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1.
Bernhard Aichner Heinz Wilkes Ulrike Herzschuh Steffen Mischke Chengjun Zhang 《Journal of Paleolimnology》2010,43(4):873-899
A sediment core from Lake Koucha (eastern Tibetan Plateau) was investigated using organic biomarkers and their stable carbon
isotope signatures. The correlation between TOC content, total amount of aquatic macrophyte-derived n-alkanes (e.g. nC23) and δ13C values of TOC and nC23 indicates that Lake Koucha was macrophyte-dominated before 8 cal ka BP. Shortly after the lake turned from a saline to a
freshwater system at 7.2 cal ka BP, a variety of algal and bacterial markers such as hopanoids and isoprenoids emerged, of
which phytane, pentamethylicosene (PMI), moretene and diploptene are particularly abundant. Phytane and PMI show different
isotopic signals (≈−18 and ≈−28‰, respectively), which indicates that they originated from different sources. Phytane may
have been derived from cyanobacteria, while methanogenic archaea may be the source of PMI. The isotopic depletion of diploptene
and moretene (≈−60‰) indicates the presence of methanotrophs. After 6.1 cal ka BP, the saturated C20 highly branched isoprenoid (HBI) became the dominant constituent of the aliphatic hydrocarbon fraction. Such dominance has
rarely been reported in lacustrine environments, and indicates a strong presence of algae (most likely diatoms) or cyanobacteria.
At 4.7 cal ka BP, the appearance of an unsaturated C25 HBI, which is a specific biomarker for diatoms, was noted. Furthermore, the level of nC17-alkane was observed to increase in abundance in the uppermost two samples. These results suggest that the lake was phytoplankton-dominated
during the last 6.1 ka. Relatively low biomarker concentrations and δ13C values at 6.0, 3.1 and 1.8 cal ka BP indicate the occurrence of cool periods, which is in agreement with inferences from
other locations on the Tibetan Plateau. The δ13C values of nC23 range from −23.5 to −12.6‰, with high values at the peak of macrophyte abundance at ca. 11 cal ka BP and at the phytoplankton
maximum between ca 6.1 and 2.8 cal ka BP. Thus, aquatic macrophyte-derived mid-chain n-alkanes have been found to be excellent indicators of carbon-limiting conditions, which lead to the assimilation of isotopically-enriched
carbon species. The limitation of carbon sources could be a localized phenomenon occurring in dense plant stands (as in the
older section of the core), or it may be induced by high primary productivity (as in the younger section). Since the δ13C value of the inorganic carbon source may vary, the offset between the δ13C values of nC23 and TIC could serve as a more precise proxy for carbon-limiting conditions in lacustrine environments, which could in turn
be interpreted with respect to lacustrine paleo-productivity. 相似文献
2.
Wei Zhong Jibin Xue Jun Ouyang Yanming Zheng Qiaohong Ma Xuefeng Yu 《Journal of Paleolimnology》2010,44(3):777-788
Two cores were recovered from raised peat bogs on the tropical northern Leizhou Peninsula, south China. Multiple sediment
variables including organic matter (OM) content, the stable carbon isotope signature of OM, low-frequency magnetic susceptibility
and degree of humification, indicate that the regional paleoclimate played an important role in determining the nature of
peat that accumulated. Based on comparison with other climate proxies, the bulk peat δ13C record was interpreted as an indicator of variation in the East Asian (EA) summer monsoon, and to a lesser extent, the Indian
summer monsoon, during the last glacial period between ~49 and 10 cal ka BP. More negative bulk δ13C values reflect wetter and warmer conditions, and thus a strong EA summer monsoon. More positive values indicate drier and
cooler conditions. A warm and wet period occurred between ~46 and 28 cal ka BP, implying a strengthening of the EA summer
monsoon. A climate shift occurred at ~22 cal ka BP and the driest and coldest period appeared between ~19 and 16 cal ka BP,
suggesting weakening of the EA summer monsoon. After ~12 cal ka BP, climate shifted towards wetter and warmer conditions again.
It has been suggested that variations in orbitally induced solar insolation played a role in the last glacial climate of the
study region. Several millennial—scale arid and cold phases characterized by C4 plants, or by more positive δ13C values during periods when C3 plants dominated, show agreement with the Greenland GISP2 ice core and the Chinese stalagmite
records. Interactions between high northern latitude cold air advection and summer moisture transported across the tropical
ocean, and the migration of the mean position of the Intertropical Convergence Zone (ITCZ) would have favored these millennial–scale
phases. Additionally, changes in heat transport to the North Atlantic would also have influenced climate in the region. 相似文献
3.
Rare earth element, major and trace element and mineralogy in the sediments representing last 50 cal kyr BP from the summer precipitation fed paleolake San Felipe identify the different association of minerals and selective transportation of different grain size fractions and relate them to the variation in pluvial discharge into the basin as well as aeolian activities in the western Sonora Desert. Period of lower pluvial discharge during 14–48 cal kyr BP is contemporary to the regime of dominant winter storms in the region. Transportation of coarser quartz and plagioclase during 40–48 cal kyr BP and dominant finer fractions during 14–40 cal kyr BP possibly mirror the variation in the frequency of winter storms. During 3–14 cal kyr BP, higher catchment erosion (sedimentation increased 4–12 times) and transportation of REE bearing heavy minerals into the basin indicate higher pluvial discharge. We relate this period to a regime of dominant summer precipitation associated with the North American Monsoon and tropical cyclones. Geochemical and mineralogical signatures of the sediments deposited during ca. 8, 12–13 and >48 cal kyr BP suggest selective mobilization of quartz and plagioclase from the surrounding sand dunes by the aeolian processes. 相似文献
4.
Following deglaciation, the long-term pattern of change in diatom communities and the inferred history of the aquatic environment are affected by a hierarchy of environmental controls. These include direct climate impacts on a lake’s thermal and hydrologic budgets, as well as the indirect affects of climate on catchment processes, such as weathering, soil development, microbial activity, fire, and vegetation composition and productivity, which affect the transfer of solutes and particulates from the terrestrial ecosystem into the lake. Some of these catchment influences on lacustrine systems operate as time-dependent patterns of primary succession that are set in motion by glacier retreat. This paper provides a conceptual model of some dominant pathways of catchment influence on long-term lake development in glaciated regions and uses a series of paleolimnological examples from arctic, boreal, and temperate regions to evaluate the relative role of direct climate influences and of catchment processes in affecting the trajectory of aquatic ecosystems during the Holocene in different environmental contexts. 相似文献
5.
Matthew A. Konfirst Charlotte Sjunneskog Reed P. Scherer Peter T. Doran 《Journal of Paleolimnology》2011,46(2):257-272
We present a diatom record from a sediment core taken in Lake Fryxell, Taylor Valley, Antarctica. Six zones were defined using
diatom assemblage changes that indicate varying limnological conditions. The early lake stage, ca. 35,000 cal years BP, is characterized by Mayamea atomus f. permitis, a species rarely reported in modern Antarctic Dry Valley environments. An extended period from ca. 35,000 to 19,000 cal years BP is characterized by low diatom abundance, with dominant taxa Luticola spp., Muelleria spp., and Diadesmis contenta. The modern assemblage was established ca. 13,000 cal years BP, after two relatively brief transitional stages. One key species
for this recent period, Navicula lineola var. perlepida, is absent in surface sediments and the modern environment, indicating an environmental change within the last several centuries.
The diatom assemblage is compared to modern diatom communities in Dry Valley streams, which provide the most complete information
on diatom distributions in this region. Although precise environmental interpretation of the core is hampered by limited knowledge
of environmental constraints on many of the diatom taxa present in the lake core, the data provide important new insights
into the history of Glacial Lake Washburn. 相似文献
6.
Juan Pablo Corella Ana Moreno Mario Morellón Valentí Rull Santiago Giralt María Teresa Rico Ana Pérez-Sanz Blas Lorenzo Valero-Garcés 《Journal of Paleolimnology》2011,46(3):351-367
Sedimentological, mineralogical and compositional analyses performed on short gravity cores and long Kullenberg cores from
meromictic Montcortès Lake (Pre-Pyrenean Range, NE Spain) reveal large depositional changes during the last 6,000 cal years.
The limnological characteristics of this karstic lake, including its meromictic nature, relatively high surface area/depth
ratio (surface area ~0.1 km2; z
max = 30 m), and steep margins, facilitated deposition and preservation of finely laminated facies, punctuated by clastic layers
corresponding to turbidite events. The robust age model is based on 17 AMS 14C dates. Slope instability caused large gravitational deposits during the middle Holocene, prior to 6 ka BP, and in the late
Holocene, prior to 1,600 and 1,000 cal yr BP). Relatively shallower lake conditions prevailed during the middle Holocene (6,000–3,500 cal years
BP). Afterwards, deeper environments dominated, with deposition of varves containing preserved calcite laminae. Increased
carbonate production and lower clastic input occurred during the Iberian-Roman Period, the Little Ice Age, and the twentieth
century. Although modulated by climate variability, changes in sediment delivery to the lake reflect modifications of agricultural
practices and population pressure in the watershed. Two episodes of higher clastic input to the lake have been identified:
1) 690–1460 AD, coinciding with an increase in farming activity in the area and the Medieval Climate Anomaly, and 2) 1770–1950 AD,
including the last phase of the Little Ice Age and the maximum human occupation in late nineteenth and early twentieth centuries. 相似文献
7.
青海湖盆地因其独特的地理位置和气候环境,对气候变化响应敏感,是研究环境变化重要的场所。本研究在青海湖湖东沙地获取两个剖面,采用AMS14C测年,结合岩性、粒径组分和常量元素氧化物及其比值等多指标分析方法,重建了湖东沙地8.4 ka BP以来的环境演变过程。结果表明:除少量沉积物处于中等化学风化阶段外,大部分沉积物处于物理风化和初级化学风化阶段,说明自8.4 ka BP以来研究区气候环境总体上相对寒冷干燥。在千年尺度上,研究区不同时间段的气候环境存在较大的差异。8.4—4.2 ka BP气候相对温暖湿润;其中,8.4—6.2 ka BP河湖相和风成相沉积互层,表明存在明显的百年尺度气候波动;6.2—4.2 ka BP化学风化和淋溶作用较强,表明季风降水较多,径流较强,发育了较为稳定的湖泊沉积环境。4.2 ka BP以来湖泊消失,化学风化和淋溶作用减弱,沉积物从砾石转变为砂质黄土,显示气候较为冷干且波动较大。对比分析表明,中晚全新世以来青海湖湖东沙地的气候变化特征与东亚季风边缘区其他区域基本一致,并主导了区域沉积环境的演变过程。 相似文献
8.
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
9.
Jennifer A. Nelson Kathy Licht Catherine Yansa Gabriel Filippelli 《Journal of Paleolimnology》2010,43(1):1-13
Records from lake sediment cores are critical for assessing the relative stability of climate and ecosystems over the Holocene.
Duck Lake in south-central Lower Michigan, USA, was the focus of a study that identified how changes in the geochemical variables
in lake sediments relate to variations in regional climate and local land use during the Holocene. More than 8.5 m of lacustrine
sediment were recovered using Livingston and freeze corers and analyzed for organic carbon, inorganic (carbonate) carbon,
total nitrogen, and trace metals. Repeating packages of sediment (1–10 cm thick) that grade from light (inorganic carbon-rich)
to dark (organic carbon-rich) were found from the surface to a depth of about 8 m. Variations in the high-resolution gray
scale data from core X-radiographs are highly correlated to the relative amount of inorganic carbon. Geochemical analyses
of the upper 8.5 m of sediment revealed a wide range of values: 0.05–10.6% for inorganic carbon (i.e. 0.5–89% calcium carbonate)
and 1.1–28% for organic carbon (i.e. 2.7–70% organic matter). Organic carbon to nitrogen ratios indicate that most of the
sediment organic matter is produced within the lake. A core chronology based on eight AMS radiocarbon dates shows low sediment
accumulation rates (0.05 cm/year) from 10,000 to 3,800 cal year BP and higher sediment accumulation rates (0.1–0.3 cm/year)
from 3,800 cal year BP to present. We suggest that carbonate accumulates during relatively dry times, whereas organic matter
accumulation dominates when nutrient input to the lake is enhanced by wetter climate. The Duck Lake core records a distinct
low point in inorganic carbon deposition that may be related to the 8.2 ka cooling event now documented from several sites
in North America. Spectral analysis of gray scale values shows significant ~200-year periodicities over the past 8,000 years,
hypothesized to result from climate changes induced by solar forcing. Concentrations of trace metals (e.g. lead, iron, copper,
zinc) indicate the onset of regional anthropogenic influence about 150 cal year BP. 相似文献
10.
Quantitative reconstructions of mean July temperatures (T
jul) based on new and previously published pollen-stratigraphical data covering the last 2000 years from 11 lakes in northern
Fennoscandia and the Kola Peninsula are presented. T
jul values are based on a previously published pollen-climate transfer function for the region with a root-mean-square error
of prediction (RMSEP) of 0.99°C. The most obvious trend in the inferred temperatures from all sites is the general decrease
in T
jul during the last 2000 years. Pollen-inferred T
jul values on average 0.18 ± 0.56°C (n = 91) higher than present (where “present” refers to the last three decades based on pollen-inferred T
jul in core-top samples) are indicated between 0 and 1100 AD (2000–850 cal year BP), and temperatures −0.2 ± 0.47°C (n = 78) below present are inferred between 1100 and 1900 AD (850–50 cal year BP). No consistent temperature peak is observed
during the ‘Medieval Warm Period’, ca. 900–1200 AD (1100–750 cal year BP), but the cooler period between 1100 and 1900 AD
(850–50 cal year BP) corresponds in general with the ‘Little Ice Age’ (LIA). Consistently with independent stable isotopic
data, the composite pollen-based record suggests that the coldest periods of the LIA date to 1500–1600 AD (450–350 cal year
BP) and 1800–1850 AD (150–100 cal year BP). An abrupt warming occurred at about 1900 AD and the twentieth century is the warmest
century since about 1000 AD (950 cal year BP).
相似文献
A. E. BjuneEmail: |
11.
瓦窑沟剖面记录的早中全新世毛乌素沙地东南缘气候变化 总被引:1,自引:1,他引:0
毛乌素沙地位于中国东亚季风边缘区,对气候变化响应敏感。区域内分布着大量河湖相沉积地层,忠实记录了全新世以来气候变化状况。选取毛乌素沙地东南缘河湖相沉积地层为主的瓦窑沟剖面(WYG),在光释光(OSL)和14C(AMS 14C)测年基础上,对粒度、色度和有机质环境代用指标进行分析,反演得出毛乌素沙地早中全新世气候经历了3个阶段:约9.8 ka BP前气候干冷,广泛沉积沙层;9.8—9.6 ka BP气候逐渐回暖,为气候干冷转为暖湿的过渡期;9.6—5.2 ka BP气候相对暖湿,发育砂质泥炭层。推测计算得出,剖面地层沉积速率沙层>过渡层>砂质泥炭层,且各地层均存在次一级旋回,曲线旋回周期砂质泥炭层(480 a)>沙层(150 a)>过渡层(130 a),指示砂质泥炭层沉积速率最慢、形成时期气候相对稳定。研究结果与东亚季风区域古气候记录具有很好的可比性,是对东亚冬夏季风强度变化的全球性响应。 相似文献
12.
Holocene millennial-scale climate variations documented by multiple lake-level proxies in sediment cores from Hurleg Lake,Northwest China 总被引:2,自引:0,他引:2
Cheng Zhao Zicheng Yu Yan Zhao Emi Ito Kenneth P. Kodama Fahu Chen 《Journal of Paleolimnology》2010,44(4):995-1008
Millennial-scale climate variability has not been well documented in arid northwest China due to the scarcity of high-resolution,
well-dated paleoclimate records. Here we present multi-proxy records from sediment cores taken in freshwater Hurleg Lake on
the northeastern Tibetan Plateau, which reveal millennial-scale lake-level and climate variations over the past 8,000 years.
This high-elevation region is very sensitive to large-scale climate change, thus allowing us to better understand Holocene
climate variations in East Asia. The lake-level record, derived from lithology, magnetic mineralogy, carbonate isotopes, ostracode
shell isotopes and trace elements, X-ray fluorescence (XRF), and gray scale data, indicates a highly variable and generally
dry climate from 7.8 to 1 ka (1 ka = 1,000 cal year BP), and a relatively stable and wet climate after 1 ka. Superimposed
on this general trend, six dry intervals at 7.6–7.2 ka, 6.2–5.9 ka, 5.3–4.9 ka, 4.4–3.8 ka, 2.7–2.4 ka, and 1.7–1.1 ka were
detected from the high-resolution carbonate content and XRF data. The generally dry climate between 7.8 and 1 ka was almost
synchronous with the decrease of East Asian and Indian monsoon intensities shortly after 8 ka. The six dry intervals can be
correlated with weak monsoon events recorded in the East Asia and Indian monsoon regions, as well as the North Atlantic cold
events. Our data suggest that millennial-scale monsoon variations could cause highly variable climate conditions in arid northwest
China during the Holocene. These millennial-scale climate variations may reflect changes in solar variation and/or changes
in oceanic and atmospheric circulation. 相似文献
13.
Inorganic minerals form a major component of lacustrine sediments and have the potential to reveal detailed information on
previous climatic and hydrological conditions. The ability to extract such information however, has been restricted by a limited
understanding of the relationships between minerals and the environment. In an attempt to fill in this gap in our knowledge,
146 surface sediment samples have been investigated from 146 lakes on the Tibetan Plateau. The mineral compositions derived
from these samples by X-Ray Diffraction (XRD) were used to examine the relationships between mineral compositions and the
environmental variables determined for each site. Statistical techniques including Multivariate regression trees (MRT) and
Redundancy Analysis (RDA), based on the mineral spectra and environmental variables, reveal that the electrical conductivity
(EC) and Mg/Ca ratios of lake water are the most important controls on the composition of endogenic minerals. No endogenic
minerals precipitate under hyper-fresh water conditions (EC lower than 0.13 mS/cm), with calcite commonly forming in water
with EC values above 0.13 mS/cm. Between EC values of 0.13 and 26 mS/cm the mineral composition of lake sediments can be explained
in terms of variations in the Mg/Ca ratio: calcite dominates at Mg/Ca ratios of less than 33, whereas aragonite commonly forms
when the ratio is greater than 33. Where EC values are between 26 and 39 mS/cm, monohydrocalcite precipitates together with
calcite and aragonite; above 39 mS/cm, gypsum and halite commonly form. Information on the local geological strata indicates
that allogenic (detrital) mineral compositions are primarily influenced by the bedrock compositions within the catchment area.
By applying these relationships to the late glacial and Holocene mineral record from Chaka Salt Lake, five lake stages have
been identified and their associated EC conditions inferred. The lake evolved from a freshwater lake during the late glacial
(before 11.4 cal. ka BP) represented by the lowest EC values (<0.13 mS/cm), to a saline lake with EC values slightly higher
than 39 mS/cm during the early and mid Holocene (ca. 11.4–5.3 cal. ka BP), and finally to a salt lake (after 5.3 cal. ka BP).
These results illustrate the utility of our mineral-environmental model for the quantitative reconstruction of past environmental
conditions from lake sediment records. 相似文献
14.
《Basin Research》2018,30(Z1):186-209
We present new data addressing the evolution, activity and geomorphic impact of three normal faults in the Southern Apennines: the Vallo di Diano, East Agri and Monti della Maddalena faults. We show that these faults have minimum total throws of ca. 1000–2000 m, and throw rates of ca. 0.7–1 mm year−1 for at least the last ca. 18 ka. We demonstrate that for the Vallo di Diano and East Agri faults, the landscape is effectively recording tectonics, with relief, channel and catchment slopes varying along fault strike in the same manner as normal fault activity does, with little apparent influence of lithology. We therefore use these data to reconstruct the time‐integrated history of fault interaction and growth. From the distribution of knickpoints on the footwall channels, we infer two episodes of base level change, which we attribute to fault interaction episodes. We reconstruct the amount of throw accumulated after each of these events, and the segments involved in each, from the fault throw profiles, and use fault interaction theory to estimate the magnitude of the perturbations and past throw rates. We estimate that fault linkage events took place 0.7 ± 0.2 Ma and 1.4 ± 0.3 Ma in the Vallo di Diano fault, and 1 ± 0.1 in the East Agri Fault, and that both faults likely started their activity between 3 and 3.5 Ma. These fault linkage scenarios are consistent with the observed knickpoint heights. This method for reconstructing fault evolution could potentially be applied for any normal faults for which there is information about throw and throw rates, and in which channels are transiently responding to tectonics. 相似文献
15.
Diatom succession related to land use during the last 6000 years: a study of a small eutrophic lake in southern Sweden 总被引:1,自引:0,他引:1
Diatoms, pollen, physical and magnetic analyses of the sediments have been used to reconstruct the development over the last 6000 years of Lake Bussjösjön, a small lake in southern Sweden. Stratigraphic variations in a core of more than 15 m reveal changes in diatom assemblages, which correspond closely to changes in pollen, loss-on-ignition, and magnetic measurements that are related to land use and vegetation changes in the catchment. From ca 6000 BP to 2700 BP, a forest surrounded what was then a slightly eutrophic lake. The sudden appearance of Cyclostephanos dubius (Fricke) Round and several epiphytic/epipsammic diatoms at 2700 BP coincides with deforestation of the catchment (2700 BP to 2500 BP). A change in land use from predominantly pasture to arable land from 1300 BP to 1100 BP caused a high level of soil erosion with a decrease of C. dubius and the increase of Stephanodiscus species. An increase of epiphytic/epipsammic species coincides with increased arable farming and the change from a field-rotation to a crop-rotation system, and shows not only an increase in eutrophication but also changes in water depth. The influence of the catchment through time resulted in a smaller, shallower and eutrophic to hypertrophic lake. 相似文献
16.
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
17.
Partitioning of the grain-size components of Dali Lake core sediments: evidence for lake-level changes during the Holocene 总被引:4,自引:0,他引:4
Jule Xiao Zhigang Chang Bin Si Xiaoguang Qin Shigeru Itoh Zaur Lomtatidze 《Journal of Paleolimnology》2009,42(2):249-260
We recovered a sediment core (DL04) from the depocenter of Dali Lake in central-eastern Inner Mongolia. The upper 8.5 m were
analyzed at 1-cm intervals for grain-size distribution to partition the grain-size components and provide a high-resolution
proxy record of Holocene lake level changes. Partitioning of three to six components, C1, C2, C3 through C6 from fine to coarse
modes within the individual polymodal distributions, into overlapping lognormal distributions, was accomplished utilizing
the method of lognormal distribution function fitting. Genetic analyses of the grain-size components suggest that two major
components, C2 and C3, interpreted as offshore-suspension fine and medium-to-coarse silt, can serve as sediment proxies for
past changes in the level of Dali Lake. Lower modal sizes of both C2 and C3 and greater C3 and lower C2 percentages reflect
higher lake stands. The proxy data from DL04 core sediments span the last 12,000 years and indicate that Dali Lake experienced
five stages during the Holocene. During the interval ca. 11,500–9,800 cal year BP, lake level was unstable, with drastic rises
and falls. Following that interval, the lake level was marked by high stands between ca. 9,800 and 7,100 cal year BP. During
the period from ca. 7,100 to 3,650 cal year BP, lake level maintained generally low stands, but displayed a slight tendency
to rise. Subsequently, the lake level continued rising, but exhibited high-frequency, high-amplitude fluctuations until ca.
1,800 cal years ago. Since ca. 1,800 cal year BP, the lake has displayed a gradual lowering trend with frequent fluctuations. 相似文献
18.
Silvina Stutz C. Marcela Borel Sonia L. Fontana Laura del Puerto Hugo Inda Felipe García-Rodriguez Marcela S. Tonello 《Journal of Paleolimnology》2010,44(3):761-775
We analyzed pollen, non-pollen palynomorphs (NPPs), calcareous microfossils, plant macrofossils, diatoms, chrysophyte cysts,
opal phytoliths and organic matter content in a 123-cm sediment sequence from Nahuel Rucá Lake, a shallow, freshwater system
in the southeastern Pampa grasslands, Argentina. Three stages in the lake evolution were identified. Before 3,680 cal year BP,
only pollen, NPPs (dinoflagellate cysts and acritarchs) and ostracods were recovered, suggesting brackish/saline conditions
in the lake and nearby areas. Freshwater conditions are, however, indicated by Myriophyllum, Pediastrum and Zygnemataceae. The brackish/saline conditions could have been caused by marine influence during a Holocene sea level
high stand that affected the area ca. 6,000 year BP. Between 3,680 and 390 cal year BP, macrophyte pollen and plant macrofossils
indicate increasingly freshwater conditions in the lake and the adjacent area. Diatom and ostracod assemblages, however, suggest
brackish and oligotrophic conditions, giving way to freshwater and meso-eutrophic conditions toward the end of this period.
The relationship between submersed macrophytes (Myriophyllum, Potamogeton, Ceratophyllum, Chara) and planktonic algae (Chlorophyta and diatoms), suggests a shift in the lake from a clear to a turbid state. This turbid
state is more evident after 390 cal year BP. High values of Pediastrum, Scenedesmus and diatoms (Cyclotella meneghiniana, Aulacoseira granulata, A. muzzanensis) observed during this stage could have reduced light penetration, with consequent loss of submersed plants. Pollen and plant
macrofossils in the uppermost 20 cm indicate a shallow, freshwater lake similar to present, though an increase in brackish/freshwater
diatoms suggests an increase in salinity, perhaps related to periodic droughts. Opal phytoliths yield a regional paleoclimatic
reconstruction that agrees closely with inferences made using pollen, mammals and sediment characteristics. 相似文献
19.
Charly Massa Bianca B. Perren émilie Gauthier Vincent Bichet Christophe Petit Hervé Richard 《Journal of Paleolimnology》2012,48(1):241-258
This is the first integrated multiproxy study to investigate climate, catchment evolution and lake ecology in South Greenland. A 4-m-long sedimentary sequence from Lake Igaliku (61o 00?? N, 45o 26?? W, 15?m asl) documents major environmental and climatic changes in south Greenland during the last 10?ka. The chronology is based on a 210Pb and 137Cs profile and 28 radiocarbon dates. The paleoenvironmental history is interpreted on the basis of magnetic susceptibility, grain size, total organic carbon, total nitrogen and sulphur, sedimentation rates, pollen, and diatom assemblages. The basal radiocarbon date at ca. 10?cal?ka BP provides a minimum age for the deglaciation of the basin, which is followed by ~500?years of high sedimentation rates in a glacio-marine environment. After the glacio-isostatic emergence of the basin ca. 9.5?cal?ka BP, limnological and terrestrial proxies suggests early warmth, which may have been interrupted by a cold, dry and windy period between 8.6 and 8.1?cal?ka BP. A dry and windy event ~5.3?C4.8?cal?ka BP preceded the Neoglacial transition at Lake Igaliku, which is characterized by a shift toward moister and perhaps cooler conditions ~4.8?cal?ka BP, causing major changes in terrestrial and aquatic ecological conditions. Significant cooling is documented after ~3?cal?ka BP. Since ~1?cal?ka BP the climatic-driven changes were overprinted by the human influence of Norse and recent agriculture. 相似文献
20.
Hongya Wang Hongyan Liu Yanhua Liu Haiting Cui Niels Abrahamsen 《Journal of Paleolimnology》2010,43(2):345-367
The Qinling Mountain Range (33°–34°30′N, 107°–111°E; 3,767 m a.s.l.) lies south of the Chinese Loess Plateau and functions
as the boundary between ‘north’ and ‘south’ China. Taibai Mountain (33°41′–34°10′N, 107°19′–107°58′E; 3,767 m a.s.l.) is the
central massif and highest part of the range and is the highest mountain in eastern and central China, east of 105°E. It is
also one of two mountains higher than the modern climatic timberline and the only one where high alpine lakes (>2,500 m a.s.l.)
exist in eastern and central China. Sediments were recovered from Foye Chi (33°57′N, 107°44′E; 3,410 m a.s.l.), a small lake
on the southern slope of the mountain, and measured for magnetic properties. Chronological control was achieved with AMS 14C dating. Combined with analyses of particle-size, TOC, C/N, δ13Corg and pollen in these sediments, and magnetic properties of catchment soils, the mineral-magnetic data reveal late Holocene
palaeoenvironmental changes on the high-altitude southern slope of Taibai Mountain. Climate gradually ameliorated about 2,300 cal yr BP
and warm and wet conditions occurred afterwards, culminating from 1,700 to 1,510 cal yr BP. The climate began to deteriorate
at 1,510 cal yr BP, but was still warmer and wetter than present until ~663 cal yr BP. Cool, arid conditions peaked and were
cooler and drier than the present at 663–290 cal yr BP, coincident with the Little Ice Age. Climate became warmer and more
humid again after 290 cal yr BP. Data from these less anthropologically-disturbed alpine-lake sediments provide a record of
late Holocene palaeoenvironmental change that supplements information from historical documents and literature for eastern
and central China. 相似文献