Andrea S. Ogston

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.

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.

Origin of Amazon mudbanks along the northeastern coast of South America

Abstract Seismic profiles, sediment cores, and water column measurements were collected along the northeastern coast of Brazil to examine the origin of mudbanks in the Amazon coastal mud belt. These 10–60-km-long, shore-attached features previously had been observed to migrate along the 1200 km coast of the Guianas in response to wave forcing. CHIRP (3.5 kHz) seismic profiles of the shoreface and inner shelf located two mudbanks updrift of the previous eastern limit in French Guiana. 210 Pb geochronology shows that these two banks are migrating to the northwest over a relict mud surface in 5–20 m water depth. The mudbanks are 3–4 m thick and are translating over a modern shoreface mud wedge deposited by previous mudbank passage in

Sediment-transport events on the northern California continental shelf

Abstract A long-term monitoring tripod has been maintained in 60-m water depth at the northern end of the STRATAFORM study site on the northern California continental shelf. As part of this ongoing study, tripod data for 1 year beginning 24 September 1995 are used to provide a sediment-transport analysis on an event-by-event basis. The objective of this paper is to highlight the energetic nature of this shelf region in terms of the frequency, duration, and magnitude of sediment-suspension events and the associated particle flux. Analyses are based on measurements from two current meters and two optical backscatterance sensors located at 30 and 100 cm above the bed. Data from these instruments and a pressure sensor were averaged over 7.5 min every hour. During the year of record, 41 distinct sediment-suspension events occurred (i.e., sustained suspended-sediment concentrations greater than 20 mg/l above background level at 100 cm above bed). Suspended-sediment events were associated with significant wave activity, tidal currents, and river discharge. The average duration of a sediment-suspension event was 3.1 days and varied from 0.7 to 8 days. During events, mean suspended-sediment concentrations of 110 mg/l were observed with peak hourly observations exceeding 1000 mg/l. Concentrations between events averaged 35 mg/l. The overall sediment flux for the period of record was directed seaward and southward. The distribution and magnitude of sediment-suspension events and particle flux showed a strong seasonality. For example, of the sediment transport during events, 98% of the net along-shelf, and 73% of the net across-shelf sediment transport occurred in the winter. Three major winter storms could account for 72% of the total along-shelf transport but only 10% of the across-shelf transport. The across-shelf flux was more evenly distributed over the year and occurred as a result of mean currents and very low-frequency seaward flows associated with mesoscale circulation patterns.

Northern Adriatic Response to a Wintertime Bora Wind Event

During winters, the northern Adriatic Sea experiences frequent, intense cold-air outbreaks that drive oceanic heat loss and imprint complex but predictable patterns in the underlying waters. This strong, reliable forcing makes this region an excellent laboratory for observational and numerical investigations of air-sea interaction, sediment and biological transport, and mesoscale wind-driven flow. Narrow sea surface wind jets, commonly known as “bora,” occur when cold, dry air spills through gaps in the Dinaric Alps (the mountain range situated along the Adriatics eastern shore). Horizontal variations in these winds drive a mosaic of oceanic cyclonic and anticyclonic cells that draw coastal waters far into the middle basin. The winds also drive intense cooling and overturning, producing a sharp front between dense, vertically homogenous waters (North Adriatic Dense Water, or NAdDW) in the north and the lighter (colder, fresher), stratified waters of the Po River plume. Once subducted at the front, the NAdDW flows southward in a narrow vein following the isobaths (contours of constant depth) of the Italian coast. In addition to governing the basins general circulation, these processes also influence sediment transport and modulate biological and optical variability

Terrigenous organic matter in sediments from the Fly River delta‐clinoform system (Papua New Guinea)

loadings (0.5–1.0 mg C m 2 ), although several samples from the outer topset region, an area of sediment bypass, were characterized by lower carbon loadings indicative of enhanced carbon losses. Overall, the organic matter in both surface and subsurface sediments appeared to have predominantly a terrigenous origin, with no evidence for dilution and/or replacement by marine carbon. The measured compositions were consistent with contributions from modern vascular plant detritus, aged soil organic matter, and very old or fossil organic matter devoid of recognizable biochemicals.

Sediment dynamics in the lower Mekong River: Transition from tidal river to estuary

A better understanding of flow and sediment dynamics in the lowermost portions of large-tropical rivers is essential to constraining estimates of worldwide sediment delivery to the ocean. Flow velocity, salinity, and suspended-sediment concentration were measured for 25 h at three cross sections in the tidal Song Hau distributary of the Mekong River, Vietnam. Two campaigns took place during comparatively high-seasonal and low-seasonal discharge, and estuarine conditions varied dramatically between them. The system transitioned from a tidal river with ephemeral presence of a salt wedge during high flow to a partially mixed estuary during low flow. The changing freshwater input, sediment sources, and estuarine characteristics resulted in seaward sediment export during high flow and landward import during low flow. The Dinh An channel of the Song Hau distributary exported sediment to the coast at a rate of about 1 t s−1 during high flow and imported sediment in a spatially varying manner at approximately 0.3 t s−1 during low flow. Scaling these values results in a yearly Mekong sediment discharge estimate about 65% smaller than a generally accepted estimate of 110 Mt yr−1, although the limited temporal and spatial nature of this study implies a relatively high degree of uncertainty for the new estimate. Fluvial advection of sediment was primarily responsible for the high-flow sediment export. Exchange-flow and tidal processes, including local resuspension, were principally responsible for the low-flow import. The resulting bed-sediment grain size was coarser and more variable during high flow and finer during low, and the residual flow patterns support the maintenance of mid-channel islands.

A video system for in situ measurement of size and settling velocity of suspended particulates

Abstract A video camera system is described which is designed to make in situ measurements of particle size and settling velocity under conditions of strong currents and high suspended sediment concentrations. The system consists of a Sony Hi8 video camera attached to a miniaturized sediment trap and an underwater light. With the camera lens set at wide angle, the total field of view is 15 mm in the vertical and the camera can resolve particle sizes ranging from 100 μm to several millimetres. The system is programmed to record a 7-s time series of images every 6 h over a 4-mo deployment. Results from the initial deployment showed settling flocs that ranged from 100 to greater than 500 μm in diameter. By comparison of sequential images, these particles were found to be settling at speeds ranging from 0.1 to 2.2 mm·s −1 and a strong correlation existed between particle size and settling velocity. Effective floc density did not significantly change with floc size. These initial tests suggest that this video system can provide valuable information on floc size and settling velocity from long-term deployments in energetic shelf environments.

Predictions of Turbidity Due to Enhanced Sediment Resuspension Resulting from Sea-Level Rise on a Fringing Coral Reef: Evidence from Molokai, Hawaii

Abstract Accelerating sea-level rise associated with global climate change will affect sedimentary processes on coral reefs and other shoreline environments by increasing energy and sediment resuspension. On reefs, sedimentation is known to increase coral stress and bleaching as particles that settle on coral surfaces interfere with photosynthesis and feeding, and turbidity induced by suspended sediment reduces incident light levels. Using relationships developed from observations of wave orbital velocity, water-surface elevation, and suspended-sediment concentration on a fringing reef flat of Molokai, Hawaii, predictions of the average daily maximum in suspended-sediment concentration increase from ~11 mg/l to ~20 mg/l with 20 cm sea-level rise. The duration of time concentrations exceeds 10 mg/l increases from 9% to 37%. An evaluation of the reduction of wave energy flux through breaking and frictional dissipation across the reef flat shows an increase of ~80% relative to the present will potentially reach the shoreline as sea level increases by 20 cm. Where the shoreline exists on low, flat terrain, the increased energy could cause significant erosion of the shoreline. Considering the sediment budget, the sediment flux is predicted to increase and removal of fine-grained sediment may be expedited on some fringing reefs, and sediment in storage on the inner reef could ultimately be reduced. However, increased shoreline erosion may add sediment and offset removal from the reef flat. The shifts in sediment availability and transport that will occur as result of a modest increase in sea level have wide application to fringing coral reefs elsewhere, as well as other shoreline environments.

On the importance of nearbed sediment flux measurements for estimating sediment transport in the surf zone

Abstract Previous sediment studies in the surf zone typically have computed suspended sediment flux profiles by pairing a single current meter measurement at an elevation of 20 cm or more with measurements of suspended sediment concentration from sensor arrays located at 4–50 cm elevation. This note reports the results of a field experiment in which small impellor current meters were paired with OBS sensors at common elevations of 4, 9 and 19 cm from the bed and across the inner surf zone to obtain concurrent velocity and concentration profiles. The objectives of this experiment were to measure suspended sediment flux profiles within 20 cm of the seabed to determine the magnitude of flux very close to the seabed in comparison to measurements at higher elevation; and to evaluate the errors associated with estimating sediment flux using current meter measurements at only one elevation compared to flux estimates based on velocity profile measurements. Results show that the total sediment flux at z = 4 cm was greater than the flux higher in the water column ( z = 9 and 19 cm) by a factor of at least 2. The flux profiles computed using a single impellor current meter at z = 19 cm and OBS sensors at z = 4, 9 and 19 cm typically were between 0.5 and 2.0 times the flux profiles computed using paired instruments. In one case the estimate of flux direction from the single current meter data predicted transport in the wrong cross shore direction. These results highlight the importance of nearbed measurements of concentration and velocity in estimating sediment transport in the surf zone.