第 6 卷 第 4 期 地球环境学报 Vol.6 No.4 2015 年 8 月 Journal of Earth Environment Aug. 2015 doi:10.7515/JEE201504001

柴达木盆地西台吉乃尔盐湖区域晚更新世环境变化

曾方明 1，张 萍 2 （1. 中国科学院青海盐湖研究所，西宁 810008；2. 中国地质大学图书馆，武汉 430074）

摘 要：本文对柴达木盆地西台吉乃尔盐湖剖面沉积物（XT 剖面，240 cm 厚）的粒度、磁化率、 色度和总有机碳（TOC）等指标记录的晚更新世环境变化进行了研究。结果表明：（1）XT 剖面 沉积物的粒度总体较细（中值粒径变化范围为 3.4 ~ 10.0 μm），但是 25 ~ 47 cm 深和 185 ~ 199 cm 深的粒度明显变粗（中值粒径变化范围为 15.7 ~ 59.0 μm）。（2）XT 剖面沉积物的中值粒径与低

频磁化率（χlf）呈正相关关系；红度（a*）与低频磁化率呈负相关关系。（3）XT 剖面低频磁化 率与全球深海 δ18O 记录试探性的对比表明西台吉乃尔盐湖区域晚更新世的气候变化可能受全球 气候影响。 关键词：环境变化；更新世；柴达木盆地；西台吉乃尔盐湖 中图分类号：P534.631 文献标志码：A 文章编号：1674-9901(2015)04-0201-07

Late Pleistocene environmental change in the Xitaijinair salt lake region, Qaidam Basin

ZENG Fang-ming1, ZHANG Ping2 (1. Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China; 2. Library, China University of Geosciences, Wuhan 430074, China)

Abstract: This study investigated late Pleistocene environmental change indicated by proxies (grain size, magnetic susceptibility, color reflectance, and total organic carbon) of the 240 cm-deep XT section in the Xitaijinair salt lake region. The results suggest that: (1) Sediments in the XT section are generally fine (median grain size ranging from 3.4 μm to 10.0 μm), but are coarse at the depth of 25 ~ 47 cm and 185 ~ 199 cm (median grain size varying from 15.7 μm to 59.0 μm). (2) Median grain size is positively

correlated with low-frequency magnetic susceptibility (χlf). Redness a* is negatively correlated with χlf . 18 (3) Tentative comparison with χlf records in the XT section and δ O records in the global deep oceans shows that the Pleistocene climate change in the Xitaijinair salt lake region is possibly influenced by global climate. Key words: environmental change; Pleistocene; Qaidam Basin; Xitaijinair salt lake

As a production of extremely arid climate with al, 2014; Han et al, 2014; Wei et al, 2015). high ratio of evaporation to precipitation, salt lake Salt lakes are widely distributed in the Qaidam sediments record paleoenvironmental information. Basin, northeastern Qinghai-Tibetan Plateau (QTP). Methods of stratigraphy, sedimentology, geochemistry Since the 1980s, evolution of salt lakes and its and micropaleontology have been applied to reconstruct paleoenvironmental implications in the Qaidam the past environment change that is recorded in salt Basin were researched on drilling cores from Qarhan lake deposits (Cai et al, 2012; Wang et al, 2013; Fan et salt plain (Chen and Bowler, 1985; Huang and Chen,

Received Date: 2015-06-24 Foundation Item: Youth Guidance Foundation of ISL-CAS (Y460261051) Corresponding Author: Zeng Fang-ming, E-mail: [email protected] 202 地球环境学报 第 6 卷

1990). The evolution of salt lake in the Qaidam magnetic susceptibility, color reflectance, and total Basin has been significantly impacted by global organic carbon proxies. climate change during the past 730 ka (Huang and 1 Samples and methods Chen, 1990). Recently, a series of drill cores from Chaka salt lake (Liu et al, 2008), Dalangtan playa 1.1 Samples (Hou et al, 2010), Chahansilatu playa (SG-1) (Zhang Xitaijinair salt lake is located in the western et al, 2012), Qarhan salt lake (ISL1A) (Fan et al, Qaidam Basin. The lake basin is a closed basin. 2014), and Gahai Lake (DG-02) (He et al, 2014) Sands, silts, clay, and salts deposited during the late in the Qaidam Basin have been studied on past Pleistocene in the Xitaijinair region. environmental changes. XT section (37°40'42.69"N, 93°32'47.73"E, Compared to the above salt lakes and playas in 2692 m above sea level), in Xitaijinair salt lake the Qaidam Basin, study of the past environmental region, is outcropped by an excavator from surface change in the Xitaijinair salt lake is scarce. Sediment downward to a 240 cm depth (Fig.1). The lithology of the XT section (240 cm depth) in the Xitaijinair of the XT section from the top to the bottom is as salt lake region formed in the Pleistocene based on follows: 0 ~ 25 cm, red silt and clayey silt; 25 ~ 47 the OSL (optically stimulated luminescence) ages and cm, green sandy silt; 47 ~ 185 cm, red silt and clayey AMS (accelerator mass spectrometry) 14C ages (Zeng silt; 185 ~ 199 cm, green sandy silt interbedded and Xiang, 2015). This study aims to investigate with red silt; 199 ~ 240 cm, red silt (Zeng and the environmental change during the Pleistocene in Xiang, 2015). Bulk samples for proxy analysis were the Xitaijinair salt lake region based on grain size, collected at 2 cm intervals.

Fig.1 Location of the Xitaijinair section (XT section) in the western Qaidam Basin (after Zeng and Xiang, 2015)

1.2 Methods 69.1 ± 6.0 ka (at the depth of 190 cm) were obtained 14 OSL dating for two samples was performed on (Zeng and Xiang, 2015). AMS C dating for bulk an automated Risø TL/OSL DA-20 reader by method organic matter of nine samples ranging between 33— of Single Aliquot Regeneration (SAR) (Murray and 40 cal ka BP were analyzed in the Center for Applied Wintle, 2000). The quartz grain size for OSL dating is Isotope Studies at the University of Georgia, USA (Zeng 90 ~ 125 μm fractions. U, Th and K concentrations were and Xiang, 2015). measured by neutron activation analysis (NAA). Two Grain size analysis was tested by Beckman OSL ages of 57.9 ± 5.7 ka (at the depth of 40 cm) and Coulter LS 13 320 laser diffraction grain size analyzer 第 4 期 ZENG Fang-ming, et al: Late Pleistocene environmental change in the Xitaijinair salt lake region, Qaidam Basin 203 at School of Earth and Space Sciences, University of The standard sample GSS-7 (TOC value is Science and Technology of China (USTC), China. 0.64% ± 0.07%) was used in the experiment. The Sample about 0.1 ~ 0.2 g was pretreated by 10 mL 30% relative error of the TOC content is less than 5%. H O to remove organic matter and 10 mL 10% HCl to 2 2 2 Results and discussion remove carbonates. The relative error of the repeated measurement is less than 5%. 2.1 Environmental change indicated by proxies in Magnetic susceptibility (MS) was measured by the Xitaijinair region a Bartington MS2 magnetic susceptibility meter at The median grain size (Md), <2 μm content (%),

USTC. Low-frequency MS (χlf) and high-frequency low-frequency magnetic susceptibility (χlf), high-

MS (χhf) values of air-dried samples were measured frequency magnetic susceptibility (χhf), frequency- at 470 Hz and 4700 Hz, respectively. Frequency- dependent magnetic susceptibility (χfd), lightness (L*), dependent susceptibility (χfd) is calculated by equation redness (a*), yellowness (b*), and TOC content of the

χfd = ( χlf – χhf )/χlf ×100. sediments in the XT section are presented in Fig.2. Color reflectance was measured by a Konica Grain size is a common index for indicating Minolta CM-2500c spectrophotometer after the the energy of the wind and water dynamics that sample was ground carefully to powder at the Key transport particles from source area to depositional Laboratory of Qinghai-Tibetan Plateau Environment zone (Yang and Ding, 2008; An et al, 2012; Hao et and Resource, Qinghai Normal University, China. al, 2012). Median grain size is the medium value of Total organic carbon (TOC) content was the particle size distribution. The median grain size determined by rapid dichromate oxidation method is remarkably higher at the depth of 25 ~ 47 cm and (Walkley and Black, 1934). The procedure for 185 ~ 199 cm (median grain size varying from 15.7 μm determination of TOC content is as follows: to 59.0 μm) than other part (median grain size ranging (1) 0.2000 g sample (ground to less than 75 μm from 3.4 μm to 10.0 μm) in the XT section (Fig.2a), by agate mortar) was weighed into a 50 mL glass showing the strong energy for transporting particles. –1 tube; (2) 5 mL 0.8000 mol∙L 1/6 K2Cr2O7 solution, Meanwhile, the <2 μm content (%) proxy shows that

0.1 g Ag2SO4, and 5 mL concentrated H2SO4 were the lowest value occurred at the depth of 25 ~ 47 cm added; (3) When the temperature of oil bath was and 185 ~ 199 cm (Fig.2b). heated to 185℃ , the glass tube was put into the oil For lake sediments in Qinghai Lake, increased bath and kept boiling at 170 ~ 180 ℃ for 5 minutes; coarse grain size (>63 μm content) indicates (4) All materials were washed by deionized water strong large volume of river water flowed into to a 250 mL triangle bottle, and the volume of Qinghai Lake, in response to high precipitation the solution was 60 ~ 80 mL; (5) 5 mL phosphate in the catchment over the last 800 years (Zhang and four drops 1% diphenylamine sulfonate (as et al, 2003). However, increased coarse fraction indicator) were added to flask; (6) 0.2 mol∙L–1 (>25 μm) of Qinghai Lake sediments indicates dry

(NH4)2Fe(SO4)2 was used for redox titration. TOC condition resulted from intensified Westerlies in content of the salt lake sediment in the XT section this region during the past 32 ka (An et al, 2012). was calculated by following empirical formula: The above studies show that the controlling factors

TOC (%) = 1.236 × (V0 – V ) /(V0 × m). of coarse grains of sediments in the Qinghai Lake –1 Where V0 is the volume (mL) of 0.2 mol∙L are complicated. The climate in the Qaidam Basin –1 (NH4)2Fe(SO4)2 used for titration of 5 mL 0.8000 mol∙L is affected by the Westerlies and probably heavy

1/6 K2Cr2O7 in control group; V is the volume (mL) of rainfall during the past. Due to the complicated –1 0.2 mol∙L (NH4)2Fe(SO4)2 used for titration of excess controlling factors of coarse grains in the Xitaijinair –1 0.8000 mol∙L 1/6 K2Cr2O7 in the sample; m is mass salt lake, the climatic event indicated by coarse grain of the sample (g). size in the XT section requires further study. 204 地球环境学报 第 6 卷

Fig.2 Proxies of the sediments in the XT section (the shaded boxes show the coarse sediments)

Magnetic susceptibility is a proxy of precipitation The correlation between L* and TOC of the sediments and East Asian summer monsoon strength on the in the XT section demonstrated that the higher TOC Chinese Loess Plateau (An et al, 1991; Song et al, content, the lower value of L* (Fig.3a), in agreement 2014). Magnetic susceptibility was also applied with the previous study (Sun et al, 2011). to reconstruct the past climate change recorded TOC content of the sediments in the XT section in lake deposits (Chen et al, 1999; Yu and Kelts, is very low, varying from 0.14% to 0.82% (Fig.2i).

2002). Similar to the grain size, obviously high χlf It is interesting that the TOC content is positively 2 and χhf values occurred at the depth of 25 ~ 47 cm correlated with yellowness b* (R is 0.21) (Fig.3b). and 185 ~ 199 cm in the XT section (Fig.2c and 2d). For the sediments in the XT section, the median grain

The variations of χlf and χhf are very similar, with size is positively correlated with χlf with correlation 2 positive linear correlation coefficient R2 of 0.99. coefficient R of 0.67 (Fig.3c). It may indicate that

χfd is sensitive to ultra-fine magnetic grains at the coarse sediments containing more magnetite and/ superparamagnetic (SP)-stable single domain (SSD) or maghemite with higher magnetic susceptibility values are transported to the Xitaijinari lake basin, boundary (Song et al, 2014). The χfd values vary from while the fine sediments in the XT section may have 0.14% to 4.66% (Fig.2e). χfd values fluctuate more more hematite with lower susceptibility values. frequently than χlf and χhf values in the sediments of the XT section. The above speculation is confirmed by negatively 2 Color reflectance (Commission Internationale de correlation (R is 0.76) between redness a* and the χlf l′Eclairage, CIE, L*, a*, b*) of the sediments reflects (Fig.3d). the sedimentary environment and paleoclimatic All proxies indicate extremely climatic events condition (Ji et al, 2005). Investigations show that probably occur at the depth of 25 ~ 47 cm and the lightness (L*) of the sediment is related to TOC 185 ~ 199 cm in the XT section. However, the process content, redness (a*) is related to hematite content, and and reason of this extremely climatic event needs yellowness (b*) is influenced by content of goethite more lines of evidence in future. and pyrite minerals (Sun et al, 2011). L* values vary 2.2 Global climate forcing of the Pleistocene climate from 53.02 to 68.24 (Fig.2f), a* values range from 4.55 change in the Xitaijinair salt lake region to 11.50 (Fig.2g), and b* values change from 15.96 The paleoclimatic change recorded in core CK-6 to 22.45 (Fig.2h) for the sediments in the XT section. of the Qarhan salt lake was comparable with global 第 4 期 ZENG Fang-ming, et al: Late Pleistocene environmental change in the Xitaijinair salt lake region, Qaidam Basin 205 climatic change implicated by the deep sea sediments, the past 730 ka was controlled by the global climate and the climatic variation in the Qarhan area during change (Huang and Chen, 1990).

Fig.3 Linear correlations between (a) TOC content and L*, (b) TOC content and b*, (c) χlf and median grain size,

(d) χlf and redness a* of the sediments in the XT section. (R2: Pearson's correlation coefficient of linear regression. P: a level (2-tailed) of significance.)

Recently, OSL ages of high lake terraces The sketchy chronological framework of the and shorelines indicate that the highest lake level XT section is between 55 ka and 73 ka on the basis during the late Quaternary occurred at 130—71 ka of two OSL ages (Zeng and Xiang, 2015). It is [marine isotope stage (MIS) 5] in the northeastern noted that the accuracy of the OSL ages of the XT

QTP (Madsen et al, 2008; Fan et al, 2010; Rhode section is still debated. Interestingly, the χlf values et al, 2010; Madsen et al, 2013; Lai et al, 2014). of sediments in the XT section and δ18O records Additionally, the OSL ages of the marine core from the global deep oceans (Lisiecki and Raymo, sediments (Lz908 and BH2) from Bohai Sea indicate 2005) are roughly compared (Fig.4). Similar to the that the highest sea level occurred at MIS 5 since high lake levels in the northeastern QTP and high the late Quaternary (Yi et al, 2013). Therefore, it is sea level in north China occurred at MIS 5 were suggested that the late Quaternary climate change influenced by the global climate, this study indicates in the northeast QTP is in accord with the pattern of that the climate change in the Xitaijinair region global climate (Lai et al, 2014). during the late Pleistocene is also possibly forced by 14 Nine AMS C ages, ranging between 33 and 40 the global climate. This study supports the previous cal ka BP, are obviously younger than the two OSL study showing that the global climate influenced the 14 ages in the XT section (57.9 ka and 69.1 ka). The C climate change in the Qaidam Basin during the past ages of the XT section are probably underestimated 730 ka (Huang and Chen, 1990). (Zeng and Xiang, 2015). The 14C dating results of XT section support the conclusion that the 14C ages larger 3 Conclusions than ~ 25 ka BP are severe underestimates, especially (1) Grain size, magnetic susceptibility, color dates on samples from arid regions (Lai et al, 2014). reflectance indicate that the Pleistocene climate Therefore, the chronology of XT section cannot be change in Xitaijinair salt lake region fluctuates established by 14C ages. frequently. 206 地球环境学报 第 6 卷

18 Fig.4 Comparisons with low-frequency magnetic susceptibility (χlf) of sediments in the XT section and δ O records from the global deep oceans (Lisiecki and Raymo, 2005)

(2) Low-frequency magnetic susceptibility the Westerlies and Asian monsoon recorded in Lake Qinghai

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