Browsing by Author "Tomonaga, Yama"

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Impact of Lake Level Change on Deep-Water Renewal and Oxic Conditions in Deep Saline Lake Van, Turkey
(Amer Geophysical Union, 2010) Kaden, Heike; Peeters, Frank; Lorke, Andreas; Kipfer, Rolf; Tomonaga, Yama; Karabiyikoglu, Mustafa
[1] 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, H-3, He-3, He-4, 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.
Noble Gases in the Sediments of Lake Van - Solute Transport and Palaeoenvironmental Reconstruction
(Pergamon-elsevier Science Ltd, 2014) Tomonaga, Yama; Brennwald, Matthias S.; Meydan, Aysegul F.; Kipfer, Rolf
Sediment samples acquired in 2010 from the long cores of the International Continental Scientific Drilling Program (ICDP) PaleoVan drilling project on Lake Van for noble-gas analysis in the pore water allow determination of the local terrestrial He-gradient as a function of depth within a sediment column of more than 200 m. These measurements yield first insights into the physical transport mechanisms of terrigenic He through the uppermost part of unconsolidated lacustrine sediments overlying the continental crust. In line with our previous work on the spatial distribution of the terrigenic He release into Lake Van, we identify a high He concentration gradient in the uppermost 10 m of the sediment column. The He concentration gradient decreases below this depth down to approx. 160 m following in general the expectations of the modelling of radiogenic He production and transport in a sediment column with homogeneous fluid transport properties. Overall the in-situ radiogenic He production due to the decay of U and Th in the mineral phases of the sediments accounts for about 80% of the He accumulation. At approx. 190 m we observe a very high He concentration immediately below a large lithological unit characterised by strong deformations. We speculate that this local enrichment is the result of the lower effective diffusivities in the pore space that relate to the abrupt depositional history of this deformed unit. This particular lithological unit seems to act as a barrier that limits the transport of solutes in the pore space and hence might "trap" information on the past geochemical conditions in the pore water of Lake Van. The dissolved concentrations of atmospheric noble gases in the pore waters of the ICDP PaleoVan cores are used to geochemically reconstruct salinity on the time scale of 0-55 ka BP. Higher salinities in the pore water at a depth of about 20 m suggest a significantly lower lake level of Lake Van in the past. (C) 2014 Elsevier Ltd. All rights reserved.
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.
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.