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TitleNearbed current and suspended sediment concentration recorded by a seabed lander deployed in the deep water at Sackville Spur, Grand Banks, Newfoundland and Labrador
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AuthorLi, M Z; Schillinger, D J; Robertson, A
SourceGeological Survey of Canada, Open File 8136, 2016, 27 pages, https://doi.org/10.4095/299384
Year2016
PublisherNatural Resources Canada
Documentopen file
Lang.English
Mediaon-line; digital
File formatpdf
ProvinceEastern offshore region
AreaSackville Spur; Flemish Cap; Flemish Pass
Lat/Long WENS -47.0000 -45.0000 49.0000 47.0000
Subjectsmarine geology; stratigraphy; paleontology; sedimentology; continental slope; sedimentation; currents; sea floor currents; bottom currents; sediment transport; submarine transport; silts; clays; sands; miocene; pliocene; unconformities; structural features; erosion; biostratigraphy; bioturbation; Quaternary; Tertiary; Cenozoic
Illustrationslocation maps; photographs; tables; profiles; plots
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Location
 
Natural Resources Canada Library - Ottawa (Earth Sciences)
 
ProgramMarine Geohazards, Public Safety Geoscience
Released2016 12 02
Abstract(Summary)
Powerful near-bed currents exist on the Grand Banks and Labrador slopes down to >1000 m. Field data is lacking in these areas for assessing near-bed current strength and the frequency and magnitude of sediment transport in deep slope waters. The Geologic Survey of Canada-Atlantic (GSCA) deployed a redesigned deep-water seabed lander from August 9 - 21, 2011 in 1200 m water depth on the northern flank of the Sackville Spur, Grand Banks, to collect near-bed currents and sediment transport data. This seabed lander was designed for free-fall type deployment and equipped with a deep-water rated AquaDopp Acoustic Doppler Current Profiler (ADCP) with an analog Optical Backscatter Sensor (OBS) and a camera capable of burst recording both video and still photos of the seabed. The goal of this redesigned lander is to expand our seabed lander technology to make observations of near-bed currents and sediment transport in deep water environments. Assessments of the recorded data indicate that the AquaDopp ADCP ran for the entire deployment and recorded velocity and backscatter profile data as well as suspended sediment concentration at 1.2 m above bottom for ~13 days. Unfortunately, the camera system only recorded seabed images intermittently for the first four days of the deployment due to the premature depletion of the battery power. Analysis of the acoustic backscatter and velocity data recorded by the AquaDopp ADCP shows acoustic interference, likely from the lander frame, contaminating the data of the range from 50 to 60 cm from the transducer. Side-lobe interference from the bottom likely corrupted the bottom three range bins to render the velocity data in the bottom 30 cm unreliable.
The seabed photos show grey mud with possible bioturbation at the lander site.. A cursory evaluation of the seabed images, recorded intermittently from August 9th to 13th, suggests that no apparent sediment transport could be observed during these four days. The preliminary analysis of the ADCP data shows several moderate current events, in which the peak bottom currents reached nearly 30 cm/s. The direction of these peak currents was predominantly to the east. During these moderate current events, the ADCP backscatter intensity (a proxy of suspended sediment concentration) was increased by nearly 50% from the mean background values. A positive correlation between the depth-averaged acoustic backscatter intensity and the depth-averaged current speed was observed. This would suggest that increased current speeds were responsible for the increase of suspended sediment concentration. Profiles of backscatter averaged over the deployment duration show that suspended sediment concentration increases with height above seabed from 0.3 m to 1.3 m above bottom. This suggests that averaged over the deployment duration, the suspended sediments were not bottom intensified and were probably advected from other areas. In the bottom 0.3 m, however, the backscatter amplitude increases rapidly towards the seabed. The profile of the averaged backscatter for current speeds > 0.2 m/s also shows that the backscatter intensity was higher near the seabed and decreased with the increasing height above bottom. This is evidence that local erosion of bottom sediments probably occurred when current speeds exceeded ~0.2 m/s. Nevertheless, the analysis of the data collected is inconclusive whether the increased sediment concentration under increased currents was due to local erosion or being advected to the deployment site from another region. Data from a two day period during the early part of the deployment shows intriguing sinusoidal variations of temperature, current speed and current direction with elevated suspended sediment concentration. Further efforts are warranted to explore the driving processes causing this observed event.
Summary(Plain Language Summary, not published)
Powerful near-bed currents exist on the Grand Banks and Labrador slopes. A redesigned deep-water seabed lander was deployed in August 2011 over muddy sediments in 1200 m depth on the northern flank of the Sackville Spur, Grand Banks, to collect near-bed currents and sediment transport data. The analysis of the current data shows occurrence of several moderate current events. The peak bottom currents reached ~30 cm/s and were predominantly to the east. During these current events, the acoustic backscatter intensity (a proxy of suspended sediment concentration) was increased by nearly 50% from the background values. A positive correlation exists between the depth-averaged backscatter intensity and current speed. This would suggest that increased currents were responsible for the increase of suspended sediment concentration. Intriguing sinusoidal variations of temperature, current speed and direction with elevated sediment concentration were also observed over a two day period.
GEOSCAN ID299384