Physical properties, structural features, and climate signals in lake sediments of the ICDP site Lake Van (Turkey) derived from downhole logging
Zusammenfassung der Projektergebnisse
Lake Van (Turkey) is the 4th largest terminal lake worldwide and is located at a key climatic position. In the frame of the ICDP project ‘PALEOVAN’, a deep drilling campaign has been executed in summer 2010. It aims to enhance the understanding of the paleoclimatic and paleoenvironmental conditions of the Middle East. Multiple coring of two sites (Northern Basin and Ahlat Ridge) in water depth up to 360 m has been performed. High quality downhole logs have been acquired at both sites. The sedimentary record is mainly composed of clayey silts and tephra deposits, supplied by four volcanic sources: 1) Süphan volcano, located on the northern shore, 2) Nemrut volcano, 15 km west of the western shore, 3) Incekaya, on the southwestern shore, and 4) intralake eruptive centers. The dominant chemical composition of the volcanic sources is known from extensive studies of land deposits. The downhole logging data from Ahlat Ridge (30 – 210 m below lake floor) have been analyzed by cluster analysis. Additionally, two parameters from x-ray fluorescence scanning on cores were used as input parameters for the analysis (calcium and zirconium) to improve the separation of the sediments. A continuous lithological profile was constructed and five cluster units (three tephra units, two clayey silt units) were derived. The physical and chemical properties were determined and interpreted after correlation with the lithological units from the composite profile (from visual core description). Separation of the two major groups of lacustrine sediments, banded clayey silts (interpreted as glacial deposits) and laminated clayey silts (interpreted as interglacial deposits) by their physical and chemical properties were hypothesized. However, the clayey silts show little differences in their physical properties. The tephra units are characterized by contrasting properties and differ mainly in their susceptibility, spectral gamma ray components (uranium, thorium, and potassium) and relative contents of calcium and zirconium. The tephra units have been linked to the dominant volcanic composition of the deposited tephra layers and partly to the volcanic source. Depth trends could be derived with prevailing of basaltic deposits in the bottom part (128 - 210 m below lake floor) and gradually outweigh of higher differentiated (dacitic and rhyolitic/trachytic) products towards the top. Further, spectral analysis was performed on gamma ray-data to estimate sedimentation rates and the time period of deposition. However, cyclostratigraphy prerequisites continuous sedimentation and cannot be applied on the complete sedimentary record of Lake Van, which is frequently intersected by tephra layers. Therefore, the tephra deposits have been removed manually and a synthetic-log was created (overall length of 170 m). High amplitude signals have been detected and correlated to orbital cycles (Milankovitch) for calculation of sedimentation rates. Their evolvement over the record was analyzed by the sliding window technique as 21.5 cm/ka in the deeper part to 40.0 cm/ka in the upper section. The yield time span for 0 to 210 m below lake floor was calculated at 612 ka. A good agreement between the results from spectral analysis on gamma ray-data and the interpretation of the cores (e.g. correlation of TOC-trends with Marine Isotope Stages) supports our estimates that the detected orbital signals can be used for cyclostratigraphy.
Projektbezogene Publikationen (Auswahl)
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(2012): 500,000 Years of Environmental History in Eastern Anatolia: The PALEOVAN Drilling Project. - Scientific Drilling, 14: 18-29
Litt, T., Anselmetti, F., Baumgarten, H.,..., Wonik, T. & PALEOVAN Scientific Team
