ALBERT

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

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 sediment transport and depositional processes associated with certain climatic background and 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, upper-level 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: El Niño-Southern Oscillation (ENSO) is an important element of earth's ocean-climate system. To further understand its past variability, proxy records from climate archives need to be studied. Ice cores from high alpine glaciers may contain high resolution ENSO proxy information, given the glacier site is climatologically sensitive to ENSO. We investigated signals of ENSO in the climate of the subtropical Andes in the proximity of Cerro Tapado glacier (30°08' S, 69°55' W, 5550 m a.s.l.), where a 36 m long ice core was drilled in 1999 (Ginot, 2001). We used annual and semi-annual precipitation and temperature time series from regional meteorological stations and interpolated grids for correlation analyses with ENSO indices and ice core-derived proxies (net accumulation, stable isotope ratio δ18O, major ion concentrations). The total time period investigated here comprises 1900 to 2000, but varies with data sets. Only in the western, i.e. Mediterranean Andes precipitation is higher (lower) during El Niño (La Niña) events, especially at higher altitudes, due to the latitudinal shift of frontal activity during austral winters. However, the temperature response to ENSO is more stable in space and time, being higher (lower) during El Niño (La Niña) events in most of the subtropical Andes all year long. From a northwest to southeast teleconnection gradient, we suggest a regional water vapour feedback triggers temperature anomalies as a function of ENSO-related changes in regional pressure systems, Pacific sea surface temperature and tropical moisture input. Tapado glacier ice proxies are found to be predominantly connected to eastern Andean summer rain climate, which contradicts previous studies and the modern mean spatial boundary between subtropical summer and winter rain climate derived from the grid data. The only ice core proxy showing a response to ENSO is the major ion concentrations, via local temperature indicating reduced sublimation and mineral dust input during El Niño years.

Description: Lake Donggi Cona is located in the north-eastern part of the Tibetan Plateau at the boundary between monsoon and westerly climate influence on an elevation of 4090 m. Donggi Cona is an oligotrophic freshwater lake with a recent open-basin system. The 30 km long, 8 km wide, and up to 98 m deep lake basin is a tectonic pull-apart structure as part of Kunlun fault. Sub-bottom profiling of the lake basin revealed the presence of graben structures, conjugate faults, subaquatic terraces, and ancient fan systems, draped by 4 to 5 m thick postglacial lacustrine sediments. The aim of our investigations is the reconstruction of palaeo-environmental changes of the lake catchment.After analysis of the seismic pre-survey, five sediment cores have been retrieved at 35-40 m and at 2 m water depth. The cores are partly laminated and composed of calcareous muds with changing amounts of carbonate micrite, organic matter and detrital silt and clay. Furthermore, the cores contains remarkable amounts of ostracods and plant remains. Dating results from the top of three sediment cores obtain a hardwater effect of about 2000 years. Nineteen AMS dates from the entire cores suggest a sedimentary history throughout the Holocene. Despite possible reservoir errors the sandy bottom sediments were deposited during or after the last glacial maximum (LGM). With a multi-proxy approach using grain size, XRF, XRD, CNS, TOC and multivariate statistical endmember-modelling (EMMA) we hereby represent an interpretation concerning changes in hydrological conditions and sedimentation processes during the last about 20 kyears: Prior to 18 kyears, terrestrial loess and sand suggest a very low lake level. From 18 to 13 kyrs. BP the lake level rise during a period of climate-driven glacier melt. For the same time period the detrital supply is at its maximum and seems to control the high sedimentation rate.Grain-size coarsening and hiatuses in four sediment cores, document a repeated lake-level fall after 13 kyrs. BP. From 12.3 to 4.3 kyrs, the lake level rose to above its present level as evidenced by the different terrace levels in the littoral of Lake Donggi Cona and by the increase of clay content within all cores. For the same time interval we assume increasing salinity because of the accumulation of aragonite. The lake changes in that time from a full ventilated to a stratified system in response to warm and moist conditions during the Holocene climate optimum. At 4.3 kyrs. BP the system shifted from an aragonite- to a calcite- dominated system. This sudden change can be attributed to a threshold response during prolonged lake-level rise and overspill at the western end of the lake, an tectonic event and/or climate induced processes.

Description: The catchment of Lake Donggi Cona, situated at north-eastern Tibetan Plateau, is a region influenced by monsoonal air masses of different origin and character, highly variable in space and time. Reconstruction of Quaternary monsoon dynamics focuses mainly on high-resolution archives (e.g. glacier ice and speleothem records) with detailed proxy information on temperature and precipitation regimes in the past. However, also sedimentological and geomorphological processes respond to and register monsoon dynamics, mainly amount and spatio-temporal variation of precipitation. In lake catchments, climatic signals affect the whole process system within the geomorphological inventory on different scales. Sediment traps store these signals and the landscapes response to them, though buffering and topological dependencies of archives have to be considered. Within temporal or final sediment storage basins, sediment properties represent a mixture of terrestrial and lacustrine processes during and after deposition (e.g. aeolian, fluvial transport, and water current directions). Each process as well as different sediment sources (e.g. sub-catchments with a certain geochemical and mineralogical fingerprint) can be regarded as an end-member.We present an approach of unmixing lake sediment properties, i.e. grain size distributions, geochemical and mineralogical compositions, to reveal end-members of geomorphological processes and sediment source. In a first step, modern surface samples from the bottom of Lake Donggi Cona are used to test a flexible iterative end-member modelling algorithm (EMMA) with different types of data transformation, eigenspace analysis and goodness-of-fit tests. Empirical end-members are derived and compared to natural end-members, i.e. terrestrial surface samples from locations dominated by a single geomorphological process regime or sediment source. Using Bayesian approaches, conditional probabilities can be given for the variation within and between sediment properties. This determines the significance of a single value against randomness and accounts for interferences and/or buffering of external signals in the sediments. As a second step, end-members derived from modern analogues as probability density functions can be applied to fossil lake sediments of high resolution reaching back to the Last Glacial Maximum. Thus, a spatio-temporal quantification of geomorphic processes and regions is facilitated probabilistically and allows a coupling with climate records.

Description: Limnogeological studies at Lake Donggi Cona on the NE Tibetan PlateauS. Opitz (1), E. Dietze (4), J. IJmker (2), K. Hartmann (4), F. Lehmkuhl (2), G. Stauch (2), B. Wünnemann (3, 4), B. Diekmann (1),(1) Alfred Wegener Institute for Polar and Marine Research, Research Unit Potsdam Germany (Stephan.Opitz@awi.de), (2) Department of Geography, RWTH Aachen University, Germany (3) School of Geography and Oceanography, Nanjing University, China, (4) Institute of Geographical Sciences, Interdisciplinary Center of Ecosystem Dynamics of Central Asia (EDCA), Freie Universität Berlin, Germany,Lake Donggi Cona is located in the north-eastern part of the Tibetan Plateau at the boundary between monsoon and westerly climate influence. The main objective is to infer late glacial to Holocene lake development in response to monsoon dynamics. Donggi Cona is an oligotrophic freshwater lake with a recent open-basin system. The 30 km long, 8 km wide, and up to 90 m deep open lake basin is a tectonic pull-apart structure, situated along the Kunlun suture. Sub-bottom profiling of the lake basin revealed the presence of graben structures, conjugate faults, subaquatic terraces, and ancient fan systems, draped by 4 to 5 m thick postglacial lacustrine muds.After analysis of the seismic pre-survey, five sediment cores have been retrieved at 35-40 m and at 2 m water depth. The cores are partly laminated and composed of calcareous muds with variable amounts of carbonate micrite, organic matter, detrital silt and clay, and are rich in ostracods and plant remains. Dating results from the top of cores obtain a hardwater effect of about 2000 years. 19 14C AMS dates from the entire cores suggest a sedimentary history throughout the Holocene. Despite possible reservoir errors the sandy bottom sediments were deposited during or after the last global glacial maximum (LGM). With a multi-proxy approach using grain size, XRF, XRD, CNS, TOC and statistical modelling (EMMA) we reconstruct the lake development and the forcing factors of the last 19 ka in relation to the palaeo-climatic development. Prior to 18 ka terrestrial loess and sand suggest a very low lake level. From 18 to 13 ka B.P the lake level rise during a period of climate-driven glacier melt. For the same time period the detrital supply is at its maximum and seems to control the high sedimentation rate.Grain-size coarsening and hiatuses in some sediment cores, document a repeated lake-level fall after 13 ka BP, possibly associated with the late-glacial Younger Dryas stadial. From 12.3 to 4.3 ka the biological productivity was high, the lake level rose to above its present level as evidenced by the different terrace levels in the littoral of Lake Donggi Cona and by the increase of the fine fraction of the sediment. For the same time interval we assume a rise in salinity because of the increasing accumulation of aragonite. The lake changes in that time from a full ventilated to a stratified system in response to warm and moist conditions during the Holocene climate optimum. The values of aragonite and calcite show for all cores an inverse distribution pattern. At 4.3 ka B.P the system shifts from an aragonite dominated to a calcite dominated system. This can be explained by a shift from a closed brackish to an open fresh water system. This sudden change can be attributed to a threshold response during prolonged lake-level rise and overspill at the western end of the lake. Lake-level rise, possibly progressed in the course of continuous climate deterioration in response to a decline in summer insolation and evaporation, leading to a positive hydrological budget.