Environmental archaeology of the Dachang region in the Daning Valley,the Three Gorges reservoir region of the Yangtze River,China

Environmental archaeology of archaeological sites on both sides of the Dachang section in the Daning River,the Three Gorges of the Yangtze River,shows that riverbed evolution has a great influence on the distribution of archaeological sites during the prehistorical and historical period,and it restricts human activity and the development of ancient culture.Field investigation,dating data,paleocurrent analysis of 100 gravels on the riverbed and archaeological excavation reveal that the riverbed of the Dachang section developed in the Shuangyantang region during the early stage,after about 30 kaBP it shifted westward gradually and reached the present place.Along with the westward shift,the focus of ancient culture changed for several times.In the Western Zhou Dynasty’s period,the east riverbank of the Daning River was the main residential area.During the Han Dynasty,along with the westward shift,the living space of ancient people was no longer confined to the east riverbank and the focus of culture moved westward.Then the Zhangjiawan site became the main dwelling district.In the late Han Dynasty, the villages were flooded and the ancients had to immigrate to a high place.Tombs of the Han and the Jin Dynasties outcropped on the flood plain of the east bank because their surface sediments were eroded by several flood events and the westward shift of the riverbed.     

Isotopic evolution of the terminal Neoproterozoic and early Cambrian carbon cycle on the northern Yangtze Platform, South China

Profound geotectonic, climatic and biological changes occur during the terminal Neoproterozoic and its transition into the early Cambrian. These are reflected in temporal variations of the chemical and isotopic composition of seawater. We are studying a sequence of sedimentary rocks at the Shatan section, northern Yangtze Platform, Sichuan Province of China. This succession comprises, in ascending stratigraphic order, predominantly calcareous sediments of the Sinian upper Dengying Formation and black shales of the lower Cambrian Guojiaba Formation (time equivalent of Niutitang Fm.). Paleoenvironmental setting represents shallow-water shelf deposits. The objective of our study is to provide temporal records for the isotopic compositions of organic and carbonate carbon throughout this time interval. Organic carbon isotope values display a range between -35.8‰ and -30.1‰ with clear stratigraphic variations. Carbonate carbon isotope data vary between -3.5‰ and +0.5‰. These secular variations are interpreted to reflect perturbations of the global carbon cycle, specifically changes in the fractional burial of organic carbon. However, local conditions have further affected the isotopic signals.     

Identifying paleoflood deposits archived in Zhongba Site, the Three Gorges reservoir region of the Yangtze River, China

Based on the principle that the present is the key to the past, detailed analyses, such as AMS ^14C dating, grain size, component and morphology of heavy minerals, micro-morphology of zircon, Rb/Sr, magnetic susceptibility and total organic carbon （TOC）, were conducted to identify paleoflood sediments archived in Zhongba Site. The results indicate that the plaeoflood sediments bear great similarities with modern flood sediments in the following aspects： （1） probability cumulative curves mainly show a pattern of 3-4 segments; （2） grain-size distribution of suspended matter ranges between 3 and 10（I）; （3） the sediments are well-sorted, most of which are suspended matter （〉50%）; （4） the same species, quantity and morphology of heavy minerals; （5） scanning electronic microscope images show that shapes of zircon are mainly oval and nearly spheral, rounded due to long-distance transport; （6） higher Rb/Sr values （0.55-0.66）than those of sediments from cultural layers （0.03-0.26）; （7） magnetic susceptibility values （133.73-433.05 10^-6m^3/kg） are lower than those of sediments from cultural layers （959.25-2442.44 10^-6 m^3/kg）; （8） TOC （0.14%- 0.33%） are lower than those of sediments from cultural layers （1.13%-2.95%）. Our results demonstrate that, except for the 1981 flood, there are at least six paleoflood events that occurred during the Qing Dynasty, the middle of Song Dynasty, the early Warring States （400BC-350BC）, the West Zhou Dynasty （920BC-900BC）, the Xia Dynasty （2070BC-1600BC）, and the late Neolithic Age （3000BC-2300BC）, respectively.     

Petrogenesis of the Neoproterozoic bimodal volcanic rocks along the western margin of the Yangtze Block： New constraints from Hf isotopes and Fe/Mn ratios

High-precision major element and Hf isotope data are reported for the Neoproterozoic Suxiong volcanic rocks along the western margin of the Yangtze Block. These volcanic rocks have variable εHf（T） values and Fe/Mn ratios. The relatively primitive basalts have high Fe/Mn ratios and high Hf-Nd isotopic compositions, indicating that they were generated by partial melting of garnet clinopyroxene in mantle plume at high pressure. Thus, the Suxiong basalts are genetically related to the proposed Neoproterozoic superplume. On the contrary, a few differentiated basalts have low Fe/Mn ratios and low Hf-Nd isotopic compositions. They are likely to experience assimilation-fractional crystallization process. The Suxiong rhyolites have consistent Hf and Nd model ages of 1.3-1.4 Ga. They are likely generated by shallow dehydration melting of pre-existing young arc igneous rocks associated with the basaltic underplating/intrusion in a continental rift.     

Zircon U-Pb dating for the earliest Neoproterozoic mafic magmatism in the southern margin of the North China Block

Geological records of Neoproterozoic magmaticevents have recently been identified in the central Phanerozoic orogenic belts of China[1]. In regions of east Qinling orogen[2], Dabie-Sulu orogen[3], north Qaidam Basin orogen[4―6], and southwest Tarim Basin…     

Initial organic geochemical investigation on Late Neoproterozoic-Early Cambrian sediments in the Yangtze region, China

Totally 19 samples of typical Upper Proterozoic-Lower Cambrian sedimentary rocks were collected and analyzed for an organic geochemical investigation. Almost all these rocks have high TOCs, super-maturities and similar biomarker distribution. As an exception, however, the Sinian Nantuo Tillite shows much lower TOCs and little phytane and pristane in comparison with those in other strata, which implies a very faint photosynthetic process, and a restricted euphotic zone and quite limited sunlight within the sedimentary water column during the Sinian glaciation age in the western Yangtze region providing an evidence for palaeo-oceanic environment of the Neoproterozoic Snowball Earth age.     

Late Neoproterozoic to early Cambrian sulphur cycle-An isotopic investigation of sedimentary rocks from the Yangtze Platform

The sulphur cycle responds to changes in seawater chemistry, biological evolution and tectonic activity. We follow an isotopic approach in order to constrain the state of the ocean/atmosphere system during late Neoproterozoic and early Cambrian. For this purpose, a sedimentary succession deposited on the Yangtze Platform, South China, was analysed for its sulphur isotopic composition in different S-bearing phases. Redox changes were defined by the degree of pyritization (DOP) values in order to show variations in the oxygenation of the depositional environment. The sulphur isotopic composition of late Neoproterozoic to early Cambrian seawater sulphate ranges from +30‰ to +35‰ as evident from trace sulphate in unaltered carbonates and phosphorites. The isotopic composition for pyrite and organic sulphur varies between -16‰ and +23‰. The apparent sulphur isotopic fractionation between seawater sulphate and pyrite as well as organically bound sulphur varies between 7‰ and 50‰. This large fractionation, as well as its variability suggests a biological origin for pyrite and organically bound sulphur. The temporal evolution of different geochemical proxy signals is comparable for different successions across the Yangtze Platform.     

Relation between human existence and environment viewing from the distribution of prehistoric archaeological sites in the Three Gorges reservoir region of the Yangtze River,China

Based on archaeological research findings in the Three Gorges region as well as the distribution of prehistoric sites in this area,the following conclusion can be derived:the unique physical and geographical conditions and variations in the Three Gorges region make local people of different ages seek their survival and development in the favorable environment along the riverbanks.Environment evolution has direct impact on the selection of location for survival.The increase of sites quantity reflects the favorable natural environment,but the decrease just reflects the consequence of such natural disasters as flood.     

Vermicular fossils in the Early Cambrian Xidashan Formation in the Quruqtagh region of Xinjiang, China

Large amounts of vermicular fossils and a minor amount of sponge animal fossils have been found in the Early Cambrian Xidashan Formation in the region of Quruqtagh, Xinjiang. Vermicular fossils are generally more than 50 mm long and 0.3?1.4 mm wide; their wrinkled lamellae are microfine with 3?10 pieces within the length of every each millimeter. The fossils are considered to be Sabellidites cambriensis Sokolov (1965). The Xidashan Formation is the highest stratum in which Sabellidites occur, as has been so far reported.     

Discussion on the Neoproterozoic glaciations in the South China Block and their related paleolatitudes

The Kaigas, Sturtian, Marinoan, and Gaskiers glaciations are widely recognized in Neoproterozoic. However, in the South China Block only the Jiangkou （Sturtian） and Nantuo （Marinoan） are symbolized by sedimentary records. The Kaigas, recorded by isotopic and chemical proxies, exhibited likely the nature of cold paleoclimate with local mountain glaciation. The correlation of the Doushantuo Formation with the Gaskiers is indicated by the carbon isotope excursion and the dated age from the interval, however the South China Block was then under non-glacial weather. With no paleomagnetic data, the position of the South China Block during the Sturtian glaciation cannot be determined. The paleolatitudes of the South China Block during the Kaigas and Nantuo glaciations are intermediate, even though the Nantuo was once rendered erratically deduced equatorial. In fact, the paleolatitudes of the South China Block during the Neoproterozoic glaciations are all likely at about 30°-40°.     

δ13Ccarb and Ceanom excursions in the post-glacial Neoproterozoic and Early Cambrian interval in Guizhou,South China

Secular δ 13Ccarb and Ceanom profiles in the post-glacial Neoproterozoic and Early Cambrian interval are reported from Guizhou on the southeasten border of the Yangtze platform, South China. Overall, the δ 13Ccarb profile drifts to negative values in the post-glacial Nantuo and lower to middle Doushantuo Formations, and then to positive values in the upper Doushantuo and Dengying Formations of Neoproterozoic, followed by negative values in the Lower Cambrian Gezhongwu and Niutitang Formations. A detailed investigation of the relationship between the δ 13Ccarb and Ceanom profiles reveals that the main anoxic and oxic episodes are coupled with negative and positive δ 13Ccarb values, respectively. This may suggest a control of alternation between ocean stratification and mixing on variations in 13C abundance in the ancient ocean of the investigated areas.     

扬子陆块西缘康定杂岩的形成时代和构造环境

攀西麻粒岩是扬子陆块西缘康定杂岩的重要代表.通过对麻粒岩Rb-Sr、Sm-Nd、40Ar/39Ar和铬石U-Pb测年结果的比较,结合麻粒岩的变质演化历史和岩石地球化学特征分析,认为角闪岩相退变质作用对麻粒岩的Rb-Sr、Sm-Nd和40Ar/39Ar同位素计时体系产生了重要影响.多个岩体锆石U-Pb测年结果的一致性,表明康定岩浆-变质杂岩形成于新元古代(860～750 Ma).岩石的地球化学性质还指示了康定杂岩形成于扬子陆块西缘新元古代活动大陆边缘的岛弧环境.     

贵州瓮安新元古代陡山沱组磷块岩中的有机化合物

对保存有多细胞藻类原植体、细菌和疑原类化石的贵州瓮安陡山沱组磷块岩所作的地球化学分析表明,磷块岩有机质演化程度较高,最高热解温度t_max达597℃。岩石抽提物中以非烃化合物和沥青质为主要成分,烃类含量不足20%。色谱-质谱(GC-MS)分析图谱显示正烷烃的碳数范围很宽,有显著的高碳数正烷烃峰群(C₁₉—C₃₁),奇偶优势明显,烃类有机组分中除正烷烃外还包含有萜烷、甾烷、芳香烃和类异戊二烯烃等。其中生物标记化合物及其组成特征指示了有机物的主要来源是当时生存的真核的多细胞藻类、细菌和古细菌,这与所观察到的磷块岩中保存的古生物化石组成相吻合。某些特征有机物和生物标记物还指示了磷块岩沉积环境特征：强还原性、高盐度、低陆源输入以及与水热活动相关的局部高温。这与沉积学和岩石学的观察相符。     

Zircon U-Pb geochronology of the volcanic rocks from Fanchang-Ningwu volcanic basins in the Lower Yangtze region and its geological implications

The latest eruptions in two important Mesozoic volcanic basins of Fanchang and Ningwu located in the middle-lower reaches of the Yangtze River formed the bimodal volcanic rocks of the Kedoushan Formation and ultrapotassic volcanic rocks of the Niangniangshan Formation, respectively. The representative volcanic rocks of the two Formations were selected for LA-ICPMS zircon U-Pb dating. The results indicate that there exist a large amount of magmatic zircons as indicated by high Th/U ratios in these volcanic rocks. The weighted mean age of 21 analyses is 130.7±1.1 Ma for the Kedoushan Formation, and that of 20 analyses is 130.6±1.1 Ma for the Niangniangshan Formation. These U-Pb ages are interpreted to represent the formation times of the volcanic rocks. In combination with other known geochronological data for Mesozoic volcanic rocks from the Lower Yangtze region, it is proposed that the latest volcanic activations in the Jinniu, Luzong, Fanchang and Ningwu volcanic basins probably came to end prior to ca. 128 Ma. There is no significant time interval between the early and later volcanic activities in the Luzong and Ningwu basins, suggesting a short duration of volcanic activities and thus implying the onset of an extensional tectonic setting at about 130 Ma in the Lower Yangtze region. Integrated studies reveal that the Early Cretaceous magmatic activities and their geochronological framework in the Lower Yangtze region are a response to progressively dynamic deep processes that started with the transformation of tectonic setting from compression to extension, followed by delaminating of the lower part of the thickened lithosphere, lithospheric thinning, asthenosphere upwelling, and crust-mantle interaction.     

扬子板块西缘盐边群及周边岩体地质年代学新格局

扬子板块西缘的盐边群及其周边岩体,作为前寒武纪构造岩浆活动带的核心,是观察和研究新元古代大陆壳生长的重要窗口.查明这一构造岩浆活动带内地质体的产出时代及构造岩浆活动的轨迹,是揭示新元古代大陆壳产生、发展地球动力学过程的重点环节.对盐边群及周边岩体的地质年代学研究表明:(1)区内大规模的岩浆事件集中在中元古代晚期至新元古...     

SHRIMP zircon U-Pb dating for volcanic rocks of the Dasi Formation in southeast Hubei Province, middle-lower reaches of the Yangtze River and its implications

The Jinniu Basin in southeast Hubei,located at the westernmost part of middle-lower val-ley of the Yangtze River,is one of the important vol-canic basins in East China. Volcanic rocks in the Jin-niu Basin are distributed mainly in the Majiashan Formation,the Lingxiang Formation and the Dasi Formation,consisting of rhyolite,basalt and basaltic andesite,(trachy)-basalt and basaltic trachy-andesite and (trachy)-andesite and (trachy)-dacite and rhyolite respectively,in which the Dasi volcanism is volumet-rically dominant and widespread. The Dasi volcanic rocks were selected for SHRIMP zircon U-Pb dating to confirm the timing of volcanism. The results indi-cate that there exist a large amount of magmatic zir-cons characterized by high U and Th contents in the volcanic rocks. The concordia ages for 13 points are 128 ± 1Ma (MSWD = 3.0). On account of the shape of zircons and Th/U ratios,this age is considered to represent the crystallization time of the Dasi volcan-ism. The volcanic rocks in the Dasi,Majiashan and Lingxiang Formations share similar trace elementand REE partition patterns as well as Sr-Nd isotopic compositions. In combination with the regional geol-ogy,it is proposed that the southeast Hubei volcanic rocks were formed mainly during the Early Creta-ceous,just like other volcanic basins in middle-lower Yangtze valley. A lithospheric extension is also sug-gested for tectonic regime in this region in the Cre-taceous Period.     

长江口南港底沙再悬浮特征及其浓度预测

利用声学悬沙浓度剖面仪（ACP-1）和Endeco流速仪、双频测深仪等对长江口南港底沙再悬浮的浓度、流速和盐度、水深等进行连续14h定点同步观测。对长江口南港底沙再悬浮特征及其影响因子进行综合分析，结果表明：转流阶段再悬浮频率较小，强度较大；涨（落）急阶段，再悬浮频率较大，强度较小；过滤阶段，再悬浮频率和强度随流速同相变化；底沙再悬浮浓度的主要影响因子是盐度、流速、水深和悬沙粒径。再悬浮浓度与流速、盐度、水深和悬沙粒径的线性回归关系显，相关系数达0.91，且流速、水深和粒径与之呈正相关，盐度与之呈负相关。