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Now showing items 1 - 16 of 17

  • A large carbon pool in lake sediments over the arid/semiarid region, NW China

    Lan, Jianghu   Xu, Hai   Liu, Bin   Sheng, Enguo   Zhao, Jiangtao   Yu, Keke  

    Carbon burial in lake sediments is an important component of the global carbon cycle. However, little is known about the magnitude of carbon sequestered in lake sediments over the arid/semiarid region of China (ASAC). In this study, we estimate both organic and inorganic carbon burial since ~ad 1800 based on nine lakes in ASAC, and discuss the most plausible factors controlling carbon burial. Our estimates show that the annual organic carbon burial rate (OCBR) ranges from 5.3 to 129.8 g cm−2 year−1 (weighted mean of 49.9 g cm−2 year−1), leading to a standing stock of 1.1–24.0 kg cm−2 (weighted mean of 8.6 kg cm−2) and a regional sum of ~108 Tg organic carbon sequestered since ~ad 1800. The annual inorganic carbon burial rate (ICBR) ranges from 11.4 to 124.0 g cm−2 year−1 (weighted mean of 48.3 g cm−2 year−1), which is slightly lower than OCBR. The inorganic carbon standing stock ranges from 2.4 to 26.0 kg cm−2 (weighted mean of 8.1 kg cm−2), resulting in a sum of ~101 Tg regional inorganic carbon burial since ~ad 1800, which is slightly lower than the organic carbon sequestration. OCBR in ASAC shows a continuously increasing trend since ~ad 1950, which is possibly due to the high autochthonous and allochthonous primary production and subsequently high sedimentation rate in the lakes. This increasing carbon burial is possibly related to both climatic changes and enhanced anthropogenic activities, such as land use change, deforestation, and eutrophication in the lake. Furthermore, OCBR and ICBR are expected to continuously increase under the scenario of increasing precipitation and runoff and enhanced anthropogenic activities. The results of this research show that the buried carbon in lake sediments of the ASAC region constitutes a significant and large carbon pool, which should be considered and integrated into the global carbon cycle.
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  • Solar activity and the westerlies dominate decadal hydroclimatic changes over arid Central Asia

    Yan, Dongna   Xu, Hai   Lan, Jianghu   Zhou, Kangen   Ye, Yuanda   Zhang, Jixiao   An, Zhisheng   Yeager, Kevin M.  

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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  

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  • Responses of cyanobacteria to climate and human activities at Lake Chenghai over the past 100 years

    Yan, Dongna   Xu, Hai   Yang, Ming   Lan, Jianghu   Hou, Weiguo   Wang, Fushun   Zhang, Jixiao   Zhou, Kangen   An, Zhisheng   Goldsmith, Yonaton  

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  • 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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  • 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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  • Responses of cyanobacteria to climate and human activities at Lake Chenghai over the past 100 years

    Yan, Dongna   Xu, Hai   Yang, Ming   Lan, Jianghu   Hou, Weiguo   Wang, Fushun   Zhang, Jixiao   Zhou, Kangen   An, Zhisheng   Goldsmith, Yonaton  

    Cyanobacteria play important roles in carbon and nutrient cycling in aquatic systems. Cyanobacteria biomass in most lakes has increased over recent decades, threatening both water quality and ecosystem management. However, due to lack of long-term monitoring, the factors that control the interannual variability and diversity of cyanobacteria are poorly known. In this study, we used genetics and geochemical record of a vertical-down sediment core to investigate the impacts of climate changes and human activities on the long-term (100-year) abundance and diversity of cyanobacteria at Lake Chenghai, southwestern China. The results show that before 1980 CE, the nutrient level of Lake Chenghai was in a generally natural state, and human impacts were relatively weak; whereas after 1980 CE the cyanobacterial biomass inferred from both qPCR and Miseq sequencing have significantly increased compared with the average of the past 100 years, suggesting that major changes in both human activities and hydrological conditions might have occurred since then. The phosphorus (P) concentration and lake level changes are thought to have significant impacts on the cyanobacterial biomass in this lake. We contend that both human activities and the decrease in lake water level (which could lead to enrichment of nutrients in the water) could have resulted in an increase in cyanobacterial biomass and total phosphorus (TP) in lake sediments. The results of this study suggest that controls on nutrient inputs and lake levels are necessary to ensure the sustainability of Lake Chenghai, especially under a continuous global warming scenario.
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  • Arid Central Asia saw mid-Holocene drought

    Xu, Hai   Zhou, Kang'en   Lan, Jianghu   Zhang, Guilin   Zhou, Xinying  

    The mid-Holocene hydroclimates and the forcing mechanisms over arid Central Asia (ACA) are hotly debated in the context of global climate change. It is widely assumed that ACA Holocene precipitation broadly followed and/or was out-of-phase with Northern Hemisphere solar insolation. However, here we show a broadly antiphase relationship between Holocene boreal solar insolation and ACA hydroclimatic trend revealed from a well-dated peat core (at the Big Black peatland; BBP) in northwestern China, southern Altai Mountains. Multiple-proxies, including peat development rate, pollen assemblages, and peat cellulose isotopic records, show wet conditions during the early and late Holocene, but drought condition during the midHolocene. This hydroclimatic pattern is similar to those extracted from other peatlands nearby and those inferred from sedimentary records in lakes in adjacent regions. The trend of delta O-18 in BBP peat cellulose is similar to that of a stalagmite in northern Xinjiang, both of which record the Holocene atmospheric precipitation delta O-18 trend over ACA areas and possibly suggest a changing proportion of glacier meltwater supply. We speculate that the mid-Holocene drought over ACA could be ascribed to: (1) the northward movement of the westerlies, such that when the westerlies moved northward under warm conditions, less water vapor was transported to ACA, and vice versa, and (2) increased evaporation under mid-Holocene warm conditions. The data from this study and the potential mechanisms suggest that drier conditions are expected over ACA areas under a continuous global warming expectation.
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  • Solar activity and the westerlies dominate decadal hydroclimatic changes over arid Central Asia

    Yan, Dongna   Xu, Hai   Lan, Jianghu   Zhou, Kangen   Ye, Yuanda   Zhang, Jixiao   An, Zhisheng   Yeager, Kevin M.  

    Hydroclimatic changes over arid central Asia (ACA) are not fully understood, primarily due to the paucity of accurate, high-resolution climatic records. Here we reconstruct hydroclimatic changes over the past 160 years at Lake Karakul, Pamir Plateau (central Asia), using multiple, high-resolution (similar to 0.8 yr) sedimentary proxies. We find that hydroclimatic changes at Lake Karakul are well correlated with meteorological records, the snow equilibrium line altitude (ELA), reconstructed regional precipitation, the westerly index, and total solar irradiance (TSI), We contend that solar activity and the westerlies were the dominant influences on ACA hydro climatic variations during the period of record: (1) solar activity dominates regional temperature variations, precipitation/evaporation, and the advance/retreat of glaciers; (2) stronger westerly intensity (corresponding to higher westerly index and higher North Atlantic Oscillation (NAO) index) brings more water vapor from the west to ACA, and vice versa; and (3) the southerly migration of the westerly jet stream, which is closely related to lower TSI and temperatures, could favor more water vapor transported to ACA areas, and vice versa. We anticipate that continued global warming will result in more rapid glacial retreat, enhanced evaporation, lake area shrinkage, and deeper aridification over ACA.
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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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  • Warming favors subtropical lake cyanobacterial biomass increasing

    Yan, Dongna   Xu, Hai   Lan, Jianghu   Yang, Ming   Wang, Fushun   Hou, Weiguo   Zhou, Kangen   An, Zhisheng  

    Subtropical lakes are increasingly subject to cyanobacterial blooms resulting from climate change and anthropogenic activities, but the lack of long-term historical data limits understanding of how climate changes have affected cyanobacterial growth in deep subtropical lakes. Using high-resolution DNA data derived from a sediment core from a deep lake in southwestern China, together with analysis of other sedimentary hydroclimatic proxies, we investigated cyanobacterial biomass and microbial biodiversity in relation to climate changes during the last millennium. Our results show that both cyanobacterial abundance and microbial biodiversity were higher during warmer periods, including the Medieval Warm Period (930-1350 CE) and the Current Warm Period (1900 CE-present), but lower during cold periods, including the Little Ice Age (1400-1850 CE). The significant increases in cyanobacterial abundance and microbial biodiversity during warmer intervals are probably because warm climate not only favors cyanobacterial growth but also concentrates lake water nutrients through water budgets between evaporation and precipitation. Furthermore, because rising temperatures result in greater vertical stratification in deep lakes, cyanobacteria may have exploited these stratified conditions and accumulated in dense surface blooms. We anticipate that under anthropogenic warming conditions, cyanobacterial biomass may continue to increase in subtropical deep lakes. (C) 2020 Elsevier B.V. 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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