Pluri-Decadal Variability of Fluvio-Glacial Dynamics Across Heinrich Stadial 1: Dinocyst Evidence for Seasonality Changes in Western Europe

Abstract

The northern Bay of Biscay has proven, in previous studies, its great potential for recording fluvio-glacial activity thanks to its proximity to the mouth of the ‘Fleuve Manche’ paleoriver, the largest Pleistocene river in Europe. In the present work, new dinoflagellate cyst (dinocyst) analyses have been carried out at pluri-decadal resolution in core MD13-3438. Our principle objective is to reconstruct the deglacial history of the ‘Fleuve Manche’ paleoriver runoff coupled with European glacial fluctuations across Heinrich Stadial 1 (HS1: 18.2-14.6 ka BP), a key extreme climatic event of the last glacial period. We identify warm conditions preceding HS1, at the end of the Last Glacial Maximum, highlighted by a significant penetration of the warm and salty North Atlantic Drift (NAD) at the study site. The onset of HS1 is marked by enhanced ‘Fleuve Manche’ paleoriver runoff leading to the formation of laminated deposits, here referred to as the ‘HS1-a laminated interval’. \xa0We also suggest a novel pluri-decadal scale subdivision of HS1 in 7 sub-phases including 5 sub-phases within the laminated interval (HS1-a1 to HS1-a5: 18.2–16.7 ka BP) and 2 sub-phases during the Heinrich Event 1 (HE1: HS1-b: 16.7–15.6 ka BP including the ‘conventional’ HE1 layer at ~16 ka and HS1-c: 15.6-14.6 ka BP). Within HS1-a, substantial terrigenous inputs are coeval with high marine primary productivity and large sea surface salinity drops, with reconstructed values as low as 29 psu at the study site. This results from substantial fluvio-glacial delivery of the ‘Fleuve Manche’ paleoriver in response to the seasonal influx of meltwater associated with the retreat of the southern limb of the European Ice Sheets. Additionally, in this interval, pluri-decadal variability is here detected. Two major events (HS1-a2: 17.8–17.6 ka BP and HS1-a4: 17.4–17 ka BP) of huge fluvial inputs (i.e., substantial erosional processes in the ‘Fleuve Manche’ basin and strong stratification of the water column) are revealed, and occur concomitantly with warm summers. In the rest of HS1-a (HS1-a1: 18.2–17.8 ka BP, HS1-a3: 17.6–17.4 ka BP and HS1-a5: 17–16.7 ka BP), fluvio-glacial discharges via the ‘Fleuve Manche’ paleoriver, although significant, were comparatively moderate induced by decreased seasonality dominated by extended cold winters that could have limited the EIS melting. We argue for the main role of seasonality (i.e., temperature contrast between summer and winter) within HS1-a in controlling fluvio-glacial dynamics. Subsequent intervals HS1-b and HS1-c (first and second phase of HE1, respectively), show a significant decrease of fluvial inputs as well as extended sea-ice cover occurrences and extremely low sea surface temperatures (-1.8°C), especially during HS1-b (associated to Laurentide-sourced Ice Rafted Debris). At the end of HS1-c, the NAD influence in the study area marks the onset of the transition to the warm Bolling/Allerod interval.Interestingly, our paleoclimatic record (i) confirms the crucial role of seasonality in controlling climate variations during the HS1 interval and (ii) shows a remarkable echo with reconstructions from the western Mediterranean Basin, highlighting common regional forcings acting on multi-scale climate variability during the last deglaciation.