25th International Meeting on Organic Geochemistry IMOG 2011

P-196
The holocene ecosystem of Lake Van, Eastern Turkey
Ozlem Bulkam 1 , Heinz Wilkes 2 , Naci Orbay 3 , M.Namık Cagatay 4 , M.Namık Yalcin 1
1 Istanbul University, Engineering Faculty, Geological Engineering Department, Avcılar/Istanbul, Turkey,
2 Helmholtz Centre, GeoForschungsZentrum-Potsdam, Section 4.3 Organic Geochemistry,
Telegrafenberg/Potsdam, Germany, 3 Istanbul University, Engineering Faculty, Geophysical Engineering
Department, Avcılar/Istanbul, Turkey, 4 Istanbul Technical University, Engineering Faculty, Geological
Engineering Department, Maslak/Istanbul, Turkey (corresponding author:bulkan@istanbul.edu.tr)
The ecosystem of Lake Van (LV) deserves special
attention as the largest soda lake of the Earth (area:
3522 km 2 , maximum water depth: 451 m; water
volume: 576 km 3 ) which exhibits an extremely high
alkalinity level (152 meq L -1 ) (Kempe, 1991; Thiel,
1997). The lake deposits are characterized by a very
distinct varving due to seasonal sedimentation
variability and the lack of bioturbation. Therefore, its
sedimentary record keeps highly sensitive paleoenvironmental
and ecological information for Eastern
Anatolia.
In order to contribute to the general knowledge on this
unique ecosystem, a multidisciplinary approach,
combining the bulk/molecular organic and inorganic
geochemical methods, was applied to LV sediments.
Two cores were studied: P01 from the eastern (4,5m;
corresponding to 10500 a BP) and P07 from western
part (5m; 9200 a BP) of the lake. A total of 65
samples were selected for basic organic geochemical
analysis (LECO and Rock-Eval) and the detailed
geochemical methods were applied to 19 of the
samples. Elemental composition of the sediments
were determined using XRF-scanner at 0.5 cm
resolution along each core.
Main organic geochemical characteristics of the EP
(eastern part) and WP (western part) of the lake are
somewhat different (Figure 1). EP sediments contain
relatively high amounts of TOC (0.69% - 5.03%) and
algal to terrestrial organic matter (OM) (HI: 62-818
mgHC/gTOC; C/N: 7-11). WP sediments contain
relatively lower TOC (0,9 to 2.8%) dominated by
terrestrial to mixed type of OM (HI: 78-330
mgHC/gTOC; C/N: 8-10). Additionally, the isotopic
compositions of the EP sediments fluctuate in a wider
range (δ 13 C: -22,9‰ to -25,5‰; δ 15 N: 3‰-12‰) than
of the WP (δ 13 C: -22,8 to -23,9‰; δ 15 N: 2‰-5‰). This
is probably a result of differences in intensity of
terrestrial OM flux and/or primary production between
the WP and EP of LV.
Lake sediments were also investigated in terms of
their n-alkane distributions. Recent EP sediments (0-
118 cm) are characterized by a n-alkane distribution,
which has its maximum at n-C17, typical for aquatic
producers (Meyers, 1997). Below 118 cm level the
main source are probably terrestrial plants, indicated
by the dominance of the C29 and C31 n-alkanes
(Meyers, 1997), except at 236 cm. Different from the
EP, terrestrial plant input was probably higher during
the entire depositional period for the WP sediments,
indicated by the general dominance of the C29 and/or
C31 n-alkanes. These conclusions are also supported
by HI values.
Figure 1. Variations of amount, type and isotopic
composition of the OM in the LV sediments.
The major and trace element profiles from XRF core
scanner analysis was partly used as proxies for water
chemistry (salinity, redox) and nutrient supply
mechanisms. The results suggest that periodical
changes in the LV ecosystem occurred over the last
10 500 a.
REFERENCES
Kempe S., Kazmirczak J., Landmann G., Konuk T.,
Reimer A., and Lipp A. 1991, Largest known
microbialites discovered in Lake Van, Turkey. Nature
349,605-608.
Meyers, P.A., 1997, Organic geochemical proxies of
paleoceanographic, paleolimnologic and paleoclimatic
processes. Organic Geochemistry 27, 213-250.
Thiel, V., Jenisch, A., Landman, G., Reimer, A.,
Michaelis, W., 1997, Unusual distributions of longchain
alkenones and tetrahymanol from the highly
alkaline Lake Van, Turkey. Geochimica et
Cosmochimica Acta, 61, 10, 2053-2064.
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