Sedimentological-stratigraphic evolution and lake level changes of Lake Van

Lake Van is a terminal lake located at an altitude of 1648 m above sea level (masl) on the tectonically active East Anatolian Plateau in eastern Turkey. The uplift of the plateau started around 12 Ma, as a result of the collision of the Arabian plate with the Eurasian plate. The lake is surrounded by Quaternary volcanoes in the west and north, the Paleozoic Bitlis Massif in the south and Plio-Quaternary sediments and rocks of accretionary complex in the east. It is also situated in a climatologically sensitive region at the crossroads of North Atlantic, Siberian high pressure and mid-latitude subtropical high pressure systems near the boundary between the continental Eastern Anatolian and continental Mediterranean zones. Because of its special location in a tectonically active and climatically sensitive region and its varved sediments, Lake Van has been the subject of intensive paleoclimate, tectonic and volcanologic studies. The Lake Van basin was formed in the east-west oriented Muş depression that evolved as a result of the collision of the Arabian and the Eurasian plates during the Cretaceous. The Paleotectonic period was proceeded by the Neotectonic period during the Miocene-Quaternary. The Lower Miocene carbonate sedimentation was followed by clastic Pliocene sedimentation. The region was affected by intensive volcanism with formation of the volcanoes during the Quaternary, such as Süphan, Nemrut, Tendürek and İncekaya. Nemrut Volcano started forming about 600 ka BP and played an important role in formation of Lake Van basin in the eastern part of the Muş Basin. The lake's basin was also influenced of the strike-slip faults under the NW-SE compressional regime that created the E-W and NW-SE trending folds. According to the recent ICDP PaleoVan project results, Lake Van was initially a freshwater lake at the beginning but became brackish-alkaline as a result of weathering and evaporation. Lake Van is the largest Lake in Turkey with a volume of 607 km3, area of 3570 km2 and a maximum depth of ca 450 m. It consists of several morphological parts, such as the 450 m-deep Tatvan Basin, 260 m-deep Northern Basin (NB), the NEE-trending Ahlat (AR) and Northern ridges in the west, Erciş Gulf and Erek Fan in the north, and Eastern Shelf and Eastern Fan in the east. For this thesis, Lake Van's varved sedimentary record was studied to understand its sedimentary, tectonic and paleoclimatic evolution, using seismic reflection, bathymetric and core data. Up to 5 m-long piston cores from the Eastern Shelf offshore west of Van city and two ICDP PaleoVan project drill cores from the northwestern part of the lake were used. The drill cores were recovered at 245 m water depth in the North Basin (NB) and 375 m water depth in the Ahlat Ridge (AR). The drill cores from the NB and AR provided 145 and 220 m-long sedimentary sections below the lake floor, respectively. In these sediment cores, physical properties such as p-wave velocity, magnetic susceptibility, gamma density and resistivity were measured using a Geotek Multi Sensor Core Logger (MSCL). Elemental geochemical analyses of sedimentary cores were carried out by μ-X-Ray Fluorescence (μ-XRF) Itrax scanner. Elemental ratios such as Ca/Fe, Ca/Ti were used to assess the role of carbonate (endogenic) and clastic sedimentation episodes and relate them to climate and lake level changes. The μ-XRF elemental data were also used to identify tephra units as the records of volcanic activity. μ-XRF elemental data such as K and Zr, together with gamma density and magnetic permeability, were utilized in tephra identification and correlation. The digital X-Ray radiography of the cores were obtained using Itrax XRF core scanner. Seasonally deposited laminae were counted from digital radiographic images using a MATLAB algorithm that was developed under the scope of this thesis. With the algorithm, the numbers and thicknesses of the annually deposited varves were determined. The chronologies of the core sequences were established by the 40Ar/ 39Ar dating of tephra units, radiocarbon dating of plant fossils and by correlation with the North Greenland Ice Core (NGRIP, intGRIP) and marine isotope stage (MIS) isotope records and their time scales. Using spectral signal analysis of varve thickness data, we determined the climate cyclicity and past climate variations. In addition to spectral analysis, varve thicknesses were plotted for different periods to determine the arid/cold and humid/warm climate intervals. The past climate reconstruction was compared with the seismic stratigraphic data and findings of other studies to assess the lake water level changes and their causal relationship to the climate. From the collected seismic reflection and bathymetry data, bathymetric map of the lake was created. This map, together with seismic data, revealed morphological, sedimentological, tectonic and volcanic features, such as wave-cut terraces and notches, submerged fluvial channels, delta clinoforms, berms, active faults and volcanic craters. MSCL p-wave velocity and gamma density were used to obtain synthetic seismograms from the age-modelled cores, which were then utilized in core-to-seismic correlation and dating some seismic reflector surfaces. This allowed us to discuss the lake level changes, sedimentological and tectonic evolution of the lake on a temporal scale. The erosional and depositional features such as wave-cut terraces and notches, submerged fluvial channels, delta clinoforms and berms are indicative of past lake level still stands. Correlation of these features on the seismic reflection lines with the core chronostratigraphy indicates the following paleoshorelines (i.e., the lake level still stands) developed during the period 400-17 ka BP and labelled as D1-D6: D1 at 200 m during 20-17 ka BP, D1a at 105 m during 65-60 ka BP, D2 at 150 m 115-106 ka BP, D3 at 225 m during 166-142 ka BP, D4 at 300 m during 195-169 ka BP, D5 at 375 m during 270-234 ka BP and D6 at 450 m during 400-350 ka BP. In addition to the older depositional features, a clinoform observed at the 60 m below the present lake water-level is dated 4-3 ka BP. Other more recent lake level terraces could not be dated because of the absence of insufficient number of high resolution seismic lines. However, these younger terraces are most likely related to the cold periods of Greek Dark Ages or Iron Cold Stage, Medieval Dark Ages and Little Ice Age. Submerged channel-network systems are well developed on the eastern shelf and Erciş Strait. They connect with the river drainage system on land and extend to water depths of more than 100 m. The channels are as deep as 50 m and as wide as 500 m. Submerged channel-network systems on the shelf and slope areas were developed during the major low-stand periods of Lake Van, the last two of which occurred during 15 ka BP and the YD with water levels at 200 mbls and 70 mbls, respectively. Annual sedimentation rates were calculated from the varve thicknesses. In the NB, lake sediments are interbedded with mass flow (turbidite) and tephra units. Ignoring the tephra and mass flow units, the sedimentation rates during the latest Pleistocene-Holocene were calculated. The lowest rates with 0.43 mm/year occurred between 5,8 and 12,65 ka BP years and the highest rates with 3,72 mm/year between 12,65 and 13,89 ka BP. The average sedimentation rate in the NB for the Holocene is found as 1,54 mm/year, which is roughly twice as fast as sedimentation in the Tatvan Basin and Eastern Fan. The sedimentation rate for the AR varies between 0,4 and 0,7 mm/year. The results of climatic periods determined from the spectral decomposition of varve thicknesses of the Van Lake sediment show compatibility with the climate events in the North Atlantic, such as the Bond, Heinrich (H), Dansgaard-Oeschger (DO) events as well as the solar activity minima and the historical records. Thus, the method based on varve thickness is a viable technique in paleoclimatic reconstructions. According to the results of such a reconstruction from Lake Van, we identify the following relatively cold periods: 82-81 ka BP, 71-69 ka BP, 62-60 ka BP, 58-55 ka BP, 38-35 ka BP, 18-16 ka BP, 5-4,8 ka BP, 4,4-4,2 ka BP, 2,7-2,6 ka BP, 1,6-1,2 ka BP (AD 450-800), 0,7-0,45 ka BP (AD 1300-1550), AD 1790-1840 and AD 1880-1950, and the followingwarm periods are: 87-82 ka BP, 65-62 ka BP, 47-43 ka BP, 32-30 ka BP, 22-21 ka BP, 8-5 ka BP, 4,8-4,4 ka BP, 3,2-3,15 ka BP, 2,6-2,5 ka BP, 1,8-1,65 ka BP (AD 200-450), 1,2-0,95 ka BP (AD 800-1050), AD 1550-1790, AD 1840-1880 and from AD 1950 to today. Widespread volcanic activity occurred in and around the lake during the Quaternary. Magma entered the window opened by the break of the oceanic crust below the Tauride block during Upper Miocene. Volcanic activity appears to be localized along the South Boundary Fault (SBF) with the presence of numerous volcanic crater and crater-like structures located along the fault that may have facilitated the passage of magma to the surface. Under the NW-SE compressional regime, NW-SE trending normal fault system developed. This fault system conveyed the carbonate-rich groundwaters in offshore Adilcevaz, to form up to 30 m-high microbialites on the shelf area. The microbialites have chimney- or three-like morphology and are clearly observed on the seismic lines and the high resolution (multi-beam) bathymetry map. A NW-SE trending, left lateral strike-slip fault with a vertical component was observed in seismic lines and the bathymetry map in the southeastern shelf of the lake. This fault is most likely the source of the Mw 5,7 Edremit Earthquake of September 9th, 2011. There is a fault control on the submerged channel system in the south from the Erciş Strait. The vertical slip rate on the fault in Erciş Strait is calculated as 0.43 mm/year.