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Edouard Bard - One of the best experts on this subject based on the ideXlab platform.
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Early Holocene Thermal Maximum recorded by branched tetraethers and pollen in Western Europe (Massif Central, France)
Quaternary Science Reviews, 2020Co-Authors: Céline Martin, Guillemette Menot, Nicolas Thouveny, Odile Peyron, Valérie Andrieu-ponel, Vincent Montade, Nina Davtian, Maurice Reille, Edouard BardAbstract:The evolution of temperatures during the Holocene is controversial, especially for the early Holocene. The occurrence of the Holocene Thermal Maximum (HTM) during the early Holocene has recently been reconsidered and seasonal biases have been suggested in the paleoclimatic proxies. High regional variability and a low number of reliable and continuous quantitative reconstructions compared with the oceanic realm further complicate study of the Holocene climate in the continental realm. We analyzed branched glycerol dialkyl glycerol tetraethers (brGDGTs), an organic paleothermometer, and palyno-logical signals as part of a multiproxy analysis of the sedimentary record from Lake St Front, in the Massif Central (France). Identification of a shift in brGDGT sources through the Holocene required removing terrigenous influences from the temperature signal. BrGDGT-and pollen-inferred paleotemperature reconstructions (based on the Modern Analog Technique and the Weighted Averaging Partial Least Squares method) were compared. Both showed a thermal maximum during the early Holocene followed by a decrease of temperatures. We evaluated biases which could potentially influence the reconstructed signal. There was no evidence for a summer temperature bias either for brGDGT-derived temperatures or for pollen-derived temperatures. The Lake St Front data, in agreement with other regional records, confirm the occurrence of the HTM as a general warm period during the early Holocene followed by mid-Holocene cooling in Western Europe and suggest that seasonal biases are not the main explanation of the Holocene conundrum d the disagreement between model simulations and proxy-based temperature reconstructions for the northern hemisphere.
Fahu Chen - One of the best experts on this subject based on the ideXlab platform.
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position and orientation of the westerly jet determined Holocene rainfall patterns in china
Nature Communications, 2019Co-Authors: Ulrike Herzschuh, Fahu Chen, Xianyong Cao, Thomas Laepple, Anne Dallmeyer, Richard J Telford, Zhaochen Kong, Guangxiu LiuAbstract:Proxy-based reconstructions and modeling of Holocene spatiotemporal precipitation patterns for China and Mongolia have hitherto yielded contradictory results indicating that the basic mechanisms behind the East Asian Summer Monsoon and its interaction with the westerly jet stream remain poorly understood. We present quantitative reconstructions of Holocene precipitation derived from 101 fossil pollen records and analyse them with the help of a minimal empirical model. We show that the westerly jet-stream axis shifted gradually southward and became less tilted since the middle Holocene. This was tracked by the summer monsoon rain band resulting in an early-Holocene precipitation maximum over most of western China, a mid-Holocene maximum in north-central and northeastern China, and a late-Holocene maximum in southeastern China. Our results suggest that a correct simulation of the orientation and position of the westerly jet stream is crucial to the reliable prediction of precipitation patterns in China and Mongolia.
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a persistent Holocene wetting trend in arid central asia with wettest conditions in the late Holocene revealed by multi proxy analyses of loess paleosol sequences in xinjiang china
Quaternary Science Reviews, 2016Co-Authors: Fahu Chen, Jia Jia, Jianhui Chen, Xiaojian Zhang, Haichao Xie, Dunsheng Xia, Wei HuangAbstract:Abstract There are significant differences in the interpretation of the moisture (precipitation) history of arid central Asia (ACA) during the Holocene, as inferred on one hand from speleothem oxygen isotope records, and on the other from lake sediments. Here we present the results of measurements of climatically-sensitive magnetic properties and soil color from four well-dated loess-paleosol sequences from the northern slopes of the Tienshan Mountains and the Yili River valley, Xinjiang, China, in the core area of ACA. Our results demonstrate that the characteristic Holocene paleosol, indicating relatively moist conditions, generally formed after ∼6 ka (1 ka = 1000 cal yr BP) in the study region, and that the accumulation of unweathered loess prevailed during the early Holocene, indicating a dry climate at that time. The magnetic proxies further reveal a trend of generally increasing moisture since the Last Glacial Maximum, with the wettest climate occurring during the late Holocene. This trend of increasing moisture during the Holocene is representative of the Xinjiang region and possibly of the whole of the core area of ACA, and is in marked contrast both to the mid-Holocene moisture maximum observed in the East Asian summer monsoon region and to the general decrease in the strength of the Indian summer monsoon since the early Holocene. Our findings are supported by the results of a climate simulation which indicate a trend of increasing summer and winter precipitation during the Holocene in the core area of ACA, caused mainly by an increase in the strength of the westerlies effected by an increasing latitudinal insolation gradient and by a negative trend of the Arctic Oscillation (AO) or North Atlantic Oscillation (NAO).
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Causes of early Holocene desertification in arid central Asia
Climate Dynamics, 2011Co-Authors: Liya Jin, Fahu Chen, Carrie Morrill, Bette L. Otto-bliesner, Nan RosenbloomAbstract:Paleoclimate records of effective moisture (precipitation minus evaporation, or P–E) show a dry (low effective moisture) period in mid-latitude arid/semi-arid central Asia during the early Holocene (11,000–8,000 years ago) relative to the middle and late Holocene, in contrast to evidence for greater-than-present precipitation at the same time in the south and east Asian monsoonal areas. To investigate the spatial differences in climate response over mid-latitude central Asia and monsoonal Asia we conducted a series of simulations with the Community Climate System Model version 3 coupled climate model for the early, middle and late Holocene. The simulations test the climatic impact of all important forcings for the early Holocene, including changes in orbital parameters, the presence of the remnant Laurentide ice sheet and deglacial freshening of the North Atlantic. Model results clearly show the early Holocene patterns indicated by proxy records, including both the decreased effective moisture in arid central Asia, which occurs in the model primarily during the winter months, and the increase in summer monsoon precipitation in south and east Asia. The model results suggest that dry conditions in the early Holocene in central Asia are closely related to decreased water vapor advection due to reduced westerly wind speed and less evaporation upstream from the Mediterranean, Black, and Caspian Seas in boreal winter. As an extra forcing to the early Holocene climate system, the Laurentide ice sheet and meltwater fluxes have a substantial cooling effect over high latitudes, especially just over and downstream of the ice sheets, but contribute only to a small degree to the early Holocene aridity in central Asia. Instead, most of the effective moisture signal can be explained by orbital forcing decreasing the early Holocene latitudinal temperature gradient and wintertime surface temperature. We find little evidence for regional subsidence related to a stronger summer Asian monsoon in forcing early Holocene aridity in central Asia, as has been previously hypothesized.
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Holocene environmental changes in Mongolia: A review
Global and Planetary Change, 2008Co-Authors: Fahu Chen, Loukas BartonAbstract:Abstract Holocene environmental change in Mongolia is reconstructed on the basis of recently published paleoclimate records, including lake levels, pollen assemblages, and eolian sediment records. These data indicate that the early Holocene of Mongolia is characterized by increasing temperature and humidity. Paleosol development, high lake-stands, and a more southward distribution of forest-steppe environments suggest the early-mid Holocene was humid. The mid-Holocene however is characterized by enhanced aridity, even though the onset and termination of the dry interval differs from place to place. Finally, humidity increased again during the late Holocene, as evaporation decreased in concert with dropping temperatures in Mongolia.
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Holocene vegetation and climate changes from fossil pollen records in arid and semi-arid China
Developments in Quaternary Sciences, 2007Co-Authors: Yan Zhao, Fahu ChenAbstract:Abstract Holocene variability of the summer monsoons has been documented in a variety of proxy records in east and southwest China. However, its influence on regional climate in other parts of China is still poorly understood, especially further inland in north China and northwest China. Here we review fossil pollen records available from arid and semi-arid areas of China (including Inner Mongolia, the northwestern Loess Plateau, the northern Tibetan Plateau, and Xinjiang) to document regional patterns of Holocene vegetation and climate change, and to understand the large-scale controls of these changes. Pollen records from the four regions reveal different vegetation and wet–dry changes during the Holocene. With the exception of the westernmost sites, vegetation at most sites in Inner Mongolia switches between forest, forest steppe, and typical steppe. There is a dry climate after ~6 ka following early to Early – Mid Holocene maximum moisture conditions. At western sites, the climate was dry in the Early Holocene, wet in the Middle Holocene and dry again in the Late Holocene. Vegetation in the northwestern Loess Plateau switches between desert steppe, steppe, and steppe forest, with corridor forests often occurring in loess valleys during the steppe forest period. These changes indicate wet–dry oscillations, from an initial dry climate to a wet Middle Holocene and then back to a dry climate in the Late Holocene. In the northern Tibetan Plateau, vegetation is characterized by steppe, desert steppe, or desert. However, the Qinghai Lake area was dominated by tree pollen during the Early- and Mid-Holocene, indicating a wet climate, until a drying trend started after 6–4.5 ka. In Xinjiang, pollen assemblages show changes between desert, desert steppe or steppe during the Holocene, with a wet period occurring briefly during the early Mid-Holocene at most sites. The highest moisture interval during the Holocene (the so-called Holocene climate optimum) occurred in the Early- to Mid-Holocene in eastern Inner Mongolia, but apparently occurred later during the Mid-Holocene at sites in the west (northern Tibetan Plateau and Xinjiang). The complex climate patterns during the Holocene in arid and semi-arid China suggest regional climate responses to large-scale climate forcing was controlled by interactions of competing factors including the monsoons, westerlies and topography. The decline of forest in the eastern Tibetan Plateau, the Loess Plateau valleys and Inner Mongolia during the Late Holocene may have been caused by human activity.
Odile Peyron - One of the best experts on this subject based on the ideXlab platform.
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Early Holocene Thermal Maximum recorded by branched tetraethers and pollen in Western Europe (Massif Central, France)
Quaternary Science Reviews, 2020Co-Authors: Céline Martin, Guillemette Menot, Nicolas Thouveny, Odile Peyron, Valérie Andrieu-ponel, Vincent Montade, Nina Davtian, Maurice Reille, Edouard BardAbstract:The evolution of temperatures during the Holocene is controversial, especially for the early Holocene. The occurrence of the Holocene Thermal Maximum (HTM) during the early Holocene has recently been reconsidered and seasonal biases have been suggested in the paleoclimatic proxies. High regional variability and a low number of reliable and continuous quantitative reconstructions compared with the oceanic realm further complicate study of the Holocene climate in the continental realm. We analyzed branched glycerol dialkyl glycerol tetraethers (brGDGTs), an organic paleothermometer, and palyno-logical signals as part of a multiproxy analysis of the sedimentary record from Lake St Front, in the Massif Central (France). Identification of a shift in brGDGT sources through the Holocene required removing terrigenous influences from the temperature signal. BrGDGT-and pollen-inferred paleotemperature reconstructions (based on the Modern Analog Technique and the Weighted Averaging Partial Least Squares method) were compared. Both showed a thermal maximum during the early Holocene followed by a decrease of temperatures. We evaluated biases which could potentially influence the reconstructed signal. There was no evidence for a summer temperature bias either for brGDGT-derived temperatures or for pollen-derived temperatures. The Lake St Front data, in agreement with other regional records, confirm the occurrence of the HTM as a general warm period during the early Holocene followed by mid-Holocene cooling in Western Europe and suggest that seasonal biases are not the main explanation of the Holocene conundrum d the disagreement between model simulations and proxy-based temperature reconstructions for the northern hemisphere.
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Precipitation changes in the Mediterranean basin during the Holocene from terrestrial and marine pollen records: a model-data comparison
Climate of the Past, 2017Co-Authors: Odile Peyron, Valérie Andrieu-ponel, Nathalie Combourieu-nebout, David Brayshaw, Simon Goring, Stephanie Desprat, Will Fletcher, Belinda Gambin, Chryssanthi Ioakim, Sebastien JoanninAbstract:Climate evolution of the Mediterranean region during the Holocene exhibits strong spatial and temporal variability, which is notoriously difficult for models to reproduce. We propose here a new proxy-based climate synthesis synthesis and its comparison -at a regional (similar to 100 km) level - with a regional climate model to examine (i) opposing northern and southern precipitation regimes and (ii) an east-to-west precipitation dipole during the Holocene across the Mediterranean basin. Using precipitation estimates in-ferred from marine and terrestrial pollen archives, we focus on the early to mid-Holocene (8000 to 6000 cal yr BP) and the late Holocene (4000 to 2000 cal yr BP), to test these hypotheses on a Mediterranean-wide scale. Special attention was given to the reconstruction of season-specific climate in-formation, notably summer and winter precipitation. The reconstructed climatic trends corroborate the north-south partition of precipitation regimes during the Holocene. During the early Holocene, relatively wet conditions occurred in the south-central and eastern Mediterranean regions, while drier conditions prevailed from 45 degrees N northwards. These patterns then reverse during the late Holocene. With regard to the existence of a west-east precipitation dipole during the Holocene, our results show that the strength of this dipole is strongly linked to the reconstructed seasonal parameter; early-Holocene summers show a clear east-west division, with summer precipitation having been highest in Greece and the eastern Mediterranean and lowest over Italy and the western Mediterranean. Summer precipitation in the east re-mained above modern values, even during the late-Holocene interval. In contrast, winter precipitation signals are less spatially coherent during the early Holocene but low precipita-tion is evidenced during the late Holocene. A general drying trend occurred from the early to late Holocene, particularly in the central and eastern Mediterranean. For the same time intervals, pollen-inferred precipita-tion estimates were compared with model outputs, based on a regional-scale downscaling (HadRM3) of a set of global climate-model simulations (HadAM3). The high-resolution detail achieved through the downscaling is intended to enable a better comparison between site-based paleo-reconstructions and gridded model data in the complex terrain of the Mediterranean; the model outputs and pollen-inferred precipitation estimates show some overall correspondence, though modeled changes are small and at the absolute margins of statistical significance. There are suggestions that the eastern Mediterranean experienced wetter summer conditions than present during the early and late Holocene; the drying trend in winter from the early to the late Holocene also appears to be simulated. The use of this high-resolution regional climate model highlights how the inherently patchy nature of climate signals and paleo-records in the Mediterranean basin may lead to local signals that are much stronger than the large-scale pattern would suggest. Nevertheless, the east-to-west division in summer precipitation seems more marked in the pollen reconstruction than in the model outputs. The footprint of the anomalies (like today, or dry winters and wet summers) has some similarities to modern analogue atmospheric circulation patterns associated with a strong westerly circulation in winter (positive Arctic Oscillation-North Atlantic Oscillation (AO-NAO)) and a weak westerly circulation in summer associated with anticy-clonic blocking; however, there also remain important differences between the paleo-simulations and these analogues. The regional climate model, consistent with other global models, does not suggest an extension of the African summer monsoon into the Mediterranean. Therefore, the extent to which summer monsoonal precipitation may have existed in the southern and eastern Mediterranean during the mid-Holocene remains an outstanding question.
Valérie Andrieu-ponel - One of the best experts on this subject based on the ideXlab platform.
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Early Holocene Thermal Maximum recorded by branched tetraethers and pollen in Western Europe (Massif Central, France)
Quaternary Science Reviews, 2020Co-Authors: Céline Martin, Guillemette Menot, Nicolas Thouveny, Odile Peyron, Valérie Andrieu-ponel, Vincent Montade, Nina Davtian, Maurice Reille, Edouard BardAbstract:The evolution of temperatures during the Holocene is controversial, especially for the early Holocene. The occurrence of the Holocene Thermal Maximum (HTM) during the early Holocene has recently been reconsidered and seasonal biases have been suggested in the paleoclimatic proxies. High regional variability and a low number of reliable and continuous quantitative reconstructions compared with the oceanic realm further complicate study of the Holocene climate in the continental realm. We analyzed branched glycerol dialkyl glycerol tetraethers (brGDGTs), an organic paleothermometer, and palyno-logical signals as part of a multiproxy analysis of the sedimentary record from Lake St Front, in the Massif Central (France). Identification of a shift in brGDGT sources through the Holocene required removing terrigenous influences from the temperature signal. BrGDGT-and pollen-inferred paleotemperature reconstructions (based on the Modern Analog Technique and the Weighted Averaging Partial Least Squares method) were compared. Both showed a thermal maximum during the early Holocene followed by a decrease of temperatures. We evaluated biases which could potentially influence the reconstructed signal. There was no evidence for a summer temperature bias either for brGDGT-derived temperatures or for pollen-derived temperatures. The Lake St Front data, in agreement with other regional records, confirm the occurrence of the HTM as a general warm period during the early Holocene followed by mid-Holocene cooling in Western Europe and suggest that seasonal biases are not the main explanation of the Holocene conundrum d the disagreement between model simulations and proxy-based temperature reconstructions for the northern hemisphere.
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Precipitation changes in the Mediterranean basin during the Holocene from terrestrial and marine pollen records: a model-data comparison
Climate of the Past, 2017Co-Authors: Odile Peyron, Valérie Andrieu-ponel, Nathalie Combourieu-nebout, David Brayshaw, Simon Goring, Stephanie Desprat, Will Fletcher, Belinda Gambin, Chryssanthi Ioakim, Sebastien JoanninAbstract:Climate evolution of the Mediterranean region during the Holocene exhibits strong spatial and temporal variability, which is notoriously difficult for models to reproduce. We propose here a new proxy-based climate synthesis synthesis and its comparison -at a regional (similar to 100 km) level - with a regional climate model to examine (i) opposing northern and southern precipitation regimes and (ii) an east-to-west precipitation dipole during the Holocene across the Mediterranean basin. Using precipitation estimates in-ferred from marine and terrestrial pollen archives, we focus on the early to mid-Holocene (8000 to 6000 cal yr BP) and the late Holocene (4000 to 2000 cal yr BP), to test these hypotheses on a Mediterranean-wide scale. Special attention was given to the reconstruction of season-specific climate in-formation, notably summer and winter precipitation. The reconstructed climatic trends corroborate the north-south partition of precipitation regimes during the Holocene. During the early Holocene, relatively wet conditions occurred in the south-central and eastern Mediterranean regions, while drier conditions prevailed from 45 degrees N northwards. These patterns then reverse during the late Holocene. With regard to the existence of a west-east precipitation dipole during the Holocene, our results show that the strength of this dipole is strongly linked to the reconstructed seasonal parameter; early-Holocene summers show a clear east-west division, with summer precipitation having been highest in Greece and the eastern Mediterranean and lowest over Italy and the western Mediterranean. Summer precipitation in the east re-mained above modern values, even during the late-Holocene interval. In contrast, winter precipitation signals are less spatially coherent during the early Holocene but low precipita-tion is evidenced during the late Holocene. A general drying trend occurred from the early to late Holocene, particularly in the central and eastern Mediterranean. For the same time intervals, pollen-inferred precipita-tion estimates were compared with model outputs, based on a regional-scale downscaling (HadRM3) of a set of global climate-model simulations (HadAM3). The high-resolution detail achieved through the downscaling is intended to enable a better comparison between site-based paleo-reconstructions and gridded model data in the complex terrain of the Mediterranean; the model outputs and pollen-inferred precipitation estimates show some overall correspondence, though modeled changes are small and at the absolute margins of statistical significance. There are suggestions that the eastern Mediterranean experienced wetter summer conditions than present during the early and late Holocene; the drying trend in winter from the early to the late Holocene also appears to be simulated. The use of this high-resolution regional climate model highlights how the inherently patchy nature of climate signals and paleo-records in the Mediterranean basin may lead to local signals that are much stronger than the large-scale pattern would suggest. Nevertheless, the east-to-west division in summer precipitation seems more marked in the pollen reconstruction than in the model outputs. The footprint of the anomalies (like today, or dry winters and wet summers) has some similarities to modern analogue atmospheric circulation patterns associated with a strong westerly circulation in winter (positive Arctic Oscillation-North Atlantic Oscillation (AO-NAO)) and a weak westerly circulation in summer associated with anticy-clonic blocking; however, there also remain important differences between the paleo-simulations and these analogues. The regional climate model, consistent with other global models, does not suggest an extension of the African summer monsoon into the Mediterranean. Therefore, the extent to which summer monsoonal precipitation may have existed in the southern and eastern Mediterranean during the mid-Holocene remains an outstanding question.
Céline Martin - One of the best experts on this subject based on the ideXlab platform.
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Early Holocene Thermal Maximum recorded by branched tetraethers and pollen in Western Europe (Massif Central, France)
Quaternary Science Reviews, 2020Co-Authors: Céline Martin, Guillemette Menot, Nicolas Thouveny, Odile Peyron, Valérie Andrieu-ponel, Vincent Montade, Nina Davtian, Maurice Reille, Edouard BardAbstract:The evolution of temperatures during the Holocene is controversial, especially for the early Holocene. The occurrence of the Holocene Thermal Maximum (HTM) during the early Holocene has recently been reconsidered and seasonal biases have been suggested in the paleoclimatic proxies. High regional variability and a low number of reliable and continuous quantitative reconstructions compared with the oceanic realm further complicate study of the Holocene climate in the continental realm. We analyzed branched glycerol dialkyl glycerol tetraethers (brGDGTs), an organic paleothermometer, and palyno-logical signals as part of a multiproxy analysis of the sedimentary record from Lake St Front, in the Massif Central (France). Identification of a shift in brGDGT sources through the Holocene required removing terrigenous influences from the temperature signal. BrGDGT-and pollen-inferred paleotemperature reconstructions (based on the Modern Analog Technique and the Weighted Averaging Partial Least Squares method) were compared. Both showed a thermal maximum during the early Holocene followed by a decrease of temperatures. We evaluated biases which could potentially influence the reconstructed signal. There was no evidence for a summer temperature bias either for brGDGT-derived temperatures or for pollen-derived temperatures. The Lake St Front data, in agreement with other regional records, confirm the occurrence of the HTM as a general warm period during the early Holocene followed by mid-Holocene cooling in Western Europe and suggest that seasonal biases are not the main explanation of the Holocene conundrum d the disagreement between model simulations and proxy-based temperature reconstructions for the northern hemisphere.
