Duo Wu

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.

Changes in the radiocarbon reservoir age in Lake Xingyun, Southwestern China during the Holocene.

Chronology is a necessary component of paleoclimatology. Radiocarbon dating plays a central role in determining the ages of geological samples younger than ca. 50 ka BP. However, there are many limitations for its application, including radiocarbon reservoir effects, which may cause incorrect chronology in many lakes. Here we demonstrate temporal changes in the radiocarbon reservoir age of Lake Xingyun, Southwestern China, where radiocarbon ages based on bulk organic matter have been reported in previous studies. Our new radiocarbon ages, determined from terrestrial plant macrofossils suggest that the radiocarbon reservoir age changed from 960 to 2200 years during the last 8500 cal a BP years. These changes to the reservoir effect were associated with inputs from either pre-aged organic carbon or 14C-depleted hard water in Lake Xingyun caused by hydrological change in the lake system. The radiocarbon reservoir age may in return be a good indicator for the carbon source in lake ecosystems and depositional environment.

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.

Nonlagged Response of Vegetation to Climate Change During the Younger Dryas: Evidence from High‐Resolution Multiproxy Records from an Alpine Lake in Northern China

The use of pollen analysis to reconstruct regional vegetation changes, and subsequently climate, is a common approach in paleoclimatic reconstruction. However, it is unclear whether or not vegetation exhibits a lagged response to climate change during shifts from rapid cooling to rapid warming. The Younger Dryas (YD) cold event, spanning the last glacial/Holocene transition, was a prominent abrupt climatic event. To investigate the vegetation response to climate change during the YD, we compared profiles of independent climatic indicators from the sediments of alpine Lake Gonghai, on the margin of the modern Asian summer monsoon, with a pollen record from the same site. Our aim was to assess the phase lag of vegetation change relative to independent climate proxies that would have responded near instantaneously to climate change. The results show that the vegetation changed synchronously with the climate proxies, and consequently, we conclude that vegetation succession around Lake Gonghai exhibited a nonlagged response to climate change during the YD. This result confirms that a previous pollen-based quantitative regional precipitation record from Lake Gonghai reliably depicts variations in the East Asian summer monsoon (EASM), especially its decreasing trend during the YD. This conclusion differs from that of related research on the Baxie loess section and Hani peat deposits. Also, we suggest that our findings provide an improved understanding of the likely response of the EASM during cooling intervals and that the YD is a useful analog for studying the response of vegetation to possible abrupt cooling events under future global warming scenarios.