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1.
The Holocene environmental history and climate are reconstructed for Råtåsjøen, a low-alpine lake in south-central Norway. The reconstructions are based on chironomids, diatoms, pollen, plant macrofossils, and sediment characteristics. From plant macrofossil evidence, birch trees (Betula pubescens) immigrated ca. 10,000 cal BP. The chironomid-inferred mean July air temperature was high, but may be unreliable during the early stages of the lakes history due to the high abundance of Chironomus anthracinus type, a taxon that may include several species. From ca. 9000 cal BP the inferred mean July temperature was lower (ca. 9 °C). Temperatures increased towards 8000 cal BP and pine (Pinus sylvestris) reached its upper limit near the lake. July temperature may have become a significant factor controlling long-term pH in the lake, starting shortly after 8000 cal BP. High pH values were associated with periods of warm summers and lower pH values occurred during periods of colder summers. Alkalinity processes within the lake and/or the catchment are possible factors controlling this relationship. A temperature decline at ca. 5400 cal BP separated two 10.6 °C temperature maxima around 6400 and 4500 cal BP. The 1.5 °C decline in July air temperatures from ca. 4400 cal BP was paralleled by a decrease of pH from 7.2 to 6.8. Following the temperature drop, first pine and then birch trees declined and disappeared from the catchment and organic accumulation in the lake increased. The increased organic accumulation rate had a positive effect on diatom production. At ca. 2700 cal BP the temperature reached a minimum (ca. 9.2 °C) and correspondingly a second pH minimum was reached. Temperature decreased again slightly at ca. 400 cal BP during the Little Ice Age, before increasing by about 0.5 °C towards the present. Percentage organic carbon as estimated by loss-on-ignition appears to be better correlated with chironomid-inferred July temperatures than organic accumulation rates, at least for the last 9000 years. Accumulation rates of organic sediments are more coupled with catchment-related processes, such as erosion and major changes in vegetation, than is percentage organic carbon.  相似文献   

2.
Climatic and environmental changes since the last glacial period are important to our understanding of global environmental change. There are few records from Southern Tibet, one of the most climatically sensitive areas on earth. Here we present a study of the lake sediments (TC1 core) from Lake Chen Co, Southern Tibet. Two sediment cores were drilled using a hydraulic borer in Terrace 1 of Lake Chen Co. AMS 14C dating of the sediments showed that the sequence spanned >30,000 years. Analyses of present lake hydrology indicated that glacier melt water is very important to maintaining the lake level. Sediment variables such as grain size, TOC, TN, C/N, Fe/Mn, CaCO3, and pollen were analyzed. Warm and moderately humid conditions dominated during the interval 30,000–26,500 cal year BP. From 26,500 to 20,000 cal year BP, chemical variables and pollen assemblages indicate a cold/dry environment, and pollen amounts and assemblages suggest a decline in vegetation. From 20,000 to 18,000 cal year BP, the environment shifted from cold/dry to warm/humid and vegetation rebounded. The environment transitioned to cold/humid during 16,500–10,500 cal year BP, with a cold/dry event around 14,500 cal year BP. After 10,500 cal year BP, the environment in this region tended to be warm/dry, but exhibited three stages. From 10,500 to 9,000 cal year BP, there was a short warm/humid period, but a shift to cold/dry conditions occurred around 9,000 cal year BP. Thereafter, from 9,000 to 6,000 cal year BP, there was a change from cold/dry to warm/humid conditions, with the warmest period around 6,000 cal year BP. After 6,000 cal year BP, the environment cooled rapidly, but then displayed a warming trend. Chemical variables indicate that a relatively warm/dry event occurred around 5,500–5,000 cal year BP, which is supported by time-lagged pollen assemblages around 4,800 cal year BP. Our lake sediment sequence exhibits environmental changes since 30,000 cal year BP, and most features agree with records from the Greenland GISP2 ice core and with other sequences from the Tibetan Plateau. This indicates that environmental changes inferred from Lake Chen Co, Southern Tibet were globally significant.  相似文献   

3.
A high-resolution paleolimnological study from Lake Brazi, a small mountain lake in the Southern Carpathian Mountains, Romania, shows distinct diatom responses to late glacial and early Holocene climate change between ca. 15,750 and 10,000?cal?year BP. Loss-on-ignition, titanium, sulphur, phosphorus, biogenic silica content, and diatom assemblage composition were used as proxies for past environmental changes. Total epilimnetic phosphorus (TP) concentrations and lakewater pH were reconstructed quantitatively using diatom-TP and pH transfer functions. The most remarkable changes in the aquatic ecosystem were found at ca. 12,870 and 10,400?cal?year BP. Whereas the onset of the Younger Dryas (YD) climatic reversal was conspicuous in our record, the beginning of the Holocene was not well marked. Two diatom assemblage zones characterize the YD in Lake Brazi, suggesting a bipartite division of this climatic oscillation. The diatom responses to the YD cooling were (1) a shift from Staurosira venter to Stauroforma exiguiformis dominance; (2) a decrease in overall diatom diversity; (3) a decrease in lake productivity, inferred from DI-TP, organic matter, and biogenic silica content; and (4) a lowering of the DI-pH. Compositional change of the diatom assemblages suggested a sudden shift towards more acidic lake conditions at 12,870?cal?year BP, which is interpreted as a response to prolonged ice cover and thus shorter growing seasons and/or enhanced outwash of humic acids from the catchment. Taking into account the chironomid-based inference of only moderate July mean temperature decrease (<1?°C), together with the pollen-inferred regional opening of the forest cover and expansion of steppe-tundra, our data suggest that ecosystem changes in the Southern Carpathians during the YD were likely determined by strong seasonal changes.  相似文献   

4.
A 341 cm long sediment sequence was recovered from the unofficially named Raffles Sø on Raffles Ø, outer Scoresby Sund region, East Greenland. The sediment sequence consists in the upper part (0–230 cm) of a stratified gyttja enriched in organic carbon and biogenic silica whereas the lower core part (235–341 cm) is composed of terrigenous, consolidated glacio-limnic sediments. 14C-AMS measurements indicate that the sediment sequence represents the entire Holocene lake history from 10,030 calibrated radiocarbon years.The geochemical parameters (opal, total organic carbon (TOC), total nitrogen (TN)) and the total diatom concentration show similar developments during the Holocene, and reflect changes in biological production and nutrient input into the lake. These records clearly reveal a broad Holocene TOC-opal-maximum interval between 5200 and 1800 cal. yrs BP.The diatom flora consisted of 66 taxa representing 20 genera but only seven taxa were abundant and, sometimes, these were monospecifically dominant during the Holocene. In the sediment core from Raffles Sø four successive stratigraphical zones can be distinguished. Accumulation of diatom valves began at 9900 cal. yrs BP with a Stephanodiscus minutulus (Kütz.) Cleve and Möller dominated assemblage (stratigraphic zone 1) followed by a diatom flora dominated by Cyclotella pseudostelligera Hustedt and, less frequently, by Diatoma tenuis Agardh (9400 until 5900 cal. yrs BP, zone 2). Cyclotella sp. A, a taxon which belongs to the Cyclotella rossii-comensis-tripartita-complex, was the dominant floral element between 5200 and 1800 cal. yrs BP (zone 3). From 1800 cal. yrs BP, the periphytic taxa Fragilaria capucina var. gracilis (Østr.) Hustedt and F. capucina var. rumpens (Kütz.) Lange-Bertalot attained highest relative abundances, also almost monospecifically (zone 4).The distribution and composition of the diatom assemblages in the sediment record from Raffles Sø probably reflect past variations in the extent of the lake-ice cover during the growing season. More or less ice-free conditions during summer may have prevailed during the early Holocene until ca. 1800 cal. yrs BP, which allowed growth of planktonic diatoms (Cyclotella taxa) in the pelagic lake region. From 1800 cal. yrs BP, colder conditions lead to a perennial lake-ice cover with a small ice-free moat in summer which favored the growth of periphytic, littoral species (Fragilaria capucina varieties).  相似文献   

5.
A consequence of predicted climate warming will be tree-line advance over large areas of the Russian tundra. Palaeolimnological techniques can be used to provide analogues of how such changes in tree-line advance and subsequent retreat affected lake ecosystems in the past. A Holocene sediment core taken from Kharinei Lake (Russia) was dated radiometrically and used for multi-proxy analyses with the aim of determining how climate and tree-line dynamics affected the productivity, community structure, carbon cycling and light regime in the lake. Pollen and macrofossil analyses were used to determine the dates of the arrival and retreat of birch and spruce forest. C:N ratios and percent loss-on-ignition were used to infer past changes in sediment organic matter. Visible-near-infrared spectroscopy and diatom analysis were used to infer past changes in lake-water carbon. Algal pigments and aquatic macrophytes were used to determine changes in lake productivity and light. Chironomids together with remains of the aquatic flora and fauna were used to provide information on past July temperature and continentality. Lake sedimentation was initiated shortly before 11,000 cal. years BP, when both chironomid- and pollen-inferred temperature reconstructions suggest higher summer temperatures than present, between 1 and 2°C warmer, and lake productivity was relatively high. A few trees were already present at this time. The spruce forest expanded at 8,000 cal. year BP remaining in the vicinity of the lake until 3,500 cal. year BP. This period coincided with a high concentration of organic material in the water column, and relatively high benthic productivity, as indicated by a high benthic: planktonic diatom ratio. After tree-line retreat, the optical transparency of the lake increased, and it became more open and exposed, and was thus subject to greater water-column mixing resulting in a higher abundance of diatom phytoplankton, especially heavily silicified Aulocoseira species. The colder climate resulted in a shorter ice-free period, the lake was less productive and there was a loss of aquatic macrophytes. Increased wind-induced mixing following forest retreat had a greater influence on the lake ecosystem than the effects of decreasing organic matter concentration and increased light penetration.  相似文献   

6.
黄旗海岩芯烧失量分析与冰后期环境演变   总被引：3，自引：1，他引：2       下载免费PDF全文
重建闭流型湖泊的水位及环境变化,可以为研究冰后期及全新世以来的气候变化及季风环流演变提供不可或缺的关键信息。对HQH4岩芯烧失量与川蔓藻化石种子的研究结果显示,黄旗海湖区在全新世到来之前的气候寒冷,湖面冰封期较长,夏季温度较低,致使湖泊的有机生产率和自生碳酸盐产量比全新世低很多。黄旗海从全新世早期起进入了稳定的湖泊发展阶段后有机生产率显著上升,自生碳酸盐产量同步大幅度增长。在10 200~6 800 a BP期间,烧失量出现显著波动,很大程度上反映了气候发生暖湿-冷干-暖湿-冷湿的变化。在6 800~3 800 a BP期间,烧失量较高而且波动较小,反映了中全新世黄旗海以稳定的温暖湿润气候为主,湖水水位变化不大,有机质和碳酸盐生产力较高。从3 800 a BP起,气候逐渐变凉,间有多次冷暖干湿的剧烈变化,水位波动频繁。  相似文献   

7.
Two sediment cores of 70 and 252 cm length were recovered from Hjort Sø, a small lake on Store Koldewey, Northeast Greenland, and studied with a multidisciplinary approach in order to reconstruct the local environmental history and to test the relevance of proxies for paleoenvironmental information. The basal sediments from the longer core are dominated by clastic matter, which was likely deposited during deglaciation of the lake basin. These clastic sediments are overlain by gyttja, which is also present throughout the shorter core. AMS radiocarbon dating was conducted on plant macrofossils of 11 samples from the gyttja in both cores. A reliable chronology was established for both cores, which dated the onset of organic accumulation at 9,500 cal. year BP. The Holocene temperature development, with an early to mid Holocene thermal maximum, is best reflected in the grain-size composition. Nutrient availability was apparently low during the early Holocene and led to low productivity in the lake and its vicinity. From ca. 7,000 cal. year BP, productivity in the lake increased significantly, probably induced by external nutrient input from goose excrements. From this time, micro- and macro-fossil remains reflect relatively well the climate history of East Greenland, with a cooling during the middle Holocene, the medieval warming, and the Little Ice Age. The amount of organic matter in the sequence seems to be more affected by lake ice cover or by nutrient supply from the catchment than by temperature changes. The record from Hjort Sø thus reveals the difficulties in interpreting sedimentary records from high arctic regions.  相似文献   

8.
《自然地理学》2013,34(5):438-456
Few long-term records of the fire history of Rocky Mountain National Park exist. Data from a lake sediment core was used to reconstruct changes in vegetation and fire frequencies over the last 7000 cal yr. Bear Lake is a high-elevation lake surrounded by subalpine vegetation in Rocky Mountain National Park, Colorado. Pollen data indicate that a warm and dry climate prevailed between ca. 7000 and 5000 cal yr BP. Temperatures increased until shortly before ca. 3500 cal yr BP when evidence for a marked decline is seen. Cooler-than-present conditions were maintained until ca. 1700 cal yr BP, when conditions transitioned to more like those of the present-day climate. Based on macroscopic charcoal analyses, fire frequency had varied between two and five episodes per 1000 years. The largest peak in charcoal was at ca. 590 cal yr BP. The fire return interval has varied with climate over time; however, we calculate a fire return interval of 325 years over the past 7000 years. Given these results, fire activity is likely to increase under current Intergovernmental Panel on Climate Change climate projections of an increase in annual temperatures.  相似文献   

9.
The lithology, radiocarbon chronology, granulometry, geochemistry and distribution of diatoms were investigated in three sediment cores from fresh-water Figurnoye Lake in the southern Bunger Hills, East Antarctica. Our paleolimnological data provide a record of Holocene environmental changes for this region. In the early Holocene (prior to 9.0 ± 0.5 kyr BP), warm climate conditions caused intensive melting of either the floating glacier ice mass or glaciers in the immediate lake surroundings, leading to the accumulation of terrigenous clastic sediments and limiting biogenic production in the lake. From ca. 9.0 ± 0.5 to 5.5 ± 0.5 kyr BP, highly biogenic sediments dominated by benthic mosses formed, indicating more distal glaciers or snowfields. A relatively cold and dry climate during this period caused weaker lake-water circulation and, likely, occurrence of lake ice conditions were more severe than present. The distribution of marine diatoms in the cores shows that, sometime between 8 and 5 kyr BP, limited amounts of marine water episodically penetrated to the lake, requiring a relative sea-level rise exceeding 10–11 m. During the last ca. 5.5 ± 0.5 kyr BP, sedimentation of mainly biogenic matter with a dominance of laminated microbial mats occurred in the lake under warm climatic conditions, interrupted by relative coolings: the first one around 2 kyr BP and then shortly before recent time. Between ca. 5.5 and 4 kyr BP, the drainage of numerous ice-dammed lakes took place in the southern Bunger Hills and, as a result, drier landscapes have existed here from about 4 kyr BP.  相似文献   

10.
Sediment cores recovered from Lago di Mezzano, central Italy, were petrographically and geochemically (dry densitity, total organic carbon) investigated. A floating chronology was established with sedimentation rates derived from varve thickness measurements, and this chronology was both supported and extrapolated with calibrated AMS-14C-datings. The profile has a length of 29.7 m and comprises a total of 34,000 years.Late Pleistocene sediments consist of minerogenic-organic mud with few benthic diatoms and an organic carbon content of 2%, thus suggesting a high allochthonous input. The onset of the Late Glacial at 14,580 cal BP is documented by a lithologic change to more organic-dominated sedimentation. The Younger Dryas cold event is recorded between 12,650 and 11,400 cal BP and exhibits higher dry densities and minerogenic input. These dates agree with records from other lacustrine archives in Europe and the Greenland ice cores.The early Holocene comprises a laminated organic diatom gyttja deposited at a time of climatic amelioration and increased primary productivity. The establishment of an anoxic hypolimnion enhanced the varve and organic matter preservation. Since 3700 varve years BP the sedimentation pattern has been strongly influenced by human impact, as documented by the increase in minerogenic sediments and turbidites as well as higher sedimention rates. The onset of this influence coincides with a Bronze Age settlement at the lake shore.  相似文献   

11.
The Holocene diatom and pollen records from Kelly’s Lough have been analysed to determine the timing and extent of the acidification in this upland lake. The pollen data during the early Holocene reflect the typical vegetation changes that occur in sediments throughout Ireland during this period. The diatom record begins by being dominated by circumneutral and acidophilous benthic forms. Later tychoplanktonic Aulacoseira species begin to expand and dominate indicating increased water transparency following the stabilization of catchment soils. Peatland development in the catchment is evident from approximately 6,450 cal year BP. The main change in the diatom assemblages at this time is the decline of Aulacoseira species and expansion of periphytic species. At around 1,450 cal year BP, loss-on-ignition (LOI) values, Calluna pollen and microscopic charcoal all increase suggesting the initiation of a major phase of peat erosion and an increased inwash of organic matter to the lake. Lake acidity changed significantly although the initial acidification is very subtle as indicated by the diatom-inferred pH record. Changes in the diatom assemblages might be largely the result of increasing erosion and inwash of organic matter from the catchment to the lake leading to reduced water transparency and more acidic conditions. The diatom flora remains relatively stable until the mid-twentieth century when more acidibiontic species increase. These diatom changes result in the reconstructed pH curve showing a moderate recent acidification from pH 5.7 to 5.1. About half of the total change in pH took place by around the late 1960s. The lowest diatom-inferred pH value occurs in the late 1970s, and parallels the peak in SO2 emissions in Ireland. Acidic conditions seem to have prevailed in Kelly’s Lough throughout its entire history and alkalinity has been low or absent for much of the time. However, soil acidification and inwash of organic acids from peatlands are not a sufficiently effective mechanism to explain the low pH levels found today in Kelly’s Lough. The effect of acid deposition on the waters of Kelly’s Lough is clear and it has probably caused these already naturally acid waters to acidify further.  相似文献   

12.
Permafrost in peatlands of subarctic Sweden is presently thawing at accelerated rates, which raises questions about the destiny of stored carbon and nutrients and impacts on adjacent freshwater ecosystems. In this study we use peat and lake sediment records from the Stordalen palsa mire in northern Sweden to address the late Holocene (5,000 cal BP-present) development of the mire as well as related changes in carbon and nutrient cycling. Formation, sediment accumulation and biogeochemistry of two studied lakes are suggested to be largely controlled by the development of the mire and its permafrost dynamics. Peat inception took place at ca. 4,700 cal BP as a result of terrestrialisation. Onset of organic sedimentation in the adjacent lakes occurred at ca. 3,400 and 2,650 cal BP in response to mire expansion and permafrost aggradation, respectively. Mire erosion, possibly due to permafrost decay, led to re-deposition of peat into one of the lakes after ca. 2,100 cal BP, and stimulated primary productivity in the other lake at ca. 1,900–1,800 cal BP. Carbonate precipitation appears to have been suppressed when acidic poor fen and bog (palsa) communities dominated the catchment mire, and permafrost-induced changes in hydrology may further have affected the inflow of alkaline water from the catchment. Elevated contents of biogenic silica and diatom pigments in lake sediments during periods of poor fen and bog expansion further indicate that terrestrial vegetation influenced the amount of nutrients entering the lake. Increased productivity in the lake likely caused bottom-water anoxia in the downstream lake and led to recycling of sediment phosphorous, bringing the lake into a state of self-sustained eutrophication during two centuries preceding the onset of twentieth century permafrost thaw. Our results give insight into nutrient and permafrost dynamics in a subarctic wetland and imply that continued permafrost decay and related vegetation changes towards minerotrophy may increase carbon and nutrient storage of mire deposits and reduce nutrient fluxes in runoff. Rapid permafrost degradation may on the other hand lead to widespread mire erosion and to relatively short periods of significantly increased nutrient loading in adjacent lakes.  相似文献   

13.
This study used organic matter in oligotrophic Lake Constance (southern Germany) to reconstruct lake environment and to disentangle the multiple factors, such as climate change and human impacts, which influence sedimentation in large lakes. A sediment core from Upper Lake Constance, which represents 16,000 years of Late Glacial and Holocene lake history, was analysed for organic biomarkers, hydrogen index and elements calcium, strontium, and magnesium. Magnetic susceptibility was measured to establish a high-resolution stratigraphic framework for the core and to obtain further information about changes with respect to relative allochthonous versus autochthonous sedimentation. Dinosterol—a biomarker for dinoflagellates—and calcium have low concentrations in Younger Dryas sediments and consistently high concentrations between 10,500 and 7,000 cal. years BP. These variations are attributed to changes in lake productivity, but are not reflected in the proportion of total organic carbon within the sediment. During the Younger Dryas and between 6,000 and 2,800 cal. years BP, concentrations and accumulation rates of land-plant-derived C29-steroids (β-sitosterol, stigmastanol and stigmasterol), in combination with a relatively low HI, indicate periods of enhanced terrigenous input to the lake. For the Younger Dryas, higher runoff can be attributed to a cold climate, leading to decreased vegetation cover and increased erosion. After 6,000 cal. years BP, high terrestrial input may be explained by enhanced precipitation. Biomarker and HI results, in combination with archaeological studies, raise the question as to whether lakeshore settlements affected sedimentation in Upper Lake Constance between 6,000 and 2,800 cal. years BP.  相似文献   

14.
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   

15.
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   

16.
Geochemical data obtained from X-ray fluorescence, physical properties, total organic and inorganic carbon content (TOC/TIC), and diatom analysis from a 6.61-m-long sedimentary sequence near the modern northern shore of Lake Zirahuen (101° 44′ W, 19° 26′ N, 2000 m asl) provide a reconstruction of lacustrine sedimentation during the last approximately 17 cal kyr BP. A time scale is based on ten AMS 14C dates and by tephra layers from Jorullo (AD 1759-1764) and Paricutin (AD 1943-1952) volcanoes. The multiproxy analyses presented in this study reveal abrupt changes in environmental and climatic conditions. The results are compared to the paleo-record from nearby Lake Patzcuaro. Dry conditions and low lake level are inferred in the late Pleistocene until ca. 15 cal kyr BP, followed by a slight but sustained increase in lake level, as well as a higher productivity, peaking at ca. 12.1 cal kyr BP. This interpretation is consistent with several regional climatic reconstructions in central Mexico, but it is in opposition to record from Lake Patzcuaro. A sediment hiatus bracketed between 12.1 and 7.2 cal kyr BP suggests a drop in lake level in response to a dry early Holocene. A deeper, more eutrophic and turbid lake is recorded after 7.2 cal kyr BP. Lake level at the coring site during the mid Holocene is considered the highest for the past 17 cal kyr BP. The emplacement of the La Magueyera lava flows (LMLF), dated by thermoluminiscence at 6560 ± 950 year, may have reduced basin volume and contributed to the relative deepening of the lake after 7.2 cal kyr BP. The late Holocene (after 3.9 cal kyr BP) climate is characterized by high instability. Extensive erosion, lower lake levels, dry conditions and pulses of high sediment influx due to high rainfall are inferred for this time. Further decrease in lake level and increased erosion are recorded after ca. AD 1050, at the peak of Purepechas occupation (AD 1300–1521), and until the eighteenth century. Few lacustrine records extend back to the late Pleistocene—early Holocene in central Mexico; this paper contributes to the understanding of late Pleistocene-Holocene paleoclimates in this region.  相似文献   

17.
Diatom species counts were conducted on 171 sediment samples from the 13-m-long core PG1351 from Lake El’gygytgyn, northeast Siberia. The planktonic Cyclotella ocellata-complex dominates the diatom assemblage through most of the core record, persisting through a variety of climate conditions. Periphytic diatoms, although less abundant, have greater diversity and greater down-core assemblage variation. During warm climate modes, longer summer ice-free conditions may have allowed more complex diatom communities to develop in shallow-water habitats, and enhanced circulation may have increased transport of these diatoms to deeper parts of the lake. Zones of low overall diatom abundance further support inferred intervals of low lake productivity during times of extended lake ice and snow cover. More data on the modern spatial and temporal distribution of diatom species in the Lake El’gygytgyn system will improve inferences from core records. This is the last in a series of eleven papers published in this␣special issue dedicated to initial studies of El'gygytgyn Crater Lake and its catchment in NE Russia. Julie Brigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue.  相似文献   

18.
Postglacial sediments from Lake Jues, located at the SW margin of the Harz Mountains, were investigated for pollen, diatoms and sediment characteristics. The paper focuses on the time period between 7600 and 1200 y cal. BP (calibrated years before present), during which human impact began to influence the environnment of the lake. Climate variability was mainly inferred from sediment characteristics and changes in algal assemblages. The observed climatic changes coincide well with those recorded from other sites. Neolithic settlement started during the warm and dry Atlantic period. Intensive land use occurred under favourable climatic conditions during the Bronze and the Iron Ages, while human activities declined during cool and wet periods around 4000 and 2700 y cal. BP and after 2300 y cal. BP. The study shows that climate strongly influenced human settlement at remote locations like Lake Jues.  相似文献   

19.
We present a paleolimnological record spanning the Holocene from a small lake on Russell Island (Lake PW02), in the central Canadian Arctic Archipelago (74.07° N, 97.77° W, 182 m asl). Fragilarioid diatom types in the genera Pseudostaurosira, Staurosira and Staurosirella constitute >90% of valves in fossil samples. Using modern biogeographic data which specify the temperature optima of the Fragilarioid diatom taxa, we present new inferences about the timing of paleoclimatic changes in the central Arctic islands. The early Holocene was characterized by maximum values for sediment organic matter, and lower ratios of Staurosirella pinnata to Staurosira construens v. venter, suggesting warm summer air temperatures between about 9500−6500 cal year BP. Influxes of biogenic silica and diatom valves decreased following 4000 cal year BP, the sediment accumulation rate slowed and diatom taxa of the littoral zone diversified, suggesting cooler summers and more persistent lake ice. Variations in the species composition of the assemblages indicate paleoclimatic changes that are in broad agreement with other paleoenvironmental records from the Arctic including melt records from the Agassiz Ice Cap. Although autecological data remain incomplete for Fragilarioid taxa, our results indicate differences in these taxa in responses to paleoenvironmental change and underline the potential for the increased use of these taxa in paleoenvironmental reconstructions. The record from Lake PW02, as in other records from Arctic lakes with low algal diversity throughout the Holocene, shows a pronounced increase in diatom diversity since the 1920s, and diatom production since the 1970s far exceeds any recorded during the Holocene.  相似文献   

20.
Analyses of lithology, organic-matter content, magnetic susceptibility, and pollen in a sediment core from Okpilak Lake, located in the northeastern Brooks Range, provide new insights into the history of climate, landscape processes, and vegetation in northern Alaska since 14,500?cal?year BP. The late-glacial interval (>11,600?cal?year BP) featured sparse vegetation cover and the erosion of minerogenic sediment into the lake from nearby hillslopes, as evidenced by Cyperaceae-dominated pollen assemblages and high magnetic susceptibility (MS) values. Betula expanded in the early Holocene (11,600?C8,500?cal?year BP), reducing mass wasting on the landscape, as reflected by lower MS. Holocene sediments contain a series of silt- and clay-dominated layers, and given their physical characteristics and the topographic setting of the lake on the braided outwash plain of the Okpilak River, the inorganic layers are interpreted as rapidly deposited fluvial sediments, likely associated with intervals of river aggradation, changes in channel planform, and periodic overbank flow via a channel that connects the river and lake. The episodes of fluvial dynamics and aggradation appear to have been related to regional environmental variability, including a period of glacial retreat during the early Holocene, as well as glacial advances in the middle Holocene (5,500?C5,200?cal?year BP) and during the Little Ice Age (500?C400?cal?year BP). The rapid deposition of multiple inorganic layers during the early Holocene, including thick layers at 10,900?C10,000 and 9,400?C9,200?cal?year BP, suggests that it was a particularly dynamic interval of fluvial activity and landscape change.  相似文献   

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