Richard W. Sternberg

The use of Pb-210 geochronology as a sedimentological tool: Application to the Washington continental shelf

Abstract In the past, marine sedimentologists have been unable to quantitatively investigate modern sediment accumulation on continental shelves, but recent development of Pb-210 geochronology provides a tool to overcome this limitation. It is used in this study to examine the accumulation of modern Columbia River sediments on the Washington continental shelf. Pb-210 profiles in Washington shelf sediments reveal three characteristic regions: a homogeneous surface layer (about 10 cm thick) where sediments are actively mixed by physical and biological processes, a region where Pb-210 activities decrease logarithmically with depth in the sediment, and a lower region of background activities. The surface mixed layer generally reflects erosion by wave and current activity in inner shelf sediments (shallower than about 60 m), and biological mixing in mid and outer shelf sediments. The region of logarithmic activities (i.e., radioactive decay) in the Pb-210 profiles provides the rate of sediment accumulation, which is on the order of mm/yr. The predominant depositional feature is a mid-shelf silt deposit in which accumulation rates progressively decrease north-northwestward away from the Columbia River. These results are consistent with other sedimentological observations, and indicate that Pb-210 geochronology not only allows determination of modern sediment accumulation rates, but also provides additional insight into processes affecting accumulation. Pb-210 geochronology can be an important sedimentological tool for future studies of continental shelf sedimentation in areas undergoing modern sediment accumulation.

Fluid-mud processes on the Amazon continental shelf

Abstract A sediment transport study conducted on the Amazon continental shelf as part of AmasSeds (A Multi-disciplinary Amazon Shelf SEDiment Study) revealed extensive regions of dense nearbed suspensions of sediment, known as fluid mud (suspended-sediment concentration > 10 g1−1 ). Fluid mud was found near the river mouth on the inner- and middle-shelf, in the region of the bottom salinity front, and was most extensive during periods of rising and high river discharge. Fluid mud, up to 7.25 m thick, but generally 1–2 m thick, appears to form by processes similar to those occurring at an estuarine turbidity maximum, i.e. enhanced settling and lateral convergence of near-bottom flows. A modeling study showed that vertical mixing was controlled by the suppression of turbulence, due to the stratification induced by suspended sediment, and established an upper bound for the total amount of suspended sediment that may be carried in suspension. Sediment leaving the Amazon River appears to go through cycles of trapping and resuspension at the river mouth, before being partially advected seaward and alongshelf, where it is largely incorporated into fluid mud along the bottom salinity front. The fluid muds have far-reaching effects on the Amazon shelf system by reducing boundary shear stresses, affecting water-column seabed exchange, and serving as the agent of outward growth of the subaqueous delta through episodic offshore transport.

Shelf-to-canyon sediment-transport processes on the Eel continental margin (northern California)

Abstract To investigate the processes by which sediment is supplied to the head of a submarine canyon, an instrumented tripod and a mooring were deployed in the northern thalweg of the Eel Canyon during autumn and winter 1999–2000. This was done as part of the STRATAFORM program, and in combination with a long time-series benthic-tripod data collection on the Eel continental shelf. Sediment-resuspension events on the shelf were forced by waves, and near-bottom suspended-sediment concentrations (SSC) were enhanced during the Eel River flood season. Periodic SSC fluctuations in intermediate waters (corresponding to water depths equal to the shelf-break depth) were predominantly recorded at semidiurnal tidal frequencies, associated with decreases of water salinity and increases of temperature. Within the Eel Canyon, increases of water turbidity were not directly related to the Eel River discharge, but they were linked to the occurrence of storms. This relationship was evident in the bottom-boundary-layer measurements at 120 m depth in the canyon head, although farther down-thalweg (280 m depth), significant increases of near-bottom SSC associated with storm events were recorded also. The highest SSC measured within the canyon coincided with a highly energetic storm on 28 October 1999, in the absence of any river flood event, but associated with a down-canyon density-driven flow. On the shelf at 60 m depth, near-bottom SSC during this storm event reached extremely high concentrations (>10 g l−1), characteristic of fluid-mud suspensions. The across-shelf sediment transport near the bottom showed a persistent off-shelf direction through the entire recording period, while the along-shelf transport fluctuated in direction, but resulted in net transport toward the Eel Canyon head. Within the canyon, near-bottom sediment fluxes were continuously directed down-canyon, while the across-canyon flux was negligible. Sediment fluxes through intermediate slope waters (above the canyon rims) were directed toward the north, following the orientation of the adjacent shelf-break. Results from this field study have identified some of the major processes controlling the off-shelf sediment export in the Eel continental margin, and corroborate previous findings that a substantial portion of the Eel River sediment discharged on the shelf can be exported into the Eel submarine canyon.

New Instrumentation for the Investigation of Sediment Suspension Processes in the Shallow Marine Environment

Abstract An instrumentation system has been developed that continuously measures: flow velocity, water depth, and suspended sediment concentration profiles in the nearshore zone. The system consists of four components: (1) an array of electronic sensors, including: an electromagnetic current meter, a surface-piercing resistance wave gauge, and a five-element array of optical back-scatterance sensors; (2) a signal processing electronics unit; (3) a guyed mast support structure; and (4) a data acquisition system. The small size of the sensors and their response characteristics make the array suitable for monitoring sediment transport to within a few centimeters of the bed under moderately large breaking waves (∼2 m). Results from an experiment on an exposed ocean beach on the coast of Washington, U.S.A., indicate that sand suspension events of long duration (15–40 sec) relative to the period of incident waves contribute a significant quantity of sand to the total littoral drift. Work is now in progress to modify the system for self-contained operation on the continental shelf. Field investigations with the modified system will focus on the acquisition of a data set to test the validity of the suspended sediment distribution equation.

Friction factors in tidal channels with differing bed roughness

Abstract During 1965–1966 a field program was conducted within six tidal channels in Puget Sound, Washington, and the Strait of Juan de Fuca. The purpose of the program was to investigate the logarithmic nature of the velocity distribution within 1.5 m of the bed, and the frictional influence that different bed configurations have upon the adjacent fluid. Measurements were made of the velocity distribution, bed configuration, and sediment textural characteristics in each of the channels. Velocity data were collected continuously for periods of several to 20 h. The bed configuration in the study areas varied from rocks and gravel to small ripples and sand deformed into irregular roughness patterns. Generally the flow within a channel can be divided into two categories conforming to “hydrodynamically rough” and “hydrodynamically transitional” flow conditions. This division is based on a significant decrease in the dispersion of the drag coefficient as the Reynolds number increased. The boundary between transitional and rough flow seems to be qualitatively related to the geometry of the bed, with the more complex beds becoming fully rough at higher Reynolds number than more simple roughness patterns. By grouping all data, a mean and 95% confidence limit was assigned to the drag coefficient, which represents a more or less average value for the shallow marine environment regardless of bed configuration and flow conditions. The grouped data are also divided into two regions (transitional and rough) according to a roughness Reynolds number, with representative values of the drag coefficient assigned to each region.

MEASUREMENT OF SIZE AND SETTLING VELOCITY OF SUSPENDED AGGREGATES ON THE NORTHERN CALIFORNIA CONTINENTAL SHELF

Abstract A high-resolution video camera attached to a miniature sediment trap was used to make time-lapse images of suspended aggregates settling through the water column. The video-trap system was placed 2 m above the seabed in the STRATAFORM study area off Eureka, California, and was programmed to take a 7-second time sequence of settling particles every 6 h over a 4-month period. The camera field of view within the trap is 15 mm and the minimum particle resolution is approximately 130 μm. Data analysis was carried out with a video frame grabber and digital imaging software to determine the particle size, shape, and settling velocity. Results of the first deployment of this system (September 1995) show that particle sizes (elliptical nominal diameter) determined from the video images ranged from 130 to 740 μm with a modal size of 300 μm. The average form factor (ratio of short to long semi-axis) of the aggregates was 0.71. Measured settling velocities varied from 0.09 to 8.13 mm/s. In terms of weight-percent, or mass, the median size of the aggregate distribution is 600 μm. Analysis of settled particles collected from the sediment trap shows disaggregated or component grains ranging from 1.0 to 63 μm with a modal size of 26 μm and a median size of 11 μm. Additionally, an analysis was carried out to separate the relative importance of particle shape and excess density on settling velocity, and these results were compared to observations. Using the equations for settling velocity of spheres and prolate spheroids, and substituting published relationships for size versus excess density into these equations, the settling velocity for combinations of shape and excess density has been calculated. Results show that differences between sphere and prolate spheroid (with major axis both parallel and perpendicular to flow) approximations caused less than a 13% difference in computed settling velocity. In contrast, the various size/excess density relationships used to compute settling velocity caused order of magnitude deviations from measured values. In all cases, however, the approximations of settling velocity for aggregates provide a significantly better fit to data than approximations based on disaggregated sediment size distribution.

Distribution of fluid muds on the Amazon continental shelf

Abstract A comprehensive study of suspended-sediment transport on the Amazon shelf was conducted as part of the AmasSeds project (A Multidisciplinary Amazon Shelf SEDiment Study) to understand the link between physical processes and the fate of Amazon River sediment. A small instrumented tripod was used to measure the fluid, flow, and suspended-sediment characteristics throughout the water column on varying time scales (e.g., semidiurnal, fortnightly, seasonal). Dense nearbed suspensions, or fluid muds (suspended-sediment concentration > 10 g/l), on the order of a few meters in thickness, were found on the inner and middle shelf covering an area ranging from ∼5,700 km2 during falling and low river discharge to ∼10,000 km2 during rising and high discharge. Fluid muds, generally observed in the region of bottom salinity fronts, extend across the topset beds to the foreset beds during rising and high discharge. Salinity and temperature anomalies frequently observed in fluid muds imply that often the muds do not result from erosion and resuspension from the seabed, but from trapping processes at the bottom salinity front and transport across the shelf. The vertical structure of fluid mud varies in space and tune. Variability close to the river mouth is predominantly fortnightly, related to the location and structure of the bottom salinity front; on the open shelf, the variability appears to be related to seasonal differences in river and sediment discharge and the location of the bottom salinity front. The total suspended sediment inventory on the shelf during the season of high discharge is approximately equal to the annual sediment discharge from the river (1.2 × 109 tons/yr) and > 90% of that inventory is within fluid mud. Suspended-sediment transport on the shelf is dominated by fluid mud.

Measurements of high concentration suspended sediments using the optical backscatterance sensor

Abstract An optical backscatterance sensor (OBS™) was used to measure extremely high concentration suspended sediments on the Amazon continental shelf as part of the AmasSeds project (A Multidisciplinary Amazon Shelf Sediment Study). Using a pumping system for obtaining water samples to calibrate the sensor in situ, the OBS output was observed to increase approximately linearly for concentrations 36 g/l. The OBS response was nearly continuous from 0 to 320 g/l making it possible to obtain continuous suspended sediment concentration profiles through fluid muds.

Sediment-transport events on the northern California continental shelf during the 1990–1991 STRESS experiment

Abstract Measurements of currents and light transmission were made at bottom tripods and moorings arrayed across the northern California continental shelf along the Coastal Ocean Dynamics Experiment (CODE) “C” transect as part of the 1990–1991 Sediment Transport Events on Shelves and Slopes (STRESS) experiment. In combination with meteorological and wave data from the National Data Buoy Center Buoy 46013, these measurements provide information about the physical forcing and resultant resuspension and transport of bottom material between 21 November and 8 March. Sixteen events were identified in the wave, wind and current-meter records for this period. Only two were local storms with southerly winds, but they caused about half of the seasonal net transport. Seven were swell events that combined long-period waves generated by distant storms with local currents. At the 90-m site, swells interacted with the mean northward flow to produce northward transport. During six northerly wind events, upwelling-favorable winds often were sufficient to slow or reverse the mean northward flow and thus caused southward transport. A single current event, which produced moderate southward transport, was observed at the 130-m site. Net transport during the winter experiment was offshore at all sites, northward at the inner- and mid-shelf sites, but southward at the outer-shelf site. The results suggest that local storms with southerly winds may dominate seasonal transport, as on the Washington shelf, but significant transport also can occur during fair weather and during periods of northerly winds.

An introduction to the geological significance of sediment transport and accumulation on the Amazon continental shelf

In order to understand the formative processes and resulting stratigraphy on the Amazon shelf and adjacent shoreline of Amapa, geological investigations were undertaken as part of A Multidisciplinary Amazon Shelf SEDiment Study (AmasSeds). The design of the study provided results of a multifaceted nature: integrated with observations in other disciplines; focused on sedimentary processes; representative of fluctuating conditions on several time scales; and broadly distributed on the shelf, including regions not investigated in the past. On short time scales, most muddy sediment is transported northwest of the river mouth. From there, it moves seaward as fluid mud to cause rapid accumulation of foreset beds, and moves northwestward to prograde the northernmost Amapa shoreline and to supply sediment to the Guianas. Fluid muds cover the shelf as far northward as ~3.5 °N and allow strong tides to propagate to shore, where tidal currents cause most of the Amapa shoreline to undergo erosion today. Averaged over decades and centuries, about half of the Amazon sediment discharge accumulates on the adjacent shelf, and another significant fraction (~one-sixth) leads to accretion of the northernmost Amapa shoreline and northwestward bypassing of sediment. The remaining sediment is hypothesized to be trapped in deltaplain deposits of the lower Amazon River system. The foreset region and shoreline represent the two common loci of sediment accumulation that alternate their predominance on time scales of millennia, and lead to a two-stage progradation of coastal-plain and subaqueous-deltaic deposits. This muddy regressive sedimentation during high sea level is replaced by formation of erosional sand layers during low sea level and transgressive conditions. Future research in the study area should address important considerations that were delineated by the present study, including: mechanisms of shoreline accretion; the Holocene history recorded in topset and coastal-plain strata; the role in local sedimentation played by the large shoal extending from Cabo Norte; and the entrapment of Amazon sediment by the delta plain.