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1.
N. Kazancı S. A. G. Leroy S. Öncel Ö. İleri Ö. Toprak P. Costa S. Sayılı C. Turgut M. Kibar 《Journal of Paleolimnology》2010,43(1):89-110
Freshwater Lake Ulubat (z
mean = 1.5–2.0 m and Area = ~138 km2), NW Anatolia, Turkey was filled in by fine-to-medium-grain silts during the late Holocene. Deposition in Lake Ulubat has
been 1.6 cm year−1 for the last 50 years, but the sedimentation rate over the last ~1,600 years was lower (0.37 mm year−1). The organic matter and carbonate contents of the infill show cyclic changes that reflect environmental fluctuations. The
silt-dominated lithology and the vertically uniform heavy metal distributions are probably due to wind-controlled sedimentation
in the lake. Heterogeneous mud, derived from a large, mountainous drainage basin, is deposited in the lake mostly during summer,
June to October, when conditions are hot and calm. Winter months are stormier and sediments are re-suspended due to the shallow
water depth and the effect of waves on the lake bottom. It is likely that re-suspended sediments, particularly fine-grained
particles, together with the heavy metals, are transported out of the lake via the outlet, especially during periods of high
lake level. This resuspension and removal process probably caused the lake sediments to become silt-dominated and depleted
in heavy metals. The role of broad shallow lakes in sequestering sediments and heavy metals can be described more accurately
when wind data are considered. Such information may also be helpful for land-use planning in downstream areas. 相似文献
2.
Fasong Yuan Yongwei Sheng Tandong Yao Chaojun Fan Junli Li Hui Zhao Yanbin Lei 《Journal of Paleolimnology》2011,46(2):291-307
Stable isotopes (δ18O and δD) are useful tracers for investigating hydrologic and climatic variability on a variety of temporal and spatial scales.
Since the early isotopic studies on mountainous glaciers in the late 1960s, a great deal of information has been generated
on the isotopic composition of rainfall, snow, ice, surface waters, and lake carbonate sediments across the Tibetan Plateau.
However, measurements of δ18O and δD values of lake water are scarce. Here we present a new dataset of δ18O and δD values of lake waters collected from 27 lakes across the plateau during a reconnaissance survey in summer 2009. δ18O and δD values of lake water range from −19.9 to 6.6‰ and from −153 to −16‰, respectively. The average values of δ18O and δD are −6.4 and −72‰, considerably greater than those of precipitation observed in this region. The derived Tibetan
lake water line, δD = 5.2δ18O − 38.9, is significantly different from the global meteoric water line. Most of the lakes, including some freshwater lakes,
contain water with negative values of d-excess (d). There is a negative correlation between d and total dissolved solids (TDS). Each of these findings indicates that evaporation-induced isotopic enrichment prevails
in Tibetan lakes. Moreover, we develop an isotope modeling scheme to calculate E/P ratios for Tibetan lakes, using a combination
of existing isotopic fractionation equations and the Rayleigh distillation model. We use the intersection of the local evaporation
line and GMWL as a first approximation of δ18O and δD values of lake water inputs to infer an E/P ratio for each lake. Our modeling calculations reveal that although variable
from lake to lake, the water budget across the plateau is positive, with an average E/P of 0.52. This is in good agreement
with other observational and model data that show varying degrees of increases in lake size from satellite imagery and significant
decreases in lake salinity in many lakes on the plateau over the last several decades. Together with the new isotopic dataset,
the proposed modeling framework can be used to examine and quantify past changes in a lake’s hydrologic balance from the isotopic
record of downcore carbonate sediments in the region. 相似文献
3.
James T. Teller Zhirong Yang Matthew Boyd William M. Buhay Kyle McMillan Hedy J. Kling Alice M. Telka 《Journal of Paleolimnology》2008,40(2):661-688
West Hawk Lake (WHL) is located within the glacial Lake Agassiz basin, 140 km east of Winnipeg, Manitoba. The small lake lies
in a deep, steep-sided, meteorite impact crater, which has been partly filled by 60 m of sediment that today forms a flat
floor in the central part of the basin below 111 m of water. Four cores, 5–11 m in length, were collected using a Kullenberg
piston gravity corer. All sediment is clay, contains no unconformities, and has low organic content in all but the upper meter.
Sample analyses include bulk and clay mineralogy, major and minor elements, TOC, stable isotopes of C, N, and O, pollen, charcoal,
diatoms, and floral and faunal macrofossils. The sequence is divided into four units based mainly on thickness and style of
lamination, diatoms, and pollen. AMS radiocarbon dates do not provide a clear indication of age in the postglacial sequence;
possible explanations include contamination by older organic inwash and downward movement of younger organic acids. A chronological
framework was established using only selected AMS dates on plant macrofossils, combined with correlations to dated events
outside the basin and paleotopographic reconstructions of Lake Agassiz. The 822 1-cm-thick varves in the lower 8 m of the
cored WHL sequence were deposited just prior to 10,000 cal years BP (∼8,900 14C years BP), during the glacial Lake Agassiz phase of the lake. The disappearance of dolomite near the top of the varved sequence
reflects the reduced influence of Lake Agassiz and the carbonate bedrock and glacial sediment in its catchment. The lowermost
varves are barren of organisms, indicating cold and turbid glacial lake waters, but the presence of benthic and planktonic
algae in the upper 520 varves indicates warming; this lake phase coincides with a change in clay mineralogy, δ18O and δ13C in cellulose, and in some other parameters. This change may have resulted from a major drawdown in Lake Agassiz when its
overflow switched from northwest to east after formation of the Upper Campbell beach of that lake 9,300–9,400 14C years ago. The end of thick varve deposition at ∼10,000 cal years BP is related to the opening of a lower eastern outlet
of Lake Agassiz and an accompanying drop in West Hawk Lake level. WHL became independent from Lake Agassiz at this time, sedimentation
rates dropped, and only ∼2.5 m of sediment was deposited in the next 10,000 years. During the first two centuries of post-Lake
Agassiz history, there were anomalies in the diatom assemblage, stable O and C isotopes, magnetic susceptibility, and other
parameters, reflecting an unstable watershed. Modern oligotrophic conditions were soon established; charcoal abundance increased
in response to the reduced distance to the shoreline and to warmer conditions. Regional warming after ∼9,500 cal years BP
is indicated by pollen and diatoms as well as C and O isotope values. Relatively dry conditions are suggested by a rise in
pine and decrease in spruce and other vegetation types between 9,500 and 5,000 cal years BP (∼8,500–4,400 14C years BP), plus a decrease in δ13Ccell values. After this, there was a shift to slightly cooler and wetter conditions. A large increase in organic content and change
in elemental concentration in the past several thousand years probably reflects a decline in supply of mineral detritus to
the basin and possibly an increase in productivity. 相似文献
4.
Robert G. Hatfield Barbara A. Maher Jacqueline M. Pates Philip A. Barker 《Journal of Paleolimnology》2008,40(4):1143-1158
We examine sediment dynamics in an upland, temperate lake system, Lake Bassenthwaite (NW England), in the context of changing
climate and land use, using magnetic and physical core properties. Dating and analysis of the sedimentary records of nine
recovered cores identify spatially variable sedimentation rates across the deep lake basin. Mineral magnetic techniques, supported
by independent geochemical analyses, identify significant variations both in sediment source and flux over the last ∼2100 years.
Between ∼100 years BC and ∼1700 AD, sediment fluxes to the lake were low and dominated by material sourced from within the
River Derwent sub-catchment (providing 80% of the hydraulic load at the present day). Post-1700 AD, the lake sediments became
dominantly sourced from Newlands Beck (presently providing ∼10% of the lake’s hydraulic load). Three successive, major pulses
of erosion and increased sediment flux appear linked to specific activities within the catchment, specifically: mining activities
and associated deforestation in the mid-late nineteenth century; agricultural intensification in the mid-twentieth century
and, within the last decade, the additional possible impact of climate change. These results are important for all upland
areas as modifications in climate become progressively superimposed upon the effects of previous and/or ongoing anthropogenic
catchment disturbance. 相似文献
5.
Modern assessment and monitoring of aquatic ecosystems is increasingly based on biota and the “reference condition” approach,
in which the observed values (O) of biological variables are compared to those expected in the absence of human disturbance (E). To use this approach, correct estimation and validation of reference conditions are critical. Because appropriate modern
or historical data are never available for this approach, palaeolimnological data offer an alternative. We used a calibration
data set from 73 profundal sites in semi-pristine Finnish lakes to construct a regression model for estimating expected values
for the chironomid Benthic Quality Index (BQI)—a macroinvertebrate metric widely used in bioassessment—from environmental
variables that are insensitive to human disturbance. For comparison, reference values were estimated using the European legislative
rationale based on a priori lake typology. Performance of the alternative approaches was assessed by internal ‘leave-one-out’
cross-validation using the calibration set and by external cross-validation using independent palaeolimnological data on BQI
values representing the historical pristine status of 24 lake basins. Additionally, for 19 of these sites, which vary in their
degree of human impact, the ratio of present BQI to that in pristine condition, which shows the degree of actual change, if
any, was calculated from palaeolimnological data and compared with the O/E ratios based on the present chironomid data and estimated E. A linear regression model with mean depth and mean/maximum depth ratio as independent variables estimated the reference
values of BQI much closer to the observed ones (r
2 = 0.58, RMSEP = 0.65 and r
2 = 0.71 RMSEP = 0.55; for internal and external cross-validation, respectively) than did the typology approach (r
2 = 0.28, RMSEP = 0.86; r
2 = 0.10, RMSEP = 0.97). The regression approach also yielded O/E ratios more similar to the actual ones (r
2 = 0.79, RMSEP = 0.09) than did the typology approach (r
2 = 0.62, RMSEP = 0.23). Our results strongly support the use of lake morphometric variables and modelling instead of categorical
lake typology for the establishment of reference conditions for profundal macroinvertebrate communities and demonstrate the
utility of palaeolimnological data in the validation of reference values and assessment methods. 相似文献
6.
Lake sediments can be significantly impacted by industrial activities. These impacts vary among sites and include both local
point sources and atmospherically-derived pollution. Here we present results of a lake sediment pollution record from Ullswater,
UK, where lead mining activities have taken place within the catchment since 1690, although large-scale mining did not begin
until 1840. Metal concentration data from 12 cores taken along a lake bed transect illustrate that lead mining at Greenside
Mine had a significant impact on the lake sediments. High Pb concentrations were identified throughout Ullswater, and exhibited
a spatial gradient from south to north, with concentrations decreasing with greater distance from the main source of input
at Glenridding. Furthermore, inter-element correlations exhibit spatial variation that reflects the processes by which they
are incorporated into the lake sediment record. Together, these observations illustrate potential shortcomings in palaeoenvironmental
reconstructions and pollution studies based on single cores from large and morphologically variable lakes. Sedimentation rates
were estimated by matching this pollution record with the historical record of mining activities. Within Ullswater, sedimentation
rates from 1840 to the present varied from 0.67 to 2.33 mm year−1 with a mean of 1.4 mm year−1; highest sedimentation rates were observed in the deepest section of the lake and close to the main inputs, and lowest sedimentation
rates were observed in the northern part of the lake, furthest from the main input. Despite the considerable changes in mining
techniques and production, there is little evidence to suggest significant changes in sedimentation rates over time. 相似文献
7.
Cladocera as indicators of trophic state in Irish lakes 总被引:1,自引:0,他引:1
We examined the impact of lake trophic state on the taxonomic and functional structure of cladoceran communities and the role
of nutrient loading in structuring both cladoceran and diatom communities. Surface sediment assemblages from 33 Irish lakes
were analysed along a gradient of total phosphorus concentration (TP; 4.0–142.3 μg l−1), using a variety of statistical approaches including ordination, calibration and variance partitioning. Ordination showed
that the taxonomic structure of the cladoceran community displayed the strongest response to changes in lake trophic state,
among 17 measured environmental variables. Trophic state variables chlorophyll-a and TP explained about 20% of the variance in both cladoceran and diatom assemblages from a set of 31 lakes. Procrustes analysis
also showed significant concordance in the structure of cladoceran and diatom communities (P < 0.001). Thus, lake trophic state affects the taxonomic structure of both primary and secondary producers in our study lakes.
We also found a significant decrease in relative abundance of taxa associated with both macrophytes and sediments, or sediments
only, along the TP gradient (r = −0.49, P = 0.006, n = 30), as well as an increase in the proportion of the planktonic group (r = 0.43, P = 0.017, n = 30). This suggests that cladoceran community structure may also be shaped by lake trophic state indirectly, by affecting
habitat properties. We found no relationship between lake trophic state and the relative abundance of each of three cladoceran
groups that display different body size. We compared community structure between bottom and top sediment samples in cores
from six Irish lakes. Results revealed similar trajectories of nutrient enrichment over time, as well as a strong shift in
cladoceran functional structure in most systems. This study confirms that Cladocera remains in lake sediments are reliable
indicators of lake trophic state. This study also highlights the fact that taxonomic and functional structure should both
be considered to account for the multiple factors that shape cladoceran communities. 相似文献
8.
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
9.
10.
Jane M. Reed Francesc Mesquita-Joanes Huw I. Griffiths 《Journal of Paleolimnology》2012,47(2):251-275
Diatoms (Bacillariophyceae; single-celled algae) and ostracods (Ostracoda; shelled microcrustacea) are known for their sensitivity
to salinity. In palaeolimnology, the potential has yet to be tested for quantifying past salinity, lake level, and by inference,
climate change, by application of multiple-indicator transfer functions. We used weighted averaging techniques to derive diatom
(n = 91; r
2
= 0.92) and ostracod (n = 53; r
2
= 0.83) conductivity transfer functions from modern diatom, ostracod and water chemistry data collected in lakes of central,
western and northern Turkey. Diatoms were better represented across the full gradient than ostracods, at intermediate levels
of conductivity in particular, but both transfer functions were statistically robust. Because transfer functions are not infallible,
we further tested the strength and simplicity of salinity response and the potential for identifying characteristic associations
of diatom and ostracod taxa in different parts of the salinity gradient, to improve palaeoclimate reconstruction. We identified
a subset of 51 samples that contained both diatoms and ostracods, collected at the same time from the same habitat. We used
Two-Way Indicator Species Analysis of a combined diatom-ostracod data set, transformed to achieve numerical equivalence, to
explore distributions in more detail. A clear ecological threshold was apparent at ~3 g l−1 salinity, rather than at 5 g l−1, the boundary used by some workers, equating to the oligosaline-mesosaline boundary. Other salinity boundaries were poorly
defined, indicating lack of a simple, well-defined salinity response. We did, however, define characteristic associations
of taxa, to facilitate the distinction to be drawn between a hydrologically open, fresh lake and an oligosaline lake, in palaeoenvironmental
reconstruction. Over the rest of the salinity gradient, we highlighted the potential for the multi-proxy approach to strengthen
ostracod-based reconstruction in particular, to overcome the problem of broad apparent tolerance ranges in common halophilic
taxa such as Limnocythere inopinata, which often dominate in low-diversity fossil assemblages. The combination of multi-proxy quantitative reconstruction, complemented
by qualitative understanding of ecological responses generated by the analysis, remains a powerful tool in Quaternary palaeoclimate
research. 相似文献
11.
Historical change in lake sedimentation in Lake Takkobu, Kushiro Mire, northern Japan over the last 300 years 总被引:1,自引:0,他引:1
Environmental degradation, including shallowing, deterioration of aquatic habitat and water pollution, has arisen from the inflow of fine sediment to Lake Takkobu in northern Japan. The lake has experienced gradual environmental degradation due to agricultural development, which has introduced both fine sediment and sediment-associated nutrients into the lake. We have reconstructed the history of sediment yield to Lake Takkobu in Kushiro Mire over the last 300 years and have examined trends with reference to land-use development. Fifteen lake sediment core samples were obtained, and various physical variables of lake sediments were analyzed and dated using 137Cs and tephrochronology. The physical variables showed that all points contained mainly silt, except for two points close to the river mouths, where the mean diameter was < 35 μm. The peaks were defined as a “signal” when the physical variables were synchronous in a profile. These were created by floods and engineering works constructing drainage systems. The signal of canal construction in 1898 was detected in all core points. Lake Takkobu core samples contained two tephra layers. From the refractive indices of dehydrated glasses, the lower tephra layer was identified as Ko-c2 (1694) and the upper tephra layer as Ta-a (1739). A clear peak in the 137Cs concentration was detected at all the sampling points, except for the site close to the Takkobu River. This site showed two peaks in the 137Cs concentration, which was attributed to perturbation from flood events and a drainage project. The maximum 137Cs concentration was identified as the sediment surface from 1963, enveloped by the 1962 and 1964 signals. The sediment yield averaged over the last 300 years for Lake Takkobu was reconstructed for four periods using the signal, tephra and 137Cs as marker layers. The sediment yield under the natural erosion condition for the first two periods was 226 tons/year from 1694 to 1739 and 196 tons/year from 1739 to 1898. The development of the Takkobu watershed started in 1880s with partial deforestation and channelization in 1898, 1959, and 1962 leading to an increased sedimentation yield of 1016 tons/year from 1898 to 1963. Continued deforestation, channelization works in 1964, road construction in 1980–1990s, as well as agriculture development caused a further increase to 1354 tons/year from 1963 to 2004. Compared to the averaged natural sedimentation yield of 206 tons/year until 1898, initial land-use development in a catchment accelerated lake sedimentation, indicated by the 5-fold sediment yield. With increasing agricultural development since 1960s, sedimentation yields were highest for 1963–2004; a 7-fold increase compared with pre-impact conditions. To reduce sediment yield, riparian buffers along the rivers should be preserved or rebuilt, and sluices may function effectively during short-term periods of flooding. Environmental management policy and laws restricting uncontrolled and inappropriate land-use might help in addition to ensure longer-term environmental health by reducing the sedimentation rate. 相似文献
12.
Neil L. Rose David Morley Peter G. Appleby Richard W. Battarbee Tiiu Alliksaar Piero Guilizzoni Erik Jeppesen Atte Korhola Jaan-Mati Punning 《Journal of Paleolimnology》2011,45(4):447-468
Sediment accumulation rate (SAR) is an important physical parameter in all lakes and increases have been observed in many
over the last c.100 years. This has been ascribed to changes in land-use and land-management causing accelerated catchment
soil erosion and an increase in autochthonous organic matter production. The EU Water Framework Directive requires that assessment
of biological, hydromorphological and chemical elements of water quality should be based on the degree to which present day
conditions deviate from those expected in the absence of significant anthropogenic influence, termed reference conditions.
Currently however, the reference condition for sediment accumulation rate for lakes of different types is undefined. To improve
our understanding of the controls on SARs we compiled SAR and lake typology data for 207 European lakes derived from 210Pb dated cores to assess how rates have changed through time (in 25 year classes) both overall and for lakes of different
types. Seventy-one percent of these sediment cores showed surface SARs higher than “basal” (mainly nineteenth century) rates,
11% showed no change while 18% showed a decline. Lakes were then classified into lake-types using four variables: alkalinity
(3 classes), altitude (3 classes), maximum depth (2 classes) and lake area (2 classes). This generated a possible 36 lake
classes of which 25 were represented in the dataset. Nine lake-types contained >10 lakes. Little change in SAR occurred prior
to 1900 and most increases occurred in more recent periods, in particular 1950–1975 and post-1975. This indicates a general
acceleration in SAR in European lakes during the second half of the twentieth century. Reference SARs were estimated for six
lake-types with the highest number of sites. European mountain lakes had the lowest reference SAR (0.005 ± 0.003 g cm−2 yr−1) while lowland, high alkalinity sites had the highest (0.03–0.04 g cm−2 yr−1). SARs for other lake-types ranged between 0.012 and 0.024 g cm−2 yr−1. Using the mountain lake-type as an example, the 1850 reference SAR appears to show good agreement with available data for
lakes beyond Europe indicating these values may be more broadly applicable. Contemporary SARs in lakes of all classes showed
exceedence over their defined reference SAR. This may be partly due to diagenetic processes. Greatest exceedences were found
in shallow, low altitude lakes and these are considered to be the ones under the greatest threat from continued elevation
of SAR. It is considered that climate change may play a progressively more important role in driving SAR in the future. 相似文献
13.
A decade of widespread increases in surface water concentrations of total organic carbon (TOC) in some regions has raised
questions about longer term patterns in this important constituent of water chemistry. This study uses near-infrared spectroscopy
(NIRS) to infer lake water TOC far beyond the decade or two of observational data generally available. An expanded calibration
dataset of 140 lakes across Sweden covering a TOC gradient from 0.7 to 24.7 mg L−1 was used to establish a relationship between the NIRS signal from surface sediments (0–0.5 cm) and the TOC concentration
of the water mass. Internal cross-validation of the model resulted in an R
2 of 0.72 with a root mean squared error of calibration (RMSECV) of 2.6 mg L−1. The TOC concentrations reconstructed from surface sediments in four Swedish lakes were typically within the range of concentrations
observed in the monitoring data during the period represented by each sediment layer. TOC reconstructions from the full sediment
cores of four lakes indicated that TOC concentrations were approximately twice as high a century ago. 相似文献
14.
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
15.
16.
Modest diatom responses to regional warming on the southeast Tibetan Plateau during the last two centuries 总被引:1,自引:0,他引:1
Juliane Wischnewski Anson W. Mackay Peter G. Appleby Steffen Mischke Ulrike Herzschuh 《Journal of Paleolimnology》2011,46(2):215-227
A general mean annual temperature increase accompanied with substantial glacial retreat has been noted on the Tibetan Plateau
during the last two centuries but most significantly since the mid 1950s. These climate trends are particularly apparent on
the southeastern Tibetan Plateau. However, the Tibetan Plateau (due to its heterogeneous mountain landscape) has very complex
and spatially differing temperature and precipitations patterns. As a result, intensive palaeolimnological investigations
are necessary to decipher these climatic patterns and to understand ecological responses to recent environmental change. Here
we present palaeolimnological results from a 210Pb/137Cs-dated sediment core spanning approximately the last 200 years from a remote high-mountain lake (LC6 Lake, working name)
on the southeastern Tibetan Plateau. Sediment profiles of diatoms, organic variables (TOC, C:N) and grain size were investigated.
The 210Pb record suggests a period of rapid sedimentation, which might be linked to major tectonic events in the region ca. 1950.
Furthermore, unusually high 210Pb supply rates over the last 50 years suggest that the lake has possibly been subjected to increasing precipitation rates,
sediment focussing and/or increased spring thaw. The majority of diatom taxa encountered in the core are typical of slightly
acidic to circumneutral, oligotrophic, electrolyte-poor lakes. Diatom species assemblages were rich, and dominated by Cyclotella sp., Achnanthes sp., Aulacoseira sp. and fragilarioid taxa. Diatom compositional change was minimal over the 200-year period (DCCA = 0.85 SD, p = 0.59); only a slightly more diverse but unstable diatom assemblage was recorded during the past 50 years. The results indicate
that large-scale environmental changes recorded in the twentieth century (i.e. increased precipitation and temperatures) are
likely having an affect on the LC6 Lake, but so far these impacts are more apparent on the lake geochemistry than on the diatom
flora. Local and/or regional peculiarities, such as increasing precipitation and cloud cover, or localized climatic phenomena,
such as negative climate feedbacks, might have offset the effects of increasing mean surface temperatures. 相似文献
17.
The main limnological features of Lake Issyk-kul are described. The lake is a large (6,236 km2), deep (zm, 668 m), closed lake in eastern Kirgizia. It lies at ∼1,607 m above sea level, but water-levels have been dropping since
the last century. It is slightly saline (salinity, ∼6g L−1), with Na+, Mg2+, Cl− and SO
4
2−
the dominant ions. Nutrient levels are low and the lake is considered ultra-oligotrophic.
Characeae dominate the macrophytes. About 300 and 117 taxa of, respectively, phytoplankton and zooplankton have been identified,
withArctodiaptomus salinus the most numerous in the zooplankton. Chironomids dominate the benthos. Several endemic taxa of fish occur, of whichLeuciscus bergi was dominant until the 1970s. The fish fauna has been supplemented by many introduced species. Three mysids were introduced
in 1965–8 and are now a significant part of the ecosystem. The present annual fish catch permitted is 320 t.
The most important value of the lake is as a recreational resource. To promote and sustain this value requires careful, ongoing
management. The most significant threats to the lake are local pollution, visitor pressure, and declining water-levels. 相似文献
18.
Swan Lake is a small kettle lake located on the Oak Ridges Moraine; a moraine that is recognized as an important source of
ground water for the nearby and rapidly expanding Greater Toronto Area. A paleolimnological reconstruction using pollen and
diatoms from the lake sediments showed significant changes in biological community composition through the last ∼400 years.
Alterations in the diatom and pollen assemblages were most dramatic ca. A.D. 1850, correlating with the highest sediment flux
in the lake between the period ca. A.D. 1850 and A.D. 1870. These changes were directly linked to regional deforestation and
agricultural activities associated with European settlement. The pollen record from ca. A.D. 1850 to present day indicated
that tree species (e.g. Pinus spp., Tsuga canadensis) were declining, while grass (Poaceae) and invasive species (e.g. Ambrosia) were increasing. Around A.D. 1850, the diatom flora changed from an assemblage dominated by large, benthic species (e.g.
Sellaphora pupula, Pinnularia cf. maior, and Stauroneis phoenicenteron) to an assemblage characterized by smaller, tychoplanktonic (e.g. Fragilaria tenera, Staurosirella pinnata) and epiphytic (e.g. Achnanthidium minutissimum, Rossithidium linearis) taxa. This diatom community change supports the intermediate disturbance hypothesis which predicts a high level of diversity
and richness following an intermediate to intense disturbance of short duration. Phosphorus concentrations in Swan Lake were
inferred using a diatom-based regional calibration model, and the results indicated marked changes in lake water chemistry
through time (from below detection limits before land clearance and settlement to 19.3 μg l−1 in the current sediments), which were concurrent with episodes of regional deforestation and land-use change. Although the
sediment and biological records indicate that the lake ecology has stabilized over the last 30–50 years, paleolimnological
records show that the water quality and biology of Swan Lake has changed dramatically and not returned to pre-settlement conditions.
Swan Lake presents a detailed record of the impact created by deforestation and urban development with a population of <50
individuals per km2. Detailed paleolimnological studies like Swan Lake, in tandem with global human footprint studies, can create realistic estimates
of land-use impacts at the global scale. 相似文献
19.
The lipids in a sediment core from Lake Valencia, a hypereutrophic freshwater lake in Venezuela, are examined to understand
environmental changes over the last ∼13,000 years. From the latest Pleistocene to the earliest Holocene, total organic carbon
(TOC) substantially increased from 2.2 to 10%, while total organic carbon over total nitrogen (TOC/TN) decreased from as high
as 34 to as low as 10. Correspondingly, the concentration of terrestrially derived triterpenoids markedly decreased, and the
dominant n-alkane shifted from C31 to C23 or C25. During the same period, algal biomarkers such as botryococcenes, dinosterol, isoarborinol, C20 HBIs and 1,15C32 keto-ol markedly increased in abundance. These changes suggested a greater contribution of algal organic matter at the onset
of the Holocene, which was concurrent with increasing rainfall and the formation of a permanent lake (Lake Valencia) in the
Aragua Valley, Venezuela. The age profile of Paq, a n-alkane based proxy, showed large oscillations (0.20–0.81), reflecting historical variations in source strength of submerged/floating
vs. terrestrial/emergent OM inputs. An abrupt increase in tetrahymanol abundance at ∼7,260 cal years BP suggests the establishment
of an oxic–anoxic boundary in the lake’s water column. After reaching its maximum abundance at ∼2,100 cal year BP, botryococcenes,
a biomarker of Botryococcus braunii, gradually decreased to below the detection limit in the uppermost sediments, while different algal/microbial biomarkers
such as diploptene, dinosterol and isoarborinol substantially increased. These different historical profiles of algal/microbial
biomarkers reflect different responses of source organisms to environmental changes throughout this period. The δ13C determinations presented exceptionally enriched values for botryococcene isomers (−7.7 to −15.1‰), indicating the utilization
of bicarbonate as carbon sources in an extremely productive ecosystem. 相似文献
20.
Spatial variability and correlation of environmental proxies during the past 18,000 years among multiple cores from Lake Pumoyum Co,Tibet, China 总被引:3,自引:0,他引:3
Junbo Wang Liping Zhu Mitsugu Nishimura Toshio Nakamura Jianting Ju Manping Xie Watanabe Takahiro Matsunaka Testsuya 《Journal of Paleolimnology》2009,42(3):303-315
Multiple cores from Lake Pumoyum Co, southern Tibet, provide an improved understanding of the spatial distribution of lake
sediments, and how well they represent the paleo-climate. Comparative study of these cores using AMS 14C dating and environmental proxies clarified their relationships with environmental changes. Our work focused on understanding
the spatial similarities among cores covering different time scales, and evaluating variations in sedimentary processes across
sites. The four studied cores demonstrate different sedimentation rates, but environmental proxies help synchronize the timing
of environmental variations. Sediment variables such as total organic carbon (TOC), inorganic carbon (IC), and grain size
in different cores correlate well and corroborate changing trends over the past 10,000 cal years. Differences in sedimentation
rates and facies among core sites probably result largely from differences in water depth. The core from the deepest site
displays the highest average sedimentation rates and the highest accumulation rates of TOC, but lowest content of IC. Two
cores from somewhat shallower sites have plant residues in their lower sections and record similar variations in both the
number of layers and their depositional ages. Our results do not indicate any significant variation in sedimentation pattern
or its related factors among the three sites. A single core from the deepest site could adequately represent the total lake
environment over the time span covered. But cores from somewhat shallower sites might reveal important shifts in the environment
over a longer time period. 相似文献