The PEPITE project therefore aims to retrace the sedimentary history of several coastal marine terraces now colonized by maerl beds (Morlaix Bay, Concarneau Bay and Belle-Île). The originality of maerl as a marker of paleoenvironmental evolution is manifold: (1) It constitutes an ecological and sedimentary signal recorded on the scale of the Armorican domain ; (2) It is contemporary with the anthropization of coasts and watersheds; (3) Its sensitivity to repeated increases in turbidity in the environment is an excellent marker of sedimentary fluxes brought in from the watershed and/or the remobilization of coastal sediments by waves or man ; (4) finally, once fossilized by silted-up sediments, it ¿traps¿ past biodiversity, which can be examined in relation to the current ecosystem to measure its evolutionary factors. Analysis of fossil maerl beds in the Rade de Brest (Ehrhold et al., 2021) shows the rapid response of these fossil biocenoses to surrounding factors over several thousand years. The choice of the 3 sites was dictated by the different hydrological, anthropic and morphological influences at each site, the aim being to monitor the evolution of these calcareous algae as a proxy for the construction of coastal zones during the Holocene. The summary results presented here, by zone, integrate both the analyses of seismic data acquired during PEPITE-HAL (30-03 to 16/04/2019) with the Haliotis launch and those of coring data acquired during PEPITE-THA (16/05 to 31/05/2019) from the N/O Thalia.

1 - The Morlaix Bay sector:

The Bay of Morlaix, like the Rade de Brest, is characterized by highly anthropized watersheds and two estuaries (Penzé and Morlaix), whose marine valleys are strongly channeled by the geometry of the bedrock. The PEPITE-HAL (30-03 to 16/04/2019) and PEPITE-THA (16/05 to 31/05/2019) offshore missions enabled the acquisition of 162 seismic profiles (sediment sounder and sparker monotrace) and 18 Kullenberg cores in Morlaix Bay (Schittulli, 2022) (Fig. 1A). The seismic data were processed (Delph Ixblue and QcSubop Ifremer) and incorporated into a Kingdom suite 8.7 project for interpretation. All cores were opened in the laboratory, described and measured on Geotek MSCL-S and XCT benches. 4 reference cores were selected (PEPITE-KS69-76-82-84) as representative of the depositional environment of the main structural compartments, on which granulometric and geochemical analyses, maerl concentration measurements and radiocarbon dating were performed.

Topographic analysis shows that the morphology of the Morlaix and Penzé valleys is asymmetrical (Fig. 1B). Their talwegs are deep and sinuous, with sedimentary deposits filling the channels. At Callot, a plateau at -11 m seems to correspond to the old riverbed. A flat area at -14 m stands out among the Morlaix talwegs (fig. 1C).

Figure 1: A: Positions of processed seismic profiles and analyzed cores; B: Morphological profile intersecting the two valleys; C: Isopaque sediment thickness map of the Guerheon zone.

The seismo-stratigraphic and sedimentological analysis of reference profiles and cores (Fig. 2A and C), based on fossil maerl beds, has enabled us to reconstruct the emplacement of Holocene infill deposits in the bay (Fig. 2B). The succession of sedimentary depositional sequences shows the existence of estuarine sedimentation from 8 ka BP onwards. The environment at Morlaix was then a vast estuary made up of contributions from the two banks, whose mouths were very close to each other. This transgressive period, with a rather slow rise in sea level, was accompanied by large quantities of clay deposits rich in organic matter. They filled the valleys and adjacent terraces at a sedimentation rate of over 3 mm/year. Up to 3 ka BP, a sandy-silty shell facies, reflecting a change to a more marine environment, is established throughout the bay, framed by two erosion surfaces at the base and top. On the marine terraces at around -11 m depth, above this transgressive procession whose MFS is difficult to determine, and sealed by sandy-shell levels resembling tempestuous deposits (around 3 ka BP), the maerl begins to form its first bioconstructions.

Figure 2: A: Analysis of the reference sparker seismic profile on Callot (no. 34); B: Stratigraphic infill model of Morlaix Bay over the 3 study areas; C: Synthetic log of the MLX-KS84 reference core

2 - Eastern terrace area in Concarneau Bay:

Concarneau Bay, and in particular the morphological terrace between Baie de la Forêt and Pointe de Trévignon, is a flat area up to 15 m deep, relatively protected from dragging gear, tidal currents and storms by the presence of numerous reefs. The PEPITE-HAL (30-03 to 16/04/2019) and PEPITE-THA (16/05 to 31/05/2019) offshore missions enabled the acquisition of 78 seismic profiles (sediment sounder and single-track sparker) and 39 Kullenberg cores on the east coast of Concarneau Bay (Bijon, 2020) (fig. 3). The seismic data were processed (Delph Ixblue and QcSubop Ifremer) and incorporated into a Kingdom suite 8.7 project for interpretation. All cores were opened in the laboratory, described and measured on Geotek MSCL-S and XCT benches. 7 reference cores were selected (PEPITE-KS49-51-52-55-56-58-60-64) as representative of the depositional environment of the main structural compartments, on which granulometric and geochemical analyses, maerl concentration measurements and radiocarbon dating were performed.

Figure 3: A: Positions of kullenberg coring stations: B: Seismic profiles (sparker and sediment sounder); C: Example of profile (n°27) chirp and sparker on the terrace.

The coastal terrace between Concarneau and Pointe de Trévignon is occupied by 4 seismo-stratigraphic units: U0b corresponding to the Variscan bedrock, U3, U4a and U4b for the sedimentary fill units (fig. 4). Morphology at the top of the bedrock is highly irregular. It features multiple incisions corresponding to small, branched NE-SW-trending paleovalleys and several-metre-high rock outcrops. Two surfaces flank the base and top of U4a. The total thickness of the sediments can reach a dozen meters in these small valleys, but is most often a few meters to 7m in line with the present-day maerl beds. The first transgressive marine deposits on the terrace are dated at 9000 years cal BP. They are rich in bivalves in a silt-clay matrix and have a rapid sedimentation rate. They are eroded at the surface, up to 6500 cal. BP, by a coarse deposit that can be interpreted as a major gully surface (horizon 1) by tidal current circulation. The second transgressive cortège, in which the first maerl beds are established, is also capped by an erosion surface (horizon 2) corresponding to a period of intense reworking of the coastal bed under the effect of repeated stormy periods. Finally, the high sea level (HST) procession seals the sedimentary fill on the terrace from 2000 cal. BP onwards, with different sedimentary facies (muddy, sandy or maerl), depending on the position of the environments in relation to hydrodynamic agents.

Figure 4: Interpretation of two profiles (12 and 26) perpendicular and longitudinal to the maerl terrace

3 - East coast of Belle-Île-en-Mer:

Belle-Île-en-Mer is the largest of the Breton islands. Located 15 kilometers off the mainland, it forms a plateau bounded by high cliffs and intersected by a large number of deep, branching valleys. Formed of volcano-sedimentary and volcanic rocks, with a slight metamorphic component, Belle-Île is part of the southern Armorican plateau. Together with the islands of Glénan and Groix, it forms morphological barriers at the front of the coast, delimiting a structural unit located at a depth of less than 50 m. Calcareous algae grow mainly on the east coast of the island, between Pointe de Taillefer and Pointe de kerdonis, on a gently sloping flat or terrace to a depth of around ten meters (fig. 5A). The PEPITE-HAL (30-03 to 16/04/2019) and PEPITE-THA (16/05 to 31/05/2019) offshore missions enabled the acquisition of 141 seismic profiles (sediment sounder and sparker monotrace) and 27 Kullenberg cores on the east coast of the island (Livramento, 2020) (fig. 5B). The seismic data were processed (Delph Ixblue and QcSubop Ifremer) and incorporated into a Kingdom suite 8.7 project for interpretation. All cores were opened in the laboratory, described and measured on Geotek MSCL-S and XCT benches. 14 reference cores were selected from 5 seismic profiles perpendicular to the coast (PEPITE-KS02-03-08-10-11-12-13-14-15-16-19-21-22-24) as representative of the depositional environment of the main structural compartments, on which granulometric and geochemical analyses, maerl concentration measurements and radiocarbon dating were performed.

   A                                                                                            B

Figure 5: A: Map of maerl cover on the east coast of Belle-Île en Mer (Dubreuil et al., 2017); B: Position of kullenberg cores and seismic profiles during the PEPITE survey.

Interpretation of the seismic data reveals the succession of 3 sedimentary sequences above bedrock, corresponding to the stacking of 6 seismic units separated by 3 regressive gully surfaces (at -8, -13 and -22m), the whole constituting the progradation prism leaning against the coast. The maximum thickness of the prism is 14 m at Le Palais (fig. 6A). Sedimentary cores allow us to characterize the last two units (U6 and U5), and even exceptionally U4 at the edge of the prism (PEPITE-BI-KS08 and KS09). Deposits at the coast, in the inner zone of the terrace, show a thick, vertically continuous maerl (several meters) in a muddy matrix. Towards the outer edge of the terrace, the maerl bioconstructions are thinner and are interspersed with sandier, shellier and even gravelly deposits over time (Fig. 6B). Further offshore, beyond a depth of 15 m, no more maerl thallus are present. The predominantly sandy-muddy facies are distinguished by their abundance of shell debris and turritella. They reflect a change in hydrodynamic conditions compared with the more sheltered environment of the inner terrace. Unlike the evolution of the maerl colonies in the other PEPITE project areas, the calcareous algae colonies on Belle-Île show a progradation in the construction of the bank from the coast towards the open sea until the Bronze Age, then a retreat towards the coast since this period, reflecting a major change in the evolution of turbidity in the environment.

   A                                                                                      B

Figure 6: A: Total thickness of the prism at the top of the bedrock (G. Jouet, in progress); B: Interpretation of core KS11 at the edge of the sedimentary prism (Livramento, 2020)

Références :

Bijon C., 2020. Sédimentation Holocène des terrasses littorales armoricaines. Analyses stratigraphiques sismiques et enjeux paléo-environnementaux : Exemple de la terrasse à maerl en baie de Concarneau. Mémoire de stage M2 (IUEM), 72p.

Dubreuil, J., Grall, J., & Barillé, A.-L., 2017. Cartographie fonctionnelle comme outil de gestion d'un habitat patrimonial : cas du banc de maërl de Belle-Île-en-Mer. Les cahiers naturalistes de l'Observatoire Marin.

Ehrhold A., Jouet G., Le Roy P., Jorry S., Grall J., Reixach T., Lambert C., Gregoire G., Goslin J., Roubi A., Penaud A., Vidal M., Siano R., 2021. Fossil maerl beds as coastal indicators of late Holocene palaeo-environmental evolution in the Bay of Brest (Western France). Palaeogeography Palaeoclimatology Palaeoecology, 577, 110525, ISSN 0031-0182.

Livramento A., 2020. Evolution sédimentaire de la terrasse a maerl au nord de Belle-Île (Morbihan). Mémoire de stage M1 (ENSEGID), 60p.

Schittulli M., 2022. Évolution Holocène des terrasses submergées de la baie de Morlaix : Analyse sismo-stratigraphique. Mémoire de stage M2 (IUEM), 75p.