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1.
The identification of fossil conifer stomata was conducted from a lake sedimentary sequence on the Liupan Mountains, southwestern Loess Plateau, in order to reconstruct detailed forest history and assess the potential of using stomata as a supplement to pollen analysis to determine the local presence of conifers over the last 3200 years. The pollen has already been analyzed in our previous study. Reference conifer stomata were prepared for the fossil stomata identification and demonstrate that the conifer stomata could be identified to at least genus level. Our stomata and pollen results reveal three clear stages of the forest recession during the late Holocene: mixed conifer-deciduous forest, steppe-forest and steppe. Combined stomata and pollen results confirm the local presence of conifer trees (Abies and Pinus) in the vicinity of the Tianchi Lake. Abundant Abies trees existed during 3200-2200 cal yr BP. Then it decreased and finally demised at ca.210 cal yr BP. Pinus trees have been continuously present with very low abundance throughout the late Holocene. Application of comprehensive studies comprising both stomata and pollen analyses allowed a detailed determination of the stages of conifer trees presence in the vicinity of the Tianchi Lake. Our study suggests that the analysis of fossil stomata is a valuable methodological tool for the provision of unambiguous evidence of the past local presence of the coniferous taxa in this region.  相似文献   

2.
Analyses of pollen, plant macrofossils, sediment mineralogy, geochemistry, and lithology of cores from Chappice Lake, southeastern Alberta, provide an outline of paleohydrological changes spanning the last 7300 radiocarbon years. Situated near the northern margin of the Great Plains, Chappice Lake is currently a small (1.5 km2), shallow (<1 m), hypersaline lake. Results of this study suggest that the lake has experienced significant changes in water level and chemistry during the Holocene.From 7300 to 6000 BP the lake oscillated between relatively high stands and desiccation. From 6000 to 4400 BP it was smaller than present and ponded highly saline water. Although extreme water level variations of the preceding period had ceased, pronounced seasonal fluctuations persisted. Between 4400 and 2600 BP, lake level was more stable but gradually rising. Carbonates were a major component of the sediments deposited during this interval. A large, relatively fresh lake existed from 2600 to 1000 BP. Illite was the dominant mineral deposited during this period, but since then has been a minor constituent in a mineral suite dominated by detrital silicates. A series of low-water, high-salinity stands occurred between 1000 and 600 BP, although these low stands were not as pronounced as low-water intervals in the middle Holocene. Relatively high water levels were sustained from 600 BP until the late 1800s. The lake declined significantly in the last one hundred years, notably during the historically documented droughts of the late 1800s, 1920s, 1930s, and 1980s.The timing of paleohydrological events at Chappice Lake corresponds closely with well documented Holocene climatic intervals, such as the Hypsithermal, Neoglaciation, Medieval Warm Period, and Little Ice Age. In addition, historic lake-level fluctuations can be related directly to climate. As a result, the Chappice Lake sedimentary succession offers a rare opportunity to obtain a high-resolution, surrogate record of Holocene climate on the northern Great Plains, and to observe the response of lake chemistry and biota to significant environmental change.Geological Survey of Canada Contribution No. 45191, Palliser Triangle Global Change Contribution No. 2This publication is the first of a series of papers presented at the Conference on Sedimentary and Paleolimnological Records of Saline Lakes. This Conference was held August 13–16, 1991 at the University of Saskatchewan, Saskatoon, Canada. Dr. Evans is serving as Guest Editor for this series.  相似文献   

3.
Transects of surface sediment samples were taken in 4 lakes from the Sylvania Wilderness Area, Upper Peninsula of Michigan. These surface samples were compared with diatom samples from a core taken in the Northwest basin of Crooked Lake, also from the Sylvania Wilderness Area. Weighted Averaging calibration was used to reconstruct lake depths in Crooked Lake using the diatom microfossils from the core and the surface samples to infer past lake depth. During the early Holocene the lake was dominated by planktonic species and diatom-inferred water depth was large – approx. 13 m. At about 6700 BP inferred water depth was 2 m and samples were dominated by Fragilaria construens var. venter – a species characteristic of shallow parts of the surface sample transects. From 6700 to 5000 BP reconstructed water level was at its shallowest. From 5000 to 3000 BP it increased. This rise in water level was marked by increasing abundances of Aulacoseira ambigua and occurred at the same time increasing percentages of hemlock pollen indicate increasing available moisture. Modern water depth was reached about 3000 BP. The water level changes at Crooked Lake are consistent with regional climate changes in the Upper Midwest during the Holocene. The lake was shallowest during the mid-Holocene warm period documented by other investigators. It deepened as the Midwestern climate became cooler and wetter during the late Holocene.  相似文献   

4.
王中  刘向军  丛禄 《盐湖研究》2017,25(2):67-75
基于野外沉积物调查、已报道的风成沉积物年代和重建的湖泊水位波动曲线,初步绘制了末次冰期冰盛期和全新世早期青海湖东岸的沙漠范围。初步认为,末次冰期冰盛期时青海湖大幅度萎缩,湖底沉积物暴露并遭受风蚀,在偏西风的搬运下,近岸粗颗粒砂堆积在日月山西侧和倒淌河河谷,形成流动沙丘,细颗粒沉积物被向东搬运沉积至河湟谷地和黄土高原。倒淌河源头的沙丘最晚是在末次冰期盛冰期(23~16ka BP)沙漠扩张时形成并残留至今的;青海湖东部日月山西侧山麓地带和倒淌河河谷中、晚全新世黄土、古土壤下部普遍沉积的风成砂是末次冰期冰盛期和全新世早期(11~9 ka BP)青海湖东岸地区大范围沙化的证据。  相似文献   

5.
Qinghai Lake and Zhuye Lake, ~400 km apart, are located in the northwest margin of the Asian summer monsoon. Water of these two lakes mostly comes from the middle and eastern parts of the Qilian Mountains. Previous studies show that the Holocene climate changes of the two lakes implied from lake records are different. Whether lake evaporation plays a role in asynchronous Holocene climate changes is important to understand the lake records. In this paper, we used modern observations beside Qinghai Lake and Zhuye Lake to test the impact factors for lake evaporation. Pan evaporation near the two lakes is mainly related to relative humidity, temperature, vapor pressure and sunshine duration. But tem-perature has different impacts to lake evaporation of the two lakes, which can affect Holocene millennial-scale lake level changes. In addition, differences in relative humidity on the millen-nial-scale would be more significant, which also can contribute to asynchronous lake records.  相似文献   

6.
We examined long-term relationships between Cladocera and limnogeological and climate variables in a late Holocene lake sediment sequence to assess cladoceran responses to climate change and reconstruct past climate variations in southern Finland, near the Baltic Sea coast. Elemental composition, organic matter, and inferred water quality variables were used to constrain paleoenvironmental (until 4500 cal BP) conditions and human impact. Fossil Cladocera assemblages, ephippia, and body size trends were utilized to estimate mean July air temperature (T Jul) variability, open-water season length, and water-temperature regimes. Results revealed stable cladoceran communities, dominated by Eubosmina, until 500 cal BP, followed by major assemblage changes including increases in Chydorus cf. sphaericus, Alonella spp., and Bosmina longirostris. Pb and Cu concentrations were associated temporally with increasing human impact and warming climate during the past few centuries, and were the most significant factors in explaining cladoceran community changes, based on redundancy analysis. A Cladocera-based T Jul reconstruction estimated elevated temperatures for the end of the Holocene Thermal Maximum, a cooler period during ~3000–2000 cal BP, slightly increased temperatures during 1200–800 cal BP corresponding to the Medieval Climate Anomaly (MCA), and lower temperatures during the Little Ice Age (LIA), 800–200 cal BP. The reconstruction also suggests that significant climate warming took place during the twentieth century. Cladocerans mostly associated with warm periods included B. longirostris and Pleuroxus uncinatus, whereas Alona affinis, A. quadrangularis, and C. cf. sphaericus were associated with colder climate. Compared to a reference Chironomidae-inferred T Jul reconstruction from the same region, discrepancies were apparent in amplitude of temperature change, as the temperature variability in the cladoceran-based record was muted until ~200 cal BP. During the LIA, increased ephippia and body size agreed with the temperature reconstruction, but suggested a severely shorter open-water season and reduced water temperatures compared to the preceding late Holocene episodes, even though the cladoceran T Jul showed quite similar temperatures for these periods. Our results suggest that fossil cladoceran assemblages, ephippia, and morphological attributes respond sensitively to long-term climate fluctuations and this record reflects well the major climate events of the late Holocene and provides realistic paleoclimatic estimates for maritime southern Finland.  相似文献   

7.
We present results of analyses on a sediment core from Lake Karakul, located in the eastern Pamir Mountains, Tajikistan. The core spans the last ~29 cal ka. We investigated and assessed processes internal and external to the lake to infer changes in past moisture availability. Among the variables used to infer lake-external processes, high values of grain-size end-member (EM) 3 (wide grain-size distribution that reflects fluvial input) and high Sr/Rb and Zr/Rb ratios (coinciding with coarse grain sizes), are indicative of moister conditions. High values in EM1, EM2 (peaks of small grain sizes that reflect long-distance dust transport or fine, glacially derived clastic input) and TiO2 (terrigenous input) are thought to reflect greater influence of dry air masses, most likely of Westerly origin. High input of dust from distant sources, beginning before the Last Glacial Maximum (LGM) and continuing to the late glacial, reflects the influence of dry Westerlies, whereas peaks in fluvial input suggest increased moisture availability. The early to early-middle Holocene is characterised by coarse mean grain sizes, indicating constant, high fluvial input and moister conditions in the region. A steady increase in terrigenous dust and a decrease in fluvial input from 6.6 cal ka BP onwards points to the Westerlies as the predominant atmospheric circulation through to present, and marks a return to drier and even arid conditions in the area. Proxies for productivity (TOC, TOC/TN, TOC Br ), redox potential (Fe/Mn) and changes in the endogenic carbonate precipitation (TIC, δ18O Carb ) indicate changes within the lake. Low productivity characterised the lake from the late Pleistocene until 6.6 cal ka BP, and increased rapidly afterwards. Lake level remained low until the LGM, but water depth increased to a maximum during the late glacial and remained high into the early Holocene. Subsequently, the water level decreased to its present stage. Today the lake system is mainly climatically controlled, but the depositional regime is also driven by internal limnogeological processes.  相似文献   

8.
Fossil diatoms were analysed from a 10.3 m core from Harris Lake, Cypress Hills, Saskatchewan, and a diatom-salinity transfer function was used to construct a history of Holocene salinity changes for the lake. The diatom paleosalinity record indicates that Harris Lake remained fresh <0.5 g l-1 throughout the Holocene, with only slight increases in salinity between approximately 6500 and 5200 years BP. This interval corresponds to the only period in the lake's history when planktonic diatoms were abundant; benthic Fragilaria taxa, mainly F. pinnata, F. construens and F. brevistriata were dominant throughout most of the Holocene. The shift from a benthic to a planktonic diatom flora between 6500 and 5200 years BP may be an indirect response to a warmer climate that reduced forest cover in the watershed and allowed greater rates of inorganic sedimentation. The small salinity increase that accompanies the floristic change is probably not the result of lower lake levels; in fact the lake was probably deeper at this point than in the later Holocene. This paleosalinity record indicates that Harris Lake did not experience episodes of hypersalinity during the mid-Holocene, as suggested by a previous study, and that the lake may have been fresh during the early Holocene as well.  相似文献   

9.
Modern climate research has shown that the Asian summer monsoon water vapor transport is limited to the eastern part of the Qilian Mountains. On the Holocene millennial-scale, whether the northwest boundary of the summer monsoon varies according to climate change is a key scientific issue. Yanchi Lake is located in the northern Qilian Mountains and the middle of the Hexi Corridor, where the modern climate is less affected by the Asian summer monsoon. It is a key research area for examining the long-term variations of the Asian summer monsoon. Paleoclimatic data, including AMS ^14C dates of pollen concentrates and bulk organic carbon, lithology, grain-size, mineral composition and geochemical proxies were acquired from sediments of Yanchi Lake. The chronological results show that the lower part of the lacustrine section is formed mainly in the Late Glacial and early Holocene period, while the proxies＇ data indicate the lake expansion is associated with high content of mineral salts. The middle part of this section is formed during the transitional period of the early and middle Holocene. Affected by the reworking effect, the pollen concentrates AMS^14C dates from the middle part of the section are generally older than those from the lower part. Since the mid-Holocene, Yanchi Lake retreated significantly and the deposition rate dropped obvi- ously. The Yanchi Lake record is consistent with the Late Glacial and Holocene lake records in the Qinghai-Tibet Plateau and the climatic records in typical monsoon domain, which indicate the lake expansion and the strong Asian summer monsoon during the Late Glacial and early Holocene. The long-term monsoonal pattern is different from the lake evolution in Central Asia on the Holocene millennial-scale. This study proves the monsoon impacts on the northwestern margin of the summer monsoon, and also proves the fact that the northern boundary of the summer monsoon moves according to millennial-scale climate change.  相似文献   

10.
Annually laminated sediments in Lake Xiaolongwan, northeast China, contain a suite of n-alkanes (C17–C33) with a strong odd over even carbon number predominance. Biomarker n-alkane proxies (average chain length, Paq ratio, grass/tree ratio, carbon preference index and compound-specific δ13C values) were used to reconstruct climate changes that occurred over the last millennium. Compound-specific δ13C values show large differences between the distinctive chain length groups of n-alkanes that originate from algae, aquatic macrophytes and terrestrial plants. Long-chain n-alkanes (C27–C33) are predominantly derived from leaf wax lipids in the forest. Variations in long-chain n-alkane δ13C values may mainly record water-use efficiency, inasmuch as the contribution from C4 plants is negligible in the Lake Xiaolongwan catchment. Short- and middle-chain n-alkanes are mainly from algae and aquatic plants. They are strongly depleted in 13C. This feature may be linked to a methane-derived, negative δ13C pool and lake overturn, which regulates dinoflagellate blooms. Parallel fluctuations are observed in δ13C27–31 values, Paq, and the grass/tree ratio throughout the record. Variations in δ13C27–31 values and Paq are in agreement with historical documents on summer and winter climate conditions. They support earlier suggestions that δ13C values in the long-chain n-alkanes and Paq may be useful indicators of effective precipitation or drought stress in this forested area. The δ13C27–31 index and Paq show distinct decadal variations. Periods with high δ13C27–31 values and a low Paq index correspond with the warm phases of the Pacific Decadal Oscillation (PDO). Values are reversed during PDO cool phases. At the decadal timescale, summer monsoon rainfall in northeast China over the last millennium may have been regulated mainly by the PDO.  相似文献   

11.
Lakes have received considerable attention as long-term sinks for organic carbon(C) at regional and global scales. Previous studies have focused on assessment and quantification of carbon sinks, and few have worked on the relationship between millennial-scale lake C sequestration, hydrological status and vegetation, which has important scientific significance in improving our understanding of lake C stocks and storage mechanisms. Here, we present a comprehensive study of pollen records, organic geochemical proxies, lake-level records, sediment accumulation rate(SAR) and organic C accumulation rate(CAR) in China since the Holocene. We also include numerical climate classification and lake-level simulations, to investigate variations of lake C sequestration, hydrological status and vegetation during the Holocene. Results indicate that the evolution of lake C accumulation showed an out-of-phase relationship with hydrological status and vegetation in China. Lake C accumulation exhibited an overall trend of increasing from the early to late Holocene in response to gradually increasing terrestrial organic matter input. However, China as a whole experienced the densest vegetation cover in the middle Holocene, corresponding to the mid-Holocene optimum of a milder and wetter climate. Optimal hydrological conditions were asynchronous in China; for example, early Holocene in Asian monsoon dominated areas, and middle Holocene in westerlies controlled regions. Our synthesis indicated that climate change was the main factor controlling the long-term variability in lake C accumulation, hydrologic conditions, as well as vegetation, and human influences were usually superimposed on the natural trends.  相似文献   

12.
A reconstruction of the hydrological and environmental evolution of the crater lake at Malha (Northern Darfur, Sudan) resulted from the mineralogical and biological study of a 9.21 m section of lake sediments, representing an uninterrupted sequence of lacustrine deposition since 8 290 14C years BP.Important changes in water supply and conditions of sedimentation are reflected in the nature of the sediments and the morphology and stratigraphical distribution of various salt minerals. Additional information on lake circulation patterns and salinity conditions are obtained from associated benthic paleocommunities, represented by ostracods and dipterid larvae. Combining both lines of evidence, the studied sequence can be divided in six distinct sections, which correspond to six successive periods in the lake's Holocene history. The first three periods, generally characterized by high lake levels, represent three generations of a meromictic lake, two of which have ended with a complete desiccation of the lake basin. Meromixis was stable during Period I, due to wind shelter and pronounced density stratification. In the course of Periods II and III stratification was repeatedly interrupted. During Period II, the disruptions were accompanied by important water budget fluctuations; a superimposed gradual decrease in net water supply eventually resulted in holomictic conditions terminating this period. Evidence of turbulence periodically affecting profundal waters is recorded in the sediments of Period III, suggesting that disruptions of stratification were now initiated by very strong winds. Between Period I and Period III, the littoral mixolimnion gradually evolved from near fresh to mesosaline. In Periods IV to VI, lake level was intermediate to low. The lake was holomictic for most of the time and meso- to hypersaline; during Period V, it repeatedly shrunk to a shallow brine pool.The Holocene evolution of Malha Crater Lake illustrates the progressive increase in aridity over most of North Africa following a well-established, early- to mid-Holocene major humid episode. The uninterrupted sedimentary sequence lends itself for detailed reconstruction of Holocene climatic evolution in arid Northeast Africa, a region where records of continuous lacustrine deposition are extremely scarce. As the chronology of critical events in the lake's history remains as yet unsupported by radiocarbon dates, correlation with other Holocene sequences in the eastern Sahara is highly tentative at this point.  相似文献   

13.
Zeekoevlei is the largest freshwater lake in South Africa and has a century-long history of anthropogenic impact that caused hyper-eutrophic conditions. We used biomarkers (alkanes and pigments), stable isotopes (δ13C and δ15N), rates of primary palaeoproduction and total inorganic carbon (TIC) accumulation rates in the lake sediments to investigate changes in plankton and macrophyte communities in response to anthropogenic activities in this shallow lake. Specific alkanes (ΣC15,17,19, pristane, phytane and n-C29/n-C17 ratio) and pigment (chlorophyll a, β,β-carotene, echinenone, fucoxanthin and zeaxanthin) concentrations in lake waters indicated the present-day hyper-eutrophic condition and seasonal fluctuations of cyanobacteria, zooplankton and diatom populations. Eutrophic conditions were initiated in the lake with the start of recreational activities and construction of a sewage treatment plant in the early 1920s. The lake transformed from a eutrophic to a hyper-eutrophic waterbody following damming, pondweed eradication and accelerated catchment-derived nutrient input. The change in lake trophic state was recorded by a sharp decline in the terrestrial to aquatic ratio (TAR) of specific n-alkanes, low carbon preference index (CPI) and increased δ13C values in the sediment core. In addition, the aquatic macrophyte n-alkane proxy (Paq) values (~1) indicated a slow takeover by floating macrophytes after the eradication of submerged pondweeds in 1951. Elevated n-alkane (ΣC15,17,19), total alkane and pigment (chlorophyll a, β,β-carotene, zeaxanthin and zeaxanthin to β,β-carotene ratio) concentrations, low δ15N values and low TIC accumulation rates in the upper middle section of the core indicated the beginning of intense cyanobacterial blooms after the dredging in 1983. Although the cyanobacterial population has decreased in recent years, hyper-eutrophic conditions are reflected by low CPI <0.04 and TAR <1 values at the top of the sediment core.  相似文献   

14.
The main aim of this study was to reconstruct the history of water level fluctuations of a small dimictic lake during the Holocene based on the lithological and geochemical composition of sediments. We studied lithological and geochemical parameters in sediment cores extracted along a transect through Lake Juusa in southeastern Estonia. Analysis revealed that LOI550 and CaCO3 content in the accumulated sediment are dependent on water depth. At depths greater than 3 m, the relationship between organic carbon and CaCO3 is inverse, whereas no relationship exists at shallower depths. Lake-level changes are well depicted on the grain-size distribution profiles and macrofossil composition. An increase in the >‰315μm fraction appeared during the initial phases of water level fluctuations, followed by an increase in the 100–315μm fraction. The >‰36μm fraction dominated the silt fraction. Higher OC/N ratios in shallow water sediments reflect the dominance of macrophytes vegetation.  相似文献   

15.
We studied mineral magnetic properties of a 6-m-long, late Pleistocene through Holocene sediment sequence from Lake Aibi in Dzungaria (Zunggary, Junggar), northern Xinjiang, China. Results were used to infer environmental changes and are compared with previously studied cores from Lake Manas. Both water bodies occupy the deepest parts of the Dzungarian Basin and are remnants of large Holocene lakes. During the Late Pleistocene, the magnetic mineralogy in both lakes was dominated by detrital, iron oxide minerals. Oxic conditions, which dominated during sedimentation and early diagenesis, persisted over the Pleistocene–Holocene transition. Later, during the middle Holocene, lake bottom conditions enabled authigenic formation of iron sulphide minerals such as pyrite (FeS2) in Lake Aibi, and pyrite and greigite (Fe3S4) in Lake Manas. This iron sulphide mineralogy suggests increased biological activity in stagnant, anoxic bottom waters. Anoxic bottom conditions started about 9.8 cal kyr BP in Lake Manas and at about 7.2 cal kyr BP in Lake Aibi. A short dry event recorded in Lake Manas between 6.8 and 5.2 cal kyr BP is not clearly observed in Lake Aibi. In the late Holocene, i.e. the last 2.8 cal kyr, sediments of both lakes are again characterised by iron oxides, suggesting well-mixed, shallow water bodies. For this recent period, it seems that the detrital material in the two lakes had a common origin. Magnetic properties of sediments in Lakes Aibi and Manas show broadly similar environmental evolution during the late Pleistocene and Holocene. Nevertheless, despite the close proximity of the two lakes (~200 km) in the same basin, they display some different magnetic properties and record environmental changes at different times.  相似文献   

16.
湖泊沉积记录的环境演变是全球变化的重要研究领域之一,通过对中国内陆区中30 个湖泊研究成果的总结和梳理,探讨了全新世以来该地区干湿变化的规律和区域分异。通过降水量结合传统分区方法将中国内陆区分为西北干旱区、东亚季风边缘区和青藏高原区。对每个湖泊样点以500 年为时间间隔,以孢粉为主要干湿指标,综合氧同位素、有机质及碳酸盐等,将湖泊干湿状况划分3 个干湿等级（干旱,半湿润,湿润）,建立区域干湿指数。结果表明,中国内陆不同区域全新世可能经历了不同的干湿变化过程：①西北干旱区基本上是早中全新世干旱,中晚全新世相对湿润,但区域差异明显;②东亚季风边缘区早全新世早期干旱,早全新世晚期和中全新世相对湿润,晚全新世干旱;③青藏高原区的湿润时期主要发生在早中全新世,但是不同地区有所不同。对比分析显示：西北干旱区的干湿变化可能受西风环流控制,但在时间和强度上区域内部差异较明显;东亚季风边缘区可能主要受东亚季风控制;青藏高原地区早中全新世的湿润可能与印度季风的增强相关。  相似文献   

17.
Diatoms, organic matter and magnetic susceptibility in a 10-m-long sediment sequence from coastal Lake Lilaste, Latvia, were analysed to evaluate Holocene environmental changes related to past sea-water intrusions. Lake Lilaste is located ~1 km from the present sea coast in an area with a low uplift rate and a threshold altitude of 0.5 m a.s.l. It was thus considered to be an appropriate site to study the influence of past sea level fluctuations on the lake and its sediments. Variations in diatom community composition, along with sediment lithostratigraphy, show that a shallow, nutrient-rich freshwater lake existed there during the early Holocene. The first brackish-water diatoms appeared concurrent with a sea level rise ca. 8700 ± 50 cal a BP, but long-term, intermittent inputs of brackish water were observed between 6700 ± 40 and 4200 ± 80 cal a BP. During those time spans, diatoms indicate increased nutrient concentrations and high conductivity, a consequence of occasional mixing of brackish and freshwater that promoted biological productivity. Lilaste was isolated from the sea at 4200 ± 80 cal a BP, after which a stable freshwater environment, dominated by planktonic diatoms such as Aulacoseira ambigua, A. granulata, A. islandica and A. subarctica, was established. At 400 ± 50 cal a BP, planktonic diatoms were gradually replaced by Fragilaria spp., indicating the beginning of anthropogenic impact. The reconstructed relative water-level curve from the lake coincides with the eustatic sea level curve from 6800 ± 40 cal a BP onwards. There was a distinct increase in abundance of brackish-water diatoms when the sea level reached the threshold of Lilaste, which at that time was probably about 3 m lower than the present sea level. According to radiocarbon-dated shifts in the diatom community composition, the Litorina Sea transgression was a long-lasting event (ca. 2200 years) in the southern part of the Gulf of Riga, where the land uplift rate was near zero. It culminated more than 1000 years later than at other sites with higher uplift, in the northern part of the Baltic Sea.  相似文献   

18.
Stable isotopes and trace elements in ostracod shells have been used widely in paleolimnological investigations of past lake hydrochemistry and climate because they provide insights into past water balance and solute evolution of lakes. Regional differences in lake characteristics and species-specific element fractionation, however, do not permit generalization of results from other regions or ostracod species to the southern Tibetan Plateau, in part because most common taxa from the southern Tibetan Plateau are endemic to the area. This study evaluated relations between present-day environmental conditions and the geochemical composition of modern ostracod shells from the southern Tibetan Plateau, to assess the suitability of using shell chemistry to infer hydrological conditions. We studied nine lakes and their catchments, located along a west–east transect in the south-central part of the Tibetan Plateau. Stable oxygen and carbon isotope values and trace element concentrations in recent shells from the four most abundant ostracod species (Leucocytherella sinensis, ?Leucocythere dorsotuberosa, Limnocythere inopinata, Tonnacypris gyirongensis) were measured, together with hydrochemical properties of host waters at the time of sampling. Results revealed significant between-species differences in stable isotope fractionation and trace element incorporation into shell calcite. Stable oxygen and carbon isotope values of ostracod shells were correlated significantly with the stable isotope composition of the respective water body \( \left( {\updelta^{18} {\text{O}}_{{{\text{H}}_{ 2} {\text{O}}}} \,{\text{and }}\updelta^{13} {\text{C}}_{{{\text{H}}_{ 2} {\text{O}}}} } \right) \), reflecting salinity and productivity, respectively. Offsets between δ18Oshell and δ13Cshell and inorganic calcite, the latter representing isotopic equilibrium, suggest shell formation of T. gyirongensis during spring melt. L. sinensis reproduces throughout the monsoon season until September and shows several consecutive generations, and L. inopinata molts to the adult stage after the monsoon season in August/September. The influence of pore water δ13C was displayed by L. inopinata, suggesting shell calcification within the sediment. Mg/Cashell is primarily influenced by water Mg/Ca ratios and salinity and confirms the use of this shell ratio as a proxy for precipitation-evaporation balance and lake level. In addition, Sr/Ca and Ba/Ca can be used to infer changes in salinity, at least in closed-basin lakes with calcite saturation. Observed effects of water Sr/Ca and salinity on Sr/Ca incorporation are biased by the presence of aragonite precipitation in the lakes, which removes bioavailable Sr from the host water, resulting in low Sr/Cashell values. Changes in carbonate mineralogy affect the bioavailability of trace elements, a process that should be considered in paleoclimate reconstructions. Oxygen isotopes and Mg/Cashell ratios were unaffected by water temperature. Positive correlations among Fe/Ca, Mn/Ca and U/Ca in ostracod shells, and their negative correlation with δ13C, which reflects organic matter decay, show the potential to infer changes in redox conditions that can be used to reconstruct past oxygen supply to bottom waters and thus past water-circulation changes within lakes. The intensity of microbial activity, associated with organic matter decomposition, can be inferred from U/Ca ratios in ostracod shells. These findings highlight the value of fossil ostracod records in lake deposits for inferring paleoenvironmental conditions on the southern Tibetan Plateau.  相似文献   

19.
This study uses the Holocene lake sediment of Lake ?ū?i (Latvia, Vidzeme Heights) for environmental reconstruction with multi-proxy records including lithology, computerised axial tomography scan, grain-size analysis, geochemistry, diatoms and macrofossils, supported by AMS radiocarbon dating. Numerical analyses (PCA; CONISS) reveal three main phases in the development of the lake. Response to the Lateglacial–Holocene transition in Lake ?ū?i took place around 11,300 cal. BP. Organogenic sedimentation started with distinctive 5-cm-thick peat layer and was followed by lacustrine sedimentation of carbonaceous gyttja. Several findings of the peat layer with similar dated age and position at different absolute altitudes indicate that lake basin was formed by glaciokarstic processes. In the Early Holocene (until around 8,500 cal. BP), the lake was shallow and holomictic, surrounded by unstable catchment with erosion and inflow events. Predominance of diatom species of Cyclotella and Tabellaria, large numbers of respiratory horns of phantom midge pupae (Chaoboridae), high Fe/Mn ratio, as well as the presence of laminated sediments indicates the transition to a dimictic and oligo-mesotrophic lake conditions with high water level, anoxia in the near-bottom and stable catchment in the Middle Holocene (8,500–2,000 cal. BP). This contrasts with many hydrologically sensitive lakes in Northern and Eastern Europe in which the water level fell several meters during this period. During the Late Holocene (from 2,000 cal. BP to the present), the lithological and biotic variables reveal major changes, such as the increase in erosion (coarser grain-size fraction) and eutrophication [diatoms Aulacoseira ambigua (Grun.) Sim., Stephanodiscus spp., Cyclostephanos dubius (Fricke) Round]. Characteristics of lake-catchment system during the Late Holocene reflect anthropogenic signal superimposed on the natural forcing factors. To date, the Late Quaternary palaeolimnological reconstructions using lake sediment has been limited in the Baltic region. Therefore, findings from Lake ?ū?i provide important information about environmental and climatic changes that took place in this part of Eastern Europe. This study shows that the relative importance of climate and local factors has varied over the time and it is essential to consider the lake basin topography, catchment size and land cover as potential dominant forcing factors for changes in sedimentary signal.  相似文献   

20.
We explored the potential for using Pediastrum (Meyen), a genus of green alga commonly found in palaeoecological studies, as a proxy for lake-level change in tropical South America. The study site, Laguna La Gaiba (LLG) (17°45′S, 57°40′W), is a broad, shallow lake located along the course of the Paraguay River in the Pantanal, a 135,000-km2 tropical wetland located mostly in western Brazil, but extending into eastern Bolivia. Fourteen surface sediment samples were taken from LLG across a range of lake depths (2–5.2 m) and analyzed for Pediastrum. We found seven species, of which P. musteri (Tell et Mataloni), P. argentiniense (Bourr. et Tell), and P. cf. angulosum (Ehrenb.) ex Menegh. were identified as potential indicators of lake level. Results of the modern dataset were applied to 31 fossil Pediastrum assemblages spanning the early Holocene (12.0 kyr BP) to present to infer past lake level changes qualitatively. Early Holocene (12.0–9.8 kyr BP) assemblages do not show a clear signal, though abundance of P. simplex (Meyen) suggests relatively high lake levels. Absence of P. musteri, characteristic of deep, open water, and abundance of macrophyte-associated taxa indicate lake levels were lowest from 9.8 to 3.0 kyr BP. A shift to wetter conditions began at 4.4 kyr BP, indicated by the appearance of P. musteri, though inferred lake levels did not reach modern values until 1.4 kyr BP. The Pediastrum-inferred mid-Holocene lowstand is consistent with lower precipitation, previously inferred using pollen from this site, and is also in agreement with evidence for widespread drought in the South American tropics during the middle Holocene. An inference for steadily increasing lake level from 4.4 kyr BP to present is consistent with diatom-inferred water level rise at Lake Titicaca, and demonstrates coherence with the broad pattern of increasing monsoon strength from the late Holocene until present in tropical South America.  相似文献   

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