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1.
A record of vegetation dynamics and lake level changes from Lake Emakat,northern Tanzania,during the last c. 1200 years 总被引:1,自引:0,他引:1
Analyses of down-core variations in pollen and charcoal in two short cores of lake sediment and wood samples taken from the
in situ remains of Nuxia congesta from Lake Emakat, a hydrologically-closed volcanic crater lake occupying the Empakaai Crater in northern Tanzania, have generated
evidence of past vegetation change and lake level fluctuations. Eight AMS radiocarbon (14C) dates on bulk samples of lake sediment provide a chronological framework for the two cores and indicate that the sediment
record analysed incorporates the last c. 1200 years. The in situ remains of a Nuxia congesta tree, now standing in deep water, were dated with three additional AMS 14C dates, suggesting tree growth within the interval ∼1500–1670 AD. Down-core variations in pollen from terrestrial taxa, particularly
the montane forest trees Hagenia abyssinica and Nuxia congesta, indicate a broad period of generally more arid conditions in the catchment to c. 1200 AD and at a prolonged period between
c. 1420 and 1680 AD. Variations in pollen from plants in lake margin vegetation indicate low lake levels, presumably as a
result of reduced effective precipitation, contemporary with indications of relatively dry conditions mentioned above, but
also during the late 18th and the late 19th centuries. The presence of charcoal throughout both cores indicates the frequent
occurrence of vegetation fires. An increase in burning, evident in the charcoal data and dated to the early to mid second
millennium AD, could relate to an expansion of human population levels and agricultural activity in the region. 相似文献
2.
This paper raises fundamental questions about the sole use of paleolimnological techniques to identify sediment sources and
develop catchment management plans. The concept of an integrated lake: catchment framework was established 30 years ago, yet
paleolimnologists occasionally fail to appreciate the dynamics of the contributing catchment. This is especially critical
when the predominant source of sediment accumulating in a lake is allochthonous. In this paper we argue that a detailed appraisal
of catchment sources and investigation of historical documentary evidence is needed to identify and evaluate the relative
significance of sediment sources. We used such an approach at Aqualate Mere, Shropshire, UK. Mineral magnetic and radionculide
signatures of potential catchment sources and accumulating lake sediments were compared in an attempt to match the sources
to sediments deposited in the Mere. Dated lake sediments indicate there has been an increase in sedimentation rate and the
relative amount of minerogenic material delivered to the Mere over the last 200 years. In contrast to a previous study at
the same site, there is no evidence to attribute this increase to an overspill from a nearby canal. Other catchment disturbances
include landscaping in parkland surrounding the Mere in the early nineteenth century and drainage systems installed to improve
catchment agriculture over the last ca. 150 years. Both activities may explain the change in sedimentation rates and types,
independent of the hypothesized canal origin. Although our results exclude the canal as a major sediment source, identifying
the contribution of other potential catchment sources remains problematic. 137Cs inventories for the lake are similar to those recorded at a local reference site, suggesting little influx of 137Cs-bearing topsoil, yet 137Cs activities remain high in the upper 20–30 cm of the lake sediment profile, indicating a topsoil origin. Combined radionuclide
and mineral magnetic signatures proved to be relatively poor discriminators of potential sources, and the high atmospheric
pollution load from the West Midland conurbation has probably altered recent lake sediment signatures. Although further research
is required to identify the origins of recent (last ca. 200 years) minerogenic sediment inputs to the Mere, we suggest that
the combined lake: catchment approach offers a more rigorous method for understanding the impact of catchment disturbance
than analysis of the paleolimnological record alone. 相似文献
3.
Diatom assemblage response to Iroquoian and Euro-Canadian eutrophication of Crawford Lake,Ontario, Canada 总被引:2,自引:0,他引:2
Erik J. Ekdahl Jane L. Teranes Chad A. Wittkop Eugene F. Stoermer Euan D. Reavie John P. Smol 《Journal of Paleolimnology》2007,37(2):233-246
Diatom and geochemical data from Crawford Lake, Ontario, have been used to document limnological responses to periods of cultural
disturbance resulting from native Iroquoian occupation of the watershed (1268–1486 AD) and Euro-Canadian agriculture and deforestation
(1867 AD–present). Here, we further develop the high-resolution nature of the Crawford Lake sediment record to examine the
physical, chemical and biological aspects of limnological response to human disturbances in the lake catchment area with exceptional
detail. We report detailed diatom abundance and flux data for individual taxa from Crawford Lake, and further describe the
relationship between assemblage composition and environmental conditions using canonical correspondence analysis (CCA). Diatom
assemblage data are used to calculate diatom inferred-total phosphorus (DI-TP) concentrations for the past ∼1,000 years. We
also examine the diatom community response during and after periods of disturbance by Iroquoian and Euro-Canadian populations,
and compare this response to existing geochemical proxies of lake production and new elemental geochemical indicators of catchment
area erosion. In particular, we explore the differing limnological response to the two distinct periods of cultural eutrophication
and examine the limnological processes that occurred during the period of␣low (or no) human activity (1487–1866 AD), when
geochemical indicators of lake production recovered to pre-disturbance conditions, but diatom assemblages notably did not.
Our results illustrate the highly susceptible nature of diatom communities to periods of anthropogenic disturbance, and emphasize
that ecological indicators (such as diatom assemblages) should be included with other proxies (such as nutrient concentrations
and physical characteristics) when assessing disturbance and recovery in lake systems. 相似文献
4.
Kathleen R. Stoof-Leichsenring Annett Junginger Lydia A. Olaka Ralph Tiedemann Martin H. Trauth 《Journal of Paleolimnology》2011,45(3):353-367
Lake Naivasha, Kenya, is one of a number of freshwater lakes in the East African Rift System. Since the beginning of the twentieth
century, it has experienced greater anthropogenic influence as a result of increasingly intensive farming of coffee, tea,
flowers, and other horticultural crops within its catchment. The water-level history of Lake Naivasha over the past 200 years
was derived from a combination of instrumental records and sediment data. In this study, we analysed diatoms in a lake sediment
core to infer past lacustrine conductivity and total phosphorus concentrations. We also measured total nitrogen and carbon
concentrations in the sediments. Core chronology was established by 210Pb dating and covered a ~186-year history of natural (climatic) and human-induced environmental changes. Three stratigraphic
zones in the core were identified using diatom assemblages. There was a change from littoral/epiphytic diatoms such as Gomphonema gracile and Cymbella muelleri, which occurred during a prolonged dry period from ca. 1820 to 1896 AD, through a transition period, to the present planktonic Aulacoseira sp. that favors nutrient-rich waters. This marked change in the diatom assemblage was caused by climate change, and later
a strong anthropogenic overprint on the lake system. Increases in sediment accumulation rates since 1928, from 0.01 to 0.08 g cm−2 year−1 correlate with an increase in diatom-inferred total phosphorus concentrations since the beginning of the twentieth century.
The increase in phosphorus accumulation suggests increasing eutrophication of freshwater Lake Naivasha. This study identified
two major periods in the lake’s history: (1) the period from 1820 to 1950 AD, during which the lake was affected mainly by
natural climate variations, and (2) the period since 1950, during which the effects of anthropogenic activity overprinted
those of natural climate variation. 相似文献
5.
The Chilean Lake District (38–42°S) is strongly influenced by Southern westerlies-driven precipitations. At 40°S Lago Puyehue
provides high resolution sedimentation rates (∼1–2 mm/yr) suitable for annual climate reconstruction. Several short and long
sediment cores were collected in this lake. Their analysis aim at a better understanding of climate mechanisms related to
ENSO in this part of the world. The recognition of ENSO related periodicities and their stability is studied through the analysis
of two short varved cores collected from underflow and interflow key sites. According to varve chronology controlled by 137Cs and 210Pb profiles and chronostratigraphical markers, the short core from underflow site (PU-I) spans 294 ± 18 years and the core
in the interflow site (PU-II) covers 592 ± 9 years. Several methods of spectral analysis were applied on the total varve thickness
to identify potential periodicities in the signal. Blackman–Tuckey, Maximum Entropy, Multi-Taper Methods (MTM) and singular
spectrum analysis were applied on the whole record. In addition, evolutive MTM and wavelet analyses allow to identify temporal
influence of some periodicities. In the PU-I studied interval (AD 1700–2000), a period at ∼3.0 years appears in a large part
of the interval, mostly in the recent part. Periods at ∼5.2 and ∼23 years also show up. PU-II record (AD 1400–2000) displays
the most robust periodicities at around 15, 9, 4.4, 3.2 and 2.4 years. These periodicities are in good agreement with the
sub-decadal periods identified by Dean and Kemp (2004) and linked to the El Nino Southern Oscillation and the Pacific Decadal
Oscillation. Differences in the recorded periodicities between PU-I and PU-II sites are consistent with different sedimentation
processes in the lake. According to climate instrumental data for the last 20 years, varves in PU-I site are mostly related
to fluvial dynamics and regional climate factors, i.e., precipitation, temperature and wind. In PU-II site, varves increment
is related to both regional and global climate forcing factors, i.e., El Nino Southern Oscillation. The evolutive MTM analysis
and the wavelet analysis suggest a striking break in the periodicities at around AD 1820. Finally relationships between El
Nino and longer term climate phase like the Little Ice Age (LIA) are also assessed.
This is the seventh in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of
Lago Puyehue, Chilean Lake District. The papers in this special issue were collected by M. De Batist, N. Fagel, M.-F. Loutre
and E. Chapron. 相似文献
6.
Records of environmental and climatic changes during the late Holocene from Svalbard: palaeolimnology of Kongressvatnet 总被引:2,自引:0,他引:2
Piero Guilizzoni Aldo Marchetto Andrea Lami Achim Brauer Luigi Vigliotti Simona Musazzi Leonardo Langone Marina Manca Federico Lucchini Natale Calanchi Enrico Dinelli Alceo Mordenti 《Journal of Paleolimnology》2006,36(4):325-351
A multi-core, multidisciplinary palaeolimnological study of the partially varved sediment of a deep, meromictic, arctic lake, Kongressvatnet (Svalbard, Western Spitsbergen), provides a record of environmental and climatic changes during last ca. 1800 years. The chronology of sedimentation was established using several dating techniques (137Cs, 210Pb, varve counts, palaeomagnetic correlation). A multiproxy record of palaeolimnological variability was compiled based on sedimentation rates, magnetic properties, varve thickness, organic matter, geochemistry, pigments from algal and photosynthetic bacteria, mineralogy and biological assemblages (diatoms, Cladocera). The major features recognised in our master core K99-3 include a shift in sediment source and supply (magnetic measurements, geochemistry) probably caused by glaciological changes in the catchment around 38–32 cm core depth (AD 700–820). Additional environmental changes are inferred at 20–18, 8–4.5 and 3–2 cm (AD ca. 1160–1255; 1715–1880; 1940–1963, respectively). During the past ca. 120 years a prominent sedimentological change from brownish-grey, partly laminated silt-clay (varves) to black organic-rich deposits was observed. From AD 1350 to AD1880 the sediment is comprised of a continuous sequence of varves, whereas the earlier sediments are mostly homogeneous with only a few short intercalated laminated sections between AD 860 and 1350. Sedimentation and accumulation rates increased during the last 30 years (modern warming). Pigment concentrations are very low in the lower ca. 32 cm of the core (AD 820) probably because of the high turbidity high energy environment. The high sulphur content in the uppermost 32 cm of sediment has given rise to two horizontally stratified populations of sulphur anaerobic photosynthetic bacteria, as inferred from their specific carotenoids. These bacteria populations are much more abundant during the Little Ice Age (LIA) than during warmer periods (e.g., during the Medieval Warm Period and 20th century). Diatoms are lacking from the core base up to 18 cm (ca. AD 1255); at this level, species indicative of mesotrophic water are present, whereas from 17 cm to the top of the core, oligotrophic taxa such as Staurosira construens/S. pinnata complex dominate, indicating extended ice coverage and more oligotrophic waters during the LIA. The concentration of Cladocera subfossil remains (dominated by Chydorus) are relatively high in the deepest sections (54–32 cm), whereas the upper 32 cm are characterized by a very low concentration of remains, possibly because of the strongly anoxic conditions, and in this upper sediment section rotifer resting eggs become prevalent. We interpret these changes as responses to climate forcing through its impact on glacial melt water, lake ice cover duration and mainly redox conditions in deep water. The observed changes suggest that at least some of our recorded changes may parallel the Greenland Ice core, although our study added more details about the inferred climatic changes. Further aspects are discussed, such as catchment processes, glacial activity, duration of the Medieval Warm Period, the Little Ice Age, local human activity, and limnology. 相似文献
7.
Christoph Butz Martin Grosjean Tomasz Goslar Wojciech Tylmann 《Journal of Paleolimnology》2017,58(1):57-72
Hypoxia in freshwater systems is currently spreading globally and putting water quality, biodiversity and other ecosystem services at risk. Such adverse effects are of particular concern in permanently stratified meromictic lakes. Yet little is known about when and how meromixis and hypoxia became established (or vanished) prior to anthropogenic impacts, or how human activities such as deforestation, erosion and nutrient cycling affected the mixing regimes of lakes. We used calibrated hyperspectral imaging (HSI) data in the visible and near infrared range from a fresh, varved sediment core taken in Lake Jaczno, NE Poland, to map sedimentary pigments at very high resolution (sub-varve scale) over the past 1700 years. HSI-inferred bacteriopheophytin a (bphe a, produced by anoxygenic phototrophic bacteria) serves as a proxy for meromixis, whereas HSI-inferred green pigments (chlorophyll a and diagenetic products) can be used as estimators of aquatic productivity. Meromixis was established and vanished long before significant human disturbance in the catchment was observed in the late eleventh century AD. Under pre-anthropogenic conditions, however, meromixis was interrupted frequently, and the lake mixing regime flickered between dimixis and meromixis. During two periods with intense deforestation and soil erosion in the catchment, characterised by sedimentary facies rich in clay and charcoal (AD 1070–1255 and AD 1670–1710), the lake was mostly dimictic and better oxygenated than in periods with relative stability and a presumably closed forest around the lake, i.e. without human disturbances. After ca. AD 1960, meromixis became established quasi-permanently as a result of eutrophication. The persistent meromixis of the last ~60 years is unusual with respect to the record of the last 1700 years. 相似文献
8.
Charles E. UmbanhowarJr Philip Camill Jeffrey A. Dorale 《Journal of Paleolimnology》2011,45(2):151-166
Few studies have assessed the relative importance of landscape, land use history, climate, and regional heterogeneity on lake
ecosystem processes, despite the likelihood that interactions among these factors must be important for controlling lake dynamics.
We used 14 sediment measures from 20 lake cores in a climatically sensitive region of the prairie-forest border in southern
Minnesota to (1) assess relationships between modern lake productivity (Carlson’s Trophic State Index [TSI]), modern land-use,
catchment, and lake morphometry, and (2) contrast regional responses to climatic transitions from the ‘Medieval Climatic Anomaly’(1000–1350)
to the ‘Little Ice Age’ (1350–1800) to ‘Modern’(~1980–1996 AD). TSI was significantly positively correlated with modern sedimentation
rate, and accumulation rates of organic matter (OM), biogenic silica (BSi), and total phosphorus (TP). TSI was not significantly
correlated with “modern” land cover, catchment, or lake morphometry characteristics, but total organic N(N) was negatively
correlated with percent cultivation in the catchment area and negatively correlated with δ15N of bulk organic matter in sediment. Regional, among-lake heterogeneity was high over the past 1,000 years, but Little Ice
Age (LIA) cooling appeared to result in an approximately 20% decline in OM, BSi and TP accumulation, while warming and cultural
eutrophication of the past 150 years corresponded to a 200–400% increase in accumulation rates as well as an 80% increase
in carbonates and a small but significant 10% drop in C/N ratios consistent with greater in-lake productivity. Our results
indicate that climate does have regional effects on lake ecosystems but that among-lake variability is high, reflecting the
importance of local factors and suggesting a need for (1) more explicit definition of what ‘regional’ means, (2) a focus on
degree as well as direction of change, and (3) estimating accumulation rates based on multiple lakes and multiple cores within
lakes. 相似文献
9.
Diatoms reveal complex spatial and temporal patterns of recent limnological change in West Greenland
Bianca B. Perren Marianne S. V. Douglas N. John Anderson 《Journal of Paleolimnology》2009,42(2):233-247
Diatoms in sediment cores were analysed across a range of stratigraphic resolutions along a transect of 23 lakes spanning
the ice-free margin of the west coast of Greenland (~67°N), to explore spatial and temporal patterns of recent (last ~150 years)
environmental change in the region. These records display heterogeneous lake development trajectories over the last several
centuries. Estimates of species composition turnover (beta-diversity) since 1850 AD are among the lowest for lakes in the
Arctic, and are comparable to “unimpacted” reference lakes from temperate regions. Most of the change that occurred in West
Greenland lakes pre-dates potential industrial anthropogenic effects, while post-1850 change is well within the natural range
of variability for these systems. Nonetheless, a spatial pattern in core “top–bottom” changes is apparent across the transect:
lakes in the arid interior of the region, adjacent to the ice sheet, and those with higher pH, register greater change than
those in the more maritime climate of the coast. This suggests that climate plays an indirect role in the recent development
of these lakes, and that recent anthropogenic forcing has not yet exceeded major ecological thresholds in this region. 相似文献
10.
This paper investigates the correlations between lake level change, rainfall variability and general atmospheric forcing in
southern Africa. The analysis of fossil diatom assemblages in a sediment sequence from the small, rain-fed Lake Nhaucati,
southern Mozambique, is presented and discussed in relation to regional palaeoclimate data. The accumulation of organic sediments
in Lake Nhaucati began 1,600 years ago when the lake level was rising. Lithology and pollen suggest a low stand at 800 AD,
which correlates with other climate proxies from the summer rainfall region of southern Africa. The diatom assemblage suggests
that lake levels were high between 900 and 1300 AD, with shorter low stands at c.1100 and 1200 AD. The period after 1400 AD
was marked by a slow rate of accumulation and consequently a low temporal resolution. The correlation with other climate proxies
in the summer rainfall region, written sources, and pollen data suggests repeated droughts corresponding to the Little Ice
Age, though the driest periods may have caused complete desiccation of the lake. Higher lake levels are suggested after 1800 AD,
though written sources suggest droughts in the beginning of the twentieth century. The analysis shows a good correlation with
palaeoclimate data from the summer rainfall region and confirms the presence of an anti-phase relationship between the summer
rainfall region of southern Africa and the bi-modal rainfall region of east tropical Africa. It also supports the general
hypothesis that variation in the intensity of the Inter Tropical Convergence Zone is the main agent modulating rainfall over
southern and eastern Africa on centennial timescales. 相似文献
11.
Lourdes L��pez-Merino Ana Moreno Manel Leira Javier Sigr�� Pen��lope Gonz��lez-Samp��riz Blas L. Valero-Garc��s Jos�� Antonio L��pez-S��ez Manola Brunet Enric Aguilar 《Journal of Paleolimnology》2011,46(3):453-467
We present a study of two short sediment cores recovered from Lago Enol, in the Picos de Europa National Park, Cantabrian Mountains, northern Iberia. We inferred past climate conditions and anthropogenic impacts using geochemical and biological (pollen and diatoms) variables in the dated sequences, in conjunction with temperature and precipitation data collected since 1871 at meteorological stations in the region. The record provides evidence of environmental changes during the last 200 years. At the end of the Little Ice Age (~1800?C1875 AD) the region was characterized by an open landscape. Long-term use of the area for mixed livestock grazing in the mountains, and cultivation of rye during the nineteenth century, contributed to the expansion of grassland at the expense of forest. Warmer temperatures since the end of the nineteenth century are inferred from a change in diatom assemblages and development of the local forest. Socioeconomic transformation during the twentieth century, such as livestock changes related to dairy specialization, planting of non-native trees, mining activities, and management of the national park since its creation in 1918, caused profound changes in the catchment and in the lake ecology. The last several decades (~1970?C2007 AD) of the Lago Enol sediment record are strikingly different from previous periods, indicating lower runoff and increasing lake productivity, particularly since AD 2000. Today, the large number of tourists who visit the area cause substantial impacts on this ecosystem. 相似文献
12.
通过对腾格里沙漠东南部昂格尔图湖岩芯AGE15A的粒度、碳酸盐的百分含量和主要化学元素等多项沉积学气候代用指标及精确定年的综合分析,重建了该区域988 AD以来的古气候变化序列。结果表明:988 AD以来的研究区的气候环境的演化过程与我国西北西风区古气候变化基本一致,呈现"冷湿-暖干"交替变化的特征,但在时间上存在一定的延迟。具体表现为:988~1383 AD,昂格尔图湖由洼地发育成小型湖泊,化学风化作用弱,湖区气候环境整体偏冷湿,期间出现几次暖波动;1383~1560 AD,研究区处在中世纪暖期,为温暖、干旱的气候特征,此时湖泊扩张,湖中水生植物生长茂盛,呼吸作用增强,造成大量的碳酸盐沉淀,最高值达到8.16%;1560~1700 AD,致使气候在昂格尔图湖形成之后出现最寒冷的时期,降水增加,气候湿冷,此时段与小冰期最盛期相对应;1700 AD之后,温度开始上升,气候回暖,在1900 AD左右达到最甚,此时沉积物中碳酸盐的百分含量也达到最大值,为10.15%,此期间湖泊继续扩张,出现几次气候冷波动。基于我国古代文献重建的历史时期气候变化基本不包括沙漠地区,所以本文利用湖相沉积记录来反演腾格里沙漠昂格尔图湖历史时期的气候环境演变,对中国沙漠研究有着重要参考价值,同时也为中国西北历史时期的气候重建提供科学依据。 相似文献
13.
L. del Puerto F. García-Rodríguez H. Inda R. Bracco C. Castiñeira J. B. Adams 《Journal of Paleolimnology》2006,36(2):151-163
Paleolimnological data are presented on Holocene climatic changes in Lake Blanca, a small (0.6 km2), coastal fresh waterbody in southern Uruguay. Using a sediment core that extended to 7,300 year BP, analyses of pooid, panicoid and chloridoid phytoliths (Poaceae) allowed us to identify 16 Opal Phytoliths Association Zones (OPAZ). Both temperature and humidity indices were calculated from relative percentages of opal phytoliths and the climatic trends were inferred. During the early Holocene, prior to the first Holocene marine transgression (∼
∼8,000–7,000 yr BP), cool-arid climatic conditions were inferred. After the first Holocene transgression, predominance of cool-humid conditions was registered until about 4,000 yr BP. A transition from cool-semiarid/arid to alternate warm-temperate and humid-very-humid conditions was detected for ∼∼4,000–1,000 yr BP. Increases in trophic state were observed because of such a climatic amelioration. After ∼
∼1,000 yr BP, a shift from warm to temperate and cool temperatures was inferred. Such a cooling episode was assigned to a “LIA” which took place by the end of the 18th century and the beginning of the 19th century. A decrease in trophic state was observed during this climatic deterioration. After such a little ice age, present temperate humid conditions were established. The humidity indices calculated from relative percentages of opal phytoliths for the last ∼
∼70 years were compared to annual rainfall data. Such a comparison showed that opal phytoliths are reliable indicators of humidity. 相似文献
14.
Fahu Chen Jianbao Liu Qinghai Xu Yuecong Li Jianhui Chen Haitao Wei Qingsong Liu Zongli Wang Xianyong Cao Shengrui Zhang 《Journal of Paleolimnology》2013,49(3):447-464
Environmental magnetic studies were conducted on a 9.42-m-long sediment core from Gonghai Lake, North China. Radiocarbon dating indicates that the record spans the last 15,000 cal year BP. The principal magnetic mineral in the sediments is pseudo-single domain magnetite of detrital origin with minimal post-depositional alteration. Although the variations in the concentration of detrital magnetic minerals and their grain size throughout the core reflect inputs from both soil erosion and eolian dust, it is shown that their climatic and environmental significance changes with time. In the lowermost part of the core, ~15,000–11,500 cal year BP, the magnetic minerals were supplied mainly by bedrock erosion, soil erosion and dust input when climate ameliorated after the cold and dusty last glacial maximum. The increasing magnetic susceptibility (χ) in this interval may indicate a combination of changes in the lake environment together with catchment-surface stabilization and a decreasing proportion of dust input. In the central part of the core, ~11,500–1,000 cal year BP, the detrital magnetic minerals mainly originated from dust inputs from outside the catchment when the lake catchment was covered by forest, and catchment-derived sediment supply (and thus the lake sediment accumulation rate) were minimal. The generally low concentration of magnetic minerals in this part of the core reflects the highest degree of soil stability and the strongest summer monsoon during the Holocene. In the uppermost part of the core, the last ~1,000 years, detrital magnetic minerals mainly originated from erosion of catchment soils when the vegetation cover was sparse and the sediment accumulation rates were high. Within this part of the core the high magnetic susceptibility reflects strong pedogenesis in the lake catchment, and thus a strong summer monsoon. This scenario is similar to that recorded in loess profiles. Overall, the results document three main stages of summer monsoon history with abrupt shifts from one stage to another: an increasing and variable summer monsoon during the last deglacial, a generally strong summer monsoon in the early and middle Holocene and a weak summer monsoon in the late Holocene. The results also suggest that different interpretational models may need to be applied to lake sediment magnetic mineral assemblages corresponding to different stages of environmental evolution. 相似文献
15.
A combination of biotic, sedimentary and biogeochemical proxies was used to investigate the timing and causes of post-18th
century changes in the stratigraphic record of a large, deep lake on the Boothia Peninsula, Nunavut, Canada (70°15′ N, 94°30′ W).
A varve chronology verified with radioisotopic dating (210Pb and 137Cs) revealed a complex pattern of environmental dynamics since c. AD 1830. An increase in the diatoms Asterionella formosa, Stephanodiscus minutulus and Cyclotella atomus and a decrease in Aulacoseira taxa in the uppermost centimetre of sediment suggested that environmental conditions have favoured the growth of smaller
and/or lighter planktonic species since the 1980s. Longer term changes in some benthic species, the chrysophyte cysts to diatom
valve ratio, %C, and C/N ratios suggest declined river inflow and a relative reduction in allochthonous inputs during the
last century. Higher than average δ15N values in the late 19th to early 20th centuries coincide with changes in bulk carbon and nitrogen profiles, and below average
values since approximately 1950 may be associated with increased atmospheric N loading or reduced productivity. Biogenic silica
and organic carbon accumulation in the sediments suggest a possible decline in lake production during the 20th century that
may be associated with changes in the river discharge regime. The short and long-term ecological and biogeochemical trends
were also reflected in the sedimentary structure through declining varve thickness for the duration of the record and an abrupt
change in sedimentology in the uppermost 1 cm, coinciding with deposition since ca. AD 1987. Together, these biological and
physical changes suggest changes in hydroclimatic conditions in the 20th century, and an increase in planktonic diatom taxa
since the 1980s that coincides with a distinct period of climate warming. 相似文献
16.
Charline Giguet-Covex Fabien Arnaud Jérôme Poulenard Dirk Enters Jean-Louis Reyss Laurent Millet Jérome Lazzaroto Olivier Vidal 《Journal of Paleolimnology》2010,43(1):171-190
Sedimentological, geochemical and particle-size analyses were used to reconstruct the evolution of both trophic state and
hypolimnetic anoxia in Lake Bourget (French Alps) during the last century. Radionuclide dating (210Pb, 137Cs and 241Am) confirmed the annual rhythm of laminations in the upper sediment profile. In Lake Bourget, biochemical varves are triplets
composed of a diatom layer (spring lamina), a bio-precipitated calcite-rich layer (spring/summer lamina), and a layer rich
in organic matter and detrital particles (winter lamina). The onset of eutrophication and the first appearance of an anoxic
facies occurred simultaneously and were dated by laminae counting to AD 1943±1 year. Persistent anoxic conditions began in
AD 1960. Eutrophication is characterised by drastic increases in the flux of biogenic silica (mostly diatoms), lacustrine
organic matter, and larger calcite crystals (15–30 μm). The increase of organic matter also represents a marker of the onset
of anoxic conditions in the hypolimnion. Our results show that eutrophication was the main factor controlling anoxia in the
hypolimnion. This eutrophication was caused mostly by the inflow of untreated sewage effluents, and to a lesser extent, by
input of fertilizer-derived phosphorus during floods of the Rhone River and run-off from the lake catchment. The Rhone River,
however, can also be a source of re-oxygenation via underflows that originate during flood events. Oxygenation of the hypolimnion
is also controlled by low winter temperatures, which enable turnover of the lake. Thus, global warming, associated with a
forecasted reduction in precipitation, might reduce the efficiency of hypolimnetic re-oxygenation in Lake Bourget. 相似文献
17.
Simon M. Hutchinson 《Journal of Paleolimnology》1994,10(3):169-180
Levels of Chernobyl radiocaesium found in the upland region of Galloway, south west Scotland were relatively high e.g. catchment mean134Cs inventories ranged from 5.72±2.64 kBq m2 to 8.72±1.46 kBq m2. The nuclear weapons test137Cs content of soils and peats was significantly augmented. Accurate assessment of spatial variations and temporal changes in the levels of contamination, however, are inhibited by its marked, short-range heterogeneity. No significant change in mean catchment inventories was detected over a twelve month period. Lake sediment inventories, however, were consistently enhanced from initially lower levels of contamination than their catchments' surfaces. Pre Chernobyl catchment and lake sediment137Cs inventories, however, show no such contrast. Both are substantially lower than rainfall based predictions. These observations of the relative levels of Chernobyl and pre-existing radiocaesium in watersheds and their waterbodies have important implications for both the assessment of the catchment and lake residence times of particle-associated contaminants and the use of lake sediment records as monitors of such pollutants. 相似文献
18.
Reconceptualising coarse sediment delivery problems in rivers as catchment-scale and diffuse 总被引:1,自引:0,他引:1
This paper assesses river channel management activities in the context of the interaction between coarse sediment delivery, climate change, river channel response and flood risk. It uses two main sources of evidence: (1) an intensive instrumentation of an upland river catchment using both traditional hydrometric and novel sediment sensing methods; and (2) a sediment delivery model that combines a treatment of sediment generation from mass failure with a treatment of the connectivity of this failed material to the drainage network. The field instrumentation suggests that the precipitation events that deliver sediment from hillslopes to the drainage network are different to those that transfer sediment within the network itself. Extreme events, that could occur at any time in the year (i.e. they are not dependent on wet antecedent conditions), were crucial for sediment delivery. However, sustained high river flows were responsible for the majority of transfer within the river itself. Application of three downscaling methods to climate model predictions for the 2050s and 2080s suggested a significant increase in the number and potential volume of delivery events by the 2050s, regardless of the climate downscaling scenario used. First approximations suggested that this would translate into annual bed level aggradation rates of between 0.10 and 0.20 m per year in the downstream main channel reaches. Second, the importance of this delivery for flood risk studies was confirmed by simulating the effects of 16 months of measured in-channel simulation with river flows scaled for climate change to the 2050s and 2080s. Short-term sedimentation could result in similar magnitude increases in inundated area for 1 in 0.5 and 1 in 2 year floods to those predicted for the 2050s in relation to increases in flow magnitude. Finally, we were able to develop an alternative approach to river management in relation to coarse sediment delivery, based upon reducing the rates of coarse sediment delivery through highly localised woodland planting, under the assumption that reducing delivery rates should reduce the rate of channel migration and hence the magnitude of the bank erosion problem. Thus, the paper demonstrates the need to conceptualise local river management problems in upland river environments as point scale manifestations of a diffuse sediment delivery process, with a much more explicit focus on the catchment scale, if our river systems are to become more insulated from the impacts of future climate changes. 相似文献
19.
Recent Environmental Change and Atmospheric Contamination on Svalbard as Recorded in Lake Sediments – Synthesis and General Conclusions 总被引:3,自引:2,他引:3
The major patterns of biostratigraphical and geochemical change detected in a multidisciplinary study on recent environmental change and atmospheric contamination on Svalbard are summarised and synthesised. The patterns discussed are changes in sediment accumulation rates, organic matter accumulation rates, atmospheric contaminants, and biological assemblages (diatoms, chrysophyte cysts, chironomids). Possible environmental factors that may have influenced these patterns are discussed, in particular, the role of atmospheric contamination (including the deposition of nitrogen-compounds), local human impact, and recent climatic change. The major conclusions are (1) sediment accumulation rates show consistent temporal and geographical patterns with rates increasing towards the present-day and highest in the south, (2) sediment organic-matter accumulation rates increase markedly in the last 50–100 years, (3) atmospheric contamination is a combination of local and regional sources, (4) sediment inorganic geochemistry suggests catchment and lake responses to climate change in the last 30–50 years, (5) all lakes show a marked increase in the rate of biotic compositional changes in the last 50–100 years, and (6) Svalbard lakes appear to be highly dynamic and show considerable biotic and sedimentary changes in recent decades. The most likely cause of many of the observed changes is recent climatic change, with some local human activity at one site. Detailed interpretation of the observed changes is problematic given current limited knowledge about high Arctic limnology, biology, and catchment processes. 相似文献
20.
Sediment fingerprinting as an environmental forensics tool explaining cyanobacteria blooms in lakes 总被引:1,自引:0,他引:1
Cyanobacteria (blue-green algae) blooms in water bodies present serious public health issues with attendant economic and ecological impacts. Llyn Tegid (Lake Bala) is an important conservation and amenity asset within Snowdonia National Park, Wales which since the mid-1990s has experienced multiple toxic cyanobacteria blooms threatening the ecology and tourism-dependent local economy. Multiple working hypotheses explain the emergence of this problem, including climate change, land management linked to increased nutrient flux, hydromorphological alterations or changing trophic structure - any of which may operate individually or cumulatively to impair lake function. This paper reports the findings of a sediment fingerprinting study using dated lake cores to explore the linkages between catchment and lake management practices and the emergence of the algal blooms problem. Since 1900 AD lake bed sedimentation rates have varied from 0.06 to 1.07 g cm−2 yr−1, with a pronounced acceleration since the early 1980s. Geochemical analysis revealed increases in the concentrations of total phosphorus (TP), calcium and heavy metals such as zinc and lead consistent with eutrophication and a rising pollution burden, particularly since the late 1970s. An uncertainty-inclusive sediment fingerprinting approach was used to apportion the relative fluxes from the major catchment land cover types of improved pasture, rough grazing, forestry and channel banks. This showed improved pasture and channel banks are the dominant diffuse sources of sediment in the catchment, though forestry sources were important historically. Conversion of rough grazing to improved grassland, coupled with intensified land management and year-round livestock grazing, is concluded to provide the principal source of rising TP levels. Lake Habitat Survey and particle size analysis of lake cores demonstrate the hydromorphological impact of the River Dee Regulation Scheme, which controls water level and periodically diverts flow into Llyn Tegid from the adjacent Afon Tryweryn catchment. This hydromorphological impact has also been most pronounced since the late 1970s. It is concluded that an integrated approach combining land management to reduce agricultural runoff allied to improved water level regulation enabling recovery of littoral macrophytes offers the greatest chance halting the on-going cyanobacteria issue in Llyn Tegid. 相似文献