The annual particle cycle in Lake Van (Turkey)

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Abstract

The varved sediments of Lake Van provide a high-quality continental archive of seasonal to decadal-scale climate variability. In order to read the natural record, modern varve formation was studied on the basis of (1) remotely-sensed total suspended-matter (TSMrs) concentrations; (2) time-series of particle flux and water temperatures; and (3) turbidity, temperature, and oxygen profiles. TSMrs, validated by contemporaneous water-column sampling, shows great temporal and lateral variations (whitings and turbidity plumes). From 2006 to 2009, sequential sediment traps recorded high particle fluxes during spring and fall, medium fluxes during summer, and almost zero flux during winter. The mean total mass flux of 403 mg m 2 day 1 comprised 33% (seasonally up to 67%) calcium carbonate, 7% aquatic organic matter, 6% biogenic opal, and 54% detrital minerals. The CaCO3 fluxes are controlled by river discharge (precipitation and snowmelt) during spring, by high productivity during summer, and by river discharge (precipitation before snowfall starts) and mixing during fall. In November 2007, an anomalously high CaCO3 flux occurred as a result of a warm water surface supersaturated with calcite coinciding with an anomalous runoff event. The results demonstrate that the couplets of light and dark laminae in the short sediment cores are true varves representing spring–summer–fall and winter conditions, respectively. Consequently, varve formation can be linked to the seasonal climate pattern, providing a calibration that can be used to interpret the partially varved paleo-record of Lake Van and related environmental processes.

Highlights

► Satellite data and sediment traps quantify the annual particle cycle in Lake Van. ► CaCO3 flux is mainly controlled by runoff; e.g. by rainfall and snowmelt. ► The annual particle cycle is deposited as varves. ► Light laminae are produced from spring to fall, and dark laminae in winter. ► The varved sediment offers a seasonally-resolved paleoenvironmental archive.

Introduction

Annually laminated lacustrine sediments (‘varves’ after de Geer, 1910) are natural, paleoenvironmental archives containing high-resolution proxy data and precise chronologies that can be used to study seasonal to decadal-scale climate variability (Brauer et al., 2008; Fig. 1). Closed-basin lakes are particularly well-suited for such studies. Assuming no groundwater interaction, the levels of such lakes are sensitive only to climate (evaporation, precipitation, and river inflow), which also strongly affects sedimentation processes. Lake Van in eastern Anatolia (Turkey), a closed-basin lake located in a transitional zone between major atmospheric circulation systems, represents an excellent continental archive to investigate the evolution of the Quaternary climate in the Near East. Because of its potential, Lake Van was designated a key site within the International Continental Scientific Drilling Program (ICDP) (Litt et al., 2009). In 2010, its sedimentary subsurface was drilled as part of the Paleovan project and varved sedimentary successions were recovered that span several glacial–interglacial cycles (Litt et al., 2011).

In order to fully exploit the potential of Lake Van's varved sediments, knowledge of the sedimentological response of the lake to the prevailing weather and climate conditions is required (Thunell et al., 1993, Broecker, 2002). A powerful approach involves monitoring modern sediment deposition in the lake basin and relating it to meteorological and limnological variables (Lotter and Birks, 1997, Lotter et al., 1997, Kienel et al., 2005). Sediment-trap studies yield valuable information about vertical and temporal particle dynamics (Sturm et al., 1982, Teranes et al., 1999, Douglas et al., 2002), while remote-sensing instruments yield information on the horizontal distribution of particles at high temporal resolution (Odermatt et al., 2009, Lahet and Stramski, 2010, Matthews et al., 2010). Seasonal sediment fluxes reflect the environmental conditions prevailing in both the water column and the catchment. This is because the flux of autochthonous particles (e.g., carbonate, diatoms) is affected by the conditions pertaining in the water column, while the flux of allochthonous particles (e.g., detrital minerals, pollen) is affected by the conditions pertaining in the catchment. In Lake Van, autochthonous carbonate precipitation in the epilimnion is recognizable as drifting, milky clouds, termed whitings (Strong and Eadie, 1978, Shinn et al., 1989, Thompson et al., 1997). The deposited carbonates are the source of several proxies that can be used in paleoenvironmental investigations; e.g., total inorganic carbon (TIC), the Mg/Ca ratio, and δ18O (Lemcke and Sturm, 1997).

Here we report the results of monitoring seasonal and interannual particle dynamics in Lake Van from July 2006 to August 2009. Well-established limnological methods involving water-column measurements and sequential sediment traps, supplemented with state-of-the-art remote-sensing methods, are used to define the annual particle cycle and to determine the nature and timing of the processes responsible for sediment formation in Lake Van. Multiple techniques are employed to provide a micro-scale characterization of the particles, to describe their appearance, and to quantify the seasonal fluxes of inorganic and organic material. In addition, the annual particle cycle is related to the meteorological conditions in the Lake Van area.

Section snippets

Study site

Lake Van (Fig. 2) is the world's fourth-largest hydrologically closed lake by volume, and the world's largest soda lake by area (volume 607 km3, area 3570 km2, maximum depth 460 m, pH ~ 9.7, salinity ~ 21 g kg 1; Kempe et al., 1991, Kaden et al., 2010). It is situated on a high plateau in eastern Anatolia, Turkey, at an altitude of 1648 m above sea level (a.s.l.). Lake Van fills the eastern part of a tectonic depression that was closed in the Upper Pliocene, probably because of volcanic activity (Sengör

Remote sensing

The medium-resolution imaging spectrometer (MERIS) for passive optical remote sensing, which is mounted on board the environmental satellite (ENVISAT), fulfills the requirements for mapping suspended particles in Lake Van (3-day resolution, 15 spectral bands, 300 m ground resolution). The Improved Contrast between Ocean and Land processor (ICOL; Santer and Zagolski, 2009) and the standard Case-2-Regional processor (C2R; Doerffer and Schiller, 2008) were applied to 40 MERIS full-resolution Level

Ground validation of MERIS data

Fig. 3 shows the highly variable TSMlab and TSMrs concentrations in the surface waters. The 19 match-ups of TSMlab and TSMrs concentrations show that the ICOL and C2R algorithms perform well at concentrations above 1 mg L 1 (measured mostly in 2010), which correspond to the occurrence of whitings, but much less well at concentrations below 1 mg L 1 (measured mostly in 2009). The sensitivity for TSMlab > 1 mg L 1 (dTSMrs/dTSMlab = 1.38) substantially exceeds that for TSMlab  1 mg L 1 (dTSMrs/dTSMlab = 0.15).

At

Spatial hydrological and sedimentological processes

The turbidity profile in August 2009 showed maximum particle concentrations occurring between 15 and 40 m water depth, at water temperatures between 5 °C and 15 °C, coinciding with an oxygen maximum (Fig. 6). This suggests that at least part of the turbidity signal is related to primary productivity. Decreasing turbidity and oxygen values below 26 m and 30 m, respectively, indicate that these depths represent the lower boundary of the photic zone. Below the photic zone, particles settle and dead

Conclusions

Three years of monitoring and tracking seasonal particle pulses in Lake Van have enabled us, for the first time in this lake, to link the seasonal particle fluxes to hydrological and meteorological forcing that is ultimately controlled by atmospheric circulation patterns. The sediment-trap data were complemented by remotely-sensed total suspended-matter concentrations, validated by simultaneous water-column sampling, that show great temporal and lateral variations resulting from whitings and

Acknowledgments

We thank Rolf Kipfer and Yama Tomonaga for their special efforts during fieldwork and for helpful discussions. Heike Kaden and Frank Peeters from the University of Konstanz, Germany, are acknowledged for providing the thermistors and processing the thermistor data. Thanks go to Sefer Örcen and Mustafa Karabiyikoglu from the Yüzüncü Yıl Üniversitesi of Van, Turkey, for their cooperation and support, and to the ship's crew, Mete Orhan, Mehmet Sahin, and Münip Kanan for their strong commitment.

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      Landmann et al. (1996) and Lemcke (1996) proposed that, due to the lower Mg/Ca ratio of the river water, calcite may be able to precipitate within the well-documented river plume whitings. River plumes can extend far into the basin (Stockhecke et al., 2012) and calcite may be transported even further due to its small crystal size (Landmann et al., 1996). The documented river plume whitings are in line with saturation index calculations by Lemcke (1996) demonstrating that modern calcite precipitation is possible within Ca-rich river plumes even when the freshwater mixes with far <20% of Lake Van's water.

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