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TitleStorm-induced turbidity currents on a sediment-starved shelf: insight from direct monitoring and repeat seabed mapping of upslope migrating bedforms
AuthorNormandeau, AORCID logo; Bourgault, D; Neumeier, U; Lajeunesse, P; St-Onge, G; Gostiaux, L; Chavanne, C
SourceSedimentology 2019 p. 1-24, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20180334
Mediapaper; on-line; digital
File formatpdf (Adobe® Reader®); html
ProvinceQuebec; Eastern offshore region
AreaSt. Lawrence Estuary; Pointe-des-Monts
Lat/Long WENS -67.5000 -67.3667 49.3667 49.2500
Lat/Long WENS -67.3667 -67.3667 49.3667 49.2500
Subjectsmarine geology; surficial geology/geomorphology; sedimentology; Nature and Environment; Science and Technology; turbidity currents; flow structures; bedforms; meteorology; storms; sediment stability; continental margins; continental shelf; estuaries; bottom currents; submarine features; submarine canyons; gullies; geophysical surveys; acoustic surveys, marine; bathymetry; depositional environment; suspended sediments; sediment dispersal; sediment volumes; monitoring; Wind; Infrastructures
Illustrationslocation maps; geoscientific sketch maps; schematic cross-sections; bar graphs; satellite images; profiles; time series; plots; models
ProgramPublic Safety Geoscience Marine Geohazards
Released2019 10 08
AbstractThe monitoring of turbidity currents enables accurate internal structure and timing of these flows to be understood. Without monitoring, triggers of turbidity currents often remain hypothetical and are inferred from sedimentary structures of deposits and their age. In this study, the bottom currents within 20 m of the seabed in one of the Pointe-des-Monts (Gulf of St. Lawrence, eastern Canada) submarine canyons were monitored for two consecutive years using Acoustic Doppler Current Profilers. In addition, multibeam bathymetric surveys were carried out during deployment of the Acoustic Doppler Current Profilers and recovery operations. These new surveys, along with previous multibeam surveys carried out over the last decade, revealed that crescentic bedforms have migrated upslope by about 20 to 40 m since 2007, despite the limited supply of sediment on the shelf or river inflow in the region. During the winter of 2017, two turbidity currents with velocities reaching 0.5 m/sec and 2.0 m/sec, respectively, were recorded and were responsible for the rapid (<1 min) upstream migration of crescentic bedforms measured between the autumn surveys of 2016 and 2017. The 200 kg (in water) mooring was also displaced 10 m down-canyon, up the stoss side of a bedform, suggesting that a dense basal layer could be driving the flow during the first minute of the event. Two other weaker turbidity currents with speeds <0.5 m/sec occurred, but did not lead to any significant change on the seabed. These four turbidity currents coincided with strong and sustained wind speed >60 km/h and higher than normal wave heights. Repeat seabed mapping suggests that the turbidity currents cannot be attributed to a canyon-wall slope failure. Rather, sustained windstorms triggered turbidity currents either by remobilizing limited volumes of sediment on the shelf or by resuspending sediment in the canyon head. Turbidity currents can thus be triggered when the sediment volume available is limited, likely by eroding and incorporating canyon thalweg sediment in the flow, thereby igniting the flow. This process appears to be particularly important for the generation of turbidity currents capable of eroding the lee side of upslope migrating bedforms in sediment-starved environments and might have wider implications for the activity of submarine canyons worldwide. In addition, this study suggests that a large external trigger (in this case storms) is required to initiate turbidity currents in sediment-starved environments, which contrasts with supply-dominated environments where turbidity currents are sometimes recorded without a clear triggering mechanism.
Summary(Plain Language Summary, not published)
Turbidity currents were recorded on the seafloor of the St. Lawrence Estuary and were associated with the migration of bedforms. Turbidity currents were triggered during storms in 2017 and led to significant change on the seafloor.

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