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Geochemical characterization of a Holocene aeolian profile in the Zhongba area (southern Tibet, China) and its paleoclimatic implications
Aeolian Research 20 (2016) 169–175
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Geochemical characterization of a Holocene aeolian profile in the Zhongba area (southern Tibet, China) and its paleoclimatic implications Tuoyu Li a,b, Yongqiu Wu a,c,⇑, Shisong Du c, Wenmin Huang c, Chengzhi Hao c, Chao Guo c, Mei Zhang c, Tianyang Fu c a b c
State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, Beijing 100875, China Editorial Department of Journal, Capital Normal University, Beijing 100048, China MOE Engineering Center of Desertification and Blown-sand Control, Beijing Normal University, Beijing 100875, China
a r t i c l e
i n f o
Article history: Received 19 February 2015 Revised 20 January 2016 Accepted 22 January 2016
Keywords: Aeolian deposits in southern Tibet OSL and 14C dating Geochemistry Paleoclimatic reconstruction
a b s t r a c t The Zhongba area lies in the valley of the Maquan River in southern Tibet, where there are both strong modern aeolian activities and ancient aeolian sand sediments. A Holocene aeolian sand and paleosol profile in the Zhongba area was selected for study and termed (Zhuzhu (ZZ) profile). The chronology of the ZZ profile was established by optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) 14C dating. Based on the grain size and geochemical elements of the ZZ profile, the geochemical characterization was analyzed, the Holocene aeolian activity processes were reconstructed in the study area, and the paleoclimatic implications were discussed. The major elements and the chemical indicators are highly correlated with different grain-sizes in the ZZ profile. The evolutionary sequence of the aeolian activities and the paleoclimate in Holocene reveal four stages: before 7.3 ka BP, the climate was warm and wet with weak winds when the sand paleosol developed; at 7.3–3.8 ka BP, the climate turned dry, with strong aeolian activities; at 3.8–0.7 ka BP, the climate became wetter and the winds weakened when the silt paleosol developed; and since 0.7 ka BP, it was cold and dry with strong aeolian activities. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Aeolian deposits are an important part of the Earth’s surface depositional system, and they are mainly distributed in low- and mid-latitude dry land environments. Aeolian deposits (dust emissions, sand transport, and dune mobility) are all strongly influenced by antecedent precipitation, which affects soil moisture and vegetation cover (Lancaster, 2013). Some studies indicated that aeolian processes coincided with drier conditions (McGowan et al., 2008; White et al., 2013; Liu et al., 2014). Aeolian deposits are a widespread occurrence on the Qinghai-Tibetan Plateau in China, where much attention has been paid to the aeolian deposits as a valuable climate archive (Kaiser et al., 2009; Liu et al., 2012a, 2013a; Yu and Lai, 2014; Pan et al., 2014; Zhang et al., 2015). However, the aeolian deposits in the southwest corner of Tibet merit more attention. ⇑ Corresponding author at: D Building, Beijing Normal University Science Park, Beijing 100875, China. Tel.: +86 10 62207162. E-mail addresses: [email protected] (T. Li), [email protected] (Y. Wu), [email protected] (S. Du), [email protected] (W. Huang), [email protected] (C. Hao), [email protected] (C. Guo), [email protected] (M. Zhang), [email protected] (T. Fu). http://dx.doi.org/10.1016/j.aeolia.2016.01.005 1875-9637/Ó 2016 Elsevier B.V. All rights reserved.
The Holocene climate change in southern Tibet can be divided into three periods according to previous researches (Gu et al., 1993; Gasse et al., 1996; Huang, 2000; Tang et al., 2000; Zhu et al., 2009; Lu et al., 2011; Guo et al., 2014). In the early Holocene (10.0–8.0 ka BP), the climate turned warm and humid with rapid climate fluctuations. The middle Holocene Epoch (8.0–3.0 ka BP) included two stages, in the early stage (8.0–6.0 ka BP), the climate was warmest and wettest; in the subsequent stage (6.0–3.0 ka BP), the climate turned from warm and humid to cool and dry. In the late Holocene (since 3.0 ka BP), the climate turned cold and dry. The geochemical characterization of aeolian deposits in the Qinghai-Tibetan Plateau has been studied to reconstruct the paleoclimate (Liu, et al., 2012b, 2013a), but there are few studies on the geochemical characterization of aeolian deposits in southern Tibet. Our study focuses on the Zhongba area in the valley of the Maquan River of southwestern Tibet, which contains extensive aeolian deposits. We selected an aeolian sand and paleosol profile (Zhuzhu profile) in the Zhongba area and established its chronology using optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) 14C dating. Using grain size and geochemical parameters, we analyzed the geochemical characterization of these aeolian deposits, reconstructed the Holocene aeolian activity
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processes in the Zhongba area, and discussed their paleoclimatic implications.
2. Site location and lithostratigraphy The Zhongba area is located in the valley of the Maquan River, in the upper reaches of the Yarlung Zangbo River in southwestern Tibet (Fig. 1). This area is situated between the Gangdise Mountains and the Himalayan Mountains, with a mean elevation above 4700 m. The middle of this area contains the Maquan River and its alluvial plain. The climate type of this study area is subfrigid and semi-arid. The annual mean temperature of the Zhongba area is 0.3 °C, and the annual mean precipitation is 290.8 mm (Li, 1999). Land desertification is serious in the Zhongba area, and the desertified land is distributed across the valley of the Maquan River (Li et al., 2001). Due to the serious desertification, the nearby Zhongba County government has been relocated several times in recent decades. Based on the field investigation of the Holocene aeolian deposits in the Zhongba area, the Zhuzhu (ZZ) profile (29°510 59.8800 N, 83°470 13.1400 E, Fig. 1) was selected as a typical aeolian sand and paleosol profile. The ZZ profile is located in the southern foot of its north mountain, where there are some aeolian deposits. The ZZ profile is 3.1 m thick with a paleosol-aeolian sequence that can be divided into nine sedimentary units (Fig. 2):
Unit A: 0–70 cm of a very pale brown (10YR7/3), fine sand unit with plant roots; Unit B: 70–110 cm of a pale brown (10YR6/3), sand paleosol unit with plant roots; Unit C: 110–150 cm of a light, yellowish brown (10YR6/4), fine sand unit with plant roots; Unit D: 150–186 cm of a yellowish brown (10YR5/4), silt paleosol unit with plants’ roots; Unit E: 186–200, a light, yellowish brown (10YR6/4), sand paleosol unit; Unit F: 200–228 cm of a yellowish brown (10YR5/4), silt paleosol unit; Unit G: 228–256 cm of a light, yellowish brown (10YR6/4), coarse sand unit; Unit H: 256–278 cm of a yellowish brown (10YR5/4), sand paleosol unit; Unit I: 278–310 cm of a light, yellowish brown (10YR6/4), medium sand unit.
3. Sampling and analytical procedures Three OSL dating samples were collected from the ZZ profile. All OSL samples were collected using metal tubes of 4 cm diameter and 30 cm long. The tubes were horizontally hammered into the profile; after removal, their ends were sealed with aluminum foil and tape. The OSL samples were measured at the School of Archaeology and Museology, Peking University. The OSL samples
Fig. 1. Location of the sample profile.
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Fig. 2. Stratigraphy of the ZZ profile with OSL and AMS
preparation was done under subdued red light in the laboratory’s dark room. The sediments at the two ends of the tubes, which may have been exposed to light during sampling, were removed and reserved for U, Th and K measurements. The samples were treated with 10% H2O2 to remove organic material, and then dilute HCl to dissolve carbonates. The fine-grain (4–11 lm) was separated, and then treated with silica saturated fluorosilicic acid (H2SiF6) for 3 days at room temperature to dissolve feldspars, followed by a treatment with 10% HCl to remove any produced fluorides. The chemically purified fine-grained quartz was prepared for luminescence measurements. The single-aliquot regenerative-dose procedure (SAR) was used to measure the equivalent dose (De) of the quartz extracts. U, Th and K contents of the OSL sediment samples were determined at the Nuclear Physics Institute of China Institute of Atomic Energy. The OSL sampled water contents cannot represent their in situ water contents because of the strong wind weather in Zhongba area, so the water content for each sample was estimated as 10%. Five AMS 14C dating samples were collected from the ZZ profile. All AMS 14C samples were horizontally collected at a thickness of 5–10 cm. The AMS 14C samples (dating the bulk organic matter) were also measured at the School of Archaeology and Museology, Peking University. The IntCal13 curve was used in calculating all dates, with the calibration performed using Calib 7.0 (http:// calib.qub.ac.uk/calib/calib.html). We also collected 155 sediment samples at 2 cm intervals from the ZZ profile for grain size and geochemical element analysis. The grain size measurements were carried out using a Mastersizer 2000 with a size range of 0.02–2000 lm at the MOE Key Laboratory of Environmental Change and Natural Disasters, Beijing Normal University. The samples were measured after pre-treatment with H2O2 (10%) to remove organic matter, HCl (10%) to eliminate secondary carbonates, and (NaPO3)6 to disperse the particles (Lu and An, 1998). The geochemical elements were determined by wavelength dispersion X-ray fluorescence spectrometry at the Cold and Arid Regions Environmental and Engineering Research Institute. The procedure was as follows: samples were dried, ground, and sifted through a 200-mesh screen, were crushed 4 g
14
C ages.
powdered samples into round discs (32 mm in diameter) with 30 tons of pressure for analysis. 4. Results 4.1. Chronology The dating results are shown in Tables 1 and 2. The 14C dates are in agreement with the OSL dates with the exception of ZZ-14C-1, whose minimum range is suspect due to the impingement of the end of the calibration data set. According to the dating results, the sediments of the Zhuzhu profile were continuous section from 70 cm (ZZ-OSL-2, 210 ± 80 yr.) to 258 cm (ZZ-14C-5, 7622 ± 41 cal. yr. BP), but a discontinuity is evident from 258 cm (ZZ-14C-5, 7622 ± 41 cal. yr. BP) and 300 cm (ZZ-OSL-6, 23930 ± 5440 yr.). This sedimentary hiatus is either due to poor vegetation cover erosion in cold, dry, and windy climatic conditions, or it is due to the erosion by glaciofluvial outwash (Sun et al., 2007). The chronological framework (Fig. 2) was calculated based on the grain-size model (Porter and An, 1995), this model assumed that the aeolian sedimentation flux was proportional to grain size, and expressed the age, which can reduce the chronology error between paleosol unit and sand unit. The dating results of the A–G sedimentary units are in Fig.2. 4.2. Grain size, major element abundances and chemical indicators of the ZZ profile Sand particle contents, and the major elements of the ZZ profile are listed in Table 3. The grain size in the ZZ profile can be divided into three populations: a very fine population (clay and silt, <63 lm), a medium population (very fine sand and fine sand, 63–250 lm), and a coarse population (medium sand and coarse sand, >250 lm) (Fig. 3). The average of the very fine population is 16.0%, with a range of 3.3–50.7%, the averages in units D–F and H are above 20%, and the others are below 20%. The average of the medium population is 57.8%, with a range of 3.8–91.6%, the averages in units A–C are
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Table 1 The OSL ages for the ZZ profile. Lab. No.
Sample. No.
Depth(cm)
Sediment type
Dose rate/Gy ka1
De/Gy
Age(yr)
20140401 20140402 20140403
ZZ-OSL-2 ZZ-OSL-5 ZZ-OSL-6
70 240 300
Fine sand Coarse sand Medium sand
4.17 ± 0.16 3.62 ± 0.14 4.00 ± 0.16
0.86 ± 0.33 16.6 ± 3.16 95.7 ± 21.4
210 ± 80 4580 ± 890 23930 ± 5440
Table 2 The AMS
14
C ages for the ZZ profile.
Lab. No. BA131806 BA131809 BA131807 BA131810 BA131811 *
Sample. No. 14
ZZ- C-1 ZZ-14C-2 ZZ-14C-3 ZZ-14C-4 ZZ-14C-5
14
Calibrated age 2r (cal. yr BP)
C age (14C yr BP)
Depth(cm)
Sediment type
AMS
80–90 160–170 197–204 217–224 255–260
Fine sand Fine sand Medium sand and silt Silt and coarse sand Coarse sand and medium sand
185 ± 30 870 ± 20 2425 ± 25 3090 ± 20 6765 ± 25
151 ± 149* 814 ± 84 2523 ± 168 3303 ± 61 7622 ± 41
This standard deviation (error) includes a lab error multiplier.
Table 3 Grain size and geochemical parameters in sedimentary units of the ZZ profile. Unit
Number
A B C D E F G H I The whole profile
35 20 20 18 7 14 14 11 16 155
<63 lm
63–250 lm
>250 lm
81.8 82.2 76.3 54.4 44.5 33.8 16.0 32.4 36.2 57.8 91.6 3.76
9.78 6.76 11.4 21.6 30.3 31.2 74.1 45.9 48.6 26.2 91.5 0.77
% Average Average Average Average Average Average Average Average Average Average Max Min
8.45 11.1 12.3 24.1 25.2 35.0 9.95 21.7 15.2 16.0 50.7 3.27
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
10.9 11.2 10.9 10.7 10.6 11.1 8.86 9.59 9.76 10.5 12.0 8.48
1.74 1.89 1.81 1.92 1.83 2.06 1.12 1.60 1.62 1.74 2.60 0.97
1.96 1.75 1.69 1.26 1.07 1.05 0.75 0.83 0.98 1.41 2.23 0.66
0.60 0.65 0.60 0.55 0.47 0.57 0.26 0.37 0.39 0.52 0.78 0.22
3.32 3.37 3.29 3.23 3.22 3.28 3.09 3.08 3.03 3.24 3.48 2.97
2.82 2.82 2.73 2.44 2.37 2.21 1.98 2.01 2.11 2.48 2.94 1.87
%
all above 70%, the average in unit G is 16.0%, and the others are between 30% and 60%. The average of the coarse population is 26.2%, with a range of 0.8–91.5%, the averages in units A–F are below 40%, and the others are above 40%. Both the medium population and the coarse population are local transport, which are sensitive to strong aeolian activity as they are higher in aeolian sand unit than that of paleosol. The very fine population is from far distant sources, and it is sensitive to weak aeolian activity, so it is higher in paleosol than that of the aeolian sand unit (Visher, 1969; Pye, 1987; Liu et al., 1997; Pan et al., 2014). These major element contents of the ZZ profile change with the coarse population and the medium population. The SiO2 contents show a positive correlation with the coarse population, the correlation coefficient is 0.933. The contents of Al2O3, Fe2O3, MgO, and K2O are the opposite of the coarse population, the correlation coefficients are 0.900, 0.659, 0.880 and 0.814. The contents of CaO and Na2O show the positive correlation with the medium population, the correlation coefficients are 0.952 and 0.981. The chemical indicators are shown in Fig. 4. The SiO2/(Al2O3 + Fe2O3) ratios are low in units A–F, with a range of 9.63–12.4, but high in units G–I, with a range of 12.7–16.4. The SiO2/(Al2O3 + Fe2O3) ratios are the lowest in unit F, and the highest in unit G. The CaO/MgO ratios are relatively lower in units D–F and H, the averages are 1.65, 1.63, 1.41 and 1.62, respectively. The CaO/MgO ratios are higher in units A–C, G, and I, the averages are 2.37, 1.92, 2.05, 2.12, and 1.82, respectively. The ratios of Ti/Sr are relatively higher in units D–F, H, and I; the averages are 15.8, 15.0, 17.2, 15.3, and 15.8, respectively. The ratios
79.3 78.5 79.7 80.9 81.2 80.3 87.3 84.5 84.5 81.2 88.1 76.6
of Ti/Sr are relatively lower in units A–C, and G; the averages are 10.7, 11.3, 12.4, and 12.0, respectively. The chemical index of alteration (CIA) is an important index for indicating sediment weathering (Nesbitt and Young, 1982). CIA = [Al2O3/(Al2O3 + CaO⁄ + K2O + Na2O)] 100, where the chemical oxide values are a molecular formulae. The CaO⁄ is the amount of CaO incorporated into the silicate fraction, if the molar CaO/Na2O ratio is more than one, mCaO⁄ = mNa2O, if the molar CaO/Na2O ratio is less than one, mCaO⁄ = mCaO (McLennan, 1993). The ratios of CIA are relatively higher in units D–F and H, the averages are 53.3, 53.3, 55.2, and 54.1, respectively. The ratios of CIA are relatively lower in units A–C, G, and I, the averages are 48.1, 49.3, 50.3, 52.7, and 53.3. There is a strong positive correlation between the SiO2/ (Al2O3 + Fe2O3) ratio and the coarse population, and the correlation coefficient is 0.911. The ratios of CaO/MgO and Ti/Sr show high correlation with the very fine population, the correlation coefficients are 0.859 and 0.853. There is a negative correlation between CIA and the medium population and a positive correlation between CIA and the very fine population, the correlation coefficients are 0.875 and 0.755. 4.3. Holocene climatic changes in the Zhongba area Based on the characteristics of the grain size and the geochemical elements in the ZZ profile, along with the OSL and 14C dating ages, the history of Holocene climatic changes and aeolian activity were reconstructed for the Zhongba area.
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Fig. 3. Variations in grain size, and other major elements of the ZZ profile.
Fig. 4. Variation of geochemical parameters and very fine population with depth in the ZZ profile.
(1) Before 7.3 ka BP, the lithology consisted of a yellowish brown medium sand paleosol, and the very fine population was relatively high, indicating strong pedogenesis intensity and weak aeolian activity. The CaO/MgO ratio was relatively low, and the Ti/Sr ratio and the CIA were relatively high, implying a relatively warm and wet climate. (2) In 7.3–3.8 ka BP, the stratum was composed of light, yellowish brown, coarse sand, and the coarse population sharply increased, which indicated strong aeolian activity. The highest SiO2/(Al2O3 + Fe2O3) ratio, the highest CaO/MgO ratio, the relatively low Ti/Sr ratio, and the relatively low CIA all reflected a cold and dry climate.
(3) In 3.8–0.7 ka BP, the silt paleosol was developed with the highest fine population, which indicated strongest pedogenesis intensity and the weakest aeolian activity. The other proxies, such as the lowest CaO/MgO ratio, the highest Ti/Si ratio, and the highest CIA all reflected a warm and humid climate. However, in 2.5–1.9 ka BP, the sand paleosol was developed, the SiO2/(Al2O3 + Fe2O3) ratio and the CaO/MgO ratio obviously increased, and the Ti/Sr ratio and the CIA significantly decreased, implying the climate turned little cold and dry. In 1.9–0.7 ka BP, the yellowish brown silt paleosol was developed with a lower fine population than that of 3.8–2.5 ka BP. The CaO/MgO ratio was higher, and
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the Ti/Sr ratio and the CIA were lower than that of 3.8–2.5 ka BP, which implied a less warm and humid climate. (4) Since 0.7 ka BP, the lithological units were comprised of fine sand with the highest medium population and the highest deposition rate, indicating the wind weakened than that in 7.3–3.8 ka BP, but more frequent windy weather. The Ti/Sr ratio and the CIA were the lowest, implying a cold and dry climate. However, in 0.5–0.2 ka BP, the lithology consisted of pale brown sand paleosol, probably indicating restored vegetation coverage and a slightly wetter climate than that of 0.7–0.5 ka BP. 5. Discussion The lithology composed of sand paleosol before 7.3 ka BP and the climatic indices indicated a relatively warm and humid climate in the Zhongba area. There was also sand paleosol development in southern Tibet during 9.3–6.6 ka BP and approximately 7.7 ka BP (Pan et al., 2014; Lai et al., 2009). The analysis of lake sediments in the southern Tibet indicated that the climate was more humid and warmer before 7.3 ka BP (Van Campo and Gasse, 1993; Gasse et al., 1996; Zhu et al., 2009). However, some studies showed the climate was warm but relatively dry (Wang et al., 2002; Ma et al., 2014). Aeolian activity was strong during 7.3–3.8 ka BP, and the climate was cold and dry in the Zhongba area. Previous studies showed that intermittent aeolian deposits were distributed in the southern Tibetan Plateau during 7.7–3.5 ka BP (Liu et al.,2013b; Pan et al., 2014), which indicated that the climate was rather dry. Many studies on lake sediments in southern Tibet showed the climate became drier after 6.0 ka BP (Gasse et al., 1996;Wang et al., 2002), the reflection of the climate change in lake sediment was late than that of aeolian deposit. The paleosols developed from 3.8 to0.7 ka BP, indicating a warm and humid climate, but it was a cold and dry event from 2.5 to 1.9 ka BP. There were also paleosols developed in southern Tibet and northeastern Tibet during this time (Lai et al., 2009; Liu et al., 2012a). However, the paleoclimatic proxies of the lake sediments in southern Tibet showed that the climate turned dry since 3.0 ka BP (Tang et al., 2000; Wang et al., 2002; Zhang et al., 2015). But some paleoclimatic records of lake sediments and peat showed humidity event happened from 3.8 to 0.7 ka BP (Gu et al., 1993; Tang et al., 2000; Wang et al., 2002;Yu et al., 2006). Some human activities also showed the climate was appropriate (Cui et al., 1995; Chen et al., 2015), human adapted to live at altitudes above 3000 m in the Tibetan Plateau after 3.6 ka BP. Since 0.7 ka BP, the aeolian activity has strengthened, and the climate has been dry and cold, certainly, humans impact in the form of overgrazing by domestic animals and firewood gathering can be accounted for the aeolian activities (Kaiser et al., 2006). During 0.5–0.2 ka, there was sand paleosol development, but the sediment weathering was not strong, this might have been a Little Ice Age where the climate was cold but relatively wet. 6. Conclusion We analyzed the geochemical characterization and grain size in the paleosol-aeolian sand profile to reconstruct the history of Holocene climate changes and aeolian sand activities in the Zhongba area. Our main conclusions are as follows: (1) The major elements and the chemical indicators vary considerably among different grain size fractions in the ZZ profile. There are high correlations between fine population and CaO/MgO, Ti/Sr, CIA, which are relatively good paleoclimatic indicators in aeolian deposits area.
(2) The climate changes and aeolian activities of the Holocene in the Zhongba area have the local features: The aridification event during 7.4–3.8 ka BP was earler than lacustrine records and the aeolian activities lasted longer than other aeolian deposits area. In 3.8–2.5 ka BP, the climate was optimum, which might be a regional character, there were many evidences on natural sediments and human activities.
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Geochemical characterization of a Holocene aeolian profile in the Zhongba area (southern Tibet, China) and its paleoclimatic implications Tuoyu Li a,b, Yongqiu Wu a,c,⇑, Shisong Du c, Wenmin Huang c, Chengzhi Hao c, Chao Guo c, Mei Zhang c, Tianyang Fu c a b c
State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, Beijing 100875, China Editorial Department of Journal, Capital Normal University, Beijing 100048, China MOE Engineering Center of Desertification and Blown-sand Control, Beijing Normal University, Beijing 100875, China
a r t i c l e
i n f o
Article history: Received 19 February 2015 Revised 20 January 2016 Accepted 22 January 2016
Keywords: Aeolian deposits in southern Tibet OSL and 14C dating Geochemistry Paleoclimatic reconstruction
a b s t r a c t The Zhongba area lies in the valley of the Maquan River in southern Tibet, where there are both strong modern aeolian activities and ancient aeolian sand sediments. A Holocene aeolian sand and paleosol profile in the Zhongba area was selected for study and termed (Zhuzhu (ZZ) profile). The chronology of the ZZ profile was established by optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) 14C dating. Based on the grain size and geochemical elements of the ZZ profile, the geochemical characterization was analyzed, the Holocene aeolian activity processes were reconstructed in the study area, and the paleoclimatic implications were discussed. The major elements and the chemical indicators are highly correlated with different grain-sizes in the ZZ profile. The evolutionary sequence of the aeolian activities and the paleoclimate in Holocene reveal four stages: before 7.3 ka BP, the climate was warm and wet with weak winds when the sand paleosol developed; at 7.3–3.8 ka BP, the climate turned dry, with strong aeolian activities; at 3.8–0.7 ka BP, the climate became wetter and the winds weakened when the silt paleosol developed; and since 0.7 ka BP, it was cold and dry with strong aeolian activities. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Aeolian deposits are an important part of the Earth’s surface depositional system, and they are mainly distributed in low- and mid-latitude dry land environments. Aeolian deposits (dust emissions, sand transport, and dune mobility) are all strongly influenced by antecedent precipitation, which affects soil moisture and vegetation cover (Lancaster, 2013). Some studies indicated that aeolian processes coincided with drier conditions (McGowan et al., 2008; White et al., 2013; Liu et al., 2014). Aeolian deposits are a widespread occurrence on the Qinghai-Tibetan Plateau in China, where much attention has been paid to the aeolian deposits as a valuable climate archive (Kaiser et al., 2009; Liu et al., 2012a, 2013a; Yu and Lai, 2014; Pan et al., 2014; Zhang et al., 2015). However, the aeolian deposits in the southwest corner of Tibet merit more attention. ⇑ Corresponding author at: D Building, Beijing Normal University Science Park, Beijing 100875, China. Tel.: +86 10 62207162. E-mail addresses: [email protected] (T. Li), [email protected] (Y. Wu), [email protected] (S. Du), [email protected] (W. Huang), [email protected] (C. Hao), [email protected] (C. Guo), [email protected] (M. Zhang), [email protected] (T. Fu). http://dx.doi.org/10.1016/j.aeolia.2016.01.005 1875-9637/Ó 2016 Elsevier B.V. All rights reserved.
The Holocene climate change in southern Tibet can be divided into three periods according to previous researches (Gu et al., 1993; Gasse et al., 1996; Huang, 2000; Tang et al., 2000; Zhu et al., 2009; Lu et al., 2011; Guo et al., 2014). In the early Holocene (10.0–8.0 ka BP), the climate turned warm and humid with rapid climate fluctuations. The middle Holocene Epoch (8.0–3.0 ka BP) included two stages, in the early stage (8.0–6.0 ka BP), the climate was warmest and wettest; in the subsequent stage (6.0–3.0 ka BP), the climate turned from warm and humid to cool and dry. In the late Holocene (since 3.0 ka BP), the climate turned cold and dry. The geochemical characterization of aeolian deposits in the Qinghai-Tibetan Plateau has been studied to reconstruct the paleoclimate (Liu, et al., 2012b, 2013a), but there are few studies on the geochemical characterization of aeolian deposits in southern Tibet. Our study focuses on the Zhongba area in the valley of the Maquan River of southwestern Tibet, which contains extensive aeolian deposits. We selected an aeolian sand and paleosol profile (Zhuzhu profile) in the Zhongba area and established its chronology using optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) 14C dating. Using grain size and geochemical parameters, we analyzed the geochemical characterization of these aeolian deposits, reconstructed the Holocene aeolian activity
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processes in the Zhongba area, and discussed their paleoclimatic implications.
2. Site location and lithostratigraphy The Zhongba area is located in the valley of the Maquan River, in the upper reaches of the Yarlung Zangbo River in southwestern Tibet (Fig. 1). This area is situated between the Gangdise Mountains and the Himalayan Mountains, with a mean elevation above 4700 m. The middle of this area contains the Maquan River and its alluvial plain. The climate type of this study area is subfrigid and semi-arid. The annual mean temperature of the Zhongba area is 0.3 °C, and the annual mean precipitation is 290.8 mm (Li, 1999). Land desertification is serious in the Zhongba area, and the desertified land is distributed across the valley of the Maquan River (Li et al., 2001). Due to the serious desertification, the nearby Zhongba County government has been relocated several times in recent decades. Based on the field investigation of the Holocene aeolian deposits in the Zhongba area, the Zhuzhu (ZZ) profile (29°510 59.8800 N, 83°470 13.1400 E, Fig. 1) was selected as a typical aeolian sand and paleosol profile. The ZZ profile is located in the southern foot of its north mountain, where there are some aeolian deposits. The ZZ profile is 3.1 m thick with a paleosol-aeolian sequence that can be divided into nine sedimentary units (Fig. 2):
Unit A: 0–70 cm of a very pale brown (10YR7/3), fine sand unit with plant roots; Unit B: 70–110 cm of a pale brown (10YR6/3), sand paleosol unit with plant roots; Unit C: 110–150 cm of a light, yellowish brown (10YR6/4), fine sand unit with plant roots; Unit D: 150–186 cm of a yellowish brown (10YR5/4), silt paleosol unit with plants’ roots; Unit E: 186–200, a light, yellowish brown (10YR6/4), sand paleosol unit; Unit F: 200–228 cm of a yellowish brown (10YR5/4), silt paleosol unit; Unit G: 228–256 cm of a light, yellowish brown (10YR6/4), coarse sand unit; Unit H: 256–278 cm of a yellowish brown (10YR5/4), sand paleosol unit; Unit I: 278–310 cm of a light, yellowish brown (10YR6/4), medium sand unit.
3. Sampling and analytical procedures Three OSL dating samples were collected from the ZZ profile. All OSL samples were collected using metal tubes of 4 cm diameter and 30 cm long. The tubes were horizontally hammered into the profile; after removal, their ends were sealed with aluminum foil and tape. The OSL samples were measured at the School of Archaeology and Museology, Peking University. The OSL samples
Fig. 1. Location of the sample profile.
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Fig. 2. Stratigraphy of the ZZ profile with OSL and AMS
preparation was done under subdued red light in the laboratory’s dark room. The sediments at the two ends of the tubes, which may have been exposed to light during sampling, were removed and reserved for U, Th and K measurements. The samples were treated with 10% H2O2 to remove organic material, and then dilute HCl to dissolve carbonates. The fine-grain (4–11 lm) was separated, and then treated with silica saturated fluorosilicic acid (H2SiF6) for 3 days at room temperature to dissolve feldspars, followed by a treatment with 10% HCl to remove any produced fluorides. The chemically purified fine-grained quartz was prepared for luminescence measurements. The single-aliquot regenerative-dose procedure (SAR) was used to measure the equivalent dose (De) of the quartz extracts. U, Th and K contents of the OSL sediment samples were determined at the Nuclear Physics Institute of China Institute of Atomic Energy. The OSL sampled water contents cannot represent their in situ water contents because of the strong wind weather in Zhongba area, so the water content for each sample was estimated as 10%. Five AMS 14C dating samples were collected from the ZZ profile. All AMS 14C samples were horizontally collected at a thickness of 5–10 cm. The AMS 14C samples (dating the bulk organic matter) were also measured at the School of Archaeology and Museology, Peking University. The IntCal13 curve was used in calculating all dates, with the calibration performed using Calib 7.0 (http:// calib.qub.ac.uk/calib/calib.html). We also collected 155 sediment samples at 2 cm intervals from the ZZ profile for grain size and geochemical element analysis. The grain size measurements were carried out using a Mastersizer 2000 with a size range of 0.02–2000 lm at the MOE Key Laboratory of Environmental Change and Natural Disasters, Beijing Normal University. The samples were measured after pre-treatment with H2O2 (10%) to remove organic matter, HCl (10%) to eliminate secondary carbonates, and (NaPO3)6 to disperse the particles (Lu and An, 1998). The geochemical elements were determined by wavelength dispersion X-ray fluorescence spectrometry at the Cold and Arid Regions Environmental and Engineering Research Institute. The procedure was as follows: samples were dried, ground, and sifted through a 200-mesh screen, were crushed 4 g
14
C ages.
powdered samples into round discs (32 mm in diameter) with 30 tons of pressure for analysis. 4. Results 4.1. Chronology The dating results are shown in Tables 1 and 2. The 14C dates are in agreement with the OSL dates with the exception of ZZ-14C-1, whose minimum range is suspect due to the impingement of the end of the calibration data set. According to the dating results, the sediments of the Zhuzhu profile were continuous section from 70 cm (ZZ-OSL-2, 210 ± 80 yr.) to 258 cm (ZZ-14C-5, 7622 ± 41 cal. yr. BP), but a discontinuity is evident from 258 cm (ZZ-14C-5, 7622 ± 41 cal. yr. BP) and 300 cm (ZZ-OSL-6, 23930 ± 5440 yr.). This sedimentary hiatus is either due to poor vegetation cover erosion in cold, dry, and windy climatic conditions, or it is due to the erosion by glaciofluvial outwash (Sun et al., 2007). The chronological framework (Fig. 2) was calculated based on the grain-size model (Porter and An, 1995), this model assumed that the aeolian sedimentation flux was proportional to grain size, and expressed the age, which can reduce the chronology error between paleosol unit and sand unit. The dating results of the A–G sedimentary units are in Fig.2. 4.2. Grain size, major element abundances and chemical indicators of the ZZ profile Sand particle contents, and the major elements of the ZZ profile are listed in Table 3. The grain size in the ZZ profile can be divided into three populations: a very fine population (clay and silt, <63 lm), a medium population (very fine sand and fine sand, 63–250 lm), and a coarse population (medium sand and coarse sand, >250 lm) (Fig. 3). The average of the very fine population is 16.0%, with a range of 3.3–50.7%, the averages in units D–F and H are above 20%, and the others are below 20%. The average of the medium population is 57.8%, with a range of 3.8–91.6%, the averages in units A–C are
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Table 1 The OSL ages for the ZZ profile. Lab. No.
Sample. No.
Depth(cm)
Sediment type
Dose rate/Gy ka1
De/Gy
Age(yr)
20140401 20140402 20140403
ZZ-OSL-2 ZZ-OSL-5 ZZ-OSL-6
70 240 300
Fine sand Coarse sand Medium sand
4.17 ± 0.16 3.62 ± 0.14 4.00 ± 0.16
0.86 ± 0.33 16.6 ± 3.16 95.7 ± 21.4
210 ± 80 4580 ± 890 23930 ± 5440
Table 2 The AMS
14
C ages for the ZZ profile.
Lab. No. BA131806 BA131809 BA131807 BA131810 BA131811 *
Sample. No. 14
ZZ- C-1 ZZ-14C-2 ZZ-14C-3 ZZ-14C-4 ZZ-14C-5
14
Calibrated age 2r (cal. yr BP)
C age (14C yr BP)
Depth(cm)
Sediment type
AMS
80–90 160–170 197–204 217–224 255–260
Fine sand Fine sand Medium sand and silt Silt and coarse sand Coarse sand and medium sand
185 ± 30 870 ± 20 2425 ± 25 3090 ± 20 6765 ± 25
151 ± 149* 814 ± 84 2523 ± 168 3303 ± 61 7622 ± 41
This standard deviation (error) includes a lab error multiplier.
Table 3 Grain size and geochemical parameters in sedimentary units of the ZZ profile. Unit
Number
A B C D E F G H I The whole profile
35 20 20 18 7 14 14 11 16 155
<63 lm
63–250 lm
>250 lm
81.8 82.2 76.3 54.4 44.5 33.8 16.0 32.4 36.2 57.8 91.6 3.76
9.78 6.76 11.4 21.6 30.3 31.2 74.1 45.9 48.6 26.2 91.5 0.77
% Average Average Average Average Average Average Average Average Average Average Max Min
8.45 11.1 12.3 24.1 25.2 35.0 9.95 21.7 15.2 16.0 50.7 3.27
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
10.9 11.2 10.9 10.7 10.6 11.1 8.86 9.59 9.76 10.5 12.0 8.48
1.74 1.89 1.81 1.92 1.83 2.06 1.12 1.60 1.62 1.74 2.60 0.97
1.96 1.75 1.69 1.26 1.07 1.05 0.75 0.83 0.98 1.41 2.23 0.66
0.60 0.65 0.60 0.55 0.47 0.57 0.26 0.37 0.39 0.52 0.78 0.22
3.32 3.37 3.29 3.23 3.22 3.28 3.09 3.08 3.03 3.24 3.48 2.97
2.82 2.82 2.73 2.44 2.37 2.21 1.98 2.01 2.11 2.48 2.94 1.87
%
all above 70%, the average in unit G is 16.0%, and the others are between 30% and 60%. The average of the coarse population is 26.2%, with a range of 0.8–91.5%, the averages in units A–F are below 40%, and the others are above 40%. Both the medium population and the coarse population are local transport, which are sensitive to strong aeolian activity as they are higher in aeolian sand unit than that of paleosol. The very fine population is from far distant sources, and it is sensitive to weak aeolian activity, so it is higher in paleosol than that of the aeolian sand unit (Visher, 1969; Pye, 1987; Liu et al., 1997; Pan et al., 2014). These major element contents of the ZZ profile change with the coarse population and the medium population. The SiO2 contents show a positive correlation with the coarse population, the correlation coefficient is 0.933. The contents of Al2O3, Fe2O3, MgO, and K2O are the opposite of the coarse population, the correlation coefficients are 0.900, 0.659, 0.880 and 0.814. The contents of CaO and Na2O show the positive correlation with the medium population, the correlation coefficients are 0.952 and 0.981. The chemical indicators are shown in Fig. 4. The SiO2/(Al2O3 + Fe2O3) ratios are low in units A–F, with a range of 9.63–12.4, but high in units G–I, with a range of 12.7–16.4. The SiO2/(Al2O3 + Fe2O3) ratios are the lowest in unit F, and the highest in unit G. The CaO/MgO ratios are relatively lower in units D–F and H, the averages are 1.65, 1.63, 1.41 and 1.62, respectively. The CaO/MgO ratios are higher in units A–C, G, and I, the averages are 2.37, 1.92, 2.05, 2.12, and 1.82, respectively. The ratios of Ti/Sr are relatively higher in units D–F, H, and I; the averages are 15.8, 15.0, 17.2, 15.3, and 15.8, respectively. The ratios
79.3 78.5 79.7 80.9 81.2 80.3 87.3 84.5 84.5 81.2 88.1 76.6
of Ti/Sr are relatively lower in units A–C, and G; the averages are 10.7, 11.3, 12.4, and 12.0, respectively. The chemical index of alteration (CIA) is an important index for indicating sediment weathering (Nesbitt and Young, 1982). CIA = [Al2O3/(Al2O3 + CaO⁄ + K2O + Na2O)] 100, where the chemical oxide values are a molecular formulae. The CaO⁄ is the amount of CaO incorporated into the silicate fraction, if the molar CaO/Na2O ratio is more than one, mCaO⁄ = mNa2O, if the molar CaO/Na2O ratio is less than one, mCaO⁄ = mCaO (McLennan, 1993). The ratios of CIA are relatively higher in units D–F and H, the averages are 53.3, 53.3, 55.2, and 54.1, respectively. The ratios of CIA are relatively lower in units A–C, G, and I, the averages are 48.1, 49.3, 50.3, 52.7, and 53.3. There is a strong positive correlation between the SiO2/ (Al2O3 + Fe2O3) ratio and the coarse population, and the correlation coefficient is 0.911. The ratios of CaO/MgO and Ti/Sr show high correlation with the very fine population, the correlation coefficients are 0.859 and 0.853. There is a negative correlation between CIA and the medium population and a positive correlation between CIA and the very fine population, the correlation coefficients are 0.875 and 0.755. 4.3. Holocene climatic changes in the Zhongba area Based on the characteristics of the grain size and the geochemical elements in the ZZ profile, along with the OSL and 14C dating ages, the history of Holocene climatic changes and aeolian activity were reconstructed for the Zhongba area.
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Fig. 3. Variations in grain size, and other major elements of the ZZ profile.
Fig. 4. Variation of geochemical parameters and very fine population with depth in the ZZ profile.
(1) Before 7.3 ka BP, the lithology consisted of a yellowish brown medium sand paleosol, and the very fine population was relatively high, indicating strong pedogenesis intensity and weak aeolian activity. The CaO/MgO ratio was relatively low, and the Ti/Sr ratio and the CIA were relatively high, implying a relatively warm and wet climate. (2) In 7.3–3.8 ka BP, the stratum was composed of light, yellowish brown, coarse sand, and the coarse population sharply increased, which indicated strong aeolian activity. The highest SiO2/(Al2O3 + Fe2O3) ratio, the highest CaO/MgO ratio, the relatively low Ti/Sr ratio, and the relatively low CIA all reflected a cold and dry climate.
(3) In 3.8–0.7 ka BP, the silt paleosol was developed with the highest fine population, which indicated strongest pedogenesis intensity and the weakest aeolian activity. The other proxies, such as the lowest CaO/MgO ratio, the highest Ti/Si ratio, and the highest CIA all reflected a warm and humid climate. However, in 2.5–1.9 ka BP, the sand paleosol was developed, the SiO2/(Al2O3 + Fe2O3) ratio and the CaO/MgO ratio obviously increased, and the Ti/Sr ratio and the CIA significantly decreased, implying the climate turned little cold and dry. In 1.9–0.7 ka BP, the yellowish brown silt paleosol was developed with a lower fine population than that of 3.8–2.5 ka BP. The CaO/MgO ratio was higher, and
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the Ti/Sr ratio and the CIA were lower than that of 3.8–2.5 ka BP, which implied a less warm and humid climate. (4) Since 0.7 ka BP, the lithological units were comprised of fine sand with the highest medium population and the highest deposition rate, indicating the wind weakened than that in 7.3–3.8 ka BP, but more frequent windy weather. The Ti/Sr ratio and the CIA were the lowest, implying a cold and dry climate. However, in 0.5–0.2 ka BP, the lithology consisted of pale brown sand paleosol, probably indicating restored vegetation coverage and a slightly wetter climate than that of 0.7–0.5 ka BP. 5. Discussion The lithology composed of sand paleosol before 7.3 ka BP and the climatic indices indicated a relatively warm and humid climate in the Zhongba area. There was also sand paleosol development in southern Tibet during 9.3–6.6 ka BP and approximately 7.7 ka BP (Pan et al., 2014; Lai et al., 2009). The analysis of lake sediments in the southern Tibet indicated that the climate was more humid and warmer before 7.3 ka BP (Van Campo and Gasse, 1993; Gasse et al., 1996; Zhu et al., 2009). However, some studies showed the climate was warm but relatively dry (Wang et al., 2002; Ma et al., 2014). Aeolian activity was strong during 7.3–3.8 ka BP, and the climate was cold and dry in the Zhongba area. Previous studies showed that intermittent aeolian deposits were distributed in the southern Tibetan Plateau during 7.7–3.5 ka BP (Liu et al.,2013b; Pan et al., 2014), which indicated that the climate was rather dry. Many studies on lake sediments in southern Tibet showed the climate became drier after 6.0 ka BP (Gasse et al., 1996;Wang et al., 2002), the reflection of the climate change in lake sediment was late than that of aeolian deposit. The paleosols developed from 3.8 to0.7 ka BP, indicating a warm and humid climate, but it was a cold and dry event from 2.5 to 1.9 ka BP. There were also paleosols developed in southern Tibet and northeastern Tibet during this time (Lai et al., 2009; Liu et al., 2012a). However, the paleoclimatic proxies of the lake sediments in southern Tibet showed that the climate turned dry since 3.0 ka BP (Tang et al., 2000; Wang et al., 2002; Zhang et al., 2015). But some paleoclimatic records of lake sediments and peat showed humidity event happened from 3.8 to 0.7 ka BP (Gu et al., 1993; Tang et al., 2000; Wang et al., 2002;Yu et al., 2006). Some human activities also showed the climate was appropriate (Cui et al., 1995; Chen et al., 2015), human adapted to live at altitudes above 3000 m in the Tibetan Plateau after 3.6 ka BP. Since 0.7 ka BP, the aeolian activity has strengthened, and the climate has been dry and cold, certainly, humans impact in the form of overgrazing by domestic animals and firewood gathering can be accounted for the aeolian activities (Kaiser et al., 2006). During 0.5–0.2 ka, there was sand paleosol development, but the sediment weathering was not strong, this might have been a Little Ice Age where the climate was cold but relatively wet. 6. Conclusion We analyzed the geochemical characterization and grain size in the paleosol-aeolian sand profile to reconstruct the history of Holocene climate changes and aeolian sand activities in the Zhongba area. Our main conclusions are as follows: (1) The major elements and the chemical indicators vary considerably among different grain size fractions in the ZZ profile. There are high correlations between fine population and CaO/MgO, Ti/Sr, CIA, which are relatively good paleoclimatic indicators in aeolian deposits area.
(2) The climate changes and aeolian activities of the Holocene in the Zhongba area have the local features: The aridification event during 7.4–3.8 ka BP was earler than lacustrine records and the aeolian activities lasted longer than other aeolian deposits area. In 3.8–2.5 ka BP, the climate was optimum, which might be a regional character, there were many evidences on natural sediments and human activities.
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