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211.
苏州东南部全新世沉积特征及海侵 总被引:2,自引:0,他引:2
苏州东南部隶属太湖沉积区,该区全新世的沉积环境尤其是海侵问题,前人曾作过研究,存在一定的争议,根据对苏州东南部分地区的实际资料及前人资料的研究,对苏州东南全新世时的沉积特征及海侵发生时间,范围等问题提出了看法。 相似文献
212.
S. Lorenz B. Grieger Ph. Helbig K. Herterich 《International Journal of Earth Sciences》1996,85(3):513-524
We present atmospheric simulations of three different time slices of the late Quaternary using the ECHAM 3 general circulation
model in T42 resolution. In this work we describe the results of model runs for the time slices 6000 years BP (last climate
optimum), 21 000 BP (last glacial maximum) and 115 000 years BP (glacial inception). Although the solar insolation is known
for all time slices, a complete data set of the other boundary conditions which are necessary for running the atmospheric
model exists only for the last glacial maximum in the form of the CLIMAP reconstruction. For the other two time slices, which
are interglacial states like the modern climate, sea surface temperatures, land albedo and ice sheet topography are kept at
modern values and only the solar insolation is changed appropriately. The response of the model to solar insolation changes
is quite reasonable. The modelled anomalies are small and roughly opposite in sign for 6000 BP and 115 000 BP, respectively.
In the case of last glacial maximum, the glacial ice sheet topography and ice albedo produce a much larger climate anomaly
in the model. However, to enable a real test of model performance under glacial boundary conditions, the CLIMAP sea surface
temperatures, which are now known to be partly inaccurate, should be replaced by an updated “state-of-the-art” reconstruction. 相似文献
213.
Kurt A. Refsnider Benjamin J.C. Laabs David M. Mickelson Marc W. Caffee 《Quaternary Research》2008,69(1):130-144
During the last glacial maximum (LGM), the western Uinta Mountains of northeastern Utah were occupied by the Western Uinta Ice Field. Cosmogenic 10Be surface-exposure ages from the terminal moraine in the North Fork Provo Valley and paired 26Al and 10Be ages from striated bedrock at Bald Mountain Pass set limits on the timing of the local LGM. Moraine boulder ages suggest that ice reached its maximum extent by 17.4 ± 0.5 ka (± 2σ). 10Be and 26Al measurements on striated bedrock from Bald Mountain Pass, situated near the former center of the ice field, yield a mean 26Al/10Be ratio of 5.7 ± 0.8 and a mean exposure age of 14.0 ± 0.5 ka, which places a minimum-limiting age on when the ice field melted completely. We also applied a mass/energy-balance and ice-flow model to investigate the LGM climate of the western Uinta Mountains. Results suggest that temperatures were likely 5 to 7°C cooler than present and precipitation was 2 to 3.5 times greater than modern, and the western-most glaciers in the range generally received more precipitation when expanding to their maximum extent than glaciers farther east. This scenario is consistent with the hypothesis that precipitation in the western Uintas was enhanced by pluvial Lake Bonneville during the last glaciation. 相似文献
214.
The environmental history of the Kootenay Valley in the southern Canadian Rockies was reconstructed using lake sediment from
Dog Lake, British Columbia, and compared to other paleoenvironmental studies in the region to understand how vegetation dynamics
and fire regimes responded to climate change during the Holocene. A pollen-based vegetation reconstruction indicates five
periods of vegetation change. At 10,300 cal yr B.P. Pinus-Juniperus parkland colonized the valley and by 7600 cal yr B.P. was replaced by mixed stands of Pinus, Picea and Pseudotsuga/Larix. Fire frequencies increased to their Holocene maximums during the 8200–4000 cal yr B.P. period. From 5500–4500 cal yr B.P.
Pseudotsuga/Larix reached its maximum extent in the Kootenay Valley under a more frequent fire regime. At 5000 cal yr B.P. Picea and Abies began to expand in the area and by 4500 cal yr B.P. the forest shifted to a closed montane spruce forest type with dramatically
reduced fire frequency. The shift to less frequent forest fires after 4500 cal yr B.P., along with a moisterPicea – dominated closed forest, corresponds to Neoglacial advances in the Canadian Rockies and Coast Mountains. Fire intervals
after 4000 cal yr B.P. are significantly longer than the shorter fire intervals of the early to mid Holocene. A return to
drier, more open forest condition occurs between 2400–1200 cal yr B.P. with a slight increase in fire activity and summer
drought events. Lower lake levels inferred by charophyte accumulation rates during the 2400–1200 cal yr B.P. interval support
this moisture regime shift. An abrupt shift toPicea dominated forest occurred from 1200–1000 cal yr B.P. and a final period of wet-closed forest cover reaches its maximum extent
from 700–150 cal yr B.P. that appears to be a response to Little Ice Age cooling. Present forests are within their natural
range of variability but are predicted to shift again to a drier more open structure with increased Pseudotsuga/Larix cover. More frequent stand replacing fires and increased area burned likely will accompany this change due to continued global
warming. 相似文献
215.
Erin M. Barley Ian R. Walker Joshua Kurek Les C. Cwynar Rolf W. Mathewes Konrad Gajewski Bruce P. Finney 《Journal of Paleolimnology》2006,36(3):295-314
Freshwater midges, consisting of Chironomidae, Chaoboridae and Ceratopogonidae, were assessed as a biological proxy for palaeoclimate in eastern Beringia. The northwest North American training set consists of midge assemblages and data for 17 environmental variables collected from 145 lakes in Alaska, British Columbia, Yukon, Northwest Territories, and the Canadian Arctic Islands. Canonical correspondence analyses (CCA) revealed that mean July air temperature, lake depth, arctic tundra vegetation, alpine tundra vegetation, pH, dissolved organic carbon, lichen woodland vegetation and surface area contributed significantly to explaining midge distribution. Weighted averaging partial least squares (WA-PLS) was used to develop midge inference models for mean July air temperature (r
boot2 = 0.818, RMSEP = 1.46°C), and transformed depth (ln (x+1); r
boot2 = 0.38, and RMSEP = 0.58). 相似文献
216.
Sherilyn C. Fritz Paul A. Baker Geoffrey O. Seltzer Ashley Ballantyne Pedro Tapia Hai Cheng R. Lawrence Edwards 《Quaternary Research》2007,68(3):410-420
A 136-m-long drill core of sediments was recovered from tropical high-altitude Lake Titicaca, Bolivia-Peru, enabling a reconstruction of past climate that spans four cycles of regional glacial advance and retreat and that is estimated to extend continuously over the last 370,000 yr. Within the errors of the age model, the periods of regional glacial advance and retreat are concordant respectively with global glacial and interglacial stages. Periods of ice advance in the southern tropical Andes generally were periods of positive water balance, as evidenced by deeper and fresher conditions in Lake Titicaca. Conversely, reduced glaciation occurred during periods of negative water balance and shallow closed-basin conditions in the lake. The apparent coincidence of positive water balance of Lake Titicaca and glacial growth in the adjacent Andes with Northern Hemisphere ice sheet expansion implies that regional water balance and glacial mass balance are strongly influenced by global-scale temperature changes, as well as by precessional forcing of the South American summer monsoon. 相似文献
217.
Giovanni Zanchetta Alessandra Borghini Anthony E. Fallick Francesco P. Bonadonna Gabriello Leone 《Journal of Paleolimnology》2007,38(2):227-239
Ten meters of lacustrine deposits retrieved from Lake Pergusa (Sicily, southern Italy) were investigated through stable isotope
composition (carbon and oxygen) of authigenic carbonate (calcareous muds) and freshwater shells. The core chronology was established
through three AMS dates, and by correlation with a previously dated nearby core. Stable isotope data show that the lake water
evolution was mainly dominated by evaporation. Between ca. 20 and 28 ka the recovered sediments have very high δ18O values, likely corresponding to very dry climatic conditions. The observed rapid oscillations in the δ18O of the recovered sediments during this period also suggest important climatic fluctuations. More humid conditions dominated
during the Holocene period, with the wettest interval occurring between ca. 9000 and 3000 years BP. Late Holocene sediments
represent a substantial return to drier conditions. The available pollen data from a nearby core substantially confirm this
general climatic trend during the Holocene. The positive correlation between δ13C of the calcareous muds and carbonate content suggests that biological activity played a key role in the carbon isotope evolution
of dissolved inorganic carbon. However, a clear climatic signal is not evident from the δ13C record. 相似文献
218.
Sedimentology,clay mineralogy and grain-size as indicators of 65 ka of climate change from El’gygytgyn Crater Lake,Northeastern Siberia 总被引:2,自引:0,他引:2
Celeste A. Asikainen Pierre Francus Julie Brigham-Grette 《Journal of Paleolimnology》2007,37(1):105-122
El’gygytgyn Crater Lake, NE Siberia was investigated for sedimentological proxies for regional climate change with a focus
on the past 65 ka. Sedimentological parameters assessed relative to magnetic susceptibility include stratigraphy, grain size,
clay mineralogy and crystallinity. Earlier work suggests that intervals of high susceptibility in these sediments are coincident
with warmer (interglacial-like) conditions and well-mixed oxygenated bottom waters. In contrast, low susceptibility intervals
correlate with cold (glacial-like) conditions when perennial ice-cover resulted in anoxia and the dissolution of magnetic
carrier minerals.
The core stratigraphy contains both well-laminated to non-laminated sequences. Reduced oxygen and lack of water column mixing
preserved laminated sequences in the core. A bioturbation index based upon these laminated and non- laminated sequences co-varies
with total organic carbon (TOC) and magnetic susceptibility.
Clay mineral assemblages include illite, highly inter-stratified illite/smectite, and chlorite. Under warm or hydrolyzing
conditions on the landscape around the lake, chlorite weathers easily and illite/smectite abundance increase, which produces
an inverse relationship in the relative abundance of these clays. Trends in relative abundance show distinct down-core changes
that correlate with shifts in susceptibility. The mean grain-size (6.92 μm) is in the silt-size fraction, with few grains
larger than 65 μm. Terrigenous input to the lake comes from over 50 streams that are filtered through storm berms, which limits
clastic deposition into the lake system. The sedimentation rate and terrigenous input grain-size is reduced during glacial
intervals. Measurements of particle-size distribution indicate that the magnetic susceptibility fluctuations are not related
to grain size.
Lake El’gygytgyn’s magnetic susceptibility and clay mineralogy preserves regional shifts in climate including many globally
recognized␣events like the Younger Dryas and Bolling/Allerod. The sedimentary deposits reflect the climatic transitions starting
with MIS4 through the Holocene transition. This work represents the first extensive sedimentological study of limnic sediment
proxies of this age from Chukotka (Fig. 1).
This is the tenth in a series of eleven papers published in this special issue dedicated to initial studies of El'gygytgyn Crater Lake and
its catchment in NE Russia. Julie Brigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue. 相似文献
219.
The minimum sieve mesh size for fossil chironomid analysis is usually set at 100 μm, to ensure adequate recovery of small
species. Yet taking into account the labor intensity of sorting and identifying fossil chironomid remains, the large numbers
of samples requiring processing in paleoclimate studies with high temporal resolution, and the increasing need to engage non-specialist
analysts in this work, it seems appealing to sieve samples through a larger mesh size that would mainly retain easier-to-see,
easier-to-extract and easier-to-identify fossil specimens. In this study we evaluated the influence of sieve mesh size on
chironomid-based quantitative paleoenvironmental reconstructions in African lakes. We developed two chironomid-salinity calibration
data sets based on either the >100 μm or >150 μm size fractions of surface-sediment fossil assemblages, and compared the performance
of salinity-inference models derived from them. We find that, despite ∼35% additional fossil loss, restriction to >150 μm
data did not appreciably affect individual taxon optima or tolerances, and resulted in only a modest reduction of mean taxon
richness per lake. Parameters of statistical model performance were as good, or better than those of models based on the >100 μm
data, albeit only after excluding two atypically dilute (<100 μS/cm) lakes from the calibration. Application of inference
models based on >150 μm or >100 μm data to a 200-year fossil record from Lake Abiyata, Ethiopia, produced very similar trends,
amplitudes and uncertainty ranges of inferred past salinity change. Restriction to >150 μm data reduced the mean fossil yield
of core samples by ∼20% on average, i.e. fossil loss was markedly lower than in surface-sediment samples. Using the larger
mesh size reduced sample processing time by up to 50%, partly by removing a significant proportion of visually obstructive
organic debris, and partly by allowing a greater fraction of chironomid fossils to be identified directly in the sorting tray.
The fraction of 1st instar group taxa in surface-sediment samples was reduced from 13% to 3%, increasing the mean taxonomic
resolution of fossil assemblages, and thus their ecological specificity. 相似文献
220.
P. S. Minyuk J. Brigham-Grette M. Melles V. Ya. Borkhodoev O. Yu. Glushkova 《Journal of Paleolimnology》2007,37(1):123-133
The inorganic geochemistry of sediments from El’gygytgyn Lake shift in phase with interpreted paleoclimatic fluctuations seen
in the record over the past 250 ka. Warm periods, when the lake was seasonally ice free and fully mixed, are characterized
by increased concentrations of SiO2, CaO, Na2O, K2O, and Rb, by decreased contents of TiO2, Fe2O3, Al2O3, and MgO, and by a lower chemical index of alteration (CIA). Increased levels of SiO2 reflect increases in limnic productivity whereas many of the other elements and the CIA likely reflect increased hydrological
activity coincident with an increase in coarser sand and silt content and a decrease in clay mineral content. For cold/cooler
periods when perennial lake ice cover lead to a stratifed water column and anoxic bottom waters, the opposite is generally
observed suggesting a decrease in hydrological activity and an increase in post-depositional chemical alteration.
Peaks in P2O3 and MnO, coincident with an increased abundance of vivianite, suggest possible linkages to the paleoproductivity of local
fish fauna regardless of climate change across the region surrounding Lake El’gygytgyn. Strontium is high in concentration
during warmer intervals and may also be linked to paleoproductivity. Enrichment of the post-Eemian portion of the sediment
record in niobium, and yttrium appears independent of glacial–interglacial change; rather it may reflect a gradual shift in
the geomorphology of the catchment, particularly the hydrology of large alluvial fans along the western side of the lake.
In contrast to some lake records, changes in Zr concentration over time suggests only a weak, if any, increase in eolian sediment
supply during colder periods.
This is the first in a series of eleven papers published in this special issue dedicated to initial studies of El'gygytgyn Crater Lake and
its catchment in NE Russia. Julie Brigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue. 相似文献