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1.
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
2.
Kaarina Weckström Jan Weckström Linda-Marie Yliniemi Atte Korhola 《Journal of Paleolimnology》2010,43(1):61-73
Subarctic and arctic lakes are the focus of many paleolimnological studies, as they are still among the least impacted lakes
by humans. Hence they provide an excellent setting for studies on long-term climatic variability without the overriding effects
of direct anthropogenic perturbation. On the other hand, these ecosystems are highly vulnerable to even moderate anthropogenic
influence like long-distance airborne transport of nutrients and pollutants. The paleolimnological studies conducted in these
areas usually include a multitude of different proxies, but so far only few have used the green algal group of Pediastrum Meyen. These algae, however, preserve well in sediments and can be identified to species level, which lends them potential
as a paleo-proxy. In this study we analysed the present Pediastrum assemblages from surface-sediments of 16 subarctic lakes in Finnish Lapland as well as bottom samples from the same sediment
cores, which are “spot” samples from the recent past. We found a total of 14 Pediastrum taxa, five of which occurred at moderate to high relative abundances. The majority of the taxa showed distinct relationships
to environmental variables measured. Of these, pH and dissolved organic carbon (DOC) explained most of the variance in the
distribution of Pediastrum and generally summarised the main environmental gradients in our data set well. Five of the studied lakes lacked Pediastrum taxa altogether, and Pediastrum occurred at low abundances in four additional lakes. All of these nine lakes have extremely low nutrient concentrations and
generally lowest pH and DOC in the data set and were defined by barren catchment areas and scarce lake macrophyte growth.
According to a top–bottom analysis of sediment cores, the Pediastrum assemblages of the study lakes have changed moderately, suggesting changed environmental conditions in the lakes. Although
these changes appear to be climate-related, more studies are needed to confirm this hypothesis. 相似文献
3.
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
4.
I. Mazzini C. Faranda M. Giardini C. Giraudi L. Sadori 《Journal of Paleolimnology》2011,45(2):243-256
We present a Holocene record of climate and environmental change in central New York (USA) inferred using lithologic and stable
isotope data from two sediment cores recovered in Cayuga Lake. The record was divided into three intervals: (1) early Holocene
(~11.6–8.8 ka), (2) Hypsithermal (~8.8–4.4 ka), and (3) Neoglacial (~4.4 ka to present). The early Holocene began abruptly,
with rising lake level and relatively deep water. Between ~10.8 and 9.2 ka, cool and dry conditions prevailed at a time of
maximum solar insolation. This anomaly has been referred to as the “post-Younger Dryas climate interval” and lasted ~1,600 years,
the approximate length of one “Bond cycle.” The Hypsithermal was the warmest, wettest and most biologically productive interval
of the Holocene in central New York. The Hypsithermal was characterized by centennial to multi-centennial-scale variability.
The 8.2 ka event is one such variation. The Neoglacial was an interval of generally cooler and dryer conditions, falling lake
levels, and several prominent climate anomalies. At approximately 2.4 ka, δ13C of bulk organic matter increased abruptly by 5‰ as lake level declined, and the lake flora was dominated by Chara sp. during the coldest interval of the Neoglacial. Numerous sediment variables display increased variability ~2.0 ka, which continues
today. Archaeological data from the literature suggest that Native American populations may have been large enough to impact
land cover by about 2.4 ka and we hypothesize that the “Anthropocene” began at about that time in central New York. We also
found paleolimnological evidence for the Medieval Warm Period (~1.4–0.5 ka), which was warmer and wetter than today, and for
the Little Ice Age (~500–150 years ago), a period with temperatures colder than today. 相似文献
5.
I. Kristen H. Wilkes A. Vieth K.-G. Zink B. Plessen J. Thorpe T. C. Partridge H. Oberhänsli 《Journal of Paleolimnology》2010,44(1):143-160
Comparing the organic matter (OM) composition of modern and past lake sediments contributes to the understanding of changes
in lacustrine environments over time. We investigate modern plant and lake-water samples as well as modern and ancient sediment
samples from the Tswaing Crater in South Africa using biomarker and stable carbon isotope analyses on bulk OM and specific
biomarker compounds. The characteristic molecular markers for higher land plants (predominantly C3-type deciduous angiosperms)
in Lake Tswaing are long-chain n-alkanes (n-C27−33), n-alkanols (n-C28+30), stigmasterol, β-sitosterol, β-amyrin, α-amyrin and lupeol. The C17
n-alkane, tetrahymanol, gammaceran-3-one and C29 sterols dominate the lipid fraction of autochthonously produced OM. By comparing stable carbon isotope analyses on bulk OM
and the characteristic biomarkers, we follow the modern carbon cycle in the crater environment and find indications for methanotrophic
activity in the lake from isotopically depleted moretene. A comparative study of core sediments reveals changes in the terrestrial
(C3 versus C4) and aquatic bioproductivity and allows insights into the variability of the carbon cycle under the influence
of changing climatic conditions for the time from the end of the last glacial (Termination I) to the late Holocene, ca. 14,000–2,000
calibrated years before present (years BP). The most pronounced changes occur in the aquatic realm after ca. 10,000 years
BP when our results imply climate swings from more humid to more arid and after 7,500 years BP to gradually more humid conditions
again, which can be related to a shift in the position of the Inter-Tropical Convergence Zone or to changes in the tropical
atmosphere–ocean interaction. Long-chain alkenones (LCAs) have been identified in ancient lake sediments from Africa for the
first time. They occur in samples older than 7,500 years BP and their distribution (dominance of C38 and of tri- over tetra-unsaturated LCAs) is distinctly different from other published records suggesting a to date unknown
source organism. 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.
Liseth Pérez Peter Frenzel Mark Brenner Jaime Escobar Philipp Hoelzmann Burkhard Scharf Antje Schwalb 《Journal of Paleolimnology》2011,46(1):59-74
We inferred late Pleistocene and early Holocene (24–10 ka BP) environmental conditions in and around Lago Petén Itzá, Guatemala
from ostracode remains in the lake sediments. Multivariate statistics were run on autecological information for 29 extant
ostracode species collected in 63 aquatic ecosystems on the Yucatán Peninsula along a steep, increasing NW–S precipitation
gradient and across a large altitudinal range. Conductivity and water depth are the most important factors that shape ostracode
communities. Transfer functions were developed and applied to fossil ostracode assemblages in a ~76-m sediment core (PI-6,
~85 ka) taken in 71 m of water from Lago Petén Itzá, to infer past shifts in conductivity and water level. Results suggest
climate was cold and wet during the Last Glacial Maximum (LGM). Alternating dry and wet conditions characterized the deglacial.
Early Holocene climate was warmer and wetter. The LGM was characterized by low ostracode species richness (4 spp.) and abundance
(<940 valves g−1), dominance of benthic over nektobenthic taxa, abundant Physocypria globula, conductivity as low as 190 μS cm−1, and clay-rich sediments with relatively high total organic carbon and low C/N ratios (<14), suggesting relatively deeper
water at the core site associated with abundant precipitation. Greatest water depth at the core site during the LGM occurred
late in the period and was ~50 m. The deglacial was characterized by drier conditions, higher ostracode species richness (6
spp.) and abundances up to 18,115 valves g−1, dominance of nektobenthic species, and presence of shallow-water and littoral-zone indicators such as Heterocypris punctata and Strandesia intrepida, conductivity up to 550 μS cm−1, C/N ratios as high as 37, and gypsum deposition. Lowest inferred lake depth at the core site during the deglacial was ~20 m.
The early Holocene was characterized by high numbers of ostracode remains, up to 25,500 valves g−1, and the presence of L. opesta and P. globula. Cytheridella ilosvayi was absent from late Pleistocene sediments, suggesting it colonized northern Central America during the Holocene. 相似文献
8.
Exposures along the lower Kaministiquia River (near Thunder Bay, Ontario, Canada) provide insight into early Holocene lake
level fluctuations and paleoenvironmental conditions in the northwestern Lake Superior basin. These exposures show at least
two large paleochannels which were downcut into offshore sediments, and were later filled with >2 m of sand, ~3 m of rhythmically
laminated silt and clay, and ~6 m of interbedded silt and sand. Buried by the rhythmically laminated silty clay unit is a
well-preserved organic deposit with abundant plant macrofossils from terrestrial and emergent taxa, including several upright
tree trunks. Three AMS radiocarbon ages were obtained on wood and conifer cones from this deposit: 8,135 ± 25 (9,130–9,010
cal), 8,010 ± 25 (9,010–8,780 cal), and 7,990 ± 20 (8,990–8,770 cal) BP. This sequence records an early postglacial high-water
phase, followed by the Houghton lowstand, and reflooding of the lower Kaministiquia River Valley. The drop in lake level associated
with the Houghton phase forced the ancestral Kaministiquia River to downcut. By ~9,100 cal (~8,100) BP, older channels eroded
into subaqueous underflow fan deposits in the Thunder Bay area near Fort William Historical Park (FWHP) were abandoned and
colonized by a Picea-Abies-Larix forest. Based on stratigraphic data corrected for differential isostatic rebound, the lake was below the Sault Ste. Marie
bedrock sill between at least 9,100 cal (8,100) and 8,900 cal (8,000) BP. Shortly after 8,900 cal BP, the lake quickly rose
and buried in situ lowland vegetation at FWHP with varved sediments. We argue that this transgression was due to overflow
from glacial Lakes Agassiz or Ojibway associated with the retreat of the Laurentide Ice Sheet from the Nakina moraine and/or
the Cochrane surge margins in the Hudson Bay Lowlands. A continued rise in lake level after 6,420 ± 20 (7,400 cal) BP at FWHP
may record uplift of the North Bay outlet above the Sault Ste. Marie bedrock sill and the onset of the Nipissing transgression
in the Lake Superior basin. 相似文献
9.
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
10.
We present a paleolimnological record spanning the Holocene from a small lake on Russell Island (Lake PW02), in the central
Canadian Arctic Archipelago (74.07° N, 97.77° W, 182 m asl). Fragilarioid diatom types in the genera Pseudostaurosira, Staurosira and Staurosirella constitute >90% of valves in fossil samples. Using modern biogeographic data which specify the temperature optima of the
Fragilarioid diatom taxa, we present new inferences about the timing of paleoclimatic changes in the central Arctic islands.
The early Holocene was characterized by maximum values for sediment organic matter, and lower ratios of Staurosirella pinnata to Staurosira construens v. venter, suggesting warm summer air temperatures between about 9500−6500 cal year BP. Influxes of biogenic silica and diatom valves
decreased following 4000 cal year BP, the sediment accumulation rate slowed and diatom taxa of the littoral zone diversified,
suggesting cooler summers and more persistent lake ice. Variations in the species composition of the assemblages indicate
paleoclimatic changes that are in broad agreement with other paleoenvironmental records from the Arctic including melt records
from the Agassiz Ice Cap. Although autecological data remain incomplete for Fragilarioid taxa, our results indicate differences
in these taxa in responses to paleoenvironmental change and underline the potential for the increased use of these taxa in
paleoenvironmental reconstructions. The record from Lake PW02, as in other records from Arctic lakes with low algal diversity
throughout the Holocene, shows a pronounced increase in diatom diversity since the 1920s, and diatom production since the
1970s far exceeds any recorded during the Holocene. 相似文献
11.
M. E. Kirby S. P. Lund W. P. Patterson M. A. Anderson B. W. Bird L. Ivanovici P. Monarrez S. Nielsen 《Journal of Paleolimnology》2010,44(3):819-839
High-resolution terrestrial records of Holocene climate from Southern California are scarce. Moreover, there are no records
of Pacific Decadal Oscillation (PDO) variability, a major driver of decadal to multi-decadal climate variability for the region,
older than 1,000 years. Recent research on Lake Elsinore, however, has shown that the lake’s sediments hold excellent potential
for paleoenvironmental analysis and reconstruction. New 1-cm contiguous grain size data reveal a more complex Holocene climate
history for Southern California than previously recognized at the site. A modern comparison between the twentieth century
PDO index, lake level change, San Jacinto River discharge, and percent sand suggests that sand content is a reasonable, qualitative
proxy for PDO-related, hydrologic variability at both multi-decadal-to-centennial as well as event (i.e. storm) timescales.
A depositional model is proposed to explain the sand-hydrologic proxy. The sand-hydrologic proxy data reveal nine centennial-scale
intervals of wet and dry climate throughout the Holocene. Percent total sand values >1.5 standard deviation above the 150–9,700 cal year
BP average are frequent between 9,700 and 3,200 cal year BP (n = 41), but they are rare from 3,200 to 150 cal year BP (n = 6). This disparity is interpreted as a change in the frequency of exceptionally wet (high discharge) years and/or changes
in large storm activity. A comparison to other regional hydrologic proxies (10 sites) shows more then occasional similarities
across the region (i.e. 6 of 9 Elsinore wet intervals are present at >50% of the comparison sites). Only the early Holocene
and the Little Ice Age intervals, however, are interpreted consistently across the region as uniformly wet (≥80% of the comparison
sites). A comparison to two ENSO reconstructions indicates little, if any, correlation to the Elsinore data, suggesting that
ENSO variability is not the predominant forcing of Holocene climate in Southern California. 相似文献
12.
A paleolimnological record of Holocene climate and environmental change in the Temagami region,northeastern Ontario 总被引:3,自引:0,他引:3
The Arcellacean (Thecamoebian) fauna was assessed in five Holocene sediment cores obtained from James and Granite lakes in the Temagami region of northeastern Ontario. In addition, palynological analysis was carried out on two of these cores, one each from James and Granite lakes. The first indication of postglacial colonization by plants was the appearance of rare Cupressaceae pollen, dated to 10,800 yr BP. Plant diversity began to increase by 10,770 yr BP when Pinus spp. and Larix migrated into the area. The first appearance of arcellaceans occurred after 9650 yr BP in assemblages dominated by Centropyxis constricta and opportunistic Centropyxis aculeata. High abundances of charophytes in the cores until 8800 yr BP indicated that macroalgae were proliferating at this time. This deposition is interpreted to have occurred during the draining of an ice-marginal lake following the retreat of the Laurentide Ice Sheet. Based on pollen analysis, warmer conditions associated with the Holocene Hypsithermal prevailed in the area from 6250 to 4115 yr BP. The stable, open Great Lakes – St. Lawrence type forest that developed here at the beginning of the Hypsithermal continues to prevail to the present. The periodic colonization of the lake by beavers (Castor canadensis) acted as a control on water-level and eutrophication through the Holocene. Evidence of eutrophication was indicated in the core samples by the abundance of high levels of the alga Pediastrum and the arcellacean Cucurbitella tricuspis. Eutrophication periodically developed when beavers dammed a site, causing the rate of flow in drainage streams to slow and stagnant conditions occurred. When the site became depleted of the nearby trees, which were preferred by beaver (Betula, Alnus and Populus), the dam would be abandoned, causing the water-level to drop. Stagnant conditions were reduced as flow levels increased, reducing eutrophication and resulting in recovering forest stands. In addition, the lowering water levels would result in encroachment of the forest along the lake shore. This cycle occurred many times in the history of this lake as indicated by fluctuations in the size of arcellacean populations. 相似文献
13.
Valentí Rull José A. López-Sáez Teresa Vegas-Vilarrúbia 《Journal of Paleolimnology》2008,40(1):399-411
This paper evaluates the potential usefulness of non-pollen palynomorphs or NPPs (microfossils other than pollen and spores
present in palynological preparations) contained in lake sediments in the paleolimnological reconstruction of high altitude
environments (>4,000 m) from the Venezuelan Andes. A synthetic, quantitative approach is employed, instead of the classical
analytical and mostly qualitative approach commonly used so far for NPPs. The main sources of variation are the Pediastrum–Botryococcus alternation and the relationship between these two algae and animal remains such as Acari legs, postabdomina of Cladocera,
mandibles of other invertebrates, and an unknown type called LVA-1. Other significant microfossils are remains of Rivularia-type and turbellarian oocytes, including Gyratrix. The sequence initiates around 15,000 calibrated years before present (cal BP) with the deglaciation of the lake catchment,
high water levels and still cold climates. A phase of lower lake levels was recorded between about 12,000 and 6,000 cal BP.
Temperatures increased by around 9,000 and 7,000 cal BP, and then decreased until 6,000 cal BP. Since that time, both lake
levels and temperature increased again and stabilized at about 4,000 cal BP, when they reach modern-like values. These results
show a good agreement with previous studies based on pollen, diatom and oxygen isotope analyses, and provide additional paleoecological
information, as for example the possibility of a previously unrecorded Younger Dryas signal. The inclusion of quantitative
NPP analysis in routine paleolimnological studies using synthetic methods is thus recommended. Possible future improvements
are suggested, mainly those related with the development and use of NPP modern analogs. 相似文献
14.
Sediment geochemistry of Lake Daihai,north-central China: implications for catchment weathering and climate change during the Holocene 总被引:2,自引:0,他引:2
Qianli Sun Sumin Wang Jie Zhou Zhongyuan Chen Ji Shen Xiuping Xie Feng Wu Peng Chen 《Journal of Paleolimnology》2010,43(1):75-87
A 12.87-m-long sediment core was retrieved from closed-basin Lake Daihai in the monsoon–arid transition zone of north-central
China. Oxides of major elements and their ratios normalized to Al in the AMS-14C-dated core were employed to evaluate chemical weathering intensity (CWI) in the lake drainage basin, which reflects hydrothermal
conditions in the study area. Lower CWI periods occurred prior to 14.5 ka BP, and during the intervals ca. 11.7–10.3, 3.5–3.2,
2.6–1.7 ka BP, and 1.2–0 ka BP, indicating relatively low temperatures and moisture availability. Greater CWI during the intervening
periods ca. 14.5–11.7, 10.3–9.0, 3.2–2.6, and 1.7–1.2 ka BP, with the maximum CWI at ca. 6.7–3.5 ka BP, imply ameliorated
hydrothermal conditions in the lake basin, i.e. higher temperatures and precipitation. Exceptionally low CWI, associated with
high CaO/MgO ratio during ca. 9.0–6.7 ka BP, suggests higher evaporation rates in the area under warmer temperature. Overall,
CWI displays in-phase variations with changes in organic matter (TOC, TN), carbonate (CaCO3) and pollen assemblages, all of which are related to variations in monsoon effective precipitation. High CWI indicates strong
monsoon-induced precipitation, whereas low CWI reflects a weak precipitation regime. The optimum hydrothermal status, recorded
by the strongest CWI and maximum monsoon effective precipitation during ca. 6.7–3.5 ka BP defines the Holocene climate optimum
(HCO) in the Lake Daihai region. These results indicate that the HCO prevails after the early Holocene in the monsoon–arid
transition zone of north-central China. Temperature and precipitation variations during most of the Holocene, inferred from
the lake sediments, are due largely to insolation forcing. Dry but warm conditions ca. 9.0–6.7 ka BP, however, probably reflect
the complex interactions between insolation and geography (e.g. altitude and local topography). 相似文献
15.
C. F. M. Lewis G. D. M. Cameron T. W. Anderson C. W. HeilJr. P. L. Gareau 《Journal of Paleolimnology》2012,47(3):493-511
Water levels in the Lake Erie basin are inferred from glacial lake times to present. An era of early to middle Holocene lowstands
is defined below outlets by a submerged paleo-beach, and truncated reflectors in glaciolacustrine sediment beneath a mud-covered
wave-cut terrace. Also, the glacial clay surface above the paleo-shore level has elevated shear strength because of porewater
drainage during subaerial exposure. Below the paleo-shore where exposure did not occur, clay strength remained normal. Sedimentation
rates were reduced during the lowstands. The distortion of once-level shore zone indicators by differential glacial rebound
was removed by computing original elevations of the indicators using an empirical model of rebound based on observations of
upwarped former lake shorelines. Erie water-level history was inferred from a plot of the original elevations of lake-level
constraints and outlets versus age. The lake history was validated by reference to ~83 water-level indicators, not used as
constraints. During the deglaciation, lake-crossing moraines were likely eroded by fluvial drainage into low-level Lake Ypsilanti
and a subsequent unnamed low lake to produce the Lorain Valley and Pennsylvania Channel. Once inflow from the upper Great
Lakes basins was directed to Ottawa Valley about 10,400 (12,270 cal BP), Erie water levels descended in a dry, evaporative
climate to a closed lowstand during which ostracode δ18O increased ~2‰ above present values. Lake level began to rise 6,000 to 7,000 (6,830 to 7,860 cal) BP in response to increased
atmospheric moisture and later, to northern inflow as the Nipissing Transgression returned upper Great Lakes drainage to Lake
Erie by about 5,200 (6,000 cal) BP. At that time, the lake overflowed the uplifted Lyell–Johnson Sill north (downstream) of
the present Niagara Falls at higher-than-present levels. After recession of the Falls breached this sill about ~3,500 (~3,770 cal)
BP, Lake Erie fell 3–4 m to its present Fort Erie–Buffalo Sill. The extended low-water phase with its isolated sub-basins
could have restricted migration of aquatic fauna. The early to middle Holocene closed-basin response highlights the sensitivity
of Lake Erie to climatic reductions in its water budget. 相似文献
16.
Christoph Mayr Andreas Lücke Nora I. Maidana Michael Wille Torsten Haberzettl Hugo Corbella Christian Ohlendorf Frank Schäbitz Michael Fey Stephanie Janssen Bernd Zolitschka 《Journal of Paleolimnology》2009,42(1):81-102
A combination of carbon-to-nitrogen ratios (TOC/TN), Rock Eval-analyses, and stable isotope values of bulk nitrogen (δ15N) and organic carbon (δ13Corg) was used to characterize bulk organic matter (OM) of a piston core from the Patagonian maar lake Laguna Potrok Aike (Argentina)
for the purpose of palaeoenvironmental reconstruction. Sedimentary data were compared with geochemical signatures of potential
OM sources from Laguna Potrok Aike and its catchment area to identify the sources of sedimentary OM. Correlation patterns
between isotopic data and TOC/TN ratios allowed differentiation of five distinct phases with different OM composition. Before
8470 calibrated 14C years before present (cal. yrs BP) and after 7400 cal. yrs BP, isotopic and organo-geochemical fingerprints indicate that
the sediments of Laguna Potrok Aike consist predominantly of soil and diatom OM with varying admixtures of cyanobacterial
and aquatic macrophyte OM. For a short phase of the early Holocene (ca. 8470–7400 cal. yrs BP), however, extremely high input
of soil OM is implied by isotopic fingerprints. Previous seismic and geochronological results indicate a severe lake-level
drop of 33 m below present-day shortly before 6590 cal. yrs BP. It is suggested that this lake level drop was accompanied
by increased erosion of shore banks and channel incision enhancing soil OM deposition in the lake basin. Thus, isotopic data
can be linked to hydrological variations at Laguna Potrok Aike and allow a more precise dating of this extremely low lake
level. An isotopic mixing model was used including four different sources (soil, cyanobacteria, diatom and aquatic macrophyte
OM) to model OM variations and the model results were compared with quantitative microfossil data. 相似文献
17.
Francine McCarthy Sarah Tiffin Adam Sarvis John McAndrews Stephan Blasco 《Journal of Paleolimnology》2012,47(3):429-445
Microfossils have been critical in unravelling the complex postglacial history of Georgian Bay. Thecamoebians (testate amoebae/rhizopods)
record paleolimnological conditions, and pollen stratigraphy allows correlation across the basin, where sedimentation has
been spatially and temporally discontinuous. Because parts of Georgian Bay have been non-depositional or erosional since the
end of the Nipissing transgression (~5,000 (5,800 cal) BP), early Holocene features are exposed on the lakebed. Among these
are shoreline features, such as submerged beaches and relict channels, associated with low-level Lake Hough that was driven
far below the level of basin overflow. Cores taken throughout Georgian Bay record the existence of closed basin conditions
that persisted several centuries around 7,500 (8,300 cal) BP, corresponding to the late Lake Hough lowstand. Evidence for
hydrologic closure includes a low-diversity centropyxid-dominated thecamoebian fauna around the boundary between pollen subzones
2a and 2b in the Flowerpot Beach core, Flowerpot and Killarney basins, and in Severn Sound. This low-diversity centropyxid-dominated
fauna is interpreted as recording the development of slightly brackish conditions as a result of a hydrologic deficit associated
with relatively arid conditions in the Great Lakes basin during the early Holocene pine zone (~8,800–7,200 (9,900–8,050 cal)
BP). The rest of the Holocene record in Georgian Bay (where it is preserved) is more diverse and dominated by difflugiid thecamoebians:
predominantly Difflugia oblonga prior to human settlement, and Cucurbitella tricuspis since high-density human occupation and agriculture (and resulting eutrophication) began with the Wendat First Nations people
around Severn Sound about 750 years ago. The implication that water budget fluctuations leading to discernible variations
in lake level and water chemistry occurred in the relatively recent geologic past is significant to studies of global climate
change and resource management in the Great Lakes, one of the world’s largest freshwater resources. 相似文献
18.
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
19.
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
20.
Pere Anadón Andrea Moscariello Julio Rodríguez-Lázaro M. L. Filippi 《Journal of Paleolimnology》2006,35(3):593-616
Stable isotopes and trace-element contents of ostracod (Candona neglecta) valves mostly from the Holocene portion of two assembled cores from Petit Lac (Lake Geneva, Switzerland-France) were analysed
in order to depict the geochemical record of post-glacial environmental changes of this lake. Additional stable isotope and
trace element data from the gastropod Bithynia tentaculata (shells and opercula) from some intervals of these cores, as well as previous data from bulk carbonate from the lower part
of the studied intervals were also considered. Mg/Ca and Sr/Ca molar ratios for the Holocene lake water have been estimated
from evaluations of the partitioning coefficients for Mg and Sr for C. neglecta and B. tentaculata taking into account the modern-lake water composition. This study shows an overall gentle trend to higher δ18O values in C. neglecta valves from the Boreal interval (mean −8.44‰) to the upper part of the core (mean −8.11‰). This trend is superimposed to
higher frequency oscillations of stable isotope values and trace element ratios, especially through the upper Older Atlantic
and the Subboreal. The overall isotopic oxygen trend includes several shifts in δ18O of about 1‰. These shifts are interpreted as major regional-global climate changes that have also been observed in other
coeval δ18O and pollen records which reflect the Holocene climate variability in other European basins. Especially well-defined peaks
in some episodes like Older Atlantic (~8200 yr BP), Younger Atlantic – Subboreal transition (~5600 yr BP) and early Subatlantic
(~ 2500 yr BP) correspond to well-recognized events in globally-distributed records. Some of these shifts are correlated with
pulses in the lake-level curve of the Lake Geneva.
An erratum to this article is available at . 相似文献