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Thermokarst lakes are assumed to develop cyclically, driven by processes that are triggered by climate and maintained by internal feedbacks that may trigger lake drainage. However, the duration of these cycles remains uncertain, as well as whether or not they affect the stabilization of lake ecosystems in permafrost regions over millennial time scales. Our research has combined investigations into modern lake-to-lake variability with a study of the long-term development of individual lakes. We have investigated the physico-chemical and diatom compositions of a set of 101 lakes with a variety of different origins in central Yakutia (Eastern Siberia), including thermokarst lakes, fluvial-erosion thermokarst lakes, fluvial-erosion lakes, and dune lakes. We found a significant relationship between lake genesis and the present-day variability in environmental and diatom characteristics, as revealed by multi-response permutation procedures, indicator species analyses, and redundancy analyses. Environmental parameters also exhibit a significant correlation with variations in the diatom data, for which they may have been to a substantial extent responsible. Mg and SO4 concentrations, together with pH and water depth, were identified as the most important parameters, influencing the variations in the diatom data almost as much as the entire environmental parameter set. We were therefore able to establish a robust Mg-diatom transfer function, which was then applied to three Holocene lake records. From these reconstructions, together with a general interpretation of the diatom record (including, e.g., the ratio between benthic/epiphytic and planktonic taxa), we have been able to infer that all three of these lakes show (1) a continuous record with no desiccation events, (2) high lake water-levels during the early Holocene, (3) centennial to millennial scale variability, and (4) high levels of variability during the early Holocene but rather stable conditions during the late Holocene (a feature that is also known from other sites around the world). We therefore concluded that the development of these three lakes was mainly driven directly by the climate, rather than by thaw lake cycling.
The first 1400-year floating varve chronology for north-eastern Germany covering the late Allered to the early Holocene has been established by microscopic varve counts from the Rehwiese palaeolake sediment record. The Laacher See Tephra (LST), at the base of the studied interval, forms the tephrochronological anchor point. The fine laminations were examined using a combination of micro-facies and mu XRF analyses and are typical of calcite varves, which in this case provide mainly a warm season signal. Two varve types with different sub-layer structures have been distinguished: (I) complex varves consisting of up to four seasonal sub-layers formed during the Allered and early Holocene periods, and, (II) simple two sub-layer type varves only occurring during the Younger Dryas. The precision of the chronology has been improved by varve-to-varve comparison of two independently analyzed sediment profiles based on well-defined micro-marker layers. This has enabled both (1) the precise location of single missing varies in one of the sediment profiles, and, (2) the verification of varve interpolation in disturbed varve intervals in the parallel core. Inter-annual and decadal-scale variability in sediment deposition processes were traced by multi-proxy data series including seasonal layer thickness, high-resolution element scans and total organic and inorganic carbon data at a five-varve resolution. These data support the idea of a two-phase Younger Dryas, with the first interval (12,675-12,275 varve years BP) characterised by a still significant but gradually decreasing warm-season calcite precipitation and a second phase (12,275-11,690 varve years BP) with only weak calcite precipitation. Detailed correlation of these two phases with the Meerfelder Maar record based on the LST isochrone and independent varve counts provides clues about regional differences and seasonal aspects of YD climate change along a transect from a location proximal to the North Atlantic in the west to a more continental site in the east
Temporally changing drivers for late-Holocene vegetation changes on the northern Tibetan Plateau
(2012)
Fossil pollen records have been widely used as indicators of past changes in vegetation and variations in climate. The driving mechanisms behind these vegetation changes have, however, remained unclear. In order to evaluate vegetation changes that have occurred in the northern part of the Tibetan Plateau and the possible drivers behind these changes, we have applied a moving-window Redundancy Analysis (RDA) to high resolution (10-15 years) pollen and sedimentary data from Lake Kusai covering the last 3770 years. Our analyses reveal frequent fluctuations in the relative abundances of alpine steppe and alpine desert components. The sedimentary proxies (including total organic carbon content, total inorganic carbon content, and "end-member" indices from grain-size analyses) that explain statistically some of the changes in the pollen assemblage vary significantly with time, most probably reflecting multiple underlying driving processes. Climate appears to have had an important influence on vegetation changes when conditions were relatively wet and stable. However, a gradual decrease in vegetation cover was identified after 1500 cal a BP, after which the vegetation appears to have been affected more by extreme events such as dust-storms or fluvial erosion than by general climatic trends. Furthermore, pollen spectra over the last 600 years are shown by Procrustes analysis to be statistically different from those recovered from older samples, which we attribute to increased human impact that resulted in unprecedented changes to the vegetation composition. Overall, changes in vegetation and climate on the northern part of the Tibetan Plateau appear to have roughly followed the evolution of the Asian Summer Monsoon. After taking into account the highly significant millennial (1512 years) periodicity revealed by time-series analysis, the regional vegetation and climate changes also show variations that appear to match variations in the mid-latitude westerlies.
A high resolution multi proxy (pollen, grain size, total organic carbon) record from a small mountain lake (Lake Khuisiin; 46.6 degrees N, 101.8 degrees E; 2270 m a.s.l.) in the south eastern Khangai Mountains of central Mongolia has been used to explore changes in vegetation and climate over the last 1200 years. The pollen data indicates that the vegetation changed from dry steppe dominated by Poaceae and Artemisia (ca AD 760-950), to Larix forest steppe (ca AD 950-1170), Larix Betula forest steppe (ca AD 1170-1380), meadow dominated by Cyperaceae and Poaceae (ca AD 1380-1830), and Larix Betula forest steppe (after similar to AD 1830). The cold-wet period between AD 1380 and 1830 may relate to the Little Ice Age. Environmental changes were generally subtle and climate change seems to have been the major driver of variations in vegetation until at least the early part of the 20th century, suggesting that either the level of human activity was generally low, or the relationship between human activity and vegetation did not alter substantially between AD 760 and 1830. A review of centennial scale moisture records from China and Mongolia revealed that most areas experienced major changes at ca AD 1500 and AD 1900. However, the moisture availability since AD 1500 varied between sites, with no clear regional pattern or relationship to present day conditions. Both the reconstructions and the moisture levels simulation on a millennium scale performed in the MPI Earth System Model indicate that the monsoon-westerlies transition area shows a greater climate variability than those areas influenced by the westerlies, or by the summer monsoon only.
A general mean annual temperature increase accompanied with substantial glacial retreat has been noted on the Tibetan Plateau during the last two centuries but most significantly since the mid 1950s. These climate trends are particularly apparent on the southeastern Tibetan Plateau. However, the Tibetan Plateau (due to its heterogeneous mountain landscape) has very complex and spatially differing temperature and precipitations patterns. As a result, intensive palaeolimnological investigations are necessary to decipher these climatic patterns and to understand ecological responses to recent environmental change. Here we present palaeolimnological results from a (210)Pb/(137)Cs-dated sediment core spanning approximately the last 200 years from a remote high-mountain lake (LC6 Lake, working name) on the southeastern Tibetan Plateau. Sediment profiles of diatoms, organic variables (TOC, C:N) and grain size were investigated. The (210)Pb record suggests a period of rapid sedimentation, which might be linked to major tectonic events in the region ca. 1950. Furthermore, unusually high (210)Pb supply rates over the last 50 years suggest that the lake has possibly been subjected to increasing precipitation rates, sediment focussing and/or increased spring thaw. The majority of diatom taxa encountered in the core are typical of slightly acidic to circumneutral, oligotrophic, electrolyte-poor lakes. Diatom species assemblages were rich, and dominated by Cyclotella sp., Achnanthes sp., Aulacoseira sp. and fragilarioid taxa. Diatom compositional change was minimal over the 200-year period (DCCA = 0.85 SD, p = 0.59); only a slightly more diverse but unstable diatom assemblage was recorded during the past 50 years. The results indicate that large-scale environmental changes recorded in the twentieth century (i.e. increased precipitation and temperatures) are likely having an affect on the LC6 Lake, but so far these impacts are more apparent on the lake geochemistry than on the diatom flora. Local and/or regional peculiarities, such as increasing precipitation and cloud cover, or localized climatic phenomena, such as negative climate feedbacks, might have offset the effects of increasing mean surface temperatures.
Inorganic minerals form a major component of lacustrine sediments and have the potential to reveal detailed information on previous climatic and hydrological conditions. The ability to extract such information however, has been restricted by a limited understanding of the relationships between minerals and the environment. In an attempt to fill in this gap in our knowledge, 146 surface sediment samples have been investigated from 146 lakes on the Tibetan Plateau. The mineral compositions derived from these samples by X-Ray Diffraction (XRD) were used to examine the relationships between mineral compositions and the environmental variables determined for each site. Statistical techniques including Multivariate regression trees (MRT) and Redundancy Analysis (RDA), based on the mineral spectra and environmental variables, reveal that the electrical conductivity (EC) and Mg/Ca ratios of lake water are the most important controls on the composition of endogenic minerals. No endogenic minerals precipitate under hyper-fresh water conditions (EC lower than 0.13 mS/cm), with calcite commonly forming in water with EC values above 0.13 mS/cm. Between EC values of 0.13 and 26 mS/cm the mineral composition of lake sediments can be explained in terms of variations in the Mg/Ca ratio: calcite dominates at Mg/Ca ratios of less than 33, whereas aragonite commonly forms when the ratio is greater than 33. Where EC values are between 26 and 39 mS/cm, monohydrocalcite precipitates together with calcite and aragonite; above 39 mS/cm, gypsum and halite commonly form. Information on the local geological strata indicates that allogenic (detrital) mineral compositions are primarily influenced by the bedrock compositions within the catchment area. By applying these relationships to the late glacial and Holocene mineral record from Chaka Salt Lake, five lake stages have been identified and their associated EC conditions inferred. The lake evolved from a freshwater lake during the late glacial (before 11.4 cal. ka BP) represented by the lowest EC values (< 0.13 mS/cm), to a saline lake with EC values slightly higher than 39 mS/cm during the early and mid Holocene (ca. 11.4-5.3 cal. ka BP), and finally to a salt lake (after 5.3 cal. ka BP). These results illustrate the utility of our mineral-environmental model for the quantitative reconstruction of past environmental conditions from lake sediment records.
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.
A sediment core from a closed basin lake (Lake Kuhai) from the semi-arid northeastern Tibetan Plateau was analysed for its pollen record to infer Lateglacial and post glacial vegetation and climatic change. At Lake Kuhai five major vegetation and climate shifts could be identified: (1) a change from cold and dry to relatively warmer and more moist conditions at 14.8 cal ka BP: (2) a shift to conditions of higher effective moisture and a stepwise warmer climate at 13.6 cal ka BP; (3) a further shift with increased moisture but colder conditions at 7.0 cal ka BP; (4) a return to a significantly colder and drier phase at 6.3 cal ka BP; (5) and a change back to relatively moist conditions at 2.2 cal ka BP. To investigate the response of lake ecosystems to climatic changes, statistical comparisons were made between the lake Kuhai pollen record and a formerly published ostracod and sedimentary record from the same sediment core. Furthermore, the pollen and lacustrine proxies from lake Kuhai were compared to a previously published pollen and lacustrine record from the nearby Lake Koucha. Statistical comparisons were done using non-metric multidimensional scaling and Procrustes rotation. Differences between lacustrine and pollen responses within one site could be identified, suggesting that lacustrine proxies are partly influenced by in-lake or local catchment processes, whereas the terrestrial (pollen) proxy shows a regional climate signal. Furthermore, we found regional differences in proxy response between lake Kuhai and Lake Koucha. Both pollen records reacted in similar ways to major environmental changes, with minor differences in the timing and magnitude of these changes. The lacustrine records were very similar in their timing and magnitude of response to environmental changes; however, the nature of change was at times very distinct. To place the current study in the context of Holocene moisture evolution across the Tibetan Plateau, we applied a five-scale moisture index and average link clustering to all available continuous palaeo-climate records from the Tibetan Plateau to possibly find general patterns of moisture evolution on the Plateau. However, no common regional pattern of moisture evolution during the Holocene could be detected. We assign this to complex responses of different proxies to environmental and atmospheric changes in an already very heterogeneous mountain landscape where minor differences in elevation can cause strong variation in microenvironments.
Diatom diversity in lakes of northwest Yakutia (Siberia) was investigated by microscopic and genetic analysis of surface and cored lake sediments, to evaluate the use of sedimentary DNA for paleolimnological diatom studies and to identify obscure genetic diversity that cannot be detected by microscopic methods. Two short (76 and 73 bp) and one longer (577 bp) fragments of the ribulose 1,5-bisphosphate carboxylase/oxygenase (rbcL) gene, encoding the large subunit of the rbcL, were used as genetic markers. Diverse morphological assemblages of diatoms, dominated by small benthic fragilarioid taxa, were retrieved from the sediments of each lake. These minute fragilarioid taxa were examined by scanning electron microscopy, revealing diverse morphotypes in Staurosira and Staurosirella from the different lakes. Genetic analyses indicated a dominance of haplotypes that were assigned to fragilarioid taxa and less genetic diversity in other diatom taxa. The long rbcL_577 amplicon identified considerable diversification among haplotypes clustering within the Staurosira/Staurosirella genera, revealing 19 different haplotypes whose spatial distribution appears to be primarily related to the latitude of the lakes, which corresponds to a vegetation and climate gradient. Our rbcL markers are valuable tools for tracking differences between diatom lineages that are not visible in their morphologies. These markers revealed putatively high genetic diversity within the Staurosira/Staurosirella species complex, at a finer scale than is possible to resolve by microscopic determination. The rbcL markers may provide additional reliable information on the diversity of barely distinguishable minute benthic fragilarioids. Environmental sequencing may thus allow the tracking of spatial and temporal diversification in Siberian lakes, especially in the context of diatom responses to recent environmental changes, which remains a matter of controversy.
A modern pollen dataset from China and Mongolia (18-52 degrees N, 74-132 degrees E) is investigated for its potential use in climate reconstructions. The dataset includes 2559 samples, 229 terrestrial pollen taxa and four climatic variables - mean annual precipitation (P-ann): 35-2091 mm, mean annual temperature (T-ann): -12.1-25.8 degrees C, mean temperature in the coldest month (Mt(co).): -33.8-21.7 degrees C, and mean temperature in the warmest month (Mt(wa)): 03-29.8 degrees C. Modern pollen-climate relationships are assessed using canonical correspondence analysis (CCA), Huisman-Olff-Fresco (HOF) models, the modern analogue technique (MAT), and weighted averaging partial least squares (WA-PLS). Results indicate that P-ann is the most important climatic determinant of pollen distribution and the most promising climate variable for reconstructions, as assessed by the coefficient of determination between observed and predicted environmental values (r(2)) and root mean square error of prediction (RMSEP). Mt(co) and Mt(wa) may be reconstructed too, but with caution. Samples from different depositional environments influence the performance of cross-validation differently, with samples from lake sediment-surfaces and moss polsters having the best fit with the lowest RMSEP. The better model performances of MAT are most probably caused by spatial autocorrelation. Accordingly, the WA-PLS models of this dataset are deemed most suitable for reconstructing past climate quantitatively because of their more reliable predictive power. (C) 2014 Elsevier B.V. All rights reserved.