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1.
Ten sites near the Snake River Plain have consistent differences in their climatic histories. Sites at low elevation reflect the “early Holocene xerothermic” of the Pacific Northwest, whereas most climatic chronologies at high elevation indicate maximum warmth or aridity somewhat later, ca. 6000 yr ago. This elevational contrast in climatic histories is duplicated at three sites from the central Snake River Plain. For sites in such close proximity, the different chronologies cannot be explained by changes in atmospheric circulation during the late Quaternary. Rather, the differences are best explained by the autecology of the plants involved and the changing seasonal climate. The seasonal climatic sequence predicted by multiple thermal maxima explains the high- and low-elevation chronologies. During the early Holocene, maximum insolation and intensified summer drought in July forced low-elevation vegetation upward. However, moisture was not a limiting factor at high elevation, where vegetation moved upward in response to increased length of growing season coincident with maximum September insolation 6000 yr ago.  相似文献   

2.
Humic acids were extracted from six Paleosols (buried soils), ranging in radiocarbon ages from about 6000 to 29,000 yr. The N distribution (total N, amino acid-N, amino sugar-N, ammonia-N and “unknown” N) in each Paleosol humic acid was determined. After 6 M HCl hydrolysis, amino acid-N and ammonia-N decreased with increasing age but relative concentrations of “unknown” N increased. Aspartic acid, valine and serine were the most stable amino acids. Concentrations of amino sugars were very small, with concentrations of galactosamine exceeding those of glucosamine. The data showed that with increasing age, identifiable N components were converted to complex polymeric compounds whose identities still remain unknown. The different N-forms in HA's of geological times are valuable geochemical markers which provide useful information on the history of these soils.  相似文献   

3.
Geochemical exploration in secondary environments can be viewed as a particular manifestation of indirect geological observation. Geochemical anomalies in complex sample media reflect dispersion signatures, generally much disguised by secondary or higher-order mechanical and physico-chemical processes such as mixing, comminution, dilution, (re)transportation, weathering etc. Such complexities often make a thorough understanding of the origin of any particular sample type difficult ot obtain. The objective of data analysis in this context is to convert the geochemical data into a meaningful “signal”, particularly useful for prospecting, and other, in this case irrelevant, variability or “noise”. The experience of the last decades of practical exploration has clearly shown that statistical as well as geographical geochemical anomaly patterns are multi-element signatures. Using suitable multivariate statistical procedures (in the present case principal components modelling), it is possible to simultaneously define both a background data model and to quantify multivariate geochemical anomalies. This type of data analysis is guided very strongly by geological interaction, in which the emphasis is on modelling the background population(s), coupled with geographic plotting facilities. This outlier-screening facility is critical for many types of geochemical data evaluation. An example of this approach is described below. Another application of indirect multivariate data analysis is represented by PLS (Partial Least Squares) regression, which is a supervised pattern recognition and regression technique. We use it here to predict modal scheelite occurrences from regional stream-sediment data.  相似文献   

4.
Understanding the influence of natural climatic variability on modern fisheries is complicated by over a century of industrial fishing. Archaeological data provide unique opportunities for assessing precolonial and preindustrial fisheries. Records show that anchoveta-vs sardine-dominated fisheries correlate with 20th-century climate change in the Pacific Basin and are linked to multidecadal climatic variability. The “anchovy regime” is characterized by cooler conditions and lower frequency El Niño/Southern Oscillation (ENSO) events, while the “sardine regime” is associated with warmer conditions and higher frequency ENSO. Fish remains excavated at Lo Demás, an Inca-period (ca. A.D. 1480–1540) fishing site at 13°25′S on the Peruvian coast, document a shift from an anchoveta-to a sardine-dominated fishery at about A.D. 1500. This shift correlates with records for increasing ENSO frequency at the same time. Middle and late Holocene sites have archaeofish assemblages that also suggest regime changes. Here we show that changes in fish regimes can result from natural variability and we support the potential role of archaeological assemblages in tracking multidecadal climate change in the Pacific Basin throughout the Holocene (0–11,500 cal yr B.P.).  相似文献   

5.
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6.
In order to examine high-frequency variations of East Asian winter monsoon in Quaternary climatic extremes, two typical loess–paleosol sequences in the Chinese Loess Plateau were investigated. Sandy layers in the loess deposits, the “Upper sand” and “Lower sand” (layers L9 and L15, respectively), which represent a high-resolution record of paleomonsoon changes, have been sampled at intervals of 5–6 cm from sections at Luochuan and Xifeng. The grain size and magnetic susceptibility was measured for all samples. The grain-size results (a proxy of winter monsoon strength) indicate that the winter monsoon strength fluctuated on a millennial timescale during cold climatic extremes, with climatic events of a few hundred to a few thousand years. However, the winter monsoon was relatively stable during warm periods. The magnetic susceptibility signal (a proxy of summer monsoon intensity) is practically constant over the same period. This is tentatively explained by the assumption that the summer monsoon intensity was too low to be recorded in the magnetic susceptibility signal. The intensified winter monsoon events show periodicities in a range of 1000 to 2770 yr, with a dominant cycle of approximately 1450 yr. The detection of this oscillation in older glacial stages strongly suggests that it may be a pervasive cycle of the cold climatic phases of the Quaternary. Millennial-scale variations of the winter monsoon may be caused by instability of the westerly jet, which is determined by temperature differences between the polar and the equatorial regions.  相似文献   

7.
A bottomland flora that prevailed between 9900 and 6000 cal yr B.P. in a North Carolina stream valley may not reflect a regionally much wetter Atlantic climate, coeval with record drought in the Great Plains region and assumed dry Gulf coastal conditions. Such conditions were inferred for 6000 ± 1000 yr ago when the Bermuda High may have consistently occupied summer positions far to the NE. Arid episodes coeval with the Little River local wet interval are known from eolian sediments and pollen spectra in the Atlantic and the Gulf coastal plain. For multiple reasons, the regional extent, intensity, and duration of coastal aridity and alternating wet phases and the Bermuda High positions are not yet adequately constrained. The climate and edaphic causes for the steadily growing predominance of southern pines over hardwoods, achieved between 8900 and 4200 cal yr B.P. at different sites at different times are similarly still unresolved. New data from Shelby Lake, AL, reconfirms that no credible field or other proxy evidence exists for a previously postulated “catastrophic Gulf hurricane phase” in the late Holocene.  相似文献   

8.
Paleoecological records from two Holocene peat bogs in northern Germany are linked by two microscopic volcanic ash layers, correlated by petrology and geochemistry to explosive volcanism on Iceland. The younger “Microlite tephra” cannot be correlated to any known eruption, while the older tephra layer is identified as a deposit of the Hekla 3 eruption. The tephra layers are dated by an age–depth regression of accelerator mass spectrometry 14C ages that have been calibrated and combined in probability distributions. This procedure gives an age of 730–664 cal yr B.C. for the “Microlite tephra” event and 1087–1006 cal yr B.C. for the Hekla 3 event. Accordingly, the tephra layers were deposited during the late Bronze Age. At this time, human settlement slowly increased pressure on the environment, as indicated by changes in woodland pollen composition at the two bogs. The tephra-marker horizons further show that the palynologically defined transition from the Subboreal to the Subatlantic Period is synchronous in the investigated area. However, the macroscopic visible marker in peat, the change from fibrous to sapric peat, the “Schwarztorf-Weißtorf-Kontakt,” is asynchronous. Bog vegetation did not immediately react in unison to a climatic change at this pollen zone boundary; instead, the timing of vegetation change depended on the location within the bog.  相似文献   

9.
Man's interference with the landscape process places him in the role of a “paraglacial” agent: He tends to duplicate ice-age stresses such as deforestation, accelerated erosion, and climate alteration. Against a background climatic control dictated by the Milankovich mechanisms, the 10,000-year history of the Holocene has seen very large secondary modulations that must be better understood so that they may be distinguished from anthropogenic effects. If they are exogenetically controlled, as it seems, then they are probably predictable from astronomic data. Four geomorphic type areas are selected for demonstrating Holocene changes in tropical regions, because they have been somewhat neglected within the framework of Quaternary science and because they include some of the most fragile and easily disrupted environments: lakes, semiarid desert margins, coastlines, and coral reefs. In a nutshell, the tropical Holocene has seen three major changes: (a) the evolution from the hyperaridity of the last pleniglacial stage to the “postglacial pluvial”; (b) the “climatic optimum,” which was highly diachronous and strongly retarded as it shifted from low latitudes to high; (c) the postoptimum “deterioration” that has involved desiccation of lakes, readvances of the deserts, fall of sea level, and truncation of coral reefs. While this deterioration is predictable in terms of an interglacial-glacial climatic transition, strong natural climatic oscillations, not yet well understood, together with man's activities, make the future a cause for concern.  相似文献   

10.
Evidence of (at least) five rapid hemispheric coolings of about 5°C during the last 105 yr has been found, each event spread over not more than about a century, as examples of a global-scale climatic intransitivity. Only some of them lead to a complete glaciation at the northern continents, others ended after a few centuries by a sudden warming (“abortive glaciation”). Starting from a modified version of Wilson's hypothesis of Antarctic ice surges, an air-sea interaction model with realistic geophysical parameters is outlined to interpret the sudden initiation of the North American ice sheet. Special attention is given to the Atlantic section, where the climatic anomalies during the last glaciation appear to have been significantly larger than in other sections.  相似文献   

11.
Palynological, paleopedological, and glacial geomorphological evidence from the area of the high plain of Bogotá in the Colombian Eastern Cordillera indicates a significant climatic warming around 18,00014C yr B.P. Comparison of dated vegetation changes, pedogenic episodes, and glacier fluctuations provides the basis for defining the so-called “La Laguna Interstadial” that lasted from 19,500 to 17,000 yr B.P. During this interstadial period, mean annual temperatures in the tropical Andes were up to 4°C higher than during the preceding and following stadial periods, when full-glacial conditions prevailed and temperatures were up to 8°C colder than at present.  相似文献   

12.
Dimensions of Holocene relict channels and sedimentological characteristics of point bars associated with these relict channels were used to reconstruct a Holocene history of long-term changes in magnitudes of 1.58-yr floods in Upper Mississippi Valley watersheds of southwestern Wisconsin. The reconstructed record of floods shows relatively large and persistent (nonrandom) departures from contemporary long-term average flood magnitudes. The flood history indicates climatic changes that are broadly similar to climatic changes indicated from fossil pollen in the same region. The Holocene floods ranged from about 10–15% larger to 20–30% smaller than contemporary floods of the same recurrence frequency. Large floods were characteristic between about 6000 – 4500 and 3000 – 2000 yr B.P., and during a brief interval after 1200 yr B.P. Small floods were common between about 8000 – 6500, 4500 – 3000, and 2000 – 1200 yr B.P. These fluvial responses were found to be closely associated with a long-term episodic mobility and storage of sediments in the Wisconsin watersheds. During periods of relatively large floods, relatively rapid lateral channel migration either reworked or removed extensive tracts of valley bottom alluvium. In contrast, during periods of relatively small floods, relatively slow lateral channel migration is apparent and the channel and floodplain system appear to have been relatively stable.  相似文献   

13.
The Richmond River Valley of northern N.S.W. contains a late Pleistocene succession dating back to approximately 250,000 yr B.P. Dinoflagellate and spore-pollen assemblages from the lowest interval, the lower “Dungarubba Clay” of Drury (1982), indicate deposition in a restricted estuarine environment at approximately 250,000 yr. Deposition in the overlying interval, the upper “Dungarubba Clay” and “Gundurimba Clay”, at approximately 120,000 yr B.P., began in a restricted estuary, but rising sea level caused inundation and deposition in a more open, marine-dominated environment. Dinoflagellate cyst assemblages from the last interglaciation (stage 5) are interpreted by analogy with those from the morphologically similar, modern Broken Bay, N.S.W. They are indicative of an open, marine-dominated environment and imply that barrier formation in the Richmond River Valley, and possibly elsewhere in northern N.S.W., did not commence until after the initial postglacial transgression. Synchronous changes in sea level and rainforest development suggest that there was no significant time lag between climate and sea-level change.  相似文献   

14.
Three samples of gem quality plagioclase crystals of An60 were experimentally deformed at 900 °C, 1 GPa confining pressure and strain rates of 7.5–8.7×10−7 s−1. The starting material is effectively dislocation-free so that all observed defects were introduced during the experiments. Two samples were shortened normal to one of the principal slip planes (010), corresponding to a “hard” orientation, and one sample was deformed with a Schmid factor of 0.45 for the principal slip system [001](010), corresponding to a “soft” orientation. Several slip systems were activated in the “soft” sample: dislocations of the [001](010) and 110(001) system are about equally abundant, whereas 110{111} and [101] in ( 31) to ( 42) are less common. In the “soft” sample plastic deformation is pervasive and deformation bands are abundant. In the “hard” samples the plastic deformation is concentrated in rims along the sample boundaries. Deformation bands and shear fractures are common. Twinning occurs in close association with fracturing, and the processes are clearly interrelated. Glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation. Grain boundaries of tiny, defect-free grains in healed fracture zones have migrated subsequent to fracturing. These grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization. Nucleation via small fragments can explain a non-host-controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi-brittle to plastic transition.  相似文献   

15.
An empirical model is presented that derives the initial isotopic composition of water used by terrestrial plants and the relative humidity that prevailed during photosynthesis from the isotopic composition of oxygen and carbon-bound hydrogen in the plant cellulose. The model uses fixed values for the fractionation factors that describe the evapotranspirative isotopic enrichment of leaf waters and the biochemical fractionations occurring during cellulose synthesis. When applied to cellulose extracted from fossil plant matter, the model can be used to generate quantitative estimates of the temperature and moisture regimes of the past. The application of the model is demonstrated by consideration of δ18O and δ2H results on cellulose from fossil wood collected at Brampton, Ontario.The Brampton data yield records of systematic changes in the temperature and moisture regimes of the eastern Great Lakes region over the past 11 500 a. Both mean annual temperature and photosynthetic humidity increased substantially from values much lower than the present at the end of the Wisconsin Stage, to values higher than the present in the mid-Holocene. The phase relation of the paleotemperature and paleohumidity curves engenders differentiation of the climatic history at Brampton into four characteristic zones: (I) a period of ameliorating climate predominantly much colder and drier than now, lasting until about 7400 B.P.; (II) an “early Hypsithermal” period of warm and dry conditions, that transformed after 6000 a B.P. into (III) the “main Hypsithermal” time of warmth and pronounced moistness. Subsequent climatic detrioration led to establishment of (IV) cool, moist conditions like the present sometime within the last few thousand years.  相似文献   

16.
It has been proposed that Victorian brown coal can be considered as a two-component structure — a lignocellulosic “host”, containing various amounts of weakly bound or entrapped “guest” material together with very small amounts of inorganic and/or mineral matter. The latter predominantly consists of wax esters and/or terpenoid material. In this paper we describe attempts to gain structural information regarding the more complex, “host” component of the coal. Our initial model compound has been humic acid that can be readily obtained from the coal by alkaline extraction. It has been found that “pure” humic acid, free from material associated with the “guest” components of the coal, can be obtained by a highly selective, low-yielding alkaline extraction. This humic acid has been studied by nmr spectroscopy and pyrolysis gas chromatography-mass spectroscopy (py-gc/ms). The products arising from py-gc/ms have been compared with those obtained from similar pyrolysis of whole coals. Alkylation of humic acids using alkyl halides in the presence of base has been successfully carried out and reactivity of the resulting materials compared with those of the parent coal and humic acid.  相似文献   

17.
Data on glacier, tree-line, tree-ring, pollen, and ice-core variations in North America, Greenland, and Europe during the last 2000 yr (up to A.D. 1800) are compared in detail on the century time scale. Only data that may be indicative of summer temperature changes are included, since these comprise most of the available paleoclimatic information, although some variations (especially of glaciers) may have been in response to precipitation changes instead. Radiocarbon dates and 14C-dated records are converted to calendar (dendrochronological) years using the calibration of M. Stuiver (1982, Radiocarbon 24, 1–26). Despite the basic uncertainties in dating, interpretation, response times, and “noise level” of proxy climatic data, there is at times good agreement among different kinds of evidence from within a region to suggest an episode of generally warmer or cooler summers. Three previously identified episodes find expression in records from all of the regions considered: the “Little Ice Age” of the last few centuries, a “Medieval Warm Period” around the 12th century A.D., and an earlier cold period some time between the 8th and 10th centuries. However, the timing of minima and maxima within these episodes seems to have varied from region to region (although the evidence is consistent within regions). In the 15th century, summers were warm in the eastern Canadian Arctic and southern Greenland while there was a cold episode in Europe and western North America.  相似文献   

18.
A laminated sequence (core BAP96-CP 24°38.12′N, 110°33.24′W; 390 m depth) from the Alfonso Basin in Bay of La Paz, southern Gulf of California, contains a record of paleoceanographic and paleoclimatic changes of the past 7900 yr. Radiolarian assemblages and magnetic susceptibility are used as proxies of oceanographic and climatic variability. The records provide a regional scenario of the middle and late Holocene, suggesting two major climatic regimes and several millennial-scale events. Conditions relatively warmer and drier than today occurred from 7700 to 2500 cal yr BP, promoting the intensification of evaporation processes and the prevalence of the Gulf of California water in the Basin. These conditions correlate with strong droughts in the middle Holocene of North America and with minimal incursion of tropical waters into the Gulf of California. Proxies indicate a warm scenario and the dominance of the Equatorial Surface Water in the Alfonso Basin from 2400 to 700 cal yr BP, suggesting the intensification of ENSO cycles. A climatic signal between 1038 and 963 cal yr BP may be correlated with global signal of the “Medieval Warm Period.” Several cooling events are recognized at 5730, 3360, 2700, 1280 and 820 cal yr BP and are associated with intensification of northwest winds leading to upwellings and enhanced productivity in the Basin.  相似文献   

19.
Kylen Lake, located within the Toimi drumlin field, is critically positioned in relation to Late Wisconsin glacial advances, for it lies between the areas covered by the Superior and St. Louis glacial lobes between 12,000 and 16,000 yr B.P. The pollen and plant-macrofossil record suggests the presence of open species-rich “tundra barrens” from 13,600 to 15,850 yr B.P. Small changes in percentages of Artemisia pollen between 14,300 and 13,600 yr B.P. appear to be artifacts of pollen-percentage data. Shrub-tundra with dwarf birch, willow, and Rhododendron lapponicum developed between 13,600 and 12,000 yr B.P. Black and white spruce and tamarack then expanded to form a vegetation not dissimilar to that of the modern forest-tundra ecotone of northern Canada. At 10,700 B.P. spruce and jack pine increased to form a mosaic dominated by jack pine and white spruce on dry sites and black spruce, tamarack, and deciduous trees such as elm and ash on moist fertile sites. At 9250 yr B.P. red pine and paper birch became dominant to form a vegetation that may have resembled the dry northern forests of Wisconsin today. The diagram terminates at 8410 ± 85 yr B.P. Climatic interpretation of this vegetational succession suggests a progressive increase in temperature since 14,300 yr B.P. This unidirectional trend in climate contrasts with the glacial history of the area. Hypotheses are presented to explain this lack of correspondence between pollen stratigraphy and glacial history. The preferred hypothesis is that the ice-margin fluctuations were controlled primarily by changes in winter snow accumulation in the source area of the glacier, whereas the vegetation and hence the pollen stratigraphy were controlled by climatic changes in front of the ice margin.  相似文献   

20.
J. -B. Edel   《Tectonophysics》2003,363(3-4):225-241
Generally, the lack of bedding criteria in basement units hampers the interpretation of paleomagnetic results in terms of geotectonics. Nevertheless, this work demonstrates that successive remagnetizations recorded in Early Carboniferous metamorphic and plutonic units, without clear bedding criteria, can be used to constrain a polyphased tectonic evolution consisting of a regional clockwise rotation, followed by a folding phase, a tilting phase and a second regional clockwise rotation.Metamorphic, ultrabasic, tonalitic and granitic rocks from different parts of Limousin (western French Massif central; 45.5°N/1.25°E), which underwent metamorphism during Devonian–Early Carboniferous or were intruded in the Early–Middle Carboniferous, were sampled in order (a) to identify the magnetic overprinting phases and the related tectono-magmatic events and (b) to constrain the regional and plate tectonic evolution of Limousin. Paleomagnetic results from 32 new and 26 sites investigated previously show that at least 90% of the magnetization isolated in rocks older than 330 Ma are overprints. In agreement with results from adjacent areas of the Variscan belt, the major overprinting phases occurred: (a) in the last stages of the major exhumation phase [332–328 Ma; mean Virtual Geomagnetic Pole (VGP) “Cp”: 37°N/70.5°E], (b) during the post-collisional syn-orogenic extension (325–315 Ma; VGP “B”: 11°N/114°E), (c) in the Latest Carboniferous and Early Permian (VGP “A1”: 27°N/149°E) and (d) in the Late Permian (VGP “A”: 48°N/146°E). The Middle–Late Carboniferous overprints “Cp” and “B” are contemporaneous with emplacement of leucogranitic, crustal derived plutons, and probably result from the hydro-thermal activity related to the magmatism. The drift from “Cp” directions to “B” directions implies that after 330 Ma, Limousin underwent a clockwise rotation by 65°, together with the Central Europe Variscides. The “Bt” components, the VGPs of which deviate from the mean apparent polar wander path (APWP) of the belt, are interpreted as “B” overprints tilted during Late Variscan tectonics, that is, in the time range 325–315 Ma. The first and most important generation of “Bt” overprints was tilted during NW–SE folding associated with NE–SW shortening, updoming and emplacement of leucogranitic plutons. The second generation reveals southeastward tilting due to NE-striking normal faulting. The drift from “B” to “A1” directions implies that Limousin has participated to the second clockwise rotation by 40° of the whole belt in Westphalian times.  相似文献   

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