トルコ中部の内陸塩性湖沼の珪藻遺骸群衆 : 第四紀後期における古塩分変動の定量的復元への応用

In arid and semi-arid areas, like inner Anatolia, small change in precipitation and evaporation rates can have marked effects on the water environment of inland lakes at closed basins. This study demonstrates the importance of detailed diatom analyses in attempt to identify lake level and lake water quality changes of inland lakes in Turkey. An important first step in using diatoms as salinity indicators is to obtain quantitative data on their ecological characteristics, such as optima and tolerances along salinity gradients. We took 51 samples of living diatoms from 38 lakes and rivers in the central part of Turley, and calculated an abundance-weighted mean salinity (AWM) for each toxin (Kashima, 1996). On the basis of a strong relationship between diatom composition and salinity, we defined the diatom-based transfer functions for salinity reconstruction, and then applied them to Late Quaternary sediments in Turkey. Our drilling surveys were done at Kaman Kalehoyük, Lake Tuz, Konya Basin and in Akgöl Marsh and its surrounding areas. The results show that there was a number of alternations between fresh and saline conditions during the Late Quaternary (Kashima et al., in press)

Impact of lake level change on deep-water renewal and oxic conditions in deep saline Lake Van, Turkey

Changes in the hydrological regime of the saline closed basin Lake Van, a large, deep lake in eastern Turkey, resulted in a lake level increase by about 2 m between 1988 and 1995, followed by a 1.5 m decrease until 2003 and a relatively constant lake level thereafter. Based on measurements of transient tracers (sulfur hexafluoride, CFC-12, 3H, 3He, 4He, Ne), dissolved oxygen, light transmission, conductivity-temperature-depth profiles, and thermistor data, we investigate the implications associated with lake level fluctuations for deep-water renewal and oxygenation. Our data suggest that deep-water renewal was significantly reduced in Lake Van between 1990 and 2005. This change in mixing conditions resulted in the formation of a more than 100 m thick anoxic deep-water body below 325 m depth. Apparently, the freshwater inflows responsible for the lake level rise between 1988 and 1995 decreased the salinity of the surface water sufficiently that the generation of density plumes during winter cooling was substantially reduced compared to that in the years before the lake level rise. Significant renewal and oxygenation of the deep water did not occur until at least 2005, although by 2003 the lake level was back to almost the same level as in 1988. This study suggests that short-term changes in the hydrological regime, resulting in lake level changes of a couple of meters, can lead to significant and long-lasting changes in deep-water renewal and oxic conditions in deep saline lakes

Lake Van Drilling Project: A Long Continental Record in Eastern Turkey

An international research group is proposing a new research initiative, the Lake Van Drilling Project ‘PaleoVan’ within the framework of the International Continental Scientific Drilling Program (ICDP). The project mainly aims at obtaining high-resolution paleoclimate records from lacustrine sediments, where biotic and abiotic parameters provide proxy climate data. Lake Van in Turkey has the potential to yield long continental records covering several glacial-interglacial cycles from annually-laminated sediments, hencemaking the lake a key site for the investigation of the Quaternary climate evolution in the Near East

Reconstructing the Sedimentary Evolution of the Miocene Aksu Basin Based on Fan Delta Development (Eastern Mediterranean-Turkey)

The Aksu Basin in southern Turkey is dominantly represented by an alluvial fan and five fan deltas (FDs) developed along the tectonically controlled margins of the basin during the Miocene. Four alternating compressional and tensional tectonic phases have influenced the basin since its formation. Strong tectonic movements caused high sedimentation rates and progradation of large debris-flow and mass-flow dominated FDs. Here we describe two FDs (the Karadag and Kargi FDs) in detail. The Karadag FD began to develop under the control of a compressional regime and continued the evolution under a tensional regime. The same tensional regime caused the separation of the Karadag FD from its source and the deposition of the Kargi FD into the newly formed accommodation area. The alternating tectonic regimes and sea-level oscillations in the Aksu Basin gave rise to the development of coral colonies on the shallow delta fronts, forming patch reefs despite the large amounts of conglomerates supplied by fan deltaic processes

Reconstruction of climatic changes during the late Pleistocene, based on sediment records from the Konya basin (Central Anatolia, Turkey)

Climatic changes during the last climatic cycle have been studied using three sediment cores from the Konya plain, a now dry, closed and semi-arid lacustrine basin at 1000 m altitude in central Anatolia, Turkey. The reconstruction of regional climatic characteristics and evolution is based on mineral, diatom, pollen and molluscan content of sediments. Correlations are made between cores using 14C and U-Th ages, stratigraphy and drought levels shown by changes in mineral contents. Environmental responses to local and regional climatic changes are traced by trends in authigenic carbonates, evaporites, detrital mineral content and by diatom-inferred salinity levels. Our data have shown that, during the period covering the end of the previous Glacial (Termination II)and the last Interglacial (between c. 150 and 117 ka), peat and freshwater shallow lakes expanded. From 117 to 66 ka, the plain was occupied by lakes, the salinity and existence of which varied in time and space. Specific events are marked by mineralogic and stratigraphic signals at (i) c. 101 ka and (ii) 66 ka. From 66 ka to 30 ka, desiccation of the lake is marked by a hiatus. At c. 27 ka, milder climtic conditions led to the extension of freshwater marshes and lakes in the central depressions while palaeosols developed on the emerged parts of the plain. From 25 to 20.5 ka 14C cal., the sediments of a freshwater to brackish lake are present in one core only. From 20.5 14C cal. onwards, strong evaporitic conditions occurred, the lake edges being transformed into playas. Upper parts of the sequences registered other lacustrine short phases, both before the Younger Dryas and during the Holocene. (Résumé d'auteur

Formation of the Upper Pleistocene terraces of Lake Van (Turkey)

International audienceSedimentological and geomorphological studies of terraces around Lake Van (1647 m) provided a preliminary framework for lake-level variations. The elevations of terraces and past lake level were measured with a differential global positioning system. A chronology is developed using 234U/230Th dating of travertines, 39Ar/40Ar dating of pyroclastites and 14C dating of organic matter. Facies and stratigraphic correlations identify four transgressions (C10, C100, C20 and C200), each followed by a regression which ended with low lake levels that caused river incision and terrace formation. Evidence of the oldest transgression (C10) is found in the uppermost reaches of valleys up to 1755 m, an altitude higher than the present lake threshold (1736 m). This C10 transgression may be related to pyroclastic flows which dammed an outlet located in the western part of the lake basin and which is dated to before 105 ka. After 100 ka, a second transgression (C100) reached 1730/1735 m, possibly related to a younger ignimbrite flow, in association with high water inflow (warm and/or wetter conditions). The two younger transgressions reached 1700–1705 m. The first one (C20) is dated to 26–24.5 cal. ka BP and the second one (C200) to 21–20 cal. ka BP. Available data suggest that the long-termlake-level changes responded mainly to climate oscillations. Additional events such as river captures caused by volcanic falls filling valleys, tectonism, erosion and karstic diversion may have impacted these long-term lake-level changes

Volume 21 Numéro 4

Organisé par l’AFEQ et l’UMR Archéologie des Sociétés Méditerranéennes (Montpellier, 26-28 Février 2008