Jiawu Zhang

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.

Paleoclimatic implications of an 850-year oxygen-isotope record from the northern Tibetan Plateau

Oxygen-isotope records from the sediments of hydrologically-closed lakes are commonly interpreted in terms of changing effective precipitation. We compare an 850-year-long oxygen-isotope record derived from ostracode carbonate from the sediments of Sugan Lake, in the northern Tibetan Plateau, with tree-ring and ice core evidence for changing temperature, precipitation and isotopic composition of the lakes inflow. Taking into account all of these independent records, we show that variations in the carbonate delta O-18 values could not have been the result of varying effective precipitation alone: changes in water temperature and in the delta O-18 of source waters also played a significant role. Where independent records of temperature, precipitation or the isotopic composition of input waters are unavailable, care should be taken to avoid simplistic interpretations of carbonate stable isotope records, as these may contribute to incorrect paleoclimatic reconstructions.

Temporal Change of Radiocarbon Reservoir Effect in Sugan Lake, Northwest China during the Late Holocene

Many lacustrine chronology records suffer from radiocarbon reservoir effects. A continuous, accurate varve chronology, in conjunction with accelerator mass spectrometry (AMS) 14 C dating, was used to determine the age of lacustrine sediment and to quantify the past 14 C reservoir effect in Sugan Lake (China). Reservoir age varied from 4340 to 2590 yr due to 14 C-depleted water in the late Holocene. However, during the Little Ice Age (LIA), 14 C reservoir age was relatively stable. According to this study, 14 C reservoir age in the late Holocene may be driven by hydrological and climatic changes of this period. Therefore, special caution should be paid to the correction of the 14 C reservoir effect by a unique 14 C reservoir age in paleoclimatic and paleolimnological study of northwest China.

Loess particle size data indicative of stable winter monsoons during the last interglacial in the western part of the Chinese Loess Plateau

Abstract Particle size data from the western part of the Chinese Loess Plateau do not support the last interglacial correlation, suggested by the data from the central part of the Loess Plateau, between the high dust-influx events in the Loess Plateau and the cool events in the high northern latitudes. The lack of marked oscillations in the particle size of the last interglacial paleosol S1 in the western part of the plateau is interpreted as an indication of stable winter monsoons. It is thus inferred that the teleconnection between the northern high latitudes and the Chinese Loess Plateau evident in the last glacial might not have existed during the last interglacial. It is argued that the western part of the plateau is a better place to establish synchroneity between parent material (eolian) deposition and S1 paleosol formation (6–8 m thick) under such cooler and drier conditions. However, to assume synchroneity between parent material (eolian) deposition and S1 paleosol formation (only 2.5 m thick) under the warmer and moister interglacial conditions in the central part of the plateau is a questionable practice. Thus, the high-resolution correlation with the data from the North Atlantic area remains problematic.

Palaeovegetational and palaeoenvironmental changes since the last deglacial in Gonghe Basin, northeast Tibetan Plateau

Paleoenvironmental history in the monsoonal margin in the northeast Tibetan Plateau provides important clue to the regional climate. Previous researches have been limited by either poor chronology or low resolution. Here we present a high-resolution pollen record from a 40.92-m-long sediment core (DLH) taken from Dalianhai, a terminal lake situated in the Gonghe Basin, the northeast Tibetan Plateau for reconstructing the vegetation and climate history since the last deglacial on the basis of a chronology controlled by 10 AMS 14C dates on plant remains preserved in the core sediments. The pollen assemblages in DLH core can be partitioned into 6 pollen zones and each zone is mainly characterized by the growth and decline of tree or herb pollen percentage. During the periods of 14.8–12.9 ka and 9.4–3.9 ka, the subalpine arboreal and local herbaceous pollen increased, indicating the subalpine forest developed in the surrounding mountains and a desert steppe or typical steppe developed in Gonghe Basin under a relatively moister climate. During the periods of 15.8–14.8 ka, 12.9–9.4 ka and 3.9–1.4 ka, the forest shrank or disappeared according to different degrees of aridity, and the desert steppe degraded to a more arid steppe desert in the basin, indicating a dry climate. After 1.4 ka, vegetation type around Dalianhai was mainly dominated by steppe suggested by increased Artemisia. Our results suggested the climate history in this region was dry from 15.8–14.8 ka, humid from 14.8–12.9 ka and dry from 12.9–9.4 ka, after which the climate was humid during 9.4–3.9 ka, followed by dry conditions during 3.9–1.4 ka and humid conditions in the last 1.4 ka. The change of pollen percentage and the evolution of palaeovegetation in Dalianhai since the last deglacial were similar to those recorded in Qinghai Lake. The forest expanded in the mountains around Dalianhai during the Bølling-Allerød period, shrank during the Younger Dryas and the early Holocene, then it developed and reached its maximum in the mid-Holocene. During the late Holocene, the vegetation began to shrink till disappearance. However, the timing of forest expansion in the Holocene lagged behind that of Qinghai Lake, and this spatial heterogeneity was probably caused by the different forest species between these two places. The maximum of forest development in the mid-Holocene was inconsistent with the period of stronger summer monsoon in the early Holocene indicated by stalagmite records, the reason might be related to the complexity of vegetation response to a large-scale climatic change.

Extension of drylands in northern China around 250 kaBP linked with the uplift of the southeast margin of Tibetan Plateau

Study on two loess sections, one located at Wuwei near the Tengger Desert in northwestern China, another located near Ganzi at the southeast margin of the Tibetan Plateau in southwest China, reveals a coeval drying step occurred at ∼250 kaBP. It is expressed by the increase in eolian grain-size at Wuwei, and by a drastic extension of C4 plants and a decrease of loess chemical weathering intensity at Ganzi. Examination of the available eolian data indicates that the event has also been clearly documented in the loess sections near the deserts in northern China, and in the eolian records from the North Pacific. On the contrary, the signal is rather weak for the central and southern Loess Plateau regions as well as for Central Asia, where the climates are influenced by the southeast Asian monsoon and the westerlies, respectively. Since the climate at Ganzi is under strong control of the southwest Asian monsoon, we interpret this drying step as a result of decreased influence of the southwest summer monsoon. This decrease in monsoon moisture is attributable to the uplift of the Hengduan Mountains, the southeast margin of the Tibetan Plateau at ∼ 250 ka ago.

Effects of climate change and human activity on lake shrinkage in Gonghe Basin of northeastern Tibetan Plateau during the past 60 years

Changes in the status of freshwater resources are a topic of major global, regional and local concern. This is especially so in the arid and semi-arid regions of China, where shortage of water resources plays a crucial role in limiting sustainable socioeconomic development, as well as in sustaining natural ecosystems. Recent climate change, as well as the effects of localized human activity, such as the use of water for irrigation agriculture, may have significant effects on the status of the water resources in the region. Here, we report the results of a study of changes in the areas of lakes in Gonghe Basin, northeastern Tibetan Plateau of China, over the last 60 years. The data were acquired from optical satellite images and demonstrate that the total water area of lakes in Gonghe Basin decreased significantly from the 1950s to 1980s. The cause is ascribed mainly to human activity including exploitation of farmland, against a background of increasing population; in addition, climatic data for the region demonstrate a minor drying trend during this period as the temperature increased slightly. After the construction of several reservoirs, significant amounts of water were redistributed to promote irrigation agriculture and we conclude that this caused a significant shrinkage of the natural lakes. However, both the area of farmland and the population size remained approximately constant after 1990. We conclude that the variation of the total area of lakes during the second period was mainly controlled by climatic factors (precipitation and temperature). As the regional temperature reached a new high, the area of some of the lakes decreased sharply before finally maintaining a relatively steady state. We emphasize that anthropogenic climate change and human activity have both significantly influenced the status of water resources in the arid and semi-arid regions of China.

Oasis evolution processes and mechanisms in the lower reaches of Heihe River, Inner Mongolia, China since 1 ka ago

Two major oases in the lower reaches of Heihe River, Inner Mongolia of China, the desertified ancient Juyanze oasis (AJO) and the modern Ejina oasis (MEO), experienced dramatic environmental changes and human migrations as documented in historic records. The processes and mechanisms of their evolution are the key for us to understand the environmental changes and human activities in this region, yet are still unclear. This study provides an evolution chronology of these two oases by analyzing the optical dating records from MEO and 14C dating records from AJO and discusses their evolution mechanisms based on drainage network analysis and field investigations. The optical dating samples collected from aeolian sands below humus layers in Populus euphratica woodlands on MEO indicate that this oasis developed between 1.05 and 0.45 ka ago, while the 14C dating samples collected from a number of dead P. euphratica trees from AJO consistently suggest that the complete desertification of AJO occurred before ~400 cal. a BP. These new dating results imply that the two oases evolved simultaneously but in opposite directions. That is, AJO desertified while MEO developed gradually in the last 1 ka. Climate change occurring over the last 1 ka in northern China cannot explain the opposite evolution directions of these two adjacent oases. The drainage network analysis based on digital elevation models (DEMs) and field observations and the dates (~0.5 ka) of the top lacustrine layer from GaxuNur Lake north of MEO suggest that the evolution of the two oases was mainly governed by Heihe River migration.