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The relationships amongst modern pollen assemblages, vegetation, climate and human activity are the basis for reconstructing palaeoenvironmental changes using pollen records. It is important to determine these relationships at regional scales due to the development of vegetation under different climatic conditions and human activities. In this paper, we report on an analysis of modern pollen assemblages of 31 surface lake samples from 31 lakes (one sample per lake) on the southwestern Tibetan Plateau where the knowledge of modern pollen and their relationships with vegetation, climate and human activities is insufficient. The region includes five vegetation zones: sub-alpine shrub steppe, alpine steppe, alpine meadow and steppe ecotone, mountain desert and alpine desert. The lakes span a wide range of mean annual precipitation (50-500 mm) and mean annual temperature (−8 to 6 °C). Modern pollen assemblages from our samples mainly consist of herb taxa ( Artemisia, Cyperaceae, Poaceae, Chenopodiaceae, etc.) and some tree taxa ( Pinus, Fagaceae, Alnus, etc.). The results indicate that modern pollen assemblages are able to reflect the main vegetation distribution. Redundancy analysis for the main pollen types and environmental variables shows that precipitation is the leading factor that influences the pollen distribution in the study area with the first axis capturing 13.7% of the variance in the pollen data set. The Artemisia/Chenopodiaceae ratio is valid for separating the desert component (<2) from the steppe and other vegetation zones (>2) but is unable to distinguish moisture variations. The Artemisia/Cyperaceae ratio is able to identify meadows (<1) and steppes (>1) and can be used as a moisture index on the southwestern Tibetan Plateau. Our results show that an appropriate range is needed for a modern pollen data set in order to perform pollen-based quantitative climate reconstructions in one region. It is essential to perform modern studies before using pollen ratios to reconstruct palaeovegetation and palaeoclimate at a regional scale. [ABSTRACT FROM AUTHOR]

Abstract: Marine oxygen isotope records show that the Cenozoic global climate has undergone remarkable fundamental changes. However, how terrestrial ecosystems respond to these changes remains unclear. The Xining Basin on the northeastern Tibetan Plateau holds continuous fine fluvial-lacustrine sediments from the early Eocene to the early Miocene (between ~52Ma and 17Ma). This paper first presents total organic carbon isotope (δ13CTOC) and pollen records from the Xiejia section in the Xining Basin, as indicators of the long-term terrestrial paleoecosystem and paleoclimate changes in the Asian interior of NW China. The δ13CTOC record shows a long term persistently decreasing trend (from ~−20‰ to −23.5‰) superimposed with obvious cycles of five distinctive phases of heavier and lighter δ13CTOC values, along with short pulses of especially low values occurring at ~32.5Ma, 23Ma, 21.5Ma, and 18Ma. These variations generally correlate with changes of gymnosperm content or the ratio of gymnosperm to angiosperm (G/A); i.e., heavier δ13CTOC values correspond with higher gymnosperm content or higher G/A, as well as with global temperatures (for both long term trends and shorter term fluctuations). We propose that global temperatures might provide the major factor controlling the evolution of the gymnosperm content (and the G/A ratio) that determines the δ13CTOC in the sediments. The East Asian monsoon may not have reached this region during the deposition of the sediments, or alternatively, its signals could not clearly be identified in these low resolution paleoecologic records. [Copyright &y& Elsevier]

Abstract: Water level and chemistry of Lake Qinghai are sensitive to climate changes and are important for paleoclimatic implications. An accurate understanding of hydrological and chemical budgets is crucial for quantifying geochemical proxies and carbon cycle. Published results of water budget are firstly reviewed in this paper. Chemical budget and residence time of major dissolved constituents in the lake are estimated using reliable water budget and newly obtained data for seasonal water chemistry. The results indicate that carbonate weathering is the most important riverine process, resulting in dominance of Ca2+ and DIC for river waters and groundwater. Groundwater contribution to major dissolved constituents is relatively small (4.2 ± 0.5%). Wet atmospheric deposition contributes annually 7.4–44.0% soluble flux to the lake, resulting from eolian dust throughout the seasons. Estimates of chemical budget further suggest that (1) the Buha-type water dominates the chemical components of the lake water, (2) Na+, Cl−, Mg2+, and K+ in lake water are enriched owing to their conservative behaviors, and (3) precipitation of authigenic carbonates (low-Mg calcite, aragonite, and dolomite) transits quickly dissolved Ca2+ into the bottom sediments of the lake, resulting in very low Ca2+ in the lake water. Therefore, authigenic carbonates in the sediments hold potential information on the relative contribution of different solute inputs to the lake and the lake chemistry in the past. [Copyright &y& Elsevier]