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Radiocarbon dating of sediments from large continental lakes (Lakes Baikal, Hovsgol and Erhel)
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 259 (2007) 565–570 www.elsevier.com/locate/nimb
Radiocarbon dating of sediments from large continental lakes (Lakes Baikal, Hovsgol and Erhel) Takahiro Watanabe b
a,*
, Toshio Nakamura a, Takayoshi Kawai
b
a Center for Chronological Research, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8602, Japan Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8602, Japan
Available online 1 February 2007
Abstract Lake Baikal (Russia) and Lake Hovsgol (Mongolia) are ancient large freshwater lakes in East Eurasia, and Lake Erhel is a saline lake (S = 20&) in Mongolia. Radiocarbon ages of the lake sediments were measured by Tandetron accelerator mass spectrometer at the Center for Chronological Research, Nagoya University. The linear sedimentation rate for the VER99G12 sediment core from Lake Baikal is estimated to be 18.2 cm/kyr, based on the conventional 14C ages. 14C measurements of the Lake Hovsgol sediment cores (HDP04) show that the sediment layers from 0 to 2 m depth (Core1-1) were disturbed during core sampling. The sediment layers below 2 m depth for HDP04 cores are available from past-environmental and biological investigations. The linear sedimentation rates of Lake Erhel sediment core (Erhel Core2) are 5.2 cm/kyr for cool periods (24–11 kyr BP) and 9.1–36.6 cm/kyr for warm periods (11–3 kyr BP). An abrupt increase of sedimentation rate about 11 kyr BP (from 5.2 to 31.0 cm/kyr) could be caused by climate humidification with warming. Ó 2007 Elsevier B.V. All rights reserved. PACS: 91.67.y; 91.67.Ty; 92.60.Iv Keywords: Radiocarbon age; Sediments; Lake Baikal; Lake Hovsgol; Lake Erhel; Climate changes
1. Introduction Paleo-environment, climate and biological records from lacustrine sediments are essential to understanding changes in Earth’s climate system. Radiocarbon dating using accelerator mass spectrometry is indispensable to performing paleo-environmental studies. Lake Baikal in southeastern Siberia is located in the tectonically active Baikal rift zone and is the largest freshwater lake in the world, with a total water volume of 23 000 km3 (about 20% of the earth’s fresh water [1]). The watershed of the lake covers about 540 000 km2, and more than 300 rivers enter the lake. Lake Baikal is located within the central Asian continent, which is far from any direct marine influence. Studies of long-
*
Corresponding author. Tel.: +81 52 789 2579; fax: +81 52 789 3092. E-mail address: [email protected] (T. Watanabe).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.200
term climate changes on Baikal area have been progressed through the Baikal Drilling Project (BDP) [2,3]. Previous climatic reconstructions have been mainly made using biogenic proxies such as diatoms, pollen, spores and organic materials, as well as abiogenic proxies such as clay minerals and grain size profiles [4–6]. For example, pollen analysis shows 57 alternations between forest taiga and subarctic desert around Lake Baikal during the past five million years [7]. Watanabe et al. [8,9] conducted a high-resolution analysis of total organic carbon, total nitrogen and total sulfur concentrations as well as biogenic opal content over the past 250 kyr in a Lake Baikal sediment core. Lake Hovsgol is located in northern Mongolia and at a high elevation (1645 m above sea level). This geographical characteristic makes Lake Hovsgol a unique place for paleoclimate studies using a continuous sediment. Analyses of organic and inorganic components have been carried out using short cores (core length: 1–2 m) from Lake Hovsgol
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[10–12]. In 2004, a long drilling core (core length: 81 m) was taken in Lake Hovsgol for a reconstruction of longterm climate changes. Lake Erhel, also in Mongolia, is a shallow (3.5 m in water depth), saline lake (S = 20&). Radiocarbon ages and environmental study with Lake Erhel sediment have not been reported. In this study, we provide AMS radiocarbon ages, total organic carbon content and water content in continental lake sediment cores from the last glacial period to the Holocene (back to 25 kyr). 2. Samples and analytical methods A piston core (VER99G12) was taken from the Buguldeika Saddle, Lake Baikal, in 1999 (52° 31 0 3600 N, 106° 09 0 0800 E; water depth, 350 m). The lake has three sedimentary basins, the Northern, Central and Southern basins (Fig. 1). The Buguldeika saddle and Academician ridge are underwater uplifts in Lake Baikal, which separate each sedimentary basin. The Buguldeika saddle was mainly formed by terrestrial materials from the Selenga river [13]. The total length of VER99G12 core is 466 cm. VER99G12 was divided into five sections at 1 m intervals just after core sampling. A drilling core (HDP04) and gravity core (GC04) were taken from the basin floor of Lake Hovsgol (50° 57 0 1900 N, 100° 21 0 3200 E; water depth, 250 m) in 2004. The lengths of the HDP04 and GC04 sediment cores were 81 m and 0.6 m, respectively. HDP04 consists of two columns (Fig. 3), Core1-1 (0–1.86 m depth) and Core2-1A– Core 48-3 (0–81 m depth). Piston coring was used for HDP04 drilling core. A 370 cm sediment core (Erhel Core2) was taken in 2004 from Lake Erhel (49° 55 0 0600 N, 99° 56 0 3600 E; water depth, 3.2 m). 14C age dating for the late Quaternary was per-
Table 1 14 C ages of total organic materials in the VER99G12 sediment core from Buguldeika saddle, Lake Baikal Sample No.
Depth (cm)
14
Lab. code (NUTA2-)
1-1-1 1-1-10 1-3-10 2-2-10 2-4-10 2-4-20 3-4-21 3-3-20 3-3-6 3-2-5 3-1-20 3-1-5 3-1-3 4-2-10 5-2-10
3.5 12.5 54.5 104.5 154.5 164.5 174.5 200.5 214.5 240.5 250.5 265.5 267.5 303.5 400.5
1793 ± 28 3219 ± 30 4792 ± 32 7673 ± 37 10 335 ± 42 10 164 ± 50 11 834 ± 53 10 440 ± 50 12 705 ± 54 15 714 ± 56 15 964 ± 68 16 139 ± 69 15 543 ± 66 19 543 ± 71 23 654 ± 91
8898 8899 8900 8901 8902 9597 9604 9603 9602 8905 9601 9600 9598 8906 8907
C age (BP)
formed for the 15 samples from Lake Baikal sediment core (VER99G12), for the 26 samples from the upper part of the HDP04 sediment cores (500 cm depth) and the GC04 gravity core, and for the 9 samples from Erhel Core2. The sediment samples were taken at every other 1 cm, and their outer rims were removed to avoid contamination. The discrete samples were dried and powdered. Subsequently, these samples were treated with 1.2 M HCl to remove carbonate. Plant residues from sediment samples were treated with 1.2 M HCl and 1.2 M NaOH. Decalcified sediment samples and plant residues were combusted at 850 °C for 6 h in evacuated bycol tubes with CuO wire. The resulting CO2 was collected and purified in a vacuum line and subsequently reduced to graphite with an iron catalyst and hydrogen at 650 °C for 6 h. Radiocarbon measurements were performed by an accelerator mass spectrometer (Model-4130 AMS, HVEE) at Nagoya
Fig. 1. Map showing the location of Lake Baikal, Lake Hovsgol and coring sites (VER99G12 and HDP04) in the lakes.
T. Watanabe et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 565–570
University. Total organic carbon (TOC) concentrations were determined using a CN analyzer (MT-700, Yanako Co.). 3. Results and discussion 3.1.
14
C dating of the Lake Baikal sediment core
Down core profiles of (a) water content, (b) total organic carbon content (TOC) and (c) 14C age for total organic materials in the VER99G12 sediment core from Buguldeika saddle are shown in Fig. 2 and Table 1. The water content in the sediment core varies between 27 and 72wt.%. Water content and density profiles for Lake Baikal sediment cores reflect changes mainly in the ratio of terrigenous (silt and clay mineral) to biogenic components (diatom frustules), corresponding to glacial–interglacial cycles [14]. Total organic carbon content varies widely from 6.0 to 31.4 mg/g dry sediment. The high water content and organic carbon accumulation (more than 50wt.% and 20 mg/g dry sediment, respectively) are observed in the sediment within the upper part of the core (150-3 cm in depth), which is dated as Holocene by AMS 14C (about 10– 1.8 kyr). 14 C age dating of the VER99G12 sediment core shows that the sediment core contains a continuous record, over the past 24 kyr BP (Fig. 2). TOC concentrations gradually increase from 16 to 13 kyr BP (from 7 to 17 mg/g dry sediment), corresponding to the climate transition period from glacial to interglacial around Lake Baikal. The small peak of TOC (up to 16.7 mg/g dry sediment) at 215–201 cm in depth (12.7 kyr BP) corresponds to the Bøllong-Allerød period, which is a warming phase at the early stage of the Holocene. The Bøllong-Allerød warming phase has been also observed in the sediment core from Posolskaya bank (south of the delta of the Selenga river) in Lake Baikal [15]. A decrease of TOC concentration (16.6 to 12.6 mg/g dry sediment) at 175–165 cm depth (about 11 kyr BP) in the VER99G12 sediment core is observed;
Lake Baikal
VER99G12 sediment core
Water content (wt.%) 0
Depth (cm)
100
10
b
20
30
age
(kyr BP)
(mg/g dry sed.)
20 30 40 50 60 70 0
a
14 C
TOC 0
5
10 15 20 25
c
200
300
400
Fig. 2. Vertical distributions for (a) water content, (b) total organic carbon (TOC) and (c) 14C ages of TOC in the VER99G12 sediment core from Buguldeika saddle, Lake Baikal.
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this characteristic corresponds to the Younger Dryas rapid cooling event during the climate transition period from the last glacial maximum (LGM) to the Holocene. The rapid decrease of organic carbon content during this climate transition period has been also observed other sediment cores from Academician ridge, Lake Baikal [16]. In the VER99G12 sediment core, the 14C age for TOC at a surface layer is 1.8 kyr BP. 14C ages of 4.5–1.6 kyr BP have also been obtained from surface sediment of Academician ridge and the Northern and Southern basins in Lake Baikal (Watanabe et al., unpublished). These relatively old ages for surface sediments from Lake Baikal could be due to the loss of the upper layer during core sampling, reworking of sediment layers, and/or input into the lake of terrestrial organic materials containing old carbon. Watanabe et al. have also studied radiocarbon ages of sedimentary lipids from Lake Baikal surface sediments (Southern basin) (unpublished). The 14C ages of sedimentary lipids (1.6–1.5 kyr BP) are younger than those of TOC (4.5–2.2 kyr BP). Therefore, it will be necessary to perform 14C measurements of organic molecules in Lake Baikal that have specific biological sources. The present result of 14C age for the VER99G12 sediment core from Lake Baikal shows that the sedimentation rate is approximately constant as 18.2 cm/kyr (3.5– 400.5 cm in depth). This sedimentation rate in the VER99G12 sediment core is 4–5 times higher than that of sediment cores from Academician ridge. Academician ridge is isolated from direct fluvial or downslope sedimentation. Several radiocarbon age studies on other sediment cores from Academician ridge show the rate of sediment accumulation to be 2–10 cm/kyr [8,17,18]. Furthermore, sedimentation rates of long cores (cores BDP96 and BDP98) from Academician ridge tentatively determined by geomagnetic polarity reversals are constant and linear (3.8–3.9 cm/kyr) over the past five million years [3,19]. The relatively higher sedimentation rate in the VER99G12 sediment core could be due to the influx of land-derived materials from Selenga river, the largest river flowing into the lake and the largest source of fluvial sediment. High total organic carbon/total nitrogen (TOC/TN) atomic ratios in the VER99G12 during the Holocene (up to 17 at a depth of 12.5 cm, unpublished) indicate that the major source of organic materials is terrestrial higher plants. 3.2.
14
C dating of sediments from Lake Hovsgol
Fig. 3 and Table 2 show radiocarbon ages for total organic materials and plant residues (wood and moss fragments) in the HDP04 sediment cores from Lake Hovsgol. The radiocarbon age of the surface layer of the gravity core (GC04 core, 0.5 cm depth) is 7.8 kyr BP, which is older by more than 7000 years than the reported values from Lake Hovsgol [11,12]. This result could be caused by loss of the upper section on the sediment core. In the deeper part of the GC04 core (from 60.5–20.5 cm), relatively small changes of the 14C age are observed. One interpretation
T. Watanabe et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 565–570
7778±24 17992±50 21351±62 21506±62 22285±65 23659±71
2913±22 2776±22 2321±22
4110±32 8132±38 13538±33
1674±21
17301±63
(wood fragment)
Core
1689±21
23236±69
2743±22
1302±21 Core
(wood fragment)
0
2-1A 1
2-2A
6622±26
GC04
Core
1-1
(originally marked GC-3)
1cm
Core
3-1
Core
3-2 21092±79
18726±83 Core
2
3
23761±95
(moss fragment?)
Lake Hovsgol
4719±24 8080±37
Core depth (m)
568
4-1
4 26341±115
HDP04 sediment core Core
4-2
Core
5-1 27587±126
5
Fig. 3. 14C ages (years BP) of TOC and plant residue in the HDP04 sediment cores from Lake Hovsgol, Mongolia.
Table 2 14 C ages of total organic materials and plant residues in the HDP04 sediment cores from Lake Hovsgol Sample No.
Depth (cm)
14
GC04,0-1 GC04,10-11 GC04,20-21 GC04,30-31 GC04,40-41 GC04,50-51 GC04,60-61 1-1,0-1 1-1,10-11 1-1,40-41 1-1,44-45a 1-1,69-72a 1-1,70-71 1-1120-121 1-1180-181 2-1A,0-1 2-1A,20-21 2-1A,40-41 2-1A,70-71 3-1,10-11 3-1,40-41 4-1,20-21 4-1,60-61 4-1,63.5b 4-1130-131 5-2,20-21
0.5 10.5 20.5 30.5 40.5 50.5 60.5 0.5 10.5 40.5 44.5 70.5 70.5 120.5 180.5 0.5 20.5 40.5 70.5 211.5 241.5 310.5 350.5 353.5 420.5 500.5
7778 ± 24 17 991 ± 50 21 351 ± 62 21 506 ± 62 22 285 ± 65 23 659 ± 71 23 236 ± 69 2743 ± 22 2913 ± 22 2776 ± 22 1674 ± 21 1302 ± 21 2321 ± 22 1689 ± 21 6622 ± 26 4110 ± 32 8132 ± 38 13 533 ± 38 17 301 ± 63 4719 ± 24 8080 ± 37 21 092 ± 79 23761 ± 95 18 726 ± 83 26 341 ± 115 27 587 ± 126
a b
C age (BP)
Lab. code (NUTA2-) 8423 8425 8426 8427 8428 8429 8430 8431 8433 8434 8440 8442 8435 8436 8437 8885 8889 8438 8890 8439 8891 8892 8893 8445 8896 8897
Wood fragment. Plant residue (moss fragment?).
is high sedimentation rate (7–64 cm/kyr, 50.5–20.5 cm depth) caused by low water levels during a cool period. Another possibility is that the sediment layers were contaminated by modern carbon after sample collection. In fact, the 14C age at the bottom (60.5 cm depth) is younger than that of the upper layer (50.5 cm depth).
In the HDP04 Core1-1 (core length; 1.86 m), 14C ages for wood fragments from the sediment (70.5 cm and 44.5 cm depth) are younger than that of TOC by approximately 1000 year (Fig. 3). In previous study, a 14C age of 7.2 kyr BP (for wood fragment at a depth of 30 cm) in Lake Hovsgol sediment has been obtained (X105 sediment core; unpublished data). The X105 core was taken near the sampling site of HDP04 drilling core in 2001 (50° 57 0 39200 N, 100° 20 0 85500 E; water depth, 248 m). The 14C ages of wood fragments in 70.5 cm (1.3 kyr BP) is younger than that of the upper layer (44.5 cm depth, 1.7 kyr BP) in the HDP04 Core1-1. Furthermore, the 14C ages of TOC in Core1-1 do not become progressively older with depth from 0.5 to 120.5 cm (Fig. 3). The 14C age at the bottom of Core1-1 (180.5 cm depth) is 6.6 kyr BP. Nara et al. [12] has reported 14 C ages of 22.5 kyr BP (138 cm depth; X104 core) and 24.0 kyr BP (128 cm depth; X106 core) for the Lake Hovsgol sediment cores. The 14C age of the bottom layer in Core1-1 (180.5 cm depth) is extremely young in comparison with the previous study [12]. These anomalous results reveal that the sedimentary layers of the HDP04 Core1-1 were disturbed during piston coring. The Core1-1 is not suitable for paleo-environmental studies. In the HDP04 Core2-1A sediment sample, 14C ages of the layers 0.5 and 70.5 cm depth are 4.1 and 17.3 kyr BP, respectively. The linear sedimentation rate for the Core21A is estimated to be 5.3 cm/kyr, based on the conventional 14C ages. The 14C dates and sedimentation rate is coincident with results of the X105 core (core length, 111 cm; average sedimentation rate, 5.6 cm/kyr). However, the 14C age of 4.7 kyr BP is obtained at the top of Core3-1 (211.5 cm depth, Fig. 3). The sediment layers of Core 2, 3, 4 and 5 were not disturbed during piston coring except surface layers, because laminated layers are observed in the cores. One interpretation is that the top of Core3-1 (200 cm depth) may be consistent with the surface layer of Lake Hovsgol sediment. The sediment layers below 2 m depth for HDP04 cores are useful to environmental and biological investigations, because the 14C age becomes progressively older with depth, and changes of the 14C age are consistent with previous studies (14C dates of X104, X105 and X106 sediment cores) [12]. On the other hand, Core2-1A and 2-2A (0–102.5 cm depth, Fig. 3) are available for the study of climate and environment changes during Holocene and last glacial periods. In the climate transition period from glacial to interglacial (17.3–13.5 kyr BP), finely laminated layers are observed in the cores. The 14C age of a moss fragment in Core4-1 (18.7 kyr BP, 353.5 cm in depth) is younger than that of TOC (23.8 kyr BP). This result could be caused by a large supply of terrestrial organic materials containing old carbon during the last glacial period. 3.3.
14
C dating of Lake Erhel sediment core
Down core profiles of the radiocarbon ages for total organic materials and a plant seed in the Erhel Core2 sed-
T. Watanabe et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 565–570 14C
Lake Erhel Erhel Core 2
0
0
age (kyr BP)
5
10 15 20 25
5mm
Depth (cm)
50
Plant residue (seed)
100
22.4 cm/kyr
150 200 250
34.6 cm/kyr 5.2 cm/kyr
300 Fig. 4. Vertical profile for 14C ages of TOC and plant seed in the Erhel Core2 from Lake Erhel, Mongolia. The 14C ages of TOC and plant residue are shown as filled and open diamonds, respectively.
Table 3 14 C ages of total organic materials and plant residue in the Erhel Core2 from Lake Erhel Sample No.
Depth (cm)
14
Lab. code (NUTA2-)
1,42–43 1,42.5a 1,68–70 1A,7–8 1A,45–46 1A,86–87 2,25.5–26.5 2,48–49 3,15–16
42.5 42.5 69.5 77.5 115.5 156.5 206 228.5 295.5
3174 ± 30 3395 ± 35 4469 ± 36 5345 ± 34 6954 ± 35 8879 ± 44 10 231 ± 41 10 958 ± 43 23 798 ± 98
8875 8882 8883 8876 9585 8880 9586 8881 8884
a
C age (BP)
Plant residue (seed).
iment samples from Lake Erhel are shown in Fig. 4 and Table 3. The radiocarbon age of plant residue (3.4 kyr BP) at a depth of 42.5 cm is slightly older than the value of TOC (3.2 kyr BP). In the bottom layer of this core at a depth of 295.5 cm, a 14C age of 23.8 kyr BP is observed. The linear sedimentation rates of the Lake Erhel sediment core are 5.2 cm/kyr for the cool period (24–11 kyr BP) and 9.1–34.6 cm/kyr for the warm period (11–3 kyr BP). The low sedimentation rate in Lake Erhel during the cool period could be due to a decline in precipitation, a prolongation of the freezing period for the lake water, or a drying of the lake basin. An abrupt increase of sedimentation rate about 11 kyr BP (from 5.2 to 31.0 cm/kyr) could be caused by climate humidification with warming. Because a relatively high sedimentation rate and thinly laminated layers are observed during the Holocene in the Lake Erhel sediment core, high time-resolution climate and environment change studies will be possible. 4. Conclusions The sediments from three continental lakes, Lake Baikal, Hovsgol and Erhel, have been dated by AMS 14C. The estimated sedimentation rate of VER99G12 sediment core
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from Lake Baikal (18.2 cm/kyr in average) is 4–5 times higher than that of sediment cores from Academician ridge, Lake Baikal. The relatively higher sedimentation rate could be due to the influx of land-derived materials from Selenga river (the largest river flowing into the lake). The millennial scale climate changes, Bøllong-Allerød (13 kyr BP) and Younger Dryas (11 kyr BP) events, during the transition period from LGM to Holocene were observed in the VER99G12 sediment core. 14 C ages of the HDP04 sediment cores (Lake Hovsgol) from 0 to 2 m depth (Core1-1) do not become progressively older with depth, which suggest that sedimentary layers were disturbed by core sampling. The sediment layers below 2 m for HDP04 cores are available for the study of climate change, because the changes in 14C age are consistent with previous studies. Radiocarbon dating of the sediment from Lake Erhel (Erhel Core2) shows that the core provides a high time-resolution record (up to 30 yr/cm) that can be used for paleo-environmental and biological studies in the continental interior. Acknowledgments We are grateful to the BDP and HDP members for the collection of the sediment core samples. We thank M. Nishimura, A. Tanaka, Y. Hase, K. Shiti, M. Shimokawara and F. Nara for preservation and division of the VER99G12 sediment cores. We would like to thank the staff of the Center for Chronological Research, Nagoya University, especially A. Ikeda and T. Ohta for their cooperation of sample preparation. This work was partly supported by a grant to T.W. from the Sumitomo Foundation (No. 043273). References [1] K.K. Falkner, C.I. Measures, S.E. Herbelin, J.M. Edmond, R.F. Weiss, Limnol. Oceanogr. 36 (1991) 413. [2] Baikal Drilling Project BDP-96 (Leg II) Members, Eos. Trans., vol. 78, 1997, pp. 601. [3] V.A. Kravchinsky, M.A. Krainov, M.E. Evans, J.A. Peck, J.W. King, H. Sakai, M.I. Kuzmin, T. Kawai, D.F. Williams, Palaeogeogr. Palaeoclimatol. Palaeoecol. 195 (2003) 281. [4] K. Kashiwaya, M. Ryogo, H. Sakai, T. Kawai, Geophys. Res. Lett. 25 (1998) 659. [5] Y. Soma, M. Soma, A. Tanaka, T. Kawai, Geochem. J. 35 (2001) 377. [6] T. Takamatsu, K. Kashiwaya, T. Kawai, in: K. Kashiwaya (Ed.), Long Continental Records from Lake Baikal, Springer-Verlag, Tokyo, 2003, p. 313. [7] K. Kawamuro, K. Shichi, Y. Hase, A. Iwauchi, K. Minoura, T. Oda, H. Takahara, H. Sakai, Y. Morita, N. Miyoshi, M.I. Kuzmin, in: K. Minoura (Ed.), Lake Baikal: A Mirror in Time and Space for Understanding Global Change Processes, Elsevier, Amsterdam, 2000, p. 101. [8] T. Watanabe, H. Naraoka, M. Nishimura, T. Kawai, Earth Planet. Sci. Lett. 222 (2004) 285. [9] T. Watanabe, A. Tanaka, T. Nakamura, R. Senda, M. Nishimura, T. Kawai, Verh. Internat. Verein. Limnol. 29 (2005) 903. [10] E. Karabanov, D. Williams, M. Kuzmin, V. Sideleva, G. Khursevich, A. Prokopenko, E. Solotchina, L. Tkachenko, S. Fedenya, E. Kerber,
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[15] E.P. Chebykin, D.N. Edgington, M.A. Grachev, T.O. Zheleznyakova, S.S. Vorobyova, N.S. Kulikova, I.N. Azarova, O.M. Khlystov, E.L. Goldberg, Earth Planet. Sci. Lett. 200 (2002) 167. [16] T. Watanabe, H. Naraoka, M. Nishimura, M. Kinoshita, T. Kawai, Geochem. J. 37 (2003) 493. [17] R.G. Deike, L. Granina, E. Callender, J.J. McGee, Mar. Geol. 139 (1997) 21. [18] K. Horiuchi, K. Minoura, K. Hoshino, T. Oda, T. Nakamura, T. Kawai, Palaeogeogr. Palaeoclimatol. Palaeoecol. 157 (2000) 95. [19] H. Sakai, S. Nomura, M. Horii, K. Kashiwaya, A. Tanaka, T. Kawai, V. Kravchinsky, J. Peck, J. King, in: K. Minoura (Ed.), Lake Baikal: A Mirror in Time and Space for Understanding Global Change Processes, Elsevier, Amsterdam, 2000, p. 35.
Nuclear Instruments and Methods in Physics Research B 259 (2007) 565–570 www.elsevier.com/locate/nimb
Radiocarbon dating of sediments from large continental lakes (Lakes Baikal, Hovsgol and Erhel) Takahiro Watanabe b
a,*
, Toshio Nakamura a, Takayoshi Kawai
b
a Center for Chronological Research, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8602, Japan Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8602, Japan
Available online 1 February 2007
Abstract Lake Baikal (Russia) and Lake Hovsgol (Mongolia) are ancient large freshwater lakes in East Eurasia, and Lake Erhel is a saline lake (S = 20&) in Mongolia. Radiocarbon ages of the lake sediments were measured by Tandetron accelerator mass spectrometer at the Center for Chronological Research, Nagoya University. The linear sedimentation rate for the VER99G12 sediment core from Lake Baikal is estimated to be 18.2 cm/kyr, based on the conventional 14C ages. 14C measurements of the Lake Hovsgol sediment cores (HDP04) show that the sediment layers from 0 to 2 m depth (Core1-1) were disturbed during core sampling. The sediment layers below 2 m depth for HDP04 cores are available from past-environmental and biological investigations. The linear sedimentation rates of Lake Erhel sediment core (Erhel Core2) are 5.2 cm/kyr for cool periods (24–11 kyr BP) and 9.1–36.6 cm/kyr for warm periods (11–3 kyr BP). An abrupt increase of sedimentation rate about 11 kyr BP (from 5.2 to 31.0 cm/kyr) could be caused by climate humidification with warming. Ó 2007 Elsevier B.V. All rights reserved. PACS: 91.67.y; 91.67.Ty; 92.60.Iv Keywords: Radiocarbon age; Sediments; Lake Baikal; Lake Hovsgol; Lake Erhel; Climate changes
1. Introduction Paleo-environment, climate and biological records from lacustrine sediments are essential to understanding changes in Earth’s climate system. Radiocarbon dating using accelerator mass spectrometry is indispensable to performing paleo-environmental studies. Lake Baikal in southeastern Siberia is located in the tectonically active Baikal rift zone and is the largest freshwater lake in the world, with a total water volume of 23 000 km3 (about 20% of the earth’s fresh water [1]). The watershed of the lake covers about 540 000 km2, and more than 300 rivers enter the lake. Lake Baikal is located within the central Asian continent, which is far from any direct marine influence. Studies of long-
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Corresponding author. Tel.: +81 52 789 2579; fax: +81 52 789 3092. E-mail address: [email protected] (T. Watanabe).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.200
term climate changes on Baikal area have been progressed through the Baikal Drilling Project (BDP) [2,3]. Previous climatic reconstructions have been mainly made using biogenic proxies such as diatoms, pollen, spores and organic materials, as well as abiogenic proxies such as clay minerals and grain size profiles [4–6]. For example, pollen analysis shows 57 alternations between forest taiga and subarctic desert around Lake Baikal during the past five million years [7]. Watanabe et al. [8,9] conducted a high-resolution analysis of total organic carbon, total nitrogen and total sulfur concentrations as well as biogenic opal content over the past 250 kyr in a Lake Baikal sediment core. Lake Hovsgol is located in northern Mongolia and at a high elevation (1645 m above sea level). This geographical characteristic makes Lake Hovsgol a unique place for paleoclimate studies using a continuous sediment. Analyses of organic and inorganic components have been carried out using short cores (core length: 1–2 m) from Lake Hovsgol
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[10–12]. In 2004, a long drilling core (core length: 81 m) was taken in Lake Hovsgol for a reconstruction of longterm climate changes. Lake Erhel, also in Mongolia, is a shallow (3.5 m in water depth), saline lake (S = 20&). Radiocarbon ages and environmental study with Lake Erhel sediment have not been reported. In this study, we provide AMS radiocarbon ages, total organic carbon content and water content in continental lake sediment cores from the last glacial period to the Holocene (back to 25 kyr). 2. Samples and analytical methods A piston core (VER99G12) was taken from the Buguldeika Saddle, Lake Baikal, in 1999 (52° 31 0 3600 N, 106° 09 0 0800 E; water depth, 350 m). The lake has three sedimentary basins, the Northern, Central and Southern basins (Fig. 1). The Buguldeika saddle and Academician ridge are underwater uplifts in Lake Baikal, which separate each sedimentary basin. The Buguldeika saddle was mainly formed by terrestrial materials from the Selenga river [13]. The total length of VER99G12 core is 466 cm. VER99G12 was divided into five sections at 1 m intervals just after core sampling. A drilling core (HDP04) and gravity core (GC04) were taken from the basin floor of Lake Hovsgol (50° 57 0 1900 N, 100° 21 0 3200 E; water depth, 250 m) in 2004. The lengths of the HDP04 and GC04 sediment cores were 81 m and 0.6 m, respectively. HDP04 consists of two columns (Fig. 3), Core1-1 (0–1.86 m depth) and Core2-1A– Core 48-3 (0–81 m depth). Piston coring was used for HDP04 drilling core. A 370 cm sediment core (Erhel Core2) was taken in 2004 from Lake Erhel (49° 55 0 0600 N, 99° 56 0 3600 E; water depth, 3.2 m). 14C age dating for the late Quaternary was per-
Table 1 14 C ages of total organic materials in the VER99G12 sediment core from Buguldeika saddle, Lake Baikal Sample No.
Depth (cm)
14
Lab. code (NUTA2-)
1-1-1 1-1-10 1-3-10 2-2-10 2-4-10 2-4-20 3-4-21 3-3-20 3-3-6 3-2-5 3-1-20 3-1-5 3-1-3 4-2-10 5-2-10
3.5 12.5 54.5 104.5 154.5 164.5 174.5 200.5 214.5 240.5 250.5 265.5 267.5 303.5 400.5
1793 ± 28 3219 ± 30 4792 ± 32 7673 ± 37 10 335 ± 42 10 164 ± 50 11 834 ± 53 10 440 ± 50 12 705 ± 54 15 714 ± 56 15 964 ± 68 16 139 ± 69 15 543 ± 66 19 543 ± 71 23 654 ± 91
8898 8899 8900 8901 8902 9597 9604 9603 9602 8905 9601 9600 9598 8906 8907
C age (BP)
formed for the 15 samples from Lake Baikal sediment core (VER99G12), for the 26 samples from the upper part of the HDP04 sediment cores (500 cm depth) and the GC04 gravity core, and for the 9 samples from Erhel Core2. The sediment samples were taken at every other 1 cm, and their outer rims were removed to avoid contamination. The discrete samples were dried and powdered. Subsequently, these samples were treated with 1.2 M HCl to remove carbonate. Plant residues from sediment samples were treated with 1.2 M HCl and 1.2 M NaOH. Decalcified sediment samples and plant residues were combusted at 850 °C for 6 h in evacuated bycol tubes with CuO wire. The resulting CO2 was collected and purified in a vacuum line and subsequently reduced to graphite with an iron catalyst and hydrogen at 650 °C for 6 h. Radiocarbon measurements were performed by an accelerator mass spectrometer (Model-4130 AMS, HVEE) at Nagoya
Fig. 1. Map showing the location of Lake Baikal, Lake Hovsgol and coring sites (VER99G12 and HDP04) in the lakes.
T. Watanabe et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 565–570
University. Total organic carbon (TOC) concentrations were determined using a CN analyzer (MT-700, Yanako Co.). 3. Results and discussion 3.1.
14
C dating of the Lake Baikal sediment core
Down core profiles of (a) water content, (b) total organic carbon content (TOC) and (c) 14C age for total organic materials in the VER99G12 sediment core from Buguldeika saddle are shown in Fig. 2 and Table 1. The water content in the sediment core varies between 27 and 72wt.%. Water content and density profiles for Lake Baikal sediment cores reflect changes mainly in the ratio of terrigenous (silt and clay mineral) to biogenic components (diatom frustules), corresponding to glacial–interglacial cycles [14]. Total organic carbon content varies widely from 6.0 to 31.4 mg/g dry sediment. The high water content and organic carbon accumulation (more than 50wt.% and 20 mg/g dry sediment, respectively) are observed in the sediment within the upper part of the core (150-3 cm in depth), which is dated as Holocene by AMS 14C (about 10– 1.8 kyr). 14 C age dating of the VER99G12 sediment core shows that the sediment core contains a continuous record, over the past 24 kyr BP (Fig. 2). TOC concentrations gradually increase from 16 to 13 kyr BP (from 7 to 17 mg/g dry sediment), corresponding to the climate transition period from glacial to interglacial around Lake Baikal. The small peak of TOC (up to 16.7 mg/g dry sediment) at 215–201 cm in depth (12.7 kyr BP) corresponds to the Bøllong-Allerød period, which is a warming phase at the early stage of the Holocene. The Bøllong-Allerød warming phase has been also observed in the sediment core from Posolskaya bank (south of the delta of the Selenga river) in Lake Baikal [15]. A decrease of TOC concentration (16.6 to 12.6 mg/g dry sediment) at 175–165 cm depth (about 11 kyr BP) in the VER99G12 sediment core is observed;
Lake Baikal
VER99G12 sediment core
Water content (wt.%) 0
Depth (cm)
100
10
b
20
30
age
(kyr BP)
(mg/g dry sed.)
20 30 40 50 60 70 0
a
14 C
TOC 0
5
10 15 20 25
c
200
300
400
Fig. 2. Vertical distributions for (a) water content, (b) total organic carbon (TOC) and (c) 14C ages of TOC in the VER99G12 sediment core from Buguldeika saddle, Lake Baikal.
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this characteristic corresponds to the Younger Dryas rapid cooling event during the climate transition period from the last glacial maximum (LGM) to the Holocene. The rapid decrease of organic carbon content during this climate transition period has been also observed other sediment cores from Academician ridge, Lake Baikal [16]. In the VER99G12 sediment core, the 14C age for TOC at a surface layer is 1.8 kyr BP. 14C ages of 4.5–1.6 kyr BP have also been obtained from surface sediment of Academician ridge and the Northern and Southern basins in Lake Baikal (Watanabe et al., unpublished). These relatively old ages for surface sediments from Lake Baikal could be due to the loss of the upper layer during core sampling, reworking of sediment layers, and/or input into the lake of terrestrial organic materials containing old carbon. Watanabe et al. have also studied radiocarbon ages of sedimentary lipids from Lake Baikal surface sediments (Southern basin) (unpublished). The 14C ages of sedimentary lipids (1.6–1.5 kyr BP) are younger than those of TOC (4.5–2.2 kyr BP). Therefore, it will be necessary to perform 14C measurements of organic molecules in Lake Baikal that have specific biological sources. The present result of 14C age for the VER99G12 sediment core from Lake Baikal shows that the sedimentation rate is approximately constant as 18.2 cm/kyr (3.5– 400.5 cm in depth). This sedimentation rate in the VER99G12 sediment core is 4–5 times higher than that of sediment cores from Academician ridge. Academician ridge is isolated from direct fluvial or downslope sedimentation. Several radiocarbon age studies on other sediment cores from Academician ridge show the rate of sediment accumulation to be 2–10 cm/kyr [8,17,18]. Furthermore, sedimentation rates of long cores (cores BDP96 and BDP98) from Academician ridge tentatively determined by geomagnetic polarity reversals are constant and linear (3.8–3.9 cm/kyr) over the past five million years [3,19]. The relatively higher sedimentation rate in the VER99G12 sediment core could be due to the influx of land-derived materials from Selenga river, the largest river flowing into the lake and the largest source of fluvial sediment. High total organic carbon/total nitrogen (TOC/TN) atomic ratios in the VER99G12 during the Holocene (up to 17 at a depth of 12.5 cm, unpublished) indicate that the major source of organic materials is terrestrial higher plants. 3.2.
14
C dating of sediments from Lake Hovsgol
Fig. 3 and Table 2 show radiocarbon ages for total organic materials and plant residues (wood and moss fragments) in the HDP04 sediment cores from Lake Hovsgol. The radiocarbon age of the surface layer of the gravity core (GC04 core, 0.5 cm depth) is 7.8 kyr BP, which is older by more than 7000 years than the reported values from Lake Hovsgol [11,12]. This result could be caused by loss of the upper section on the sediment core. In the deeper part of the GC04 core (from 60.5–20.5 cm), relatively small changes of the 14C age are observed. One interpretation
T. Watanabe et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 565–570
7778±24 17992±50 21351±62 21506±62 22285±65 23659±71
2913±22 2776±22 2321±22
4110±32 8132±38 13538±33
1674±21
17301±63
(wood fragment)
Core
1689±21
23236±69
2743±22
1302±21 Core
(wood fragment)
0
2-1A 1
2-2A
6622±26
GC04
Core
1-1
(originally marked GC-3)
1cm
Core
3-1
Core
3-2 21092±79
18726±83 Core
2
3
23761±95
(moss fragment?)
Lake Hovsgol
4719±24 8080±37
Core depth (m)
568
4-1
4 26341±115
HDP04 sediment core Core
4-2
Core
5-1 27587±126
5
Fig. 3. 14C ages (years BP) of TOC and plant residue in the HDP04 sediment cores from Lake Hovsgol, Mongolia.
Table 2 14 C ages of total organic materials and plant residues in the HDP04 sediment cores from Lake Hovsgol Sample No.
Depth (cm)
14
GC04,0-1 GC04,10-11 GC04,20-21 GC04,30-31 GC04,40-41 GC04,50-51 GC04,60-61 1-1,0-1 1-1,10-11 1-1,40-41 1-1,44-45a 1-1,69-72a 1-1,70-71 1-1120-121 1-1180-181 2-1A,0-1 2-1A,20-21 2-1A,40-41 2-1A,70-71 3-1,10-11 3-1,40-41 4-1,20-21 4-1,60-61 4-1,63.5b 4-1130-131 5-2,20-21
0.5 10.5 20.5 30.5 40.5 50.5 60.5 0.5 10.5 40.5 44.5 70.5 70.5 120.5 180.5 0.5 20.5 40.5 70.5 211.5 241.5 310.5 350.5 353.5 420.5 500.5
7778 ± 24 17 991 ± 50 21 351 ± 62 21 506 ± 62 22 285 ± 65 23 659 ± 71 23 236 ± 69 2743 ± 22 2913 ± 22 2776 ± 22 1674 ± 21 1302 ± 21 2321 ± 22 1689 ± 21 6622 ± 26 4110 ± 32 8132 ± 38 13 533 ± 38 17 301 ± 63 4719 ± 24 8080 ± 37 21 092 ± 79 23761 ± 95 18 726 ± 83 26 341 ± 115 27 587 ± 126
a b
C age (BP)
Lab. code (NUTA2-) 8423 8425 8426 8427 8428 8429 8430 8431 8433 8434 8440 8442 8435 8436 8437 8885 8889 8438 8890 8439 8891 8892 8893 8445 8896 8897
Wood fragment. Plant residue (moss fragment?).
is high sedimentation rate (7–64 cm/kyr, 50.5–20.5 cm depth) caused by low water levels during a cool period. Another possibility is that the sediment layers were contaminated by modern carbon after sample collection. In fact, the 14C age at the bottom (60.5 cm depth) is younger than that of the upper layer (50.5 cm depth).
In the HDP04 Core1-1 (core length; 1.86 m), 14C ages for wood fragments from the sediment (70.5 cm and 44.5 cm depth) are younger than that of TOC by approximately 1000 year (Fig. 3). In previous study, a 14C age of 7.2 kyr BP (for wood fragment at a depth of 30 cm) in Lake Hovsgol sediment has been obtained (X105 sediment core; unpublished data). The X105 core was taken near the sampling site of HDP04 drilling core in 2001 (50° 57 0 39200 N, 100° 20 0 85500 E; water depth, 248 m). The 14C ages of wood fragments in 70.5 cm (1.3 kyr BP) is younger than that of the upper layer (44.5 cm depth, 1.7 kyr BP) in the HDP04 Core1-1. Furthermore, the 14C ages of TOC in Core1-1 do not become progressively older with depth from 0.5 to 120.5 cm (Fig. 3). The 14C age at the bottom of Core1-1 (180.5 cm depth) is 6.6 kyr BP. Nara et al. [12] has reported 14 C ages of 22.5 kyr BP (138 cm depth; X104 core) and 24.0 kyr BP (128 cm depth; X106 core) for the Lake Hovsgol sediment cores. The 14C age of the bottom layer in Core1-1 (180.5 cm depth) is extremely young in comparison with the previous study [12]. These anomalous results reveal that the sedimentary layers of the HDP04 Core1-1 were disturbed during piston coring. The Core1-1 is not suitable for paleo-environmental studies. In the HDP04 Core2-1A sediment sample, 14C ages of the layers 0.5 and 70.5 cm depth are 4.1 and 17.3 kyr BP, respectively. The linear sedimentation rate for the Core21A is estimated to be 5.3 cm/kyr, based on the conventional 14C ages. The 14C dates and sedimentation rate is coincident with results of the X105 core (core length, 111 cm; average sedimentation rate, 5.6 cm/kyr). However, the 14C age of 4.7 kyr BP is obtained at the top of Core3-1 (211.5 cm depth, Fig. 3). The sediment layers of Core 2, 3, 4 and 5 were not disturbed during piston coring except surface layers, because laminated layers are observed in the cores. One interpretation is that the top of Core3-1 (200 cm depth) may be consistent with the surface layer of Lake Hovsgol sediment. The sediment layers below 2 m depth for HDP04 cores are useful to environmental and biological investigations, because the 14C age becomes progressively older with depth, and changes of the 14C age are consistent with previous studies (14C dates of X104, X105 and X106 sediment cores) [12]. On the other hand, Core2-1A and 2-2A (0–102.5 cm depth, Fig. 3) are available for the study of climate and environment changes during Holocene and last glacial periods. In the climate transition period from glacial to interglacial (17.3–13.5 kyr BP), finely laminated layers are observed in the cores. The 14C age of a moss fragment in Core4-1 (18.7 kyr BP, 353.5 cm in depth) is younger than that of TOC (23.8 kyr BP). This result could be caused by a large supply of terrestrial organic materials containing old carbon during the last glacial period. 3.3.
14
C dating of Lake Erhel sediment core
Down core profiles of the radiocarbon ages for total organic materials and a plant seed in the Erhel Core2 sed-
T. Watanabe et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 565–570 14C
Lake Erhel Erhel Core 2
0
0
age (kyr BP)
5
10 15 20 25
5mm
Depth (cm)
50
Plant residue (seed)
100
22.4 cm/kyr
150 200 250
34.6 cm/kyr 5.2 cm/kyr
300 Fig. 4. Vertical profile for 14C ages of TOC and plant seed in the Erhel Core2 from Lake Erhel, Mongolia. The 14C ages of TOC and plant residue are shown as filled and open diamonds, respectively.
Table 3 14 C ages of total organic materials and plant residue in the Erhel Core2 from Lake Erhel Sample No.
Depth (cm)
14
Lab. code (NUTA2-)
1,42–43 1,42.5a 1,68–70 1A,7–8 1A,45–46 1A,86–87 2,25.5–26.5 2,48–49 3,15–16
42.5 42.5 69.5 77.5 115.5 156.5 206 228.5 295.5
3174 ± 30 3395 ± 35 4469 ± 36 5345 ± 34 6954 ± 35 8879 ± 44 10 231 ± 41 10 958 ± 43 23 798 ± 98
8875 8882 8883 8876 9585 8880 9586 8881 8884
a
C age (BP)
Plant residue (seed).
iment samples from Lake Erhel are shown in Fig. 4 and Table 3. The radiocarbon age of plant residue (3.4 kyr BP) at a depth of 42.5 cm is slightly older than the value of TOC (3.2 kyr BP). In the bottom layer of this core at a depth of 295.5 cm, a 14C age of 23.8 kyr BP is observed. The linear sedimentation rates of the Lake Erhel sediment core are 5.2 cm/kyr for the cool period (24–11 kyr BP) and 9.1–34.6 cm/kyr for the warm period (11–3 kyr BP). The low sedimentation rate in Lake Erhel during the cool period could be due to a decline in precipitation, a prolongation of the freezing period for the lake water, or a drying of the lake basin. An abrupt increase of sedimentation rate about 11 kyr BP (from 5.2 to 31.0 cm/kyr) could be caused by climate humidification with warming. Because a relatively high sedimentation rate and thinly laminated layers are observed during the Holocene in the Lake Erhel sediment core, high time-resolution climate and environment change studies will be possible. 4. Conclusions The sediments from three continental lakes, Lake Baikal, Hovsgol and Erhel, have been dated by AMS 14C. The estimated sedimentation rate of VER99G12 sediment core
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from Lake Baikal (18.2 cm/kyr in average) is 4–5 times higher than that of sediment cores from Academician ridge, Lake Baikal. The relatively higher sedimentation rate could be due to the influx of land-derived materials from Selenga river (the largest river flowing into the lake). The millennial scale climate changes, Bøllong-Allerød (13 kyr BP) and Younger Dryas (11 kyr BP) events, during the transition period from LGM to Holocene were observed in the VER99G12 sediment core. 14 C ages of the HDP04 sediment cores (Lake Hovsgol) from 0 to 2 m depth (Core1-1) do not become progressively older with depth, which suggest that sedimentary layers were disturbed by core sampling. The sediment layers below 2 m for HDP04 cores are available for the study of climate change, because the changes in 14C age are consistent with previous studies. Radiocarbon dating of the sediment from Lake Erhel (Erhel Core2) shows that the core provides a high time-resolution record (up to 30 yr/cm) that can be used for paleo-environmental and biological studies in the continental interior. Acknowledgments We are grateful to the BDP and HDP members for the collection of the sediment core samples. We thank M. Nishimura, A. Tanaka, Y. Hase, K. Shiti, M. Shimokawara and F. Nara for preservation and division of the VER99G12 sediment cores. We would like to thank the staff of the Center for Chronological Research, Nagoya University, especially A. Ikeda and T. Ohta for their cooperation of sample preparation. This work was partly supported by a grant to T.W. from the Sumitomo Foundation (No. 043273). References [1] K.K. Falkner, C.I. Measures, S.E. Herbelin, J.M. Edmond, R.F. Weiss, Limnol. Oceanogr. 36 (1991) 413. [2] Baikal Drilling Project BDP-96 (Leg II) Members, Eos. Trans., vol. 78, 1997, pp. 601. [3] V.A. Kravchinsky, M.A. Krainov, M.E. Evans, J.A. Peck, J.W. King, H. Sakai, M.I. Kuzmin, T. Kawai, D.F. Williams, Palaeogeogr. Palaeoclimatol. Palaeoecol. 195 (2003) 281. [4] K. Kashiwaya, M. Ryogo, H. Sakai, T. Kawai, Geophys. Res. Lett. 25 (1998) 659. [5] Y. Soma, M. Soma, A. Tanaka, T. Kawai, Geochem. J. 35 (2001) 377. [6] T. Takamatsu, K. Kashiwaya, T. Kawai, in: K. Kashiwaya (Ed.), Long Continental Records from Lake Baikal, Springer-Verlag, Tokyo, 2003, p. 313. [7] K. Kawamuro, K. Shichi, Y. Hase, A. Iwauchi, K. Minoura, T. Oda, H. Takahara, H. Sakai, Y. Morita, N. Miyoshi, M.I. Kuzmin, in: K. Minoura (Ed.), Lake Baikal: A Mirror in Time and Space for Understanding Global Change Processes, Elsevier, Amsterdam, 2000, p. 101. [8] T. Watanabe, H. Naraoka, M. Nishimura, T. Kawai, Earth Planet. Sci. Lett. 222 (2004) 285. [9] T. Watanabe, A. Tanaka, T. Nakamura, R. Senda, M. Nishimura, T. Kawai, Verh. Internat. Verein. Limnol. 29 (2005) 903. [10] E. Karabanov, D. Williams, M. Kuzmin, V. Sideleva, G. Khursevich, A. Prokopenko, E. Solotchina, L. Tkachenko, S. Fedenya, E. Kerber,
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