Steffen Mischke

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.

Middle Pleistocene Ostracoda from a large freshwater lake in the presently dry Qaidam Basin (NW China)

Ostracods of Middle Pleistocene age were recovered from an escarpment at the northeastern margin of the large Qarhan playa surface in the Qaidam Basin (NW China). The Middle Pleistocene age of the sampled Quan Ji section was determined by means of Optically Stimulated Luminescence (OSL) dating of seven sediment samples. Among the 31 ostracod taxa identified in total, only Eucypris gyirongensis, Leucocythere dorsotuberosa and Prionocypris gansenensis seem to be restricted to high-altitude sites in Central Asia whereas most taxa are known from a number of European and Siberian (Palaeoarctic) or even Holarctic sites. Laterally widespread sediments and the ostracods from the Quan Ji section suggest the formation of a large freshwater to oligohaline lake of at least several metres in depth in the Qaidam Basin, which is presently dry apart from a few shallow salt lakes.

Re-assessment of the paleoclimate implications of the Shell Bar in the Qaidam Basin, China

The Shell Bar in the Qaidam Basin, China, is a prominent geological feature composed of millions of densely packed Corbicula shells. Since the mid 1980s, it has been regarded as evidence for existence of a large lake during Marine Isotope Stage (MIS) 3 in the presently hyper-arid Qaidam Basin. Early studies suggested the bivalve shells accumulated at the shore of a large lake, whereas more recent work led to the conclusion that the Shell Bar was formed within a deeper water body. Based on our re-assessment of sediments and fossils from the Shell Bar, investigation of exposed fluvio-lacustrine sections upstream of the Shell Bar and study of nearby modern streams, we infer that the Shell Bar represents a stream deposit. Corbicula is a typical stream-dweller around the world. Preservation of Corbicula shells of different sizes, as well as occurrence of many articulated shells, provide evidence against post-mortem transport and accumulation along a lake shore. Additionally, the SE-NW alignment of the Shell Bar is similar to modern intermittent stream beds in its vicinity and corresponds to the present-day slope towards the basin centre further NW, and furthermore, the predominantly sandy sediments also indicate that the Shell Bar was formed in a stream. Abundant ostracod shells in the Shell Bar sediments originated from stream-dwelling species that are abundant in modern streams in the vicinity of the Shell Bar, or in part from fluvio-lacustrine sediments exposed upstream of the Shell Bar, as a result of erosion and re-deposition. Deflation of alluvial fine-grained sediments in the Shell Bar region and protection of the stream deposits by the large and thick-walled Corbicula shells reversed the former channel relief and yielded the modern exposure, which is a prominent morphological feature. Occurrence of Corbicula shells in the Qaidam Basin indicates climate was apparently warmer than present during the formation of the Shell Bar because Corbicula does not live at similar or higher altitudes in the region today. Because the Shell Bar is no longer considered a deposit formed within a lake, its presence does not indicate paleoclimate conditions wetter than today.

Quaternary Ostracods from the Tibetan Plateau and Their Significance for Environmental and Climate-Change Studies

Abstract Tibetan Plateau lakes are diverse in basin morphology, water chemistry and geological history. Limnological parameters including salinity, ion composition and water depth are closely linked to climatic conditions in this continental, alpine and relatively dry part of Central Asia. Analyses of Quaternary ostracods focused initially on the biostratigraphical assessment of lacustrine sediments of the Qaidam Basin for oil and gas exploration, but more recent palaeoenvironmental and palaeoclimatic studies have focused on lakes Qinghai and Nam Co. Stratigraphical and palaeoenvironmental inferences have been made from analyses of ostracod assemblages, stable isotopes and trace element ratios of ostracod shells and shell length variations. Consideration of species’ ecological characteristics facilitates the use of ostracods as indicator taxa and the application of transfer functions for quantitative palaeoenvironmental reconstructions. Ostracod shell chemistry has been used successfully in environmental and climate-change studies, but assemblage composition data have provided the clearest evidence for sometimes dramatic hydrological changes.

Sedimentary record of coseismic subsidence in Hersek coastal lagoon (Izmit Bay, Turkey) and the late Holocene activity of the North Anatolian Fault

This research was funded by the European Union in the framework of the REL.I.E.F. (Reliable Information on Earthquake Faulting) project (EVG1‐CT‐2002‐00069). Copyright @ 2011 American Geophysical Union.

What drives the recent intensified vegetation degradation in Mongolia - Climate change or human activity?

This study examines the course and driving forces of recent vegetation change in the Mongolian steppe. A sediment core covering the last 55 years from a small closed-basin lake in central Mongolia was analyzed for its multi-proxy record at annual resolution. Pollen analysis shows that highest abundances of planted Poaceae and highest vegetation diversity occurred during 1977–1992, reflecting agricultural development in the lake area. A decrease in diversity and an increase in Artemisia abundance after 1992 indicate enhanced vegetation degradation in recent times, most probably because of overgrazing and farmland abandonment. Human impact is the main factor for the vegetation degradation within the past decades as revealed by a series of redundancy analyses, while climate change and soil erosion play subordinate roles. High Pediastrum (a green algae) influx, high atomic total organic carbon/total nitrogen (TOC/TN) ratios, abundant coarse detrital grains, and the decrease of δ13Corg and δ15N since about 1977 but particularly after 1992 indicate that abundant terrestrial organic matter and nutrients were transported into the lake and caused lake eutrophication, presumably because of intensified land use. Thus, we infer that the transition to a market economy in Mongolia since the early 1990s not only caused dramatic vegetation degradation but also affected the lake ecosystem through anthropogenic changes in the catchment area.

Holocene climate and landscape change in the northeastern Tibetan Plateau foreland inferred from the Zhuyeze Lake record

Pollen and grain-size data from the Holocene Zhuyeze Lake record in arid Central Asia were re-assessed and combined with new ostracod species assemblage data to improve inferences of the lake history and controlling climate conditions. Zhuyeze Lake was a perennial freshwater–oligohaline lake since its establishment ca. 13 cal. ka BP. The lake level fell below the position of the QTL02 section site at 2.1 cal. ka BP after the beginning of the Han Dynasty, and we assume that significantly intensified land use upstream of Zhuyeze Lake was at least partly causing the lake-level decline. Most stable lake conditions and lowest salinities were recorded in the mid Holocene between 7.5 and 5.5 cal. ka BP, providing additional evidence for the inference of the highest moisture availability in the mid Holocene in arid Central Asia. The most striking feature of analyses of grain-size and ostracod data is the inference of more or less unchanged lake levels and lake water chemistry during a period of aeolian sand accumulation in the lake between 7.8 and 7.5 cal. ka BP. Similar conspicuous and apparently contemporaneously formed sand layers were recorded in other sections in the ancient lake basin and farther upstream, and mobilization of aeolian sands must have occurred for a few hundred years in the region. Indications for the 8.2 ka event from our section and other climate records in Central and eastern Asia support the hypothesis that a short-lived cold-dry climate initiated the increased activation of aeolian sands which was later on gradually enhanced as a result of burial of previously vegetated land by dunes and sand sheets. Further work is required to determine the regional extent of sand mobilization at ca. 7.5 cal. ka BP in the northeastern Tibetan Plateau foreland and to examine the timing and controls of the self-enhancing aeolian sand mobilization, vegetation degradation and subsequent recovery.

Reply to comment by Zhang (2014): The Shell Bar in the Qaidam Basin: fluvial or lake deposit, and OSL versus 14C age data

Zhang’s (2014) comments on the formation and age of the Shell Bar in the Qaidam Basin go far beyond our specific Shell Bar papers (Lai et al. 2014; Mischke et al. 2014) and include questions about a number of other lake basins in China. Although we disagree with many of Zhang’s comments about these other basins (e.g. the Qinghai Lake Basin), we cannot hope to address them all here, so we stick to several main points. First, we address specific comments about the Shell Bar, including: (1) Shell Bar sediments, (2) geomorphological history and implications, (3) Corbicula shells as a major Shell Bar constituent, and (4) OSL age data. Second, we address what is surely the more scientifically important, and as yet unsettled issue at the core of the debate, i.e. the relative reliability of OSL versus radiocarbon dating for shoreline sediments in the range from MIS 5 to early MIS 3.

The world’s earliest Aral-Sea type disaster: the decline of the Loulan Kingdom in the Tarim Basin

Remnants of cities and farmlands in China’s hyperarid Tarim Basin indicate that environmental conditions were significantly wetter two millennia ago in a region which is barren desert today. Historical documents and age data of organic remains show that the Loulan Kingdom flourished during the Han Dynasty (206 BCE–220 CE) but was abandoned between its end and 645 CE. Previous archaeological, geomorphological and geological studies suggest that deteriorating climate conditions led to the abandonment of the ancient desert cities. Based on analyses of lake sediments from Lop Nur in the eastern Tarim Basin and a review of published records, we show that the Loulan Kingdom decline resulted from a man-made environmental disaster comparable to the recent Aral Sea crisis rather than from changing climate. Lop Nur and other lakes within the Han Dynasty realm experienced rapidly declining water levels or even desiccation whilst lakes in adjacent regions recorded rising levels and relatively wet conditions during the time of the Loulan Kingdom decline. Water withdrawal for irrigation farming in the middle reaches of rivers likely caused water shortage downstream and eventually the widespread deterioration of desert oases a long time before man initiated the Aral Sea disaster in the 1960s.

Carbon and Oxygen Isotopic Compositions: How Lacustrine Environmental Factors Respond in Northwestern and Northeastern China

Surface lake sediments,28 from Hoh Xil,24 from northeastern China,99 from Lake Bosten,31 from Ulungur and 26 from Heihai were collected to determine δ13C and δ18O values.Considering the impact factors,conductivity,alkalinity,pH,TOC,C/N and carbonate-content in the sediments,Cl,P,S,and metal element ratios of Mg/Ca,Sr/Ca,Fe/Mn of bulk sediments as environmental variables enable evaluation of their influences on δ13C and δ18O using principal component analysis(PCA) method.The closure and residence time of lakes can influence the correlation between δ13C and δ18O.Lake water will change from fresh to brackish with increasing reduction and eutrophication effects.Mg/Ca in the bulk sediment indicates the characteristic of residence time,Sr/Ca and Fe/Mn infer the salinity of lakes.Carbonate formation processes and types can influence the δ13C–δ18O correlation.δ18O will be heavier from Mg-calcite and aragonite formed in a high-salinity water body than calcite formed in freshwater conditions.When carbonate content is less than 30%,there is no relationship with either δ13C or δ18O,and also none between δ13C and δ18O.More than 30%,carbonate content,however,co-varies highly to δ13C and δ18O,and there is also a high correlation between δ13C and δ18O.Vegetation conditions and primary productivity of lakes can influence the characteristics of δ13C and δ18O,and their co-variance.Total organic matter content(TOC) in the sediments is higher with more terrestrial and submerged plants infilling.In northeastern and northwestern China,when organic matter in the lake sediments comes from endogenous floating organisms and algae,the δ13C value is high.δ13C is in the range of 4‰ to 0‰ when organic matter comes mainly from floating organisms(C/N6);in the range of 4‰ to 8‰ when organic matter comes from diatoms(C/N=6 to 8);and 8‰ to 4‰ when organic matter comes from aquatic and terrestrial plants(C/N8).