The PEPITE-HAL campaign is the first step in the study of fossil melobesian algae that colonized the Finisterian coast about 2000 years ago. Recent work in Brest harbour (Grégoire et al., 2017; Reixach 2017) shows that terrigenous coastal sedimentation is interspersed with major episodes of multi-centimetre biogenic constructions (maerl banks). These very recently acquired results show that they are an ecological and temporal marker, particularly sensitive to the rapid fluctuations in paleo-climatic, sedimentary and anthropogenic conditions that have impacted the coastal domain in Western Brittany. The objective of this first mission is to identify two shallow coastal sites (< 5 m CM), in the south near the tip of Trévignon and in the north near Carantec. Both are known to host Maerl deposits now and in the past (Wehrmann 1998, Pinot, 1970). The seismic data acquired by the launch will thus make it possible to identify the core drilling targets that will be carried out in a second step from the N/O Thalia (PEPITE-THA). The objectives of the PEPITE project are:

• to establish the chrono-stratigraphic framework for the establishment and disappearance of maerl colonies (dates by 14C AMS, estimation of sedimentation rates by 210Pb, sedimentary analyses and geochemical tracers (qualitative XRF measurements, analyses of trace elements such as barium) of the last deposition sequence to trace the origin of sources and the importance of continental and marine flows).
• to identify the species that make up the primitive maerl and the more recent sequences of colonization of the seabed (macroscopic morphological identification, by electron microscopy and DNA barcoding). A challenge will be to extract DNA from fossil maerl to determine genetic information (collaboration with the University of Corunna and the PALMIRA project) and compare it between sites or more broadly with maerl deposits in Europe. A core recently acquired as part of PALMIRA in the commercial port of Brest (April 2017), with interlayered levels of maerl, was sub-sampled using the palaeogenetic approach of Klouch et al (2016) and will complete the study design.
• to understand the dynamics of the evolution of the environment, by reading the possible paleo-climatic, paleo-ecological and anthropogenic correlations between the Concarneau, Brest and Morlaix sites, as well as the paleo-environmental data available on the Rade (C. Lambert) and data from other biological communities such as marine micro-eukaryotes (PALMIRA project).