Although archaeological findings show the synchronous collapses of major well-documented Chinese Neolithic cultures around 4000 cal. yr BP, the driving mechanism for the phenomenon is still unclear ...and debatable. Spatial climatic features in China spanning this time period suggest a generally cold-dry setting. This is evidenced by 130 well-dated geological records at 97 sites located in climatically and topographically diverse regions, with occurrences of some extreme hydrological events like severe floods in the Chinese Loess Plateau, and in basins of the lower Yellow River and the middle-to-lower Yangtze River. The weakening of the Asian Summer Monsoon (ASM) since the mid-Holocene would have made Neolithic subsistence living unfavourable by decreasing the warmth and wetness in arid and semi-arid regions. However, it might not have been the sole factor that destroyed the Neolithic cultures in the vast territories of China ca. 4000 cal. yr BP. Environmental alterations in the major cultural territories of China reacted in response to precipitation anomalies caused by high variability of the ASM and the westerlies, which were modulated by centennial- to inter-annual- scale driving factors such as solar insolation, the North Atlantic Oscillation (NAO), the Pacific Decadal Oscillation (PDO) and El Niño-Southern Oscillations (ENSO). This most likely accounted for the nearly synchronous Chinese Neolithic cultural collapses.
A 741-cm-long laminated sediment core, covering the last 10,800 years was collected from Lake Zigetang, central Tibetan Plateau (90.9°E, 32.0°N, 4560
m a.s.l.), and analysed palynologically at 69 ...horizons. Biome reconstruction suggests a dominance of temperate steppe vegetation (mainly
Artemisia and Poaceae) on the central Tibetan Plateau during the first half of the Holocene (10.8–4.4
cal.
ka
BP), while alpine steppes with desert elements (mainly Cyperaceae, Poaceae, Chenopodiaceae, and characteristic high-alpine herb families) tend to dominate the second half (4.4–0
cal.
ka
BP). The
Artemisia/Cyperaceae ratio—a semi-quantitative measure for summer temperature—indicates a general cooling trend throughout the Holocene. Dense temperate steppe vegetation and maximum desert plant withdrawal, however, indicate that a suitable balance of wet and warm conditions for optimum vegetation growth likely occurred during the middle Holocene (7.3–4.4
cal.
ka
BP). Severe Early Holocene cold events have been reconstructed for 8.7–8.3 and ∼7.4
cal.
ka
BP.
Abstract
High-altitude ecosystems react sensitively to hydroclimatic triggers. Here we evaluated the ecological and hydrological changes in a glacier-influenced lake (Hala Hu, China) since the last ...glacial. Rapid fluctuations of aquatic biomarker concentrations, ratios, and hydrogen isotope values, from 15 to 14,000 and 8 to 5000 years before present, provided evidence for aquatic regime shifts and changes in lake hydrology. In contrast, most negative hydrogen isotope values of terrestrial biomarkers were observed between 9 and 7,000 years before present. This shows that shifts of vapour sources and increased precipitation amounts were not relevant drivers behind ecosystem changes in the studied lake. Instead, receding glaciers and increased meltwater discharge, driven by higher temperatures, caused the pronounced ecological responses. The shifts within phytoplankton communities in the Late Glacial and mid Holocene illustrate the vulnerability of comparable ecosystems to climatic and hydrological changes. This is relevant to assess future ecological responses to global warming.
Ostracod assemblage changes and stable isotopes of ostracod shells were used to reconstruct the salinity history of Lake Donggi Cona on the northeastern Tibetan Plateau in response to the late ...glacial and Holocene climate history. Brackish and relatively unstable conditions were inferred for the earlier part of the late glacial between 18.9 and 13.4
cal
ka
BP and for the early to mid Holocene (11.9–6.8
cal
ka
BP). A saline lake existed in the intervening period probably representing colder and drier conditions during the Greenland Stadial 1 (=
Younger Dryas in North Atlantic region). Freshwater conditions similar to the present stage of Lake Donggi Cona were established for the first time at 6.8
cal
ka
BP. This inference and the implied increase in moisture availability in the latter half of the Holocene is in contrast to most climate records from the Tibetan Plateau which suggest drier and colder conditions. However, three lake records from the eastern Tibetan Plateau including our new record show that the Holocene insolation-driven monsoonal weakening and temperature decrease caused a catchment-specific, opposite response of the moisture availability in comparison to those at the majority of sites. Catchments with relatively low precipitation at present experienced apparently a moisture availability increase during the colder second half of the Holocene in contrast to the inferred drier conditions at predominantly generally wetter sites. The increase in moisture availability at relatively dry sites mainly results from the lowered temperature and evaporation but is probably further controlled by a complex of factors including the amount and timing of precipitation, the altitudes of the lake basins and upper reaches, and the existence or development of glaciers, snow fields and frozen ground in a given catchment. Our record from Lake Donggi Cona suggests that the recently observed reduction in Yellow River discharge in response to present global warming will likely continue or even increase under warmer climatic conditions in the future.
In strong tectonic areas such as the eastern margin of the Tibetan Plateau, phyllite is prone to fragment during the uplifting of the earth crust and resulting in widely distributed bimrocks. ...Bimrocks are inhomogeneous and loose geomaterial, which are comprised of impermeable blocks and porous matrix. The inhomogeneous and block spatial distribution of bimrocks makes the evaluation of their flow behavior a challenging task. In this research, we study the flow behavior of bimrocks with the orientation of blocks. To test the permeability property of the bimrocks, a large-scale permeability apparatus was designed and 10 bimrock specimens have been made with various block arrangements. These bimrock specimens contain blocks with different dip angles and dip directions. The permeability test was used to analyze the flow behavior of bimrocks. From the test results, the non-Darcy flow behavior of bimrocks was proposed. The Forchheimer equation is valid used to analyze the flow behavior of bimrocks. The testing results indicate that the orientation of blocks significantly affects the flow behavior of bimrocks. With the increase of the inclinations of blocks, the macroscopic permeability coefficient increases linearly. Besides, as the number of oriented blocks increases, the permeability of the bimrock specimens increases. Furthermore, the orientation of blocks also has a substantial impact on the seepage failure of bimrocks. All the preliminary results throw light on the nature of the flow behavior of bimrocks, which will be helpful to predict the seepage failure of bimrocks with the orientation of blocks.
Extra-tropical circulation systems impede poleward moisture advection by the Indian Summer Monsoon. In this context, the Himalayan range is believed to insulate the south Asian circulation from ...extra-tropical influences and to delineate the northern extent of the Indian Summer Monsoon in central Asia. Paleoclimatic evidence, however, suggests increased moisture availability in the Early Holocene north of the Himalayan range which is attributed to an intensification of the Indian Summer Monsoon. Nevertheless, mechanisms leading to a surpassing of the Himalayan range and the northern maximum extent of summer monsoonal influence remain unknown. Here we show that the Kunlun barrier on the northern Tibetan Plateau ~36°N delimits Indian Summer Monsoon precipitation during the Holocene. The presence of the barrier relocates the insulation effect 1,000 km further north, allowing a continental low intensity branch of the Indian Summer Monsoon which is persistent throughout the Holocene. Precipitation intensities at its northern extent seem to be driven by differentiated solar heating of the Northern Hemisphere indicating dependency on energy-gradients rather than absolute radiation intensities. The identified spatial constraints of monsoonal precipitation will facilitate the prediction of future monsoonal precipitation patterns in Central Asia under varying climatic conditions.
Potential field data are of great significance to the study of geological characteristics. Downward continuation of the potential field converts potential field data from a high plane to a low plane. ...Since this method is mathematically an inverse problem solution, it is unstable. The Tikhonov regularization strategy is an effective means of the downward continuation of the potential field. However, achieving high-precision requirements in the stage of precise geophysical exploration is still challenging. Deep learning can effectively solve unstable problems with excellent nonlinear mapping capabilities. Inspired by this, for the downward continuation of the potential field, we propose a new neural network architecture for downward continuation named D-Unet. This study uses the potential field data of a high horizontal plane and the initial model as the network’s input, with the corresponding low-level data serving as the output for supervised learning. Moreover, we add noise to 10% of the data in the training dataset. Model testing shows that our D-Unet has higher accuracy, validity, and stability. In addition, adding noise to the training data can further improve the robustness of the model. Finally, we use the actual potential data of a particular place in northeast China to test our model and satisfactory results have been obtained.
Sediments of Lake Donggi Cona on the northeastern Tibetan Plateau were studied to infer changes in the lacustrine depositional environment, related to climatic and non-climatic changes during the ...last 19kyr. The lake today fills a 30×8km big and 95m deep tectonic basin, associated with the Kunlun Fault. The study was conducted on a sediment-core transect through the lake basin, in order to gain a complete picture of spatio-temporal environmental change. The recovered sediments are partly finely laminated and are composed of calcareous muds with variable amounts of carbonate micrite, organic matter, detrital silt and clay. On the basis of sedimentological, geochemical, and mineralogical data up to five lithological units (LU) can be distinguished that document distinct stages in the development of the lake system.
The onset of the lowermost LU with lacustrine muds above basal sands indicates that lake level was at least 39m below the present level and started to rise after 19ka, possibly in response to regional deglaciation. At this time, the lacustrine environment was characterized by detrital sediment influx and the deposition of siliciclastic sediment. In two sediment cores, upward grain-size coarsening documents a lake-level fall after 13cal ka BP, possibly associated with the late-glacial Younger Dryas stadial. From 11.5 to 4.3cal ka BP, grain-size fining in sediment cores from the profundal coring sites and the onset of lacustrine deposition at a litoral core site (2m water depth) in a recent marginal bay of Donggi Cona document lake-level rise during the early to mid-Holocene to at least modern level. In addition, high biological productivity and pronounced precipitation of carbonate micrites are consistent with warm and moist climate conditions related to an enhanced influence of summer monsoon. At 4.3cal ka BP the lake system shifted from an aragonite- to a calcite-dominated system, indicating a change towards a fully open hydrological lake system. The younger clay-rich sediments are moreover non-laminated and lack any diagenetic sulphides, pointing to fully ventilated conditions, and the prevailing absence of lake stratification. This turning point in lake history could imply either a threshold response to insolation-forced climate cooling or a response to a non-climatic trigger, such as an erosional event or a tectonic pulse that induced a strong earthquake, which is difficult to decide from our data base.
► In this study we reconstruct with lake sediments the development of lake Donggi Cona. ► We base our study on sedimentological, mineralogical and geochemical analyses. ► The data reveal marked changes during postglacial to Holocene times. ► Changing carbonate phases give clues of changes in the hydrological system. ► Holocene Climate Optimum took place during 11.5, and 6.5cal kyr BP.
A set of 55 recent pollen spectra from the Alashan Plateau and the Qilian Mountains (Qilianshan), northwestern China has been analyzed. The established relationships between the pollen spectra and ...modern vegetation and precipitation patterns of the studied area have been applied to 65 fossil pollen spectra from a 825 cm long sediment record collected in the topographic depression formerly occupied by Eastern Juyan palaeolake (41.89°N; 101.85°E; 892 m a.s.l.). Several qualitative approaches (e.g. indicator plant species;
Artemisia/Chenopodiaceae and
Ephedra fragilis-type s.l./
Ephedra distachya-type pollen ratios) and quantitative methods of vegetation and climate reconstruction have been tested with the surface pollen and then applied to the Eastern Juyan pollen record. Fossil pollen data reveal pronounced environmental changes in the studied area of the Alashan Plateau between ca. 10,700 and 1700 cal. yr B.P. Desert taxa, as Chenopodiaceae,
Nitraria,
Calligonum,
Reaumuria were abundant in the vegetation cover around the study site through the whole record. Pollen spectra dated to ca. 10,700–5400 cal. yr B.P. are characterised by highest values of Chenopodiaceae,
E. fragilis-type and other desert indicating taxa, suggesting rather dry climate. Most favourable conditions are reconstructed between 5400 and 3900 cal. year B.P. on the basis of relative increase in abundance of
Artemisia pollen. A return to dry conditions occurred at about 3900 cal. yr B.P. The lake finally desiccated after 1700 cal. yr B.P. Reconstructed dry climate oscillations between 5900 and 5400 and 3100 cal. yr B.P. correlate well with similar events found in the published records from northern and western China and central Mongolia. However, Eastern Juyan record does not show the early Holocene humidity maximum, as suggested by the sedimentary data from monsoon-influenced areas of China.
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