Ji Junfeng

Variations in chemical compositions of the eolian dust in Chinese Loess Plateau over the past 2.5 Ma and chemical weathering in the Asian inland

Major and trace elements as well as strontium isotopic composition have been analyzed on the acid-insoluble (AI) phase of the loess-paleosol sequence from Luochuan, Shaanxi Province, China. Results show that the chemical composition of AI phase of loess and paleosols is distinctive to the average composition of upper continental crust (UCC), characterized by depletion of mobile elements Na, Ca and Sr. The distribution pattern of elements in AI phase reveals that initial dust, derived from a vast area of Asian inland, has suffered from Na- and Ca-removed chemical weathering compared to UCC. Some geochemical parameters (such as CIA values, Na/K, Rb/Sr and87Sr/86Sr ratios) display a regular variation and evolution, reflecting that the chemical weathering in the source region of loess deposits has decreased gradually since 2.5 Ma with the general increase of global ice volume. This coincidence reflects that the aridity of Asian inland since the Quaternary is a possible regional response to the global climate change.

Holocene vegetational and climatic variation in westerly-dominated areas of Central Asia inferred from the Sayram Lake in northern Xinjiang, China

Changes in the vegetation and climate of the westerly-dominated areas in Central Asia during the Holocene were interpreted using pollen-assemblages and charcoal data from a 300-cm-long sediment core of the Sayram Lake, northern Xinjiang. Accele-rator Mass Spectrometry (AMS) radiocarbon dating methods were applied to bulk organic matter of the samples. Artemisia spp./Chenopodiaceae ratios and results from principal component analysis were used to infer that the lake basin was dominated by desert vegetation before ca. 9.6 cal. ka BP, which suggests a warm and dry climate in the early Holocene. Desert steppe/steppe expanded during 9.6-5.5 cal. ka BP, indicating a remarkable increase both in the precipitation and temperature during the mid-Holocene. Desert vegetation dominated between 6.5 and 5.5 cal. ka BP, marking an extreme warmer and drier interval. The steppe/meadow steppe recovered, and temperatures decreased from 5.5 cal. ka BP in the late Holocene, as indicated by the increased abundance of Artemisia and the development of meadows. Holocene temperatures and moisture variations in the Sayram Lake areas were similar to those of adjacent areas. This consistency implies that solar radiation was the main driving factor for regional temperature changes, and that the effect of temperature variations was significant on regional changes in humidity. The evolution of climate and environment in the Sayram Lake areas, which were characterized as dry in the early Holocene and relatively humid in the middle-late Holocene, are clearly different from those in monsoonal areas. Dry conditions in the early Holocene in the Sayram Lake areas were closely related to decreased water vapor advection. These conditions were a result of reduced westerly wind speeds and less evaporation upstream, which in turn were caused by seasonal changes in solar radiation superimposed by strong evaporation following warming and drying local climate.

Weak chemical weathering during the Little Ice Age recorded by lake sediments

Low magnetic susceptibility, low Sr content and hence high Rb/Sr ratio in the lake sediment sequence indicate a weak chemical weathering process under arid and cold climate of the Little Ice Age in a single closed lake watershed. According to different geochemical behavior between rubidium and strontium in earth surface processes, variation of Rb/Sr ratios in the lake sediment sequence can be used as an effective geochemical proxy with definite climatic significance of chemical weathering in watershed. Unlike chemical weathering process in tropic zone and modern temperate-humid climate, concordant changes in both Sr content and magnetic susceptibility with δ18O values of Dunde ice core suggest that the weak chemical weathering was controlled by air temperature during the Little Ice Age maximum. After the Little Ice Age, chemical weathering intensity was controlled also gradually by precipitation with increasing in temperature.

Ore-forming fluid constraints on illite crystallinity (IC) at Dexing porphyry copper deposit, Jiangxi Province

Illite, a distinctive kind of clay minerals of potassium alteration within the hydrothermal alteration zone, frequently occurs at the Tongchang porphyry copper deposit ore field. The illite crystallinity (IC) value and expandability are mainly affected by water/rock ratio or fluid flux. It was formed by illitization of plagioclase and micas during hydrothermal fluid-rock interaction within the porphyry body and near the contact zone with wall rocks. Moreover, the negative correlation between illite index (IC) and copper grade indicates that within the alteration zone, the smaller the illite crystallinity value, the higher the alteration degree, and the higher the copper grade due to higher water/rock ratio. At lower levels of the porphyry body, however, the illite crystallinity (IC) values are mainly controlled by temperature and time duration.

Aragonite-calcite transformation in fossil snail shells of loess sequences in Loess Plateau, Central China

The methods of X-ray diffraction (XRD) and ICP-AES are applied to analyzing the mineral composition of modern and fossil snail shells in Luochuan section and Xifeng section. The results show that the mineral phase of calcium carbonate in modern snail shells is aragonite, but for some fossil snail shells in certain layers of loess sequences, a part of aragonite is transformed into calcite. In Luochuan and Xifeng sections, the stratigraphic borderline of aragonite-calcite transformation appearing obviously is between L5 and L6. Under the earth surface condition, the aragonite-calcite transformation is influenced by the factor of temperature only in a long time scale. It seems that the pressure is not the factor influencing the aragonite-calcite transformation. The results also show that existing age of snail shells is possibly the dominant and principal factor for the aragonite-calcite transformation. To a certain extent, the degree of aragonite-calcite transformation in snail shell is controlled by the content of trace element, such as Mg2+. The trace element can improve the stability of snail shell aragonite and impede the process of aragonite transforming into calcite.

Environmental changes recorded by lake sediments from Lake Jili,Xinjiang during the past 2500 years

An 80-cm-long sedimentary core from Lake Jili,which lies in westerly area of China,provided detailed information about paleoclimatic and paleoenvironmental changes in the past 2500 years.The temporal sequence of sedimentary core was constructed using the ~(210)Pb,~(137)Cs and AMS ~(14)C dating methods on the bulk organic matter.Based on analyses of grain-size,total organic carbon,total nitrogen and organic carbon isotope,climatic and environmental evolution history in the past 2500 years was reconstructed. The sequence of climatic changes in the region since 2.5ka BP was established.The climatic changes experienced five stages:warm-wet climate during 500BC-50BC;cold-wet climate during 50BC-650AD;warm-dry climate during 650-1350AD warm period(MWP);cold-dry climate period during 1350-1850AD(LIA),and moderate-dry since 1850AD,including climate warming after 1920s,especially since 1970s.The records of climatic and environmental evolution of Lake Jili were in good according with the historical literature records and other geologic records of adjacent areas.