Morphosedimentary evidences of lake level variations in the terraces of Lake Van (Turkey) [Indicateurs morphosédimentaires des variations de niveau lacustre dans les terrasses du Lac de Van (Turquie)]

摘要

In the eastern Anatolian Mountains, the relatively high-magnitude variations exhibited by the level of Lake Van in recent years indicate, as the lake has no outlet, that this level can be considered as a sensitive indicator of climate if these variations respond to changes in water input/output. Until now, previous palaeoclimatic research has focused mainly on deep sediments in the lake, which provided an environmental sequence which is today a reference for the Late Glacial and the Holocene in the Middle East. In spite of many landforms and deposits around the lake pointing out to the occurrence of very high levels in the past, studies on the lacustrine sediments preserved in terrace records around the lake have remained quite rare since a first exhaustive overview realized in the 1970's. We here present the first results of a field work coupled with sedimentology studies (stratigraphy, facies analyses), DGPS elevation measurements, spatialized reconstructions. Chronostratigraphy takes advantage of the presence of interstratified tephras layers dated by ~(39)Ar/~(40)Ar, of travertines dated by U-Th series, as well as of organic deposits and shells dated by ~(14)C. Results evidence the occurrence of two major and rapid transgressions, the first one happening at the beginning of the Last Glacial, the second one in the millennia preceding the Last Glacial Maximum (LGM). Each identified lacustrine cycle started with a very rapid transgression followed by slower regressions with intermediate oscillations. Regressions started immediately after the highest stand was reached (+80 m and +55 m). Regressions lasted long, with some stand-by during the descent, allowing erosion surfaces to truncate the surface of the emerging lake deposits. During Upper Pleistocene, tectonic and volcanic activities interfered with the climatic record of lake levels, both by deforming deposits locally (uplift, faulting) and by modifying the drainage area and river network of the lake basin (closure by damming, captures, isolation). Our preliminary results thus enlighten the complexity of the processes triggering the high magnitude transgressions of the Upper Pleistocene Van Lake, as well as the difficulty to establish correspondences between climatic changes and water volume changes as evidenced by past shoreline deposits preserved in the terrace sedimentary records.