Bill was a gifted scientist whose wisdom, dedication, and boundless energy had led him to a preeminent position in his principal field of research on bottom boundary layers beneath waves and currents. In nine short years, Bill had emerged from the completion of his doctoral research at the Massachusetts Institute of Technology to the rank of Senior Scientist in the Department of Ocean Engineering at the Woods Hole Oceanographic Institution. At the time of his death, Bill was recognized as one of the world's leading authorities on the hydrodynamics of bottom boundary layers on continental shelves. Bill enjoyed the application of sound theoretical models to the explanation of geological and oceanographic phenomena observed in the marine environment. His scientific interests spanned a broad range of topics including the nature and structure of turbulent bottom boundary layers in both shallow continental shelf regions like CODE and deep oceanic domains like the High Energy Benthic Boundary Layer Experiment (HEBBLE), and the role that these bottom layers have in affecting both oceanic circulation and sediment transport patterns. Bill had the unique ability to generate physically realistic models which he then tested through intensive field investigations. He was at once a theoretician, laboratory experimentalist, and seagoing ocean scientist, and he excelled in all areas of his research.

A hydrodynamically modified, abyssal isopod fauna

At the High Energy Benthic Boundary Layer Experiment (HEBBLE) site (4820 m depth in the western North Atlantic) periods of strong currents and sediment erosion alternate with periods of weak flow and massive deposition. We investigated the impact of this atypical hydrodynamic environment on the isopod fauna by testing for a difference in composition between the HEBBLE site and a tranquil location, the Deep Ocean Mining Environmental Study (DOMES) site A ( ca. 5000 m) in the equatorial Pacific. Epifaunal isopods were significantly less abundant at the HEBBLE site than at the comparison site despite significantly greater total isopod numbers at the HEBBLE site. We suggest that the hydrodynamic regime puts epifaunal isopods at risk, making them rare at the HEBBLE site.

Long‐term in situ calculations of kinetic energy and Reynolds stress in a deep sea boundary layer

Long‐term measurements of Reynolds stress and kinetic energy are necessary to characterize the bottom boundary layer during storms which transport sediment in the High Energy Benthic Boundary Layer Experiment area on the Nova Scotia Rise. The benthic acoustic stress sensor current meter system employed an on‐board microcomputer to process 2‐Hz velocity vector data into 20‐min averages of velocity vectors and the turbulent Reynolds stress tensor. The data indicate a bottom boundary layer, which is usually transitional, with bottom roughness scaling between viscous (z0 = v/9u*) and rough (z0 = ks/30). During the periods of transitional flow the logarithmic layer was not a useful indicator of boundary stress. Direct stress and energy measurements showed that the stress was much less than that indicated by the slope of the transitional mean logarithmic velocity profile. Several westward high‐velocity events, which were modulated by a significant tidal component, were recorded. It appears from this data set that the mean westward flow must be accompanied by a train of internal tidal waves to achieve the velocities necessary to be the sediment transport events identified as “benthic storms”.

Deep ocean sediment transport—preliminary results of the High Energy Benthic Boundary Layer Experiment [HEBBLE] : Nowell, A.R.M. and C.D. Hollister (eds.), 1985. Special issue. Mar. Geol., 66(1–4):1–408; 23 papers

Deep ocean sediment transport—preliminary results of the High Energy Benthic Boundary Layer Experiment [HEBBLE] : Nowell, A.R.M. and C.D. Hollister (eds.), 1985. Special issue. Mar. Geol., 66(1–4):1–408; 23 papers

Sedimentation under deep-sea current systems: Pre-HEBBLE ideas

Studies of the interaction of deep ocean current systems with the sea bed are briefly traced from their beginnings in the decade of the mid-1950's to mid-1960's. Subsequent lines of study in physical oceanography, suspended sediment measurement, echocharacter mapping, microphysiography, sedimentology and developments of instrumentation are noted up to 1978 when the High Energy Benthic Boundary Layer Experiment started.

Evidence for control of abyssal agglutinated foraminiferal community structure by substrate disturbance: Results from the HEBBLE Area

Deep-sea agglutinated foraminifera have been quantitatively examined from 28 coretop samples from seven box cores collected in June, 1983, at the HEBBLE (High Energy Benthic Boundary Layer Experiment) Site (40°27′N, 62°20′W, 4815–4825 m depth) and HEBBLE Shallow Site (40°53′N, 63°44′W, 4185 m depth) on the Nova Scotian continental rise. Samples from the relatively tranquil HEBBLE Shallow Site can be distinguished from the HEBBLE Site on the basis of their microfauna. A Q-mode VARIMAX factor analysis of species-frequency data delineates three major faunal assemblages: 1. (1) A faunal assemblage from the HEBBLE Site dominated by Ammobaculties sp. cf. A. americanus. This assemblage exhibits lowest species diversity (suggesting a disturbed community) and strongly associated with box cores containing fine-grained, laminated sediments. 2. (2) An assemblage associated with box cores from the HEBBLE Meadow dominated by Saccammina tubulata, Psammosphaera sp., cf. P. fusca, a coarsely grained species of Psammosphaera and Reophax sp. 1. 3. (3) A fauna occurring at the HEBBLE Shallow Site, dominated by Hormosinella distans, with Reophax bacillaris and Ammobaculites agglutinans unique to the assemblage. This fauna displays a higher ratio of calcareous to agglutinated species, significantly higher species diversity, lower variability and a greater proportion of hormosinids and species utilizing finer grains than samples from the HEBBLE Site, which is interpreted as reflecting environmental stability. Marked heterogeneity exists among agglutinated foraminiferal assemblages within box cores. At the HEBBLE Meadow, within-box-core faunal variability exceeds between-box-core variability, suggesting that environmental factors influencing the distribution of species are patchy on a scale of approximately 40 cm.

Sedimentology and stratigraphy of box cores from the HEBBLE site on the Nova Scotian continental rise

Fifty box cores were obtained from the 2 × 4 km site of the High Energy Benthic Boundary Layer Experiment (HEBBLE) on the lower Nova Scotian continental rise at 4820 m. The oldest unit (occurring in two gravity cores) is a pre-Holocene dark grey gravelly mud, probably of debris flow origin. Above this is a soft grey mud overlain by brown sandy mud whose sand is largely foraminiferal with some terrigenous material from interbedded turbidites. In some places this unit has a stiff brown gravelly mud at its base which is always overlain by sandy mud containing dispersed gravel. It is suggested that the dispersed gravel originated from the underlying gravelly mud and has been dispersed upwards by the action of burrowing organisms. Foraminifera indicate a Holocene age down to about 65 cm and thus a net accumulation rate of about 6 cm ka −1. The surface unit of brown mud, named the HEBBLE Mud, is spatially variable in both thickness and sand content which average about 4 cm and 6%, respectively. Sand percentage is not related to silt size or percentage but varies inversely with deposition rate and thickness, suggesting a relation to local winnowing. The size and peak height of the main silt mode (10–17 μm) are closely correlated and the peak height is inversely related to the percentage of clay. This is attributed to fractionation during deposition of the mud wherein fine silt and clay deposition is suppressed at flow speeds from 10 to 18 cm s −1 leaving coarser silt. There is more clay on the upstream and downstream sides of a low spur suggesting spatial variation of shear stress that may be due to a standing internal wave added to the mean flow. Silt-mode size is variable downcore although clay percentages are similar to those at the surface. The overall silt size is probably controlled by turbidite input, thus silt size is likely to be an unreliable indicator of current speeds down core, though peak height of the silt mode may be satisfactory.

Theoretical analysis of ABSS data for HEBBLE

As part of HEBBLE (the High Energy Benthic Boundary Layer Experiment), a 5 MHz Acoustic Backscatter System (ABSS) has been employed to determine remotely the concentration profile of the suspended sediments in the first meter above the benthic boundary. In order to extract this information from the raw data, an ABSS computer simulator has been created. The simulator mimics the ABSS output by solving the sonar equation in a forward modeling approach with the concentration being essentially the only free parameter available for matching simulated and actual output. Both theoretical and experimental results are presented, and directions for future work are discussed.

Deep Western Boundary Undercurrent delineated by sediment texture at base of North American continental rise

A Deep Western Boundary Undercurrent (DWBUC) flows south along the North American continental slope and rise between 1,000 m and 5,000 m with the exact location differing between investigators. The silt-mean particle-size of seafloor samples, however, defines a ∼300-km wide highvelocity zone within the DWBUC below ∼4,500 m. Analysis of silt subfractions shows that the DWBUC high-velocity zone winnows both fine and very fine silt, with preferential deposition of medium and coarse silt. Silt textural analyses indicate that velocity increases with depth in the DWBUC, reaching a maximum at 4,900 to 5,000 m consistent with results of the High Energy Benthic Boundary Layer Experiment (HEBBLE).