Jianbao Liu

East Asian summer monsoon precipitation variability since the last deglaciation

The lack of a precisely-dated, unequivocal climate proxy from northern China, where precipitation variability is traditionally considered as an East Asian summer monsoon (EASM) indicator, impedes our understanding of the behaviour and dynamics of the EASM. Here we present a well-dated, pollen-based, ~20-yr-resolution quantitative precipitation reconstruction (derived using a transfer function) from an alpine lake in North China, which provides for the first time a direct record of EASM evolution since 14.7 ka (ka = thousands of years before present, where the “present” is defined as the year AD 1950). Our record reveals a gradually intensifying monsoon from 14.7–7.0 ka, a maximum monsoon (30% higher precipitation than present) from ~7.8–5.3 ka, and a rapid decline since ~3.3 ka. These insolation-driven EASM trends were punctuated by two millennial-scale weakening events which occurred synchronously to the cold Younger Dryas and at ~9.5–8.5 ka, and by two centennial-scale intervals of enhanced (weakened) monsoon during the Medieval Warm Period (Little Ice Age). Our precipitation reconstruction, consistent with temperature changes but quite different from the prevailing view of EASM evolution, points to strong internal feedback processes driving the EASM, and may aid our understanding of future monsoon behaviour under ongoing anthropogenic climate change.

Holocene stalagmite δ18O records in the East Asian monsoon region and their correlation with those in the Indian monsoon region

We compare well-dated Holocene stalagmite δ18O records from the East Asian Summer Monsoon (EASM) region and from the Indian Summer Monsoon (ISM) region. We found that the pattern of latitudinal change of speleothem δ18O values from the Indian monsoon region to the East Asian monsoon region is almost identical to that of modern precipitation. This suggests that the modern moisture transport route from the Indian Ocean to the East Asian monsoon region has existed since at least the early Holocene. The δ18O records from both regions exhibit a remarkably similar trend of variation in that the values are more negative during the early Holocene, rapidly become heavy from the mid-Holocene, and are heaviest during the late Holocene. The stalagmite δ18O changes in the East Asian monsoon region are statistically well correlated with those in the Indian monsoon region, both over the entire Holocene and in detail over the last 2000 years. However, in contrast to the obvious consistency of the δ18O values in all of the speleothem records, both instrumental and historical climate records indicate significant spatial variations in rainfall over eastern China. The early-Holocene strong EASM pattern referred from speleothem δ18O in the East Asian monsoon region is quite different to that of other paleoclimatic records such as Holocene paleosol development in the Chinese Loess Plateau, eolian activity in the northern Chinese sandlands, and lake sediments in EASM-dominated region in China, in which the strongest EASM was documented during the mid-Holocene. These findings suggest that the speleothem δ18O record from the East Asian monsoon region may not record EASM variability, but rather that it is controlled by variations in the isotopic composition of precipitation, which is determined mainly by rainfall variability in the ISM region.

Chinese cave δ 18 O records do not represent northern East Asian summer monsoon rainfall

In a recent study, Goldsmith et al. reconstructed the Holocene lake-level history of Lake Dali in Inner Mongolia from fossil beach ridges and assume it reflected East Asian summer monsoon (EASM) rainfall (1). In addition, they note a good correlation between the lake fluctuations and Chinese speleothem δ18O records, and therefore conclude that Chinese δ18O records could be used to represent the monsoonal rainfall of northern China. However, to draw these conclusions, it is necessary to consider factors such as lake hydrology, water source, and the evidence of other extant EASM rainfall records in northern China. In fact, the level of Lake Dali was not controlled solely by monsoonal rainfall because it was also fed by snow/ice melt from the Da Hinggan Range (2). For example, previous studies have demonstrated that Lake Dali reached a highstand in the early … [↵][1]2To whom correspondence should be addressed. Email: fhchen{at}lzu.edu.cn. [1]: #xref-corresp-1-1

On the timing of the East Asian summer monsoon maximum during the Holocene—Does the speleothem oxygen isotope record reflect monsoon rainfall variability?

The evolution of the East Asian summer monsoon (EASM) during the Holocene has long been of significant interest. Knowledge of past EASM variability not only increases our understanding of monsoon dynamics on a long timescale, but it also provides an environmental and climatic background for research into Chinese cultural development. However, the timing of the EASM maximum remains controversial. The popular concept of an “early Holocene maximum” is mainly based on speleothem δ18O (δ18Oc) records from caves in southern China; however, the interpretation of δ18Oc as a reliable proxy for EASM intensity is being increasingly challenged. The present paper is a critical review of the climatic significance of the δ18Oc record from China. Firstly, we suggest that precipitation in northern China is an appropriate index of EASM intensity, the variation of which clearly indicates a mid-Holocene monsoon maximum. Secondly, an interregional comparison demonstrates that the precipitation record in northern China is quite different from that in southern China on a range of timescales, and is inconsistent with the spatial similarity exhibited by speleothem oxygen isotope records. Furthermore, both modeling and observational data show that the δ18Oc records from southern China indeed reflect changes in precipitation δ18O (δ18Op) rather than precipitation amount, and therefore that their use as an EASM proxy is inappropriate. Finally, we address several significant monsoon-related issues—including the driving mechanism of the EASM on an orbital timescale, the climatic significance of speleothem oxygen isotopes, and the relationship between atmospheric circulation and precipitation in monsoonal regions.

Changing intensity of human activity over the last 2,000 years recorded by the magnetic characteristics of sediments from Xingyun Lake, Yunnan, China

AbstractWe report mineral magnetic measurements (magnetic susceptibility, anhysteretic and isothermal remanence, magnetic hysteresis and thermomagnetic properties) from a 2.46-m-long sediment core taken in Xingyun Lake, Yunnan, China. Results demonstrate that magnetic minerals in the lake sediment are dominated by stable single-domain (SSD) ferrimagnetic minerals derived mainly from in-washed catchment soil. Magnetic susceptibility increases sharply during the period AD ~500–1050, and the grain size of magnetic minerals decreases. These changes are accompanied by increases in the concentrations of Fe, Ti, Zr and Si, and by decreased tree pollen percentages. δ18O measures in speleothems from South China demonstrate that climate was relatively uniform during this interval. Documentary evidence, however, indicates a large increase in the human population of Yunnan Province at that time. We conclude that the late Holocene sediment record from Xingyun Lake reflects a dramatic increase in soil erosion that was caused by a growing human population, which in turn was responsible for increased deforestation and agricultural activity. Our results from Xingyun Lake are consistent with sediment magnetic records from Erhai Lake, located near our study site. We therefore conclude that the interval AD ~500–1050 corresponded to a regional-scale increase in the intensity of human activity on the Yunnan-Guizhou Plateau.

Weakening of the East Asian summer monsoon at 1000–1100 A.D. within the Medieval Climate Anomaly: Possible linkage to changes in the Indian Ocean‐western Pacific

Monsoon droughts, especially on a decadal-to-centennial timescale, may have a profound impact on the populations of East Asia. Previous work has suggested that the East Asian summer monsoon (EASM) was synchronously strong across East Asia during the Medieval Climate Anomaly (MCA, 900-1300A.D.); however, there is a dearth of studies addressing the issue of whether or not the EASM varied significantly during the entire duration of the MCA. Here we present results from a diverse range of proxy paleoclimatic records from the monsoonal and temperate Asian region in order to evaluate the occurrence of such short timescale variability within the MCA. Within the context of an overall strong EASM during the MCA, a weakening of the monsoon was detected in many of the records during the period 1000-1100A.D. Comparison of the timing of this event with variations of sea surface temperature (SST) of the Indian Ocean-western Pacific and with proxy records of solar activity reveals a significant covariation, suggesting that the driver of the event may have resulted from changes in the Indian Ocean-western Pacific, related to changes in solar activity. To further address the issue of a terrestrial-oceanic linkage, we used the ECHAM and the global Hamburg Ocean Primitive Equation (ECHO-G) coupled climate model to simulate the variation of EASM precipitation over the last millennium. The model results suggest an interval of weak East Asian summer monsoon at 1000-1100A.D., and they also reveal a significant positive correlation with the SST of the Indian Ocean-western Pacific. Key Points Weakening of the East Asian Summer monsoon at 1000-1100 AD The MCA was not a period of uniformly strong summer monsoonal activity An atmospheric-oceanic driver of the weakening of East Asia summer monsoon

Vegetation succession and East Asian Summer Monsoon Changes since the last deglaciation inferred from high-resolution pollen record in Gonghai Lake, Shanxi Province, China:

A cal. 20-year-resolution pollen record from Gonghai Lake presented the detailed process of mountain vegetation succession and East Asian Summer Monsoon (EASM) changes since the last deglaciation in Shanxi Province, North China. Modern vegetation distribution and lake surface pollen assemblages suggested that the fossil pollen mainly came from local and surrounding vegetation in Gonghai Lake, which reflected the elevational changes of plant communities in study area. From 14,700 to 11,100 cal. yr BP, open forests and mountain meadows dominated by shrubs and herbaceous species in surrounding area, suggesting a weak EASM with less precipitation. In the period between 11,100 and 7300 cal. yr BP, bushwoods and grasses were gradually replaced by mixed broadleaf-conifer forest, first developed by pioneer species of Betula and Populus and then replaced by Picea, Pinus, and Quercus, implying an enhanced EASM and increased temperature and precipitation. During the period of 7300–5000 cal. yr BP, warm-fitted trees became expanded and widespread, indicating a climax community of mixed broadleaf-conifer forest and warm and humid climate with higher temperature and sufficient precipitation and the strongest period of EASM. From 5000 to 1600 cal. yr BP, Pinus pollen increased, but Quercus pollen decreased, showing the breakup of the climax community and the recession of the EASM. Since 1600 cal. yr BP, under the threats of land reclamation and deforestation, forest cover sharply decreased, and mountain grass lands were developed. The EASM changes inferred from pollen record of Gonghai Lake were asynchronous to the oxygen isotope records of stalagmites from southern China. We suggest that the existence of remnant Northern Hemisphere ice sheets and relative low sea levels might hampered the northward penetration of the EASM in early Holocene, which caused the maximum monsoon precipitation to reach northern China until mid-Holocene.

Sedimentary and OSL dating evidence for the development of the present Hobq desert landscape, northern China

The sedimentary sequence and landscape of the Hobq desert was observed through twelve profiles along five N-S sections extending through the entire desert. Aeolian sands were found to overlie the alluvial, diluvial, and lacustrine sediments in this desert. Optically stimulated luminescence (OSL) dating results were obtained from post-IR OSL signals of quartz fractions of sand samples at the bottom of sand dunes and hills by using the Double-SAR protocol. The OSL ages reveal that the latest sand accumulation began around 19 ka in the northwestern part and 9 ka in the eastern part of the Hobq Desert. Around 7 ka, the sand accumulation was strengthened and extended southward and northward 2 ka to develop the present desert landscape. Our study suggests that the aeolian sand accumulation around 19, 9 and 7 ka resulted from the weakening of the Asian summer monsoon over this region. However, the rapid development of the present Hobq desert landscape since 2 ka was likely triggered by human activities in this desert.

Reliability of radiocarbon dating on various fractions of loess-soil sequence for Dadiwan section in the western Chinese Loess Plateau

The accurate radiocarbon dating of loess-soil sequences plays an essential role in the reconstruction of the environmental and climatic changes in continental settings during the last glaciation and Holocene. However, our knowledge about the reliability of radiocarbon ages of various fractions of soil and loess samples is still insufficient. Here, we present our study results on radiocarbon ages based on bulk organic matter, humin fraction, and carbonate of samples collected from a loess-paleosol section in the western Chinese Loess Plateau. We compare these observations with the optically stimulated luminescence ages and charcoal radiocarbon ages to evaluate the reliability of these fractions. We observed that the radiocarbon ages of humin fraction are very close to those of charcoal and are consistent with the optically stimulated luminescence ages within the experimental errors. We observed a significant deviation in the radiocarbon ages of carbonate and bulk organic matter from those of charcoal and optically stimulated luminescence ages, likely due to the dilution of these fractions during the pedogenetic process. Our results reveal that, except for charcoal, the humin fraction may yield reliable 14C ages for the Chinese loess-soil sequence.

A 14.7 Ka record of earth surface processes from the arid-monsoon transitional zone of China

The stability of Earths critical zone is intimately linked with erosion, weathering and vegetation type and density. Therefore, it affects global biogeochemical processes which in turn affect the global climate by absorbing and reflecting solar radiation, and by altering fluxes of heat, water vapour, carbon dioxide and other trace gases through various feedback mechanisms. However, there is a lack of knowledge about how Earths critical zone processes have changed over time and their link with past monsoon variability, especially in Asia. The study of lake sediments, which contain a suite of inorganic elemental and isotopic proxies, may facilitate the understanding of the Earths critical zone processes on millennial timescales. Here we reconstruct the history of erosion-weathering-vegetation interactions since similar to 14.7 ka using geochemical records from a radiocarbon-dated sediment core from Lake Gonghai in the monsoon-arid transitional zone of north China. Detrital (Al, Ti, K, Rb) and authigenic (Ca, Sr) elemental records reveal distinct, millennial-scale, late deglacial-Holocene erosion and weathering patterns and transitions with the former (latter) elements showing higher (lower) values in warm intervals and vice versa. Chemical Index of Alteration (CIA) molar, a humidity proxy, suggests low humidity during the late deglacial similar to 11.5-14.7 ka, high humidity during the early-mid Holocene similar to 11.5-3.2 ka, and intermediate humidity during the late Holocene interval since similar to 3.2 ka. The results of cross-spectral analysis and comparison of our records with other climate reconstructions also suggest a pattern of orbitally-phased humidity changes in north China. Overall, our results provide evidence for the solar-forcing of Earths surface processes in mid-latitude China under natural climatic conditions. Copyright (C) 2017 John Wiley & Sons, Ltd.