Holocene

摘要： The evolution of Holocene climate was investigated using grain size and magnetic susceptibility of the Holocene paleosols from Baicaoyuan(BCY),Xifeng(XF) and Linyou(LY) sections in the northwest,central and southern Chinese Loess Plateau(CLP).The results show that the Holocene in the B CY,XF and LY paleosol sections could be divided into three phases:during the early Holocene(11.8-10.5 kyr B.P.),increased magnetic susceptibility(χ) and frequency-dependent magnetic susceptibility(χ) and decreased median grain size(Md) indicate that the East Asian Summer Monsoon(EASM) has become more intense and the climate has changed from cold to warm.During the middle Holocene(10.5-5.0 kyr B.P.),the values of χ,χand 2-5 μm grain-size fraction(GT2/5) are higher and Md and 30-63 μm grain-size fraction(GT3 0/6 3) are the lowest,which reflect a warm and humid regional climate.At~8.0 kyr B.P.,there was a transient dry-cold climatic spike corresponding to a Heinrich Event,the cold event was likely due to the collapse of the Laurentide ice sheet.During the late Holocene(5.0-0 kyr B.P.),χ and χvalues are the lowest,while Md and GT30/63 are high,as proxies of a weakened EASM,when the dry-cold climate prevailed in the region.Decreased irradiance since 5.0 kyr B.P.may have caused climatic cooling and drying.Spatially,the increased main peak values and skewness from BCY,XF to LY sections show that the grain size became fine,the East Asian Winter Monsoon(EAWM) reduced,and climate was warmer and wetter from northwest to southeast.In addition,Md can be used as alternative proxy for EAWM,while χis positive with the intensity of EASM under semiarid climate conditions in CLP.

摘要： Topographic/bathymetric conditions of the continental shelf can significantly influence the long-term growth of river deltas.In particular,these conditions constrain the accommodation space for sedimentation in the deltaic areas.In this study,we use a conceptual geometric model to evaluate the role played by this factor,on the basis of the principle of mass conservation.The Ganges-Brahmaputra,Mekong and Nile deltas are examined as three typical examples,in terms of their different original seabed morphologies.The control variate method is applied to eliminate the effect of the difference in model input variables.The results show that,assuming a constant sediment load,the delta growth rate will decrease with time;a higher value of the original seabed slope leads to a lower shoreline progradation rate for the subaerial delta and a higher growth rate for the subaqueous delta.Thus,the original seabed morphology represented by slope is a critical factor affecting the evolution of Holocene large-river deltas.These results explain the interrelationships between sediment load,deltaic plain area,and the original seabed slopes for the 27 large-river deltas worldwide,located in the middle/low latitudes,with different tectonic backgrounds.In the future,the conceptual geometric model may be combined with sediment dynamic modeling to identify more details of the evolution of these deltas.

摘要： Pastoralism is considered a crucial factor in the eventual year-round occupation of high-altitude regions(>3000 m asl)of the Qinghai-Tibetan Plateau,and the northeastern Qinghai-Tibetan Plateau(NE-QTP)was an important corridor for early human occupation.We analyzed fossil fungal spore records for the last 8500 years from a high resolution aeolian section at Langgeri(LGR)on the NE-QTP.Thirty-two fungal spore types were identified in the LGR section,including seven coprophilous types.We combined analysis of coprophilous fungal spores,Cyperaceae,Artemisia,and Hippophae pollen,and the charcoal>50µm fraction to explore the timing and controls of pastoralism on the NE-QTP since the middle Holocene.Pastoralism commenced at LGR shortly before ca.5.5 ka and gradually increased between ca.5.5–3.5 ka,but markedly intensified after ca.2.2 ka,with three periods of growth at ca.2.1–1.9,1.4–1.2,and 0.6–0 ka,and a decline at ca.1.1–0.6 ka.The timing of changes in pastoral activity on the NE-QTP based on the coprophilous fungal spore record is supported by regional archeology and historical documents.Technological and political developments,rather than climate change,played key roles in the long-term fluctuations of regional pastoralism on the NE-QTP in the late Holocene.

摘要： Lake level and its inferred East Asian summer monsoon(EASM)evolution in northern boundary of EASM during the Holocene are highly debated.Here,we present a 15-ka record of glycerol dialkyl glycerol tetraethers(GDGTs)in a closed lake in northern China to address this issue.Surface and downcore sediment data demonstrate sedimentary GDGT-0 and branched GDGTs(brGDGTs)are of aquatic production.Contents of GDGT-0 and brGDGTs increase but brGDGT-based paleo-temperature proxy decreases with water depth in surface sediments along a nearshore to offshore transect.This trend is likely due to an increasingly hypoxic condition,favored by anaerobic microbes producing these GDGT-0 and brGDGTs,in bottom water.Accordingly,brGDGT-derived water temperature and pH would reflect bottom water conditions that are largely regulated by water depth of the lake.Downcore GDGT data and their derived temperature and pH consistently reveal a gradually increasing lake level from the late Pleistocene to the early Holocene,a high level during 9.5-5 ka BP and a decreasing trend afterward.The lake-level records from independent and robust proxies in this study largely agree with pollen records in the region,supporting a mid-Holocene maximum EASM precipitation.

摘要： Quantitative climate reconstruction on long timescales can provide important insights for understanding the climate variability and providing valuable data for simulations. Unfortunately, the credibility of some attempts was hampered by incomplete reconstruction procedures. We here establish a comprehensive framework resting on high-quality Chinese modern pollen database, including modern pollen data screening, calibration set selection, major climate factor analysis, appropriate model selection, strict statistical assessment of results and ecological interpretation. The application of this framework to three high-resolution pollen records from the eastern Tibetan Plateau allows accurate quantitative inferences of Holocene temperature changes, which is the major control of regional vegetation. The results show that the mean warmest month temperature(MTwa)during the early Holocene was ca. 10.4°C and reached the highest value at 8.5–6 ka BP(ca. 11°C). The early and mid-Holocene(11–5 ka BP) warmth was followed by 1.2°C temperature decrease, culminating in the coolest temperatures of the Holocene during the Neoglacial cooling. Superimposing on the general cooling trend, MTwareveals a significant 500-yr periodicity with varying intensities through time, showing that warm(cold) intervals are in phase with solar maxima(minima) periods. This spectral similarity indicates a possible connection of multi-century scale climate fluctuations with solar forcing.

摘要： The fluctuation of a single lake level is a comprehensive reflection of water balance within the basin, while the regional consistent fluctuations of lake level can indicate the change of regional effective moisture. Previous researches were mainly focused on reconstructing effective moisture by multiproxy analyses of lake sediments. We carried out a series of experiments, including a transient climate evolution model, a lake energy balance model and a lake water balance model to simulate continuous Holocene effective moisture change represented by variability of virtual lake level in East and Central Asia.The virtual lake level, area, water depth and salinity are not equivalent to actual values, but we estimated relative changes of the regional effective moisture. We also explored the driving mechanisms of effective moisture change in different geographical regions. Our results indicated that gradually falling effective moisture during the Holocene in northern China was due to the combined effects of high lake evaporation caused by longwave and shortwave radiation, and low precipitation caused by reductions of summer solar insolation. A decline in effective moisture through the Holocene in the Tibetan Plateau and southern Central Asia resulted from decreased precipitation because of the weakening of the Asian summer monsoon. Increased precipitation induced by the strengthening of the westerly circulation contributed to the effective moisture rise during the Holocene in northern Central Asia.

摘要： The impact of global warming on the climate of northern China has been investigated intensively, and the behavior of the East Asian monsoon during previous intervals of climatic warming may provide insight into future changes. In this study, we use paleovegetation records from loess and lake sediments in the marginal zone of the East Asian summer monsoon(EASM) to reconstruct the EASM during the interval of warming from the Last Glacial Maximum(LGM) to the Holocene. The results show that during the LGM, desert steppe or dry steppe dominated much of northern China;in addition, the southeastern margin of the deserts east of the Helan Mountains had a distribution similar to that of the present-day, or was located slightly further south, due to the cold and dry climate caused by a strengthened East Asian winter monsoon(EAWM) and weakened EASM. During the last deglaciation, with the strengthening of the EASM and concomitant weakening of the EAWM, northern China gradually became humid. However, this trend was interrupted by abrupt cooling during the Heinrich 1(H1) and Younger Dryas(YD) events. The EASM intensified substantially during the Holocene, and the monsoon rain belt migrated at least 300 km northwestwards, which led to the substantial shrinking of the desert area in the central and eastern part of northern China, and to the large expansion of plants favored by warm and humid conditions. Paleoclimatic records from the marginal zone of the EASM all show that the EASM reached its peak in the mid-Holocene, and past global climatic warming significantly strengthened the EASM, thereby greatly improving the ecological environment in northern China. Thus, northern China is expected to become wetter as global warming continues. Finally, high resolution Holocene vegetation records are sparse compared with the numerous records on the orbital timescale, and there is a need for more studies of Holocene climatic variability on the centennial-to-decadal scale.

摘要： This article presents the information of the Georgian Caucasus glaciation at the Late Pleistocene and Holocene period (~126,000-11,700 calendar years ago). Our primary aim was to numerically reproduce the ice extent deduced from geological and geomorphological mapping. We used the analog method with the 30 m resolution SRTM DEM (Shuttle Radar Topography Mission Digital Elevation Model). In addition, the rates of glaciation of those times are identified based on the stadial moraines and erratic boulders. The current investigation has revealed that in the Late Pleistocene, the central and western Caucasus characterized the highest glaciation, while the eastern Caucasus boasted the lowest glaciated area, and in the southern Georgia’s highland glaciation has almost the same form as there is in the Eastern Caucasus today. The longest glaciers were located in the Enguri (Nenskra ~36 km, Mulkhura ~35.1 km, Dolra ~34.5 km), Kodori (Sakeni ~25 km, Klichi ~20 km, Marukhi ~17.3 km) Rioni (Buba ~23 km, Kirtisho ~20.5 km, Jejora ~17.5 km) and Tergi (Devdoraki ~38.5 km, Suatisi ~32 km) river basins. We found, that topography thresholds related to the elevation and hypsometry of individual catchments controlled the gradient of the rate of glacier expansion in the domain at that time.

摘要： Biostratigraphic Zones Bb. Bz. H. I-V distinguished in C14 dated Peat, peaty clay sediments above arid, Barren zone have identified distinct environment of deposition as fresh water mixed brackish water to shallow marine to brackish water mangrove swamp, brackish water mixed fresh water swamp followed by colonization of non-littoral species to fresh water swamp during Holocene in the Bengal basin, India in chronological succession. The successive phases of depositional environment have identified the events of sea level rise, marine transgression and sea ward movement of the sea. The unique database has explored successive changes in the geomorphology of South Bengal from upland dry to marine deltaic environment to fresh water upland condition.

摘要： Temporal and spatial variations in accommodation (i.e., paleo-water depth) and sediment accumulation (amount of deposition) in the intra-arc Osaka Basin, Japan, were reconstructed from the post-glacial transgression through the sea-level highstand, a total of 9000 years. At the beginning of the marine transgressive stage (about 11,000 cal y BP), paleo-water depths were shallow and the sediment accumulation was large. The area occupied by Osaka Bay gradually extended and sediment deposition decreased from 11,000 to 6000 cal y BP. During the period of maximum transgression (6000 - 5000 cal y BP), an inner bay, Kawachi Bay with a water depth of 5 - 10 m, was expanded in the inland eastern Osaka area, and paleo-water depths reached a maximum and depositional rates reached a minimum. During the subsequent highstand and small regression (about 5000 cal y BP to the present), however, deposition increased rapidly as a result of river delta and shoreline progradations. Regional differences were observed in accommodation and accumulation between the outer bay area and the inner bay area. During both the transgressive and regressive stages, deposition decreased in the inner bay area. In contrast, in the outer bay area and in the basin overall, deposition was high during the first part of the transgressive stage but it decreased during the maximum transgression, before reaching a maximum during the subsequent highstand and regression. During the regressive stage, fluvial delta progradation led to the formation of a thick sequence of delta body sediments. Sediment accumulation was 30% - 40% higher during the regressive stage than that during the transgressive stage.