We discuss vegetation and climate changes across western Europe over the last deglaciation, especially zooming on the Heinrich Stadial 1 (HS1) interval in the northern Bay of Biscay. Our study focuses on the marine palynological investigation of core MD13-3438, with new pollen analyses, here compared to a set of paleohydrographical marine proxies (including dinoflagellate cysts / dinocysts) obtained on the same core.

We show that the recorded pollen signature is, at that time, tightly related to the fluvial dynamics of the ‘Fleuve Manche’ paleoriver, the latter being coupled with European Ice Sheet fluctuations, as well as influenced by fluvial deliveries from closer French rivers of the Atlantic coast.

The onset of HS1 (18.2–16.7 ka BP), marked by enhanced ‘Fleuve Manche’ paleoriver seasonal runoff, is characterized by: i) two episodes of substantial fluvio-glacial input, concomitant with warmer summers and marked by increases of temperate forest pollen grain percentages; ii) and three episodes of moderate runoff, concomitant with extended colder winters and corresponding to increases of boreal forest pollen grain percentages. We suggest an important role of the variability of the North Atlantic atmospheric circulation in explaining those multidecadal changes in pollen sources recorded in the marine realm. When westerlies are deflected southward (respectively northward), they would bring higher humidity to the southern-western and closer (respectively northern-eastern and more distal) distributaries/rivers of the ‘Fleuve Manche’ paleoriver system, associated to moderate (respectively intense) runoff. Then, extreme cold and dry climate conditions are recorded at the onset of Heinrich Event 1 (16.7–15.6 ka BP) with marked occurrences of steppic pollen grains, followed by more humid conditions at the end of HS1, with mixed signatures of open-vegetation and temperate forest taxa. The wettest conditions are recorded during the Bølling-Allerød, concomitantly with the sea level rise and the advection of warm and salty waters by the North Atlantic Current to the study site.