|Titre||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|
|Auteur||Li, M Z; King, E L; Schillinger, D; Robertson, A G; Wu, Y|
|Source||Commission géologique du Canada, Dossier public 8553, 2019, 57 pages, https://doi.org/10.4095/314645 (Accès ouvert)|
|Éditeur||Ressources naturelles Canada|
|Media||en ligne; numérique|
|Formats||pdf (Adobe® Reader®)|
|Province||Région extracotière du nord|
|SNRC||107F/13NE; 107F/13SE; 107F/14NW; 107F/14SW|
|Lat/Long OENS||-135.2333 -134.9667 70.8833 70.8167|
|Sujets||marges continentales; plate-forme continentale; talus continental; levés géophysiques; levés sismiques marins; levés acoustiques marins; vélocités acoustiques; analyses spectrales; instruments
d'observation; photographie; milieu sédimentaire; courants de fond; sédiments marins; transport des sediments; érosion; sediments en suspension; température de l'eau; géochimie de l'eau de mer; salinité; méthodes d'échantillonage; pression;
conductivité; densités; bathymétrie; océanographie; Plate-Forme de Beaufort; géologie marine; géologie des dépôts meubles/géomorphologie; géophysique; Phanérozoïque; Cénozoïque; Quaternaire|
|Illustrations||tables; location maps; geoscientific sketch maps; photographs; profiles; plots; time series; spectra|
|Programme||Géo-risques marins, Géoscience pour la sécurité publique|
|Programme||Le Programme de
recherche et de développement énergétiques (PRDE)|
|Diffusé||2019 04 25|
|Résumé||(Sommaire disponible en anglais seulement)|
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.
|Résumé||(Résumé en langage clair et simple, non publié)|
Un module benthique instrumenté a été déployé au rebord de la plateforme continentale de Beaufort d'août 2015 à août 2016 afin d'obtenir la
première observation sur un an des courants de fond et du transport de sédiments sous la glace à la rupture de pente continentale prononcée de la mer de Beaufort. Les courants de fond dépassaient périodiquement 0,3 m/s, atteignant un maximum de 1,1
m/s. La direction des forts courants était principalement vers le nord-est, à peu près parallèle aux courbes bathymétriques le long de la plateforme continentale. Les courants de fond remettaient des sédiments vaseux en suspension 14 % du temps, et
la concentration observée de sédiments en suspension a atteint un maximum de 150 mg/L. Les courants de fond sont donc assez forts et assez fréquents pour causer de l'érosion ou empêcher le dépôt de sédiments fins au rebord et dans le haut de la pente
de la plateforme de Beaufort. Les profils de vitesse verticaux, la direction des courants et leur association avec la turbidité de l'eau ont permis de distinguer deux types de courants : les courants indépendants de la profondeur et les courants
intensifiés par le fond.