RRS Discovery DY173
Cruise summary report
| Cruise Info. | |
| Ship name (ship code) | RRS Discovery (74EQ) |
| Cruise identifier | DY173 |
| Cruise period | 2024-02-05 — 2024-03-13 |
| Status | Completed |
| Port of departure | Walvis Bay, Namibia |
| Port of return | Mindelo, Cape Verde |
| Purpose | Research |
| Objectives | The study can be subdivided into three main areas, which are (i) the termination of the submarine channel and transition to unconfined flow (termed the `lobe') in international waters, (ii) the main submarine channel within international waters, and (iii) upper canyon in Angolan and DRC waters.
Lobe area: This DY-173 research cruise initially sought to understand how turbidity currents evolve at the end of the submarine channel system, in an area termed the lobe, beyond the final mooring deployed in 2019-2020 (Talling et al., 2022). This and older lobes have previously been studied by Ifremer (e.g. Dennielou et al., 2017). The objective was to understand the final stages of turbidity currents as they leave a submarine channel and become less well confined. Sensors deployed in 2019-2020 recorded turbidity currents with front speeds of 5-8 m/s within the submarine channel (Talling et al., 2022; Baker et al., in prep), and we wish to determine what happens at the end of this channel, and as flows then become less confined. This lobe area was also chosen because all previous monitoring of turbidity currents has been in confined canyons and channels (Talling et al., 2023), and this would be the first monitoring of turbidity currents as they leave a channel and become unconfined. The Congo River also had an exceptionally large flood with a peak discharge of 79,000 m3 on 9th Jan 2024, the largest flood since the 1960s, and this flood occurred only a few weeks before the DY-173 cruise. It seems likely this exceptional river flood may bring a lot of sediment into the canyon head, thus causing powerful turbidity currents in the canyon-channel (Talling et al., 2022), which might break moorings sited further up the system. Situating moorings further down the system was thus felt to be prudent, and this distal lobe area is also situated in international waters to simplify permitting. A detailed swath multibeam survey of the end of the channel was thus completed initially by DY-173. In general, it was found that the submarine channel becomes much less deeply (< 20m) incised, as compared to its upper (>20-to-200 m) reaches. However, the new survey also showed there are two reaches of deeper (~20m) incision, interspersed with less confined (< 5-8 m) flow, along the final part of the channel. The more deeply incised reaches are linked to knickpoints, likely caused by seabed failure with `ragged edges? seen in mapping, and consistent with previous work by Ifremer on the lobe (Dennielou et al., 2017). The final deepest-water knickpoint has developed between 2019-2020, as it is not visible in earlier Ifremer surveys (Dennielou et al., 2017), and it seems to have been formed by the unusually powerful turbidity currents in January and March 2020. Beyond this final knickpoint, the channel ends and flows become unconfined. This unconfined area has blocky topography in sub-bottom profiler data suggesting a debris flow, and this is consistent with sediment facies observed in DY173 cores. The bathymetric survey of the lobe was extended towards the south and west to include the un-channelised area immediately beyond the channel mouth, and in order to provide a base map for the 2025 cruise that can then determine whether the channel extends further in 2025. The 4 NERC moorings were then deployed at sites along the axis of the distal channel on the lobe, at sites where it was more deeply (moorings M1, M3, and M4) or more poorly (mooring M2) incised. The two Ifremer moorings (PG1 and PG2) were deployed outside the channel on levees, on either side of the most proximal M1 mooring. |
| Chief scientist | Megan L Baker (University of Durham Department of Geography), Peter J Talling (University of Durham Department of Geography) |
| Cruise report |  (19.54 MB) |
| Ocean/sea areas | |
| General | South East Atlantic Ocean (limit 20W) Gulf of Guinea |
| Specific | Congo Submarine Fan, southern Gulf of Guinea, eastern Atlantic Ocean, offshore West Africa. |
| Track charts | (0.02 MB) |
| Measurements | |
| Physical oceanography | |
| CTD stations | Quantity: number of profiles = 8 Description: CTD casts for to calibrate swath multibeam bathymetry surveys. |
| Geology and geophysics | |
| Core - soft bottom | Quantity: number of cores = 8 Description: 8 multicores, with individual barrels used for Micro-plastics, Micro-plastic Leachates, Sedimentology & OC ?13C (top 10 cm extruded), Archive 1 and Archive 2. In siome cases 6 barrels were not recovered at a site. See cruise report for details and locations of all multicores. Archived at BOSCORF. |
| Core - soft bottom | Quantity: number of cores = 30 Description: 30 piston cores with lengths of up to 10m, although in some other cases there was no recovery apart from the core catecher. See cruise report for details and locations of all piston cores. Archived at BOSCORF. Used to quantify organic carbon, microplastics and understand depositional processes. |
| Multi-beam echosounding | Quantity: number of surveys = 4 Description: Swath multibeam survey data was collected using a ship-mounted EM122 system, at a speed of 5-6 knots, for 4 areas. These are (i) the deep-water lobe at the end of the Congo Channel, (ii) deep-water channel within international waters, (iii) upper canyon axis in Angolan water, and (iv) upper canyon axis in DRC water The sounder is used in DEEP mode and with a 45/45 angular aperture for lobe and channel areas, as well for canyon zone in the Angola part. For the upper part of the canyon, in DRC waters, the angular aperture has been widened to 60/60, and the multibeam set to MEDIUM mode. The positioning system is a differential GPS system. The sound velocity profiles entered into the multibeam echosounder acquisition software come from the CTD stations set up during the cruise. See cruise report for details and locations of multibeam bathymetry surveys, and how these survey data were processed and cleaned. |
| Moorings, landers, buoys | |
| Physical oceanography | |
| Current profiler (eg ADCP) | 6° 30' 56" S 5° 55' 37" E — NERC Mooring 1: Mooring with downward-pointing 300kHz ADCP deployed 42m above seabed, with SBE on frame. Deployed on 11th Feb 2024 at Depth 4812m. Between Ifremer moorings at end of main channel. |
| Current profiler (eg ADCP) | 6° 30' 43" S 5° 48' 26" E — NERC Mooring 2: Mooring with downward-pointing 300kHz ADCP deployed 42m above seabed, with SBE on frame. Deployed on 12th Feb 2024 at Depth 4883m on channel across lobe. |
| Current profiler (eg ADCP) | 6° 20' 37" S 5° 55' 50" E — Peer Gynt Mooring 1 (from Ifremer): Mooring with downward pointing 300kHz ADCP deployed 79m above the seabed. Sediment trap is at 17.5m. Deployed on 12th Feb 2024 at 4791m on levees just outside end of main channel. |
| Current profiler (eg ADCP) | 6° 44' 54" S 5° 26' 22" E — NERC Mooring 4: Mooring with downward-pointing 300kHz ADCP deployed 42m above seabed, with SBE on frame 20m. Deployed on 13th Feb 2024 at Depth 4975m on channel across lobe. |
| Current profiler (eg ADCP) | 6° 20' 37" S 5° 55' 50" E — Peer Gynt Mooring 1 (from Ifremer): Mooring with downward pointing 300kHz ADCP deployed 79m above the seabed. Sediment trap is at 17.5m. Deployed on 12th Feb 2024 at 4791m on levees just outside end of main channel. |
| Current profiler (eg ADCP) | 6° 36' 50" S 5° 41' 30" E — NERC Mooring 3: Mooring with downward-pointing 300kHz ADCP deployed 42m above seabed, with sediment trap at 20m, and SBEs at ~22 and 32m. Deployed on 13th Feb 2024 at Depth 4906m on channel across lobe. |
