Isotopic characterization of rapid climatic events during OIS3 and OIS4 in Villars Cave stalagmites (SW-France) and correlation with Atlantic and Mediterranean pollen records

摘要

We present a new mid-latitude speleothem record of millennial-scale climatic variability during OIS3 from the Villars Cave that, combined with former published contemporaneous samples from the same cave, gives a coherent image of the climate variability in SW-France between ～55 ka and ～30 ka. The 0.82 m long stalagmite Vil-stm27 was dated with 26 TIMS U-Th analyses and its growth curve displays variations that are linked with the stable isotopes, both controlled by the climatic conditions. It consists in a higher resolved replicate of the previously published Vil-stm9 and Vil-stm14 stalagmites where Dansgaard-Oeschger (DO) events have been observed. The good consistency between these three stalagmites and the comparison with other palaoeclimatic reconstructions, especially high resolution pollen records (ODP 976 from the Alboran Sea, Monticchio Lake record from southern Italy) and the nearby MD04-2845 Atlantic Ocean record, permits to draw a specific climatic pattern in SW-France during the OIS3 and to see regional differences between these sites. Main features of this period are: 1) warm events corresponding to Greenland Interstadials (GIS) that are characterized by low speleothem δ~(13)C, high temperate pollen percentages, warm temperatures and high humidity; among these events, GIS#12 is the most pronounced one at Villars characterized by an abrupt onset at ～46.6 ka and a duration of about 2.5 ka. The other well individualized warm event coincides with GIS#8 which is however much less pronounced and occurred during a cooler period as shown by a lower growth rate and a higher δ~(13)C; 2) cold events corresponding to Greenland Stadials (GS) that are clearly characterized by high speleothem δ~(13)C, low temperate pollen abundance, low temperature and enhanced dryness, particularly well expressed during GS coinciding with Heinrich events H5 and H4. The main feature of the Villars record is a general cooling trend between the DO#12 event ～45.5 ka and the synchronous stop of the three stalagmites at ～30 ka ±1, with a first well marked climatic threshold at ～41 ka after which the growth rate and the diameter of all stalagmites slows down significantly. This climatic evolution differs from that shown at southern Mediterranean sites where this trend is not observed. The ～30 ka age marks the second climatic threshold after which low temperatures and low rainfalls prevent speleothem growth in the Villars area until the Lateglacial warming that occurred at ～16.5 ± 0.5 ka. This 15 ka long hiatus, as the older Villars growth hiatus that occurred between 67.4 and 61 ka, are linked to low sea levels, reduced ocean circulation and a southward shift of the Polar Front that likely provoked local permafrost formation. These cold periods coincide with both low summer 65°N insolation, low atmospheric CO_2 concentration and large ice sheets development (especially the Fennoscandian).