M&M - Turbidity and Suspended Sediment Concentration (SSC)

An autonomous optical backscatter sensor (NKE STBD 1200) was installed on the CTD probe in 2014 during the BATHYCOR1 cruise in order to collect ten vertical turbidity  profiles along the canyon axis (23^th August 2014) (Fig. 1, Table 2). The profiles had to be interrupted at an average altitude of 40 to 70 mab (m above the bottom) for safety issues.

The sensor was previously calibrated with Formazin as recommended by NKE marine electronics (www.nke-marine-electronics.com). The results are given in Nephelometric Turbidity Units (NTU).

In order to link the NTU values to the Suspended Sediment Concentration we built a calibration curve of our sensor using bottom sediment that had been previously sampled with a push core (CT9) during ROV dive 407-11, cruise ESSROV2010 (Fontanier et al., 2012). Particle grain sizes were analyzed using laser diffraction and median grain size (D50) was 11 µm, although some particles were as large as 150 µm (see (Fontanier et al., 2012) - figure 2D). We first measured the turbidity in a 10-liter container of distilled water in which we added NaCl to obtain a salinity concentration of 38.4 g/l. We checked the concordance with a laboratory turbidimeter (Hatch 2100n IS - Iso Method 7027) to confirm the starting point (0.04 NTU). Then we progressively added a known quantity of particles in the container of salty distilled water. After each addition of particles, the water was mixed and left for 30 seconds before recording the turbidity every second for 5 minutes. We tried to keep a uniformly mixed suspension with no sediment settling but failed to prevent large particles from sinking to the bottom of the container. We then calculated the mean of the recorded turbidity in NTU for each known concentration of particles. This allowed building a calibration curve, starting from 0.04 NTU to 3 NTU, which corresponded to the range of our in situ values (online supp. mat. Fig. A1).

Results - Turbidity and Suspended Sediment Concentration (SSC)

The ten vertical turbidity profiles (NKE STBD 1200) collected at the CTD stations in the canyon axis show turbidity values ranging from 0.15 NTU on the shelf and in the surface layer to 2.2 NTU at the bottom of the canyon exit ;

Turbidity values were very low above 300 m depth (<0.2 NTU) on the whole set of profiles (on the shelf, flanks and canyon axis) while the stations located downstream of the bauxite residue effluent in the canyon axis (Stations A, B, C, D, E) displayed maximum turbidity in the lower layer.

Maximum turbidity values were measured above the bottom at the deepest station (A), exceeding 2 NTU at the canyon exit. However, the maximum particle load does not always lie just above the bottom: at stations B and C there was an intermediate maximum between 600 and 700 m, and to a lesser extent at ~500 m at station A. The turbid layer was thicker closer to the pipeline effluent. As the effluent is a mixture of particles of different sizes in freshwater, it is expected that downstream it generates several layers according to water density and particle size.

The east-west orientation of the bathymetry around stations C, E, G might also contribute to the dispersion of the particles (cross-axis current), as shown by the multi-layered turbidity profile at station K (bottom depth 770 m; not shown). Thus the coral colonies at Cassis-500 are impacted by the turbidity of the red mud.

According to the calibration of our sensor, we roughly correlated a turbidity value of 1 NTU to an SSC=0.6 mg/l, and the maximum measurement of around 2.5 NTU to an SSC=2 mg/l.