| 摘要: |
| 受西风控制的天山地区黄土广泛分布，是中亚重要的黄土分布区之一。这些陆相风成沉积物为研究中亚干旱区气候环境变化历史提供了重要的信息载体。本文选取天山北麓博乐地区一个厚为12.7 m的黄土剖面作为研究对象，分析了其磁化率、粒度指标的变化特征，并探讨了其古气候意义。结果表明：（1）博乐黄土粒度组成以粉砂为主，是典型的风成沉积物，成壤作用很弱；（2）博乐黄土磁化率变化主要受控于风力强度；（3）剖面粒度的环境敏感粒级为<31.7 μm、31.7 — 282.5 μm，分别可能代表风暴过后的浮尘堆积和风暴过程中风力近距离搬运的沉积物组分；（4）博乐黄土MIS（海洋氧同位素）3阶段记录了多次千年尺度气候事件，能与北大西洋Heinrich事件和D-O事件对应，表明中纬度西风环流在传递北大西洋信号到东亚的过程中扮演着重要角色。 |
| 关键词: 黄土 磁化率 粒度 敏感组分 气候事件 天山 |
| DOI：10.7515/JEE182009 |
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| 基金项目:国家自然科学基金项目（41572162）；中国科学院国际合作局对外合作重点项目（132B61KYS20160002）；中国科学院俄乌白项目 |
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| Magnetic susceptibility and grain size records of Bole loess section in the northern piedmont of Tianshan Mountains and their implications for paleoclimatic changes |
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SUN Huanyu, SONG Yougui, LI Yue, CHEN Xiuling, OROZBAEV Rustam
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1. State Key Laboratory Breeding Base of Humid Subtropical Mountain Ecology, School of Geographical Sciences, Fujian Normal University, Fuzhou 350007, China
2. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
3. CAS Research Center for Ecology and Environment Central Asia, Urumqi 830011, China
4. College of Earth Science, University of Chinese Academy of Sciences, Beijing 100049, China
5. Institute of Geology, National Academy of Sciences of Kyrgyz Republic, Bishkek 720040, Kyrgyzstan
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| Abstract: |
| Background, aim, and scope Loess-paleosol sequences provide key aeolian archives for Quaternary climate changes. Loess as an aeolian sediment are closely related to climate changes during mobilization, transportation and deposition process. Grain size (GS) and magnetic susceptibility (MS) of loess sediments are regarded as important paleoclimatic proxies in the Chinese Loess Plateau. Loess sediments are also extensively distributed in westerly-dominated Tianshan Mountains area. However, their paleoclimate proxies are not fully understood by now. In this paper, we selected a loess section from north piedmont of Tianshan Mountains to investigate the paleoclimatic significances of these proxies. This section provides an important archive for climatic and environmental change history in the arid Central Asia. Materials and methods Bole loess section has a thickness of 12.7 m. A total of 635 samples were collected at 2 cm intervals for GS and MS analyses. GS distribution was analyzed using a Malvern 2000 laser instrument, and calculated for 100 grain size classes within a measuring range of 0.02 — 2000 μm. MS was measured with a Bartington MS2 meter both low- (0.47 kHz) and high- (4.7 kHz) frequency magnetic susceptibility (χlf and χhf, respectively). All measurements were completed at the State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences. Seventeen age samples were measured in the Xi’an AMS facility, and all radiocarbon dates were calibrated to calendar years. The visual curve matching method is used to establish age-depth model of this section, due to deficiencies and shortcomings of 14C age. The grain size-standard deviation method here has been employed to extract the environmental sensitive components. Results The results show that (1) Variations in χlf range from 38.8×10−8 m3 ∙ kg−1 to 84.7×10−8 m3 ∙ kg−1 with an average of 59.52×10−8 m3∙kg−1. Frequency-dependent magnetic susceptibility ( χfd) and frequency-dependent magnetic susceptibility ( χfd%) yield consistently low values throughout the BL section; (2) The BL loess is dominated by fine silt-sized particles (21.69% — 69.65%, with an average of 44.58%). Coarse silt-sized particles vary from 17.95% to 40.60%, averaging 31.68%. Sand fraction component account for 6.78% — 40.5%, averaging of 17.33%. BL loess contains minor clay-sized particles (1.66% — 14.69%, 6.41%); (3) The 14C ages almost stop increasing by ca. 25 cal ka BP at ~1 m depth). (4) Two sensitive components (<31.7 µm and 31.7 — 282.5 µm) were distinguished. Discussion Variations in χfd and χfd% throughout the BL section suggest weak pedogenesis, and MS is more strongly influenced by allogenetic magnetic minerals than pedogenesis. Therefore, wind strength may control on the MS enhancement of this section. The BL loess GS is dominated by silt-sized particles, suggesting a typical aeolian sediment. Two sensitive components represent suspension after dust storm and proximal deposit component mainly transported by dust storms, respectively. We compare the 31.7 — 282.5 µm sensitive component to δ18O records from Greenland ice core and the July solar insolation at 65°N based the age-depth model, variations in grain size of this section indicate good correlations to the global climate change especially millennial scale climate events during MIS3. Conclusions The BL loess is a typical aeolian sediment, which experiences very weak pedogenesis. MS variabilities are controlled by wind strength, and wind vigor model can apply to explain the MS enhancement of the BL loess. Two sensitive components (<31.7 µm and 31.7 — 282.5 µm) represent suspension after dust storm and proximal deposit component mainly transported by dust storms, respectively. Our GS results also support that the Westerlies plays an important role in transmitting the signal from the North Atlantic to the East Asia. Recommendations and perspectives This study is helpful to understand the climate and environment change in this area. Because of complexity and uncertainty of paleoclimatic proxies there. it is necessary to carry out multidisciplinary comprehensive analyses and pay more attention to their mutual verification. |
| Key words: loess magnetic susceptibility grain size sensitive components climate events Tianshan Mountains |
