Jule Xiao

Holocene climate changes over the desert/loess transition of north-central China

A 2-m trench section at the desert/loess transition of north-central China was analyzed at 2-cm intervals for grain-size distribution (Md, μm) and total organic carbon (TOC, %). AMS 14C time series of the Md and TOC records provides direct information on variations of the East Asian monsoon climate during the last 10 000 yr. Md is considered to represent the wind strength and high Md reflects strong winter monsoon winds. TOC denotes the degree of vegetation cover, a function of the intensity of summer monsoon circulations. From ca. 10 000 to 7000 cal yr BP, the climate was dominated by a strong winter monsoon and a very weak summer monsoon as indicated by high Md and low TOC values. Subsequently climatic conditions became warmer gradually from ca. 7000 to 5500 cal yr BP. The period between ca. 5500 and 2700 cal yr BP displayed the highest TOC and lowest Md values over the last 10 000 yr, marking a stable state of a warm, humid climate, i.e. the Holocene climatic optimum of China. During the interval of ca. 2700–1500 cal yr BP, the expansion and intensification of agrarian activities could have led to desertification and acceleration of deposition of coarse-grained particles, resulting in a decreased organic carbon accumulation rate. However, the TOC during this interval still has values as high as during the episode of ca. 7000–5500 cal yr BP, reflecting a relatively stronger summer monsoon circulation. Since ca. 1500 cal yr ago, high-amplitude fluctuations of TOC that fluctuate in phase with the clay-size fraction imply that the climate experienced three major oscillations with a prominent peak of enhanced summer monsoon circulation at ca. 1300 cal yr ago and with two intervals of increased winter monsoon strength occurring around ca. 1000 and 300 cal yr ago. The Md and TOC records over the past 5000 yr demonstrate a good correlation with the archaeological, phenological and historical records of China. Climate changes in the East Asian monsoon region were not only related to the changing seasonality of solar insolation resulting from progressive changes in the Earth’s orbital parameters but were also controlled by the pattern and intensity of the atmospheric circulation.

Pollen-Based Quantitative Reconstruction of Holocene Climate Changes in the Daihai Lake Area, Inner Mongolia, China

Abstract Vegetation around the Daihai Lake, northern China, is very sensitive to climate changes. In this paper, pollen-based quantitative climate reconstructions using three methods [weighted averaging partial least squares method (WAPLS), modern analog technique (MAT), and pollen response surface method (PRS)] were conducted to obtain robust reconstructions of Holocene climate changes in the Daihai Lake area. The result obtained by the three methods all consistently show the annual precipitation to have been 50–100 mm lower in the early Holocene, 100–200 mm higher in the Mid-Holocene, and 50–100 mm lower again in the late Holocene than at present. The WAPLS and the MAT methods also show quasi-synchronous oscillations of the mean annual temperature (Ta); 1°–2°C lower in the Early Holocene and 1°–3°C higher in the Mid-Holocene than today. The time period from 6200 to 5100 cal yr BP was the wettest and the warmest interval, with an annual precipitation (Pa) greater than 550 mm and mean annual temperature T...

Three large shifts in East Asian monsoon circulation indicated by loess-paleosol sequences in China and late Cenozoic deposits in Japan

Abstract Correlations between loess–paleosol sequences in China and late Cenozoic deposits in Japan suggest that the East Asian monsoon circulation experienced three large shifts during the late Cenozoic. About 2.5 Ma ago, the onset of loess accumulation in north-central China accompanied by a dramatic increase in eolian dust flux to the Sea of Japan marked a pronounced development of the East Asian winter monsoon winds. Since ca. 1.2 to 1.1 Ma ago, the obvious alternation of loess and paleosols in the Chinese loess sequence with increased coarse grain-size fraction in the loess units and higher-than-average magnetic susceptibility values in the paleosols, and the repeated occurrence of interstratified marine clay layers in the Osaka Group of central Japan, indicate that the East Asian monsoon circulation strengthened and the contrast between the summer and winter monsoons increased. The paleosols of the Chinese loess sequence were more strongly developed from ca. 550 ka ago, and loess started to accumulate in areas of the mid-lower reaches of the Yangtze River and in Lake Biwa of central Japan from ca. 450 ka ago, reflecting more vigorous summer and winter monsoon circulations since ca. 550 to 450 ka ago. These large changes in East Asian monsoon circulation were not only related to the changing solar radiation, global glacial/interglacial cycles and the changing configuration of land and sea but were also directly controlled by the uplift of the Tibetan Plateau.

Holocene weak monsoon intervals indicated by low lake levels at Hulun Lake in the monsoonal margin region of northeastern Inner Mongolia, China

A 170 cm long sediment core spanning the last 10 000 years was recovered from Hulun Lake in the northeastern margin of the East Asian summer monsoon. The sediment core was analyzed at 1 cm intervals for grain-size distribution and sedimentary component partition. These data provide a proxy record of the monsoon variability on millennial to centennial scales during the Holocene. We used a lognormal distribution function fitting method to partition three to six components from fine to coarse modes within the individual polymodal distributions into overlapping lognormal distributions. Three coarse components representing nearshore suspension, saltation and traction, together with the sand-fraction percentage and the median grain size of bulk samples, indicate the lake levels that fluctuated in response to the intensity of the monsoonal precipitation. Higher percentages of the nearshore components accompanied by more sand-fraction proportions and coarser median grain sizes reflect lower lake stands resulting from weaker monsoon circulations. The results show low levels at Hulun Lake c. 8000—7850, 6400—6050, 5150—4900, 4500—3800, 3050—2800, 1650—1400, 1150—900, 700—600, and 400—350 cal. yr BP, indicating the weakened East Asian summer monsoon during these intervals. We suggest that these weak monsoon events would not only result from the reduced ocean—atmosphere interacting processes in the western tropical Pacific, but could also be related to cold climatic conditions in the Northern Hemisphere high latitudes.

Age of the fossil Dali Man in north-central China deduced from chronostratigraphy of the loess-paleosol sequence

Dali Man, an archaic type of early Homo sapiens, is of great significance to the origin of Homo sapiens. Achievements have been made during the past decades in the understanding of the fossil human skull as well as the accompanying mammalian fossils and stone artifacts. However, the absolute age of the fossil Dali Man still remains unclear. Based on the magnetic susceptibility of loess sediments and the relationship of terracing process with climatic condition, we correlated the loess sequence and the subjacent terrace alluvium at the Dali Man site with the well-studied loess-paleosol sequence at Luochuan in the central part of the Chinese Loess Plateau. The correlation indicates that the loess-paleosol sequence at the Dali Man site extends down to the paleosol S2, and the underlying fluvial deposits correspond to the loess U. Because the Dali Man skull appears in the basal gravel layer of the terrace, and because the deposition of the terrace gravels would occur during the transition from S3 paleosol formation to L3 loess accumulation, the age of the fossil Dali Man is inferred to be ca 270 ka, which is equivalent to the age of the boundary between the loess L3 and paleosol S3 on the magnetic susceptibility time series of the Luochuan loess-paleosol sequence

Insolation driven biomagnetic response to the Holocene Warm Period in semi-arid East Asia

The Holocene Warm Period (HWP) provides valuable insights into the climate system and biotic responses to environmental variability and thus serves as an excellent analogue for future global climate changes. Here we document, for the first time, that warm and wet HWP conditions were highly favourable for magnetofossil proliferation in the semi-arid Asian interior. The pronounced increase of magnetofossil concentrations at ~9.8 ka and decrease at ~5.9 ka in Dali Lake coincided respectively with the onset and termination of the HWP, and are respectively linked to increased nutrient supply due to postglacial warming and poor nutrition due to drying at ~6 ka in the Asian interior. The two-stage transition at ~7.7 ka correlates well with increased organic carbon in middle HWP and suggests that improved climate conditions, leading to high quality nutrient influx, fostered magnetofossil proliferation. Our findings represent an excellent lake record in which magnetofossil abundance is, through nutrient availability, controlled by insolation driven climate changes.

Spatial heterogeneity of the population age structure of the ostracode Limnocythere inopinata in Hulun Lake, Inner Mongolia and its implications

Most existing studies of lacustrine ostracodes have been focused on species composition, and little attention has been paid to population structure. In this study, 39 surface-sediment samples from Hulun Lake, Inner Mongolia, were analyzed for ostracode species composition and 17 of them also for population structure of Limnocythere inopinata, which is the dominant ostracode in the lake. A total of nine ostracode species were recovered, most of which show high abundances at sites with rich plant detritus, implying the possible control of food supply on ostracode species occurrence in the lake. On the other hand, both the subfossil and the living populations of L. inopinata have greater numbers of early instars in deeper waters, while middle and late instars occur mostly in shallower waters, which is related to the grainsize composition of the substrate, denoting a hydraulic control on population structure. Pre- and post-mortem transport of ostracodes may be responsible for the observed within-lake changes in the population structure, although other factors such as migration, intrinsic population dynamics, and life cycle could also be involved. Our data provide insights into the interpretation of lake conditions when using ostracodes in paleolimnological research of lakes with similar natural settings.

Differential transport and preservation of the instars of Limnocythere inopinata (Crustacea, Ostracoda) in three large brackish lakes in northern China

Population age structure, preservation, and carapace abundance of the valve remains of Limnocythere inopinata in 88 surface-sediment samples from Lakes Daihai, Dali, and Hulun were examined to better understand taphonomy of limnic ostracods. The spatial changes in population structure of L. inopinata match the within-lake hydraulic gradients, with the older instars preferentially deposited in the littoral zone while the younger instars being more abundant in deeper waters. We propose that molt and mortem remains of ostracods, especially those of the small juveniles, experience downslope transport. The magnitude of transport in a lake is controlled by wind-driven hydrodynamics. There is no chemical dissolution of ostracod valves in these alkaline lakes, and degradation is mainly represented by physical breakage. The preservation of L. inopinata is related to sedimentation rate and hydraulic stability, and the spatial pattern of preservation varies between lakes. Spatial and seasonal changes in sedimentation rate and hydraulic condition, and the ostracod life cycle may have contributed to this complexity. Adult and A-7 carapaces are the most frequent among the instars. We suggest that the preferential preservation of juvenile carapaces is either related to high mortality rate or to the good preservation in deep waters.

The manifestation of the Younger Dryas event in the East Asian summer monsoon margin: New evidence from carbonate geochemistry of the Dali Lake sediments in northern China:

The processes and mechanisms of the Younger Dryas (YD) event in the modern northern margin of the East Asian summer monsoon (EASM) are still heatedly debated. This study presents new high-resolution (~25 years) records of elements and stable isotopes of <38-µm calcites from a sediment core from Dali Lake in order to investigate the climatic change in the EASM margin at the last glacial–interglacial transition. The <38-µm calcites in the Dali Lake sediments are cubical or rhombohedral, implying that they are predominated by endogenic calcites precipitated within the water body of the lake. High values of Ca and Mg concentrations of the endogenic calcites are interpreted as strong evaporation and low dissolved CO2 concentration of the lake water related to high regional temperature. Concurrent increases in δ13C and δ18O values of the endogenic calcites are interpreted as the result of intensified evaporation associated with high temperature or low precipitation in the region. These data indicate that the climate in the Dali Lake region was relatively warm and wet from 15,500 to 12,800 and from 11,550 to 10,000 cal. yr BP, and cold and dry from 12,800 to 11,550 cal. yr BP, which was generally supported by the evidence from the data of sedimentary organic matter from the same sediment core. In addition, the abruptness of the temperature change in the Dali Lake region from 12,800 to 11,550 cal. yr BP could be corresponded, within age uncertainties, to the YD cold reversal occurring over northern high latitudes. The atmospheric coupling between the North Atlantic region and the EASM margin was proposed as the dominant pattern influencing the climatic change in the Dali Lake region during the YD event.

Spatial patterns of vegetation and climate in the North China Plain during the Last Glacial Maximum and Holocene climatic optimum

Reconstructing the spatial patterns of regional climate and vegetation during specific intervals in the past is important for assessing the possible responses of the ecological environment under future global warming scenarios. In this study, we reconstructed the history of regional vegetation and climate based on six radiocarbon-dated pollen records from the North China Plain. Combining the results with existing pollen records, we reconstruct the paleoenvironment of the North China Plain during the Last Glacial Maximum (LGM) and the Holocene Climatic Optimum (HCO). The results show that changes in the regional vegetation since the LGM were primarily determined by climatic conditions, the geomorphic landscape and by human activity. During the LGM, the climate was cold and dry; mixed broadleaf-coniferous forest and deciduous-evergreen broadleaf forest developed in the southern mountains, and cold-resistant coniferous forest and mixed broadleaf-coniferous forest were present in the northern mountains. The forest cover was relatively low, with mesophytic and hygrophilous meadow occupying the southern part of the plain, and temperate grassland and desert steppe were distributed in the north; Chenopodiaceae-dominated halophytes grew on the exposed continental shelf of the Bohai Sea and Yellow Sea. During the HCO, the climate was warm and wet; deciduous broadleaf forest and deciduous-evergreen broadleaf forest, with subtropical species, developed in the southern mountains, and deciduous broadleaf forest with thermophilic species was present in northern mountains. Although the degree of forest cover was greater than during the LGM, the vegetation of the plain area was still dominated by herbs, while halophytes had migrated inland due to sea level rise. In addition, the expansion of human activities, especially the intensification of cultivation, had a significant influence on the natural vegetation. Our results provide data and a scientific basis for paleoclimate modelling and regional carbon cycle assessment in north China, with implications for predicting changes in the ecological environment under future global warming scenarios.