Browsing by Author "Meydan, Aysegul Feray"

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Evidence of Extensive Carbonate Mounds and Sublacustrine Channels in Shallow Waters of Lake Van, Eastern Turkey, Based on High-Resolution Chirp Subbottom Profiler and Multibeam Echosounder Data
(Springer, 2015) Cukur, Deniz; Krastel, Sebastian; Cagatay, M. Namik; Damci, Emre; Meydan, Aysegul Feray; Kim, Seong-Pil
In Lake Van of eastern Turkey, the fourth largest soda lake in the world, high-resolution subbottom profiles and bathymetric data acquired in 2004 and 2012 revealed several hundreds of topographic mounds in shallow waters (< 130 m) off the historical town of Adilcevaz in the northern lake sector. These structures are characterized by strong top reflections of transparent internal character, and are 10-300 m wide and 0.5-20 m high. Consistent with previous work, they are interpreted as carbonate mounds formed by precipitation from CO2-rich groundwater discharge into the highly alkaline lake. Their age remains to be determined but their alignment along faults suggests tectonic control on their growth. Several sublacustrine channel networks were observed on the eastern shelf of the lake, which connects with onshore rivers. The channels are up to 500 m wide and 20 m deep, and plausibly were formed by fluvial processes during the major lake level drop reported to have occurred by 14 ka in earlier publications. Erosion is common on the channel walls flanked by levees. The channels are presently inactive or abandoned. At a water depth of 100 m, they all merge into a single larger channel; this channel has a sinuous course initially trending southwestward and then northwestward at a water depth of 130 m. Numerous closely spaced small channels (similar to 10-200 m wide, 1-10 m deep) are also seen on the eastern lacustrine shelf, interpreted as denditric and parallel channel systems formed during lake level fall terminating at similar to 14 ka. Bathymetric data provide evidence of numerous sublacustrine canyons on the western slope of the lake's northern basin, most likely remnants of relict rivers formed during this lowstand.
Paleoenvironmental Reconstruction of Lake Van and Lake Erçek Over the Last Millennium Using Varved Sediments (Eastern Türkiye)
(Pergamon-elsevier Science Ltd, 2024) Meydan, Aysegul Feray; Kalugin, Ivan Aleksandrovich; Darin, Andrey Victorovich; Babich, Valery Vasilevich; Markovich, Tatiana Ivanovna; Rogozin, Denis Yurevich; Akkol, Suna
Detailed lithological and geochemical studies of the bottom sediments were conducted based on the original sampling of brackish adjacent lakes: Lake Van and Lake Er & ccedil;ek in Eastern Anatolia, T & uuml;rkiye. Seasonal variations in the accumulation of weather-driven element content were determined using high-resolution Synchrotron Radiation X-Ray Fluorescence (SR-XRF) analysis of the annual layers in the solid prepare. The results were utilized to approximate the regional paleo-temperature of both lakes and the water level of Lake Van. Visual counting of the varve as well as annual oscillation of measured geochemical series was employed to create a time scale. The element content series were synchronized with recent changes in external climatic environments and lake water level fluctuations. Subsequently, the geochemical series were transformed into climate units, using calibration on synchronous meteorological data. Finally, the original quantitative reconstruction of the regional climate variations was performed over the last millennium period with annual time resolution; for Lake Er & ccedil;ek (840 years ago) and Lake Van (1300 years ago). It was revealed that the sediments of these adjacent lakes (Lake Van and Lake Er & ccedil;ek), located 30 km apart from each other, provided synchronous responses to regional environmental changes, which confirmed the reliability of the reconstructions.
Seasonal Temperature Dynamics of Lake Ercek Water Column, Eastern Anatolia/Turkey
(Pamukkale Univ, 2020) Meydan, Aysegul Feray; Akkol, Suna
Lake Ercek is located in eastern Turkey which has a key point where different climatic systems intersect and after Lake Van it is the second largest closed basin lake in the basin. Previous studies in the water column are not sufficient to explain changes in the water column's temperature throughout the year. With this study's scope, temperature measurements were carried out in the water coloumn vertically from the lakes deepest point for 12 months between 2016-2017 with CTD device. In this study, the lake water temperature change was investigated and it was revealed that the lake water column showed thermal stratification between May and November, and the water body mixed in winter. All data showed that Lake Ercek is included in the "Holomictic Lake" class.
Seismic Evidence of Shallow Gas From Lake Van, Eastern Turkey
(Elsevier Sci Ltd, 2013) Cukur, Deniz; Krastel, Sebastian; Tomonaga, Yama; Cagatay, M. Namik; Meydan, Aysegul Feray
Analysis of multi-channel seismic reflection and chirp data from Lake Van (eastern Turkey) reveals various shallow gas indicators including seismic chimneys, enhanced reflections, bright spots, mud volcanoes, pockmarks, and acoustic blanking. The enhanced reflections, suggesting the presence of free gas, are most dominant and observed at more than 200 locations. They are characterized by very-high amplitude reflections and occur in both deep and shallow sedimentary sections. Some enhanced reflections are accompanied by very subtle seafloor expressions such as mounds, which may suggest active venting activity. Seismic chimneys or columnar zones of amplitude blanking have been observed in much of the surveyed area. Seismic chimneys in the study area cannot be associated with any known faults that would act as migration pathways for deep fluids. This suggests that the observed structures in Lake Van sediments allow the preferential emission of gases which might be for a large share of biogenic origin. The acoustic blanking, characterized by transparent or chaotic seismic facies, is seen in the eastern part of the lake. The lakeward edge of the acoustic blanking largely coincides with the 100 m water depth contour, indicating that (past) changes of the hydrostatic pressure may be responsible for the distribution of these anomalies. Mound-like features, interpreted as mud volcanoes, occur in a few locations. The presence of these features may suggest active gas emission. Very strong amplitude anomalies or bright spots with negative polarity, indicating gas-charged zones, are also seen in a number of locations. Pockmarks are observed only in the northeastern part of the study area. The scarce occurrence of pockmarks in the study area might be ascribed to a higher permeability of the lake sediments or to the absence of the substrate/reservoir providing the critical mass of gases necessary to produce such features. Turbidites, tephra layers, and deltaic deposits have the potential to provide ideal conditions to allow the sediments to act as a gas reservoir. (c) 2013 Elsevier Ltd. All rights reserved.
Seismic Stratigraphy of Lake Van, Eastern Turkey
(Pergamon-elsevier Science Ltd, 2014) Cukur, Deniz; Krastel, Sebastian; Schmincke, Hans-Ulrich; Sumita, Mari; Cagatay, M. Namik; Meydan, Aysegul Feray; Stockhecke, Mona
More than 1500 km of multi-channel seismic reflection profiles combined with ICDP (International Continental Scientific Drilling Program) drilling data, provide important insights into the stratigraphic evolution of Lake Van, eastern Turkey. Three major basins (Tatvan, Northern and the Deveboynu basins) comprise the main lake basin and are separated by morphological highs (Ahlat ridge and Northern ridge). Moreover, NE SW faults, parallel to the general tectonic lineament of the area, dominate the entire basin and are in charge of creating graben and half-graben structures. Well-developed prograding deltaic sequences on top of the basement were recognized by seismic stratigraphy analysis. Most likely, they formed during the initial flooding of Lake Van similar to 600 ka. The Tatvan basin sediments are dominated by mass-flow deposits of various origins alternating with undisturbed lacustrine sediments including distinct tephra layers. Faulting along the Tatvan basin margins may have triggered margin-wide slope failures. Ahlat ridge started to form between ca 340 ka-290 ka. Since then, Ahlat ridge was sheltered from major mass-flows due to its elevation. Hence, slow lacustrine sedimentation has prevailed throughout lake history on Ahlat ridge, which was the location of the main drill site during the ICDP. Several lake level fluctuations are evident on the eastern slope area but the deep basins were permanently covered by water. A significant lake-level low stand (ca 600 ka BP) was found at similar to 610 m below present lake level. The setting of the lake changed at about 30 ka. Tectonic activity appears to have waned significantly as the mass-transport deposition decreased across the Tatvan basin while normal undisturbed lacustrine sedimentation prevailed. A different setting is found in the Northern basin from ca 90 ka to Present, especially due to the strong influx of mostly volcaniclastic turbidites causing sedimentation rates to be about 3.5 times higher (drill Site 1), than at Site 2 (Ahlat ridge). (C) 2014 Elsevier Ltd. All rights reserved.
Structural Characteristics of the Lake Van Basin, Eastern Turkey, From High-Resolution Seismic Reflection Profiles and Multibeam Echosounder Data: Geologic and Tectonic Implications
(Springer, 2017) Cukur, Deniz; Krastel, Sebastian; Tomonaga, Yama; Schmincke, Hans-Ulrich; Sumita, Mari; Meydan, Aysegul Feray; Horozal, Senay
The structural evolution of Lake Van Basin, eastern Turkey, was reconstructed based on seismic reflection profiles through the sedimentary fill as well as from newly acquired multibeam echosounder data. The major sub-basins (Tatvan Basin and Northern Basin) of Lake Van, bound by NE-trending faults with normal components, formed during the past similar to 600 ka probably due to extensional tectonics resulting from lithospheric thinning and mantle upwelling related to the westward escape of Anatolia. Rapid extension and subsidence during early lake formation led to the opening of the two sub-basins. Two major, still active volcanoes (Nemrut and Suphan) grew close to the lake basins approximately synchronously, their explosive deposits making up > 20 % of the drilled upper 220 m of the ca. 550-m-thick sedimentary fill. During basin development, extension and subsidence alternated with compressional periods, particularly between similar to 340 and 290 ka and sometime before similar to 14 ka, when normal fault movements reversed and gentle anticlines formed as a result of inversion. The similar to 14 ka event was accompanied by widespread uplift and erosion along the northeastern margin of the lake, and substantial erosion took place on the crests of the folds. A series of closely spaced eruptions of Suphan volcano occurred synchronously suggesting a causal relationship. Compression is still prevalent inside and around Lake Van as evidenced by recent faults offsetting the lake floor and by recent devastating earthquakes along their onshore continuations. New, high-resolution bathymetry data from Lake Van reveal the morphology of the Northern Ridge and provide strong evidence for ongoing transpression on a dextral strike-slip fault as documented by the occurrence of several pop-up structures along the ridge.