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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 C-14 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 C-14 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 C-14 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 C-14 activity of the year 1950. LRE data reported in this way represent unrealistic minimum estimates. (C) 2013 Elsevier Ltd and INQUA.
Ostracod distribution and habitat relationships in the Kunlun Mountains, northern Tibetan Plateau
(2013)
Surface sediment samples were collected from the lakes Heihai, Kusai, Haiding Nuur and Yan Hu, and from streams and ponds in the Kunlun Mountains at the northern margin of the Tibetan Plateau to investigate the sub-fossil ostracod (micro-crustacean) fauna of the region. Among 65 collected samples, 46 ostracod shell-rich samples were used to study the relationship between the ostracod distribution and specific conductivity (SC) of the water, which ranged from 0.6 to 53.0 mS cm(-1). A total of eleven ostracod species was identified from this region, with about half of the species restricted to the Tibetan Plateau and its adjacent mountain areas, and the other half representing Holarctic taxa. Tonnacypris cf. estonica and Tonnacypris tonnensis are reported from the Tibetan Plateau for the first time. Leucocythere sp. is the dominant species and Ilyocypris cf. bradyi is also relatively abundant. The other seven species were recorded with limited abundances apparently due to lower SC tolerances. Leucocythere sp. was recorded over the full SC range from 0.6 to 53 mS cm(-1). Eucypris mareotica is a typical brackish and saline water species, which was found at sample sites with high SC (2.8-53.0 mS cm(-1)). In contrast, Leucocythere dorsotuberosa, Candona candida and Eucypris afghanistanensis prefer freshwater to slightly oli-gohaline waters with SC < 1.8 mS cm(-1). The SC optimum and tolerance range for each species were determined and compared to earlier reported data from other regions of Central Asia. The results indicate that species assemblage data from fossil ostracod shells have a large potential to provide information on past SC levels and more general climate-determined moisture conditions. (C) 2013 Elsevier Ltd and INQUA. All rights reserved.
The apparent isotope enrichment factor epsilon(macrophyte) of submerged plants (epsilon(macrophyte-DIC) = delta C-13(macrophyte) - delta C-13(DIC)) is indicative of dissolved inorganic carbon (DIC) supply in neutral to alkaline waters and is related to variations in aquatic productivity (Papadimitriou et al. in Limnol Oceanogr 50:1084-1095, 2005). This paper aims to evaluate the usage of epsilon(macrophyte) inferred from isotopic analyses of submerged plant fossils in addition to analyses of lake carbonate as a palaeolimnological proxy for former HCO3 (-) concentrations. Stable carbon isotopic analysis of modern Potamogeton pectinatus leaves and its host water DIC from the Tibetan Plateau and Central Yakutia (Russia) yielded values between -23.3 and +0.4aEuro degrees and between +14.0 and +6.5aEuro degrees, respectively. Values of epsilon (Potamogeton-DIC) (range -15.4 to +1.1aEuro degrees) from these lakes are significantly correlated with host water HCO3 (-) concentration (range 78-2,200 mg/l) (r = -0.86; P < 0.001), thus allowing for the development of a transfer function. Palaeo-epsilon (Potamogeton-ostracods) values from Luanhaizi Lake on the NE Tibetan Plateau, as inferred from the stable carbon isotope measurement of fossil Potamogeton pectinatus seeds (range -24 to +2.8aEuro degrees) and ostracods (range -7.8 to +7.5%) range between -14.8 and 1.6aEuro degrees. Phases of assumed disequilibrium between delta C-13(DIC) and delta C-13(ostracods) known to occur in charophyte swards (as indicated by the deposition of charophyte fossils) were excluded from the analysis of palaeo-epsilon. The application of the epsilon (Potamogeton-DIC)-HCO3 (-) transfer function yielded a median palaeo-HCO3 (-) -concentration of 290 mg/l. Variations in the dissolved organic carbon supply compare well with aquatic plant productivity changes and lake level variability as inferred from a multiproxy study of the same record including analyses of plant macrofossils, ostracods, carbonate and organic content.
This paper aims to highlight the potential of using elemental and stable isotope analyses of aquatic macrophytes in palaeolimnological studies. Potamogeton pectinatus material was collected from modem plants (n=68) and from late glacial and Holocene-aged sediments from Koucha Lake (northeastern Tibetan Plateau; 34.0 degrees N; 97.2 degrees E; 4540 m a.s.l.). It was analyzed for delta C-13(Potamogeton) (modern: -23 to 0 parts per thousand, fossil: -19 to -4 parts per thousand) and delta N-15(Potamogeton) (modern: -11.0 to +13.8 parts per thousand, fossil: -9.5 to +6.7 parts per thousand) in addition to elemental carbon and nitrogen (modem C/N-Potamogeton: 7 to 29; fossil: 13 to 68) and sulfur (fossil: 188-899 mu mol/g dry weight). Fossil data were interpreted in terms of palaeo-nutrient availability and palaeo-productivity based on the modem relationships between various proxies and certain environmental data. Productivity of Potamogeton pectinatus mats at Koucha Lake as indicated by palaeo-epsilon(Potamogeton-TIC) (i.e. the enrichment of delta C-13(Potamogeton) relative to the delta(CTIC)-C-13) was reduced during periods of high conductivity, especially between 10.3 and 7.4 cal kyr BP. Potamogeton pectinatus material from these periods was also characterized by high S-Potamogeton indicating high sulfide concentrations and anoxic conditions within the sediments. However, C/N- Potamogeton ratios and delta N-15(Potamogeton) from the lower core section were found to have been altered by decompositional processes. A pronounced shift in the aquatic productivity of Lake Koucha occurred at similar to 7.4 cal kyr BP when the hydrological conditions shifted towards an open lake system and water depth increased. At this time a strong increase in productivity led to a strong decrease in the water HCO3- concentration as inferred from the application of a epsilon-(Potamogeton-TIC)-InHCO3- transfer function. A comparison of reconstructed productivity changes from Koucha Lake with further environmental proxies suggests that primary productivity changes are probably a function of internal lake dynamics and were only indirectly triggered by climate change.
Aim: Fossil pollen spectra from lake sediments on the Tibetan Plateau have been used for qualitative climate reconstruction, but no modern pollen-climate calibration set based on lake sediments is available to infer past climate quantitatively. This study aims to develop such a dataset and apply it to fossil data. Location: The Tibetan Plateau, between 30 and 40 degrees N and 87 and 103 degrees E. Methods: We collected surface sediments from 112 lakes and analysed them palynologically. The lakes span a wide range of mean annual precipitation (P-ann; 31-1022 mm), mean annual temperature (T-ann; -6.5 to 1 degrees C), and mean July temperature (T-July; 2.6-19.7 degrees C). Redundancy analysis showed that the modern pollen spectra are characteristic of their respective vegetation types and local climate. Transfer functions for P-ann, T-ann and T-July were developed with weighted averaging partial least squares. Model performance was assessed by leave-one-out cross-validation. Results: The root mean square errors of prediction (RMSEP) were 104 mm (P-ann), 1.18 degrees C (T-ann) and 1.17 degrees C (T-July). The RMSEPs, when expressed as percentages of the gradient sampled, were 10.6% (P-ann), 15.7% (T-ann) and 11.9% (T-July). These low values indicate the good performance of our models. An application of the models to fossil pollen spectra covering the last c. 50 kyr yielded realistic results for Luanhaizi Lake in the Qilian Mountains on the north-eastern Tibetan Plateau (modern P-ann 480 mm; T-ann-1 degrees C). T-ann and P-ann values similar to present ones were reconstructed for late Marine Isotope Stage 3, with minimum values for the Last Glacial Maximum (c. 300 mm and 2 degrees C below present), and maximum values for the early Holocene (c. 70 mm and 0.5 degrees C greater than present). Main conclusions: The modern pollen-climate calibration set will potentially be useful for quantitative climate reconstructions from lake-sediment pollen spectra from the Tibetan Plateau, an area of considerable climatic and biogeographical importance.
Quantitative information on vegetation and climate history from the late glacial-Holocene on the Tibetan Plateau is extremely rare. Here, we present palynological results of a 4.30-m-long sediment record collected from Koucha Lake in the Bayan Har Mountains, northeastern Tibetan Plateau. Vegetation change has been traced by biomisation, ordination of pollen data, and calculation of pollen ratios. The application of a pollen-climate calibration set from the eastern Tibetan Plateau to Koucha Lake pollen spectra yielded quantitative climate information. The area was covered by alpine desert/steppe, characteristic of a cold and dry climate (with 50% less precipitation than today) between 16,700 and 14,600 cal yr BP. Steppe vegetation, warm (similar to 1 degrees C higher than today) and wet conditions prevailed between 14,600 and 6600 cal yr BR These findings contradict evidence from other monsoon-influenced areas of Asia, where the early Holocene is thought to have been moist. Low effective moisture on the northeastern Tibetan Plateau was likely due to high temperature and evaporation, even though precipitation levels may have been similar to present- day values. The vegetation changed to tundra around 6600 cal yr BP, indicating that wet and cool climate conditions occurred on the northeastern Tibetan Plateau during the second half of the Holocene.
A 13.94-m-long sediment core, collected from a medium-sized lake in the Qilian Mountains (NE Tibetan Plateau, China), was analysed palynologically at 81 horizons. The interpretation of indicator taxa yielded various vertical shifts of the vegetation belts. These palaeovegetation results have been checked with lake surface pollen spectra from 8 takes representing different altitudinal vegetation belts. Our main findings are the following: A short period of the late Marine Isotope Stage 3 (around similar to 46,000 yr ago) was characterized by interglacial temperature conditions with a tree line above its present-day altitude. During the LGM, the vicinity of the lake was not covered by ice but by sparse alpine vegetation and alpine deserts, indicating that the climate was colder by similar to 4-7 degrees C than today Markedly higher temperatures were inferred from higher arboreal pollen frequencies between similar to 13,000 and similar to 7000 yr ago with a Holocene temperature optimum and a maximal Picea-Betula mixed-forest expansion between similar to 9000 and similar to 7000 yr ago, when temperatures exceeded the present-day conditions by at least 1-2 degrees C. Alpine steppes and meadows and sub-alpine shrub vegetation dominated around the lake since the middle Holocene, suggesting that vegetation and climate conditions were exceptionally stable in comparison to previous periods.