The intensified monsoon increases summer rainfall and creates wet conditions in the Asian summer monsoon region during the early Holocene. Along with millennial-scale changes of the monsoon intensity, it is still uncl...The intensified monsoon increases summer rainfall and creates wet conditions in the Asian summer monsoon region during the early Holocene. Along with millennial-scale changes of the monsoon intensity, it is still unclear whether the boundary of the monsoon region changes according to monsoon variability. Investigations into the early Holocene environment in monsoon marginal zones are crucial for understanding the monsoon boundary changes. Zhuye Lake is located at the northwest edge of the Asian summer monsoon, the northern Qilian Mountains, which are less affected by modern summer monsoon water vapor. Previous studies have reached different conclusions regarding the early Holocene climatic and environmental changes based on different dating methods（^14C and OSL（optically stimulated luminescence）） and materials（shells, carbonate, pollen concentrates and bulk organic carbon）. In this study, we synthesized 102 ^14C dates and 35 OSL dates from ten Holocene sedimentary sections and ten paleo-shorelines in the lake basin. A comparison between ages from different dating methods and materials generally shows that carbon reservoir effects are relatively slight in Zhuye Lake while the disordered chronologies are mainly related to the erosion processes and reworking effects. In addition, proxy data, including lithology, pollen, total organic carbon and carbonate, were collected from different sites of Zhuye Lake. According to the new synthesis, the early Holocene environment was relatively humid, associated with high runoff and lake water levels. The result indicates that the monsoon boundary moves to the north during the period of the intensified monsoon. A typical arid-area lake was formed during the mid-Holocene when carbonate accumulation and high organic matter contents were the main features of this period. The lake retreated strongly during the late Holocene, showing a drought trend. Overall, the lake evolution is generally consistent with the Holocene Asian summer monsoon change, showing the monsoon influence to monsoon marginal zones.展开更多
17 samples were collected from aeolian and lacustrine profiles within the environment sensitive zone of the Loess Plateau, and an experimental method was established which is suitable for pollen extraction from aeolia...17 samples were collected from aeolian and lacustrine profiles within the environment sensitive zone of the Loess Plateau, and an experimental method was established which is suitable for pollen extraction from aeolian sediment. A comparative study of pollen dating was carried out using the accelerator mass spectrometry （AMS） <sup>14</sup>C dating of known age samples, and then an experiment with the pollen concentrates was performed. The results indicate that pollen that has been deposited simultaneously with sediment in a stable environment can provide reliable ages. This technique will provide a way of improving the chronological framework for the Loess Plateau since the late Pleistocene. The <sup>14</sup>C dating was combined with field investigations, and from the geological record within this zone, evidence was extracted of four major monsoon precipitation changes during the transition from the late Pleistocene to Holoeene.展开更多
The Monards Marsh is located on the northeastern bank of the Gironde Estuary in France. Lithological, sedimentolngical and mi- cropalacontological investigations were made on four cores to determine the evolution of H...The Monards Marsh is located on the northeastern bank of the Gironde Estuary in France. Lithological, sedimentolngical and mi- cropalacontological investigations were made on four cores to determine the evolution of Holocene sedimentary environments and processes in this area over the last 6 000 a. Three main lithological facies are distinguished from bottom to top： （ 1 ） grey laminated silty-sandy clay ; （2） homogeneous dark grey silty clay ; and （ 3 ） compact silty clay. About 26 benthic foraminifera species are identified and divided into six groups according to their ecological characteristics. In association with lithology, sedimentary structures and grainuiometry, the distribution of foraminifera group is used to define external slikke, internal slikke, external schorre, internal schorre, and continental marsh facies. Combined with 14C（AMS） dating, these data indicate four successive paleoenvironments in the Monards Marsh： （ 1 ） Holocene transgression resuited in the development of a basal schorre facies overlying fluvial deposits that transformed to slikke facies sedimentation; the transgression maximum occurred around 5 600 to 5 4430 a BP and was inferred to be associated with the last phase of the rapid Holocene sea-level rise; （2） post-trangressive maximum sedimentation resuited in a regressive sequence of deposits prograding towards the estuary, corresponding to the stahilisation of sea level after 5 4430 a BP; （3） a slight positive tendency in the sea level around 2 800 a BP recorded in the central part of the marsh; and （4） the wetland to a continental marsh environment. The sequential pattern for the evolution of wetlands in this estuarine area during the Holocene is fluvial facies - blackish schorre facies - slikke facies - blackish schorre facies - continental marsh facies. Characteristics of sedimentary facies distribution and evolution reveal that the development of Holocene salt marsh in this area was controlled by the sea-level change and tidal range. The sedimentary facies show an obvious surficial and vertical banding distribution in which the texture and structure, authigenie mineral, foraminifera distribution and bioturbation indicate many distinct difference characteristics.展开更多
文摘政和蛙岩崩塌是一处大型古岩崩遗迹,其堵塞闽江支流七星溪河谷形成外屯—湖屯一带长度超过10.5 km的古堰塞湖,其后的溃坝使堰塞湖水体泄空,细粒堆积物露出地表形成万亩良田。为了约束该古堰塞湖形成与溃坝的时代,对古堰塞湖沉积施工了1个深度8.5 m的浅钻,沿不同深度采集了8件AMS^(14)C测年样品,结果表明:堰塞湖沉积从地表至8.5 m深的^(14)C年龄从930~795 Cal BP至1528~1376 Cal BP,根据堰塞湖沉积顶部的最小年龄推测堰塞湖溃坝的时代(~0.8±0.1)ka BP,由沉积速率推算堰塞湖形成时代不早于(2.8±0.1)ka BP,表明蛙岩古堰塞湖形成于全新世晚期,这为古堰塞湖形成与消亡演化提供了一个研究实例。
基金supported by the National Natural Science Foundation of China (41371009)the Fundamental Research Fund for the Central Universities (lzujbky-2013-127)
文摘The intensified monsoon increases summer rainfall and creates wet conditions in the Asian summer monsoon region during the early Holocene. Along with millennial-scale changes of the monsoon intensity, it is still unclear whether the boundary of the monsoon region changes according to monsoon variability. Investigations into the early Holocene environment in monsoon marginal zones are crucial for understanding the monsoon boundary changes. Zhuye Lake is located at the northwest edge of the Asian summer monsoon, the northern Qilian Mountains, which are less affected by modern summer monsoon water vapor. Previous studies have reached different conclusions regarding the early Holocene climatic and environmental changes based on different dating methods（^14C and OSL（optically stimulated luminescence）） and materials（shells, carbonate, pollen concentrates and bulk organic carbon）. In this study, we synthesized 102 ^14C dates and 35 OSL dates from ten Holocene sedimentary sections and ten paleo-shorelines in the lake basin. A comparison between ages from different dating methods and materials generally shows that carbon reservoir effects are relatively slight in Zhuye Lake while the disordered chronologies are mainly related to the erosion processes and reworking effects. In addition, proxy data, including lithology, pollen, total organic carbon and carbonate, were collected from different sites of Zhuye Lake. According to the new synthesis, the early Holocene environment was relatively humid, associated with high runoff and lake water levels. The result indicates that the monsoon boundary moves to the north during the period of the intensified monsoon. A typical arid-area lake was formed during the mid-Holocene when carbonate accumulation and high organic matter contents were the main features of this period. The lake retreated strongly during the late Holocene, showing a drought trend. Overall, the lake evolution is generally consistent with the Holocene Asian summer monsoon change, showing the monsoon influence to monsoon marginal zones.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 49894170, 49725308)Chinese Academy of Sciences (Nos. KZ-951-A1-402 & KZ-952-S1-419)State Committee of Science and Technology of China.
文摘17 samples were collected from aeolian and lacustrine profiles within the environment sensitive zone of the Loess Plateau, and an experimental method was established which is suitable for pollen extraction from aeolian sediment. A comparative study of pollen dating was carried out using the accelerator mass spectrometry （AMS） <sup>14</sup>C dating of known age samples, and then an experiment with the pollen concentrates was performed. The results indicate that pollen that has been deposited simultaneously with sediment in a stable environment can provide reliable ages. This technique will provide a way of improving the chronological framework for the Loess Plateau since the late Pleistocene. The <sup>14</sup>C dating was combined with field investigations, and from the geological record within this zone, evidence was extracted of four major monsoon precipitation changes during the transition from the late Pleistocene to Holoeene.
文摘The Monards Marsh is located on the northeastern bank of the Gironde Estuary in France. Lithological, sedimentolngical and mi- cropalacontological investigations were made on four cores to determine the evolution of Holocene sedimentary environments and processes in this area over the last 6 000 a. Three main lithological facies are distinguished from bottom to top： （ 1 ） grey laminated silty-sandy clay ; （2） homogeneous dark grey silty clay ; and （ 3 ） compact silty clay. About 26 benthic foraminifera species are identified and divided into six groups according to their ecological characteristics. In association with lithology, sedimentary structures and grainuiometry, the distribution of foraminifera group is used to define external slikke, internal slikke, external schorre, internal schorre, and continental marsh facies. Combined with 14C（AMS） dating, these data indicate four successive paleoenvironments in the Monards Marsh： （ 1 ） Holocene transgression resuited in the development of a basal schorre facies overlying fluvial deposits that transformed to slikke facies sedimentation; the transgression maximum occurred around 5 600 to 5 4430 a BP and was inferred to be associated with the last phase of the rapid Holocene sea-level rise; （2） post-trangressive maximum sedimentation resuited in a regressive sequence of deposits prograding towards the estuary, corresponding to the stahilisation of sea level after 5 4430 a BP; （3） a slight positive tendency in the sea level around 2 800 a BP recorded in the central part of the marsh; and （4） the wetland to a continental marsh environment. The sequential pattern for the evolution of wetlands in this estuarine area during the Holocene is fluvial facies - blackish schorre facies - slikke facies - blackish schorre facies - continental marsh facies. Characteristics of sedimentary facies distribution and evolution reveal that the development of Holocene salt marsh in this area was controlled by the sea-level change and tidal range. The sedimentary facies show an obvious surficial and vertical banding distribution in which the texture and structure, authigenie mineral, foraminifera distribution and bioturbation indicate many distinct difference characteristics.
