Research ArticleGEOLOGY

Newly recognized turbidity current structure can explain prolonged flushing of submarine canyons

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Science Advances  04 Oct 2017:
Vol. 3, no. 10, e1700200
DOI: 10.1126/sciadv.1700200
  • Fig. 1 Mooring locations in the Congo Canyon.

    (A) Map of the Congo Canyon showing study area (rectangle), with bathymetric contours in meters. (B) Map showing the location of the two moorings deployed in 2013 (6, 57). (C) Map showing location of 2010 mooring (5). (D) Cross-canyon profile showing ADCP suspended 85 m above the canyon floor. Location of cross section indicated in (C).

  • Fig. 2 Turbidity currents that flush the Congo Canyon are far more prolonged than any previously monitored oceanic turbidity current.

    This figure compares the duration of the Congo Canyon flows studied here and oceanic turbidity currents that have been monitored previously using ADCPs in other shallower water locations (3034).

  • Fig. 3 Turbidity current structure and duration from ADCP measurements at the 2009–2010 mooring site.

    (A) Full velocity time series for 3 months in 2009–2010 (5), showing that flows are active for ~33% of the time. Single flow in (B) to (D) is shown by the red box. (B) Horizontal velocity (30-min averages). (C) Intensity of large-scale velocity fluctuations, defined as the root mean square of differences between individual velocity measurements and 1-min averages. (D) Sediment concentrations inverted from 300- and 75-kHz ADCP acoustic backscatter (see Materials and Methods and figs. S3 to S10). This analysis assumes that the flow contains a single grain size, and variations in grain size may cause artefacts such as higher concentrations above lower concentrations. (E to I) Velocity profiles for different parts of the flow. (J) All velocity profiles combined in one plot.

  • Fig. 4 Turbidity current structure in laboratory experiments and in the Congo Canyon.

    Structure is shown by temporal changes in velocity measured at one spatial position, with red solid line denoting highest flow velocities. (A) Typical laboratory experiment with a finite-volume (surge-like) release (1316) or previous oceanographic measurements (3034); more sustained input would produce a better developed body. Arrows denote relative movement of sediment-laden fluid with respect to the flow front, with flow front velocity thus subtracted from measured velocities. The arrows show whether the body feeds the head. Temporal changes in maximum flow velocity are shown by red lines, and velocity profile shapes are shown by blue dotted lines. (B) Turbidity current structure in the Congo Canyon.

  • Fig. 5 Structure of the frontal-cell during the first 120 min of the flow.

    (A) Horizontal flow velocity. Sidelobe interference area (see Materials and Methods) is covered by the gray box. (B) Velocity relative to the flow front propagation velocity (which is here fixed at 1.35 m/s). (C) Sediment concentration as derived from the acoustic inversion. (D) Variation of the calibration constant (Kt) is used to indicate either grain size variation or higher sediment concentrations (see Materials and Methods). (E) Velocity fluctuations.

  • Fig. 6 Timing and triggers of turbidity currents.

    (A) Flow velocities measured at 18 m above the seafloor by ADCPs at two mooring sites in 2013. Site 2013a is located 22-km up-canyon from site 2013b (Fig. 1B) (6). (B) Turbidity currents of the 2010 deployment plotted against two potential triggers: significant wave height in red and Congo River discharge at the Kinshasa gauging station in green. (C) Plot of changes in Congo River discharge (blue line), periods when measurement instruments were present in the canyon (gray bars) (5, 19, 58), and timing of turbidity currents (red dots). The green star is turbidity current shown in Fig. 3.

  • Table 1 Summary of flow properties of the 2010 deployment (numbering correspond to Fig. 3A).
    Flow123456Mean
    Duration (days)10.15.55.26.66.36.36.7
    Maximum thickness (m)53574869776862
    Maximum ADCP velocity (m/s)1.21.212.41.91.41.5
    Average ADCP velocity (m/s)0.40.40.30.70.50.40.5
    Front propagation velocity (m/s)0.80.80.71.61.51.01.1
    Average height of maximum velocity above the bed (m)6.86.95.814.211.810.09.3
    Time of maximum velocity after arrival of the flow front (min)25341008252536
    Average sediment concentration (%vol)*0.0180.0200.0200.0230.0200.0170.020
    Peak sediment concentration (%vol)*0.0760.0470.0860.1630.1680.1550.116
    Maximum flow discharge (103 m3/s)4.64.92.714.916.310.49.0
    Average flow discharge (103 m3/s)2.42.81.66.96.03.73.9
    Maximum sediment discharge (103 kg/s)*3.12.72.013.29.06.16.0
    Average sediment discharge (103 kg/s)*1.21.58.84.33.21.72.1
    Water volume displaced (km3)2.11.40.74.03.32.12.3
    Sediment volume displaced (Mt)*1.00.70.42.51.70.91.2
    Organic carbon displaced (Mt)0.040.030.020.100.070.040.05

    *Assuming a uniform grain size of 4.23 μm for inverting ADCP backscatter to sediment concentration (see Materials and Methods).

    †Assuming an average carbon content of 3 to 5% weight, as measured within turbidity current deposits on the Congo Fan (see Materials and Methods) (37).

    Supplementary Materials

    • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/10/e1700200/DC1

      fig. S1. Distance to the seafloor in different directions measured by an individual ADCP beam at the up-canyon 2013a site and the down-canyon 2013b site in 2013.

      fig. S2. Illustration of the method used to calculate flow front velocity.

      fig. S3. Raw backscatter plot.

      fig. S4. The bed echo attenuation for 300- and 75-kHz ADCP during the turbidity current.

      fig. S5. Sediment attenuation coefficient (ξ) for 300- and 75-kHz frequencies, by particles with diameters between 1 and 1000 μm.

      fig. S6. Difference between the bed echo attenuation (Abed) and the predicted cumulative echo attenuation (Aprofile) within the water column from the 75-kHz ADCP data.

      fig. S7. The suspended grain size results derived from the comparison between the 75-kHz ADCP bed echo.

      fig. S8. Cores from floor of Congo Canyon.

      fig. S9. Sediment concentration (g/liter) derived using the ADCP backscatter magnitudes.

      fig. S10. The calibration constant Kt.

      fig. S11. Bed shear stresses generated by the flow.

      fig. S12. Comparisons of the instantaneous sediment and water discharges in the Congo Canyon turbidity current shown in Fig. 2, with the mean annual discharges of water and sediment in major rivers.

      fig. S13. Comparison of turbidity current arrival times with possible triggering factors in the Congo Canyon.

      fig. S14. Increase in flow duration caused by flow stretching, which is due to a difference in the speed of the front and tail of the flow.

      table S1. Flow durations, thicknesses, and peak velocity measured at heights in excess of 18 m above the bed in 2013.

    • Supplementary Materials

      This PDF file includes:

      • fig. S1. Distance to the seafloor in different directions measured by an individual ADCP beam at the up-canyon 2013a site and the down-canyon 2013b site in 2013.
      • fig. S2. Illustration of the method used to calculate flow front velocity.
      • fig. S3. Raw backscatter plot.
      • fig. S4. The bed echo attenuation for 300- and 75-kHz ADCP during the turbidity current.
      • fig. S5. Sediment attenuation coefficient (ξ) for 300- and 75-kHz frequencies, by particles with diameters between 1 and 1000 μm.
      • fig. S6. Difference between the bed echo attenuation (Abed) and the predicted cumulative echo attenuation (Aprofile) within the water column from the 75-kHz ADCP data.
      • fig. S7. The suspended grain size results derived from the comparison between the 75-kHz ADCP bed echo.
      • fig. S8. Cores from floor of Congo Canyon.
      • fig. S9. Sediment concentration (g/liter) derived using the ADCP backscatter magnitudes.
      • fig. S10. The calibration constant Kt.
      • fig. S11. Bed shear stresses generated by the flow.
      • fig. S12. Comparisons of the instantaneous sediment and water discharges in the Congo Canyon turbidity current shown in Fig. 2, with the mean annual discharges of water and sediment in major rivers.
      • fig. S13. Comparison of turbidity current arrival times with possible triggering factors in the Congo Canyon.
      • fig. S14. Increase in flow duration caused by flow stretching, which is due to a difference in the speed of the front and tail of the flow.
      • table S1. Flow durations, thicknesses, and peak velocity measured at heights in excess of 18 m above the bed in 2013.

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