Search

Holocene environmental and climate change on the Tibetan Plateau is intensively studied and discussed with the aim to better understand the factors controlling the hydrology of individual river catchments and especially the availability of water which is of utmost significance for the communities downstream in times of rapid climate change. Thus, a late glacial and Holocene sediment record from Lake Heihai in the Kunlun-Pass region was investigated using ostracod and geochemical analyses. Cold and dry conditions were inferred between ca. 12.9 and 12.3 cal ka BP and higher temperatures before and afterwards. The cold spell probably corresponds to the Younger Dryas (YD) event in the North Atlantic region. Warmer and wetter conditions with highest lake levels and decreased lake-water salinity were recorded from ca. 10.8 to 7.0 cal ka BP when the summer monsoon was strengthened. The cold 8.2 cal ka BP event is not significantly recorded in the region probably due to the predominance of the summer monsoon over the westerlies. A declined lake level and increased lake-water salinity as the result of cold and dry conditions are inferred from ca. 7.0 to 4.5 cal ka BP when the strengthening of the mid-latitude westerly circulation probably triggered glacier advances in the catchment. An even lower lake level existed during cold conditions with glacier advances from ca. 4.5 to 1.2 cal ka BP. The level of Lake Heihai rose again after ca. 1.2 cal ka BP due to warmer conditions, causing the retreat of glaciers and higher runoff. Our record from the Kunlun Pass region provides further evidence for the catchment-specific response of hydrographical systems which are partly controlled by glaciers as major water sources. [ABSTRACT FROM AUTHOR]

To elucidate the dynamics of terrestrial leaf waxes in a high-altitude lake system, we performed compound-specific radiocarbon analysis (CSRA) of long-chain n-alkanes in two sediment core sections from Lake Karakul (Pamirs, Tajikistan) and in surface soil samples from the catchment area. We aimed to answer the question whether the n-alkanes are delivered into the lake sediment with substantial delay due to storage in soils, which may cause a potential bias when used as paleoenvironmental proxies. In the surface soils, the CSRA results reveal an age range of n-alkanes from modern to 2278 ± 155 cal BP. In the two sediment core samples, three of the four n-alkane ages fell on the lower ends of the 1σ-uncertainty ranges of modeled ages of the sediments (based on AMS 14C-TOC and OSL dating results). We conclude that sedimentary leaf waxes represent compounds with intermediate turnover time in soils, for example originating from alluvial plains close to the shores. Overall, the results provide evidence that sedimentary leaf wax compounds in this cold and arid setting are potentially older than the conventional age model indicates, but these findings need to be interpreted in context of the generally large uncertainty ranges of such age models. [ABSTRACT FROM AUTHOR]

Lake Balkhash is a large and relatively shallow closed-basin lake in the arid part of Central Asia. The lake experienced an ecological crisis in the 1970s and 1980s when the filling of the Kapchagay Reservoir in the middle reaches of its main tributary caused a significant lake-level decline and salinity rise, and resulting biodiversity loss. A sediment core representing 2900 years of lake history was obtained to assess man-made alterations of the lake in the last decades in comparison to the natural status of Lake Balkhash. Analyses of sediments and of ostracod (micro-crustacean) species assemblages and shell chemistry revealed that the lake was probably relatively shallow and more brackish ca. 2900–2200 calibrated years before present (cal a BP) in comparison to the period when the Kapchagay Reservoir was filled. A relatively high lake level and lower salinity prevail since ca. 2200 cal a BP. The uppermost sediments of the obtained core and nearby surface-sediment samples provide evidence for the establishment of hypoxic conditions in Lake Balkhash since the middle of the 1970s. The lake-level decline during the filling of the Kapchagay Reservoir probably caused the redistribution of organic-matter rich littoral sediments to greater depth in the lake where dissolved oxygen became consumed due to the decomposition of organic remains. The spatial and temporal distribution of hypoxia in Lake Balkhash is not known, and systematic studies are required to assess the status of the benthic life and potential impacts of future lake-level, nutrient influx and temperature changes. [ABSTRACT FROM AUTHOR]

Abstract: Lake Karakul in the Pamirs (Tajikistan) is a deep brackish-water lake in a closed basin at almost 4000m above sea level. Water samples from the catchment area and Lake Karakul, and a 104-cm sediment core from its shallow eastern sub-basin, were investigated and provide a first lake record from the region spanning the last 4200cal yr BP. Multi-proxy analyses revealed the following: A relatively high meltwater inflow from glaciers, snow fields and frozen ground in the catchment as a result of relatively warm conditions was reconstructed for the period between 4200 and 3500cal yr BP. The shift to colder climatic conditions around 3500cal yr BP was probably abrupt; total organic carbon values displayed the most dramatic drop for the entire core within less than 40years. In contrast, the subsequent re-establishment of warmer conditions occurred gradually over several centuries. A higher meltwater supply to the lake and warmer conditions were recorded since ca. 1900cal yr BP with two slightly colder intervals between 1200 and 800cal yr BP and between 400 and 100cal yr BP. The abrupt shift to significantly colder and drier conditions around 3500cal yr BP in the eastern Pamirs is seen also in records from the Aral Sea and the Guliya ice core from the western Tibetan Plateau. However, more palaeoclimate studies in this highly continental part of Central Asia are needed to assess the spatial and temporal patterns of the Holocene climate in the region. [Copyright &y& Elsevier]

Abstract Considerable efforts have been devoted to decipher the late Quaternary moisture and thermal evolution of arid central Asia. However, disparate interpretations still exist concerning different proxies. The spatial and temporal heterogeneities have inhibited a holistic understanding of general patterns and underlying mechanisms. To address these issues, two parallel cores (ONW I, 6.00 m; ONW II, 13.35 m) were retrieved in the Gobi Desert of Mongolia from lake Orog Nuur. Multidisciplinary investigations including geomorphological mapping, radiocarbon dating, sedimentological, palynological and ostracod analyses enabled us to gain a comprehensive dataset for vegetation development and hydrological variability over the last ~45 kyr. Higher lake levels during the Marine Isotope Stage (MIS) 3 and early MIS 2 (~35–~24 kyr) were probably caused by increased precipitation. The sharp transition of Termination I (~11 kyr) is indicated by sedimentological, palynological, and ostracod data. During the late Pleistocene, the lower area of the Orog Nuur catchment was dominated by Artemisia steppe and gradually altered to Chenopodiaceae desert steppe in the Holocene. The early Holocene is also characterized by a relatively humid environment. The humid pulses during the MIS 3 and the early Holocene were also recorded in other archives and are possibly the trait of a larger scale phenomenon in arid central Asia. Four major harsh climatic periods were documented in the core at ~43 kyr, ~36 kyr, during the global Last Glacial Maximum, and the Younger Dryas as playa phases. Reduced westerlies' moisture transport and a retreated East Asian Summer Monsoon influence probably caused those dry phases in the Gobi Desert of Mongolia. Highlights • The Gobi Desert experienced wet pluses during MIS 3/early MIS 2 (~35–~24 kyr) and early Holocene. • Four major playa phases (~43 kyr, ~36 kyr, ~19–~18 kyr, ~15–~11 kyr) exist in the Gobi Desert. • Higher pollen concentration and CaCO 3 content mark the transition of Termination I. [ABSTRACT FROM AUTHOR]

Arid Central Asia represents a key region for understanding climate variability and interactions in the Northern Hemisphere. Patterns and mechanisms of Holocene climate change in arid Central Asia are, however, only partially understood. Multi-proxy data combining diatom, ostracod, sedimentological, geochemical and stable isotope analyses from a ca. 6000-year-old lake sediment core from Son Kol (Central Kyrgyzstan) show distinct and repeated changes in species assemblages. Diatom- and ostracod-inferred conductivity shifts between meso-euhaline and freshwater conditions suggest water balance and regime shifts. Organism-derived data are corroborated by stable isotope, mineralogical and geochemical records, underlining that Son Kol was affected by strong lake level fluctuations of several meters. The δ 13 C carb /δ 18 O carb correlation shows repeated switchovers from a closed to an open lake system. From 6000 to 3800 and 3250 to 1950 cal. yr BP, Son Kol was a closed basin lake with higher conductivities, increased nutrient availability and a water level located below the modern outflow. Son Kol became again a hydrologically open lake at 3800 and 1950 cal. yr BP. Comparisons to other local and regional paleoclimate records indicate that these regime shifts were largely controlled by changing intensity and position of the Westerlies and the Siberian Anticyclone that triggered changes in the amount of winter precipitation. A strong influence of the Westerlies ca. 5000–4400, 3800–3250 and since 1950 cal. yr BP enhanced the amount of precipitation during spring, autumn and winter, whereas cold and dry winters prevailed during phases with a strong Siberian Anticyclone and southward shifted Westerlies at ca. 6000–5000, 4400–3800 and 3250–1950 cal. yr BP. Similarities between variations in winter precipitation at Son Kol and records of the predominant NAO-mode further suggest a teleconnection between wet (dry) winter climate in Central Asia and a positive (negative) NAO-mode. Thus, this study identifies climate fluctuations as the main driver for hydrological regime shifts in Son Kol controlling physicochemical conditions and consequently causing abrupt species assemblage changes. This emphasizes the importance of multi-proxy approaches to identify triggers, thresholds and cascades of aquatic ecosystem transformations. [ABSTRACT FROM AUTHOR]