Kai Hartmann

DILEMMA OF DATING ON LACUSTRINE DEPOSITS IN AN HYPERARID INLAND BASIN OF NW CHINA

Conventional and accelerator mass spectrometry (AMS) radiocarbon, TL, OSL, and IRSL dating results on samples from the cores D100 and I70 from Ejina Basin, one of the most important inland basins in arid-hyperarid NW China, show that it is difficult to determine the ages of sediments at different depths. AMS ages of core D100 samples demonstrate that the sediments at depths from 10 to 90 m were formed between 14 to 30 kyr BP. The inverted ages from both the D100 and I70 cores imply that there was a strong reworking of the sediments during and after deposition processes. The inverted ages also indicate drastic fluctuations of groundwater bearing soluble organic matters, which might be related to neotectonic activities and climate changes during the period. Consequently, it is impossible to establish an accurate and reliable chronology for the cores based only on these dates. All AMS ages, if they are reliable and acceptable, indicate a high deposition rate (5~8 mm/yr), and since all TL, OSL, and IRSL ages are much older than those given by AMS, it makes these methods questionable for determining the ages of lacustrine-fluvial-alluvial deposits.

22. Interglacial and glacial fingerprints from lake deposits in the Gobi Desert, NW China

Abstract A 230-m long sediment core from the centre of the Gaxun Nur Basin, Gobi Desert, NW China provides evidence for climate induced changes in water balance during the last glacial cycle. Millennial scale and short-term variations of geochemical precipitates and grain size show that freshwater fluxes from the Tibetan Plateau by surface run-off were the main controlling factors for lake evolution in the Tibetan dry forelands for about the last 250 kyr. Periods of positive water balance with strong lake extension and reverse developments generally coincide with changes in the global ice volume and with oxygen-18 records from Tibet and Greenland as well, documenting the close relationship between environmental conditions in remote desert regions of NW China and orbitally forced Northern Hemisphere high mountain mid-latitude and high-latitude climates on a regional and global scale. Our data imply that both the East Asian summer monsoon and the extra-tropical westerlies are the major feedback mechanisms for effective moisture supply over NW China. During the 10-kyr long interglacial warmmoist substage 5.5, summer monsoon moisture dominated owing to its strong northward shift beyond the modern limit. At that time, a large and slightly saline lake filled the entire Gaxun Nur basin as a result of strong river inflow from the Tibetan catchment by melt water supply and by enhanced summer monsoon precipitation. Aeolian transport was weak. The Eemian interglacial in the Gaxun Nur region started at about 129 kyr, with warm and moist environmental conditions between 128 and 121 kyr BP and terminated around 119 kyr, documented by a strong climate shift towards dry conditions and enhanced mobilization of aeolian sand. During interstadial climates, contemporaneous with D/O events in Greenland ice cores, both wind systems most likely supplemented each other, while in transitional phases towards cold conditions, moisture supply by the westerlies seems to have dominated. Cold-dry stages, recorded in the Gaxun Nur core, are synchronous with the global climate. They induced strong lake-level declines and promoted aeolian transport of exposed lake sediments southwards due to the enhanced winter monsoon. Loess records from the Chinese Loess Plateau confirm that the temporal distribution of loess mobilization recorded in the Gaxun Nur sediments was synchronous with depositional phases on the Loess Plateau.

A persistent northern boundary of Indian Summer Monsoon precipitation over Central Asia during the Holocene

Extra-tropical circulation systems impede poleward moisture advection by the Indian Summer Monsoon. In this context, the Himalayan range is believed to insulate the south Asian circulation from extra-tropical influences and to delineate the northern extent of the Indian Summer Monsoon in central Asia. Paleoclimatic evidence, however, suggests increased moisture availability in the Early Holocene north of the Himalayan range which is attributed to an intensification of the Indian Summer Monsoon. Nevertheless, mechanisms leading to a surpassing of the Himalayan range and the northern maximum extent of summer monsoonal influence remain unknown. Here we show that the Kunlun barrier on the northern Tibetan Plateau [~36°N] delimits Indian Summer Monsoon precipitation during the Holocene. The presence of the barrier relocates the insulation effect 1,000 km further north, allowing a continental low intensity branch of the Indian Summer Monsoon which is persistent throughout the Holocene. Precipitation intensities at its northern extent seem to be driven by differentiated solar heating of the Northern Hemisphere indicating dependency on energy-gradients rather than absolute radiation intensities. The identified spatial constraints of monsoonal precipitation will facilitate the prediction of future monsoonal precipitation patterns in Central Asia under varying climatic conditions.

Neotectonic constraints on the Gaxun Nur inland basin in north–central China, derived from remote sensing, geomorphology and geophysical analyses

Abstract The endorheic Gaxun Nur Basin (GNB, local name: Ejina basin), which is located north of the Tibetan Plateau between the tectonic stress fields of the Qilian Shan and the Gobi-Tienshan, has evolved as a large inland basin filled with deltaic sediments during the past 250 ka. Here we present selected examples of geomorphological, sedimentological and geophysical evidence of tectonic activity and discuss a possible time frame of selected occurrences. We used medium-scale geomorphological mapping supported by analyses of Landsat ETM images, Corona images and an Aster Digital Terrain Model (Aster-DTM), combined with field surveys, dated sediment sections, and geophysical investigations using electromagnetic methods. The spatio-temporal distribution of radiocarbon-dated lake sediments within the northern GNB indicates a non-even distribution of neotectonic activity with west–east increasing amplitude of subsidence rates from 0.8–1.1 m/ka in the western part and more than three times higher rates in the eastern part. Our data indicate that tectonic has strongly amplified climate-induced environmental changes and may be regarded as an example of non-climatic pulses affecting lake-hydrology and basin development.

Seismites as indicators for Holocene seismicity in the northeastern Ejina Basin, Inner Mongolia

Abstract This paper contributes to the understanding of palaeoseismology in slowly deforming regions by demonstrating the interpretation of soft-sediment deformation to be a valuable tool in identifying palaeoseismicity where the present-day seismicity is low and the geomorphology initially does not indicate active tectonics by the presence of fault scarps, surface ruptures or diverted rivers. The Ejina Basin (also known as the Gaxun Nur Basin or Juyan Lake Basin) in Inner Mongolia exhibits highly variable endorheic lakes, such as the Late Quaternary Juyanze Palaeolake. We assessed the Holocene tectonic activity of the Western Juyanze Palaeolake Basin, situated within the Ejina Basin, by identifying seismites and a previously unknown normal fault. Deformation mechanisms other than earthquakes, such as landslides or cryoturbation in permafrost, can be excluded. Far-field and near-field seismic sources are considered to have generated the soft-sediment deformation structures. As the occurrence of seismites is restricted to the Western Juyanze Palaeolake Basin and there is Pleistocene–Holocene activity on the Tienehu Fault, a near-field source is most likely.

Looking back - Looking forward: A novel multi-time slice weight-of-evidence approach for defining reference conditions to assess the impact of human activities on lake systems

Lake ecosystems are sensitive recorders of environmental changes that provide continuous archives at annual to decadal resolution over thousands of years. The systematic investigation of land use changes and emission of pollutants archived in Holocene lake sediments as well as the reconstruction of contamination, background conditions, and sensitivity of lake systems offer an ideal opportunity to study environmental dynamics and consequences of anthropogenic impact that increasingly pose risks to human well-being. This paper discusses the use of sediment and other lines of evidence in providing a record of historical and current contamination in lake ecosystems. We present a novel approach to investigate impacts from human activities using chemical-analytical, bioanalytical, ecological, paleolimnological, paleoecotoxicological, archeological as well as modeling techniques. This multi-time slice weight-of-evidence (WOE) approach will generate knowledge on conditions prior to anthropogenic influence and provide knowledge to (i) create a better understanding of the effects of anthropogenic disturbances on biodiversity, (ii) assess water quality by using quantitative data on historical pollution and persistence of pollutants archived over thousands of years in sediments, and (iii) define environmental threshold values using modeling methods. This technique may be applied in order to gain insights into reference conditions of surface and ground waters in catchments with a long history of land use and human impact, which is still a major need that is currently not yet addressed within the context of the European Water Framework Directive.

Lake-System Response to Late Quaternary Environmental Dynamics onthe northeastern Tibetan Plateau

Palaeoclimatic reconstructions in central China often rely on proxy information, obtained from single palaeoenvironmental archives at a distinct study site, such as lake sediments. Although they provide a high temporal resolution of information, their context within the acting depositional and geomorphological processes in a distinct landscape unit often is difficult to ascertain. The nature of lake systems strongly depends on catchment processes in addition to direct responses to climate forcing. In the scope of palaeoclimatic studies on the northern Tibetan Plateau and its foreland, we investigate the complex interaction of landscape evolution in response to monsoon dynamics. Our approach follows a source-to-sink approach along sediment cascades, to infer palaeoclimatic responses and to decipher internal non-climatic dynamics of the landscape system. Lake sediments represent the final and mostly complete sedimentary products, while adjacent landforms and offshore deposits provide insights into the spatial dimension of palaeoenvironmental change. The approach of concerted geomorphological and limnogeological research was successfully demonstrated at Lake Donggi Cona on the NE Tibetan Plateau at 4,090 m elevation. The 30 km long, 8 km wide, and 90 m deep lake occupies a pull-apart basin. Sub-bottom profiling revealed the presence of tectonic structures as well as subaquatic terraces and ancient fan systems from former low lake stands, draped by younger lake sediments. A prominent moraine arc in the vicinity of the lake can be attributed to a glacial advance during an early stage of the last glacial cycle. Several terrace levels and fossil lake sediments of early to mid-Holocene age around the lake shore document former stages of higher lake level of about 10 m above present level. In addition to alluvial processes, dunes and loess-like sediments in the catchment of the lake give evidence of differing stages in the intensity of aeolian sediment mobilisation or fixation. Lake development is documented by multi-proxy data in a series of sediment cores across the lake basin. The oldest recovered sediments, younger than 18 ka BP, comprise siliciclastic laminates and lateglacial saline shallow-lake sediments, overlying basal sands. The deposition of fine-grained marls and enhanced biological production characterized the deep lacustrine environment of the Holocene. A switch from a closed lake basin with brackish conditions towards an open oligotrophic fresh-water basin after 6.8 ka BP is indicated by ostracod assemblages and changes in carbonate chemistry. This change in hydrology likely was triggered by tectonic pulses along the Kunlun Fault, an area with severe earthquake activity. Pollen records exhibit climate signals and indicate moist conditions in the early Holocene. During the same time, loess sediments have been preserved from a period from 10.5 to 7.5 ka BP, related to the strengthening of the insolation-driven Asian summer monsoons. This time was characterized by wetter and warmer climate and supported the trapping of dust. A dry and cooler climate resulted in the reactivation of dune sands from 3 ka to present, possibly in combination with stronger human influence.