洞穴CO_2浓度是影响洞穴次生化学沉积物沉积和溶蚀的重要因素之一,洞穴CO_2浓度与洞内外空气交换、裂隙流(滴)水以及洞穴空间形态等因素有关。因此,洞穴空气CO_2空间分布特征及其调控因素不仅是洞穴古气候重建研究的基础,同时也是洞穴...洞穴CO_2浓度是影响洞穴次生化学沉积物沉积和溶蚀的重要因素之一,洞穴CO_2浓度与洞内外空气交换、裂隙流(滴)水以及洞穴空间形态等因素有关。因此,洞穴空气CO_2空间分布特征及其调控因素不仅是洞穴古气候重建研究的基础,同时也是洞穴旅游开发设计和洞穴资源保护的基础。对福建仙云洞的整个洞穴系统的CO_2浓度、温度进行了系统的野外现场测试,获得该洞穴夏季CO_2浓度的空间分布特征,结合洞穴结构、地下河分布等要素初步分析了夏季仙云洞洞穴空气CO_2浓度空间分布的主要影响因素及其对古气候重建和洞穴旅游开发的指示意义。结果表明:(1)夏季洞穴空气CO_2浓度变化介于400～2 800 u L·L-1,近洞口洞道、北洞和南洞之间空间差异显著,主要受洞穴结构特征和裂隙水分布控制;(2)近洞口(0～200 m)洞道宽大,总体倾斜向下,受洞穴内外空气交换的影响,CO_2浓度低,自洞口约400 u L·L-1逐渐增高到约550 uL·L-1,温度则快速下降。在距洞口约200 m,洞穴分叉,进入南北2个支洞系统后,CO_2浓度快速上升,但南北2个支洞系统CO_2浓度变化特征迥异;(3)北洞CO_2浓度相对较低,介于400～2 000 u L·L-1之间变化,不同洞道之间波动幅度大,这与北洞多变的洞道形状、时断时续的地下河以及不均匀的裂隙水分布有关,在部分裂隙水发育的洞道CO_2浓度异常增高,而在通风较好的平直洞道以及无裂隙水发育的地下河洞道CO_2浓度异常降低;(4)南洞则非常稳定,CO_2基本维持在2 250 u L·L-1左右,除了相对封闭且裂隙水发育的田园洞厅浓度稍高;(5)南洞相对封闭的洞穴环境,其石笋较北洞更适合开展古温度和古环境的重建工作;北洞更适合进行旅游开发活动。展开更多
A large number of lacustrine sedimentary records indicate that global warming is the main factor leading to significant changes in diatom communities in lakes of the northern hemisphere.However,due to the intensificat...A large number of lacustrine sedimentary records indicate that global warming is the main factor leading to significant changes in diatom communities in lakes of the northern hemisphere.However,due to the intensification of human activities since 1850,some scholars have emphasized that the increasing lake trophic level may be the main reason for the changes in diatom communities.The debate is ongoing.In order to avoid falling into the complex relationship between diatom changes and the seasonal cycle that characterizes lakes in mid and high latitudes,we chose a lake located at a low latitude,where the relationship between diatoms and temperature is not indirect but direct.The diatom record spans the past ca.100 years and reveals that the abundance of Aulacoseira granulata increased from 1900 until 1985,replacing the previously dominant Aulacoseira ambigua.These changes are in agreement with the increasing trend in global temperature.Since 1985,the percentages of the small-celled Discostella stelligera and the benthic diatom Navicula heimansioides have increased,while Aulacoseira granulata has decreased.This latest shift is caused by further global warming.We conclude that warming is the main factor leading to changing diatom communities in Douhu Lake.展开更多
本研究基于云南香格里拉石卡雪山高原面洞穴钙板SK下部约8.2 cm厚的7个^(230)Th年龄、81组碳氧同位素和9个Δ47温度数据,恢复了MIS 7.2时段的气候变化历史。SK-δ^(18)O序列重建了213.75~199.74 ka B.P.时段的印度夏季风演变过程,平均...本研究基于云南香格里拉石卡雪山高原面洞穴钙板SK下部约8.2 cm厚的7个^(230)Th年龄、81组碳氧同位素和9个Δ47温度数据,恢复了MIS 7.2时段的气候变化历史。SK-δ^(18)O序列重建了213.75~199.74 ka B.P.时段的印度夏季风演变过程,平均分辨率约180 a,该序列显示δ^(18)O值在209.17 ka B.P.和205.00 ka B.P.前后显著正偏,清晰记录了两个千年尺度季风减弱事件。通过与亚洲季风区石笋记录的对比发现,在MIS 7.2开始阶段印度夏季风与东亚夏季风同步变化,但在结束阶段,SK记录早于三宝洞记录约4000 a,考虑到样品年代误差及分辨率等因素,认为该时段轨道尺度上亚洲季风具有一致变化,但是准确时间还有待进一步厘清。Δ47古温度曲线与δ^(18)O、δ^(13)C记录的对比显示出MIS 7.2结束时段水热异步变化特征,与黄土-古土壤序列结论类似,不同的是SK记录的温度直接响应北半球太阳辐照度变化,而不是响应南极温度变化。SK记录显示夏季温度从207.36 ka B.P.左右逐渐回升,同期印度夏季风缓慢增强,但被大幅度的千年尺度季风减弱事件中断,印度夏季风显著增强滞后温度变化约6000 a,该过程与末次消冰期中海因里希事件(Heinrich events)或新仙女木事件(YD)类似,其机制可能与高纬气候变化有关,即冰盖融解导致大量淡水注入北大西洋,影响了北大西洋深水(NADW)和南极底层流(AABW)的生成,从而导致温盐环流(THC)减弱或中断,并通过大气-海洋环流传递到低纬地区,触发千年尺度印度夏季风减弱事件。展开更多
Using gas chromatography-mass spectrometry (GC-MS) technique, a series of biomarkers were identi- fied, including n-alkanes, n-alkane-2-ones, isoprenoid etc. from the loess-paleosol samples collected from the S4 and a...Using gas chromatography-mass spectrometry (GC-MS) technique, a series of biomarkers were identi- fied, including n-alkanes, n-alkane-2-ones, isoprenoid etc. from the loess-paleosol samples collected from the S4 and adjacent L5, L4 of the Luochuan loess section, Northwestern China. Based on these data, especially n-alkanes and high-resolution magnetic susceptibility and grain size data, the pa- leoenvironment and paleovegetation history during S4 was reconstructed. The CPI (Carbon Predomi- nance Index) and correlation between n-alkanes and magnetic susceptibility and grain size data dem- onstrated that the molecular fossils in paleosol and loess layers can reflect the vegetation condition during the loess-paleosol formation, if the allochthonous organic inputs could be excluded reasonably. The ACL (average chain length) index is correlated well with paleomagnetic susceptibility and grain size variations, displaying their good synchrony with warm and humid climate. However, it relatively lagged behind the paleomagnetic susceptibility and the grain size variations when the climate began to deteriorate. During the formation period of paleosol, the n-alkanes was dominated by C31 homologue, indicating that the primary organic input originated from herbs. Our study also demonstrated that the herbs were more flourish than wood plants in Loess Plateau, especially in the Luochuan area during the warm and humid phase, and there was no typical forest vegetation developed in the studied period.展开更多
Gas chromatography-mass spectrometry was used to identify a series of n-alkanes in the sediments of a typical glacially eroded lake in the eastern Tibetan Plateau.By comparing the distribution patterns of n-alkanes in...Gas chromatography-mass spectrometry was used to identify a series of n-alkanes in the sediments of a typical glacially eroded lake in the eastern Tibetan Plateau.By comparing the distribution patterns of n-alkanes in lake sediments,surface soils and cow manure,it was shown that n-C 27-n-C 33 alkanes in the soil ecosystem of Ximen Co are derived from vascular plant species and that the distribution pattern of n-C 27-n-C 33 alkanes remains unchanged during the feeding and digestion processes of herbivores.The relative percentage of C 27,C 29 and C 31 n-alkanes decreased from the bottom to the top of the sediment core showing a trend of degradation of higher plants in the Ximen Co lake region during the formation of the 44 cm core.210 Pb dating,combined with pre-existing AMS 14 C dating results showed that the depositional core reflects climatic and environmental variations since about 900 years before present.The n-alkane indexes (ACL 27-33,P aq,P wax) are comparable with regional temperature variation,especially recording the Little Ice Age event (LIA).This study highlights that n-alkanes are valid proxies for paleo-climate and paleo-environment reconstruction,despite the same distribution patterns in n-alkane molecular fossils derived from a typical glacially eroded lake.展开更多
Based on the concentration and distribution pattern analysis of the rare earth elements (REEs) at the Shell Bar section from Qaidam Basin, we studied the geochemical fractionations of REE in paleolake deposits and the...Based on the concentration and distribution pattern analysis of the rare earth elements (REEs) at the Shell Bar section from Qaidam Basin, we studied the geochemical fractionations of REE in paleolake deposits and their paleo-environmental significance. Our results show that the REE concentration in AS (acid soluble) and AR (acid residual) fractions are 20.9 μg/g and 95.4 μg/g (except element Y) individually, showing a strong REE differentiation between AS and AR fractions. However, the two types of fractions (AS & AR) have similar REE distribution patterns, which are slightly rich in light earth rare elements (LREEs), with slightly right-tilting and negative Eu anomaly. The LREE of AR is richer than that of AS. There were no significant correlations between the REE in AS, AR and other proxies. It indicated that the lacustrine deposition had different material sources and experienced varying geochemical procedures. Correlation analysis between the REE and the content of fine grain-size (<4 μm) of the sediments, Rb/Sr ratio and Mn concentration showed high correlation coefficients. Our results demonstrated that the REE in acid soluble and residual fractions bear different environmental significances, which are strongly dependent on local environment. The REE of acid soluble fraction is closely related to the paleoclimatic changes in the lake catchment and the evolutionary processes of lake itself. Our results suggest that δCe and (La/Yb)n could be used as proxies of the reduction-oxidation conditions and furthermore the temperature change and the lake level fluctuations. Using these proxies, we reconstructed the paleoclimate and water level fluctuation history during the high lake level period lasting between 43.5 and 22.4 cal. ka BP.展开更多
Biomarkers of paleolake deposits from Qarhan Salt Lake in Qaidam Basin,northwest China were sys-tematically analyzed and the A―C series compounds of branched aliphatic alkanes with quaternary substituted carbon atom(...Biomarkers of paleolake deposits from Qarhan Salt Lake in Qaidam Basin,northwest China were sys-tematically analyzed and the A―C series compounds of branched aliphatic alkanes with quaternary substituted carbon atom(BAQCs) were identified. The homologous distinguished three series,A―C,were identified as 5,5-diethylalkanes,6,6-diethylalkanes and 5-butyl,5-ethylalkanes series,and their relative abundance was A > B > C. Series A and C were characterized by odd carbon numbers,whereas series B was characterized by even carbon numbers. The high values of series A corresponded with the high values of series B and C. Therefore,it can be concluded that series A,B and C possess a similar biological origin. The abundance of series A was relatively low in the lower part of the section compared with that in the upper part,implying that these series originated from bacteria and/or algae more prevalent in fresh-mesohaline water,and such kinds of bacteria and/or algae are most likely to be thermophilous species. The A25/nC25 ratio differences in the section show that such branched aliphatic alkanes can be treated as one kind of environmental change proxy for paleolake evolution and may provide important information for the climate reconstruction of the Late Pleistocene.展开更多
Here we combine n-alkanes preserved in a shell bar section from Qarhan paleolake, Qaidam Basin with the other sedimentary proxies to elucidate the lake evolution process during the period 39.7 to 17.5 14C ka BP (calib...Here we combine n-alkanes preserved in a shell bar section from Qarhan paleolake, Qaidam Basin with the other sedimentary proxies to elucidate the lake evolution process during the period 39.7 to 17.5 14C ka BP (calibrated age ranges from 43.5 to 22.4 cal. ka BP). In different stages, the n-alkane homologues exhibited different distribution modes indicative of variations in the surrounding vegetation and the hydrologic condition of the lake. The n-alkanes proxies (CPIh, ACLh, Paq) have the same trends as the summer solar insolation variation, implying that the summer insolation is the most important climatic factor driving the environmental changes and also indirectly controls lake evolution on the Tibetan Plateau. CPIh and ACLh as well as the total pollen concentration appear to show a trend comparable with methane concentration record from the GRIP ice core that reflected the Dansgaard-Oeschger events. This demonstrates that the paleoclimate variations in Qarhan area generally agree with global climate change and show rapid oscillations in late MIS3. These findings provide the latest molecular fossil evidence from paleolake sediments to confirm that lake evolution on Tibetan Plateau in late MIS3 was closely associated with enhanced summer insolation.展开更多
Reconstructing the evolutionary history of the Gobi deserts developed from alluvial sediments in arid regions has great significance in unraveling changes in both tectonic activity and climate.However,such work is lim...Reconstructing the evolutionary history of the Gobi deserts developed from alluvial sediments in arid regions has great significance in unraveling changes in both tectonic activity and climate.However,such work is limited by a lack of suitable dating material preserved in the Gobi Desert,but cosmogenic 10Be has great potential to date the Gobi deserts.In the present study,10Be in quartz gravel from the Gobi deserts of the Ejina Basin in Inner Mongolia of China has been measured to assess exposure ages.Results show that the Gobi Desert in the northern margin of the basin developed 420 ka ago,whereas the Gobi Desert that developed from alluvial plains in the Heihe River drainage basin came about during the last 190 ka.The latter developed gradually northward and eastward to modern terminal lakes of the river.These temporal and spatial variations in the Gobi deserts are a consequence of alluvial processes influenced by Tibetan Plateau uplift and tectonic activities within the Ejina Basin.Possible episodes of Gobi Desert development within the last 420 ka indicate that the advance/retreat of alpine glaciers during glacial/interglacial cycles might have been the dominant factor to influencing the alluvial intensity and water volume in the basin.Intense floods and large water volumes would mainly occur during the short deglacial periods.展开更多
Crossing the Sino-Russian boundary, Xingkai Lake is the largest freshwater lake in Northeast Asia. In addition to the lakeshore, there are four sand hills on the north side of the lake that accumulated during a period...Crossing the Sino-Russian boundary, Xingkai Lake is the largest freshwater lake in Northeast Asia. In addition to the lakeshore, there are four sand hills on the north side of the lake that accumulated during a period of sustainable and stable lacustrine trans gression and were preserved after depression. Analysis of well-dated stratigraphic sequences based on 18 OSL datings combined with multiple index analysis of six sites in the sand hills revealed that the north shoreline of Xingkai Lake retreated in a stepwise fashion since the middle Pleistocene, and that at least four transgressions (during 193-183 ka, 136-130 ka, 24-15 ka and since 3 ka) and three depressions occurred during this process. The results of this study confirmed that transgressive stages were concur- rent with epochs of climate cooling, whereas the period of regression corresponded to the climatic optima. Transgressions and regressions were primarily caused by variations in the intensity of alluvial accumulation in the Ussuri River Valley and fluctuations in regional temperature and humidity that were controlled by climatic change. Moreover, one obvious transgressive process that occurred in MIS3 may have been related to enhanced precipitation that was reportedly widespread in the west of China, while shortterm fluctuations in the lake level might well be a direct response to regional precipitation variations on the millennial scale.展开更多
Lake Zigetang is located on the central Tibetan Plateau(TP) and represents a rare but typical meromictic lake in China.The lake's stable meromixis sustains microflora communities, and changes in these communities ...Lake Zigetang is located on the central Tibetan Plateau(TP) and represents a rare but typical meromictic lake in China.The lake's stable meromixis sustains microflora communities, and changes in these communities are relatively independent of climate. Therefore, these communities can be used as paleoclimate proxies. In this paper, the stratification properties and their relationships with the microflora of Lake Zigetang were analyzed. We found that water depth and climate conditions were two important factors for maintaining meromixis in Lake Zigetang. Generally, stratification was enhanced during warm periods, while temperature differences between the mixolimnion and monimolimnion were decreased during cold periods. The presence of anoxygenic phototrophic bacteria(APB) was demonstrated by the discovery of bacteriopheophytin-a(Bph-a) in the sediments.This bacterial community is mainly concentrated at the bottom of the chemocline and the top of the monimolimnion, where it forms a thin APB layer. Moreover, total APB productivity is mainly affected by the light intensity penetrating to the APB layer,which exponentially increases as the thermocline becomes shallow. Therefore, high Bph-a values in the lake corresponded to a shallow thermocline and warm periods, low Bph-a values corresponded to cold periods, and zero changes indicated that the water was completely mixed and reflected an extreme cold climate or low lake level period. Thus, Bph-a can be used as a climate proxy to reconstruct the history of lake stratification and climate changes.展开更多
文摘洞穴CO_2浓度是影响洞穴次生化学沉积物沉积和溶蚀的重要因素之一,洞穴CO_2浓度与洞内外空气交换、裂隙流(滴)水以及洞穴空间形态等因素有关。因此,洞穴空气CO_2空间分布特征及其调控因素不仅是洞穴古气候重建研究的基础,同时也是洞穴旅游开发设计和洞穴资源保护的基础。对福建仙云洞的整个洞穴系统的CO_2浓度、温度进行了系统的野外现场测试,获得该洞穴夏季CO_2浓度的空间分布特征,结合洞穴结构、地下河分布等要素初步分析了夏季仙云洞洞穴空气CO_2浓度空间分布的主要影响因素及其对古气候重建和洞穴旅游开发的指示意义。结果表明:(1)夏季洞穴空气CO_2浓度变化介于400～2 800 u L·L-1,近洞口洞道、北洞和南洞之间空间差异显著,主要受洞穴结构特征和裂隙水分布控制;(2)近洞口(0～200 m)洞道宽大,总体倾斜向下,受洞穴内外空气交换的影响,CO_2浓度低,自洞口约400 u L·L-1逐渐增高到约550 uL·L-1,温度则快速下降。在距洞口约200 m,洞穴分叉,进入南北2个支洞系统后,CO_2浓度快速上升,但南北2个支洞系统CO_2浓度变化特征迥异;(3)北洞CO_2浓度相对较低,介于400～2 000 u L·L-1之间变化,不同洞道之间波动幅度大,这与北洞多变的洞道形状、时断时续的地下河以及不均匀的裂隙水分布有关,在部分裂隙水发育的洞道CO_2浓度异常增高,而在通风较好的平直洞道以及无裂隙水发育的地下河洞道CO_2浓度异常降低;(4)南洞则非常稳定,CO_2基本维持在2 250 u L·L-1左右,除了相对封闭且裂隙水发育的田园洞厅浓度稍高;(5)南洞相对封闭的洞穴环境,其石笋较北洞更适合开展古温度和古环境的重建工作;北洞更适合进行旅游开发活动。
基金funded by a granted from National Key R&D Program of China(Grant No.2017YFA0603400)the National Basic Research Program of China(Grant No.2015CB953801)the National Natural Science Foundation of China(Grant No.41772379)。
文摘A large number of lacustrine sedimentary records indicate that global warming is the main factor leading to significant changes in diatom communities in lakes of the northern hemisphere.However,due to the intensification of human activities since 1850,some scholars have emphasized that the increasing lake trophic level may be the main reason for the changes in diatom communities.The debate is ongoing.In order to avoid falling into the complex relationship between diatom changes and the seasonal cycle that characterizes lakes in mid and high latitudes,we chose a lake located at a low latitude,where the relationship between diatoms and temperature is not indirect but direct.The diatom record spans the past ca.100 years and reveals that the abundance of Aulacoseira granulata increased from 1900 until 1985,replacing the previously dominant Aulacoseira ambigua.These changes are in agreement with the increasing trend in global temperature.Since 1985,the percentages of the small-celled Discostella stelligera and the benthic diatom Navicula heimansioides have increased,while Aulacoseira granulata has decreased.This latest shift is caused by further global warming.We conclude that warming is the main factor leading to changing diatom communities in Douhu Lake.
文摘本研究基于云南香格里拉石卡雪山高原面洞穴钙板SK下部约8.2 cm厚的7个^(230)Th年龄、81组碳氧同位素和9个Δ47温度数据,恢复了MIS 7.2时段的气候变化历史。SK-δ^(18)O序列重建了213.75~199.74 ka B.P.时段的印度夏季风演变过程,平均分辨率约180 a,该序列显示δ^(18)O值在209.17 ka B.P.和205.00 ka B.P.前后显著正偏,清晰记录了两个千年尺度季风减弱事件。通过与亚洲季风区石笋记录的对比发现,在MIS 7.2开始阶段印度夏季风与东亚夏季风同步变化,但在结束阶段,SK记录早于三宝洞记录约4000 a,考虑到样品年代误差及分辨率等因素,认为该时段轨道尺度上亚洲季风具有一致变化,但是准确时间还有待进一步厘清。Δ47古温度曲线与δ^(18)O、δ^(13)C记录的对比显示出MIS 7.2结束时段水热异步变化特征,与黄土-古土壤序列结论类似,不同的是SK记录的温度直接响应北半球太阳辐照度变化,而不是响应南极温度变化。SK记录显示夏季温度从207.36 ka B.P.左右逐渐回升,同期印度夏季风缓慢增强,但被大幅度的千年尺度季风减弱事件中断,印度夏季风显著增强滞后温度变化约6000 a,该过程与末次消冰期中海因里希事件(Heinrich events)或新仙女木事件(YD)类似,其机制可能与高纬气候变化有关,即冰盖融解导致大量淡水注入北大西洋,影响了北大西洋深水(NADW)和南极底层流(AABW)的生成,从而导致温盐环流(THC)减弱或中断,并通过大气-海洋环流传递到低纬地区,触发千年尺度印度夏季风减弱事件。
基金the Hundred Talent Project of Chinese Academy of Sciences and the National Natural Science Foundation of China (Grant No. 40371117)
文摘Using gas chromatography-mass spectrometry (GC-MS) technique, a series of biomarkers were identi- fied, including n-alkanes, n-alkane-2-ones, isoprenoid etc. from the loess-paleosol samples collected from the S4 and adjacent L5, L4 of the Luochuan loess section, Northwestern China. Based on these data, especially n-alkanes and high-resolution magnetic susceptibility and grain size data, the pa- leoenvironment and paleovegetation history during S4 was reconstructed. The CPI (Carbon Predomi- nance Index) and correlation between n-alkanes and magnetic susceptibility and grain size data dem- onstrated that the molecular fossils in paleosol and loess layers can reflect the vegetation condition during the loess-paleosol formation, if the allochthonous organic inputs could be excluded reasonably. The ACL (average chain length) index is correlated well with paleomagnetic susceptibility and grain size variations, displaying their good synchrony with warm and humid climate. However, it relatively lagged behind the paleomagnetic susceptibility and the grain size variations when the climate began to deteriorate. During the formation period of paleosol, the n-alkanes was dominated by C31 homologue, indicating that the primary organic input originated from herbs. Our study also demonstrated that the herbs were more flourish than wood plants in Loess Plateau, especially in the Luochuan area during the warm and humid phase, and there was no typical forest vegetation developed in the studied period.
基金supported by the National Natural Science Foundation of China (40871096)Frontier Projects of the Nanjing Institute of Geography and Limnology,CAS (NIGLAS2010XK01)
文摘Gas chromatography-mass spectrometry was used to identify a series of n-alkanes in the sediments of a typical glacially eroded lake in the eastern Tibetan Plateau.By comparing the distribution patterns of n-alkanes in lake sediments,surface soils and cow manure,it was shown that n-C 27-n-C 33 alkanes in the soil ecosystem of Ximen Co are derived from vascular plant species and that the distribution pattern of n-C 27-n-C 33 alkanes remains unchanged during the feeding and digestion processes of herbivores.The relative percentage of C 27,C 29 and C 31 n-alkanes decreased from the bottom to the top of the sediment core showing a trend of degradation of higher plants in the Ximen Co lake region during the formation of the 44 cm core.210 Pb dating,combined with pre-existing AMS 14 C dating results showed that the depositional core reflects climatic and environmental variations since about 900 years before present.The n-alkane indexes (ACL 27-33,P aq,P wax) are comparable with regional temperature variation,especially recording the Little Ice Age event (LIA).This study highlights that n-alkanes are valid proxies for paleo-climate and paleo-environment reconstruction,despite the same distribution patterns in n-alkane molecular fossils derived from a typical glacially eroded lake.
基金Supported by Hundred Talent Project by Chinese Academy of Sciences and National Natural Science Foundation of China (Grant Nos. 40871096, 40371117)
文摘Based on the concentration and distribution pattern analysis of the rare earth elements (REEs) at the Shell Bar section from Qaidam Basin, we studied the geochemical fractionations of REE in paleolake deposits and their paleo-environmental significance. Our results show that the REE concentration in AS (acid soluble) and AR (acid residual) fractions are 20.9 μg/g and 95.4 μg/g (except element Y) individually, showing a strong REE differentiation between AS and AR fractions. However, the two types of fractions (AS & AR) have similar REE distribution patterns, which are slightly rich in light earth rare elements (LREEs), with slightly right-tilting and negative Eu anomaly. The LREE of AR is richer than that of AS. There were no significant correlations between the REE in AS, AR and other proxies. It indicated that the lacustrine deposition had different material sources and experienced varying geochemical procedures. Correlation analysis between the REE and the content of fine grain-size (<4 μm) of the sediments, Rb/Sr ratio and Mn concentration showed high correlation coefficients. Our results demonstrated that the REE in acid soluble and residual fractions bear different environmental significances, which are strongly dependent on local environment. The REE of acid soluble fraction is closely related to the paleoclimatic changes in the lake catchment and the evolutionary processes of lake itself. Our results suggest that δCe and (La/Yb)n could be used as proxies of the reduction-oxidation conditions and furthermore the temperature change and the lake level fluctuations. Using these proxies, we reconstructed the paleoclimate and water level fluctuation history during the high lake level period lasting between 43.5 and 22.4 cal. ka BP.
基金Supported by the Hundred Talent Project of Chinese Academy of Sciences, the National Natural Sciences Foundation of China (Grant No.40371117)the Inno-vation Team Project of National Natural Sciences Foundation of China (Grant No.40421001)
文摘Biomarkers of paleolake deposits from Qarhan Salt Lake in Qaidam Basin,northwest China were sys-tematically analyzed and the A―C series compounds of branched aliphatic alkanes with quaternary substituted carbon atom(BAQCs) were identified. The homologous distinguished three series,A―C,were identified as 5,5-diethylalkanes,6,6-diethylalkanes and 5-butyl,5-ethylalkanes series,and their relative abundance was A > B > C. Series A and C were characterized by odd carbon numbers,whereas series B was characterized by even carbon numbers. The high values of series A corresponded with the high values of series B and C. Therefore,it can be concluded that series A,B and C possess a similar biological origin. The abundance of series A was relatively low in the lower part of the section compared with that in the upper part,implying that these series originated from bacteria and/or algae more prevalent in fresh-mesohaline water,and such kinds of bacteria and/or algae are most likely to be thermophilous species. The A25/nC25 ratio differences in the section show that such branched aliphatic alkanes can be treated as one kind of environmental change proxy for paleolake evolution and may provide important information for the climate reconstruction of the Late Pleistocene.
基金supported by National Natural Science Foundation of China (Grant Nos. 40871096, 40371117)
文摘Here we combine n-alkanes preserved in a shell bar section from Qarhan paleolake, Qaidam Basin with the other sedimentary proxies to elucidate the lake evolution process during the period 39.7 to 17.5 14C ka BP (calibrated age ranges from 43.5 to 22.4 cal. ka BP). In different stages, the n-alkane homologues exhibited different distribution modes indicative of variations in the surrounding vegetation and the hydrologic condition of the lake. The n-alkanes proxies (CPIh, ACLh, Paq) have the same trends as the summer solar insolation variation, implying that the summer insolation is the most important climatic factor driving the environmental changes and also indirectly controls lake evolution on the Tibetan Plateau. CPIh and ACLh as well as the total pollen concentration appear to show a trend comparable with methane concentration record from the GRIP ice core that reflected the Dansgaard-Oeschger events. This demonstrates that the paleoclimate variations in Qarhan area generally agree with global climate change and show rapid oscillations in late MIS3. These findings provide the latest molecular fossil evidence from paleolake sediments to confirm that lake evolution on Tibetan Plateau in late MIS3 was closely associated with enhanced summer insolation.
基金supported by the Knowledge Innovation Program of the Chinese Academy of Sciences(KZCX2-YW-117)the National Natural Science Foundation of China(40841022 and 40373046)
文摘Reconstructing the evolutionary history of the Gobi deserts developed from alluvial sediments in arid regions has great significance in unraveling changes in both tectonic activity and climate.However,such work is limited by a lack of suitable dating material preserved in the Gobi Desert,but cosmogenic 10Be has great potential to date the Gobi deserts.In the present study,10Be in quartz gravel from the Gobi deserts of the Ejina Basin in Inner Mongolia of China has been measured to assess exposure ages.Results show that the Gobi Desert in the northern margin of the basin developed 420 ka ago,whereas the Gobi Desert that developed from alluvial plains in the Heihe River drainage basin came about during the last 190 ka.The latter developed gradually northward and eastward to modern terminal lakes of the river.These temporal and spatial variations in the Gobi deserts are a consequence of alluvial processes influenced by Tibetan Plateau uplift and tectonic activities within the Ejina Basin.Possible episodes of Gobi Desert development within the last 420 ka indicate that the advance/retreat of alpine glaciers during glacial/interglacial cycles might have been the dominant factor to influencing the alluvial intensity and water volume in the basin.Intense floods and large water volumes would mainly occur during the short deglacial periods.
基金supported by the National Natural Science Founda-tion of China (40872117)
文摘Crossing the Sino-Russian boundary, Xingkai Lake is the largest freshwater lake in Northeast Asia. In addition to the lakeshore, there are four sand hills on the north side of the lake that accumulated during a period of sustainable and stable lacustrine trans gression and were preserved after depression. Analysis of well-dated stratigraphic sequences based on 18 OSL datings combined with multiple index analysis of six sites in the sand hills revealed that the north shoreline of Xingkai Lake retreated in a stepwise fashion since the middle Pleistocene, and that at least four transgressions (during 193-183 ka, 136-130 ka, 24-15 ka and since 3 ka) and three depressions occurred during this process. The results of this study confirmed that transgressive stages were concur- rent with epochs of climate cooling, whereas the period of regression corresponded to the climatic optima. Transgressions and regressions were primarily caused by variations in the intensity of alluvial accumulation in the Ussuri River Valley and fluctuations in regional temperature and humidity that were controlled by climatic change. Moreover, one obvious transgressive process that occurred in MIS3 may have been related to enhanced precipitation that was reportedly widespread in the west of China, while shortterm fluctuations in the lake level might well be a direct response to regional precipitation variations on the millennial scale.
基金supported by the Chinese Academy of Sciences Strategic Priority Research Program(Grant No.XDA05080402)the National Natural Science Foundation of China(Grant No.41361008)+1 种基金Yunnan Provincial Government Leading Scientist Program(Grant No.2015HA024)Yunnan Provincial Government Senior Talent Program(Grant No.2010CI111)
文摘Lake Zigetang is located on the central Tibetan Plateau(TP) and represents a rare but typical meromictic lake in China.The lake's stable meromixis sustains microflora communities, and changes in these communities are relatively independent of climate. Therefore, these communities can be used as paleoclimate proxies. In this paper, the stratification properties and their relationships with the microflora of Lake Zigetang were analyzed. We found that water depth and climate conditions were two important factors for maintaining meromixis in Lake Zigetang. Generally, stratification was enhanced during warm periods, while temperature differences between the mixolimnion and monimolimnion were decreased during cold periods. The presence of anoxygenic phototrophic bacteria(APB) was demonstrated by the discovery of bacteriopheophytin-a(Bph-a) in the sediments.This bacterial community is mainly concentrated at the bottom of the chemocline and the top of the monimolimnion, where it forms a thin APB layer. Moreover, total APB productivity is mainly affected by the light intensity penetrating to the APB layer,which exponentially increases as the thermocline becomes shallow. Therefore, high Bph-a values in the lake corresponded to a shallow thermocline and warm periods, low Bph-a values corresponded to cold periods, and zero changes indicated that the water was completely mixed and reflected an extreme cold climate or low lake level period. Thus, Bph-a can be used as a climate proxy to reconstruct the history of lake stratification and climate changes.
