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Radiocarbon and optically stimulated luminescence dating of sediments from Lake Karakul, Tajikistan
(2017)
Lake Karakul in the eastern Pamirs is a large and closed-basin lake in a partly glaciated catchment. Two parallel sediment cores were collected from 12 m water depth. The cores were correlated using XRF analysis and dated using radiocarbon and OSL techniques. The age results of the two dating methods are generally in agreement. The correlated composite core of 12.26 m length represents continuous accumulation of sediments in the lake basin since 31 ka. The lake reservoir effect (LRE) remained relatively constant over this period. High sediment accumulation rates (SedARs) were recorded before 23 ka and after 6.5 ka. The relatively close position of the coring location near the eastern shore of the lake implies that high SedARs resulted from low lake levels. Thus, high SedARs and lower lake levels before 23 ka probably reflect cold and dry climate conditions that inhibited the arrival of moist air at high elevation in the eastern Pamirs. Low lake levels after 6.5 ka were probably caused by declining temperatures after the warmer early Holocene, which had caused a reduction in water resources stored as snow, ice and frozen ground in the catchment. Low SedARs during 23-6.5 ka suggest increased lake levels in Lake Karakul. A short-lived increase of SedARs at 15 ka probably corresponds to the rapid melting of glaciers in the Karakul catchment during the Greenland Interstadial le, shortly after glaciers in the catchment had reached their maximum extents. The sediment cores from Lake Karakul represent an important climate archive with robust chronology for the last glacial interglacial cycle from Central Asia. (C) 2017 Elsevier B.V. All rights reserved.
Ostracod shells in surface sediments from Ulungur Lake (Xinjiang, China) belong mainly to Limnocythere inopinata as the dominant species, and Candona neglecta and Darwinula stevensoni as accompanying, less abundant taxa. Shells of an additional nine species were recorded only sporadically. The three most abundant ostracods have wide tolerance ranges in terms of salinity, substrate and water depth. The similarly recorded bivalve Pisidium subtruncatum, and the gastropods Gyraulus chinensis and Radix auricularia belong to the most tolerant representatives of the genera. The bivalve and gastropods, in addition to the ostracod assemblage, reflect the fact that Ulungur Lake has experienced strong lake level and salinity variations due to water withdrawal in the catchment and the counteracting diversion of river waters to the lake in recent decades. The substrate in Ulungur Lake is typically fine-grained, apart from the delta region of the Ulungur River channel, which is marked by relatively coarse-grained detrital sediments barren of ostracod shells. This channel was created 40 years ago to divert water to Ulungur Lake and support its local fisheries and recreational facilities. A reassessed Holocene ostracod record from the lake shows that a significantly higher salinity and lower lake level existed in the early Holocene before 6.0 ka in response to the regional climate. In contrast, a higher lake level and lowest salinity is inferred for the late Holocene period between ca. 3.6 and 1.3 ka before present. Afterwards, the lake level declined and salinity increased in response to regional moisture reduction, although conditions similar to the early Holocene lake status were not re-established. Our surface-sediment-derived data provide a baseline for analysis of future environmental variations due to global climate change and regional water management.
We present the results of our investigations on the radiocarbon dated core sediments from the Lake Tso Moriri, NW Himalaya aimed at reconstructing palaeohydrological changes in this climatically sensitive region. Based on the detailed geochemical, mineralogical and sedimentological analysis, we recognise several short-term fluctuations superimposed upon seven major palaeohydrological stages identified in this lake since similar to 26 cal ka. Stage I (>20.2 cal ka): shallow lake characterised by input of coarse-grained detrital sediments; Stage II (20.2-16.4 cal ka): lake deepening and intensification of this trend ca. 18 cal ka; Stage III (16.4-11.2 cal ka): rising lake levels with a short term wet phase (13.1-11.7 cal ka); Stage IV (11.2-8.5 cal ka): early Holocene hydrological maxima and highest lake levels inferred to have resulted from early Holocene Indian monsoon intensification, as records from central Asia indicate weaker westerlies during this interval; Stage V (8.5-5.5 cal ka): mid-Holocene climate deterioration; Stage VI (5.5-2.7 cal ka): progressive lowering of lake level; Stage VII (2.7-0 cal ka): onset of modern conditions. The reconstructed hydrological variability in Lake Tso Moriri is governed by temperature changes (meltwater inflow) and monsoon precipitation (increased runoff). A regional comparison shows considerable differences with other palaeorecords from peninsular India during late Holocene. (C) 2014 Elsevier Ltd and INQUA. All rights reserved.
High resolution isotopic (delta O-18 and delta C-13) investigations on endogenic carbonates (calcite/aragonite) from Tso Moriri Lake, NW Himalaya show dramatic fluctuations during the late glacial and the early Holocene, and a persistent enrichment trend during the late Holocene. Changes in this lake are largely governed by the [input (meltwater + monsoon precipitation)/evaporationj (WE) ratio, also reflected in changes in the carbonate mineralogy with aragonite being formed during periods of lowest I/E. Using new isotopic data on endogenic carbonates in combination with the available data on geochemistry, mineralogy, and reconstructed mean annual precipitation, we demonstrate that the late glacial and early Holocene carbonate delta O-18 variability resulted from fluctuating Indian summer monsoon (ISM) precipitation in NW Himalaya. This region experienced increasing ISM precipitation between ca. 13.1 and 11.7 cal ka and highest ISM precipitation during the early Holocene (11.2-8.5 cal ka). However, during the late Holocene, evaporation was the dominant control on the carbonate delta O-18. Regional comparison of reconstructed hydrological changes from Tso Moriri Lake with other archives from the Asian summer monsoon and westerlies domain shows that the intensified westerly influence that resulted in higher lake levels (after 8 cal ka) in central Asia was not strongly felt in NW Himalaya. (C) 2015 Elsevier B.V. All rights reserved.
This study presents a reconstruction of the Late Holocene climate in Kamchatka based on chironomid remains from a 332 cm long composite sediment core recovered from Dvuyurtochnoe Lake (Two-Yurts Lake, TYL) in central Kamchatka. The oldest recovered sediments date to about 4500 cal years BP. Chironomid head capsules from TYL reflect a rich and diverse fauna. An unknown morphotype of Tanytarsini, Tanytarsus type klein, was found in the lake sediments. Our analysis reveals four chironomid assemblage zones reflecting four different climatic periods in the Late Holocene. Between 4500 and 4000 cal years BP, the chironomid composition indicates a high lake level, well-oxygenated lake water conditions and close to modern temperatures (similar to 13 degrees C). From 4000 to 1000 cal years BP, two consecutive warm intervals were recorded, with the highest reconstructed temperature reaching 16.8 degrees C between 3700 and 2800 cal years BP. Cooling trend, started around 1100 cal years BP led to low temperatures during the last stage of the Holocene. Comparison with other regional studies has shown that termination of cooling at the beginning of late Holocene is relatively synchronous in central Kamchatka, South Kurile, Bering and Japanese Islands and take place around 3700 cal years BP. From ca 3700 cal years BP to the last millennium, a newly strengthened climate continentality accompanied by general warming trend with minor cool excursions led to apparent spatial heterogeneity of climatic patterns in the region. Some timing differences in climatic changes reconstructed from chironomid record of TYL sediments and late Holocene events reconstructed from other sites and other proxies might be linked to differences in local forcing mechanisms or caused by the different degree of dating precision, the different temporal resolution, and the different sensitive responses of climate proxies to the climate variations. Further high-resolution stratigraphic studies in this region are needed to understand the spatially complex pattern of climate change in Holocene in Kamchatka and the surrounding region.
Holocene climate variability is generally characterized by low frequency changes than compared to the last glaciations including the Lateglacial. However, there is vast evidence for decadal to centennial scale oscillations and millennial scale climate trends, which are within and beyond a human lifetime perception, respectively. Within the Baltic realm, a transitional zone between oceanic and continental climate influence, the impact of Holocene and Lateglacial climate and environmental change is currently partly understood. This is mainly attributed to the scarcity of well-dated and high-resolution sediment records and to the lacking continuity of already investigated archives.
The aim of this doctoral thesis is to reconstruct Holocene and Late Glacial climate variability on local to (over)regional scales based on varved (annually laminated) sediments from Lake Czechowskie down to annual resolution. This project was carried out within the Virtual Institute for Integrated Climate and Landscape Evolution Analyses (ICLEA) and funded by the Helmholtz Association and the Helmholtz Climate Initiative REKLIM (Regional Climate Change). ICLEA intended to gain a better understanding of climate variability and landscape evolution processes in the Northern Central European lowlands since the last deglaciation. REKLIM Topic 8 “Abrupt climate change derived from proxy data” aims at identifying spatiotemporal patterns of climate variability between e.g. higher and lower latitudes. The main aim of this thesis was (i) to establish a robust chronology based on a multiple dating approach for Lake Czechowskie covering the Late Glacial and Holocene and for the Trzechowskie palaeolake for the Lateglacial, respectively, (ii) to reconstruct past climatic and environmental conditions on centennial to multi-millennial time scales and (iii) to distinguish between local to regional different sediments responses to climate change.
Addressing the first aim, the Lake Czechowskie chronology has been established by a multiple dating approach comprising information from varve counting, tephrochronology, AMS 14C dating of terrestrial plant remains, biostratigraphy and 137Cs activity concentration measurements. Those independent age constraints covering the Lateglacial and the entire Holocene and have been further implemented in a Bayesian age model by using OxCal v.4.2. Thus, even within non-varved sediment intervals, robust chronological information has been used for absolute age determination. The identification of five cryptotephras, of which three are used as unambiguous isochrones, is furthermore a significant improvement of the Czechowskie chronology and currently unique for the Holocene within Poland. The first findings of coexisting early Holocene Hässeldalen and Askja-S cryptotephras within a varved sequence even allowed differential dating between both volcanic ashes and stimulated the discussion of revising the absolute ages of the Askja-S tephra.
The Trzechowskie palaeolake chronology has been established by a multiple dating approach comprising varve counting, tephrochronology, AMS 14C dating of terrestrial plant remains and biostratigraphy, covers the Lateglacial period (Allerød and Younger Dryas) and has been implemented in OxCal v.4.2. Those age constraints allowed regional correlation to other high-resolution climate archives and identifying leads and lags of proxy responses at the onset of the Younger Dryas.
The second aim has been accomplished by detailed micro-facies and geochemical analyses of the Czechowskie sediments for the entire Holocene. Thus, especially micro-facies changes had been linked to enhanced productivity at Lake Czechowskie. Most prominent changes have been recorded at 7.3, 6.5, 4.3 and 2.8 varve kyrs BP and are linked to a stepwise increasing influence of Atlantic air masses. Especially, the mid-Holocene change, which had been widely reported from palaeohydrological records in low latitudes, has been identified and linked to large scale reorganization of atmospheric circulation patterns. Thus, especially long-term changes of climatic and environmental boundary conditions are widely recorded by the Czechowskie sediments. The pronounced response to (multi)millennial scale changes is further corroborated by the lack of clear sediment responses to early Holocene centennial scale climate oscillations (e.g. the Preboreal Oscillation).
However, decadal scale changes at Lake Czechowskie during the most recent period (last 140 years) have been investigated in a lake comparison study. To fulfill the third aim of the doctoral thesis, three lakes in close vicinity to each other have been investigated in order to better distinguish how local, site-specific parameters, may superimpose regional climate driven changes. All lakes haven been unambiguously linked by the Askja AD1875 cryptotephra and independent varve chronologies. As a result, climate warming has only been recorded by sedimentation changes at the smallest and best sheltered lake (Głęboczek), whereas the largest lake (Czechowskie) and the shallowest lake (Jelonek) showed attenuated and less clear sediment responses, respectively. The different responses have been linked to morphological lake characteristics (lake size and depth, catchment area). This study highlights the potential of high-resolution lake comparison for robust proxy based climate reconstructions.
In summary, the doctoral thesis presents a high-resolution sediment record with an underlying age model, which is prerequisite for unprecedented age control down to annual resolution. Sediment proxy based climate reconstructions demonstrate the importance of the Czechowskie sediments for better understanding climate variability in the southern Baltic realm. Case studies showed the clear response on millennial time scale, while decadal scale fluctuations are either less well expressed or superimposed by local, site-specific parameters. The identification of volcanic ash layers is not only used for unambiguous isochrones, those are key tie lines for local to supra regional archive synchronization and establish the Lake Czechowskie as a key climate archive.
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.
Concerns about the regional impact of global climate change in a warming scenario have highlighted the gaps in our understanding of the Indian Summer Monsoon (ISM, also referred to as the Indian Ocean summer monsoon) and the absence of long term palaeoclimate data from the central Indian core monsoon zone (CMZ). Here we present the first high resolution, well-dated, multiproxy reconstruction of Holocene palaeoclimate from a 10 m long sediment core raised from the Lonar Lake in central India. We show that while the early Holocene onset of-intensified monsoon in the CMZ is similar to that reported from other ISM records, the Lonar data shows two prolonged droughts (PD, multidecadal to centennial periods of weaker monsoon) between 4.6-3.9 and 2-0.6 cal ka. A comparison of our record with available data from other ISM influenced sites shows that the impact of these PD was observed in varying degrees throughout the ISM realm and coincides with intervals of higher solar irradiance. We demonstrate that (i) the regional warming in the Indo-Pacific Warm Pool (IPWP) plays an important role in causing ISM PD through changes in meridional overturning circulation and position of the anomalous Walker cell; (ii) the long term influence of conditions like El Nino-Southern Oscillation (ENSO) on the ISM began only ca. 2 cal ka BP and is coincident with the warming of the southern IPWP; (iii) the first settlements in central India coincided with the onset of the first PD and agricultural populations flourished between the two PD, highlighting the significance of natural climate variability and PD as major environmental factors affecting human settlements.
One of the most significant Late Holocene climate shifts occurred around 2800 years ago, when cooler and wetter climate conditions established in western Europe. This shift coincided with an abrupt change in regional atmospheric circulation between 2760 and 2560 cal years BP, which has been linked to a grand solar minimum with the same duration (the Homeric Minimum). We investigated the temporal sequence of hydroclimatic and vegetation changes across this interval of climatic change (Homeric climate oscillation) by using lipid biomarker stable hydrogen isotope ratios (ED values) and pollen assemblages from the annually-laminated sediment record from lake Meerfelder Maar (Germany). Over the investigated interval (3200-2000 varve years BP), terrestrial lipid biomarker ED showed a gradual trend to more negative values, consistent with the western Europe long-term climate trend of the Late Holocene. At ca. 2640 varve years BP we identified a strong increase in aquatic plants and algal remains, indicating a rapid change in the aquatic ecosystem superimposed on this long-term trend. Interestingly, this aquatic ecosystem change was accompanied by large changes in ED values of aquatic lipid biomarkers, such as nC(21) and nC(23) (by between 22 and 30%(0)). As these variations cannot solely be explained by hydroclimate changes, we suggest that these changes in the Wag value were influenced by changes in n-alkane source organisms. Our results illustrate that if ubiquitous aquatic lipid biomarkers are derived from a limited pool of organisms, changes in lake ecology can be a driving factor for variations on sedimentary lipid MN values, which then could be easily misinterpreted in terms of hydro climatic changes. (C) 2017 Elsevier Ltd. All rights reserved.
Holocene temperature proxy records are commonly used in quantitative synthesis and model-data comparisons. However, comparing correlations between time series from records collected in proximity to one another with the expected correlations based on climate model simulations indicates either regional or noisy climate signals in Holocene temperature proxy records. In this study, we evaluate the consistency of spatial correlations present in Holocene proxy records with those found in data from the Last Glacial Maximum (LGM). Specifically, we predict correlations expected in LGM proxy records if the only difference to Holocene correlations would be due to more time uncertainty and more climate variability in the LGM. We compare this simple prediction to the actual correlation structure in the LGM proxy records. We found that time series data of ice-core stable isotope records and planktonic foraminifera Mg/Ca ratios were consistent between the Holocene and LGM periods, while time series of Uk'37 proxy records were not as we found no correlation between nearby LGM records. Our results support the finding of highly regional or noisy marine proxy records in the compilation analysed here and suggest the need for further studies on the role of climate proxies and the processes of climate signal recording and preservation.
Holocene temperature proxy records are commonly used in quantitative synthesis and model-data comparisons. However, comparing correlations between time series from records collected in proximity to one another with the expected correlations based on climate model simulations indicates either regional or noisy climate signals in Holocene temperature proxy records. In this study, we evaluate the consistency of spatial correlations present in Holocene proxy records with those found in data from the Last Glacial Maximum (LGM). Specifically, we predict correlations expected in LGM proxy records if the only difference to Holocene correlations would be due to more time uncertainty and more climate variability in the LGM. We compare this simple prediction to the actual correlation structure in the LGM proxy records. We found that time series data of ice-core stable isotope records and planktonic foraminifera Mg/Ca ratios were consistent between the Holocene and LGM periods, while time series of Uk'37 proxy records were not as we found no correlation between nearby LGM records. Our results support the finding of highly regional or noisy marine proxy records in the compilation analysed here and suggest the need for further studies on the role of climate proxies and the processes of climate signal recording and preservation.
A better understanding of past variations of the Indian Summer Monsoon (ISM), that plays a vital role for the still largely agro-based economy in India, can lead to a better assessment of its potential impact under global climate change scenarios. However, our knowledge of spatiotemporal patterns of ISM strength is limited due to the lack of high-resolution, continental paleohydrological records. Here, we reconstruct centennial-scale hydrological variability during the Holocene associated to changes in the intensity of the ISM based on a record of lipid biomarker abundances and compound-specific stable isotopic composition of a 10 m long sediment core from saline alkaline Lonar Lake, situated in the core 'monsoon zone' of central India.
We identified three main periods of distinct hydrology during the Holocene in central India. The period between 10.1 and 6 cal ka BP was likely the wettest during the Holocene. Lower average chain length (ACL) index values (29.4-28.6) and negative delta C-13(wax) values (-34.8 parts per thousand to -27.8 parts per thousand) of leaf wax n-alkanes indicate the dominance of woody C-3 vegetation in the catchment, and negative delta D-wax values (concentration weighted average) (-171 parts per thousand to -147 parts per thousand) suggest a wet period due to an intensified monsoon. After 6 cal ka BP, a gradual shift to less negative delta C-13(wax) values (particularly for the grass derived n-C-31) and appearance of the triterpene lipid tetrahymanol, generally considered as a marker for salinity and water column stratification, mark the onset of drier conditions. At 5.1 cal ka BP an increasing flux of leaf wax n-alkanes along with the highest flux of tetrahymanol indicate a major lowering of the lake level. Between 4.8 and 4 cal ka BP, we find evidence for a transition to arid conditions, indicated by high and strongly variable tetrahymanol flux. In addition, a pronounced shift to less negative delta C-13(wax) values, in particular for n-C-31 (-25.2 parts per thousand to -22.8 parts per thousand), during this period indicates a change of dominant vegetation to C-4 grasses. In agreement with other proxy data, such as deposition of evaporite minerals, we interpret this period to reflect the driest conditions in the region during the last 10.1 ka. This transition led to protracted late Holocene arid conditions after 4 ka with the presence of a permanent saline lake, supported by the sustained presence of tetrahymanol and more positive average delta D-wax values (-122 parts per thousand to -141 parts per thousand). A late Holocene peak of cyanobacterial biomarker input at 1.3 cal ka BP might represent an event of lake eutrophication, possibly due to human impact and the onset of cattle/livestock farming in the catchment.
A unique feature of our record is the presence of a distinct transitional period between 4.8 and 4 cal ka BP, which was characterized by some of the most negative delta D-wax values during the Holocene (up to -180 parts per thousand), when all other proxy data indicate the driest conditions during the Holocene. These negative delta D-wax values can as such most reasonably be explained by a shift in moisture source area and/or pathways or rainfall seasonality during this transitional period. We hypothesize that orbital induced weakening of the summer solar insolation and associated reorganization of the general atmospheric circulation, as a possible southward displacement of the tropical rainbelt, led to an unstable hydroclimate in central India between 4.8 and 4 ka.
It is widely accepted that climate has a strong impact and exerts important feedbacks on erosional processes and sediment transport mechanisms. However, the extent at which climate influences erosion is still a matter of debate. In this paper we test whether frost-cracking processes and related temperature variations can influence the sediment production and surface erosion in a small catchment situated in the eastern Italian Alps. To this extent, we first present a geomorphic map of the region that we complement with published Be-10-based denudation rates. We then apply a preexisting heat-flow model in order to analyze the variations of the frost-cracking intensity (FCI) in the study area, which could have controlled the sediment production in the basin. Finally, we compare the model results with the pattern of denudation rates and Quaternary deposits in the geomorphic map. The model results, combined with field observations, mapping, and quantitative geomorphic analyses, reveal that frost-cracking processes have had a primary role in the production of sediment where the intensity of sediment supply has been dictated and limited by the combined effect of temperature variations and conditions of bedrock preservation. These results highlight the importance of a yet poorly understood process for the production of sediment in mountain areas. (C) 2015 Elsevier B.V. All rights reserved.
A sediment core from Khatanga-12 Lake (Taimyr Peninsula, Krasnoyarsk krai) has been studied. The 131.5-cm-long core covers ca. 7100 years of sedimentation. Chironomid analysis, a qualitative reconstruction of the paleoenvironment in the region, and a quantitative reconstruction of variations of the mean July air temperature and in the water depth of the lake have been performed using Northern Russia chironomid-inferred mean July temperature models (Nazarova et al., 2008, 2011, 2015). Khatanga-12 Lake was formed during the Middle Holocene warming as a result of thermokarst processes. The development of the lake ecosystem at different stages of its development was influenced by climatic and cryolithogenic factors. The Middle Holocene warming, which occurred around 7100-6250 cal. years BP, activated thermokarst processes and resulted in the formation of the lake basin. Later, between 6250 and 4500 cal. years BP, a period of cooling took place, as is proved by chironomid analysis. The bottom sediments of the lake during this period were formed by erosion processes on the lake shores. The reconstructed conditions were close to the modern after 2500 cal. years BP.
Temporal and spatial stability of the vegetation climate relationship is a basic ecological assumption for pollen-based quantitative inferences of past climate change and for predicting future vegetation. We explore this assumption for the Holocene in eastern continental Asia (China, Mongolia). Boosted regression trees (BRT) between fossil pollen taxa percentages (Abies, Artemisia, Betula, Chenopodiaceae, Cyperaceae, Ephedra, Picea, Pinus, Poaceae and Quercus) and climate model outputs of mean annual precipitation (P-ann) and mean temperature of the warmest month (Mt(wa)) for 9 and 6 ka (ka = thousand years before present) were set up and results compared to those obtained from relating modern pollen to modern climate. Overall, our results reveal only slight temporal differences in the pollen climate relationships. Our analyses suggest that the importance of P-ann compared with Mt(wa) for taxa distribution is higher today than it was at 6 ka and 9 ka. In particular, the relevance of P-ann for Picea and Pinus increases and has become the main determinant. This change in the climate tree pollen relationship parallels a widespread tree pollen decrease in north-central China and the eastern Tibetan Plateau. We assume that this is at least partly related to vegetation climate disequilibrium originating from human impact. Increased atmospheric CO2 concentration may have permitted the expansion of moisture-loving herb taxa (Cyperaceae and Poaceae) during the late Holocene into arid/semi-arid areas. We furthermore find that the pollen climate relationship between north-central China and the eastern Tibetan Plateau is generally similar, but that regional differences are larger than temporal differences. In summary, vegetation climate relationships in China are generally stable in space and time, and pollen-based climate reconstructions can be applied to the Holocene. Regional differences imply the calibration-set should be restricted spatially.
The Chew Bahir Drilling Project (CBDP) aims to test possible linkages between climate and evolution in Africa through the analysis of sediment cores that have recorded environmental changes in the Chew Bahir basin. In this statistical project we consider the Chew Bahir palaeolake to be a dynamical system consisting of interactions between its different components, such as the waterbody, the sediment beneath lake, and the organisms living within and around the lake. Recurrence is a common feature of such dynamical systems, with recurring patterns in the state of the system reflecting typical influences. Identifying and defining these influences contributes significantly to our understanding of the dynamics of the system. Different recurring changes in precipitation, evaporation, and wind speed in the Chew Bahir basin could result in similar (but not identical) conditions in the lake (e.g., depth and area of the lake, alkalinity and salinity of the lake water, species assemblages in the water body, and diagenesis in the sediments). Recurrence plots (RPs) are graphic displays of such recurring states within a system. Measures of complexity were subsequently introduced to complement the visual inspection of recurrence plots, and provide quantitative descriptions for use in recurrence quantification analysis (RQA). We present and discuss herein results from an RQA on the environmental record from six short (< 17 m) sediment cores collected during the CBDP, spanning the last 45 kyrs. The different types of variability and transitions in these records were classified to improve our understanding of the response of the biosphere to climate change, and especially the response of humans in the area.
Abrupt or gradual?
(2018)
We used a change point analysis on a late Pleistocene-Holocene lake-sediment record from the Chew Bahir basin in the southern Ethiopian Rift to determine the amplitude and duration of past climate transitions. The most dramatic changes occurred over 240 yr (from similar to 15,700 to 15,460 yr) during the onset of the African Humid Period (AHP), and over 990 yr (from similar to 4875 to 3885 yr) during its protracted termination. The AHP was interrupted by a distinct dry period coinciding with the high-latitude Younger Dryas stadial, which had an abrupt onset (less than similar to 100 yr) at similar to 13,260 yr and lasted until similar to 11,730 yr. Wet-dry-wet transitions prior to the AHP may reflect the high-latitude Dansgaard-Oeschger cycles, as indicated by cross-correlation of the potassium record with the NorthGRIP ice core record between similar to 45-20 ka. These findings may contribute to the debates regarding the amplitude, and duration and mechanisms of past climate transitions, and their possible influence on the development of early modern human cultures.
The past climate in central Asia, and especially on the Tibetan Plateau (TP), is of great importance for an understanding of global climate processes and for predicting the future climate. As a major influence on the climate in this region, the Asian Summer Monsoon (ASM) and its evolutionary history are of vital importance for accurate predictions. However, neither the evolutionary pattern of the summer monsoon nor the driving mechanisms behind it are yet clearly understood. For this research, I first synthesized previously published Late Glacial to Holocene climatic records from monsoonal central Asia in order to extract the general climate signals and the associated summer monsoon intensities. New climate and vegetation sequences were then established using improved quantitative methods, focusing on fossil pollen records recovered from Tibetan lakes and also incorporating new modern datasets. The pollen-vegetation and vegetation-climate relationships on the TP were also evaluated in order to achieve a better understanding of fossil pollen records. The synthesis of previously published moisture-related palaeoclimate records in monsoonal central Asia revealed generally different temporal patterns for the two monsoonal subsystems, i.e. the Indian Summer Monsoon (ISM) and East Asian Summer Monsoon (EASM). The ISM appears to have experienced maximum wet conditions during the early Holocene, while many records from the area affected by the EASM indicate relatively dry conditions at that time, particularly in north-central China where the maximum moisture levels occurred during the middle Holocene. A detailed consideration of possible driving factors affecting the summer monsoon, including summer solar insolation and sea surface temperatures, revealed that the ISM was primarily driven by variations in northern hemisphere solar insolation, and that the EASM may have been constrained by the ISM resulting in asynchronous patterns of evolution for these two subsystems. This hypothesis is further supported by modern monsoon indices estimated using the NCEP/NCAR Reanalysis data from the last 50 years, which indicate a significant negative correlation between the two summer monsoon subsystems. By analogy with the early Holocene, intensification of the ISM during coming decades could lead to increased aridification elsewhere as a result of the asynchronous nature of the monsoon subsystems, as can already be observed in the meteorological data from the last 15 years. A quantitative climate reconstruction using fossil pollen records was achieved through analysis of sediment core recovered from Lake Donggi Cona (in the north-eastern part of the TP) which has been dated back to the Last Glacial Maximum (LGM). A new data-set of modern pollen collected from large lakes in arid to semi-arid regions of central Asia is also presented herein. The concept of "pollen source area" was introduced to modern climate calibration based on pollen from large lakes, and was applied to the fossil pollen sequence from Lake Donggi Cona. Extremely dry conditions were found to have dominated the LGM, and a subsequent gradually increasing trend in moisture during the Late Glacial period was terminated by an abrupt reversion to a dry phase that lasted for about 1000 years and coincided with the first Heinrich Event of the northern Atlantic region. Subsequent periods corresponding to the warm Bølling-Allerød period and the Younger Dryas cold event were followed by moist conditions during the early Holocene, with annual precipitation of up to about 400 mm. A slightly drier trend after 9 cal ka BP was then followed by a second wet phase during the middle Holocene that lasted until 4.5 cal ka BP. Relatively steady conditions with only slight fluctuations then dominated the late Holocene, resulting in the present climatic conditions. In order to investigate the relationship between vegetation and climate, temporal variations in the possible driving factors for vegetation change on the northern TP were examined using a high resolution late Holocene pollen record from Lake Kusai. Moving-window Redundancy Analyses (RDAs) were used to evaluate the correlations between pollen assemblages and individual sedimentary proxies. These analyses have revealed frequent fluctuations in the relative abundances of alpine steppe and alpine desert components, and in particular a decrease in the total vegetation cover at around 1500 cal a BP. The climate was found to have had an important influence on vegetation changes when conditions were relatively wet and stable. However, after the 1500 cal a BP threshold in vegetation cover was crossed the vegetation appears to have been affected more by extreme events such as dust storms or fluvial erosion than by the general climatic trends. In addition, pollen spectra over the last 600 years have been revealed by Procrustes analysis to be significantly different from those recovered from older samples, which is attributed to an increased human impact that resulted in unprecedented changes to the composition of the vegetation. Theoretical models that have been developed and widely applied to the European area (i.e. the Extended R-Value (ERV) model and the Regional Estimates of Vegetation Abundance from Large Sites (REVEALS) model) have been applied to the high alpine TP ecosystems in order to investigate the pollen-vegetation relationships, as well as for quantitative reconstructions of vegetation abundance. The modern pollen–vegetation relationships for four common pollen species on the TP have been investigated using Poaceae as the reference taxa. The ERV Submodel 2 yielded relatively high PPEs for the steppe and desert taxa (Artemisia Chenopodiaceae), and low PPEs for the Cyperaceae that are characteristic of the alpine Kobresia meadows. The plant abundances on the central and north-eastern TP were quantified by applying these PPEs to four post-Late Glacial fossil pollen sequences. The reconstructed vegetation assemblages for the four pollen sequences always yielded smaller compositional species turnovers than suggested by the pollen spectra, indicating that the strength of the previously-reported vegetation changes may therefore have been overestimated. In summary, the key findings of this thesis are that (a) the two ASM subsystems show asynchronous patterns during both the Holocene and modern time periods, (b) fossil pollen records from large lakes reflect regional signals for which the pollen source areas need to be taken into account, (c) climate is not always the main driver for vegetation change, and (d) previously reported vegetation changes on the TP may have been overestimated because they ignored inter-species variations in pollen productivity.
The concept of a Global Monsoon (GM) has been proposed based on modern precipitation observations, but its application over a wide range of temporal scales is still under debate. Here, we present a synthesis of 268 continental paleo-moisture records collected from monsoonal systems in the Eastern Hemisphere, including the East Asian Monsoon (EAsM), the Indian Monsoon (IM), the East African Monsoon (EAfM), and the Australian Monsoon (AuM) covering the last 18,000 years. The overall pattern of late Glacial to Holocene moisture change is consistent with those inferred from ice cores and marine records. With respect to the last 10,000 years (10 ka), i.e. a period that has high spatial coverage, a Fuzzy c-Means clustering analysis of the moisture index records together with "Xie-Beni" index reveals four clusters of our data set. The paleoclimatic meaning of each cluster is interpreted considering the temporal evolution and spatial distribution patterns. The major trend in the tropical AuM, EAfM, and IM regions is a gradual decrease in moisture conditions since the early Holocene. Moisture changes in the EAsM regions show maximum index values between 8 and 6 ka. However, records located in nearby subtropical areas, i.e. in regions not influenced by the intertropical convergence zone, show an opposite trend compared to the tropical monsoon regions (AuM, EAfM and IM), i.e. a gradual increase. Analyses of modern meteorological data reveal the same spatial patterns as in the paleoclimate records such that, in times of overall monsoon strengthening, lower precipitation rates are observed in the nearby subtropical areas. We explain this pattern as the effect of a strong monsoon circulation suppressing air uplift in nearby subtropical areas, and hence hindering precipitation. By analogy to the modern system, this would mean that during the early Holocene strong monsoon period, the intensified ascending airflows within the monsoon domains led to relatively weaker ascending or even descending airflows in the adjacent subtropical regions, resulting in a precipitation deficit compared to the late Holocene. Our conceptual model therefore integrates regionally contrasting moisture changes into the Global Monsoon hypothesis. (C) 2017 Elsevier Ltd. All rights reserved.