|Title||Observation of bottom currents and sediment transport from the 2015-2016 year-long deployment of a seabed lander on the shelf edge of the Beaufort Sea, offshore Northwest Territories|
|Author||Li, M Z; King, E L; Schillinger, D; Robertson, A G; Wu, Y|
|Source||Geological Survey of Canada, Open File 8553, 2019, 57 pages, https://doi.org/10.4095/314645 (Open Access)|
|Publisher||Natural Resources Canada|
|File format||pdf (Adobe® Reader®)|
|Province||Northern offshore region|
|NTS||107F/13NE; 107F/13SE; 107F/14NW; 107F/14SW|
|Lat/Long WENS||-135.2333 -134.9667 70.8833 70.8167|
|Subjects||marine geology; surficial geology/geomorphology; geophysics; continental margins; continental shelf; continental slope; geophysical surveys; seismic surveys, marine; acoustic surveys, marine; acoustic
velocities; spectral analyses; in-field instrumentation; photography; depositional environment; bottom currents; marine sediments; sediment transport; erosion; suspended sediments; water temperature; sea water geochemistry; salinity; sampling
methods; pressure; conductivity; densities; bathymetry; oceanography; Beaufort Shelf; CCGS Amundsen 2015804 Expedition; CCGS Amundsen 2016805 Expedition; geological hazards; backscatter measurements; shelf break; Phanerozoic; Cenozoic;
|Illustrations||tables; location maps; geoscientific sketch maps; photographs; profiles; plots; time series; spectra|
|Program||Marine Geohazards, Public Safety
|Program||Program of Energy Research and Development
|Released||2019 04 25|
Measurements of near-bed current and sediment transport on the Beaufort shelf break and upper slope are lacking. This limits our understanding of magnitude and frequency of bottom
currents and seabed erosion and in particular whether these are responsible for the observed erosion and non-deposition of modern sediments on the Beaufort shelf edge and upper slope. Two instrumented tripods with limited sensor packages and
process-dependent sampling methods were respectively deployed on the Beaufort shelf break and upper slope from August 2015 to August 2016. They were deployed for long term observation of bottom currents, and suspended sediment concentration as well
as temperature and salinity. This experiment thus represents the first ever year-long under-ice deployments of seabed landers in the deep water depths of the Beaufort Sea. Both tripods were equipped with a Nobska Modular Acoustic Velocity Sensor
(MAVS) current meter, a Nortek Aquadopp acoustic Doppler current profiler (ADCP), a Conductivity-Temperature-Depth (CTD) sensor, and two optical backscatter sensors (OBS) attached respectively to the MAVS and the ADCP instruments. The tripod on the
shelf break was recovered on 31 August 2016. Efforts to retrieve the tripod on the upper slope, however, were not successful.
MAVS4D and attached OBS recorded data from 8/27/2015 to 7/20/2016. Aquadopp ADCP and attached OBS recorded data from
8/27/2015 to 8/31/2016. SeaBird CTD worked from 8/27/2015 to 9/11/2016. The OBS data obtained in this deployment were of poor quality. The OBS on ADCP likely failed after 10 November 2015. Data from the MAVS OBS looked suspicious after 3 May 2016
likely attributed to bio-fouling. Roll, pitch and compass data suggest that the tripod was stable through the deployment duration, although the frame settled ~10 cm into the soft muddy sediments on the shelf break. Temperature varied between 0.5 to
-2 degrees from September 2015 to early May 2016, and after that time the temperature stayed constant at approximately -2 degrees.
The vertical profiles of velocity and backscatter intensity from the Aquadopp ADCP demonstrate short bursts of
strong northeastward currents that extended through the entire bottom boundary layer of ~1 m and were typically associated with increased backscatter (sediment concentration). The ADCP measurements also show occurrences of bottom-confined (<0.3 m
above bottom (mab)) current events which were not associated with any increase of backscatter intensity. The profiles of the velocity components averaged over the deployment duration give a northeastward residual current of ~6 cm/s. The mean vertical
velocity was ~4 cm/s indicating that downwelling condition was dominant for the deployment period. The 1-min averaged current data recorded by the ADCP at 1 m above bottom (mab) indicate frequent occurrence of strong bottom currents that reached a
maximum of ~60 cm/s. The 2.5-min averaged bottom currents recorded by the MAVS were even stronger and gave a maximum of 1.1 m/s likely due to MAVS sampling a smaller volume at a higher frequency. The direction of the strong currents was dominantly to
the northeast. The periodic occurrence of strong currents caused seabed erosion ~14% of the time over the year-long deployment on the Beaufort shelf break.
Qualitative correlation between the ADCP backscatter and the suspended sediment
concentration (SSC) measured by the OBS sensors was observed lending some confidence of the usefulness of the OBS data. The backscatter data recorded by the ADCP were good through the entire deployment duration and showed qualitative correlation with
the peaks of bottom currents. The ADCP backscatter and the OBS data both demonstrate periodic resuspension of bottom sediments. Maximum SSC of the OBS data were ~40 mg/l at 1 mab and reached ~150 mg/l at 0.5 mab. The backscatter intensity during
strong current events reached up to ~60 arb. dB and were ~35 arb. dB higher than the background value. The increase of backscatter extended through the entire bottom layer, and was the strongest immediately above the seabed and decreased with the
increase of height above bottom.
Spectral analysis of the hourly mean current speed and velocity component data suggest that the energy at the tidal frequencies (12 and 24 hours) was much lower than that at the lower frequencies at ~20 days and
1.5 months periods respectively. Energy also increased over the 2 - 5 day periods suggesting contribution from meteorological storms. The significant wave height and peak wave period data derived from the MAVS burst data look suspicious and likely
indicate that the effect of waves was not significant in 168 m water depth along the shelf break. However, the spectra of the along and cross-shelf velocity components did show elevated energy at the swells (~15 s) and infra-gravity (>33 s) bands
suggesting that these longer-period waves do have some impact on the seabed on the Beaufort shelf break.
Two distinctive types of current events were identified based on the velocity profile data recorded by the ADCP: Type ii depth-independent
events in which increased currents extended through the entire bottom layer and Type iii bottom-intensified events in which the increase of current speeds was confined to the bottom ~0.3 mab. Depth-independent Type ii events are recognized in strong
horizontal velocity throughout the bottom layer, current direction dominantly to the NE, increased backscatter and little change of vertical velocity. Bottom-intensified Type iii events, however, are mainly shown by strong horizontal shear between
the horizontal currents of the upper and lower bins, variable or clock-wise rotating current direction, large near bottom vertical velocities and non-correlation with the increases of backscatter intensity. Bottom intensified Type iii events are also
generally associated with large-amplitude temperature changes while temperature changes for Type ii current events were characterized by high frequency but low amplitude variability. The occurrences of Type ii and Type iii events were mutually
exclusive. Type iii events mostly occurred over the ice-covered winter months while Type ii events seem to occur throughout the year, but events with the strongest currents mainly occurred in the late spring and summer of 2016. The bottom-intensified
Type iii current events represent an unusual phenomena as they were confined close to the seabed, energetic and yet causing little increase of backscatter intensity. The cause of these unusual bottom-confined strong current events should be further
explored in future research.
|Summary||(Plain Language Summary, not published)|
An instrumented seabed lander was deployed on the Beaufort shelf break from August 2015 to August 2016 to obtain the first ever year-long under ice
observation of bottom currents and sediment transport for the deep-water setting in the Beaufort Sea. The bottom currents were found to periodically exceed 0.3 m/s and reach a maximum of 1.1 m/s. The direction of the strong currents was dominantly to
the northeast roughly parallel with the along-shelf bathymetry. The strong bottom currents could resuspend the muddy sediments in 14% of the time and the maximum observed suspended sediment concentration reached 150 mg/L. Therefore bottom currents
are strong enough and occur frequently to cause erosion and/or prevent deposition of fine size sediments on the Beaufort shelf break and upper slope. Depth-independent and bottom-intensified current event types were recognized based on vertical
velocity profile patterns, current direction and association of increased sediment concentration.