Description: Sedimentary archives on the Tibetan Plateau are a reliable source of information on palaeoenvironmental change in central Asia. Reconstructing environmental changes offers the opportunity to gain information on the dynamics of major circulation systems of the Earth´s atmosphere which influence the climatic conditions in central Asia. However, supra-regional comparisons of proxy records inferred from sedimentary archives reveal individual responses of single archives to superimposed environmental change throughout the Holocene (Wischnewski et al., 2011). This observation gave rise to an ongoing debate whether the individual response of sedimentary archives results from high age uncertainties resulting from radiocarbon based dating techniques (earlier proposed by Mischke and Zhang, 2010) or from a non-linear response of sedimentary systems to superimposed environmental change (Wischnewski et al., 2011). Here we present new insights into the dynamics of allogenic sediment supply to lake systems under changing environmental conditions. To reconstruct sediment dynamics across the Tibetan Plateau, we analyzed the elemental composition via non-destructive X-Ray Fluorescence measurements of lacustrine sediment cores retrieved from three lake systems situated on the southern and northern Tibetan Plateau. By applying a factor analysis on the bulk elemental composition of lacustrine sediments we found a common factor as expressed by similar loadings of the elements Ti, Rb, Fe and Zr. Their link to heavy mineral compounds suggests an allogenic origin and hence a coupling of factor scores to sediment routing systems within each catchment. To test a non-linear response of individual sedimentary systems to environmental change we reconstructed phase spaces from time series data in form of factor scores via a time-lag embedding method. A phase space is an abstract space which maps the entire temporal succession of evolutionary states (called trajectory) in a multidimensional Euclidean space. This so-called phase space reconstruction offers the opportunity to identify and characterize the underlying dynamics by analyzing the geometry of trajectories within the reconstructed space. The results suggest that all lake systems share a similar long-term trend in the dynamics of detrital input. Thereby, lake trajectories tend to converge towards a common attracting state in the Late Holocene. After reaching the basin of attraction, all trajectories remain in a cyclic orbit around the attracting state with a frequency spectrum similar to major variations in solar irradiance and the North Atlantic overturning circulation. These results shed new light on the comparability of low dimensional proxy-data and the environmental signal processing of complex sedimentary systems.

Description: 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.

Description: Unmixing of grain-size distributionswithmultivariate statistical analysis gives indications of themain sediment transport processes and related environmental conditions in an area. We performed end-member mixing analysis (EMMA) of sedimentological data from 912 terrestrial sediment samples collected in the Donggi Cona catchment, north-eastern Tibetan Plateau. Up to the present, this is the largest sedimentological dataset on the Tibetan Plateau. EMMA resulted in the characterisation of three end-members that explain 88% of the variance within the dataset. The end-members all represent aeolian deposits. The first end-member EM 1 represents very fine dune sediments that were deflated from a former lake system. EM 2 represents medium sand deposits that were blown out from playa and alluvial fan sediments. EM 3 represents fine loess(−like) sediments mainly found at higher elevations. Different transformations, adding of a fourth end-member and adding of up to 200 loess samples do not change the composition of the end-members, demonstrating the robustness of themodel. EMMA allows the synchronous interpretation of very large datasets, resulting in a general characterisation of sediment transport in a particular area. Performing EMMA on the dataset demonstrates the importance of aeolian transport in this part of the world.

Description: Grain-size distributions offer powerful proxies of past environmental conditions that are related to sediment sorting processes. However, they are often of multimodal character because sediments can get mixed during deposition. To facilitate the use of grain size as palaeoenvironmental proxy, this study aims to distinguish the main detrital processes that contribute to lacustrine sedimentation across the Tibetan Plateau using grain-size end-member modelling analysis. Between three and five robust grain-size end-member subpopulations were distinguished at different sites from similarly–likely end-member model runs. Their main modes were grouped and linked to common sediment transport and depositional processes that can be associated with contemporary Tibetan climate (precipitation patterns and lake ice phenology, gridded wind and shear stress data from the High Asia Reanalysis) and local catchment configurations. The coarse sands and clays with grain-size modes 〉250 μm and 〈2 μm were probably transported by fluvial processes. Aeolian sands (� 200 μm) and coarse local dust (60 μm), transported by saltation and in near-surface suspension clouds, are probably related to occasional westerly storms in winter and spring. Coarse regional dust with modes 25 μm may derive from near-by sources that keep in longer term suspension. The continuous background dust is differentiated into two robust end members (modes: 5–10 and 2–5 μm) that may represent different sources, wind directions and/or sediment trapping dynamics from long-range, upperlevel westerly and episodic northerly wind transport. According to this study grain-size end members of only fluvial origin contribute small amounts to mean Tibetan lake sedimentation (19±5 %), whereas local to regional aeolian transport and background dust deposition dominate the clastic sedimentation in Tibetan lakes (contributions: 42±14% and 51±11 %). However, fluvial and alluvial reworking of aeolian material from nearby slopes during summer seems to limit end-member interpretation and should be crosschecked with other proxy information. If not considered as a stand-alone proxy, a high transferability to other regions and sediment archives allows helpful reconstructions of past sedimentation history.

Description: Palaeoclimate reconstruction on the northern Tibetan Plateau resulted in a large spectrumof different and partly divergent interpretations for the climate evolution during the late glacial and the Holocene. In some cases this is caused by incomplete understanding of the geomorphological processes influencing the different proxies used. To overcome these limitations and to enhance the understanding of the complex process interactions in a sensitive and highly dynamical environment a detailed analysis of different members of the sedimentary system at Lake Heihai on the northern Tibetan Plateau was conducted. Lake level variations during the late Pleistocene were influenced by sediment supply to an alluvial fan. This sediment surplus resulted in the temporary blocking of the outflowof LakeHeihai.High sediment supply presumably occurred during or shortly after large glaciations in the Kunlun Shan. The spatial distribution of aeolian sediments revealed a strong relationship to possible source areas. This resulted in a spatially heterogeneous distribution of the aeolian sediments. Furthermore, topographic effects have an important influence on the preservation of the sediments. Aeolian sediments deposited in sheltered positions might not be comparable with other archives with a similar grain size. Nevertheless, deposition of loess during the mid-Holocene indicates a shift to wetter climate conditions on the northern Tibetan Plateau. This might be caused by the intrusion of the East Asian Summer Monsoon into the area. During the late Holocene, the Asian summer monsoon retreated and aeolian sediments were reactivated.

Description: Lake systems on the Tibetan Plateau reveal complex responses to late Quaternary climate variations, driven by monsoon dynamics and the influence of the westerlies regime. A limnogeological case study from Lake Heihai, situated at 4440 m a.s.l. on the northern Tibetan Plateau, gives evidence for marked changes in the depositional environment related to climatic and orographic effects. The 10 km long, 4 km wide, and 22 m deep lake is situated in a basin north of the Kunlun Mountain Range. Sub-bottom profiling revealed the presence of subaquatic terraces and ancient fan systems from a former low lake stand, draped by younger sediments. Sediment cores above basal sands comprise lacustrine sediments of late glacial to Holocene age. A prominent lake terrace about 6 m above modern level gives evidence of a higher lake level in the early Holocene. The terrace structure includes fossil lake sediments, which include ground ice and are distorted by permafrost structures. Proxy records document marked changes in allogenic sediment provenance and endogenic carbonate precipitation during the last 12 ka. Thus dominant detrital sediment and water supply from proximal alluvial fans appeared only during times, when precipiation was not blocked by the Kunlun Mountains. After a prolonged dry and cold phase during the Late Glacial, this situation of enhanced precipitation-generated sediment supply ocurred between 10.7 cal. ka BP and 7.9 cal. ka BP and decreased afterwards. This Holocene trend is in phase with known variations in atmospheric circulation systems over monsoonal Asia.