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Now showing items 1 - 9 of 9

  • Vegetation and climate evolution during the Last Glaciation at Tengchong in Yunnan Province, Southwest China

    Zhang, Jixiao   Xu, Hai   Gosling, William D.   Lan, Jianghu   Dodson, John   Lu, Fengyan   Yu, Keke   Sheng, Enguo   Liu, Bin  

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  • Late Holocene Indian summer monsoon precipitation history at Lake Lugu, northwestern Yunnan Province, southwestern China

    Sheng, Enguo   Yu, Keke   Xu, Hai   Lan, Jianghu   Liu, Bin   Che, Shuai  

    Highlights • We obtained a robust age model by using the 137Cs and radiocarbon dates. • Remarkable aridity prevailed during the MWP and last 200 years in the study region. • Drier hydroclimatic conditions prevailed during the MWP over most of ISM areas. • There is asynchronous hydroclimatic variability during the MWP in ASM region. Abstract Knowledge of late Holocene precipitation variability in the northwestern Yunnan Province is important for understanding the Indian summer monsoon (ISM) dynamics. In this study, we present a proxy record of the ISM precipitation history at Lake Lugu covering the last ~ 2,900 years based on grain size and carbon/nitrogen (C/N) ratio in the northwestern Yunnan Province. The results show centennial variations in ISM precipitation, with two remarkably dry intervals from 750 to 1167 AD and from 1733 AD to the present and two relatively wet intervals from 898 BC to 750 AD and from 1167 to 1733 AD. A comparison between the record at Lake Lugu and the pollen record at Lake Erhai (both in the northwestern Yunnan Province) suggests that dry climate conditions prevailed during the Medieval Warm Period (MWP) and over the last 100–200 years, whereas relatively humid conditions prevailed during the Little Ice Age (LIA). These characteristics are generally similar to those of the climate patterns in extensive areas dominated by the ISM (e.g., southwestern-southern China, southeastern-southern Tibetan Plateau, northeastern India–Himalayas, southern Oman, and equatorial eastern Africa), but anti-phased with the climate patterns in the wide areas dominated by the East Asian summer monsoon (EASM) (e.g., northern-northeastern China and north-central Japan). We speculate that both the variations in the sea surface temperature (SST) over the tropical Indo-Pacific Ocean, the ocean–atmosphere coupling processes and the migration of the mean position of the Intertropical Convergence Zone (ITCZ) should be responsible for the hydroclimatic contrasts over the Asian summer monsoon (ASM) region on centennial timescales during the last 2,000 years.
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  • Holocene peatland development along the eastern margin of the Tibetan Plateau

    Xu, Hai   Liu, Bin   Lan, Jianghu   Sheng, Enguo   Che, Shuai   Xu, Sheng  

    Knowledge of peatland initiation, accumulation, and decline or cessation is critical in understanding peatland development and the related carbon source/sink effect. In this study, we investigated the development of three peat profiles along the eastern margin of the Tibetan Plateau (ETP) and compared the results with those of our previous work along this transect. Our work showed that the initiation over the northern ETP is later and the slowdown/cessation earlier than in the middle to southern ETP. The timing of optimum peatland formation over the northern ETP lags the Holocene climatic optimum. These spatio-temporal differences are likely to be related to the intensity of Asian summer monsoon. Our work suggests that some peatlands along the ETP transect have returned or are now returning their previously captured carbon to the atmosphere and thus act as carbon sources. Some peatlands still have net accumulation at present, but the rates have been reduced concomitant with the decreasing summer monsoon intensity. We speculate that more of the previously stored carbon in the ETP peatlands will be re-emitted to the atmosphere if the aridity continues, as might occur under a continuous global-warming scenario. (C) 2013 Published by Elsevier Inc. on behalf of University of Washington.
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  • Climate changes reconstructed from a glacial lake in High Central Asia over the past two millennia

    Lan, Jianghu   Xu, Hai   Sheng, Enguo   Yu, Keke   Wu, Huixian   Zhou, Kangen   Yan, Dongna   Ye, Yuanda   Wang, Tianli  

    Climatic changes in Arid Central Asia (ACA) over the past two millennia have been widely concerned. However, less attention has been paid to those in the High Central Asia (HCA), where the Asian water tower nurtures the numerous oases by glacier and/or snow melt. Here, we present a new reconstruction of the temperature and precipitation change over the past two millennia based on grain size of a well-dated glacial lake sediment core in the central of southern Tianshan Mountains. The results show that the glacial lake catchment has experienced cold-wet climate conditions during the Dark Age Cold Period (similar to 300-600 AD; DACP) and the Little Ice Age (similar to 1300-1870 AD; LIA), whereas warm-dry conditions during the Medieval Warm Period (similar to 700-1270 AD; MWP). Integration of our results with those of previously published lake sediment records, stalagmite delta O-18 records, ice core net accumulation rates, tree-ring based temperature reconstructions, and mountain glacier activities suggest that there has a broadly similar hydroclimatic pattern over the HCA areas on centennial time scale during the past two millennia. Comparison between hydroclimatic pattern of the HCA and that of the ACA areas suggests a prevailing 'warm-dry and cold-wet' hydroclimatic pattern over the whole westerlies-dominated central Asia areas during the past two millennia. We argue that the position and intensity of the westerlies, which are closely related to the phase of the North Atlantic Oscillation (NAO), and the strength of the Siberian High pressure (SH), could have jointly modulated the late Holocene central Asia hydroclimatic changes. (C) 2017 Elsevier Ltd and INQUA. All rights reserved.
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  • Abrupt Holocene Indian Summer Monsoon failures: A primary response to solar activity?

    Xu, Hai   Yeager, Kevin M.   Lan, Jianghu   Liu, Bin   Sheng, Enguo   Zhou, Xinying  

    Knowledge of the millennial abrupt monsoon failures is critical to understanding the related causes. Here, we extracted proxy indices of Indian Summer Monsoon (ISM) intensity during the early to mid-Holocene, from peat deposits at Lake Xihu, in southwestern China. There are a series of abrupt, millennial-scale episodes of ISM weakening inferred from the Lake Xihu records, which are generally synchronous with those inferred from other archives over ISM areas. An important feature is that the ISM failures inferred from the Lake Xihu proxy indices synchronize well with abrupt changes in solar activity. We argue that changes in solar activity play a primary role in producing most of these millennial ISM failures, while some other causes, including freshwater outbursts into the North Atlantic Ocean and changes in sea surface temperatures of the eastern tropical Pacific Ocean, may have also exerted influences on parts of the millennial ISM failures.
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  • Modern carbon burial in Lake Qinghai, China

    Xu, Hai   Lan, Jianghu   Liu, Bin   Sheng, Enguo   Yeager, Kevin M.  

    The quantification of carbon burial in lake sediments, and carbon fluxes derived from different origins are crucial to understand modern lacustrine carbon budgets, and to assess the role of lakes in the global carbon cycle. In this study, we estimated carbon burial in the sediment of Lake Qinghai, the largest inland lake in China, and the carbon fluxes derived from different origins. We find that: (1) The organic carbon burial rate in lake sediment is approximately 7.23 g m (2) a (1), which is comparable to rates documented in many large lakes worldwide. We determined that the flux of riverine particulate organic carbon (POC) is approximately 10 times higher than that of dissolved organic carbon (DOC). Organic matter in lake sediments is primarily derived from POC in lake water, of which approximately 80% is of terrestrial origin. (2) The inorganic carbon burial rate in lake sediment is slightly higher than that of organic carbon. The flux of riverine dissolved inorganic carbon (DIC) is approximately 20 times that of DOC, and more than 70% of the riverine DIC is drawn directly and/or indirectly from atmospheric CO2. (3) Both DIC and DOC are concentrated in lake water, suggesting that the lake serves as a sink for both organic and inorganic carbon over long term timescales. (4) Our analysis suggests that the carbon burial rates in Lake Qinghai would be much higher in warmer climatic periods than in cold ones, implying a growing role in the global carbon cycle under a continued global warming scenario. (C) 2013 Elsevier Ltd. All rights reserved.
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  • Climate change and soil erosion in a small alpine lake basin on the Loess Plateau,China

    Yu, Keke   Xu, Hai   Lan, Jianghu   Sheng, Enguo   Liu, Bin   Wu, Huixian   Tan, Liangcheng   Yeager, Kevin M.  

    Multi-proxy indices retrieved from sediments in Lake Chaonaqiu, an alpine lake on the western Loess Plateau (LP) of China, were used to reconstruct a precipitation history over the last similar to 300 years. The results correlate well with records from tree rings and historical documents in neighboring regions. We show that the lake oscillated between two states, i.e. wetter climatic conditions, which favored denser vegetation cover, and promoted weaker catchment soil erosion; and drier climatic conditions, which lead to less vegetation coverage, correlate with stronger surface soil erosion. Several intensive soil erosion events were identified in the sediment cores, and most of these occurred during decadal/multi-decadal dry periods, and correlate well with flood events documented in historical literature. The results of this study show that soil erosion by flood events is particularly intense during dry periods, and further highlights the role of vegetation cover in the conservation of water and soil in small lake basins on the Chinese LP. Copyright (c) 2016 John Wiley & Sons, Ltd.
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  • Late Holocene hydroclimatic variation in central Asia and its response to mid-latitude Westerlies and solar irradiance

    Lan, Jianghu   Zhang, Jin   Cheng, Peng   Ma, Xiaolin   Ai, Li   Chawchai, Sakonvan   Zhou, Kang'en   Wang, Tianli   Yu, Keke   Sheng, Enguo   Kang, Shugang   Zang, Jingjie   Yan, Dongna   Wang, Yaqin   Tan, Liangcheng   Xu, Hai  

    Hydroclimatic variations in central Asia are widely recognized to be tele-connected with the North Atlantic climate via the mid-latitude Westerlies. However, the long-term hydroclimatic variation and its response to North Atlantic climate changes are not fully understood. Here we report multi-decadal and centennial hydroclimatic variations in central Asia during the late Holocene using high-resolution oxygen and carbon isotopes of bulk carbonate from a remote hydrologically closed alpine lake in Tianshan Mountains, northwestern China. The hydroclimatic variations inferred from the covariance between delta O-18(carb) and delta C-13(carb) and published multi-proxy in the Lake Sayram together agree well with the records from the central Asia, showing enhanced effective moisture at the intervals of 4000-3780, 3590-3210, 2800-2160, 1700-1370 and 890-280 cal yr BP during the negative phase of North Atlantic Oscillation (NAO). Comparison with European hydroclimatic changes on centennial scale, our records not only provide valuable insight into the hydroclimatic variations in central Asia, but also indicate all broadly contemporaneous changes with that of southern Europe whereas antiphase with that of northern Europe. We then conclude that the observed antiphase hydroclimatic pattern between northern Europe and southern Europe-central Asia on centennial time scale during the late Holocene were persistently controlled by the north-south migration of mid-latitude Westerlies and changes of NAO phase in response to solar irradiance forcing. However, further research is necessary to fully disentangle the natural and anthropogenic forcing mechanisms for enhanced effect moisture and/or precipitation in central Asia. (C) 2020 Elsevier Ltd. All rights reserved.
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  • Late Holocene hydroclimatic variations and possible forcing mechanisms over the eastern Central Asia

    Lan, Jianghu   Xu, Hai   Yu, Keke   Sheng, Enguo   Zhou, Kangen   Wang, Tianli   Ye, Yuanda   Yan, Dongna   Wu, Huixian   Cheng, Peng   Abuliezi, Waili   Tan, Liangcheng  

    Hydroclimatic variations over the eastern Central Asia are highly sensitive to changes in hemispheric-scale atmospheric circulation systems. To fully understand the long-term variability and relationship between hydroclimate and atmospheric circulation system, we present a high-resolution lascustrine record of late Holocene hydroclimate from Lake Sayram, Central Tianshan Mountains, China, based on the total organic carbon, total nitrogen, and carbonate contents, carbon/nitrogen ratios, and grain size. Our results reveal four periods of substantially increased precipitation at the interval of 4000-3780, 3590-3210, 2800-2160, and 890-280 cal yr BP, and one period of slightly increased precipitation from 1700-1370 cal yr BP. These wetter periods broadly coincide with those identified in other records from the mid-latitude Westerlies-dominated eastern Central Asia, including the northern Tibetan Plateau. As such, a similar hydroclimatic pattern existed over this entire region during the late Holocene. Based on a close similarity of our record with reconstruction of North Atlantic Oscillation indices and solar irradiance, we propose that decreased solar irradiance and southern migration of the entire circum-North Atlantic circulation system, particularly the main pathway of the mid-latitude Westerlies, significantly influenced hydroclimate in eastern Central Asia during the late Holocene. Finally, the inferred precipitation at Lake Sayram has increased markedly over the past 100 years, although this potential future changes in hydroclimate in Central Asia need for further investigation.
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