Environmental and climatic changes in the Jura mountains (eastern France) during the Lateglacial-Holocene transition: a multi-proxy record from Lake Lautrey.
Résumé
This paper presents a multi-proxy reconstruction of the climatic and environmental changes during the Last Glacial-Interglacial transition as recorded by a sediment sequence from Lake Lautrey (Jura, eastern France). This reconstruction is based on analysis of pollen, chironomid, organic matter, oxygen-isotope, mineralogical, magnetic susceptibility and inferred lake-level data at a high temporal resolution. The chronology is derived using AMS radiocarbon dates, the position of the Laacher See Tephra (LST), and of correlation between the Lautrey and GRIP oxygen-isotope records. This data set reveals a detailed sequence of environmental changes in the Jura mountains from Greenland Stadial 2a to the early Holocene. Biotic and abiotic indicators allow the recognition of major abrupt changes associated with the GS-2a/GI-1e, GI-1a/GS-1 and GS-1/Preboreal transitions, and other minor fluctuations related to the cold events GI-1d, GI-1b and the Preboreal oscillation (PBO). They also suggest additional cooling spells at ca 14,550 and 14,350 cal yr BP (Intra-Bølling Cold Periods), at ca 13,500 and ca 12,700 cal yr BP just before the GS-1 onset, and at ca 11,350 cal yr BP just before the PBO, as well as an intriguing brief warming episode within GS-1 at ca 12,080 cal yr BP. Summer temperature increased by ca 5 °C at the start of GI-1e, and by 1.5-3 °C at the Holocene onset, while it decreased by ca 3-4 °C at the beginning of GS-1. Major changes in local hydrology and in seasonality appear to be also associated with the GS-2a/GI-1e, GI-1a/GS-1 and GS-1/Preboreal transitions. Pollen and abiotic indicators suggest a greater sensitivity of the vegetation cover to climatic oscillations during the first part of the Lateglacial Interstadial than during the second part (after ca 13,700 cal yr BP), when a closed forest had been restored in this area. By contrast, the restoration of forest cover took less than 300 yr after the end of GS-1. At the beginning of GI-1e and GS-1, no lag occurs (within the sampling resolution of 20-50 yr) in the responses of aquatic (chironomids) and terrestrial (pollen) ecosystems, while, at the onset of the Holocene, the response of the vegetation appears slightly delayed in comparison with that of the chironomid community. Finally, the recognition of two successive tephra layers, which were deposited just before the LST at ca 12,950 cal yr BP and which originated from Le Puy de la Nugère (Massif Central, France), provides an additional tephrochronological tool for correlation between Lateglacial European sequences.
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