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1.
This study tests the hypothesis that Fourier-transform infrared spectroscopy (FTIRS) of lake sediments can be used to infer
past changes in tree-line position and total organic carbon (TOC) content of lake water. A training set of 100 lakes from
northern Sweden spanning a broad altitudinal and TOC gradient from 0.7 to 14.9 mg/l was used to assess whether vegetation
zones and TOC can be modelled from FTIR spectra of surface sediments (0–1 cm) using principal component analysis (PCA) and
partial least squares (PLS) regression. Preliminary results show that FTIRS of lake sediments can be used to reconstruct past
changes in tree line and the TOC content of lake water, which is hardly surprising since FTIRS registers the properties of
organic and minerogenic material derived from the water mass and the drainage area. The FTIRS model for TOC gives a root mean
squared error (RMSECV) of calibration of 1.4 mg/l (10% of the gradient) assessed by internal cross-validation (CV) yielding
an Rcv2 of 0.64. This should be compared with a near-infrared spectroscopy (NIRS) and diatom transfer function for TOC from the same
set of lakes, which have a Rcv2 of 0.61 and 0.31, and RMSECV of 1.6 and 2.3 mg/l, respectively. The FTIRS-TOC model was applied to a Holocene sediment core
from a tree-line lake and the results show similar trends as inferences from NIRS and pollen from the same core. Overall,
the results indicate that changes in FTIR spectra from lake sediments reflect differences in catchment vegetation and TOC,
and that FTIRS-models based on surface-sediment samples can be applied to sediment cores for retrospective analysis. 相似文献
2.
On the Matmata Plateau, the sedimentological characteristics of the peri-desert loess and the interbedded palaeosols (particle-size distribution; chemical and mineralogical composition of major components; trace elements; clay mineral assemblage; heavy mineral assemblage; SEM-examined surficial textures of the quartz and palygorskite grains) show notable homogeneity: they also indicate its allochthonous origins (the main sedimentary source being the Great Eastern Erg), its transport as an aeolian suspension sediment prior to deposition, and moderate pedogenesis acting during deposition. The emplacement of the loess was synchronous with a ‘pluvial’ steppe palaeoenvironment moister than at present, as shown by 18O and 13C data. The palaeosols point to a weak pedogenesis and colluvial and alluvial reworking that were contemporary with the aeolian deposition. 14C radiometric dating suggests a main period of pluvial dust-fall/loess deposition in the Upper Pleistocene (until 10 000 yr BP) and a shorter one in the Middle Holocene. 相似文献
3.
ChineselOeSSischaracterizedbythegreatthicknessandgabscontinuity.ItrererdsQuaternalyclimaticoscillations(Liu,1985).Inrecentyears,scientistshavetriedtOusephysicalorchemicalparameterstOrecostructthepastclimaticchanges.FOrinstance,theparticlesizeisUSedasproxyindicatorforthewintermo~intensity(Liu,1993),stableoxygenisotopicrecordofCarbenatenodulesisusedtoreflectthetemperaturevallationsOfsoilformation(Han,1995),andrnsgneticsuSCeptibilityandCarhanatecontentareughasproxyparametersforsuxnmermourn… 相似文献
4.
A. Eisenhauer Z. R. Zhu L. B. Collins K. H. Wyrwoll R. Eichstätter 《International Journal of Earth Sciences》1996,85(3):606-614
U-series ages measured by thermal ionisation mass spectrometry (TIMS) are reported for a Last Interglacial (LI) fossil coral
core from the Turtle Bay, Houtman Abrolhos islands, western Australia. The core is 33.4 m long the top of which is approximately
5 m a.p.s.l. (above present sea level). From the232Th concentrations and the reliability of the U-series ages, two sections in the core can be distinguished. Calculated U/Th
ages in core section I (3.3 m a.p.s.l to 11 m b.p.s.l) vary between 124±1.7 ka BP (3.3 m a.p.s.l.) and 132.5±1.8 ka (4 m b.p.s.l.,
i.e. below present sea level), and those of section II (11–23 m b.p.s.l.) between 140±3 and 214±5 ka BP, respectively. The
ages of core section I are in almost perfect chronological order, whereas for section II no clear age-depth relationship of
the samples can be recognised. Further assessments based on the ϖ234U(T) criteria reveal that none of the samples of core section II give reliable ages, whereas for core section I several samples
can be considered to be moderately reliable within 2 ka. The data of the Turtle Bay core complement and extend our previous
work from the Houtman Abrolhos showing that the sea level reached a height of approximately 4 m b.p.s.l at approximately 134
ka BP and a sea level highstand of at least 3.3 m a.p.s.l. at approximately 124 ka BP. Sea level dropped below its present
position at approximately 116 ka BP. Although the new data are in general accord with the Milankovitch theory of climate change,
a detailed comparison reveals considerable differences between the Holocene and LI sea level rise as monitored relative to
the Houtman Abrolhos islands. These observation apparently add further evidence to the growing set of data that the LI sea
level rise started earlier than recognised by SPECMAP chronology. A reconciliation of these contradictionary observations
following the line of arguments presented by Crowley (1994) are discussed with respect to the Milankovitch theory. 相似文献
5.
Analyses of Eemian climate dynamics based on different reconstruction methods were conducted for several pollen sequences in the northern alpine foreland. The modern analogue and mutual climate sphere techniques used, which are briefly presented, complement one another with respect to comparable results. The reconstructions reveal the occurrence of at least two similar thermal periods, representing temperate oceanic conditions warmer and with a higher humidity than today. Intense changes of climate processes become obvious with a shift of winter temperatures of about 15 °C from the late Rissian to the first thermal optimum of the Eemian. The transition shows a pattern of summer temperatures and precipitation increasing more rapidly than winter temperatures. With the first optimum during the Pinus–Quercetum mixtum–Corylus phase (PQC) at an early stage of the Eemian and a second optimum period at a later stage, which is characterised by widespread Carpinus, climate gradients across the study area were less intense than today. Average winter temperatures vary between −1.9 and 0.4 °C (present-day −3.6 to 1.4 °C), summer temperatures between 17.8 and 19.6 °C (present-day 14 to 18.9 °C). The timberline expanded about 350 m when compared to the present-day limit represented by Pinus mugo. Whereas the maximum of temperature parameters is related to the first optimum, precipitation above 1100 mm is higher during the second warm period concomitant to somewhat reduced temperatures. Intermediate, smaller climate oscillations and a cooling becomes obvious, which admittedly represent moderate deterioration but not extreme chills. During the boreal semicontinental Eemian Pinus–Picea–Abies phase, another less distinct fluctuation occurs, initiating the oscillating shift from temperate to cold conditions. 相似文献
6.
In order to improve the reliability of climate reconstruction, especially the climatologies outside the modern observed climate
space, an improved inverse vegetation model using a recent version of BIOME4 has been designed to quantitatively reconstruct
past climates, based on pollen biome scores from the BIOME6000 project. The method has been validated with surface pollen
spectra from Eurasia and Africa, and applied to palaeoclimate reconstruction. At 6 cal ka BP (calendar years), the climate
was generally wetter than today in southern Europe and northern Africa, especially in the summer. Winter temperatures were
higher (1–5°C) than present in southern Scandinavia, northeastern Europe, and southern Africa, but cooler in southern Eurasia
and in tropical Africa, especially in Mediterranean regions. Summer temperatures were generally higher than today in most
of Eurasia and Africa, with a significant warming from ∼3 to 5°C over northwestern and southern Europe, southern Africa, and
eastern Africa. In contrast, summers were 1–3°C cooler than present in the Mediterranean lowlands and in a band from the eastern
Black Sea to Siberia. At 21 cal ka BP, a marked hydrological change can be seen in the tropical zone, where annual precipitation
was ∼200–1,000 mm/year lower than today in equatorial East Africa compared to the present. A robust inverse relationship is
shown between precipitation change and elevation in Africa. This relationship indicates that precipitation likely had an important
role in controlling equilibrium-line altitudes (ELA) changes in the tropics during the LGM period. In Eurasia, hydrological
decreases follow a longitudinal gradient from Europe to Siberia. Winter temperatures were ∼10–17°C lower than today in Eurasia
with a more significant decrease in northern regions. In Africa, winter temperature was ∼10–15°C lower than present in the
south, while it was only reduced by ∼0–3°C in the tropical zone. Comparison of palaeoclimate reconstructions using LGM and
modern CO2 concentrations reveals that the effect of CO2 on pollen-based LGM reconstructions differs by vegetation type. Reconstructions for pollen sites in steppic vegetation in
Europe show warmer winter temperatures under LGM CO2 concentrations than under modern concentrations, and reconstructions for sites in xerophytic woods/scrub in tropical high
altitude regions of Africa are wetter for LGM CO2 concentrations than for modern concentrations, because our reconstructions account for decreased plant water use efficiency. 相似文献
7.
Kaarina Sarmaja-Korjonen Seija Kultti Nadia Solovieva Minna Väliranta 《Journal of Paleolimnology》2003,30(4):415-426
Mid-Holocene changes in vegetation, palaeohydrology and climate were investigated from the sediments of Lake Vankavad in the northern taiga of the Usa Basin, NE European Russia, through the analysis of pollen, plant macrofossils, Cladocera and diatoms. Lake Vankavad was probably formed at ca. 5000 BP (ca. 5600 cal. BP) and initially it was shallow with a littoral cladoceran fauna. Macrofossil and pollen results suggest that dense Betula-Picea forests grew in the vicinity and the shore was close to the sampling point. At ca. 4600 BP (ca. 5400 cal. BP) the water level rose coincident with the decrease in the density and area of forests, probably caused by cooling climate and accelerated spread of mires. There was also a further rise in the water level at ca. 3500 BP (ca. 3800 cal. BP). The initiation of the lake, followed by two periods of rising water-level, as well as the increase in mire formation, was a consequence of a rise in groundwater level. This probably reflects lower evapotranspiration in a cooling mid-Holocene climate and/or higher precipitation in the lowland area. Also the decreased forest density and area may have contributed to the lower evapotranspiration. It is also possible that permafrost aggradation or changes in peat ecosystems might have affected the hydrological conditions in the area. 相似文献
8.
根据最新研究成果，对从古气候学角度展望未来全球气候变化问题的可能性和方法进行了讨论和总结。过去温室气体含量、温度、降水、海平面和突然气候变化等古资料记录及其分析加深了我们对于气候系统物理过程的理解，古气候类比方法以及古资料在气候模式校准中的应用也显示了古气候学对于未来气候预测的潜在意义。 相似文献
9.
Aquatic Biota and the Detection of Climate Change: Are there Consistent Aquatic Ecotones? 总被引：4，自引：5，他引：4
In this study, we analyse the cumulative rate of compositional change along an altitudinal gradient in the Swiss Alps for
three different groups of aquatic organisms – Cladocera, chironomids, and diatoms. In particular, we are interested in the
magnitude of unusually large changes in species composition that allows the detection of critical ecotones for each of these
three organism groups. The estimated rate-of-change is the distance in ordination space using principal coordinate analysis
based on chord distance and chi-square distance. These analyses highlight the cumulative rate-of-change and the cumulative
relative rate-of-change, as the chi-square distance is relative to the total species composition. We found that the major
changes in taxonomic composition for the three organism groups and therefore also the major ecotones are just below the modern
tree-line (1900–2000 m a.s.l.), which may indirectly be an effect of the tree-line. For diatoms and Cladocera (only chi-square
distance) there is also an ecotone at 2055 m a.s.l., which may be a direct or indirect response to climate. Further, the ecotone
region below the modern tree-line is much wider for chironomids, with an extension downwards due to a shift in relative abundance
patterns. For diatoms there is a stronger rate-of-change above 1650 m a.s.l. when chi-square distance is used. Coupled with
the even distribution of diatom richness, this suggests that at higher altitudes the change is more strongly associated with
a few species becoming dominant compared to lower elevations. Hence, there are considerable differences among the three organism
groups, suggesting that different environmental factors may influence the rates of compositional change within and among groups.
This supports the general usefulness of multi-proxy studies, namely the study of several independent groups of organisms to
reconstruct past environmental conditions but also points to the importance of careful site selection in such studies. 相似文献
10.
Climate friction and the Earth's obliquity 总被引：1，自引：0，他引：1