Part 1. Results on aquitanian shelf

The data from serie of SEDYMAQ campaigns, combined with data from cruises of our partners at SHOM (PROTEVS mission in 2013) and Ifremer (VOLT missions in 2015 and 2016), have helped to profoundly revise our understanding of sediment dynamics on the Aquitaine shelf and the proximal zone of the Capbreton canyon.

On the Aquitaine shelf, the results of the SEDYMAQ 1 (2008), SEDYMAQ 2 (2010) and SEDYMAQ 3 (2012) cruises can be summarised as follows:

• On the middle and inner shelves, with the exception of small objects (ripples and oscillation megarides), the surface sediment cover has been stable over several decades (1980-2008) (Gillet et al., 2009).
• On the inner shelf, the description of the surface cover as a 'palimpsest' as defined by (Cirac et al., 2000) has been refined (Figure 1). From the concept of discontinuous sandy veneers simply laid down on a coarse substrate, we have moved on to the concept of sorted bedforms (bodies resulting from a sedimentary feedback sorting process) for which we have demonstrated that fine and coarse sediments are imbricated rather than superimposed (Mazières, 2014; Mazières et al., 2015).
• On the inner shelf, the superficial sandy stock tends to migrate gently towards the coast over time under the combined effect of swells and episodic pulses of current. The displacement speeds measured vary according to the size of the sedimentary objects. For example, sorted bed forms (ten to one kilometre long and one metre thick) can migrate at speeds of around 5 to 10 m/year, while hydraulic dunes (hectometre long) migrate at speeds of up to 30 m/year (Mazières, 2014; Mazières et al., 2015)(Figure 2).

The data collected on the Aquitaine plateau during the SEDYMAQ programme were used in Alais Mazières' thesis (Mazières, 2014) and in several Masters intership (Darsaut, 2011; Gehin, 2009; Schmidt, 2012).

This work has been published (Mazières et al., 2015), presented at conferences (Cirac et al., 2009; Gillet et al., 2009; Mazières et al., 2011; Mazières et al., 2014b; Mazières et al., 2013c; Sterckeman et al., 2009) and used to produce a map of the surface sediment cover of the Aquitaine shelf (Cirac et al., 2016).

All of the initial objectives set for the Aquitaine shelf were achieved following the completion of the SEDYMAQ 1, 2 and 3 missions. The SEDYMAQ programme was therefore considered closed in this area at the time of the SEDYMAQ 4 mission, which focused exclusively on the Capbreton area.

Figure 1. A, Lateral sonar mosaics collected in the Salie-Biscarrosse area during the SEDYMAQ 2 and 3 missions; B, interpretation of these data; C, oscillation megarides; E, F, G: sedimentary facies sampled.

Figure 2: Example of rapid migration of hydraulic dunes in the Biscarrosse area between 2010 and 2012.

Part 2. Results on the Capbreton Canyon

Data from the four SEDYMAQ cruises in the Capbreton canyon (head and proximal part, at depths of between 10 and 300 m) have been analysed and used in two PhD thesis, by Alais Mazières (defended in 2014) and Léa Guiatrennec-Faugas (defended in 2020), as well as in 8 Masters internships. This data has been used in 5 A-rank publications, has been the subject of a total of 26 conference papers (see list of references) and has been presented 6 times at vulgarisation events (Journées du Gouf and CPIE).

Backed up by the historical campaigns of the ISTAS series and the cruises carried out by our partners SHOM (PROTEVS mission in 2013) and Ifremer (VOLT missions in 2015 and 2016), the SEDYMAQ cruises series has first enabled us to acquire an extraordinary set of temporal bathymetric data on the head and upstream part of the Capbreton canyon. This data set has made it possible to carry out an unprecedented morpho-bathymetric "time-lapse survey" over a 20-year period from 1998 to 2018 (figure 3).

The analysis of this unique data set yielded the following key results:

This rare time series exposes the morphological evolution of this outstanding dynamic area over the last 20 years. Our work shows that much of the changes are located in the canyon floor and head (figure 4). Following a period characterised by a unique flat floor thalweg, the canyon was affected by an incision with low lateral terraces which resulted in a narrow axial talweg (figure 5). The deepening of the narrow thalweg was induced by retrogressive erosion according to the presence of upstream-migrating knickpoints (figure 6), while low elevation residual terraces formed as the canyon reached a local equilibrium profile.

The flat thalweg observed in 1998 is likely a result of a partial filling of the canyon thalweg by a substantial emptying of the canyon head and significant mass transfer to the proximal part of the canyon. A flat floor thalweg was not observed again in the remaining of our time series (since 2010), suggesting a possible quieter working mode of the canyon. We also propose the first accurate volume quantification of sediment displacement on the canyon floor. Our findings underline the alternation of filling and erosive periods in the canyon axis and an unexpected continuous sediment deposition in the canyon head during the last 20 years.

In addition to these bathymetric differential data, the analysis of sediment sampling, imaging, seismic reflection and SMF water column data acquired during the SEDYMAQ campaigns, as well as the hydrodynamic modelling that we were able to base on these data, led to the following additional results:

The north-south coastal drift from the Aquitaine coast periodically feeds sand directly into the head of the Capbreton canyon, particularly during storm swells.

The feeding of the canyon in its upstream part is not limited to its head. Small eroding lateral tributary gullies also appear to play a part in feeding the canyon by directly transferring sediment from the inner plateau to this deep sedimentary conduit (figure 5).

Beneath the inner plateau to the north of the canyon, Sparker data revealed some traces of a network of filled-in incisions. Only some of these incisions could correspond to the Adour fluvial quaternary palaeo-network connected to a palaeo-head of the canyon. One of these incisions, which is much wider and deeper, is interpreted as a paleo-position of the canyon, Miocene in age.

Finally, the SMF water column data from the SEDYMAQ 4 cruise revealed active cold fluid seeps (bubble plumes) on the flanks of the Capbreton canyon (figure 7). These active cold fluid seeps are related to the MDACs (Methane Derived Authigenic Carbonates) identified in the same area by the HaPoGé cruise (Gillet et al. 2017).

Figure 3. Bathymetric surveys of the study area acquired between 1998 and 2018 (Guiastrennec-Faugas et al., 2020)

Figure 4. Depth differences calculated through raster subtraction of bathymetric grids on canyon head. Warm colors show erosion of over 1 m. Cool colors show areas of deposition of over 1 m. A: Raster subtraction of bathymetric grids corresponding to August 1998 and May 2001. B: Raster subtraction of bathymetric grids corresponding to May 2001 and June 2010. C: Raster subtraction of bathymetric grids corresponding to June 2010 and June 2012. (Mazieres et al., 2014)

Figure 5. (A) Bathymetric survey of the study area acquired in May 2018 during the SEDYMAQ 4 cruise. (B) Relative relief to mean thalweg longitudinal profile. (C) Morphological interpretation of the Capbreton canyon upper part. Blue area corresponds to a specific area of consolidated sediments and authigenic carbonates used for statistics on repeat survey observations to validate the depth uncertainty (Guiastrennec-Faugas et al., 2020).

Figure 6. Evolution of knickpoints in a distal meander of the upper part of the Capbreton canyon. Left illustrations (B, D, F, H): representations of the bathymetric differentials between 2013/2012 (B), 2015/2013 (D), 2016/2015 (F) and 2018/2016 (H). Blue arrows represent the upstream migration of knickpoints (m). Right illustrations (A, C, E, G, I): relative relief to mean thalweg longitudinal profile in 2012 (A), 2013 (C), 2015 (E), 2016 (G) and 2018 (I) (Guiastrennec-Faugas et al., 2020).

Figure 7. SMF water column data: plume of bubbles associated with an active cold fluid seeps in the Duprat rocks area, flank of the upstream part of the Capbreton canyon (Gillet et al., 2022).