Holocene environmental changes as recorded by mineral magnetism of sediments from Anguli-nuur Lake, southeastern Inner Mongolia Plateau, China

Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 30–49

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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o

Holocene environmental changes as recorded by mineral magnetism of sediments from Anguli-nuur Lake, southeastern Inner Mongolia Plateau, China Hongya Wang a,⁎, Hongyan Liu b, Jiangling Zhu b, Yi Yin b a b

Department of Resources, Environments and Geography, School of Urban and Environmental Sciences, Peking University, Beijing 100871, China Department of Ecology, School of Urban and Environmental Sciences, Peking University, Beijing 100871, China

a r t i c l e

i n f o

Article history: Received 21 October 2008 Received in revised form 10 October 2009 Accepted 19 October 2009 Available online 27 October 2009 Keywords: Holocene Lake sediments Mineral magnetism Inner Mongolia

a b s t r a c t Two cores, one 1141-cm long (An-S) and the other 885-cm long (An-A), were retrieved from Anguli-nuur Lake (41°18′–24′N, 114°20′–27′E, ∼1315 masl), one of the largest lakes in the transition zone between a semi-humid and semi-arid climate parallel to the present limit of the southeast monsoon along the southeastern Inner Mongolia Plateau in north China. Mineral-magnetic parameters (χlf, ARM, IRM300mT, SIRM and IRM− 300mT) were measured on An-S and two additional parameters (χARM and HIRM) and four inter-parametric ratios (χARM/SIRM, IRM300mT/SIRM, IRM− 300mT/SIRM and SIRM/χlf) were calculated. Potential sources of these lake sediments (catchment soils and dune materials close to the lake and in a distant sand plain) were sampled, and the magnetic properties of the surface-material specimens were measured. A chronological model was developed for An-S by comparing and combining AMS14C dates of An-S with 137Cs, 210Pb and AMS14C dates of An-A. With the help of surface-material magnetism, the magnetic data of An-S in combination with particle size, TOC and C/N and pollen analyses indicate the environmental changes during the last ∼10,000 years around this lake. Conditions began to ameliorate at 10,900 cal. yr BP (9600 14C yr BP) and thus relatively wet and warm environments prevailed during 10,900–8900 cal. yr BP (9600–8000 14C yr BP). The Holocene optimum or the wettest and warmest conditions, was during 8900–7400 cal. yr BP (8000–6500 14C yr BP). The environment began to deteriorate from 7400 cal. yr BP (6500 14C yr BP) and the driest and coolest conditions occurred during 2200–480 cal. yr BP. There may have been a minor amelioration after 480 cal. yr BP. The inferred changes in palaeoenvironmental conditions around Anguli-nuur Lake are broadly in agreement with those around most other sites on the Inner Mongolia Plateau. © 2009 Elsevier B.V. All rights reserved.

1. Introduction In climatic transition zones and associated ectones, landscapes are particularly sensitive to climatic changes and have thus been particularly focused on in palaeoclimatic studies (e.g. Clarke and Rendell, 1998; Peck et al., 2002). The southeastern edge of the Inner Mongolia Plateau in northern China is such a climatic transition zone and associated ectone that have resulted from the northwestern limit of the southeast monsoon (Fig. 1). Specifically, this monsoon-controlled narrow belt stretching in the southwest–northeast direction is a climatic transition zone from a subhumid to a semi-arid climate and an associated ectone between forests and steppes. It is thus reasonable to assume that the large-scale climatic changes and the resultant changes in the strength of the southeast monsoon which occurred during the Quaternary must have repeatedly altered the environmental and vegetation conditions in this climatic transition zone, and the imprints left by these changes should allow us to track the changes in the strength of the southeast monsoon (e.g. Li et al., 1990).

⁎ Corresponding author. Tel.: +86 10 62755009; fax: +86 10 62751187. E-mail address: [email protected] (H. Wang). 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.10.020

There are several hundred lakes of different sizes in the southeastern Inner Mongolia Plateau. Anguli-nuur (or Angulinuo) Lake is one of the largest (Fig. 2a). Compared to those small lakes in the Inner Mongolia Plateau, Anguli-nuur Lake may have rarely desiccated in past times and, therefore, sediments deposited in this lake are likely to provide a comparatively complete record of environmental changes in this part of the plateau and thus of changes in the strength of the southeast monsoon. Largely due to this reason, Anguli-nuur Lake has been studied in palaeoenvironmental investigations during the last decades. Geomorphologic features and sediment particle size were used to infer fluctuations in the lake's water level during the late Pleistocene and Holocene (Li et al., 1990; Qiu et al., 1999). The Holocene history of wind activities around this lake was reconstructed from annual laminations of particle size in sediments (Zhai et al., 2002, 2006). Climatic and environmental changes during the last 400 years were postulated from particle size, carbonate content and total organic carbon (TOC) content of the sediments of Anguli-nuur Lake (Jiang et al., 2004). Even so, compared to some other large lakes in the Inner Mongolia Plateau (e.g. Li et al., 1990), Anguli-nuur Lake has been less intensively investigated. Applications of some commonly used methods (e.g. mineral-magnetism measurement and pollen analysis) have not yet been reported on reconstructions of past environments around this lake.

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Fig. 1. Study area (A), Inner Mongolia Plateau, the boundary between the semi-humid and semi-arid climate regions, and the northern limit of the southeast (SE) monsoon in China.

We have cored sediments from Anguli-nuur Lake and carried out mineral-magnetism measurements on them. Together with the results of accelerator mass spectrometry (AMS) 14C, 137Cs and 210Pb dating, analyses of particle size, TOC and the ratio of carbon to nitrogen (C/N) on the lake sediments, the magnetic data have revealed significant environmental changes around this lake during the Holocene. This paper reports the palaeolimnological study for Anguli-nuur Lake. 2. Study area Anguli-nuur Lake (41°18′–24′N, 114°20′–27′E, ∼1315 masl) occupies a shallow depression with flat floor between rolling small hills of basalt forming during the late Miocene Epoch. The water surface of the lake covers 47.6 km2, with a length of 11.6 km, maximum width of 7.6 km and average width of 4.1 km. The maximum depth of the lake is 4.0 m while its average depth is 2.5 m. The volume of the water stored in the lake is ∼1.19 × 108 m3 with a pH of ∼8.3. The water surface is frozen from mid-October to mid-April (∼ 180 days) and the drainage area is ∼483 km2. Rain falling on the lake and catchment surface water replenishes it. In addition, Heishui River in the east and Santai River in the south empty into the lake. Water is lost through evaporation as no rivers or streams flow from the lake. Anguli-nuur Lake and its catchment are now in a semi-arid climate. The mean annual precipitation of 350 mm is highly seasonally distributed, and 60–70% of rain falls during June, July and August. The mean annual temperature is 4.0 °C and evaporation is

1849.9 mm. Using data for 1980–2000 from 25 meteorological stations, precipitation and temperature were interpolated for a broad area around the lake (Fig. 2b and c). Annual precipitation decreases from 400 to 50 mm from the southeast to northwest due to the decline of the southeast monsoon along this direction (Fig. 2b). Annual temperature ranges from 2 to 8 °C, but shows no southeast– northwest gradient (Fig. 2c). Located in the marginal zone of the southeast monsoon influences, this part of the Inner Mongolia Plateau is also strongly affected by north and northwest winds associated with the Mongolia–Siberia high-pressure cell particularly during winter and spring. Monthly mean wind velocity ranges from 2.8 m/s in December to 4.8 m/s in April. The Otindag (Otindaq or Hunshandake) Sand Plain (or Sandy Land), composed of barchan-formed dunes normally 5–10 m in height with a maximum of 20 m (e.g. Yang et al., 2004), is situated in the north of Anguli-nuur Lake (Fig. 2d). During winter and spring, the prevailing north and northwest winds may deflate dunes in this sand plain and transport and lay down dust and other material onto the lake and its catchment. Across this part of the Inner Mongolia Plateau, there are forests, meadow-steppes, typical steppes and desert steppes from the southeast to northwest with the decrease of annual precipitation and weakening of the southeast monsoon. The zonal vegetation in the catchment of Anguli-nuur Lake is typical steppes and dominated by Stipa krylovii and Leymus chinensis (Liu et al., 2008). However, as the catchment is very close to the meadow-steppe zone located to its southeast, elements of the plant community dominated by Stipa baicalensis with abundant forbs and legumes are also present (Liu et

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H. Wang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 30–49

Fig. 2. Anguli-nuur Lake and sampling sites in it, precipitation and temperature around it and Otindag Sand Plain. (a) Sampling sites of An-S and An-A in Anguli-nuur Lake. (b) Mean annual precipitation (mm) in study area. (c) Mean annual temperature (°C) in study area. (d) Anguli-nuur Lake with Otindag Sand Plain.

al., 2008). Birch trees normally found in the further southeast forest zone occasionally appear (Liu et al., 2008). Around Anguli-nuur Lake, chestnut soil developed on basalts is common, particularly on the slopes of small hills in the catchment. A few sand dunes are present in lower parts of the southwestern or

windward shore within the depression. On these dunes, cambisols have weakly developed and mixed with the chestnut soil eroded and deflated from upper basaltic slops. Saline soils have also formed on the lacustrine sediments exposed above the water surface around the lake.

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 1+

 . In

3. Methods

(HIRM) was calculated from the expression

3.1. Fieldwork, sampling and subsampling

addition, four inter-parametric ratios, χARM/SIRM, IRM300mT/SIRM, IRM− 300mT/SIRM and SIRM/χlf were calculated. χlf reflects the concentration of ferrimagnetic minerals. χARM is basically an indicator of the concentration of the fine, stable single domain (SSD) (∼ 0.02–0.04 μm) ferrimagnetic grains. IRM300mT is also used as a manifestation of the concentration of all the ferrimagnetic minerals (Oldfield, 1991). HIRM is indicative of the concentration of anti-ferromagnetic minerals. SIRM is used as an estimation of the concentration of all the magnetic minerals. χARM/SIRM, IRM300mT/ SIRM and IRM− 300mT/SIRM manifest the relative proportion of SSD grains, ferrimagnetic minerals and anti-ferromagnetic minerals in total magnetic minerals, respectively. SIRM/χlf is an approximation of the grain size of magnetic minerals and is lower if the proportion of relatively fine grains is higher and vice versa (e.g. Oldfield, 1991; Creer and Morris, 1996). Magnetic minerals of lake sediments are at least partly derived by erosion, deflation and other surface processes taking place within and beyond catchments (e.g. Thompson and Oldfield, 1986; Dearing, 1999). Thus, magnetic concentration-independent variables of these sediments may be, to some extent, indicative of the relative intensity of these processes, particularly erosion around lakes. Some of interparametric ratios of lake sediments are likely to suggest their sources and/or modes of transportation and deposition (e.g. Dearing, 1999). Surface materials may be sorted in their transportation toward lakes and, as a result, fine SSD grains may be relatively abundant in the sediments deposited in lakes (Rosenbaum and Reynolds, 2004). Such sorting is likely due to less surface water or dense vegetation in catchments which may somehow reduce the hydrological capacity to erode and transport coarse surface materials (Li et al., 2006). In other words, relatively high χARM/SIRM of lake sediments may be related to relatively weak erosion. Variations in bulk sediment particles and magnetic grain size in the sites which receive significant amounts of windblown materials are predominately influenced by source material, wind strength and distance from the source area (e.g. Valg et al., 2004; Reynolds et al., 2006). Winds, particularly when they are not very strong, may tend to preferentially deflate relatively fine surface materials, including fine magnetic grains. Even when winds have considerably strengthened, aeolian materials transported to deposition sites from distant source areas may still be rather fine, though coarser particles from local soils may be also moved to these sites in saltation by winds (e.g. Zhai et al., 2006). Dust originating from South America has a higher percentage of very fine magnetic (SP) grains (Oldfield et al., 1985). In northwest China, including the northwest parts of the Inner Mongolia Plateau subject to strong north and northwest winds, the surface materials at some sites south and southeast have higher χARM/SIRM, which have been attributed partly to selective deflation or sorting by north and northwest winds (Xia et al., 2006). Therefore, high χARM/SIRM of sediments in lakes, and particularly those receiving significant amounts of aeolian materials, may imply remarkable sorting and thus the role of winds in the transportation and deposition of sediments. Ferrimagnetic minerals are usually relatively abundant in soils, and particularly in topsoil in catchments, compared to dusts and other materials originating from dunes (Oldfield et al., 1985; Peck et al., 2004). Therefore, the high IRM300mT/SIRM of sediments may suggest that they are mainly derived from catchment soils. In contrast, increased IRM−300mT/ SIRM of sediments is likely to indicate that more material comes from dunes in arid and semi-arid regions. However, post-depositional dissolution, authigenesis or diagenesis and formation of bacterial magnetosome may change the magnetic characteristics of sediments that originated from detrital input. χARM/ SIRM, IRM− 300mT/SIRM and SIRM/χlf are likely to be useful for assessing if such in-lake processes have ever significantly influenced sedimentary magnetism.

Sediments were first sampled from Anguli-nuur Lake in December 2004 when the surface lake was covered with ice. With a Chinese-made HZ-100Y corer, a 1141-cm long core (hereafter referred to as “An-S”) was retrieved from the southwestern portion of the lake (Fig. 2a). Another 885-cm long core (hereafter referred to as “An-A”) was subsequently recovered in a site southeast of the site of An-S with a gravity corer in June 2007 when the lake was completely dried out (Fig. 2a). The core sections were wrapped in cling-film, aluminum foil and plastic sheeting immediately following collection, transported to Beijing in sealed plastic tubes and stored at 4 °C. An-S was subsampled at intervals of 5.0 cm in the laboratory, and a total of 229 specimens were obtained. The upper part (0–135 cm) of An-A was subsampled at an interval of 0.5 cm or 1.0 cm and 231 specimens were acquired in total. The sites where An-S and An-A were obtained are far from the inlets of the two main inflowing rivers. Thus, as also indicated by our field survey, material carried into the lake by the rivers is generally absent in the two sampling sites. Sediments deposited at the two sites may have come mainly from only three sources. First, the soils developed on the slopes of the basaltic hills and developed and/or accumulated on the dunes closely around the lake may be eroded and deflated and transported into the lake by water and winds. Second, the sands of these littoral dunes may be transported to the lake floor when gentle wave actions erode the bases of these dunes during times of high lake level or humid phases. Third, particularly during arid phases, strong winds from the north and northwest may have collected dust and other materials from the inland deserts and sand plains lying further north and northwest (e.g. Otindag Sand Plain), transported them a long distance and finally unloaded them onto the lake. Therefore, materials of the three sediment sources have also been sampled. A profile of soil (“K5A”) developed on basalt was sampled with an interval of 10 cm on the slope of the catchment and five soil specimens were acquired. A 25-cm soil profile (“An-south”) on a littoral dune was sampled and five soil specimens were obtained. A 40-cm deep soil profile (0–40 cm) and the underlying dune 60-cm deep (40–100 cm) were sampled with an interval of 5 cm in the southwestern shore. Thus, a total of eight soil and 12 sand specimens were acquired from this soil–sand profile (“An-2”). Sands were also sampled from five locations in Otindag Sand Plain (Fig. 2d), the closest desert or sand plain to Anguli-nuur Lake. The soil and sand specimens were packed individually into small polythene bags, sealed and then transported to Beijing.

3.2. Mineral-magnetic measurements Mineral-magnetic measurements were taken for all 229 sediment specimens from An-S. In addition, measurements were conducted for five soil specimens from K5A, five soil specimens from An-south, eight soil specimens and 12 sand specimens from An-2, and five sand specimens from Otindag Sand Plain. Low-frequency magnetic susceptibility (χlf) was measured with a Bartington MS2 System at 0.46 kHz. Anhysteretic remanence magnetisation (ARM) was impacted with a Molspin AF demagnetiser with ARM attachment at a peak AF field of 100 mT and DC field of 0.04 mT. Isothermal remanence magnetisation was acquired at field intensities of 300 mT and 1000 mT and backfield intensity of 300 mT (referred to as “IRM300mT”, “IRM1000mT” and “IRM− 300mT”) with a Molspin pulse magnetiser. IRM1000mT is used as saturation isothermal remanence magnetization (SIRM). ARM, IRM300mT, SIRM and IRM− 300mT were measured with a Molspin Minispin magnetometer. ARM susceptibility (χARM) was derived by normalising the ARM with the DC bias field. Hard isothermal remanence magnetization

SIRM 2

IRM−300mT SIRM

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Dissolution may selectively alter ferrimagetic minerals and particularly their fine grains leaving anti-ferromagnetic minerals almost unchanged in reducing conditions caused by slow deposition of sediments in lakes (e.g. Snowball, 1991). Thus very low or nearly zero concentrations of ferrimagnetic minerals as indicated by very low χlf and SIRM in combination with very low χARM/SIRM and high IRM− 300mT/SIRM of sediments may suggest significant dissolution of fine ferrimagnetic grains in lakes (e.g. Snowball, 1993; Dearing, 1999). Authigenic or diagenetic magnetic minerals may form at the sediment–water interface or within the sediment column and thus affect overall magnetic properties of sediments (Snowball, 1991; Dearing, 1999; Oldfield, 1999; Reynolds et al., 1999; Peck et al., 2004). SIRM/χlf has been used to indicate the presence of iron sulphide greigite (Fe3S4), one of the most common authigenic or diagenetic magnetic minerals found in lake sediments (e.g. Snowball, 1991; Williams et al., 1996). Values of SIRM/χlf higher than 30 or 40 × 103 A m− 1 suggest a significant contribution of greigite to sedimentary magnetic characteristics (e.g. Snowball, 1991; Oldfield, 1999). Bacterial magnetite or magnetosome has been also found in lake sediments (e.g. Snowball, 1994; Van der Post et al., 1997). χARM/SIRM has been used to assess if bacterial magnetite or magnetosome is significantly abundant in lake sediments. Values of this ratio higher than 2 × 10− 3 m A− 1 imply a significant presence of bacterial magnetite or magnetosome (Oldfield, 1999; Foster et al., 2008). 3.3. Particle-size analysis Particle-size analysis was performed for the 229 sediment specimens from An-S and the 231 sediment specimens from An-A with a Malvern Mastersize2000. Variations in particle size of lake sediments are likely to indicate changes in the relative intensity of erosion in catchments (e.g. Schmidt et al., 2002; Chen et al., 2004). Finer (coarser) sediment particles may be due to weaker (stronger) soil erosion in catchments of lakes. Variations in erosion intensity can be in turn related to changes of climate, vegetation and their interactions (e.g. Langbein and Schumm, 1958; Walling and Webb, 1983). Particle size of the sediments deposited in lakes in the Inner Mongolia Plateau has been also related to winds (e.g. Jiang et al., 2004). Sediment particles larger than1.4 μm in a core recovered from Anguli-nuur Lake were regarded as aeolian materials (Zhai et al., 2006). Among them, the relatively fine fractions, 1.4–4.2 μm and 4.2–14.0 μm, with a similar variation trend, may be transported in suspension by winds; the coarser ones,14.0–42.0 μm and N42.0 μm, change in a similar fashion and are likely to be moved into the lake in saltation by winds (Zhai et al., 2006). 3.4. TOC and C/N analysis TOC and C/N analyses were carried out for the 468 subsamples from the two cores with an EA 1108 elemental analyser. The TOC of lake sediments reflects the organic input from catchments and productivity within lakes (e.g. Beuning et al., 1997). Flourishing terrestrial plants and/or aquatic organisms may increase the organic input and/or productivity and hence increase the TOC of sediments, and these are likely to be further related to wet and/or warm climatic conditions, and vice versa. The nitrogen of lake sediments is mainly derived from aquatic organisms, while organic matter in these sediments is derived from both the terrestrial, particularly arboreal plants, and aquatic organisms. Thus, increases in C/N are indicative of relative increases in terrestrial contributions to the organic matter, whereas decreases suggest increasing contributions from aquatic sources (e.g. Beuning et al., 1997). In addition, extremely low TOC and C/N may also imply prolonged or repeated

sub-aerial exposures of sediments or desiccations and extraordinarily arid conditions (e.g. Talbot and Livingstone, 1989).

3.5. Pollen analysis Pollen analysis was completed only for the 229 specimens subsampled from An-S. Standard preparation techniques were used to prepare the pollen samples (Moore et al., 1991). A minimum of 250 pollen grains was counted for all the samples. The results of pollen analysis for surface soil samples collected from 84 sites throughout the typical steppe in the Inner Mongolia Plateau indicate that the modern pollen compositions are dominated by Artemisia and Chenopodiaceae, with very low percentages of Pinus and Betula also present (Liu et al., 2006). Surface soil samples from the other vegetation zones lying southeast in the plateau have higher percentages of Pinus and Betula (Liu et al., 2006). Pollen of other coniferous trees (Picea and Abies) and deciduous broadleaved trees (e.g. Quercus, Tilia and Ulmus) were also identified in these surface soils while their percentages are much lower than those of Pinus and Betula. In other words, the amount of pollen from the deciduous broadleaved and coniferous trees in the surface soils decreases, while that from herbs increases from the southeast to northwest with the transition from forests to grasslands and weakening of the southeast monsoon. During the times when the southeast monsoon was stronger than it is currently, more elements of plant communities of the vegetation zones further southeast might occur and/or flourish around Angulinuur Lake. Thus, the occurrence and/or increase of pollen from trees, particularly deciduous broadleaved trees, in some parts of An-S may suggest that the environment was wetter and warmer around the lake when these sediments were deposited. By contrast, trees and particularly deciduous broadleaved trees might disappear and/or decline around the lake when the southeast monsoon was weaker. Thus, the disappearance and/or decrease of pollen of these trees with the increase of herbs dominated by Artemisia and Chenopodiaceae in the intervals of this core are likely to imply deterioration toward dry and cool conditions.

3.6. AMS

14

C dating

There were no macrofossils found in An-S and An-A. Therefore, the humid acid fraction of bulk sediments and pollen were used for AMS 14 C dating. Sediment specimens were selected from five different levels and pollen grains were from two levels in An-S and from three levels in the top 40 cm of An-A (Table 1). From each of the chosen sediment specimens, particularly the one close to the uppermost parts of An-S, roots of modern plants were carefully removed in a preliminary treatment to minimize modern carbon contamination. Table 1 Radiocarbon dates for the An-S and An-A Core from Anguli-nuur Lake, the southeast Inner Mongolia Plateau, China. Lab no.

Sediment core

Depth (cm)

Dated materials

14 C age (yr BP) (± 1σ)

Calibrated age (cal. yr BP) (± 1σ)

BA05363 BA05364 BA05365 BA05367 BA05370 BA05372 BA05374 BA071162 BA071163 BA071164

An-S An-S An-S An-S An-S An-S An-S An-A An-A An-A

4.5 73.5 128.5 382.5 592.5 817.5 1082.5 9.0 20.0 40.0

Sediments Sediments Sediments Sediments Sediments Pollen Pollen Pollen Pollen Pollen

Modern 4425 ± 30 4460 ± 90 5560 ± 30 6480 ± 30 8155 ± 40 9620 ± 50 3810 ± 45 3975 ± 35 4185 ± 40

Modern 5050–4960 5290–5100 6350–6305 7360–7330 9130–9020 10960–10790 4260–4140 4515–4470 4770–4690

H. Wang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 30–49

Fig. 3. Variations in mineral magnetism for An-S. (a) χlf, χARM, IRM300mT, HIRM and SIRM. (b) χARM/SIRM, IRM300mT/SIRM, IRM− 300mT/SIRM and SIRM/χlf.

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H. Wang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 30–49

Fig. 3 (continued).

36

H. Wang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 30–49 Fig. 4. Variations in mineral magnetism for K5A, An-2 and sand specimens from Otindag Sand Plain. (a) Variations in mineral magnetism for K5A (a profile of soils developed on the basalts in the slope of the catchment). (b) Variations in mineral magnetism for An-2 (a profile of soils and underlain littoral dune closely around the lake). The vertical lines denote average values for the sand specimens sampled from five locations in Otindag Sand Plain.

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Table 2 Mineral magnetism of sediments from Anguli-nuur Lake and soil and sand specimens collected in its catchment and Otindag Sand Plain (OSP).

An-S

Range

K5A

χlf χARM IRM300mT HIRM SIRM χARM/SIRM IRM300mT/ (10− 8 m3 kg− 1) (10− 8 m3 kg− 1) (10− 6 A m2 kg− 1) (10− 6 A m2 kg− 1) (10− 6 A m2 kg− 1) (10− 3 m A− 1) SIRM

IRM− 300mT/ SIRM/χlf SIRM (103 A m− 1)

0.2–42.8

−0.97878 ∼ −0.83908 −0.91550 −0.91282 ∼ − 0.88849 − 0.90266 − 0.92997 ∼ − 0.89815 − 0.91383 − 0.92664 ∼ − 0.87798 − 0.89829 − 0.89229 ∼ − 0.85436 − 0.87247 − 0.88592 ∼ − 0.84619 − 0.86410

0.8–341.2

24–5796

1–198

26–6342

Average 6.2 Range 16.7–30.4

14.2 123.4–201.7

227 3217–4473

10 149–273

244 3416–4902

An-south

Average 22.8 Range 21.1–27.5

172.5 102.1–128.9

3675 3017–3983

195 145–180

3952 3285–4273

An-2 soil

Average 25.7 Range 26.4-36.8

110.6 131.2–277.9

3603 3749–11510

165 249–612

3862 4080–12286

Average 31.7 An-2 sand Range 10.0– 16.4

201.1 54.2–96.2

7544 1446–3521

400 105–242

8124 1542–3916

Average 13.0 Sand from Range 6.9–32.9 OSP Average 15.7

69.3 44.5–295.0

2140 1242–4510

149 388–1552

2346 1377–7351

114.9

2467

722

3225

4. Results 4.1. Mineral magnetism The magnetism–depth curves of An-S are presented in Fig. 3 and those of K5A and An-2 in Fig. 4. The curves of An-south have not been presented as they are very similar to those of the upper An-2. In addition, the ranges and averages of the magnetic parameters and ratios of An-S, the three profiles and five specimens from Otindag Sand Plain are shown in Table 2. The lowest χlf values of An-S occur at 500–390 cm, fluctuating from 0.165 to 5.703 m3 kg− 1 with an average of 3.851 × 10− 8 m3 kg− 1. The lowest SIRM values are also at this depth range, fluctuating from 26 to 225 A m 2 kg− 1 with an average of 94 × 10− 6 A m2 kg− 1. However, at this depth range, χARM/SIRM is not low at all except at 390 cm while IRM− 300mT/SIRM, though having increased, is not particularly high. We thus assume that the influence of dissolution on magnetic characteristics is relatively insignificant for An-S even for its lowest χlf and SIRM interval. In An-S, SIRM/χlf is generally low below 600 cm, ranging from 1.484 to 6.022 A m− 1 with the average 3.037 × 103 A m− 1. Above 600 cm, and particularly above 195 cm, SIRM/χlf has apparently increased with occurrences of several remarkable spikes. However, even the highest SIRM/χlf is only 15.585 × 103 A m− 1, still far lower than 30 or 40 × 103 A m− 1, the threshold value indicating a significant presence of greigite formed by reductive diagenesis. Hence impacts of authigenic or diagenetic processes on sedimentary magnetism may also be rather slight in An-S. Also in An-S, χARM/SIRM ranges from 0.10910 to 1.57805 m A− 1 with an average of 0.59449 × 10− 3 m A− 1, lower than the threshold value of 2 × 10− 3 m A− 1 which suggests that bacterial magnetosome dominates sediment magnetism. So the influence of bacterial magnetosome on sediment magnetism may be not significant in this core in general. Therefore, the magnetic characteristics of this sediment core are predominately the manifestation of detrital magnetic minerals transported somehow into the lake. Compared to the soil and sands, the concentrations of magnetic minerals are generally low in the sediments as suggested by all the concentration-dependent parameters (Table 2). The sediments have higher χARM/SIRM than any of these potential source materials. Dusts or surface materials sorted and transported by winds may have higher proportions of fine magnetic grains (SP or SSD) (Oldfield et al., 1985; Xia et al., 2006). So the rather remarkable enrichment of the fine SSD

0.10910– 1.57805 0.59449 0.34573– 0.49891 0.43773 0.24649– 0.32817 0.28916 0.19282– 0.35431 0.26462 0.24569– 0.35143 0.35143 0.24958– 0.49591 0.34547

0.86989– 0.96691 0.93523 0.90735– 0.95007 0.93191 0.91830– 0.94171 0.93249 0.90541– 0.94269 0.92583 0.88687– 0.93746 0.91333 0.61349– 0.90175 0.81132

0.7–15.6 3.2 16.1–21.4 17.7 12.8–16.2 15.1 15.5–35.2 24.9 15.4–23.9 17.8 12.7–27.9 20.5

grains in the whole of An-S may imply that these sediments had been markedly sorted, and thus winds were significant in their transportation and deposition. It is also likely that erosion is rather weak in general and thus tends to preferentially move the fine SSD grains. The soils on the basaltic slopes and littoral dunes have a relatively high proportion of ferrimagnetic minerals as indicated by the relatively high IRM300mT/SIRM (Fig. 4 and Table 2). By contrast, sand from the dunes around the lake and in Otindag Sand Plain has a relatively high proportion of anti-ferromagnetic minerals as shown by their comparatively high IRM− 300mT/SIRM (Fig. 4 and Table 2). The magnetic characteristics of the soils and sands are likely to be useful for interpreting mineral magnetism of the sediments. Catchment soils can be eroded and deflated and transported into the lake. So the increasing IRM300mT/SIRM of the sediments may indicate increasing contribution from catchment soils to the sediments by erosion and/or deflation. As also demonstrated by studies elsewhere, dust, sand and other aeolian materials originating from dunes in arid and semi-arid regions usually have a high proportion of antiferromagnetic minerals (e.g. Oldfield et al., 1985; Peck et al., 2004). Thus, the high proportion of anti-ferromagnetic minerals in sediments have been used as an indicator of strong aeolian activity and marked aridity (e.g. Peck et al., 1994, 2004). However, for the lakes close to dunes in arid and semi-arid regions, sands may also enter by means of additional to wind deposition (Li et al., 1990; Wang et al., 2008). The increasing IRM− 300mT/SIRM of An-S may imply the increasing contribution of sands and other materials originating from dunes. Nevertheless, their increases in the sediments may be due to two totally different causes and thus have two opposite environmental implications. First, and rather intuitively, strengthening winds may deflate more sands and dusts from the dunes in the relatively distant deserts and sand plains (e.g. Otindag Sand Plain) and deposit them in the lake. Thus, the increasing IRM− 300mT/SIRM of sediments may hint at enhanced aridity. Second, and quite counter-intuitively, rising water level may allow waves on the lake surface to erode the base of the littoral dunes and move and deposit the sand grains on the lake floor. Hence, an increase in IRM− 300mT/SIRM of the sediments, in fact, implies increasing humidity. The χARM/SIRM of the specimens from Otindag Sand Plain appears somewhat higher than that of littoraldune sands (Table 2). These specimens from Otindag Sand Plain were sampled simply in situ in the sand plain itself and are thus not “typical” dusts. Even so the dune materials in the sand plain may still had been somehow transported and sorted by winds more frequently than the sands of the littoral dunes covered by soils. That may result in the relative enrichment of the fine SSD grains in the sand-plain

H. Wang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 30–49

materials compared to the littoral-dune sands. After being transported towards Anguli-nuur Lake and hence further sorted by winds, the materials that originated from Otindag Sand Plain may have even higher proportion of the SSD grains and χARM/SIRM values than those from the littoral-dune sands. We will use such difference in χARM/ SIRM in combination with other proxies to distinguish the contributions from the two different sources of dune materials to the sediments in the lake. Similarly, in most cases, the materials from catchment soils may also have relatively abundant SSD grains and high χARM/SIRM after being windily winnowed and sorted. 4.2. Particle size The accumulative percentages of different particle grades and mean particle size of An-S are presented (Fig. 5). For An-A, only mean particle size is presented (Fig. 7a). In An-S, particles of sediments are generally fine below 390 cm though they are relatively coarse at 800–600 cm. From 390 cm upwards, sediments have dramatically coarsened. They are the coarsest at 130– 20 cm and show a trend of becoming fine above 20 cm. The sediments of An-S were divided into four particle grades: 1.4– 4.5 μm, 4.5–14.2 μm, 14.2–44.8 μm and N44.8 μm, to correspond approximately to the four classes (1.4–4.2 μm, 4.2–14.0 μm, 14.0– 42.0 μm and N42.0 μm) used by Zhai et al. (2006) for their core retrieved from this lake. Like the two relatively fine classes (1.4– 4.2 μm and 4.2–14.0 μm) in Zhai et al.'s (2006) core, the percentages of the grades of 1.4–4.5 μm and 4.5–14.2 μm in An-S change similarly (Fig. 5). We also assume that these particles were moved into the lake by winds in suspension. The coarse particles (N44.8 μm) of An-S, like the fraction of N42.0 μm of Zhai et al.'s (2006) core, might be transported into the lake in saltation by winds. In addition, the sands of the littoral dunes may enter the lake due to the dune-base erosion by lake waves when the water level is high, which may also increase the percentage of the coarse particles in An-S. However, unlike the fraction of 14.0–42.0 μm in Zhai et al.'s (2006) core, the variations in the fraction of 14.2–44.8 μm are not similar to those in the coarse fraction below 390 cm in An-S. In either case, they are not similar to those in the two fine fractions below this depth. So these particles below 390 cm in An-S might be not transported predominately by winds in either suspension or saltation. We assume that these particles are transported into the lake mainly by surface water. Above 390 cm, the variations of their percentage are similar to those of the

39

particles measuring 1.4–4.5 μm and 4.5–14.2 μm, and they might be transported into the lake mainly by winds in suspension. 4.3. TOC and C/N Both TOC and C/N of An-S are presented (Fig. 5). Only TOC is presented for An-A (Fig. 7b). TOC of An-S is generally high below 600 cm and highest at 800– 600 cm. It has declined at 600–130 cm and begun to further decrease from 130 cm upward, and is thus very low at 130–0 cm. Variations in C/N of An-S closely resemble those in TOC and its values are high below 600 cm. However, decreases in C/N are much less pronounced than those in TOC at 510–130 cm in this core. From 130 cm upward, C/ N has again tracked TOC and sharply declined and is lowest at 0– 60 cm. 4.4. Pollen The major pollen taxa identified from An-S are Pinus, Picea, Abies, Betula, Quercus, Ulmus, Artemisia, Chenopodiaceae, Polygonum, Ephedra, Gramineae, Nitraria, Cyperaceae, Compositae, Leguminosae and Labiateae. Pollen percentages of some of the individual taxon and taxonomic groups and ratio of arboreal to non-arboreal plants (AP/NAP) (Fig. 6) are presented. Great variability of pollen percentages at some levels was suspected to be caused by variable deposition of windblown regional Pinus pollen from the southeast. Pollen of trees is rare while pollen of herbs is abundant at 1140– 1080 cm. At 1080–800 cm, pollen of conifers has apparently increased while that of deciduous broadleaved trees, though having increased, is still scarce. From 800 to 600 cm, pollen of deciduous broadleaved trees has increased considerably and is thus most abundant. There is generally plentiful pollen of conifers, mainly Pinus, and herbs but sparse pollen of deciduous broadleaved trees at 600–60 cm. Further upward, at 60–20 cm, herb pollen is very abundant while tree pollen is extremely rare. However, pollen of deciduous broadleaved trees has increased though pollen of coniferous trees is still scant above 20 cm. 4.5. Chronology Seven AMS radiocarbon dates have been obtained for An-S and three for An-A and converted to calendar years (cal. yr BP) using OxCal v3.10(2) with IntCal04(1) (Table 1). However, ages are quoted

Fig. 5. Variations in particle size, total organic carbon (TOC) and carbon/nitrogen ratio (C/N) for An-S.

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H. Wang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 30–49

Fig. 6. Pollen diagrams for An-S. (a) Percentages of pollen of Pinus, Picea and Abiea, Betula and broadleaved trees. (b) Percentages of pollen of Artemisia and Chenopodiaceae, total trees and total herbs and ratio of pollen of arboreal plants (AN) to non-arboreal plants (NAP).

in both 14C years (yr BP) and calendar years (cal. yr BP) whenever available in this paper to be conveniently compared to the previouslypublished chronologies which are in either yr BP or cal. yr BP. The corresponding depths in An-S to 20.0 and 40.0 cm in An-A are ∼ 15.0 and 50.0 cm as suggested by correlating the two cores with their mean particle size and TOC (Fig. 7a and b). So the radiocarbon dates for the two depths (BA071163 and BA071164) in An-A might be also applicable to An-S. Due to the absence of the mean particle size and TOC data for the topmost 10 cm in An-S, the depth in An-S corresponding to 9.0 cm in An-A has not been determined and thus the radiocarbon date for 9.0 cm in An-A (BA071162) is not applicable to An-S. Analyses of the short-lived radioisotopes 137Cs and 210Pb have been also performed for 40 sediment specimens from the top 20 cm of An-A (Yin et al., submitted for publication). The age suggested by a 137 Cs peak for 6.0 cm is 1963 AD and estimated with 210Pb concentrations for 20.0 cm (corresponding to 15.0 cm in An-S) is 1666 AD (284 cal. yr BP) (Fig. 7a and b). Although the 137Cs age in An-

A is not directly applicable to An-S as there are no mean particle size and TOC data for the topmost 10 cm in An-S, it nevertheless still confirms that the superficial interval of An-S is “modern” sediments as indicated by the AMS date for 4.5 cm (BA05363) in An-S (Table 1). Furthermore, the 137Cs and 210Pb ages also suggest that the radiocarbon dates for 9.0, 20.0 and 40.0 cm in An-A and possibly 73.5 cm (BA05364) in An-S are too old due to a comparatively strong carbon reservoir effect. In a core retrieved near the centre of this lake, very coarse particles (N0.25 mm) sharply increased upwards from the depth of ∼25.0 cm (Qiu et al., 1999) (Fig. 7c). A 14C age for a slightly shallower depth (∼23.5 cm) is 4635 ± 154 yr BP (Qiu et al., 1999), which is roughly coincident with a varve age of 4472 years (Zhai et al., 2002). In An-S, a corresponding increase of the very coarse particles (N0.2 mm) occurs at the depth of 130 cm. The 14C AMS date (BA05365) for a somewhat shallower depth (128.5 cm) is 4460 ± 90 yr BP in An-S and thus roughly coincides with the radiocarbon and varve age for the corresponding

H. Wang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 30–49

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Fig. 8. Chronology model for An-S. (a) Chronology model in 14C years (14C yr BP). (b) Chronology model in calendar years (cal. yr BP). ○ An-A 210Pb dating. ⋄ An-A 14C dating. ■ An-S 14C dating.

construct a chronological model for this core by assuming constant deposition rates between adjacent dated levels (Fig. 8). The model suggests that the average accumulation rate is much lower for the uppermost 128.5 cm. This lake totally dried out in 1936, 1972, 1985 and 2004. Usually 3–4 years after each desiccation, the lake has been inundated. Similar desiccation may have also happened during earlier periods. The very high concentrations of magnetic minerals (Fig. 3a) and very low TOC and C/N (Fig. 5) above 60 cm may imply such desiccation. As shown by the very high percentage of the coarsest particles (N44.8 μm) above 128.5 cm (Fig. 5), abundant coarse particles were windily blown in saltation into the lake and deflation might greatly intensified and largely replaced erosion. So both short but repeated desiccations and dominance of deflation over erosion may have led to the average low deposition rate for this part of An-S.

5. Interpretation

Fig. 7. Correlating An-S to other cores from Anguli-nuur Lake. (a) Correlating An-S to An-A with mean particle size. (b) Correlating An-S to An-A with TOC. (c) Correlating An-S to a core from Anguli-nuur Lake recovered by Qiu (Qiu et al., 1999) and Zhai (Zhai et al., 2002).

depth in the core of Qiu et al. (1999) and Zhai et al. (2002). Hence, the radiocarbon date for the depth of 128.5 cm (BA05365) may be less influenced by the old carbon effect and thus still relatively reliable. Therefore, only the 210Pb age applicable to 15.0 cm and 5 radiocarbon dates at and below 128.5 cm in An-S were used to

In term of variations in mineral magnetism as well as in particle size, TOC and C/N and pollen percentage, An-S has been divided into 9 units (Figs. 3, 5 and 6). Values of the concentration-dependent parameters are relatively constant below 390 cm in An-S. Thus the influx of surface materials into the lake may be less variable during the earlier phases of the last ∼ 10,000 years. By contrast, variations in the inter-parametric ratios appear more impressive throughout the whole core. Hence changes in sediment sources and/or the mode of sediment transportation and deposition may be more remarkable and will be given relatively more attention in the following interpretation. Scatter plots of IRM− 300mT/SIRM versus χARM/SIRM and IRM300mT/SIRM were made for each of these sediment units with the potential source materials to facilitate inferring sediment sources for each of the units (Fig. 9).

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5.1. An-S-1 (1140–1080 cm; N10,900 cal. yr BP or 9600 yr BP) IRM300mT/SIRM is high and hence ferrimagnetic minerals are relatively enriched and the sediments are derived mainly from catchment soils. χlf, IRM300mT, HIRM and SIRM are low, suggesting that ferrimagnetic, anti-ferromagnetic and total magnetic minerals moved into the lake are rare. Both χARM and χARM/SIRM are relatively high. Thus, of the magnetic minerals transported into the lake, only the fine SSD grains are comparatively abundant in absolute and relative terms, which may imply rather weak erosion. Alternatively or additionally, winds are still capable of deflating the fine magnetic grains from catchment soils and deposit them onto the lake. IRM− 300mT/SIRM is low, suggesting low relative proportion of anti-ferromagnetic minerals and low dune material input to the lake. Therefore, although the aeolian materials from Otindag Sand Plain is rare or absent in the lake, the water level of the lake is still low. These mineral-magnetic

characteristics suggest that the climate is still rather dry. The slight erosion and/or moderate deflation are due to the dearth of rainfalls and/ or occurrence of modest winds. The fine sediment particles (1.4–4.5 μm and 4.5–14.2 μm) transported in suspension by winds are abundant and the relatively coarse ones (14.2–44.8 μm) transported by water are rare, indicating moderate deflation and comparatively weak erosion. TOC and C/N are low, implying that vegetation particularly arboreal plants are relatively sparse and environmental conditions are relatively dry and cool. Trees are generally rare as shown by the low percentages of pollen of Pinus, Picea and Abiea, Betula, broadleaved trees and total trees and low AP/NAP. As indicated by the high percentages of pollen of Artemisia and Chenopodiaceae and total herbs, herbs are abundant. So the pollen data have also confirmed that the environments are quite arid and cool. However, the inferred moderate wind activities imply that

Fig. 9. Scatter plots of IRM− 300mT/SIRM versus χARM/SIRM and IRM− 300mT/SIRM for each of sediment units with potential source materials. (a) to (i) for An-S-1 to An-S-9 with potential source materials. ♦ Sands from Otindag Sand Plain. ⋄ Sands of the dunes around Anguli-nuur Lake. ▲ An-south Profile. △ Soils accumulated and/or developed on the dunes around Anguli-nuur Lake. □ K5A soil profile. + Sediments of An-S.

H. Wang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 30–49

43

Fig. 9 (continued).

the aridity is not particularly high and thus the extremely dry and cool conditions dominating the late Pleistocene may have already ended. 5.2. An-S-2 (1080–1010 cm; 10,900–10,400 cal. yr BP or 9600–9200 yr BP) IRM300mT/SIRM is still high as in the underlain unit and hence the sediments transported into the lake are still predominately derived from catchment soils. χlf, IRM300mT, HIRM and SIRM have either slightly increased or are almost the same as in An-S-1. Both χARM and χARM/SIRM have increased and are higher than in the underlain unit. In other words, the fine SSD grains moved into the lake are even more abundant in absolute and relative terms. So, soil erosion may have somewhat intensified but be still weak; or alternatively wind deflations may have slightly strengthened. IRM− 300mT/SIRM is still low, indicating still low input materials from dunes in the distant Otindag Sand Plain and closely around the lake. Thus, aeolian activities are still not intensive in general but the lake level is still low. The fine fractions of the sediments (1.4–4.5 μm and 4.5–14.2 μm) have decreased and the relatively coarse one (14.2–44.8 μm) somewhat increased, suggesting the weakened deflation and slightly intensified

erosion. Though the increase in TOC is less impressive, C/N has rather remarkably increased, suggesting flourishing terrestrial vegetation particularly higher plants and thus ameliorating environments. More definite and reliable evidence is given by pollen data on the climatic conditions. Trees particularly Pinus, Picea and Abiea have apparently increased though Betula and other deciduous broadleaved trees have only slightly increased and Artemisia, Chenopodiaceae and other herbs are still rather abundant. Nevertheless, the remarkably increased AP/NAP has also indicated the increased arboreal plants. Hence, the pollen data have also indicated denser vegetation and wetter and warmer conditions than during the previous phase. Thus the increased SSD grains may be attributed to slightly increased rainfall and erosion rather than deflation. 5.3. An-S-3 (1010–800 cm; 10,400–8900 cal. yr BP or 9200–8000 yr BP) IRM300mT/SIRM is still quite high and thus the sediments are still mainly contributed by catchment soils. χlf, IRM300mT, HIRM and SIRM are almost the same as in An-S-2. χARM and χARM/SIRM have decreased and thus the fine SSD grains are rarer in both absolute

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H. Wang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 30–49

Fig. 9 (continued).

and relative terms than in the underlain unit. So erosion may be the same as or even weaker than during the previous phase and deflation may be also slight. IRM− 300mT/SIRM has increased and hence dune materials moved into the lake may have increased. As shown by the low χARM/SIRM, these materials have not been strongly sorted and thus are unlikely to be originated from the remote Otindag Sand Plain. Instead they are only possibly derived from the littoral dunes. In other words, the water level of the lake may have risen and the lake waves have thus eroded the base of the dunes and transported some of the sands to the sampling site. The percentage of the coarsest particles (N44.8 μm) of the sediments has increased compared to An-S-2. However, as implied by the mineral magnetism, this increase is probably mainly due to the increasing input of sands and other materials from the littoral dunes. TOC and particularly C/N are higher, suggesting increased vegetation and particularly higher plants in the catchment and hence further ameliorated climate. As shown by the pollen data particularly the pollen percentage of total trees and AP/NAP, trees seem to have increased. Therefore, in

general, the climate may have further ameliorated. Although the rainfall and/or water surface may have somewhat increased as implied by the inferred rising lake level, soil erosion has not intensified or even slight weakened probably due to the increased vegetation.

5.4. An-S-4 (800–600 cm; 8900–7400 cal. yr BP or 8000–6500 yr BP) IRM300mT/SIRM has remarkably decreased, indicating that the contribution to the sediments from catchment soil may have relatively decreased. HIRM has rather apparently increased while χlf, χARM, IRM300mT and SIRM have only slightly increased or remain the same as in An-S-3. Thus, anti-ferromagnetic minerals transported into the lake have particularly increased. IRM− 300mT/SIRM has sharply increased and is thus high while χARM/SIRM remains low as in the underlain unit. Hence, abundant sands and other materials have been transported into the lake from the littoral dunes. In other words, the water level is very high and lake waves are thus capable of more frequently eroding the bases of the littoral dunes.

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The coarsest fraction (N44.8 μm) of the sediment particles has further increased, which probably resulted mainly from dune-bases erosion by lake-wave rather than input of coarse windblown particles. TOC and C/N are the highest, indicating very flourishing land particularly arboreal plants and thus very wet and warm conditions. Pollen percentages of Betula and other deciduous broadleaved trees have apparently increased and attained the highest, confirming very wet and warm climates. Although Picea and Abiea are rarer than during the previous phase, Pinus is still abundant. Therefore, arboreal plants particularly trees are still abundant in general as shown by the high AP/ NAP and vegetation is still very dense. The pollen composition as well as the highest TOC and C/N and inferred very high lake level suggest that climate is wettest and warmest during this phase. 5.5. An-S-5 (600–500 cm; 7400–6900 cal. yr BP or 6500–6100 yr BP) IRM300mT/SIRM has increased and the contribution to the sediments from catchment soils has thus relatively increased. IRM− 300mT/SIRM has decreased and χARM/SIRM is still low, suggesting that dune-base erosion has weakened and the lake level dropped or effective precipitation declined. The coarsest particles (N44.8 μm) of the sediments have decreased. So, the sand input from the littoral dunes has decreased and water level dropped as also implied by mineral magnetism. TOC and C/N have also declined, suggesting reduced vegetation and hence deteriorated conditions. Arboreal plants have decreased in general as shown by AP/NAP. The pollen percentages of Betula and other deciduous broadleaved trees have most remarkably decreased though those of Pinus, Picea and Abies have increased. Nevertheless, pollen data indicate that the environments are less wet and warm than during the previous phase. 5.6. An-S-6 (500–390 cm; 6900–6400 cal. yr BP or 6100–5600 yr BP) IRM300mT/SIRM is still high in general. So the sediments are still mainly derived from catchment soils. As shown by the decreased and thus the lowest χlf, χARM, IRM300mT, HIRM and SIRM, magnetic minerals transported into the lake are very rare and soil erosion is thus very slight in the catchment. χARM/SIRM has rather remarkably increased, suggesting intensified deflation. Though somewhat higher than in An-S-5, IRM− 300mT/SIRM is still low, implying still low material input from both the littoral dunes and Otindag Sand Plain. So the lake level is still low though the wind activities are not particularly strong. The fine fractions (1.4–4.5 μm and 4.5–14.2 μm) of the sediments have increased, indicating that deflation has moderately strengthened. The lowest percentage of the coarsest fraction (N44.77 μm) also confirms the very low input from the littoral dunes and very low lake level. TOC has further decreased. However, C/N seems to have somewhat increased and thus been even slightly higher than in An-S-5. The percentages of herbaceous pollen particularly those of Artemisia and Chenopodiaceae have considerably increased. The percentages of Pinus, Picea and Abies have particularly remarkably decreased though the percentages of pollen of Betula and other broadleaved trees have even slightly increased but remain low in general. As a result, trees have decreased generally as shown by the further decreased pollen percentage of total trees and AP/NAP. The pollen data may have also indicated even drier and cooler environmental conditions than during the previous phase. The further weakening of erosion is most likely due to decrease of rainfall/overflow. 5.7. An-S-7 (390–60 cm; 6400–2200 cal. yr BP or 5600–? yr BP) As shown by the generally high IRM300mT/SIRM, the sediments are still derived predominately from catchment soils. Though fluctuating, χARM/SIRM has persistently increased upwards particularly above

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195 cm (∼ 5500 cal. yr BP or 4700 yr BP). Correspondingly, the percentages of the sediment particles of 1.4–4.5 μm, 4.5–14.2 μm and 14.2–44.8 μm have decreased while the one of the particles of N44.8 μm has steadily increased upwards. Thus, although winds still tend to only pick up the fine SSD magnetic grains, nevertheless they are capable of pushing more coarse soil particles by saltation into the lake and hence further strengthened. IRM− 300mT/SIRM is still low, indicating that aeolian materials from Otindag Sand Plain are still rare or absent. TOC particularly above 100 cm and C/N particularly above 195 cm are low and keep decreasing in general, suggesting declining vegetation and thus deteriorating conditions. The extremely low TOC and C/N at 90–60 cm may be partly due to very low water level or even desiccation. Pollen percentages of broadleaved trees remain low. In particular, the pollen percentage of Betula is even lower than in An-S-6. Among the coniferous trees, Picea and Abies are sparse and Pinus has slightly increased. Pollen percentages of Artemisia, Chenopodiaceae and total herbs are high. Thus, environments are generally dry and cool and probably drier and cooler particularly since ∼ 5500 cal. yr BP or 4700 yr BP (195 cm) than during the previous phase. 5.8. An-S-8 (60–20 cm; 2200–480 cal. yr BP) IRM300mT/SIRM has apparently decreased, suggesting a relative decrease of the contribution from catchment soils to the sediments. By contrast, IRM− 300mT/SIRM has substantially increased coinciding with moderately high χARM/SIRM. So the materials derived from Otindag Sand Plain have increased or occurred in the sediments. Therefore, winds may have greatly intensified in the whole region. χARM and other concentration-dependent parameters have considerably increased. So not only SSD grains but also other magnetic materials were blown into the lake by the tremendously strengthened winds. The abundant magnetic materials may be partly contributed by pedogenic magnetic minerals. In other words, the lake might desiccate. The percentage of the coarsest sediment particles (N44.8 μm) transported by winds in saltation is the highest, confirming extremely intensive aeolian activities. TOC and C/N have dropped to the lowest, implying the very sparse vegetation. The extraordinarily low TOC and C/N are also in part caused by desiccation events. Pollen percentages of all the trees have declined to the lowest and those of herbs reached the highest. AP/NAP has also dropped to the minimum. So, vegetation is very scarce and environments are very dry and cool. The pollen composition in combination with the inferred very strong wind activities and lowest TOC and C/N indicate that climate is driest and coolest during this phase. 5.9. An-S-9 (20–0 cm; b480 cal. yr BP) IRM300mT/SIRM has pronouncedly increased and hence the contribution from catchment soils has again increased. χARM/SIRM has decreased but is still fairly high. IRM− 300mT/SIRM has apparently decreased and is very low. So dusts and other materials from Otindag Sand Plain have remarkably decreased and winds may have somewhat weakened. The extremely abundant magnetic minerals as shown by the extraordinarily high χlf, χARM, IRM300mT, HIRM and SIRM may be at least partly due to pedogenic processes and thus imply rather frequent desiccation. The inferred weakened winds appear to be somehow contrary to the strengthened desiccation. Thus, the even more frequent desiccations may be in part caused by human activities (e.g. cultivating around the lake). The percentages of the fine sediment particles (4.5–14.2 μm) and particularly relatively fine ones (14.2–44.8 μm) have increased while the one of the coarsest particles (N44.8 μm) decreased. Hence the coarse particles transported in saltation by winds into the lake have relatively declined, corroborating reduced aeolian activities.

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Both TOC and C/N tend to increase slightly but are still very low, suggesting slightly increasing vegetation and thus ameliorating but still dry and cool environments in general. The very low TOC and C/N are also likely to be caused by prolonged or repeated desiccations. Although coniferous trees are still rare as shown by the low pollen percentages of Pinus and Picea and Abiea, Betula and other broadleaved trees have increased. Artimisia, Chenopodiaeae and other herbs are also still abundant. The pollen composition indicates a less dry and cool climate than during the previous phase. 6. Discussion Marked changes in environmental conditions have happened during the Holocene in China (An et al., 2000; Feng et al., 2006). These changes in parts of the Inner Mongolia Plateau have been largely controlled by the southeast monsoon (e.g. Sun et al., 2006; Sun et al., 2009). As in many other parts of China, it was generally very dry and cold in the Inner Mongolia Plateau during the late Pleistocene. Annual temperature might be 10 °C lower and annual precipitation 200 mm or even 300 mm lower during 25,000–13,000 yr BP than now in the eastern and southern part of the Plateau (Li et al., 1990). In the more remote northwest part of the Plateau, the environment was persistently arid until ∼13,400 yr BP (Chen et al., 2003). The Holocene amelioration began to happen predominately during 10,500–9500 yr BP in the Inner Mongolia Plateau (Feng et al., 2006). Around Anguli-nuur Lake, conditions started to become wet and warm at 10,900 cal. yr BP (9600 yr BP) as shown by An-S. Geomorphologic evidence around Anguli-nuur Lake indicates that the water level began to rise at 9490 ± 125 yr BP (Li et al., 1990) or 10,000 yr BP (Qiu et al., 1999), which is coincident with the onset of the amelioration suggested by An-S. Commencement of the corresponding amelioration has been also detected at other sites in the Inner Mongolia Plateau (Table 3 and Fig. 10). Around Dali Nor (Dalang-nur or Dalainoer) Lake, a site situated in the far northeast, the onset of this amelioration is apparently earlier, at ∼12,000 yr BP (Geng and Zhang, 1988). Further south to Dali Nor Lake, the amelioration started at 10,300 and 9400 yr BP around Haoluku and Liuzhouwan respectively, two small palaeo-lakes, later than around Dali Nor Lake (Wang et al., 2001). A similar humidification began at 10,500 cal. yr BP around Lake Bayanchagan, a site also located south of Dali Nor Lake (Jiang et al., 2006), which is very close to Anguli-nuur Lake

in timing. Around Daihai Lake, a site lying further southwest in the Inner Mongolia Plateau, the corresponding amelioration commenced at 10,250 cal. yr BP (Xiao et al., 2004). Following such an onset of environmental amelioration, conditions were relatively warm generally during 10,500–8000 or 9500–7500 yr BP in the Inner Mongolia Plateau (Feng et al., 2006). As indicated by An-S, quite wet and warm environments prevailed around Angulinuur Lake during 10,900–8900 cal. yr BP (9600–8000 yr BP), which may be regarded as a transition from relatively dry and cool to wet and warm conditions. Geomorphologic features around this lake suggest that the water level was rapidly rising during 10,000–7300 yr BP (Qiu et al., 1999), hence confirming such a transition toward wetness and warmth. However, this transition seems to have not been clearly distinguished at most of the other aforementioned sites. Only around Daihai Lake have relatively humid and warm environments been inferred during 10,250–7900 cal. yr BP (Xiao et al., 2004). With further enhancement of the amelioration, the Holocene Climatic Optimum occurred in the Inner Mongolia Plateau (Feng et al., 2006). Around Anguli-nuur Lake, the wettest and warmest conditions or the optimum persisted during 8900–7400 cal. yr BP (8000–6500 yr BP) as indicated by An-S. As shown by the geomorphologic features around the lake, the highest water level was reached during 7300–6230 yr BP (Qiu et al., 1999). Annual laminations of sediments suggest that the north and northwest winds were weak around this lake during 8507–5429 yr BP though the environmental conditions were unstable during the later part of this period (Zhai et al., 2002). Therefore, both geomorphologic and sedimentary characteristics presented by other researchers have also suggested the occurrence of the optimum though there are minor differences in timing. At the other sites, similar conditions were reconstructed, whether or not there was a preceding transitional phase distinguished. The highest level of Dali Nor Lake, the most northeast site, persisted during 12,000–7000 yr BP (Geng and Zhang, 1988). Around Haoluku and Liuzhouwan, the two small palaeo-lakes lying south of Dali Nor Lake, the optimal conditions prevailed respectively during 10,300– 5600 yr BP and 9400–4700 yr BP (Wang et al., 2001). The most humid conditions occurred around Lake Bayanchagan during 10,500–6500 cal. yr BP (Jiang et al., 2006). Around Daihai Lake, the most southwest site, the Holocene optimum persisted in 7900–4450 cal. yr BP during which the maximum humidity and warmth started and ended respectively at 6050 and 5100 cal. yr BP (Xiao et al., 2004).

Table 3 The onsets and terminations of the Holocene climatic changes inferred for several lakes and palaeo-lakes in the Inner Mongolia Plateau. Climatic conditions

Beginning to become wet and warm Relatively wet and warm The Holocene optimum conditions Still wettest but less warm Beginning to become dry and cool Dry and cool Slightly ameliorating Driest and coolest Slightly ameliorating

Dali Nor Lake

Liuzhouwan Lake Palaeo-lake Bayanchagan

Anguli-nuur Lake

Geng and Wang et al. Zhang (1988) (2001)

Wang et al. (2001)

Jiang et al. (2006)

An-S

Li et al. (1990)

12,000 yr BP

9400 yr BP

10,500 cal. yr BP

10,900 cal. yr BP (9600 yr BP)

9490 ± 10,000 yr BP 125 yr BP

12,000– 7000 yr BP

7000 yr BP

Haoluku Palaeo-lake

10,300 yr BP

10,300– 5600 yr BP

9400– 4700 yr BP

10,500– 7900 cal. yr BP

5600 yr BP

4700 yr BP

7900– 6500 cal. yr BP 6500 cal. yr BP

5600– 4500 yr BP 4500– 3000 yr BP 3000– 1200 yr BP b1200 yr BP

4700– 2100 yr BP 2100– 700 yr BP b 700 yr BP

Daihai Lake Qiu et al. (1999)

Zhai et al. (2002)

Xiao et al. (2004) 10,250 cal. yr BP

10,900–8900 cal. yr BP (9600–8000 yr BP)

10,000– 7300 yr BP

8900–7400 cal. yr BP (8000–6500 yr BP)

7300– 6230 yr BP

8507– 5429 yr BP

7900–4450 cal. yr BP

6400 cal. yr BP (5600 yr BP)

6230 yr BP

5429 yr BP

4450 cal. yr BP

6400–2200 cal. yr BP (5600–? yr BP)

6230– 5300 yr BP

5429– 3244 yr BP 3244– 2494 yr BP 2494– 1165 yr BP b1165 yr BP

4450–2900 cal. yr BP

2200–480 cal. yr BP b 480 cal. yr BP

10,250–7900 cal. yr BP

2900–1700 cal. yr BP 1700–1350 cal. yr BP

H. Wang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 30–49

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Fig. 10. Anguli-nuur Lake and other sites in the Inner Mongolia Plateau.

As shown by An-S, the water level of the Anguli-nuur Lake was apparently dropping during 7400–6900 cal. yr BP (6500–6100 yr BP), which somehow coincides with a rapid water level decline during 6230–5300 yr BP indicated by geomorphologic features around the lake (Qiu et al., 1999). A notable sand deflation process began at ∼ 6000 yr BP in Otindag Sand Plain (Fig. 2d) (e.g. Yang and Song, 1992). Corresponding to such an enhanced drought, wind activities apparently intensified around Anguli-nuur Lake during 6900–6400 cal. yr BP (6100–5600 yr BP). Further deterioration appeared around this lake afterwards. During 6400–2200 cal. yr BP, abundant coarse particles were blown in saltation into Anguli-nuur Lake by winds. Laminations of sediments imply that north and northwest winds were strong at this site during 5323–3244 yr BP (Zhai et al., 2002). Corresponding deteriorations have been inferred at other sites in the Inner Mongolia Plateau (Feng et al., 2006). The water level of Dali Nor Lake, the most northeast site, began to drop at 7000 yr BP (Geng and Zhang, 1988). The deterioration started later, respectively at 5600 and 4700 yr BP, and rather dry and cool conditions dominated during 5600–4500 yr BP and 4700–2100 yr BP around Haoluku and Liuzhouwan (Wang et al., 2001), the two small lakes lying further south. Around Lake Bayanchagan, the optimum conditions ended at 6500 cal. yr BP (Jiang et al., 2006). Around Daihai Lake, the most southwest site, environments began to deteriorate at 4450 cal. yr BP, and hence were relatively cool and dry during 4450–2900 cal. yr BP (Xiao et al., 2004). The deterioration might be shortly punctuated by a brief amelioration as inferred at several sites (Table 3). With the overwhelming dominance of arid conditions, a widespread reactivation of aeolian sand has persisted since ∼2500 yr BP in Otindag Sand Plain (Fig. 2d) (Yang and Song, 1992). The dry and cool conditions climaxed around Anguli–nuur Lake during 2200–480 cal. yr BP when it was extraordinarily windy and aeolian materials from Otindag Sand Plain reached or peaked at the lake as indicated by An-S. The sedimentary laminations indicate that north and northwest winds were strongly prevailing around this lake during 2449–1165 yr BP (Zhai et al., 2002). Extremely arid conditions were also inferred to have occurred around Haoluku and Liuzhouwan palaeo-lakes during 3000–1200 yr BP and after 700 yr BP (Wang et al., 2001). A cool and dry climate prevailed around Daihai Lake during 2900–1700 cal. yr BP (Xiao et al., 2004). A slight amelioration may have occurred following the severe cool and dry conditions in the Inner Mongolia Plateau (Table 3). After 480 cal. yr BP, wind activities began to wane around Anguli-nuur Lake as shown by An-S. Lamination of sediments from the lake suggests that the aeolian activities have weakened after 1165 yr BP (Zhai et al.,

2002). A similar change toward less cool and dry conditions has happened around Haoluku (Wang et al., 2001) after 1200 yr BP and Daihai Lake during 1700–1350 cal. yr BP (Xiao et al., 2004). As described above, An-S suggests a trend of palaeoenvironmental change around Anguli-nuur Lake broadly similar to that reconstructed by other researchers around the same lake and other sites in the Inner Mongolia Plateau. These changes can be somehow related to fluctuations in the southeast monsoon strength. Nevertheless, there were also differences in the timing of the environmental changes inferred for around these sites. In particularly, the onset and termination of the Holocene optimum around Anguli-nuur Lake appeared later than around Dali Nor Lake lying ∼ 300 km northeast of it (Table 3). The optimum started during 8000–7400 yr BP and ended during 5900–3000 yr BP at five of the seven sites in Inner Mongolia dealt with in a review on the Holocene climates (Feng et al., 2006). The onset of the optimum around Anguli-nuur Lake (at 8000 yr BP) seems coincident with those at the five sites, though its termination (at 6500 yr BP) around this lake appears to be earlier. There are several possible causes resulting in these differences in timing. The Holocene Climatic Optimum may have occurred almost contemporaneously in the Inner Mongolia Plateau and another two regions in China (Feng et al., 2006), and discrepancies in its timing at different sites may have been caused largely by problems in the validity or uncertainties of chronologies at some of these sites (Feng et al., 2006; Sun et al., 2006). So firstly, the apparent differences in the timing of the Holocene optimum around Anguli-nuur Lake and other sites may be attributed at least partly to uncertainties or errors in dating the sediments. Alternatively, the Holocene optimum may have started and ended earlier in northwest parts and later in southeast parts of China due to the southeastern migration of the maximum monsoon rainfall belt during the last ∼10,000 years (An et al., 2000; He et al., 2004). Thus secondly, some of these differences in timing, in particular, the apparently later occurrence of the optimum around Anguli-nuur Lake compared to that at Dali Nor Lake may be somehow a manifestation of the asynchronous nature of this climatic event. Additionally, due to the influence of local factors, the response of the landscape to regional climatic changes may differ, and timing of the reflection of the same or corresponding climate events thus may appear different in different sites even close to each other in semi-arid regions (e.g. Holmes et al., 1999; Wang et al., 2008). Hence thirdly, the apparent differences in timing of the Holocene optimum around Anguli-nuur Lake and other sites may be in part caused by the effects of some site-specific factors.

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7. Conclusion Mineral magnetism in combination with particle size, TOC, C/N and pollen composition of An-S has revealed the environmental changes occurring during the last ∼10,000 years around Anguli-nuur Lake. The environment began to ameliorate at 10,900 cal. yr BP (9600 yr BP). Conditions were thus relatively wet and warm during 10,900–8900 cal. yr BP (9600–8000 yr BP). The wettest and warmest or optimum conditions were during 8900–7400 cal. yr BP (8000– 6500 yr BP). The climate became less wet and warm during 7400– 6900 cal. yr BP (6500–6100 yr BP) and further deteriorated during 6900–6400 cal. yr BP (6100–5600 yr BP). From 6400 cal. yr BP (5600 yr BP) onwards, the environment has persistently deteriorated. The driest and coolest conditions prevailed during 2200–480 cal. yr BP. After 480 cal. yr BP conditions may have somewhat ameliorated. The environmental changes inferred from An-S are generally in agreement with what were previously reconstructed for Anguli-nuur Lake and other sites in the Inner Mongolia Plateau by other researchers though there are some discrepancies in timing. Acknowledgements This research is supported by a grant from the Natural Sciences Foundation of China (40771208). We are grateful to Mr. Qinglu Li and his staff for their help in coring sediments from Anguli-nuur Lake and Dr. Minghui Lu, Mr. Xubo Sun and Ms. Chunmei Li for their help in doing mineral-magnetic measurements. We have considerably benefited from discussions with Prof. Haiting Cui in preparing this manuscript. We also thank two anonymous reviewers, Professor A. Peter Kershaw and Dr. Fred Kop whose comments and suggestions greatly improved the manuscript. References An, Z., Porter, S.C., Kutzbach, J.E., Wu, X., Wang, S., Liu, X., Liu, X., Zhou, W., 2000. Asynchronous Holocene optimum of the East Asian monsoon. Quaternary Science Reviews 19, 743–762. Beuning, K.R.M., Talbot, M.R., Kelts, K., 1997. A revised 30, 000-year palaeoclimatic and palaeohydrologic history of Lake Albert, East Africa. Palaeogeography Palaeoclimatology Palaeoecology 136, 259–279. Chen, C.-T.A., Lan, H.-C., Lou, J.-Y., Chen, Y.-C., 2003. The dry Holocene Megathermal in Inner Mongolia. Palaeogeography Palaeoclimatology Palaeoecology 193, 181–200. Chen, J., Wan, G.J., Zhang, F., Zhang, D.D., Huang, R., 2004. Environmental records of lacustrine sediments in different time scales: sediment grain size as an example. Science in China (Series D) 47 (10), 954–960. Clarke, M.L., Rendell, H.M., 1998. Climate change impacts on sand supply and the formation of desert sand dunes in the southwest USA. Journal of Arid Environments 39, 517–531. Creer, K.M., Morris, A., 1996. Proxy-climate and geomagnetic palaeointensity records extending back to ca. 75, 000 BP derived from sediments cored from Lago Grande di Monticchio, southern Italy. Quaternary Science Reviews 15, 167–188. Dearing, J.A., 1999. Holocene Environmental Change from Magnetic Proxies in Lake Sediments. In: Maher, B.A., Thompson, R. (Eds.), Quaternary Climates, Environments and Magnetism. Cambridge University Press, Cambridge, pp. 231–278. Feng, Z.D., An, C.B., Wang, H.B., 2006. Holocene climatic and environmental changes in the arid and semi-arid areas of China: a review. The Holocene 16, 119–130. Foster, I.D.L., Oldfield, F., Flower, R.J., Keatings, K., 2008. Mineral magnetic signatures in a long core from Lake Qarun, Middle Egypt. Journal of Paleolimnology 40 (3), 835–849. Geng, K., Zhang, Z., 1988. Geomorphologic features and evolution of the Holocene lakes in Dali Nor Area, the Inner Mongolia. Journal of Beijing Normal University (Natural Science) 4, 94–100 (in Chinese). He, Y., Theakstone, W.H., Zhang, Z., Zhang, D., Yao, T., Chen, T., Shen, Y., Pang, H., 2004. Asynchronous Holocene climatic changes across China. Quaternary Research 61, 52–63. Holmes, J.A., Street-Perrot, F.A., Perrot, R.A., Stokes, S., Waller, M.P., Huang, Y., Eglinton, G., Ivanovich, M., 1999. Holocene landscape evolution of the Manga grasslands, NE Nigeria: evidence from palaeolimnology and dune chronology. Journal of the Geological Society, London 156, 357–368. Jiang, J., Wu, J., Shen, J., 2004. Lake sediment records of climatic and environmental change in Angulinao Lake. Scientia Geographica Sinica 24 (3), 346–351 (in Chinese with English abstract). Jiang, W., Guo, Z., Sun, X., Wu, H., Chu, G., Yuan, B., Hatte, C., Guiot, J., 2006. Reconstruction of climate and vegetation changes of Lake Bayanchagan (Inner Mongolia): Holocene variability of the East Asian monsoon. Quaternary Research 65, 411–420. Langbein, W.B., Schumm, S.A., 1958. Yield of sediment in relation to mean annual precipitation. Transactions American Geophysical Union 39, 1076–1084.

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