Holocene paleoenvironmental reconstructions in western Brittany Bay of Brest: Part I – Understanding the spatial distribution of palynological records

The Bay of Brest (BB) is a shallow estuarine environment in NW France. This semi-enclosed basin of 180 km² is subject to multiple hydrodynamic factors including the dual influence of oceanic currents and fluvial discharges (Aulne and Elorn main rivers) and resulting in complex hydro-climatic and hydro-sedimentary processes. This study investigates with palynological data (continental: pollen grains and marine: dinoflagellate cysts) two kinds of different materials: (i) modern surface sediments collected over the whole BB as well as (ii) three new BB sediment cores (core ‘F’ from the mouth of the Aulne river and cores PALM-KS05 and PALM-KS06 from the Brest harbour). While modern data are analysed from a statistical point of view to highlight the influence of hydrodynamic forcing on the modern distribution of palynomorphs, the cores allow for spatial comparisons of palynological data on three windows over the Early (~9.5 and ~8.5 ka BP), Middle (~4.4–4.3 ka BP interval) and Late (~1–0.9 ka BP interval) Holocene. For each time intervals, two cores located along a transect from west (more pronounced oceanic influence) to east (more intense fluvial influence from the Aulne river) are compared, located on either side of a limit that we referred to as the river-induced palynological signal (RIPS) limit. These different comparisons reveal a high degree of spatial homogeneity in BB pollen records over time, with exceptions for environments east of the RIPS limit, for which rainfall-induced fluvial discharges have a stronger impact especially considering riparian taxa (i.e. Alnus). This is intended to improve understanding of the palynological signals recorded at different BB coring sites, a first step of crucial relevance before the establishment of a palynological stack covering the Holocene from several cores collected in different shallow bays of the BB (see Valero et al., submitted – PART II).