Zhang Jiawu

Sub-Recent Ostracoda from Qilian Mountains (NW China) and their ecological significance

Abstract 23 ostracod species have been recorded from surface and short core samples from the Qilian Mountains, NW China. Brooks and rivers, small shallow meadow and oxbow pools and a lake were sampled at an altitude ranging from 2900 m to 3570 m asl. Brooks were dominated by Candona rawsoni , Ilyocypris cf. bradyi and/or Ilyocypris lacustris and Limnocythere inopinata . In river habitats Candona candida and Eucypris sp. were most abundant. Meadow pools with peaty, dystrophic waters mainly contained valves of Ilyocypris cf. bradyi and/or Ilyocypris lacustris and Eucypris sp. whereas the latter was replaced by Heterocypris incongruens in a shallow oxbow pool. Candona rawsoni , Cyclocypris ovum and Ilyocypris echinata were the most frequent species in the freshwater Lake Luanhaizi. The recorded taxa are mainly distributed in the holarctic realm but Fabaeformiscandona danielopoli and Ilyocypris echinata appear to be restricted to the cold mountainous region in China.

The discovery of annually laminated sediments (varves) from shallow Sugan Lake in inland arid China and their paleoclimatic significance

Detailed examination of sedimentary cores retrieved from Sugan Lake in the northern Qaidam Basin of northwest China’s Tibetan Plateau reveal that fine laminated beddings form in the sediments where water depth exceeds 3 m. Seasonal surface sediments trapped at the bottom of the lake suggest that sediments deposited during summer and autumn are mainly light colored monohydrocalcites, while those deposited in winter are dark organic matter, indicating that varve layers form under modern limnological conditions. Continuous varve sediments comprising four types have accumulated in the upper 5.5 m of Core SG03I from the center of the lake. All types exhibit clear seasonality indicative of annual deposition. Varve counts correspondence with 210Pb dates on recent sediments in the upper core suggest the continuous varves of the upper 5.5 m of the core formed in the late Holocene (2670 a BP). The Sugan Lake varve sequence is the first demonstration of annually laminated sediments reported in arid western China.

Different climatic controls of soil delta C-13(org) in three mid-latitude regions of the Northern Hemisphere since the Last Glacial period

Paleoecological records of soil δ13Corg from three regions in the middle latitudes of the Northern Hemisphere, including the Chinese Loess Plateau (CLP), the Great Plains and adjacent areas of North America and northwestern Europe, showed different variations since the Last Glacial period. An attempt was made to evaluate the causes for the difference in δ13Corg on the basis of the modern climatic data collected in these regions and of the modern C3 and C4 plant distributions. The analysis indicates that temperature, especially the growing season temperature, has a dominant control on the growth of C4 plants. When the mean annual or growing season temperatures are below the “threshold value”, the growth of C4 plants is limited. When the temperature is above the “threshold value”, C4 plants can grow under a wide range of precipitation. However, when the precipitation is high enough to favor the growth of trees, the proportions of C4 plants in local biomass will decline. The implicit control factor recovered by sedimentary records is consistent with the control factor on modern C3/C4 distribution. Pure C3 plants have been dominating the local biomass since the Last Glacial period in European loess region, mainly owing to the low local temperature. The increases in C4 plants from the late Pleistocene to the Holocene in the Chinese Loess Plateau, the Great Plains and adjacent areas, mainly reflect the influence of increasing temperature.

Surface sediments of Lake Sugan and dust storm in the Northern Qaidam Basin, China: pre- liminary results of elemental tracers

To understand the role of atmospheric dust in the global climate system further, it is necessary that a longer sequence of dust storm history should be established using geological archives in dust provenance area. Hydrologically closed lakes in the arid regions of China could be ideal objectives to trace dust storm evolution since the most of Northwest China has been recognized as an important source area for global dust loading. Lake Sugan, a closed lake located at the northern margin of the Qinghai-Tibetan Plateau, would be employed to study on the dust storm history in the Qaidam Basin. Here, 8 elements and 11 rare-earth elements were analyzed for three different sediments/deposits in the Sugan Lake region, namely, the surface sediments of Lake Sugan, the settled sand dust including materials collected during dust storm durations, and the surface deposits of the region. The results show the element of Al has been enriched by 11% relative to the surface ground deposits. The composition of the element in lake sediments is nearly half of the settled dust, which may be diluted by the carbonate accounting for about 50% of the lake sediment. Two of Al, Fe and Sc exhibit apparent positive correlations in the lake sediments. Also, for one of the three elements positive correlation was found between the settled sand dust and the lake sediments. However, there are negative correlations between the catchment deposits and the settled sand dust, and between the catchment deposits and the lake sediment for Al and Sc. Variations in Fe and Sc compositions exhibit an apparent positive correlation not only with intensity of dust storm events, but also with frequency of the events. The Fe and Sc compositions seem to be changeless for the three kinds of sediments/ deposits. In addition, the results of rare-earth elements analyses show that the lake sediments, the settled sand dust and the surface ground deposits may derive from a homologous source. The elements of Al, Fe, Sc are most likely to be transported into lake through strong wind and dust storm processes. Therefore, sediments of Lake Sugan could he used to trace dust storm in this region, and the elements of Al, Fe, So could be applied in reconstruction of dust storm history.

The differences in organic carbon isotope of different size components in lake sediments and its impact on experimental results

The organic carbon isotope( OCI) of lake sediment has often been used to identify the source of organic matter in sediments or watershed vegetation,and gradually becomes a routine proxy indicator. However,when organic matter content changes significantly and in different forms,the OCI results from the unified approach( bulk sample or a graded composition) will produce deviation,no studies have evaluated this problem so far. This research chose sediment samples from rump lake in arid areas,analyzed the organic content and isotope using bulk sample and fine-grained components( screened through mesh size of 120 and360),respectively. The influence on OCI results based on different sediment grain sizes and with different organic matter forms was evaluated. Results reveal different OCI values in those sediment even with similar organic matter from different sediment components. Obviously,this relates to the forms of organic matter. It shows that fine-grained components( screened through mesh size of 360) is more advantageous to full reaction,and obtain a more stable value. When comparing the δ13Corgvalue in different lake sediments,researchers should adapt same methods for sample pretreatment. The result has an important implication for the research on organic carbon isotope in lake sediments.