Evaluation of boron isotopes in halite as an indicator of the salinity of Qarhan paleolake water in the eastern Qaidam Basin, western China

In this study, nineteen brine samples from the Qarhan Salt Lake(QSL) in western China were collected and analyzed for boron(B) and chlorine(Cl) concentrations, total dissolved solids(TDS), pH values and stable B isotopic compositions. The B concentrations and δ^(11) B values of brines in the QSL range from 51.6 mg/L to138.4 mg/L, and from +9.32& to +13.08&, respectively. By comparison of B concentrations and TDS of brines in QSL with evaporation paths of brackish water, we found that B enrichment of brines primarily results from strong evaporation and concentration of Qarhan lake water. Combining with comparisons of B concentrations, TDS, p H values and δ^(11) B values of brines, previously elemental ratios(K/Cl, Mg/Cl, Ca/Cl, B/Cl) and δ^(11) B values of halite from a sediment core(ISL1 A), we observe good correlations between B concentrations and TDS, TDS and pH values, pH and δ^(11) B values of brines, which demonstrate that higher B concentrations and more positive δ^(11) B values of halite indicate higher salinity of the Qarhan paleolake water as well as drier paleoclimatic conditions. Based on this interpretation of the δ^(11) B values of halite in core ISL1 A, higher salinity of the Qarhan paleolake occurred during two intervals, around 46-34 ka and26-9 ka, which are almost coincident with the upper and lower halite-dominated salt layers in core ISL1 A,drier climate phases documented from the δ^(18) O record of carbonate in core ISL1 A and the paleomoisture record in monsoonal central Asia, and a higher solar insolation at 30°N. These results demonstrate that the δ^(11) B values of halite in the arid Qaidam Basin could be regarded as a new proxy for reconstructing the salinity record of paleolake water as well as paleoclimate conditions.

Definition of the Quaternary Qiangtang Paleolake in Qinghai-Tibetan Plateau, China

Since the Quaternary, many lakes have been present in the Qinghai-Tibetan Plateau. As peculiar geological processes in the evolution of the uplifting of Qinghai-Tibetan Plateau, the distributions and evolutions of the Quaternary paleolakes in the Qinghai-Tibetan Plateau have been the focus of interest among the international geosciences circle. Comparisons of the newly obtained and existing data from field surveys, remote sensing images, characteristics of tectonic landforms and distribution of the lacustrine strata, the author have, for the first time, defined a large-sized Quaternary Qiangtang Paleolake. The paleolake starts from the east-westerly direction at Rutog in western Tibet, passing through Gerze, and finally ends at Nagqu in eastern Tibet. Its length is approximately 1,200 kin; it is about 420 km at its widest point （north-southerly）. The Paleolake forms an E-W （or NWW） ellipsoid with an estimated area of 354920 km2. The Paleolake is bordered by the Mts. Gangdise and Nyainqentanglha to the south and the Karakorum Pass-Toze Kangri-Zangbagangri- Tanggula Pass to the north. It generally appears as a basinal landform with low mountains and valleys in the central part （altitudes of 4400 m） and higher altitudes （5000 m） in the peripheries. The formation and development of the Paleolake was controlled by the nearly E-W trending structures. There are three east-westerly extending tectonic sutures inside the Paleolake area, from north to south： （1） the Shuanghu Suture; （2） the Bangong Co-Nujiang River Suture; （3） the Shiquanhe-Lhari Suture. These three sutures have generally controlled the spreading features of the Paleolake and act as the first grade lake-controlling structures. In the southern Paleolake basin, there are a series of south-northerly rift basins, which are controlled by the normal faults and exist as a series of south-northerly grabens and semi-graben faulted basins. The south- northerly rift zones have clearly exerted control over the south margin of the Paleolake as well as the distribution of the residual lake basins after the Paleolake＇s break up. They are the secondary lake- controlling tectonics. Discoveries of the lacustrine strata inside the Plateau, especially the successive discoveries of many high-stand lacustrine strata, are direct evidence supporting the existence of the paleolakes in the Plateau. The dating results of the lake-eroded travertine in the Dong Co in the hinterland of the Quaternary Qiangtang Paleolake have revealed that the lake-eroded travertine is of Late Pleistocene sediments. By considering the analysis of the lacustrine strata as well, the researchers have revealed that the Quaternary Qiangtang Paleolake begins its embryonic form in the Early Pleistocene, successively develops till the Middle Pleistocene, and reaches its full blossom in the middle stage of the Late Pleistocene; it further shrinks and separates by the end of the Late Pleistocene, and finally ceases its whole life as the paleolake. After the Holocene, the huge Late Pleistocene Qiangtang Paleolake no longer exists, leaving a series of widely distributed smaller lakes on the Qiangtang Plateau. These smaller, isolated water systems receive the Holocene lacustrine sediments.

Paleoenvironmental Record from Wuqi Paleolake in Northern Chinese Loess Plateau

Jinding core(ZK04,36°47′35.36″N,108°19′05.24″E)is located in the northern area of Luohe river basin,northern Chinese loess plateau,lying to the southeast of Wuqi County about 20 km.The total depth of Core ZK04 is 92.20

Climate variability recorded by n-alkanes of paleolake sediment in Qaidam Basin on the northeast Tibetan Plateau in late MIS3

Here we combine n-alkanes preserved in a shell bar section from Qarhan paleolake, Qaidam Basin with the other sedimentary proxies to elucidate the lake evolution process during the period 39.7 to 17.5 14C ka BP (calibrated age ranges from 43.5 to 22.4 cal. ka BP). In different stages, the n-alkane homologues exhibited different distribution modes indicative of variations in the surrounding vegetation and the hydrologic condition of the lake. The n-alkanes proxies (CPIh, ACLh, Paq) have the same trends as the summer solar insolation variation, implying that the summer insolation is the most important climatic factor driving the environmental changes and also indirectly controls lake evolution on the Tibetan Plateau. CPIh and ACLh as well as the total pollen concentration appear to show a trend comparable with methane concentration record from the GRIP ice core that reflected the Dansgaard-Oeschger events. This demonstrates that the paleoclimate variations in Qarhan area generally agree with global climate change and show rapid oscillations in late MIS3. These findings provide the latest molecular fossil evidence from paleolake sediments to confirm that lake evolution on Tibetan Plateau in late MIS3 was closely associated with enhanced summer insolation.

The episodic geomorphological-sedimentary evolution of different basins in the Fenwei Graben and its tectonic implication

There are a series of basins in the Fenwei Graben. Field survey found that there took place several paleolake regressions or intensive stream down-incisions in all basins during the Mid-Late Quaternary. The lowest and oldest paleosol/Ioess units overlying three of the lacustrine terraces or alluvial ones and some paleomagenetism data from the lacustrine sediment indicate that the onset times of three paleolake regressions or intensive stream down-incisions are synchronous with the formation of Lg, L6 and L2 respectively in the Weihe Basin, $8, S~ and $1 respectively in the Linfen-Taiyuan-Xingding Basins, and Ls, Ls and L1 respectively in the Datong Basin. The difference in the onset time of each lake regressions or intensive stream down-incision in different basins reveals that the farther the basin is from the Tibetan Plateau, the later it took place. Taking these field facts and the former research re- sults in terms of the regional tectonic movement into account, it is inferred that the tectonic movement of the Tibetan Plateau most probably controlled such geomorphological- sedimentary evolution in the graben.