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1.
Sabine Hanisch Martin Wessels Frank Niessen Antje Schwalb 《Journal of Paleolimnology》2009,41(3):393-406
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
2.
A late Pleistocene and Holocene mineral magnetic record from sediments of Lake Aibi,Dzungarian Basin,NW China 总被引:1,自引:0,他引:1
Alina Tudryn Piotr Tucholka Elisabeth Gibert Francoise Gasse Keiquin Wei 《Journal of Paleolimnology》2010,44(1):109-121
We studied mineral magnetic properties of a 6-m-long, late Pleistocene through Holocene sediment sequence from Lake Aibi in
Dzungaria (Zunggary, Junggar), northern Xinjiang, China. Results were used to infer environmental changes and are compared
with previously studied cores from Lake Manas. Both water bodies occupy the deepest parts of the Dzungarian Basin and are
remnants of large Holocene lakes. During the Late Pleistocene, the magnetic mineralogy in both lakes was dominated by detrital,
iron oxide minerals. Oxic conditions, which dominated during sedimentation and early diagenesis, persisted over the Pleistocene–Holocene
transition. Later, during the middle Holocene, lake bottom conditions enabled authigenic formation of iron sulphide minerals
such as pyrite (FeS2) in Lake Aibi, and pyrite and greigite (Fe3S4) in Lake Manas. This iron sulphide mineralogy suggests increased biological activity in stagnant, anoxic bottom waters. Anoxic
bottom conditions started about 9.8 cal kyr BP in Lake Manas and at about 7.2 cal kyr BP in Lake Aibi. A short dry event recorded
in Lake Manas between 6.8 and 5.2 cal kyr BP is not clearly observed in Lake Aibi. In the late Holocene, i.e. the last 2.8 cal
kyr, sediments of both lakes are again characterised by iron oxides, suggesting well-mixed, shallow water bodies. For this
recent period, it seems that the detrital material in the two lakes had a common origin. Magnetic properties of sediments
in Lakes Aibi and Manas show broadly similar environmental evolution during the late Pleistocene and Holocene. Nevertheless,
despite the close proximity of the two lakes (~200 km) in the same basin, they display some different magnetic properties
and record environmental changes at different times. 相似文献
3.
Jörn Parplies Andreas Lücke Heinz Vos Jens Mingram Martina Stebich Ulrich Radtke Jingtai Han Gerhard H. Schleser 《Journal of Paleolimnology》2008,40(1):471-487
Total organic carbon (TOC) content, total nitrogen (TN) content, stable nitrogen isotope (δ15N) and stable organic carbon isotope (δ13Corg) ratios were continuously analysed on a high resolution sediment profile from Lake Sihailongwan (SHL), covering the time
span between 16,500 and 9,500 years BP. Strong variations of the investigated proxy parameters are attributed to great climatic
fluctuations during the investigated time period. Variations in organic carbon isotope ratios and the ratio of TOC/TN (C/N
ratio) are discussed with respect to changing proportions of different organic matter (OM) sources to bulk sedimentary OM.
Phases of high TOC content, high TN content, depleted δ13Corg values and high δ15N values are interpreted as times with increased productivity of lacustrine algae in relation to input of terrigenous organic
matter. Two distinct phases of enriched nitrogen isotope ratios from 14,200 to 13,700 and 11,550 to 11,050 years BP point
towards a reduced phytoplankton discrimination against 15N due to a diminished dissolved inorganic nitrogen pool. The combination of geochemical (TOC, TN, C/N ratio) and isotopic
(δ13Corg, δ15N) proxy parameters points to a division of climate development into four stages. A cold and dry stage before 14,200 years
BP, a warm optimum stage with high phytoplankton productivity from 14,200 to 12,450 BP, a colder and drier stage from 12,450
to 11,600 BP and a stage of climatic amelioration with high variability in TOC and TN contents after 11,600 BP. These results
are discussed in relation to monsoon variability and Northern Hemisphere climate development of the late glacial. 相似文献
4.
Lourdes Vargas-Ramirez Emile Roche Philippe Gerrienne Henry Hooghiemstra 《Journal of Paleolimnology》2008,39(2):197-217
A pollen record from Puyehue area (40°S; 72°W) in the southern Lake District, Chile, indicates that prior to 13,410 14C yr BP (ca. 16,500–15,200 cal yr BP), cold resistant and hygrophilous vegetation, particularly Nothofagus forest and myricaceous vegetation, covered the area. From ca. 15,000 cal yr BP onward, the forest became increasingly dense.
Between 10,010 and 7450 14C yr BP (ca. 11,000–8000 cal yr BP), the expansion of Nothofagus obliqua and the spread of grasses suggests the climate became warmer and semi-arid. Lowland deciduous forest (Nothofagus obliqua, Aextoxicon punctatum, Laurelia sempervirens) and Valdivian rainforest (Nothofagus dombeyi, Eucryphia cordifolia, Caldcluvia paniculata, Aextoxicon punctatum, Laureliopsis philippiana) were abundant. During the next two thousand years, stable warm climatic conditions prevailed, and the diversity of the vegetation
increased. From 5760 to 1040 14C yr BP (ca. 6500–900 cal yr BP), the North Patagonian rainforest expanded. The presence of Pilgerodendron/Fitzroya, together with Nothofagus forest, suggests that humid conditions prevailed. During the last millennium, human impact intensified and regional vegetation
was disturbed, particularly the lowland deciduous forest and Valdivian rainforest. North-Patagonian and subantartic taxa,
such as Podocarpus nubigena, Pilgerodendron/Fitzroya, Nothofagus dombeyi type, Austrocedrus chilensis and Drimys winteri, occupied the low and high-altitude parts of the Cordillera. Five hundred years ago, shrub and grasses expanded in the Nothofagus forest, suggesting that forest became more open under cool–cold, and humid climatic conditions. These conditions prevail
to the present day.
This is the fourth 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
5.
Serpent River Bog lies north of North Channel, 10 m above Lake Huron and 15 m below the Nipissing Great Lake level. A 2.3 m
Holocene sequence contains distinct alternating beds of inorganic clastic clay and organic peat that are interpreted as evidence
of successive inundation and isolation by highstands and lowstands of the large Huron-Basin lake. Lowstand phases are confirmed
by the presence of shallow-water pollen and plant macrofossil remains in peat units. Twelve 14C dates on peat, wood and plant macrofossils combined with previously published 14C ages of lake-level indicators confirm much of the known early Holocene lake-level history with one notable exception. A
new Late Mattawa highstand (8,390 [9,400 cal]–8,220 [9,200 cal] BP) evidenced by a sticky blue-grey clay bed is tied to outburst
floods of glacial Lake Minong during erosion of the Nadoway drift barrier in the eastern Lake Superior basin. A subsequent
Late Mattawa highstand (8,110 [9,040 cal]–8,060 [8,970 cal] BP) is attributed to enhanced meltwater inflows that first had
deposited thick varves throughout Superior Basin. Inundation by the Nadoway floods and possibly the last Mattawa flood were
likely responsible for termination of the Olson Forest (southern Lake Michigan). A pollen diagram supports the recognized
progression of Holocene vegetation, and defines a subzone implying a very dry, cool climate about 7.8–7.5 (8.6–8.3 cal) ka
BP based on the Alnus crispa profile during the Late Stanley lowstand. A new date of 9,470 ± 25 (10,680–10,750 cal) BP on basal peat over lacustrine clay
at Espanola West Bog supports the previous interpretation of the Early Mattawa highstand at ca. 9,500 (10,740 cal) BP. The
organic and clastic sediment units at these two bogs are correlated with other records showing coherent evidence of Holocene
repeated inundation and isolation around northern Lake Huron. Taken together the previous and new lake-level data suggest
that the Huron and Georgian basin lakes were mainly closed lowstands throughout early Holocene time except for short-lived
highstands. Three of the lowstands were exceptionally low, and likely caused three episodes of offshore sediment erosion which
had been previously identified as seismo-stratigraphic sequence boundaries. 相似文献
6.
Reader Lake and Elbow Lake, two high-altitude lakes in the Uinta Mountains of Utah, are located approximately 2 km apart,
at similar elevations, and within identical vegetation communities. Loss on ignition, carbon to nitrogen ratios, biogenic
silica, and sediment grain size were analyzed throughout percussion cores retrieved from both lakes to construct continuous
time series spanning 14 to ca. 2 ka BP. Given the proximity of the lakes, it is assumed that both were subjected to the same
climatic forcing over this time. Accordingly, the first goal of this study was to consider these two multiproxy datasets in
concert to yield an integrated paleoclimate record for this region. Close inspection of the records identified discrepancies
indicating that the lakes responded to climate changes in different ways despite their proximity and similar setting. Clarifying
these differences and understanding why the two lakes behaved differently at certain times was the second goal of this study.
Overall, the paleoclimatic records document lake formation in the latest Pleistocene following glacier retreat. Buried glacier
ice at the location of Reader Lake may have persisted through the Younger Dryas. Both lakes became biologically productive
ca. 11.5 ka BP, and the first appearance of conifer needles indicates that trees had replaced alpine tundra in these watersheds
by 10.5 ka BP. The interval from 10 to 6 ka BP was marked by a dramatic increase in precipitation, perhaps related to enhanced
monsoonal circulation driven by the insolation maximum. The two lakes recorded this event in notably contrasting ways given
their differing hydrogeomorphic settings. Precipitation decreased from 6 to 4 ka BP, and low water levels and drought conditions
marked the interval from 4.0 to 2.7 ka BP. The integrated paleoclimate record developed from these cores provides a useful
point of comparison with other records from the region. The differences between the records from these closely spaced lakes
underscore the need to consider hydrogeomorphic setting when evaluating the suitability of a lake for a paleolimnological
study. 相似文献
7.
Liping Zhu Xiaolin Zhen Junbo Wang Houyuan Lü Manping Xie Hiroyki Kitagawa Göran Possnert 《Journal of Paleolimnology》2009,42(3):343-358
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
8.
Beatriz Ortega Gabriel Vázquez Margarita Caballero Isabel Israde Socorro Lozano-García Peter Schaaf Esperanza Torres 《Journal of Paleolimnology》2010,44(3):745-760
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
9.
Barbara Wohlfarth Pavel Tarasov Ole Bennike Terri Lacourse Dmitry Subetto Peter Torssander Fedor Romanenko 《Journal of Paleolimnology》2006,35(3):543-569
Three lake sediment sequences (lakes Nero, Chashnitsy, Zaozer’e) from the Rostov-Jaroslavl’ region north of Moscow were studied
to provide information on palaeoclimatic and palaeoenvironmental changes during the past 15,000 cal yr. The multi-proxy study
(i.e., pollen, macrofossils, mineral magnetic measurements, total carbon, nitrogen and sulphur) is chronologically constrained
by AMS 14C measurements. Lake Nero provided the longest sedimentary record back to ca. 15,000 cal yr BP, while sediment accumulation
began around ca. 11,000 cal yr BP in the two other lakes, possibly due to melting of permafrost. Limnic plant macrofossil
remains suggest increased lake productivity and higher mean summer temperatures after 14,500 cal yr BP. While the late glacial
vegetation was dominated by Betula and Salix shrubs and various herbs, it appears that Betula sect. Albae became established as early as 14,000 cal yr BP. Major hydrological changes in the region led to distinctly lower lake levels,
starting 13,000 cal yr BP in Lake Nero and ca. 9000 cal yr BP in lakes Chashnitsy and Zaozer’e, which are situated at higher
elevations. These changes resulted in sedimentary hiatuses in all three lakes that lasted 3500–4500 cal yr. Mixed broad-leaved
– coniferous forests were widespread in the area between 8200 and 6100 cal yr BP and developed into dense, species-rich forests
between 6100 and 2500 cal yr BP, during what was likely the warmest interval of the studied sequences. Agricultural activity
is documented since 500 cal yr BP, but probably began earlier, since Rostov was a major capital by 862 A.D. This apparent
gap may be caused by additional sedimentary hiatuses around 2500 and 500 cal yr BP. 相似文献
10.
Sub-bottom profiling and coring were undertaken at eight sub-basins along the lower French River and at five small lakes near
North Bay, Ontario, to collect stratigraphical and chronological evidence to investigate whether lakes occupying the Huron–Georgian
basins during the early- to mid-Holocene became hydrologically closed. All of the coring sites are located within the route
of the North Bay outlet that carried outflow from the upper Great Lakes during this period. Sand beds containing organic detritus
are present within five cores from Muskrat, Crombie and Deep bays that otherwise are composed of glaciolacustrine rhythmites
or fine-grained lacustrine deposits. These sand beds are interpreted to represent intervals when water levels within the sub-basins
were lower than present, based on chronology, sediment texture, and macrofossil assemblages. It is inferred that the water
surface in the Huron–Georgian basins fell below the level of the Dalles Rapids sill isolating the lower French River sub-basins
from the large lake. A core from Depensier Lake, North Bay, contains an organic-rich sand interval within a thicker sand unit
barren of organic materials. Macrofossils within this organic-rich interval are interpreted to be evidence of substantially
diminished flow through the North Bay outlet channel. Radiocarbon dates of terrestrial macrofossils provide correlation of
the sand beds between the French River cores as well as with the organic-rich sand in the Depensier Lake core. The possibility
that the sand beds in the French River cores represent flood deposits rather than evidence of hydrologically closed conditions
is considered, but rejected, based on the occurrence of multiple peaty layers and the record of shallow water conditions inferred
from macrofossils within the upper sand bed of core MUS1, Muskrat Bay, in combination with the evidence of quiescent depositional
conditions from similarly aged macrofossils in the core from Depensier Lake. Eight radiocarbon dates from the French River
cores are incorporated into an elevation-age plot of paleo-indicators of water levels in the Huron–Georgian basins, using
additional data from the literature. This plot and stratigraphic evidence from the Muskrat Bay cores indicates that separate
closed-basin intervals occurred between 9.0 and 8.4, and 9.5 and 9.3 ka cal BP (~ 8.1 and 7.6, and ~ 8.5 and 8.3 ka BP).
The occurrence of these two closed-basin intervals between 9.6 and 8.4 ka cal BP (~ 8.7 and 7.6 ka BP) implies that run-off
derived exclusively from precipitation within the non-glaciated portions of the upper Great Lakes drainage basins was likely
insufficient at this time to support an open-basin lake hydrology during the contemporary climate, which was colder and drier
than present, without being supplemented from glacial Lake Agassiz overflow and/or Laurentide Ice Sheet meltwater. 相似文献
11.
A temperature reconstruction using chironomids was attempted at Egelsee, Switzerland, a site where pollen and macrofossil
records showed a correspondence between vegetation and climatic changes inferred by other proxies in Europe. The general pattern
of temperature changes inferred from chironomids during the Late Glacial [i.e. cold temperatures between ca. 16,500 and 14,800 cal
BP, close to present-day temperature between 15,000 and 13,000 cal BP and colder temperatures during the Younger Dryas (YD)],
and the major temperature changes of the Holocene (i.e. the Younger Dryas–Holocene transition and the Late Holocene cooling
trend) at Egelsee, were mirrored in other European climate reconstructions using various proxies. However, the amplitude of
temperature changes during the YD was smaller than reconstructed by other proxies at various sites, and the 8,200 years BP
event was not apparent. These differences between records were probably due to the dominance of Corynocera ambigua, with percentages reaching 60% in parts of the Egelsee sequence. This taxon was not present in any of the 103 lakes used
for the transfer function and its absence may have yielded less accurate inferences. Its presence in samples only associated
with cold inferences at Egelsee suggests that this taxon is a cold indicator. However, it was also found in warm Danish lakes
and the factors that determine the presence of C. ambigua remain unexplained. Most samples had a poor fit to temperature and instead, dissolved organic carbon seemed to be a factor
influencing the chironomid assemblages during the Holocene. These results illustrate the need to better understand the ecology
of chironomids and to disentangle the various factors that affect chironomid communities through time. Ultimately, such information
will lead to more accurate temperature reconstructions. 相似文献
12.
Late-Glacial and Holocene Record of Lake Levels of Mathews Pond and Whitehead Lake,Northern Maine,USA 总被引:1,自引:0,他引:1
Paleohydrology studies at Mathews Pond and Whitehead Lake in northern Maine revealed synchronous changes in lake levels from
about 12,000 14C yrs BP to the present. We analyzed gross sediment structure, organic and carbonate content, mineral grain size, and macrofossils
of six cores from each of the two lakes, and obtained 72 radiocarbon dates. Interpretation of this paleo-environmental data
suggests that the late-glacial and Younger Dryas climate was dry, and lake levels were low. Early Holocene lake levels were
considerably higher but declined for an interval from about 8000 to 7200 14C yrs BP. Sediment of both lakes contains evidence of a dry period at ∼7400 14C yrs BP (8200 cal yr). Lake levels of both sites declined abruptly about 4800 14C yrs BP and remained low until 3000 14C yrs BP. Modern lake levels were achieved only within the past 600 years. The west-to-east, time-transgressive nature of
lake-level changes from several sites across northeastern North America suggests periodic changes in atmospheric circulation
patterns as a driving force behind observed moisture balance changes.
Electronic supplementary material to this article is available at and accessible for authorized users. 相似文献
13.
Ricarda Voigt 《Geografiska Annaler: Series A, Physical Geography》2006,88(2):97-105
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
14.
Bernd Wagner André F. Lotter Norbert Nowaczyk Jane M. Reed Antje Schwalb Roberto Sulpizio Verushka Valsecchi Martin Wessels Giovanni Zanchetta 《Journal of Paleolimnology》2009,41(3):407-430
Lake Ohrid is considered to be of Pliocene origin and is the oldest extant lake in Europe. A 1,075-cm-long sediment core was
recovered from the southeastern part of the lake, from a water depth of 105 m. The core was investigated using geophysical,
granulometric, biogeochemical, diatom, ostracod, and pollen analyses. Tephrochronology and AMS radiocarbon dating of plant
macrofossils reveals that the sediment sequence spans the past ca. 39,500 years and features a hiatus between ca. 14,600 and
9,400 cal. year BP. The Pleistocene sequence indicates relatively stable and cold conditions, with steppe vegetation in the
catchment, at least partial winter ice-cover of the lake, and oxygenated bottom waters at the coring site. The Holocene sequence
indicates that the catchment vegetation had changed to forest dominated by pine and summer-green oak. Several of the proxies
suggest the impact of abrupt climate oscillations such as the 8.2 or 4.0 ka event. The observed changes, however, cannot be
related clearly to a change in temperature or humidity. Human impact started about 5,000 cal. year BP and increased significantly
during the past 2,400 years. Water column mixing conditions, inflow from subaquatic springs, and human impact are the most
important parameters influencing internal lake processes, notably affecting the composition and characteristics of the sediments. 相似文献
15.
Diatom-based inferences of post-glacial hydrological change from a sedimentary record from Felker Lake, British Columbia,
show millennial-scale pacing of climate over the past approximately 11670 calendar years with change at ca. 8140 cal. year BP,
ca. 6840 cal. year BP, ca. 5700 cal. year BP, and ca. 2230 cal. year BP. Early postglacial diatom assemblages are dominated
by fragilaroid taxa, suggesting that cool and moist climate conditions and relatively high lake levels prevailed at this time.
Early Holocene warming near ca. 8140 cal. year BP promoted Cyclotella bodanica var. lemanica, a fall bloomer competitive in limnological conditions associated with warmer water and stratified conditions. Short-lived
peaks of Stephanodiscus parvus/minutulus between ca. 6340 cal. year BP and ca. 5860 cal. year BP indicate periodic increases in nutrient availability and prolonged
mixing likely associated with long cool and moist spring seasons. The diatom-inferred depth of Felker Lake increased during
the mid-Holocene to reach a record high-stand at ca. 5860 cal. year BP. Large changes in hydrological variability and terrestrial
vegetation at Felker Lake occurred after ca. 2230 cal. year BP when high-amplitude centennial-scale fluctuations in diatom-inferred
lake depth and salinity are observed. Change is first documented in terrestrial vegetation at this time by a shift from open
Pinus parklands to a landscape that periodically supported populations of Cupressaceae. Three record low-stand high-salinity events
are reconstructed between ca. 1910 cal. year BP and ca. 1800 cal. year BP, ca. 1030 cal. year BP and ca. 690 cal. year BP,
and ca. 250 cal. year BP and ca. 140 cal. year BP. The low lake-level episode of ca. 1030 cal. year BP–ca. 690 cal. year BP
is coeval with the Medieval Warm Period (ca. 1000 cal. year BP–ca. 600 cal. year BP), a period of intense drought in western
North America. Post-glacial hydrological change at Felker Lake is coherent with regional, hemispherical, and global paleoclimate
events, suggesting that millennial-and centennial-scale shifts in water availability are a persistent feature of the climate
of western North America. 相似文献
16.
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
17.
High-resolution quantitative analysis of ostracod assemblages from 4.3-m-thick freshwater tufa-rich sediments, deposited during the last 12.8 ka in Lake Sinijärv, northern Estonia, yielded information on water level, trophic state conditions, and temperature changes since the late glacial. AMS 14C dates from aquatic mosses provided time constraints on the palaeoenvironmental development of the region. In the ostracod assemblage structure, four faunal zones (OFZ) were determined. The most significant change in the ostracod fauna occurred at 10,590 cal. y BP, when a typical littoral, polythermophilic fauna was replaced by a mostly sublittoral, species-rich meso- to stenothermophilic fauna. The ostracod data indicate two major low-water-level periods in the lake at 12,800–10,590 and 7,600–3,700 cal. y BP. Sediment analysis indicates the most intensive tufa precipitation occurred during these low stand periods, rather than during the warmest climate in Estonia between 8,000 and 4,500 cal. y BP. The late glacial low water level in the groundwater-fed Lake Sinijärv at 12,800–10,590 cal. y BP coincides partly with the regression in the Lake Peipsi basin (14,000–12,100 cal. y BP) and with the last drainage event of the Baltic Ice Lake at 11,600 cal. y BP. The low-water-level period in Lake Sinijärv occurred earlier than in lakes in the SE sector of Scandinavian glaciation. The change from low to high water level in Lake Sinijärv at 10,590 cal. y BP preceded the first post-glacial transgression events in the small lowland lakes of Estonia, southern Sweden, Poles`ye in Belarus, and Valday in NW Russia. In general, the mid-Holocene low-water-level period in Lake Sinijärv between 7,600 and 3,700 cal. y BP is concurrent with the regressions in the lakes of the SE sector of Scandinavian glaciation. 相似文献
18.
The ostracode record from Harris Lake, southwestern Saskatchewan: 9200 years of local environmental change 总被引:2,自引:0,他引:2
Holocene paleoenvironments of Harris Lake, southwestern Saskatchewan, are reconstructed from the ostracode stratigraphy of a 10.4 m sediment core. Twenty three taxa, representing nine genera, were identified and counted from 113 samples. At each depth, a theoretical faunal assemblage was derived from the raw counts. The mean and variance of chemical, climatic and physical variables were inferred from modern analogues of the fossil assemblages, using existing autecological data from 6720 sites, mostly in western Canada. These data suggest four paleoenvironments: an early-Holocene (9240–6400 years BP) variable climate supporting aspen parkland vegetation; the warm dry hypsithermal (6400–4500 years BP); a short transitional period of ameliorating climate and expanding subboreal forest (4500–3600 years BP); and the present environment since 3600 years BP. A change in regional climate with the draining of Glacial Lake Agassiz (ca. 8500 years BP) and landsliding in the watershed (ca. 4000 years BP) caused relatively rapid environmental change. The ostracode record generally corroborates the interpretations of other proxy data previously published for Harris Lake. Most of the discrepancy involves the timing and severity of maximum Holocene warmth and aridity. Peak aridity interpreted from the pollen data is earlier than in the other proxy records. Both the diatoms and ostracodes indicate highest paleosalinity between ca. 6500 and 5000 years BP, but maximum salinity in the diatom record occurs between ca. 6000–5700 years BP, whereas the ostracode-inferred salinity is relatively low at this time and peaks later at ca. 5000 years. Neither of these reconstructions suggests the short episodes of hypersalinity interpreted from the mineralogy. 相似文献
19.
Sediment cores collected from embayed lakes along the east-central coast of Lake Michigan are used to construct aeolian sand
records of past coastal dune mobility, and to constrain former lake levels in the Lake Michigan basin. Time series analysis
of sand cycles based on the weight-percent aeolian sand within lacustrine sediment, reveals statistically significant spectral
peaks that coincide with established lake level cycles in Lake Michigan and the Gleissberg sunspot cycle of minima. Longer
cycles of ~ 800 and ~ 2200 years were also identified that correspond to solar cycles. Shorter cycles between 80 and 220 years
suggest a link between coastal dune mobility, climate, and lake levels in the Lake Michigan basin. Radiocarbon-dated sedimentary
contacts of lacustrine sediment overlying wetland sediment record the Nipissing transgression in the Lake Michigan basin.
Lake level rise closely mimics the predicted uplift of the North Bay outlet, with lake level rise slowing when outflow was
transferred to the Port Huron/Sarnia outlet. The Nipissing highstand was reached after 5000 cal (4.4 ka) BP. 相似文献
20.
Palaeoecological tools for improving the management of coastal ecosystems: a case study from Lake King (Gippsland Lakes) Australia 总被引:2,自引:0,他引:2
Krystyna M. Saunders Dominic A. Hodgson Jennifer Harrison Andrew McMinn 《Journal of Paleolimnology》2008,40(1):33-47
Since European settlement began over 200 years ago, many southeast Australian coastal lakes and lagoons have experienced substantial
human impacts, including nutrient enrichment. Present day management and conservation efforts are often hampered by a lack
of data on pre-impact conditions. We used a palaeoecological approach at Lake King, Gippsland Lakes, southeast Australia in
order to determine its pre-impact condition and to establish the nature and direction of subsequent environmental changes,
including responses to the construction of a permanent entrance to the sea in 1889. A 120 cm sediment core was analysed for
diatoms, chlorophyll a, total carbon, nitrogen and sulphur, and dated using 210Pb. Past phosphate and salinity concentrations were reconstructed using diatom-phosphate and diatom-salinity transfer functions
developed from a calibration set based on 53 sites from 14 southeast Australian coastal lakes and lagoons. Results show changes
in the diatom assemblage that record a shift from a brackish-water to marine diatom flora since construction of the permanent
entrance. Phosphate concentrations increased at the same time and experienced major peaks in the 1940s and 1950s to >100 μg/l.
Chlorophyll a concentrations were generally below 24 μg/l/gTOC in the core, but there has been a clear increase since the 1980s, peaking
at 120 μg/l/gTOC, likely associated with a recorded increase in the frequency of nuisance algal blooms. These results indicate
that the Lake King environment is now very different to that present during early European settlement. We conclude that by
identifying the nature and direction of environmental change, palaeoecological studies can contribute towards developing realistic
and well-informed management, conservation and restoration strategies in Australian coastal ecosystems. 相似文献