Holocene hydrological and climatic change on the northern Mongolian Plateau based on multi-proxy records from Lake Gun Nuur

Palaeogeography, Palaeoclimatology, Palaeoecology 323–325 (2012) 75–86

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Holocene hydrological and climatic change on the northern Mongolian Plateau based on multi-proxy records from Lake Gun Nuur Chengjun Zhang a,⁎, Wanyi Zhang a, Zhaodong Feng b, Steffen Mischke c, d, Xiang Gao e, Dou Gao a, Feifei Sun a a

College of Resources and Environmental Sciences, Lanzhou University, Lanzhou, 730000, China College of Resource and Environmental Sciences, Xinjiang University, Urumqi, 830046, China Institute of Earth and Environmental Science, University of Potsdam, Potsdam, 14476, Germany d Institute for Geological Sciences, Freie Universität Berlin, Berlin, 12249, Germany e Laboratory Center of Geology, China University of Geosciences, Beijing, 100083, China b c

a r t i c l e

i n f o

Article history: Received 28 June 2011 Received in revised form 3 January 2012 Accepted 24 January 2012 Available online 2 February 2012 Keywords: Multi-proxy record Sediment geochemistry Mineralogy Paleohydrology Holocene Mongolia

a b s t r a c t A multi-proxy study including analyses of δ13Corg for the lake sediment core GN-02 and grain size, TOC, CaCO3 content, δ13Ccarb and δ18Ocarb of bulk carbonate, and the mineralogy of the parallel core GN-04 from Gun Nuur was performed to reconstruct the Holocene hydrology and climate on the northern Mongolian Plateau. The chronology was established using 40 14C dates of bulk organic matter in addition to nine previously published radiocarbon dates for core GN-02, and further five 14C dates for the new core GN-04. A lake reservoir effect of 1060 14C years was determined as the intercept of the high-resolution GN-02 age-depth model at the modern sediment surface. The size of the reservoir effect is supported by the age of the core-top sample (1200 ± 40 14 C years) and the determined difference between a wood-derived radiocarbon age from the GN-02 core base and the age-model inferred age for bulk organic matter at the same stratigraphic level (1000 14C years). Low lake level and prevailing aeolian sediment deposition at Gun Nuur under dry conditions were recorded during the earliest Holocene (>10,800–10,300 cal a BP). Gun Nuur expanded under significantly wetter conditions between 10,300 and 7000 cal a BP. Unstable climate conditions existed in the mid Holocene (7000–2500 cal a BP) and three periods of low lake-levels and significantly drier conditions were recorded between 7000–5700, 4100–3600 and 3000–2500 cal a BP. Intermediate lake levels were inferred for the intervening periods. Around 2500 cal a BP, the climate change and wetter conditions were established again. As a consequence, the lake level of Gun Nuur rose again due to higher effective moisture and the relatively wet present conditions were achieved ca. 1600 cal a BP. Our results suggest that the initial Holocene climate change on the northern Mongolian Plateau was not accompanied by a rapid increase in precipitation as on the Tibetan Plateau. The establishment of wetter conditions in northern Mongolia lagged behind the early Holocene moisture increase on the Tibetan Plateau by ca. 1000 years. Subsiding dry air in the north of the Tibetan Plateau resulted from the strengthened summer monsoon on the Tibetan Plateau during the period of maximum summer insolation and probably inhibited a significant precipitation increase in Mongolia. The significant moisture increase in the Gun Nuur region at ca. 10.3 cal ka BP is probably not related to the northward shift of the present summer monsoon boundary or the moisture delivery from the northern Atlantic through the westerlies. Instead, water from melting snow, ice and frozen ground and the generation of precipitation from the local recycling of moisture are discussed as possible moisture source for the early onset of wetter conditions on the Mongolian Plateau. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The close relationship between the recent drying and increasing temperature trends in north-central Mongolia affirms the sensitivity of the Mongolian Plateau to natural and human-induced climate

⁎ Corresponding author. E-mail address: [email protected] (C. Zhang). 0031-0182/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2012.01.032

changes and shows that this region is at high risk with respect to future water resource availability (Batima et al., 2005). Environmental change in response to future global warming may be assessed if the natural variability of sensitive landscapes is known and if the driving forces of climate change at present and in the past were identified. Therefore, the understanding of the Holocene (i.e., the past ~11,700 years) environmental and climate changes is of particular relevance because similar natural boundary conditions might exist in the future (Wanner et al., 2008). In contrast to many regions in Central Asia and especially in

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China, Mongolia is poorly studied so far and paleoclimate records from this region urgently required to assess the Holocene environmental variability (Wang et al., 2010). The Mongolian Plateau is dominated by the Siberian–Mongolian High and the resulting winter monsoon during the cold season, associated with high-latitude forcing. Its eastern margin or possibly even the entire Mongolian Plateau may have been influenced by the East Asian summer monsoon during the early and/or mid Holocene in response to low-latitude forcing (Shi et al., 1994; Dorofeyuk and Tarasov, 1998; An et al., 2000; Tarasov et al., 2000, 2007; Feng et al., 2005). Chen et al. (2008) recently reviewed eleven selected Holocene climate records from the region and concluded that the entire Central Asian arid zone including the Mongolian Plateau experienced synchronous moisture changes during the Holocene under the influence of North Atlantic sea-surface temperature (SST) which extended its influence into Central Asia through eastward signal transmission via the westerlies. However, all of the three climate systems (westerlies, winter monsoon, and summer monsoon) may have operated with different strengths in different time intervals during the Holocene and their influence on the specific regions in Mongolia and Central Asia in general remains poorly understood. This study focuses on multi-proxy analyses of the new lake sediment core GN-04 and previously obtained core GN-02 from Lake Gun Nuur (Nuur = lake) in northern Mongolia to retrieve the Holocene hydro-limnological conditions and to better understand the Holocene large-scale climate dynamics (Fig. 1). Our Holocene climate reconstruction is based on granulometrical and mineralogical analyses of core GN-04 which was recovered close to the position of core GN-02, and on geochemical analyses and 54 radiocarbon dates for both cores (Feng et al., 2005). 2. Physiographic setting Gun Nuur (50°15′N, 106°36′E, 600 m a.s.l.) is a closed-basin lake situated in the east of the Orhon–Selenga Depression (Fig. 1). The

lake has eutrophic alkaline water with a pH of 8, and a maximum water depth of ~ 5 m. The lake area is 2.5 km 2. The lake is presently closed and not connected to the Buryn Gol (Gol = river), an eastern tributary of the Orhon River. However, overflowing to the Orhon River probably occurs after periods of pronounced precipitation in the region. Atmospheric precipitation and ground water are the main alimentation sources of the lake (Dorofeyuk and Tarasov, 1998). The lower reaches around the lake are covered by steppe vegetation (Gramineae and forbs) while the upper reaches of the catchment display forest steppe. The wide and flat alluvial plain around the lake is covered by low dunes. Trees of Pinus sylvestris grow on the dunes associated with Ulmus pumila and Salix (Dorofeyuk and Tarasov, 1998). Annual precipitation at the Shaamar station ca. 40 km to the southwest of the lake is 330 mm with the majority of the precipitation falling during the summer season (70% in June–August, Worldclimate, http:// www.worldclimate.com). Mean January, July and annual temperatures are −23.5, 18.6 and −0.6 °C. The pronounced seasonal precipitation and temperature pattern reflect the continental position of the northern Mongolian Plateau, and dominance of cold and dry air masses of the Siberian–Mongolian High during the cold season and of westerlies-controlled air during the warm season. 3. Materials and methods 3.1. Coring A core with a length of 928 cm (GN-04) was recovered with an UWITEC piston corer ‘Niederreiter 60’ and a platform at 4.5 m water depth in 2004 (Fig. 1). The position of core GN-04 in the lake center is about 200 m to the northeast of the previously obtained core GN02 (745 cm length; Feng et al., 2005). The core was opened, visually described and subsampled by slicing of 2-cm thick sediment sections immediately after core retrieval in the field.

Fig. 1. The studied Gun Nuur in Mongolia: A, Location of Lake Gun Nuur (black arrow) close to the Mongolian–Russian border and other lake record sites discussed in the text. The dotted gray line indicates the modern Asian summer monsoon limit with the westerlies (gray arrows in the west) and the Asian summer monsoon (gray arrows in the east) as the major components of air circulation during summer (Gao, 1962). The broken gray line indicates the maximum extent of the Holocene summer monsoon (Winkler and Wang, 1993). B, position of Gun Nuur near the main valley of the Orkhon River. C, bathymetry of Gun Nuur and position of cores.

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3.2. Radiocarbon dating and age-depth model The radiocarbon-based chronology of core GN-02 was improved by adding 40 new AMS radiocarbon samples to the previously analyzed nine samples of Feng et al. (2005). Additionally, five samples from the new core GN-04 were used for dating (Tables 1, 2). The potentially contaminated outer sediment core material of 1–5 cm thick core slices was removed before radiocarbon dating. Bulk sediment was used for all radiocarbon samples due to the lack of macroscopic terrestrial organic matter except for sample GN 14 which represents wood remains from the base of core GN-02 (Table 1). All samples were pretreated with aqueous solutions of hydrochloric acid (HCl), dilute sodium hydroxide (NaOH), and HCl again. The chronologies for cores GN-02 and GN-04 were established through the following procedure. All 48 dates for bulk sediment samples from core GN-02 (i.e., excluding the wood-derived sample age which is not supposed to be subject to the lake reservoir effect) were used to establish the age-depth model. A fifth order polynomial

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regression was used to describe the age-depth relationship. The intercept of the regression curve at 0-cm core depth of 1060 a BP was regarded as present-day lake reservoir effect. The uppermost dating sample from 5 to 0 cm core depth yielded a slightly larger age of 1200 ± 40 a BP. Considering that this sample integrates the uppermost 5 cm of sediments at the core site, the obtained radiocarbon dating result corresponds well with the determined intercept age for the core top of 1060 a BP. Furthermore, the difference between the woodderived radiocarbon age from the core base and the age-model inferred age at the same stratigraphic level (744–743 cm core depth) of 1000 years apparently indicates that the lake reservoir effect was similar for the lowermost sediments of core GN-02. Assuming that the lake reservoir effect varied only insignificantly between the base and top of core GN-02, 1060 years were subtracted from the bulk sediment radiocarbon data to account for the lake reservoir effect. The resulting lake-reservoir corrected radiocarbon years were then calibrated using 2σ probability ranges in CALIB 6.0 and INTCAL 09 (Reimer et al., 2009).

Table 1 Radiocarbon dating results of core GN-02. Lab code

Sample no.

Depth (cm)

Material

δ13C (‰)

14

Beta-182732a Beta-182733 Beta-182734 Beta-182735 Beta-171822a Beta-171823a Beta-171824a Beta-171825a Beta-171826a Beta-198046 Beta-198047 Beta-198048 Beta-198049 Beta-171827a KIA25401 KIA25402 KIA25403 KIA25404 KIA23783 KIA23782 KIA23781 KIA23780 KIA23779 KIA23778 AA51939 AA51940 AA51941 AA51942 AA51943 AA51944 AA51945 AA51946 AA51947 AA51948 AA51949 AA51950 AA51951 AA51952 AA51953a AA51954 AA51955 AA51956a AA51957 AA51958 AA51959 AA51960 AA51961 AA51962 AA51963

GN 01 GN 02 GN 03 GN 04 GN 05 GN 06 GN 07 GN 08 GN 09 GN 10 GN11 GN 12 GN 13 GN 14 GN 15 GN 16 GN 17 GN 18 GN 19 GN 20 GN 21 GN 22 GN 23 GN 24 GN 25 GN 26 GN 27 GN 28 GN 29 GN 30 GN 31 GN 32 GN 33 GN 34 GN 35 GN 36 GN 37 GN 38 GN 39 GN 40 GN 41 GN 42 GN 43 GN 44 GN 45 GN 46 GN 47 GN 48 GN 49

0–5 20–25 40–45 60–65 64–65 151–152 240–242 342–344 391–392 469–470 607–608 649–650 697–698 743–744 388–389 430–431 508–509 551–552 604–605 624–625 646–647 654–655 674–675 694–695 79–80 83–84 117–118 132–133 159–160 198–199 212–214 258–260 286–288 316–318 363–364 411–412 448–450 469–470 485–486 510 534 561–562 603–604 638–639 668–669 684–685 703–704 705–706 737–738

Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Wood Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment

− 18.2 − 20.2 − 21.4 − 22.1 − 23.1 − 24.1 − 21.0 − 22.7 − 20.9 − 21.2 − 23.5 − 25.9 − 24.5 − 26.8 − 17.00 − 20.81 − 22.82 − 22.92 − 23.93 − 25.14 − 25.32 − 24.30 − 26.08 − 21.13 − 15.8 − 17.0 − 17.4 − 20.9 − 16.1 − 21.6 − 16.2 − 19.9 − 17.4 − 14.8 − 9.4 − 15.1 − 10.3 − 8.0 − 5.2 − 11.5 − 5.6 − 5.6 − 11.0 − 16.4 − 15.0 − 21.0 − 14.7 − 28.9 − 27.6

1200 ± 40 1620 ± 40 1800 ± 40 1870 ± 40 1900 ± 40 2530 ± 40 3250 ± 40 4910 ± 40 5820 ± 50 7330 ± 50 8080 ± 50 8380 ± 50 9330 ± 50 9500 ± 50 5795 ± 30 6670 ± 30 7725 ± 35 8105 ± 35 8135 ± 45 8065 ± 40 8415 ± 45 8610 ± 45 9075 ± 45 9409 ± 46 2030 ± 37 1975 ± 41 2560 ± 37 2338 ± 37 2500 ± 140 3016 ± 39 3201 ± 38 3617 ± 40 4226 ± 42 4721 ± 44 5843 ± 47 6458 ± 47 7040 ± 61 7991 ± 69 7836 ± 63 8660 ± 66 8242 ± 63 8324 ± 63 8858 ± 83 8397 ± 62 8500 ± 60 9356 ± 64 9439 ± 88 9528 ± 58 10,047 ± 70

C age

Reservoir-corrected 14 C age

Calibrated age

Calibrated median age

140 560 740 810 840 1470 2190 3850 4760 6270 7020 7320 8270 – 4735 5610 6665 7045 7075 7005 7355 7550 8015 8349 970 915 1500 1278 1440 1956 2141 2557 3166 3661 4783 5398 5980 6931 6776 7600 7182 7264 7798 7337 7440 8296 8379 8468 8987

3–282 515–648 568–734 673–788 678–900 1294–1479 2068–2331 4152–4410 5326–5591 7014–7301 7730–7951 8008–8292 9091–9428 10,588–11,079 5326–5583 6310–6444 7472–7589 7794–7950 7794–7977 7735–7936 8029–8311 8212–8422 8662–9016 9154–9479 790–952 741–919 1306–1420 1089–1290 1018–1691 1824–1988 1999–2304 2490–2756 3269–3469 3863–4142 5329–5600 6009–6291 6667–6955 7624–7931 7509–7733 8214–8544 7871–8163 7962–8184 8408–8972 8014–8313 8070–8385 9091–9466 9136–9530 9320–9544 9893–10,254

139 587 682 721 748 1355 2225 4268 5504 7200 7856 8114 9260 10,786 5484 6371 7535 7885 7898 7842 8164 8368 8879 9367 861 838 1379 1220 1352 1905 2126 2631 3391 3983 5515 6208 6817 7765 7626 8403 7999 8085 8585 8137 8264 9297 9373 9484 10,118

Labs are indicated by Beta for Beta Analytic Inc., KIA for the Leibniz Laboratory of University of Kiel, and AA for the NSF AMS Facility of the University of Arizona; all ages in a BP. a Dates published in Feng et al. (2005).

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Table 2 Radiocarbon dating results of core GN-04 (NSF AMS Facility of the University of Arizona). The uppermost sample is apparently not influenced by the lake reservoir effect and was thus, not corrected for a reservoir effect. Lab code AA64236 AA64237 AA64238 AA64239 AA64240 a

Sample no.

Depth (cm)

Material

GN GN GN GN GN

0–2 114–115 270–271 450–451 650–651

Sediment Sediment Sediment Sediment Sediment

50 51 52 53 54

δ13C (‰)

14

− 17.3 − 22.7 − 25.9 − 22.6 − 24.2

70 ± 48 1838 ± 48 3379 ± 50 5412 ± 52 8245 ± 87

C age

Reservoir corrected – 778 2319 4352 7185

14

C age

Calibrated age

Calibrated median age

Age-model derived age

Age differencea

– 659–785 2154–2485 4832–5212 7841–8179

– 707 2334 4929 8007

– 694 2278 4900 7944

– 13 56 29 63

Age difference for reservoir corrected calibrated age and age-model derived age.

We transplanted the age-depth relationship for GN-02 to the GN-04 core using major lithological changes in both cores as first-order agecontrolling tie points. That is, the modeled ages at the boundaries of four stratigraphic units from GN-02 core were directly transplanted to core GN-04 (Fig. 2). Four significant minima and maxima in the CaCO3 curves for both cores were used as second-order age-controlling tie points (Fig. 2). Finally, the calibrated age at any given stratigraphic level of core GN-04 was determined by linear interpolation between the age-controlling tie points. The resulting age-depth model for core GN-04 is in good agreement with the results of 14C dating of four samples from core GN-04 which were similarly corrected by the assumed lake reservoir effect of 1060 years and calibrated to obtain calendar years (Table 2). The core top sample from GN-04 provided a low 14C age of 70 ± 40 a BP indicating that the uppermost sediments of core GN-04 are not influenced by the lake reservoir effect. Loss of core-top sediments during the recovery of core GN-02 and a consequently larger age for the uppermost sediments of the core in comparison to those of GN-04 is not regarded as likely. The uppermost sediments of GN-04 and GN-02 show a similar minimum in carbonate content at 35 cm core depth and generally lower values in comparison to the sediments between 100 and 50 cm core depth (Fig. 2). Therefore, we assume

that organic matter from either emergent aquatic plants or even terrestrial plants, or both, may have caused the low 14C age of the uppermost GN-04 sample. Phragmites and Scirpus are present at the modern lake shores (Dorofeyuk and Tarasov, 1998). Emergent or terrestrial plant matter influx to the lake center may have resulted from recent touristic activities on the northern shore of the lake. However, the low differences between reservoir corrected (corrected by 1060 years) and calibrated ages for the lower four samples from core GN-04 and agedepth model inferred results shows that (1) the lake reservoir effect correction by 1060 years is appropriate for the lower four radiocarbon samples from core GN-04, and (2) that the application of the agedepth relationship for GN-02 to the new core GN-04 is providing a consistent chronology for core GN-04. 3.3. Laboratory analyses Multi-proxy analyses of this study were performed on core GN-04 except the δ 13Corg analysis on core GN-02 due to the lack of sufficient sediment material from core GN-02 after the application of highresolution 14C dating. The particle size was measured at 2-cm intervals using a Malvern Co. Ltd. Mastersizer 2000 laser diffraction

Fig. 2. Lithologies of cores GN-02 and GN-04, mean grain size for GN-04, CaCO3 contents and radiocarbon dating results for the cores. 14C age data published in Feng et al. (2005) are shown in age-depth plot in red and additionally marked by horizontal tick marks next to the right vertical age scale. (Broken lines indicate first order age-controlling tie points; dotted lines indicate second order age-controlling tie points).

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particle size analyzer (size range 0.02–2000 μm). Sample pretreatment included (1) adding H2O2 to remove organic matter and soluble salts, (2) using diluted 1 N HCl to remove carbonate, and (3) using Na-hexametaphosphate to disperse aggregates (Singer and Janitzky, 1987). Mean grain size and sorting were calculated according to Folk and Ward (1957). Magnetic susceptibility was measured according to Thompson and Oldfield (1986) with a Bartington MS2 susceptibility meter linked to an MS2B Dual Frequency Sensor (470 and 4700 Hz) at 2-cm intervals below 125 cm core depth and at 4-cm intervals above. Frequencydependent magnetic susceptibility (χFD) was expressed as percentage χFD (%) = (χLF − χHF) / χLF × 100, where χLF and χHF are the low- and high-frequency values. Total organic matter content (TOC) was determined at 2 cm intervals using the antititration method with concentrated sulfuric acid (H2SO4) and potassium dichromate (K2Cr2O7). The carbonate content was determined at 2-cm intervals by treating bulk sediment samples with dilute 1 N HCl and measuring the generated CO2 volume. All above analyses were conducted in the National Laboratory of Western China's Environmental Systems at Lanzhou University. The mineral composition was determined at 8-cm intervals with a Rigaku D/Max-2400 diffractometer operated at 40 kV voltage and a 60 mA current with Cu Kα radiation in the Material Department of Lanzhou University. Sub-samples were powdered and scanned from 3° to 70°. The fine sediment fraction of GN-04 samples was separated by sieving with a 40 μm mesh for δ13Ccarb and δ18Ocarb analysis of authigenic carbonate. Phosphoric acid was added to the samples and the produced CO2 were purified and transferred into a Finnigan MAT 252 mass spectrometer at the Lanzhou Institute of Geology (Chinese Academy of Sciences). Samples for δ 13Corg analysis of the organic fraction of GN-02 sediments were collected at 2 cm intervals to improve the 8 cm interval resolution of Feng et al. (2005). The samples were combusted with O2 at ca. 800 °C and the derived CO2 was analyzed using a Finnigan MAT 252 mass spectrometer. Calibration measurements using the standard

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reference NBS-19 (δ13C = +1.95‰, δ18O = −2.2‰) were conducted every 10 measurements resulting in a calculated standard deviation smaller than 0.2‰ for δ 13Corg, δ13Ccarb, and δ 18Ocarb. The isotope data are reported in the standard δ-per mil notation with respect to V-PDB (Pee Dee Formation Belemnite standard). 4. Results 4.1. Chronology of cores GN-02 and GN-04 The 40 radiocarbon age samples from core GN-02 provided 14C ages between 1620 ± 40 a BP and 10,047 ± 70 a BP (Table 1). The obtained age data are mainly stratigraphically consistent and in agreement with the age data determined for GN-02 previously (Feng et al., 2005; Fig. 2). Five radiocarbon samples from core GN-04 provided 14C ages between 70 ± 48 a BP for the core top and 8245 ± 87 a BP for the lowermost sample from 651 to 650 cm core depth (Fig. 2). 4.2. Sedimentological and geochemical data for core GN-04 The sediments of core GN-04 are divided into five units based on major lithological differences (substrate type, bedding, color) and major changes in mean grain size, low frequency magnetic susceptibility, TOC and CaCO3 contents, and δ 13Ccarb and δ 18Ocarb values (Figs. 3, 4). The basic characteristics and general trends of the results for GN-04 are summarized in Table 3. 5. Discussion 5.1. Comparison of cores from Gun Nuur The recovered sediments of core GN-04 show the same sequence of basal sands and a thick unit of carbonate-rich sediments in the lower third, more silty and sandy sediments in the middle third, and laminated silt with gastropod shells overlain by homogenous

Fig. 3. The grain size characteristics, low frequency magnetic susceptibility (χLF) and frequency-dependent magnetic susceptibility (χFD) of core GN-04.

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Fig. 4. Comparison of the environmental proxies including grain size, TOC, carbonate content, mineral composition, δ13Ccarb and δ18Ocarb of carbonate of core GN-04, and carbonate content, δ13Corg, TOC, and diatom abundances of core GN-02. Note that planktonic and benthic diatom abundances were labeled incorrectly in Fig. 2 of Feng et al. (2005).

clayey silt in the upper part as the core GN-02 (Feng et al., 2005; Fig. 2). Thickness variations of the individual lithological units in both cores result from local differences in delivery of suspension load, authigenic carbonate precipitation and thus, sediment accumulation rates, and in preservation conditions. However, thickness differences between the major lithological units in core GN-04 and GN-02 are not large (Fig. 2). The CaCO3 profiles for both cores show similarities of the general trends, e.g., the rapid rise at the basal sand-carbonate-rich silt boundary, the continuous increase and CaCO3 drop in the middle of unit 3, and the slight increase in the upper half. However, minor maxima and minima and intervening sections within individual units do not display a high correspondence. The application of the age-model for core GN-02 to core GN-04 via lithological and CaCO3 content tie points provided age data which correspond well with the age-data determined by direct radiocarbon dating of GN-04 sediments at five stratigraphic levels apart from the core-top sample which is apparently not influenced by a lake reservoir effect (Fig. 2, Table 2). The differences between the age-model inferred ages which are based on 49 14C samples for GN-02 and the radiocarbon dating results for the four samples from GN-04 (ignoring the core-top sample) are between 13 and 63 years and thus, negligible. The low differences are evidence that the age-model for GN-02 is not only robust but was also reliably applied onto core GN-04. A sediment core from a marginal position 100 m from the shore was described by Dorofeyuk and Tarasov (1998); Fig. 1. Marly, apparently carbonate-rich lake sediments were obtained between ca. 480 and 350 cm core depth which probably correspond to the carbonate-rich sediments of unit 2 in cores GN-04 and GN-02. Sediments described as ‘sandy-clayey gyttja’ and ‘sandy gyttja’ probably correspond to the mainly silty sediments above unit 2 in GN-04 and GN-02. Mollusc shells are reported for a core depth of 250–220 cm likely corresponding to the individual gastropod-bearing bed in cores GN-04 (270–260 cm) and GN-02 (240–230 cm; Dorofeyuk and Tarasov, 1998). Gravels at around 80 cm in the more marginal core are not paralleled by similar sediments in GN-04 and GN-02 and are apparently restricted to the region closer to the lake shore.

However, the differences between the more central GN-04 and GN02 cores and the core from the more marginal position are not significant considering the major sediment types and general architecture including carbonate-rich sediments in the lower third and the mollusc bed recovered in all three cores. A spatially relatively uniform sediment accumulation is inferred for Gun Nuur during the period represented by the obtained sediments. 5.2. The Holocene history of Gun Nuur 5.2.1. GN-04 unit 1: ~10,800–9800 cal a BP (928–816 cm) Fine grained, moderately sorted sands and lowest TOC and CaCO3 contents in the lowermost unit of core GN-04 indicate a depositional setting dominated by the accumulation of detrital sediments (Figs. 3, 4). Relatively high χLF (low-frequency magnetic susceptibility) values suggest the delivery of unweathered fresh sediment material containing detrital ferromagnetic minerals such as (titano-)magnetite or (titano-) maghemite (Dearing, 1999). The relatively fine grain size and good sorting indicate that aeolian sand rather than fluvial material was deposited (Fig. 3). The frequency-dependent magnetic susceptibility χFD of ~5% shows that ultrafine (b0.03 μm) super-paramagnetic (SP) grains are present in the sediments of unit 1. SP grains derived from magnetically enhanced soil material were probably produced by magnetotactic bacteria or indirectly by Fe-reducing bacteria (Dearing et al., 1996; Dearing, 1999). Arid conditions probably prevailed although the core position was already covered by water. The latter assumption is based on the steadily increasing δ 18O values possibly reflecting a marked evaporative enrichment trend of a shallow water body. In addition, high abundances of benthic diatoms were recorded in the lowermost part of core GN-02 (Fig. 3). A rapid rise of the water level at ca. 10,300 cal a BP is indicated by a drop in mean grain size and δ 18O values and a step-like rise in TOC and CaCO3 contents (Figs. 3, 4). A very shallow stagnant water body on an aeolian sand-covered plain is reconstructed for the period before ca. 10,300 cal a BP similar to inferences by Dorofeyuk and Tarasov (1998). Sediments from a

Table 3 Results of analyses of core GN-04 (χLF and χFD is the low frequency and frequency dependent magnetic susceptibility, σI is the dispersion or sorting parameter according to Folk and Ward (1957)). TOC content

Mineralogy

δ13Ccarb

δ18Ocarb

χLF mainly 2–8, relatively low, constantly ca. 8 at base, decrease in mid and top of unit, χFD ca. 5% at base and near top, ca. 8–10% in lower half χLF constantly between 5 Mean grain size constantly ca. 50 μm, clay content ca. 10%, sorting and 8, relatively low, χFD poor and decreasing toward top highly variable, increase from 5 to 9%

Steady strong increase at base from 20 to 40%, further increase in mid to 50%, decrease near top Moderate, 30–40%, general increase from base to top

Steady decline from 20 near base to 15% in upper half and return to higher values at top

Calcite is abundant and decreasing from base to top, pyrite is rarely detected in lower half and abundant in upper half, aragonite was detected in upper half Calcite and pyrite are abundant, aragonite is absent

Continuous slight increase from 0 to 5‰, low variability

Continuous slight increase from − 8 to − 6‰, slightly higher variability in upper half

Highly variable, moderate values − 3 to 5‰

Steady increase from − 9 to − 7‰ and return to low values at top

χLF highly variable between 1 and 14, four gradual decreasing trends, three rapid increases, χFD mainly 5–9%, distinct minimum in lower half χLF low, mainly 1–4, decreasing from base toward mid unit, increasing in top, χFD mainly ca. 6%, three distinct maxima of 10–20% Mean grain size ca. 150 μm, ≥80% χLF high, 10–16, high of sand fraction, mainly moderately variability with general sorted increase in mid part of unit, χFD is low (6–8%)

Highly variable, 0–50%, three steady increases and rapid drops at end

Three times gradual steady increases from b5% to >20%, rapid declines after maxima, constantly low values of ca. 2% in mid part Moderate constant values of ca. 12%

Pyrite only in lower third, calcite shows twofold increase in unit 3 and rapid drop in between, aragonite only at base and a single sample in middle third Calcite and pyrite in lower half, few aragonite there, calcite and pyrite less abundant in upper half and aragonite with higher abundance

Slight further increase to 10‰ at mid of unit, rapid drop and constant values of ca. 5‰ afterwards Continuous slight increase from 0 to 7‰

Two gradual decreases from − 8 to − 10‰ with rapid increases at its ends in lower half, stable values around − 8‰ in upper half Highly variable values around − 11‰ in lower half, gradual increase, decrease and increase again to − 8‰ in upper half Strong increase from − 11 to − 7‰ and drop to − 12‰ near top

Lithology

Grain size

5

Carbonate-rich dark gray clayey silt with higher organic content

205–0 1.6–0

4 280–205 2.5–1.6

3 560–280 7.0–2.5

2 816–560 9.8–7.0

1

Laminated grayishbrown clayey silt, gastropod shells (mainly Gyraulus, few Radix) in 270–260 cm Grayish-brown sand to grayishblack silt with higher carbonate and organic content Massive grayishblack (below 792 cm) to layered brownish-yellow clayey silts

Homogenous gray to brownish and 928–816 >10.8–9.8 grayish black sand

Magnetic properties

Mean grain size in narrow range 20–50 μm, clay content ca. 10%, constantly poorly sorted. low variability

Mean grain size in three sub-units 100–200 μm, two steady and one rapid increases, 40–100 μm in between, large sorting variations from very poorly sorted to almost well sorted Mean grain size ca. 40 μm, increasing above 625 cm, poorly sorted

Decline from 25 to 15%

High, 40–70%, maxima at base, in upper half and at top of unit, gradual decrease starting at base Very low (b 2%), sharp Very low (b 2%), increase at top sharp increase at top

Calcite and pyrite was detected Lowest for the entire section, only in samples near the unit mainly − 10 to top − 5‰, sharp increase at top

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CaCO3 content

Unit core depth (cm) age (cal ka BP)

81

82

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Holocene aeolian sand and paleosol sequence 100 km to the west of Gun Nuur show that aeolian sands have very low TOC and CaCO3 contents and a mean grain size of ~ 100 μm in this area (Feng et al., 2005). Slightly higher mean grain sizes at Gun Nuur are probably related to the local source of aeolian sand from the surrounding alluvial plain. The shallow water body of Gun Nuur before 10,300 cal a BP was not characterized by abundant aquatic macrophytes or phytoplankton indicated by very low TOC contents. A near-shore unstable depositional environment is inferred. Generally arid conditions and a low water level changed at about 10,300 cal a BP when a rapid water level rise occurred (Fig. 4). 5.2.2. GN-04 unit 2: 9800–7000 cal a BP (816–560 cm) Carbonate rich silty sediments accumulated in Gun Nuur between 9800 and 7000 cal a BP. The shift toward TOC contents of 10% at 9800 cal a BP results from the establishment of a deeper lake with significant productivity, organic matter accumulation and preservation at the lake floor (Fig. 4). The detrital fraction mainly represents poorly sorted silty material which probably arrived as dust and/or suspension load at the core site reflecting the expansion of the lake, the larger distance to the shore and the significantly diminished influx of aeolian sand (Fig. 3). Low χLF values reflect the ‘dilution’ of the magnetic signal of detrital grains by high carbonate and organic matter contents in unit 2 (Figs. 3, 4). A significantly increased lake level is also indicated by highest abundances of planktonic diatoms in core GN-02 (Fig. 4). Carbonate occurs mainly as calcite before 8900 cal a BP. Frequently recorded pyrite regarded to indicate anaerobic decomposition of organic matter, abundant calcite and generally low δ 18Ocarb values in the lower half of unit 2 indicate that freshwater and relatively deep conditions with temporary anoxia of bottom waters prevailed before ca. 8500 cal a BP (Berner, 1985). Accordingly, Wang (2004) detected framboidal pyrite in sediments of core GN02 by scanning electron microscope (SEM) analysis. Higher aragonite contents, only sporadically detected pyrite and higher δ 18Ocarb values after 8500 cal a BP indicate drier conditions and a reduction in lake level. Aragonite may originate from mollusc shells and shell debris and from authigenic carbonate precipitation from the water column. Wang (2004) identified authigenic aragonite needles as the dominating aragonite form in GN-02 sediments by SEM analysis. Authigenic aragonite typically occurs in Mg-rich, warm and cold alkaline lake waters (Stein, 2001; Yu and Zhang, 2008; Mischke et al., 2010a; Zhang et al., 2011). Thus, a relatively high residence time and evaporative enrichment of lake waters causing higher δ 18Ocarb values and higher Mg contents are inferred. Three distinct χFD maxima centered at 8400 cal a BP (686 cm), 8100 cal a BP (652 cm) and 7700 cal a BP (614 cm) and generally slightly higher values of ~6% after 8500 cal a BP reflect pulses and a generally increased influx of pedogenic ultrafine magnetite/maghemite grains into the lake. However, the three χFD maxima are not accompanied by significant increases in mean grain size, sand content or χLF values, and strong soil erosion cannot be inferred although a slight change in vegetation density and resulting soil exposure and degradation is assumed. The sand content and mean grain size increases continuously after 8500 cal a BP, CaCO3 and TOC content decrease, and the δ 18Ocarb and χLF values increase too, all indicating the delivery of more detrital and coarser sediments to the core site and a lake level decline. Gun Nuur was a relatively deep, permanent freshwater lake between around 9800 and 8500 cal a BP in contrast to the earlier suggestion of Dorofeyuk and Tarasov (1998) who inferred the onset of wetter conditions and a higher lake level not before 9300 cal a BP, about 1000 years later than the initial lake level rise deduced from our study. A minor increase in salinity and a slight reduction in lake level are recorded after ca. 8500 cal a BP but generally stable and moist conditions prevailed in the region between 9800 and 7000 cal a BP (Fig. 4). In contrast, Feng et al. (2005) proposed warm climate conditions and a low lake level for Gun Nuur during this period.

5.2.3. GN-04 unit 3: 7000–2500 cal a BP (560–280 cm) Sandy and silty sediments accumulated in Gun Nuur between 7000 and 2500 cal a BP. The sands accumulated between 7000 and 5700 cal a BP (560–510 cm), 4100 and 3600 cal a BP (440–398 cm) and 3000 and 2500 cal a BP (330–280 cm). The sands are characterized by relatively low CaCO3, calcite and TOC contents, mean grain sizes of 80–180 μm, moderate sorting and relatively high χLF values indicating the input of aeolian material to the lake again. A significantly closer distance between the core site and the former shoreline and low lake levels are assumed although conditions represented by unit 1 were not achieved again. The accumulation of silty sediments between 5700 and 4100 cal a BP (510–440 cm) with high TOC, CaCO3 and calcite contents and the occurrence of pyrite reflect a freshwater period and a higher lake level of about 1600 years duration. The absence of aragonite apart from a single sample and planktonic diatoms indicates that the lake level and regional moisture availability were probably not as high and climatic conditions not as warm as during the period from 9800 to 7000 cal a BP (Fig. 4). TOC, CaCO3 and calcite contents were lower in the upper subunit of silty sediments between 3600 and 3000 cal a BP in comparison to those between 5700 and 4100 cal a BP, and a less pronounced lake level rise is inferred. A generally lower lake level during the period of unit 3 is also inferred from high δ 13Corg values in core GN-02 which probably reflect the presence of macrophytes instead of phytoplankton (LaZerte and Szalados, 1982; Håkansson, 1985; Zhang et al., 2003). Accordingly, C/N ratio analysis of GN-02 sediments provided values between 7 and 10 indicating the aquatic origin of organic matter in the sediments of Gun Nuur (unpubl. data Chengjun Zhang). Three distinct periods of low lake levels between 7000 and 5700, 4100 and 3600 and 3000 and 2500 cal a BP were recorded with intervening phases of higher lake levels and increased available moisture in the region. Gun Nuur experienced pronounced shifts in terms of lake level and probably lake area in the mid Holocene with the two dry–wet periods between 7000 and 4100 cal a BP lasting ca. 1500 years and the three dry–wet–dry periods between 4100 and 2500 cal a BP lasting ca. 500 years (Fig. 4). This inference for the period before 3500 cal a BP is in broad agreement to previous studies of Gun Nuur lake records since either moderate lake levels (Feng et al., 2005) or mainly lower but variable lake levels between ca. 6200 and 3500 cal a BP were suggested (Dorofeyuk and Tarasov, 1998). In contrast to our inference of moderate and low lake levels between 3600 and 2500 cal a BP, Dorofeyuk and Tarasov (1998) suggested that Gun Nuur lake levels were generally lowest between 3500 and 2100 cal a BP while Feng et al. (2005) interpreted that Gun Nuur experienced the highest lake levels between ca. 3500 and 1200 cal a BP. 5.2.4. GN-04 unit 4: 2500–1600 cal a BP (280–205 cm) Laminated silty sediments were deposited between 2500 and 1600 cal a BP in Gun Nuur. Moderately high TOC and CaCO3 contents, a low mean grain size, the reoccurrence of planktonic diatoms and the detection of pyrite indicate a relatively high lake level again. Carbonate was formed as calcite and freshwater conditions apparently prevailed. Relatively high δ 13Corg values before 2300 cal a BP indicate the prevalence of macrophytes in favor of phytoplankton. The corresponding occurrence of abundant gastropod shells is a further evidence that Gun Nuur was probably a clear-water lake during this initial phase of unit 4. Significantly lower δ 13Corg values between 2300 and 2000 cal a BP suggest a shift to phytoplankton dominated waters. A slight increase in mean grain size, high δ 18Ocarb and higher δ 13Corg values after 2000 cal a BP indicate a reduction in lake level, stronger evaporative effects on the lake and the predominance of aquatic macrophytes. Variable χFD values with four distinct and successively increasing maxima at 2300 cal a BP (268 cm), 2000 cal a BP (256 cm), 1800 cal a BP (238 cm) and 1600 cal a BP (212 cm) point to the temporary influx of soil-derived ultrafine SP grains into the lake. The lower three χFD maxima correspond to peaks in sand

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influx and low TOC contents, and short periods of vegetation degradation and soil erosion on centennial-scale cycles are inferred. However, it remains open whether these periods reflect human impact or climatic spells. Gun Nuur maintained a relatively high lake level and freshwater conditions between 2500 and 1600 cal a BP (Fig. 4). In contrast, Dorofeyuk and Tarasov (1998) reconstructed a relatively high lake level not before 2100 cal a BP again. The inference of Feng et al. (2005) is in clear contradiction since they suggested highest lake levels for Gun Nuur between 3500 and 1200 cal a BP. 5.2.5. GN-04 unit 5: 1600 cal a BP to present (205–0 cm) Silty sediments with high TOC and CaCO3 contents accumulated in Gun Nuur since 1600 cal a BP (Fig. 4). Pyrite and aragonite were predominantly formed after 1000 cal a BP indicating a relatively deep and stable freshwater environment before which probably changed gradually into a slightly brackish lake. Continuously increasing δ 18Ocarb values suggest the long-term isotopic enrichment in a stable water body comparable to the δ 18Ocarb evolution in unit 2. Higher χFD values between 1500 cal a BP (176 cm) and 1300 cal a BP (162 cm) and 1200 cal a BP (134 cm) and 700 cal a BP (110 cm) core depth reflect the enhanced delivery of ultrafine pedogenic soil particles to the lake and thus, soil erosion in the catchment of Gun Nuur. Low χLF values between 800 and 400 cal a BP and the occurrence of pyrite probably reflect the dissolution of ferrimagnetic iron oxides under anoxic conditions at the lake floor (Figs. 3, 4). Lower CaCO3 contents, the absence of pyrite and the re-occurrence of benthic diatoms near the core top are evidence for a slight lake level reduction since ca. 500 years (Fig. 4). The corresponding lack of aragonite, decrease of calcite, increases of the TOC content and the silt fraction in the uppermost part of the core possibly reflect a shift to the accumulation of more organic-rich and detrital sediments in the most recent centuries. Gun Nuur was a relatively stable freshwater to slightly brackish lake in the last 1600 years. Two periods of enhanced soil erosion were recorded between 1500–1300 and 1200–700 cal a BP. However, the shallow or declining lake levels since ca. 1300 cal a BP inferred by Dorofeyuk and Tarasov (1998) and Feng et al. (2005) are not supported by our data. 5.3. Hydrological and climatic implications for Mongolia and northern Central Asia The shift from a predominating aeolian sediment accumulation in a shallow lake to organic carbon and carbonate rich sediment accumulation in a deeper and stable lake environment reflects the onset of wet climatic conditions in the Gun Nuur area at ca. 10.3 cal ka BP. Significantly drier conditions existed probably in the initial period of the early Holocene before 10.3 cal ka BP. A comparable timing of the early Holocene moisture increase was recorded at the lakes Daihai (10.3 cal ka BP) and Eastern Juyanze (10.7 cal ka BP) in Inner Mongolia, and at Lake Hovsgol (11.4–11.0 cal ka BP) and Hoton Nuur in Mongolia (10.0 cal ka BP; Fig. 1; Prokopenko et al., 2007; Hartmann and Wünnemann, 2009; Rudaya et al., 2009; Murakami et al., 2010; Sun et al., 2010). Correspondingly, the onset of lake sediment formation at Ugii Nuur was recorded at 10.6 cal ka BP (Schwanghart et al., 2008). An age of 8.5 cal ka BP was determined for high shorelines of Adgiyn Tsagaan Nuur in the Valley of the Gobi Lakes (Fig. 1; Lehmkuhl and Lang, 2001). Grunert et al. (2000) see evidence for even earlier rising and high lake levels of the northwestern Mongolian lakes Uvs and Bayan Nuur. Accordingly, high lake levels as early as 12.0 cal ka BP were reported from the Baikal region in the north (Fig. 1; Shichi et al., 2009). The resulting implication of a widespread significant moisture increase already distinctly before 8 cal ka BP is in contradiction to the recent review of eleven Holocene paleoclimate records from arid Central Asia by Chen et al. (2008) who concluded

83

that a significant Holocene moisture increase did not occur before 8 cal ka BP in the north of the monsoon-influenced region of Central Asia. Chen et al. (2008) related the coinciding moisture increase at 8 cal ka BP and the sea surface temperature (SST) rise in the northern Atlantic and Norwegian Sea between 9 and 8 cal ka BP to the linkage of the northern Atlantic region and arid Central Asia through the westerlies moisture transport (Koç et al., 1993; Kaplan and Wolfe, 2006). However, our results from Gun Nuur and other, mainly very recently published studies reviewed by Wang et al. (2010) show that dry conditions before 8 cal ka BP in arid Central Asia cannot be longer regarded as a characteristic paleoclimatic feature of this region. We may follow the reasoning of Chen et al. (2008) that westerlies-transported moisture with northern Atlantic origin could have contributed to precipitation on the Mongolian Plateau after 8 cal ka BP. According to their suggestion that higher northern Atlantic SSTs were prerequisite for enhanced evaporation from sea waters and transport of moisture to the interior of Central Asia through westerly cyclonic storms, the northern Atlantic region was probably not a significant moisture source during the early Holocene period of lower SST before ca. 9 cal ka BP (Koç et al., 1993; Kaplan and Wolfe, 2006). A very significant northward shift of the current summer monsoon boundary beyond the Gun Nuur region in the early Holocene as alternative mechanism to explain the early Holocene moisture increase on the Mongolian Plateau is not regarded as a likely scenario (Winkler and Wang, 1993; Herzschuh, 2006; Xiao et al., 2006, 2008; Chen et al., 2008; Wen et al., 2010; Fig. 1). Thus, an alternative moisture source or several sources have probably existed. An explanation which has not been considered so far is the widespread delivery of meltwater from snow, ice and frozen ground to local, immature basins in Central Asia during the initial Holocene. The summer (June) insolation at 30° north was at maximum at the transition from the late glacial to the Holocene and may has not only caused rapid warming of the Tibetan Plateau and the strengthening of the summer monsoon but also significant warming in the most continental interior of Central Asia in the north of the Tibetan Plateau (Berger and Loutre, 1991). Rapid warming in the Mongolian Plateau region may have triggered meltwater discharge to local basins. Water course networks were probably not fully established shortly after the glacial period, inhibiting the draining of local basins. In addition, permafrost may have impeded infiltration in flat basins. As a result, many local basins have possibly hosted relatively large and shallow stagnant water bodies which have acted as local sources of evaporation and may have caused precipitation on the Mongolian Plateau through recycling of local moisture. The described scenario explains the relatively early increase of effective moisture in Central Asia far from the region of the current summer monsoon influence as a result of insolation-driven heating of the land surface, meltwater supply to the basins and generation of precipitation from local moisture recycling. This scenario is speculative, and field evidence and moisture balance modeling data are required for a critical assessment. However, we do not necessarily expect to find evidence for widely distributed water-laid sediments on the Mongolian Plateau as a consequence. The resulting sediments may have been largely altered by pedogenesis or may have been removed by fluvial erosion and deflation. The significant moisture increase on the Mongolian Plateau occurred at many places not in phase with the Pleistocene–Holocene transition but apparently ca. 1 ka later (Prokopenko et al., 2007; Rudaya et al., 2009; Shichi et al., 2009). In contrast, most lake and climate records from the Tibetan Plateau show that wet conditions were already established at the beginning of the Holocene (e.g., Herzschuh, 2006; Zhang and Mischke, 2009; Mischke et al., 2010b; Wang et al., 2010). The apparent delay in response time on the Mongolian Plateau possibly reflects the more efficient monsoon precipitation and convective rain over the Tibetan Plateau and the enhanced subsidence of dry air over northern Central Asia during the time of the maximum summer insolation (Herzschuh, 2006; Chen et al., 2008; Sato, 2009;

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Wang et al., 2010). The precipitation increase as a result of local moisture recycling over the Mongolian Plateau became probably only significant after the first waning of the insolation and the related most pronounced subsidence of air. Lower lake levels and apparently drier conditions after 7.0 cal ka BP at Gun Nuur are roughly consistent with the reduction in moisture availability in the area outside the present monsoon boundary at ca. 7.5 cal ka BP discussed in the review paper by Wang et al. (2010). Low lake levels at Gun Nuur between 7.0 and 5.7 cal ka BP correspond to drier conditions between 6.6 and 5.8 cal ka BP at Dali Lake and between 6.4 and 6.0 cal ka BP at Hulun Lake (Xiao et al., 2008, 2009). Further, dry conditions prevailed at Lake Eastern Juyanze between 7.5 and 5.4 cal ka BP (Mischke et al., 2005; Hartmann and Wünnemann, 2009). In contrast, this mid Holocene dry episode is not seen in lake records from western Mongolia or Xinjiang (Hoton Nuur: Rudaya et al., 2009; Lake Ulungur: Mischke and Zhang, 2010a; Lake Bosten: Mischke and Wünnemann, 2006; Wünnemann et al., 2006; Huang et al., 2009) although Zhang et al. (2010) see evidence for a dry period at Lake Bosten between 6.4 and 5.2 cal ka BP. Based on their review of 92 Holocene climate records from 72 sites in Central Asia, Wang et al. (2010) concluded that moisture levels decreased in the area outside the present monsoon boundary ca. 7.5 cal ka BP and thus, earlier than in the region of the Indian summer monsoon (6.5 cal ka BP) and East Asian summer monsoon (4.5 cal ka BP). The Holocene decrease of the moisture availability in Central Asia is commonly assigned to the reduced insolation in the mid Holocene (Herzschuh, 2006; Wang et al., 2010). However, the earlier moisture decrease in the area outside the present monsoon boundary cannot be explained with lower SSTs in the northern Atlantic region which were at maximum 7.5 cal ka BP (Koç et al., 1993; Kaplan and Wolfe, 2006). Thus, the early reduction in moisture availability in the area outside the present monsoon boundary might be related to the reduced precipitation generation from local moisture recycling as a result of diminished meltwater sources, the establishment of efficient drainage networks, and the reduction in permafrost regions and increased infiltration after the initial warming during the early Holocene. Higher lake levels and wetter conditions at Gun Nuur between 5.7 and 4.1 cal ka BP were similarly recorded in central Inner Mongolia (Lake Eastern Juyanze: Herzschuh et al., 2004; Mischke et al., 2005; Hartmann and Wünnemann, 2009) and at Hulun Lake (6.0 to 4.5 cal a BP, Xiao et al., 2009). Accordingly, ordination analysis of paleoclimate records from the area outside the present monsoon boundary by Wang et al. (2010) show strong signals during the period 5.5 to 4.0 cal ka BP. The shift from wet to drier conditions recorded at 4.1 cal ka BP at Gun Nuur is seen in a wide region and apparently a paleoclimatically significant event in Mongolia and on the Eurasian continent in general (Staubwasser et al., 2003; Arz et al., 2006; Wang et al., 2010). It was recorded at 4.3 cal ka BP at Balikun (=Barkol) Lake, at 4.2 cal ka BP at Ugii Nuur, at 4.4 cal ka BP at Hulun and Dali lakes, and between 4.1 and 3.9 cal ka BP at Lake Eastern Juyanze (Herzschuh et al., 2004; Mischke et al., 2005; Schwanghart et al., 2008; Xiao et al., 2008; Hartmann and Wünnemann, 2009; Tao et al., 2010; Wen et al., 2010). A spell of wetter conditions between 3.6 and 3.0 cal ka BP in the Gun Nuur area may correspond to wetter conditions inferred between 3.8 and 3.0 cal ka BP at Hulun Lake (Xiao et al., 2009). However, a similar short period of wet conditions was not recorded from other sites in Mongolia or adjacent areas although mid Holocene short-term climate fluctuations were recorded at more eastern and southern sites too (Hulun Lake, lakes Dali, Daihai and Eastern Juyanze; Mischke et al., 2005; Xiao et al., 2008, 2009; Sun et al., 2010). A relatively stable and moderately high lake level and slightly wetter conditions were inferred from the Gun Nuur record for the last 2.5 cal ka BP. Similarly wetter conditions for the most recent portion of the late Holocene were recorded at Ugii Nuur (since 2.8 cal ka

BP) but further evidence to support a relatively stable longer period with higher moisture availability in the late Holocene is only rarely seen (Fowell et al., 2003; Schwanghart et al., 2008). Spells of wetter conditions were recorded at Lake Daihai between 1.7 and 1.2 cal ka BP (Xiao et al., 2006; Sun et al., 2010). A higher shoreline above the currently almost desiccated lake Adgiyn Tsagaan Nuur provided an age of 1.5 cal ka BP but it remains unclear whether a longer wet period persisted in this area or whether the shoreline rather represents a spell of wetter climate (Lehmkuhl and Lang, 2001). The uncertain pattern of climate change in the late Holocene may partly arise from the fact that lakes experienced different degrees of human impact and that lakes responded in a non-linear way due to the specific catchment characteristics. Thus, the need for more, well-dated Holocene climate records becomes evident together with the requirement to assess human impact based on pollen indicator taxa, livestockrelated coprophilous fungi, nutrient condition indicators or accompanying archaeological studies. 6. Conclusions The onset of significantly wetter Holocene conditions was recorded at Gun Nuur ca. 10.3 cal ka BP, roughly in phase with many other lake records from Mongolia and adjacent regions. The establishment of wetter conditions in Mongolia is probably not related to high-latitude forcing linked to the northern Atlantic region since SSTs were still low before 9 cal ka BP. Therefore, local sources of water from melting snow, ice and frozen ground may have generated precipitation in the area outside the present monsoon boundary through local moisture recycling. The suggested meltwater delivery and local moisture recycling in Mongolia lagged behind the early Holocene climate change on the Tibetan Plateau probably as a result of subsiding dry air over Mongolia which was generated by strong insolation-driven heating on the Tibetan Plateau where it caused heavy monsoonal and convective rains. A mid Holocene drier period between 7.0 and 5.7 cal ka BP inferred from Gun Nuur apparently corresponds to lower lake levels and a reduction in moisture availability in Inner Mongolia (China) in the east and south whereas uneven climate conditions were reported from sites in western Mongolia and China's northwestern province Xinjiang. Similarly, the following wetter period at Gun Nuur between 5.7 and 4.1 cal ka BP was recorded at several sites in Inner Mongolia too. The shift from relatively wet to drier conditions at 4.1 cal ka BP in the Gun Nuur region was recorded at the majority of lake record sites in Mongolia and possibly reflects the large-scale climate deterioration at 4.2 cal ka BP (Staubwasser et al., 2003; Arz et al., 2006; Mischke and Zhang, 2010b). The final period of slightly wetter conditions since 2.5 cal ka BP at Gun Nuur is not supported by evidence for a regional moisture increase from a number of lake record sites. Instead, uneven climate conditions were inferred from the available records in Mongolia. However, the number of paleoclimate studies from this region is still low and a better spatial coverage and better chronologies are required for a thorough assessment of Holocene climate change on the Mongolian Plateau. Acknowledgments We are indebted to Yuzhen Ma, Xinwei Zhai, Wei Wang, Qili Yang, Yixin Hu and Aizhi Sun for their help during coring in the field and for core sampling in the laboratory of Lanzhou University. Two anonymous reviewers provided very constructive comments on an earlier version of the manuscript. Funding was provided by two China NSFC grants (40930102 and 40773064) and two US NSF grants (BCS-06-52304 and ATM 04-02509).

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