Search

Jin, Z. D., Bickle, M. J., Chapman, H. J., Yu, J., An, Z., Wang, S. & Greaves, M. J. 2010: Ostracod Mg/Sr/Ca and Sr/Sr geochemistry from Tibetan lake sediments: Implications for early to mid-Pleistocene Indian monsoon and catchment weathering. Boreas, 10.1111/j.1502-3885.2010.00184.x. ISSN 0300-9483 Lacustrine sediment serves as a valuable archive for tracing catchment weathering processes associated with past climatic and/or tectonic changes. High-resolution records of fossil ostracod Mg/Ca, Sr/Ca and Sr/Sr ratios from a lake sediment core from the central Tibetan Plateau reveal a temporal link between lake-water chemistry and catchment weathering and distinct monsoonal oscillations over the early to mid-Pleistocene. Between 2.01 and 0.95 Ma, lake-water chemistry was dominated by a high proportion of carbonate weathering related to variations in the Indian monsoon, resulting in relatively low and constant ostracod Sr/Sr but obvious fluctuations in Mg/Ca, Sr/Ca and δO. Across the mid-Pleistocene transition (MPT), a significant increase in Sr/Sr and frequently fluctuating ratios of ostracod Mg/Ca, Sr/Ca and δO are coincident with increases in both Chinese loess grain size and Arabian Sea lithogenic flux. This correlation indicates an increased glaciation and a strong monsoon seasonal contrast over the plateau. The increase in lake-water Sr/Sr across the MPT highlights a change in catchment weathering patterns, rather than one in climate-enhanced weathering intensity, with an increased weathering of Sr-rich minerals potentially induced by marked extensive glaciation and strong seasonality in the central plateau. [ABSTRACT FROM AUTHOR]

Dating lake sediment using sedimentary event is the supplement and calibration to traditional dating by radionuclide such as 210Pb and 137Cs. Based on the change of spheroidal carbonaceous particle (SCP) concentration, the age sequence of lake sediments can be deduced. It is one of the dating methods using sedimentary event. SCP is formed from combustion of fossil fuel at high temperature up to 1750°C and at a rate of heating of approaching 104 °C/s. It can be dispersed to several hundred kilometers away from its source and deposited with precipitation or dryly deposited, and kept in sediments. Compared with Cs or Pb, there is no evidence for SCP that it decays in lake sediments and is un-removable once stored except by physical disturbance because it is mainly composed of element carbon. Handy method to extract, identify and calculate has been developed. Although fossil fuel has been used early in China, combustion at high temperature started later since emergence of electricity generation. The productivity of SCP is positively related with the generated thermal power, which is reflected as the SCP concentration in lake sediments increases with the increase of generated thermal power. Therefore, reliable sediment markers from the start of the SCP record and the remarkable variation can be used for dating purpose. In China, electricity industry started from the 1950s, and rapid increase of generated power took place since 1978. Based on these time markers, SCP time sequences of lake sediment cores LH and LL-4 from Longgan Lake, the middle reach of the Yangtze River, have been established, which is comparable with the results from 137Cs and 210Pb, and has eliminated the errors of dating using 137Cs and 210Pb. [ABSTRACT FROM AUTHOR]

Abstract: The lightness (L*) and concentrations of Rb, Sr and organic carbon (C org) have been measured in the age-constrained lake sediment cores recovered from Co Ngoin in the central Tibetan Plateau. Dissolved Sr flux is a dominant control on the variation of Rb/Sr ratios in the sediments. Variations in color and geochemical proxies of Co Ngoin sediments display a continuous history of late glacial to mid-Holocene chemical versus physical weathering intensity in response to past climatic changes between approximately 13,500 and 4500 cal yr B.P. A lower chemical weathering under a late glacial climate was followed by a higher weathering during the Holocene Optimum. Weathering intensity in the central Tibetan Plateau catchment also responds to well-known climatic events, such as the Younger Dryas (YD), and possibly the Holocene Event 5 (HE-5). Although there are differences in time or duration of the climatic events, many of the well-known late glacial to mid-Holocene events occurred in high-elevation Co Ngoin where atmospheric circulation might play a hemispherical role in climatic forcing. The sediment hiatus since c. 4200 14C yr B.P. in the Co Ngoin indicates a period of desiccation that was probably associated with a sharp decrease in summer monsoon strength. Our lascustrine results not only imply catchment weathering variations in response to late glacial to mid-Holocene climatic conditions in the central plateau, but also provide further evidence for global connections between regional climates. [Copyright &y& Elsevier]

Simulated research of internal loading and collecting and analyzing the samples from the lakes were carried out before and after dredging in polluted suburb lakes, Wuli Lake (Wuxi City) and Xuanwu Lake (Nanjing City). The research results showed that dredging can inhibit internal loadings in a certain degree in a short term. The discrepancy of dredging effect and technical level, namely dredging quality, by different dredging methods will result in a difference of control of lake internal loadings. The internal loadings' reversion will gradually appear along with the biogeochemical processes, including suspended particle precipitation, hydrodynamic disturbance and microbio-transformation. The reversion rate mainly depends on the dredging method and the change of interracial processes on the newborn surface layer. The higher nutrient contents and organic matter in the sediment will enhance water-sediment interfacial processes and nutrients regeneration. It is very important to study the physicochemical and biological character of lacustrine sediments before dredging for determining the dredging methods and predicting their environmental effect. [ABSTRACT FROM AUTHOR]

The &sup87;Sr/&sup86;SR ratios of silicate (acid-insoluble, AI) and carbonate (acid-soluble, AS) of the lake-sediments from the Daihai Lake, Inner Mongolia, since the last 500 years are measured respectively, indicating that chemical weathering of silicate minerals was in an early stage since the Little Ice Ae within the Daihai watershed by combination with mineral constitute, Rb/Sr ratio and CaCO&sub3; content in the sediment. During the Little Ice Age maximum, an evident peak in the &sup87;Sr/&sup86;Sr ratios of both silicate and carbonate in sediments suggested that a cold climate condition is unfavorable to dissolving radiogenic strontium from silicate minerals. Meanwhile, the variation of &sup87;Sr/&sup86;sr ratios of silicate and carbonate also reflects a projected warming climate favorable to intensifying chemical weathering after the Little Ice Age. Consequently chemical weathering after the Little Ice Age. Consequently, the &sup87;Sr/&sup86;Sr ratio of both silicate and carbonate in inland lake sediments can be used as an effective proxy of the past climate in single watershed.

Abstract: To provide accurate analogues for fossil pollen records from lake cores, we analyzed surface sediments from 42 lakes of Yunnan and southwestern Sichuan Province, China (located between about 1500 and 4600m elevation) for pollen and spores. The pollen analysis reveals that although large amounts of arboreal pollen, such as Pinus, Picea and Abies, Betula, Juglans, deciduous oaks, evergreen oaks, and Rosaceae, from lower elevations are introduced into sub-alpine and alpine lakes by upslope winds, the pollen assemblages of surface lake sediments among and within different vegetation types can still be distinguished by their pollen spectra and indicator species. For example, Alnus is an indicator of the pollen assemblages of lakes among the Pinus yunnanensis forest-Evergreen broadleaved forest or P. yunnanensis forest-Mixed Pinus and Quercus forest. Tsuga and Ericaceae are indicators of the pollen assemblages of lakes in the Picea and Abies forest or the Picea and Abies forest-Alpine (Rhododendron) shrubland. Little pollen from Picea, Abies, Larix and Tsuga is transported downhill in lake sediments, indicating that lake cores near the lower limit of the Picea and Abies forest more correctly reflect climatic changes than lake cores near the tree line (namely the upper limit of the Picea and Abies forest) when using fossil pollen data. Principal component analysis basically separates surface lake samples below and above 3200m a.s.l. However, it does not distinctly distinguish between samples from two different vegetation types above 3200m a.s.l. because percentages of some dominant pollen types, such as Pinus, Picea and Abies, Betula, Juglans, deciduous oaks, evergreen oaks, and Rosaceae, do not markedly change with strong upslope transport. [Copyright &y& Elsevier]

Abstract: A series of 14C ages were made in two lake sediment cores from Co Ngoin and Zigê Tangco, central Tibetan Plateau, China, using bulk organic carbon, plant remains, chitin of cladocera, and carbonates. The old 14C ages of the surface sediments in both cores suggests that there is a significant reservoir effect in these lakes. The magnitude of the reservoir age of different material is variable. Authigenic carbonate has the largest reservoir age, while plant remains and chitin show smaller deviations. Due to a lack of terrestrial macrofossils at most depths, a regression approach was used to obtain the approximate 14C reservoir correction for bulk organic matter, which is 3260 14CBP and 2010 14CBP for Co Ngoin and Zigê Tangco respectively. [Copyright &y& Elsevier]

Abstract: As one of the pioneering projects of the Chinese Environmental Scientific Drilling (CESD) Program, a 206.5m long sediment core (CE) was retrieved from Co Ngoin (Co = lake) Basin in the central Tibetan Plateau. Limnic records of the abundance of ostracod shells and their oxygen and carbon isotope measurements, together with the geochemical and the sedimentological investigations, were carried out on the interval between 143 and 52m core depths, corresponding to ca. 2.01–0.84Myr. Distinct oscillations of the abundance and the chemical compositions of ostracod shells have yielded the original information about variations in lakewater chemistry in association with the change in the Indian monsoon over the period of 2.01 to 0.84Myr. The climate conditions during the Early Pleistocene were characterized by frequent oscillations and the ostracod records had expressed its synchronous response to these oscillations. A major climate change occurred in the Co Ngoin area at about 0.93Myr. A pronounced fluctuation in ostracod δ 18O and δ 13C during the Mid-Pleistocene transition (MPT) is possibly associated with an increase in continental ice sheets. The compositional and the sedimentological characters of the immediately overlying sediments imply a shallow aquatic environment and also a harsh (salinity?) condition beyond the tolerance limits for ostracod survival, resulting in the disappearance of the Qinghaicypris crassa Huang 1979 around 0.84Myr. [Copyright &y& Elsevier]

Abstract: To advance the understanding of sediment distribution, catchment weathering, hydraulic sorting, and sediment provenance in a tectonically stable basin, the geochemistry of surface sediment samples from Daihai Lake in north China is presented. Mud bulk sediments were analyzed for 10 major and 30 trace elements, organic carbon, and nitrogen and for 87Sr/86Sr ratios in silicate fraction (acid insoluble, AI) and carbonates (acid soluble, AS). Enrichment in Fe2O3, CaO, MnO, MgO, K2O, and P2O5 and their positive correlation with Al2O3, Rb/Sr ratios, and bivariate discrimination diagrams of K vs. Rb and Th/U vs. Th suggest low chemical weathering intensity under semiarid conditions in the Daihai catchment. Accumulation of clays in deeper water sites by sedimentary sorting results in an evident distribution of sediment composition with the lake water depth. Although similar chondrite-normalized REE (rare earth element) patterns of lake sediment samples (with LREE (light rare earth element) enrichment, negative Eu anomalies, and an almost flat HREE (heavy rare earth element) pattern) may suggest similar sedimentary source rocks, original sediment provenance can be recognized on the basis of their distinctive 87Sr/86Sr (AI) ratios and immobile (Th, Zr, Ti, and Nb) trace element signature, separating from weathering and sedimentary processes as well as from grain size effect. [Copyright &y& Elsevier]