Holocene paleoenvironmental reconstructions in western Brittany (Bay of Brest) with a focus on the Neolithic-Bronze transition

The Bay of Brest (BB, western Brittany, NW France) is a semi-enclosed basin of 180 km² subject to macro-tidal dynamics and to the fluvial influences of the rivers Aulne and Elorn, which combined drain watersheds of 2,600 km². This coastal environment is subject to natural climate oscillations overlaid on the long-term landscape transformations inherited from the post-glacial sea level rise and increasing anthropogenic forcing since the Neolithic (6.9 ka BP), and especially from the Bronze Age (4.2 ka BP) onwards. The BB therefore appears suitable for reconstruction of the interactions between climate, environment and human dynamics across the Holocene. In this study, a palynological stack was created based on eight cores, allowing us to discuss the BB paleolandscape evolution over the last 7 kyrs. Since the Neolithic period, the forest cover of this areas has decreased in favour of open and agro-pastoral landscapes. This trend is not uniform, however: forest cover first declined slowly around 4 ka BP, then strongly decreased at the end of the Iron Age, before experiencing a revival of about 5 centuries at the end of the Roman period (1.7–1.2 ka BP). Finally, a drastic fall of tree pollen taxa is recorded at the start of the Middle Ages, related to the population dynamics around Brest Harbour and to intensive agro-pastoral practices. This study is the first on long-term Holocene trends in the BB paleolandscapes that allows discussion of both climatic and anthropogenic forcing over the last 7 kyrs BP at an unprecedent study resolution of 35 years. We also place this local evolution in a wider regional context so as to detail interactions between natural and anthropic forcings over time.