Subfossil trees suggest enhanced Mediterranean hydroclimate variability at the onset of the Younger Dryas

International audienceNearly 13,000 years ago, the warming trend into the Holocene was sharply interrupted by a reversal to near glacial conditions. Climatic causes and ecological consequences of the Younger Dryas (YD) have been extensively studied, however proxy archives from the Mediterranean basin capturing this period are scarce and do not provide annual resolution. Here, we report a hydroclimatic reconstruction from stable isotopes (δ 18 O, δ 13 C) in subfossil pines from southern France. Growing before and during the transition period into the YD (12 900-12 600 cal BP), the trees provide an annually resolved, continuous sequence of atmospheric change. Isotopic signature of tree sourcewater (δ 18 O sw) and estimates of relative air humidity were reconstructed as a proxy for variations in air mass origin and precipitation regime. We find a distinct increase in inter-annual variability of sourcewater isotopes (δ 18 O sw), with three major downturn phases of increasing magnitude beginning at 12 740 cal BP. The observed variation most likely results from an amplified intensity of North Atlantic (low δ 18 O sw) versus Mediterranean (high δ 18 O sw) precipitation. This marked pattern of climate variability is not seen in records from higher latitudes and is likely a consequence of atmospheric circulation oscillations at the margin of the southward moving polar front. During the abrupt and intense climate change from the Allerød warm phase to the YD cold reversal in the North Hemisphere (ca. 12 700-11 600 cal BP) 1,2 sea-ice production and drifting enhanced 3 , alpine glaciers advanced 4 , storm intensity strengthened 5 , and a reorganization of the atmosphere 6,7 may have occurred. Greenland ice core data (NGRIP) reveal temperature drops of 10-15 °C with simultaneous reductions in snow accumulation and amplifications in atmospheric dust within less than a decade 6,8. During the rapid cooling, lake sediment records across Europe signal intensified wind stress, aridity and detrital input, alongside drastic ecological changes 5,9,10. The results are spatially heterogeneous in terms of hydrological change, as other European lake records find more humid conditions and/or increased quantity and intensity of precipitation (associated with higher lake levels in certain cases) 11,12. Model simulations constrained by proxy data indicate no single factor could cause the observed YD cold reversal, but rather a complex combination of weakened Atlantic meridional overturning circulation (AMOC), altered atmospheric circulation patterns, and moderate negative radiative forcing as most plausible driving factors 13. Nevertheless, mechanisms of climate variability in the North Atlantic region remain under intense debate 8 , even for the most recent past 14