Lake Donggi Cona Research Articles

Holocene climatic and tectonic forcing of decadal hydroclimate variation on the north-eastern Tibetan Plateau

A better understanding of past climate and landscape variability in high resolution on the Tibetan Plateau is of critical importance for the understanding of fundamental mechanisms and interactions in the global climate system. However, due to relatively few datasets in decadal-resolution available for analyzing possible seasonal effects and seismic activity on the northeastern Tibetan Plateau, further high-resolution lake records as an integration need to be explored to test the significance of the seasonal variations of surface processes and climatic phenomena as well as tectonic impact on a decadal scale. The rhomb-shaped pull-apart Lake Donggi Cona along the modern monsoon marginal zone on the north-eastern TP is considered an excellent archive that monitors neotectonic and climatic changes through time. We obtained new high-resolution (mean 20 yr) proxy records from a 488 cm-long sediment core from the lake and compare them directly with paleoclimate records from nearby lakes. A decadal-resolution oxygen stable isotope (δ18O) record co-varies positively with carbonate content and demonstrates the profound and varying contribution of glacier/snow melt (winter/spring moisture) to lake hydrology at different lake evolution stages. This is also indicated by the grain size composition and related endmember modelling (EMMA) results. During 11.8–9.4 cal Ka BP, the lake experienced a swamp status, apart from the major lake body followed by water rise towards the littoral zone until 7.5 cal Ka BP. Glacier melt water supply started to be active since the late Younger Dryas and led to the strong excursions of carbonate contents and δ18O values according to insolation increase. This phase was dominated by the westerlies and characterized as low seasonality with long winter duration. Water rose to its highest level during 7.5–6.5 cal Ka BP and fluctuated with deep water phases until 4.7 cal Ka BP, due to glacier melt maxima and potential increase of summer (convective) moisture preserved in the oxygen isotopic signals. Water cover dropped and remained low until 2.9 cal Ka BP due to the reduced glacier melt and colder climate. However, the low δ18O values at a reduced water budget points to the still active contribution of spring glacier/snow melt that kept the lake not dry-up unlike other lakes with no glacier melt water supply. After 2.9 cal Ka BP water level dropped towards a pond/lagoon situation until 2.0 cal Ka BP, albeit temperature rise after a long cold climatic interlude. It might be caused by another phase of lake deepening triggered by subsidence and related sediment relocation from the shore, rather than by climatic factors. However, water level increased again afterwards and fluctuated towards the present level due to non-summer moisture supply in long freezing winters. We assume profound signatures of high seasonality in climate after 7.5 cal Ka BP and the lake became open since1.3 cal Ka BP. We suggest that combined thermal forcing by glacier/snow melt and tectonics play a significant role together with solar forcing, governing hydroclimate changes on the north-eastern Tibetan Plateau with decadal to millennial cycles.

Late quaternary glacial history of the La Ji Shan: Implications for the LGM on the northeastern Tibetan Plateau

Previous studies have suggested that the Bayan Har Shan and Lake Donggi Cona areas on the northeastern Tibetan Plateau were not glaciated during the Last Glacial Maximum (LGM; MIS2). However, the La Ji Shan, which is at a lower elevation than the Bayan Har Shan and Lake Donggi Cona areas but has similar annual precipitation, experienced a glacial event during the LGM. To investigate this discrepancy, factors controlling glacier development in the Bayan Har Shan, Lake Donggi Cona, and La Ji Shan were examined. First, a coupled mass balance and ice flow model was used to reconstruct the LGM climatic conditions in the La Ji Shan, and then the factors controlling glacier development were assessed based on the modeled LGM climatic conditions. With LGM precipitation being 70%–80% of present-day values, the modeled LGM temperature decrease was 3.9°C–4.3°C, which is consistent with other reconstructed LGM climatic conditions on the Tibetan Plateau. A comparison of the topography and climate of the La Ji Shan, Bayan Har Shan and Lake Donggi Cona areas indicates that a lower LGM summit height above the LGM equilibrium line altitude (ELA) and LGM annual precipitation at the LGM ELA are the main reason for the lack of LGM glacial expansion in the Bayan Har Shan and Lake Donggi Cona areas.

Neotectonic Subsidence Along the Cenozoic Kunlun Fault (Tibetan Plateau)

AbstractMany lake basins on the Tibetan Plateau are affected by tectonic activity. It is therefore important to consider seismotectonic influence on lake evolution when discussing climate impact on hydrology and sediment budgets. Lake Donggi Cona serves as an example to demonstrate the influence of the Kunlun left‐lateral strike‐slip fault on lake formation with subsidence in the pull‐apart basin. The results show that comparable fluvial sandy sediments generated at low lake levels during the Late Glacial and early Holocene occur at different water depths within a double lake‐internal pull‐apart structure, suggesting mean subsidence rates of 2.6–4.3 mm/a since 13.5 cal. ka BP. Such pulses prevailed throughout the Holocene, pointing to repeated seismic events and ongoing neotectonic activity. Sediment distribution and changing morphological shape by subsidence are important factors that affected other lakes on the Tibetan Plateau and enable us to distinguish between neotectonic and paleoclimatic impacts.

Prehistoric human activity and its environmental background in Lake Donggi Cona basin, northeastern Tibetan Plateau

Lake Donggi Cona is a key area for the exploration of the prehistoric settlement of the inner Tibetan Plateau because of its location in the joint of the higher inner Tibetan Plateau and the upper Yellow River valley. Here, we carried out archeological investigation on the lake basin, and a total of 256 pieces of stone artifacts were collected from the surface of the DJCN 3-2-2 site on the northern shore terraces of the lake. In addition, AMS14C and OSL dating were performed on hearths and section (DJCN 3-2-2), respectively; meanwhile environmental proxies, including grain size, charcoal, magnetic susceptibility, fungal spore, and pollen, were analyzed. The results indicate that the AMS14C ages of the charcoal range between ~5.4 and 5.0 cal. ka BP, consistent with OSL dating from the charcoal layers in the DJCN 3-2-2 section (~5.5 ka BP). The functional analysis of the stone artifacts and dating results showed that the site was a seasonal, relative long-term, and central camp which was used for processing of stone artifacts, cutting and consuming food and the production of daily necessities. Charcoal, magnetic susceptibility, and fungal spores from the DJCN 3-2-2 section revealed that prehistoric human activities began at ~5.8 ka BP and significantly intensified during the period of ~5.6–5.5 ka BP and then gradually weakened. Pollen assemblages indicated that the vegetation was dominated by alpine steppe during the period of ~5.8–5.0 ka BP, indicative of a relatively warmer and wetter climatic condition during this period. Taken together, we infer that under the impact of the Neolithic culture in lower altitude of upper Yellow River valley and Microlithic culture in the higher altitude inner Plateau, a Neolithic-Zongri culture had emerged in the transitional region between these two cultures during middle Holocene.

Lithium isotopes and implications on chemical weathering in the catchment of Lake Donggi Cona, northeastern Tibetan Plateau

This study presents lithium (Li) isotope ratios (δ7Li) for rocks, sediments, suspended particulate material, and dissolved Li from the Lake Donggi Cona catchment, located on the northeastern Tibetan Plateau, China. The average δ7Li=+1.9‰ of the bedrocks is estimated from local loess. δ7Li values decrease progressively within the sediment cascade from loess, to river and lake floor sediments. The lake floor sediments average at −0.7‰. The difference between bedrock and lake sediments reflects the preferential fractionation of dissolved 6Li into clay minerals (mostly illite) in the weathering zone and grain-size sorting during fluvial sediment transport. The δ7Li values of stream and lake water samples range from +13.6 to +20.8‰, whereas thermal waters fall between +5.9 and +11.6‰. The δ7Li values of lake water samples are close to +17‰ and reflect mixing of waters from two perennial inflows and thermal waters. Dissolved Li in streams represents an integrated isotopic signal derived from soil solutions in the weathering zone. An apparent isotopic fractionation of −17.8±1.6‰ (αsec-sol∼0.982) between secondary minerals and solution was determined. An inflow that drains a sub-catchment in the north carries a high proportion of thermal waters. Despite of the high proportion of admixed thermal waters with high Li concentrations and low δ7Li, this stream has the highest δ7Li values of about +21‰. This is consistent with admixing of thermal waters to solutions in the weathering zone and subsequent fractionation by preferential uptake of isotopically light dissolved Li into secondary phases. Based on Li isotope ratios of the dissolved and solid export flux from the weathering zone we calculated that around five times more Li is exported in particles than dissolved in streams. An average δ7Li value of about +17‰ of most streams and the lake is reflecting a low weathering intensity and chemical weathering rate of about 4t/km2/a. Low weathering rates and an erosion dominated weathering system are consistent with moderate precipitations, the cold climate, and the high relief of the study area.

Late Pleistocene glaciations at Lake Donggi Cona, eastern Kunlun Shan (NE Tibet): early maxima and a diminishing trend of glaciation during the last glacial cycle

The Burhan Budai Shan in NE Tibet represents a key location for examining the variable influence of the mid‐latitude westerly and monsoonal circulations on late Quaternary glaciations in this sector of the Tibetan Plateau. Our study investigates the glacial history of mountains near Lake Donggi Cona (35°17′N, 98°33′E) using field mapping in combination with 10Be surface exposure dating and numerical reconstructions of former glacial equilibrium line altitudes (palaeo‐ELA). A set of 23 new exposure ages, collected from moraines in four glacial valleys, ranges from 45 to 190 ka, indicating ice expansion during the early and middle part of the last glacial cycle, and during the penultimate and possibly an earlier Mid‐Pleistocene glaciation. Ice advances reaching 12–15 km in length occurred at around 190–180 ka (≥MIS 6), between 140–100 ka (late MIS 6/MIS 5), and 90–65 ka (late MIS 5/early MIS 4), with a maximum ELA depression of 400–500 m below the estimated modern snowline. Exposure ages from the valley headwaters further indicate a small glaciation between c. 60–50 ka (late MIS 4/early MIS 3), which was essentially restricted to the cirque areas. Significantly, we find no evidence for any subsequent glaciation in the area during MIS 2 or the Holocene period. These results indicate a diminishing trend of glaciation in the region since at least MIS 4, and corroborate the case of a ‘missing LGM’ in the more interior parts of the northeastern Tibetan Plateau. The emerging pattern suggests that the most favourable conditions for glaciation during the Late Pleistocene correspond to periods of relatively moderate cooling combined with an intermediate or rising East Asian monsoon strength.

Climate variability in the past ∼19,000 yr in NE Tibetan Plateau inferred from biomarker and stable isotope records of Lake Donggi Cona

Abstract We investigated 4.84-m-long sediment record spanning over the Late Glacial and Holocene from Lake Donggi Cona to be able to reconstruct circulation pattern on the Tibetan Plateau (TP). Presently, Lake Donggi Cona is located at the boundaries of Westerlies and Asian monsoon circulations in the northeastern TP. However, the exact timing and stimulating mechanisms for climatic changes and monsoon shifts in this region are still debated. We used a 19-ka-long stable isotope record of sedimentary n -alkanes to address this discrepancy by providing insights into paleohydrological conditions. The δD of n C 23 is influenced by lake water evaporation; the δD values of sedimentary n C 29 are mainly controlled by moisture source and temperature changes. Long-chain n -alkanes dominate over the core whereas three mean clusters (i.e. microbial, aquatic and terrestrial) can be inferred. Multi-proxies suggest five major episodes in the history of Lake Donggi Cona. The Lake Donggi Cona record indicates that the Late Glacial (18.4–14.8 cal ka BP) was dominated by low productivity of mainly microbial and aquatic organisms. Relatively low δD values suggest low temperatures and moist conditions eventually caused by stronger Westerlies, winter monsoon and melt-water influence. Likely, the shift (∼17.9 cal ka BP) from microbial to enhanced aquatic input suggests either a change from deep to shallow water lake or a break in local stratification. Between 14.8 and 13.0 cal ka BP, variable climatic conditions prevailed. Although the Westerlies weekend, the increase in temperature enhanced the permafrost and snow melting (displayed by a high sedimentary accumulation rate). Higher δD values indicate increasingly arid conditions with higher temperatures which eventually lead to high evaporative conditions and lowest lake levels. Low vegetation cover and high erosion rates led to high sediment accumulation resulting in stratification followed by anoxia in the terminal lake. From 13.0 to 9.2 cal ka BP, lowered values of δD along with high contents of terrestrial organic matter marked the early-Holocene warming indicating a further strengthening of summer precipitation and higher lake levels. A cooling trend was observed in the mid-Holocene between 9.2 and 3.0 cal ka BP accompanied by higher moisture availability (displayed by lowered δD values) caused by reduced evaporative conditions due to a drop in temperature and recovering Westerlies. After 3.0 cal ka BP, a decrease in lake productivity and cold and semi-arid conditions prevailed suggesting lower lake levels and reduced moisture from recycled air masses and Westerlies. We propose that the summer monsoon was the predominant moisture source during the Bolling-Allerod warm complex and early-Holocene followed by Westerlies in mid-to-late Holocene period. Stable carbon isotope values ∼ - 32‰ indicate the absence of C 4 -type vegetation in the region contradicting with their presence in the Lake Qinghai record. The δD record from lake Donggi Cona highlights the importance of the interplay between Westerlies and summer monsoon circulation at this location, which is highly dynamic in northeastern plateau compared to the North Atlantic circulation and insolation changes. Consequently lake Donggi Cona might be an important anchor point for environmental reconstructions on the Tibetan Plateau.

A Complex System of Glacial Sub-Refugia Drives Endemic Freshwater Biodiversity on the Tibetan Plateau.

Although only relatively few freshwater invertebrate families are reported from the Tibetan Plateau, the degree of endemism may be high. Many endemic lineages occur within permafrost areas, raising questions about the existence of isolated intra-plateau glacial refugia. Moreover, if such refugia existed, it might be instructive to learn whether they were associated with lakes or with more dynamic ecosystems such as ponds, wetlands, or springs. To study these hypotheses, we used pulmonate snails of the plateau-wide distributed genus Radix as model group and the Lake Donggi Cona drainage system, located in the north-eastern part of the plateau, as model site. First, we performed plateau-wide phylogenetic analyses using mtDNA data to assess the overall relationships of Radix populations inhabiting the Lake Donggi Cona system for revealing refugial lineages. We then conducted regional phylogeographical analyses applying a combination of mtDNA and nuclear AFLP markers to infer the local structure and demographic history of the most abundant endemic Radix clade for identifying location and type of (sub-)refugia within the drainage system. Our phylogenetic analysis showed a high diversity of Radix lineages in the Lake Donggi Cona system. Subsequent phylogeographical analyses of the most abundant endemic clade indicated a habitat-related clustering of genotypes and several Late Pleistocene spatial/demographic expansion events. The most parsimonious explanation for these patterns would be a scenario of an intra-plateau glacial refugium in the Lake Donggi Cona drainage system, which might have consisted of isolated sub-refugia. Though the underlying processes remain unknown, an initial separation of lake and watershed populations could have been triggered by lake-level fluctuations before and during the Last Glacial Maximum. This study inferred the first intra-plateau refugium for freshwater animals on the Tibetan Plateau. It thus sheds new light on the evolutionary history of its endemic taxa and provides important insights into the complex refugial history of a high-altitude ecosystem.

Chemical and isotopic (O, H, C) composition of surface waters in the catchment of Lake Donggi Cona (NW China) and implications for paleoenvironmental reconstructions

The oxygen, hydrogen, and carbon isotope ratios and major and trace element concentrations of surface waters are reported for the catchment of Lake Donggi Cona, Qinghai Province, China. The chemistry of the surface waters in the southeastern catchment is reflecting the dissolution of carbonate rocks, whereas thermal waters add sodium and chlorine to the waters in the northern catchment. The Dongqu River, draining the southeast catchment, contributes 87 to 94% to the water budget of the lake. Thermal waters and waters from the northern catchment add 6 to 13%. The combination of hydrogen and oxygen isotopes provides evidence that only small amounts of recycled lake water contribute to the precipitation in the catchment. Most of the moisture may be transported by the East Asian summer monsoon or an eastern branch of the South Asian monsoon into the Donggi Cona region. From oxygen isotope ratios it is calculated that about 45% of the lake water input is evaporated and 55% leaves the lake through a channeled outflow. The carbon isotope ratios identifies microbial respiration of organic matter, probably in soils and sediments of the catchment, as the major source of dissolved inorganic carbon in most surface waters and the lake. An isotope effect, from biological processes, in the lake is minor, and can be neglected in order to explain isotope ratios of inorganic carbon in lake water. The results imply that oxygen isotope ratios in the lake sediment archives mainly mirror the relation between precipitation and evaporation (P/E), which may be controlled by changes in the global water cycle like monsoon intensity or by tectonically driven local changes affecting the outflow flux. A comparison between lake archives based on ostracod shells and the modern system indicates similar δ18O values for the lake water since 4.3ka but different δ13CDIC values. This discrepancy between carbon isotope ratios in ostracod shells and modern lake water samples may be due to thermal waters with high δ13CDIC emerging near the coring site. It is suggested that carbon isotopes of ostracods from a sediment drill core reflect the activity of a local thermal spring at the lake bottom, whereas oxygen isotopes provide evidence for an open lake system since about 11ka and that the climate and environment of the Donggi Cona region changed very little over the last 4300years.

Spatio-temporal pattern of detrital clay-mineral supply to a lake system on the north-eastern Tibetan Plateau, and its relationship to late Quaternary paleoenvironmental changes

Clay-mineral fingerprints from modern and fossil lake sediments from Lake Donggi Cona, situated on the north-eastern Tibetan Plateau were studied to determine changes in sediment provenance and environmental development since the late Glacial. To infer the source areas of the lake sediments, characteristic facies units (cover sediments and soils) from the catchment of the lake were studied concerning their clay-mineral compositions and through statistical analyses using the Fuzzy C-Means algorithm.Sediment contributions from the various geological provinces in the catchment of the lake are reflected in a systematic spatial pattern of variable clay-mineral assemblages, which is most pronounced in modern fluvial and lake sediments. Downcore changes in the clay-mineral fingerprints of lake sediments provide information on the spatio-temporal variability of detrital sediment supplies, related to climate-influenced changes in environmental boundary conditions.Since the late Glacial, water-level fluctuations driven by glacier melt appear to have exerted the greatest control over the transport paths of detrital sediments. Probably glacial melt-waters carried mainly illitic rich pre-Cenozoic debris into the lake from the northern and north-western catchment areas. During the Holocene the detrital clay mineral supply was mainly controlled from pre-Cenozoic and Neogene sources. However, because of the increased summer monsoon precipitation the lake level rose dramatically to at least modern levels, resulting in a relative increase in the supply from pedogenic karstic weathering sources mainly from the east, which during the Holocene was possibly flooded more widely than today. During the late Holocene a decreasing input of smectite suggests a relative reduction in the supply of far-traveled aquatic suspensions into the lake. In contrast, during the same interval kaolinite concentrations increased, which suggests a relatively strong sediment supply from the east, possibly triggered by a current flowing from east to west.

Quantitative reconstruction of precipitation changes on the NE Tibetan Plateau since the Last Glacial Maximum – extending the concept of pollen source area to pollen-based climate reconstructions from large lakes

Abstract. Pollen records from large lakes have been used for quantitative palaeoclimate reconstruction, but the influences that lake size (as a result of species-specific variations in pollen dispersal patterns that smaller pollen grains are more easily transported to lake centre) and taphonomy have on these climatic signals have not previously been systematically investigated. We introduce the concept of pollen source area to pollen-based climate calibration using the north-eastern Tibetan Plateau as our study area. We present a pollen data set collected from large lakes in the arid to semi-arid region of central Asia. The influences that lake size and the inferred pollen source areas have on pollen compositions have been investigated through comparisons with pollen assemblages in neighbouring lakes of various sizes. Modern pollen samples collected from different parts of Lake Donggi Cona (in the north-eastern part of the Tibetan Plateau) reveal variations in pollen assemblages within this large lake, which are interpreted in terms of the species-specific dispersal and depositional patterns for different types of pollen, and in terms of fluvial input components. We have estimated the pollen source area for each lake individually and used this information to infer modern climate data with which to then develop a modern calibration data set, using both the multivariate regression tree (MRT) and weighted-averaging partial least squares (WA-PLS) approaches. Fossil pollen data from Lake Donggi Cona have been used to reconstruct the climate history of the north-eastern part of the Tibetan Plateau since the Last Glacial Maximum (LGM). The mean annual precipitation was quantitatively reconstructed using WA-PLS: extremely dry conditions are found to have dominated the LGM, with annual precipitation of around 100 mm, which is only 32% of present-day precipitation. A gradually increasing trend in moisture conditions during the Late Glacial is terminated by an abrupt reversion to a dry phase that lasts for about 1000 yr and coincides with "Heinrich event 1" in the North Atlantic region. Subsequent periods corresponding to the Bølling/Allerød interstadial, with annual precipitation (Pann) of about 350 mm, and the Younger Dryas event (about 270 mm Pann) are followed by moist conditions in the early Holocene, with annual precipitation of up to 400 mm. A drier trend after 9 cal. ka BP is followed by a second wet phase in the middle Holocene, lasting until 4.5 cal. ka BP. Relatively steady conditions with only slight fluctuations then dominate the late Holocene, resulting in the present climatic conditions. The climate changes since the LGM have been primarily driven by deglaciation and fluctuations in the intensity of the Asian summer monsoon that resulted from changes in the Northern Hemisphere summer solar insolation, as well as from changes in the North Atlantic climate through variations in the circulation patterns and intensity of the westerlies.

Interaction of geomorphological processes on the north-eastern Tibetan Plateau during the Holocene, an example from a sub-catchment of Lake Donggi Cona

Geomorphological landforms on a large alluvial fan and neighboring areas on the eastern side of the Donggi Cona show a complex spatial pattern. Sediment availability is an important factor in the formation of these archives and is partly associated with lake level fluctuation. Different sedimentary archives therefore show a similar geomorphological signal while in some cases similar archives are related to different forcing. The chronology of the processes is based on 22 optical stimulated luminescence (OSL) ages obtained from coarse-grained quartz or potassium-feldspar. The base of the alluvial fan formed during the late Pleistocene but deposition of cover sediments on the fan only started at around 7ka ago. Silty sediments form a thin cover on the uppermost terraces and have been available only during short timespans. Most of the sediments are sandy deposits which form an internal sediment cycle on the fan. They developed throughout the Holocene. Sandy deposits on the footslopes of the neighboring hills have been preserved since the Pleistocene-Holocene transition. Thicker eolian deposits accumulated during the late Holocene. These sediments were washed into the Donggi Cona and formed sandy lake sediments which were susceptible to remobilization during lower lake levels. In combination with drier climatic conditions during the late Holocene these sediments formed small dunes on the fan. Sediments on the alluvial fan are highly active, for which erosion and deposition vary in space and time. The interaction of fluvial and eolian processes is an important part of this dynamic high mountain system.

Early to mid-Holocene lake high-stand sediments at Lake Donggi Cona, northeastern Tibetan Plateau, China

Lake high-stand sediments are found in three onshore terraces at Lake Donggi Cona, northeastern Tibetan Plateau, and reveal characteristics of hydrological changes on lake shorelines triggered by climate change, geomorphological processes, and neo-tectonic movements. The terraces consist of fluvial–alluvial to littoral-lacustrine facies. End-member modeling of grain-size distributions allowed quantification of sediment transport processes and relative lake levels during times of deposition. Radiocarbon dating revealed higher than modern lake levels during the early and mid Holocene. Lake levels follow the trend of Asian monsoon dynamics, and are modified by local non-climatic drivers. Site-specific impacts explain fluctuations during the initial lake-level rise ~11calka BP. Maximum lake extension reached ~9.2calka BP, at ~16.5m above present lake level (a.p.l.l.). Littoral and lacustrine sediment deposition paused during a phase of fluvial activity and post-depositional cryoturbations at ~8.5calka BP, when the lake level fell to ~8m a.p.l.l. After a second maximum at ~7.5calka BP, lake level declined slightly at ~6.8calka BP, probably due to a non-climatic pulse that caused lake opening. The level remained high until a transition towards drier conditions ~4.7calka BP. Though discontinuous, high-stand sediments provide a unique, high-resolution archive.

Spatial variability of 14C reservoir effects in Tibetan Plateau lakes

Radiocarbon dating of lake sediments is often hampered by the presence of a lake reservoir effect (LRE, also ‘dead carbon’ or ‘old carbon’ effect) especially in dry and cold regions with a sparse plant cover in the catchment. The Tibetan Plateau became a hotspot of the palaeoenvironmental and climate research community in recent years and the assessment of present-day LREs is a crucial prerequisite for the establishment of reliable radiocarbon age–depth relationships for lake sediment cores. This paper examines the spatial variability of LREs within individual lakes, through a discussion of new data for Lake Donggi Cona and a compilation of previously published data for five additional lakes where LRE data are available for different sites. Lake reservoir effects for Lake Donggi Cona on the northeastern Tibetan Plateau were determined for shells of aquatic snails collected alive close to the lake's shore. The largest determined LRE of 20,000 14C years is significantly larger than previously reported LREs from the central part of the lake, and larger than any previously published LRE for the Tibetan Plateau. Relatively low LREs in the central regions of lakes, higher LREs towards the margins, and high LREs in tributaries and spring waters are apparently a common pattern of Tibetan Plateau lakes. The differences in LREs within individual lakes or catchment areas are attributed to the more prolonged exchange of the lake water's dissolved inorganic carbon with the atmospheric CO2 in central lake regions on the one hand and the increasing influence of 14C free or poor stream and groundwater due to the dissolution of carbonaceous basement rocks towards its margins. Generally higher LREs were recorded in the three tectonically active lake regions of the six examined catchments, and it is speculated that rising crustal CO2 further contributes to the LREs in these catchments. In addition to these observations and inferences, elevated 14C levels of the atmosphere as a result of nuclear bomb testing are often ignored if LREs for modern materials are reported by convention relative to the atmospheric 14C activity of the year 1950. LRE data reported in this way represent unrealistic minimum estimates.

Higher lake stands at Lake Donggi Cona, NE Tibetan Plateau, China – combining geomorphological and sedimentological approaches

Higher lake stands at Lake Donggi Cona, NE Tibetan Plateau, China – combining geomorphological and sedimentological approaches

Postglacial detrital sediment sources and transport processes in Lake Donggi Cona, northeastern Tibetan Plateau, revealed by multivariante statistical endmember-modelling (EMMA) of sedimentological data

Postglacial detrital sediment sources and transport processes in Lake Donggi Cona, northeastern Tibetan Plateau, revealed by multivariante statistical endmember-modelling (EMMA) of sedimentological data

Late glacial and Holocene climate evolution over north-eastern Tibetan Plateau, quantitatively reconstructed from Lake Donggi Cona pollen record

Late glacial and Holocene climate evolution over north-eastern Tibetan Plateau, quantitatively reconstructed from Lake Donggi Cona pollen record

Late Glacial and Holocene development of Lake Donggi Cona, north-eastern Tibetan Plateau, inferred from sedimentological analysis

Sediments of Lake Donggi Cona on the northeastern Tibetan Plateau were studied to infer changes in the lacustrine depositional environment, related to climatic and non-climatic changes during the last 19kyr. The lake today fills a 30×8km big and 95m deep tectonic basin, associated with the Kunlun Fault. The study was conducted on a sediment-core transect through the lake basin, in order to gain a complete picture of spatio-temporal environmental change. The recovered sediments are partly finely laminated and are composed of calcareous muds with variable amounts of carbonate micrite, organic matter, detrital silt and clay. On the basis of sedimentological, geochemical, and mineralogical data up to five lithological units (LU) can be distinguished that document distinct stages in the development of the lake system.The onset of the lowermost LU with lacustrine muds above basal sands indicates that lake level was at least 39m below the present level and started to rise after 19ka, possibly in response to regional deglaciation. At this time, the lacustrine environment was characterized by detrital sediment influx and the deposition of siliciclastic sediment. In two sediment cores, upward grain-size coarsening documents a lake-level fall after 13cal ka BP, possibly associated with the late-glacial Younger Dryas stadial. From 11.5 to 4.3cal ka BP, grain-size fining in sediment cores from the profundal coring sites and the onset of lacustrine deposition at a litoral core site (2m water depth) in a recent marginal bay of Donggi Cona document lake-level rise during the early to mid-Holocene to at least modern level. In addition, high biological productivity and pronounced precipitation of carbonate micrites are consistent with warm and moist climate conditions related to an enhanced influence of summer monsoon. At 4.3cal ka BP the lake system shifted from an aragonite- to a calcite-dominated system, indicating a change towards a fully open hydrological lake system. The younger clay-rich sediments are moreover non-laminated and lack any diagenetic sulphides, pointing to fully ventilated conditions, and the prevailing absence of lake stratification. This turning point in lake history could imply either a threshold response to insolation-forced climate cooling or a response to a non-climatic trigger, such as an erosional event or a tectonic pulse that induced a strong earthquake, which is difficult to decide from our data base.

An end-member algorithm for deciphering modern detrital processes from lake sediments of Lake Donggi Cona, NE Tibetan Plateau, China

Deciphering significant sedimentological processes from a set of sediment samples is an important step in reconstructing environmental changes. One approach going beyond classical methods is the unmixing of grain-size distributions. This paper presents a flexible end-member modelling algorithm that is based on eigenspace analysis and considers inherent uncertainties. It has been applied to the detrital grain-size components of lacustrine surface sediment samples of Lake Donggi Cona, Qinghai Province, China. It allows up to five grain-size end-members to be characterised and quantified in an optimal model. An end-member with a major mode in the clay domain accounts for 34% of variance within the grain-size data set. It may represent sedimentation of suspension load from linear and laminar runoff during heavy precipitation events in summer. Three end-members in the fine sand to medium silt domains make up 60% of lacustrine sedimentation. They may represent local to remote aeolian processes that peak in wintertime. A multimodal end-member explaining the remaining 6% of variance may represent further fluvial and littoral dynamics or random fluctuations and measurement errors. Several model runs of different scaling and numbers of end-members provided a suitable way to determine uncertainties inherent to the model. A comparison of 12 different model runs and their respective uncertainties yielded a distinct model of robust end-members. The clay and medium silt end-members are robust features of detrital sedimentation within Lake Donggi Cona. They alone explain 54.4% of total variance in the data. However, no spatial pattern or relation to water depth is found for any of the grain-size end-members. Thus, when past detrital sedimentation at Lake Donggi Cona is reconstructed, a special focus should be on the robust features attributed to aeolian and suspension-related sedimentation processes as well as on effective sediment mixing processes impeding a distinct correlation between grain size and spatial attributes. Further applications of the end-member modelling algorithm to other depositional environments are encouraged to demonstrate its universal applicability.

Ecological development of Lake Donggi Cona, north-eastern Tibetan Plateau, since the late glacial on basis of organic geochemical proxies and non-pollen palynomorphs

Organic geochemical proxy data from surface sediment samples and a sediment core from Lake Donggi Cona were used to infer environmental changes on the northeastern Tibetan Plateau spanning the last 18.4 kyr. Long-chain n-alkanes dominate the aliphatic hydrocarbon fraction of the sediment extract from most surface sediment samples and the sediment core. Unsaturated mid-chain n-alkanes ( nC 23:1 and nC 25:1) have high abundances in some samples, especially in core samples from the late glacial and early Holocene. TOC contents, organic biomarker and non-pollen-palynomorph concentrations and results from organic petrologic analysis on selected samples suggest three major episodes in the history of Lake Donggi Cona. Before ca. 12.6 cal ka BP samples contain low amounts of organic matter due to cold and arid conditions during the late glacial. After 12.6 cal ka BP, relatively high contents of TOC and concentrations of Botryococcus fossils, as well as enhanced concentrations of mid-chain n-alkanes and n-alkenes suggest a higher primary and macrophyte productivity than at present. This is supported by high contents of palynomorphs derived from higher plants and algae and was possibly triggered by a decrease of salinity and amelioration of climate during the early Holocene. Since 6.8 cal ka BP Lake Donggi Cona has been an oligotrophic freshwater lake. Proxy data suggest that variations in insolation drive ecological changes in the lake, with increased aquatic productivity during the early Holocene summer insolation maximum. Short-term drops of TOC contents or biomarker concentrations (at 9.9 cal ka BP, after 8.0 and between 3.5 and 1.7 cal ka BP) can possibly be related to relatively cool and dry episodes reported from other sites on the north-eastern Tibetan Plateau, which are hypothesized to occur in phase with Northern Hemisphere cooling events.