From a coastal plain to an anthropized fluvial valley (NW Brittany, France): 7.3 kyr of paleoenvironmental evolution from sedimentological, palynological and paleogenomic perspectives

Over the Holocene (last 11.7 kyr), the Brittany coastal region has undergone major changes in response to rising sea level, climatic fluctuations and increasing anthropogenic influence. This study aims to i) accurately reconstruct the paleoenvironmental evolution of a site located on the north Brittany coast (NW France), a sector that has not been previously investigated thoroughly, and ii) improve the detection of the anthropogenic signature in paleoenvironmental records by using a multidisciplinary approach. To achieve these goals, we made the first cross-correlated study in coastal Brittany region, integrating sedimentological (grain-size, X-ray fluorescence), palynological (pollen grains), and paleogenomic (on plant and mammal sedimentary ancient DNA or sedaDNA) data, which we based two 7.3 kyr-long sedimentary sequences recovered from the Kerallé valley (Plouescat, NW Brittany).

First, sedimentary and vegetation trajectories allowed us to reconstruct the evolution of past depositional environments in the inner part of the Kerallé valley. The data show a progressive sedimentary infilling of the former alluvial plain, subjected to marine incursion between 7.3 and 6 cal ka BP, up to a continental area, totally disconnected from tidal influence since 5.4 cal ka BP. Second, the analysis of plant and mammal sedaDNA provided a complementary tool to the pollen analysis, enhancing our ability to accurately detect past land-use practices. In the upstream part of the Kerallé system, anthropogenic influence remained scarce until the late Neolithic and seemed to be restricted to the coast, as also indicated by archaeological remains. The first significant human-driven changes in plant community composition likely date back to 4.5 cal ka BP (i.e., late Neolithic), as also shown by the presence of domesticated mammal sedaDNA, both being in agreement with the documented settlement by the first metal-working societies (Bell Beaker culture) in the Plouescat area. From then until 4 cal ka BP (i.e., early Bronze Age), human pressure became more obvious, with the steadily increase in anthropogenic pollen indicators, while sedaDNA of plants and mammals were affected by preservation biases. At 2.7 cal ka BP (i.e., Iron Age), a new anthropic threshold was characterized by maximal forest clearing leading to significant human-induced soil erosion across the Kerallé watershed. Since then, at the local scale of our study, both vegetation dynamics and the presence of domesticated mammals observed by sedaDNA attest to the development of pastoralism.