排序方式: 共有7条查询结果,搜索用时 0 毫秒
1
1.
2.
A carbonate buildup of Middle Triassic age, the Esino Limestone, outcrops in the Southern Calcareous Alps of Lombardy (N Italy). Along its margin and within the open subtidal facies, the Esino Limestone contains calcite cement-filled cavities of cm to m size. These features, known as evinosponges, may form pervasive networks within the host rock. The filling consists of concentric, isopachous layers of fibrous low-Mg calcite crystals characterized by strong undulose extinction and bent cleavages. The cement crusts are non-luminescent under cathodoluminescence, but both cements and host rock are cross-cut by micro-fractures filled with bright-luminescent calcite, related to late void-filling sparite. Mixing of different carbonates is reflected in stable isotope data. On the hand specimen scale, the oxygen and carbon isotope compositions of cements and host rock show little variation. When compared on a regional scale, the values cover a broad range from δ18O(PDB)=?5‰ to ?12‰ and from δ13O =0‰ to +3‰. The linear covariant trends defined by the oxygen and carbon isotope data for different sampling regions reflect the admixture of late, isotopically depleted calcite with an isotopically enriched non-luminescent calcite of early diagenetic origin. The Esino Limestone fibrous cements, which were probably precipitated in the marine or marine-meteoric phreatic environment, were affected by late diagenetic processes that caused mineral deformation and isotopic depletion through recrystallization and the admixture of a later calcite. These later calcites precipitated from penetrative fluids possibly related to Late Triassic volcanic activity and/or to the Late Cretaceous/Early Palaeogene alpine orogeny. 相似文献
3.
MATTHEW R. BENNETT DAVID HUDDART RICHARD I. WALLER NICHOLAS G. MIDGLEY SILVIA GONZALEZ ALEXANDRE TOMIO 《Boreas: An International Journal of Quaternary Research》2004,33(2):97-107
This paper describes the internal architecture of a push moraine formed by a winter-spring surge of Hagafellsjökull-Eystri (Iceland) in 1998/99. The sedimentary architecture of this push moraine consists of a multilayered slab of glaciofluvial sediments with a monoclinal structure that has been displaced laterally by the advancing ice margin. The crest and ice-distal face of the moraine consist of subhorizontal sediment sheets, while the ice-proximal face dips steeply (45° to 90°) towards the ice margin. The core of the moraine consists of frozen sediment and thin slabs of glacier ice are embedded in its proximal face. The sediment slabs are characterized by both brittle and ductile styles of deformation. We argue that the observed variation in deformation style is dependent on whether the glacial foreland was frozen or unfrozen at the time of displacement. Frozen foreland would behave in a brittle fashion, while unfrozen foreland is likely to have deformed in a more ductile manner. The associated spatial variations in the degree of foreland freezing could be explained by variation in ice-marginal snow cover. We conclude that the thermal regime of the foreland, and the timing of the ice advance, is of importance to the style of internal deformation found within ice-marginal push moraines. 相似文献
4.
5.
Climate variability in the SE Alps of Italy over the past 17 000 years reconstructed from a stalagmite record 总被引:1,自引:0,他引:1
SILVIA FRISIA REA BORSATO CHRISTOPH SPÖTL IGOR M. VILLA FRANCO CUCCHI 《Boreas: An International Journal of Quaternary Research》2005,34(4):445-455
Stalagmite SV1 from Grotta Savi, located at the SE margin of the European Alps (Italy), is the first Alpine speleothem that continuously spans the past c . 17kyr. Extension rate and δ18 Oc record for the Lateglacial probably reflect a combination of temperature and rainfall, with rainfall exerting the dominant effect. Low speleothem calcite δ18 Oc values were recorded from c . 14.5 and 12.35 kyr, during GI-1 (Bølling— Allerød) interstadial, which in our interpretation, was warm and wet. The GS-1 (Younger Dryas) was characterized by a shift to heavier δ18 Oc , coinciding with δ13 Cc enrichment and extremely low extension rate (<8 μm/year). These characteristics indicate that GS-1 climate was cool and dry in the SE Alps. Calibration using historical data revealed that there is a positive δ18 Oc /dT relationship. A 1°C rise in mean annual temperature should correspond to c . 2.85% increase of SV-1 δc18 Oc . We reconstructed a slow and steady temperature rise of c . 0.5°C since 10 kyr BP, in broad agreement with reconstructions from pollen data for SE Europe. Stalagmite SV1 indicates that climate variability in the SE Alps has been influenced by the Mediterranean Sea for the past c . 17 kyr. 相似文献
6.
DOMENICO ANFOSSI ENRICO FERRERO DAVIDE SACCHETTI SILVIA TRINI CASTELLI 《Boundary-Layer Meteorology》1997,82(2):193-218
Vertical velocity fluctuations were measured in theatmospheric surface layer by means of an ultrasonic anemometer andhigher order correlations were calculated on two time series, recordedin unstable and neutral conditions, and selected for the wholemeasurement period on the basis of the inversion test (stationaritytest). Comparisons have been made between observed and predictedcorrelations by considering Gaussian joint-PDF and Gram-Charlierseries expansions truncated to the fourth and sixth order as doneearlier by Frenkiel and Klebanoff. A bi-Gaussian PDF, given by amixture of two Gaussian PDFs, has also been considered. This lasthas been constructed assuming that either the first three or the firstfour moments are given, and the relationships between correlationfunctions of different order are derived. The departure from Gaussianbehaviour in both stability conditions is derived. Though Gram-Charlier series expansions show a good correspondence toexperimental reality, their use as non-Gaussian probabilitydistributions cannot be suggested in theoretical approaches andshould be considered with care in practical applications, due topossible occurrences of small negative probabilities. The resultsshown in this paper support the applicability of the bi-Gaussian PDFcreated using up to the fourth moment. 相似文献
7.
Origin and palaeoenvironmental significance of lamination in stalagmites from Katerloch Cave,Austria
The origin and environmental dependencies of lamination in stalagmites from Katerloch, common in speleothems from other cave sites, are examined in detail. Petrographic observations and chemical analyses (including isotopes) of stalagmites and modern calcite were combined with multi‐annual cave monitoring. All investigated stalagmites are composed of low‐Mg calcite and show white, porous laminae and typically thinner, translucent dense laminae. The binary lamination pattern results from changes in the calcite fabric: white, porous laminae are characterized by a high porosity and abundant fluid inclusions and also by enhanced vertical growth and thinning towards the flanks. Translucent, dense laminae exhibit a compact fabric and constant thickness of individual growth layers. U‐Th dating supports an annual origin of the lamination and the seasonally changing intensity of cave ventilation provides a robust explanation for the observed relationships between lamination, stable C isotopic compositions and trace elements (Mg, Sr and Ba). The seasonally variable air exchange, driven by temperature contrasts between the cave interior and outside atmosphere, modulates the rate and amount of CO2 degassing from the drip water and affects the hydrochemistry and consequently the fabric of the precipitating calcite. Although cave air composition and drip rate are both major variables in controlling CO2 degassing from the drip water, the seasonally changing ventilation in Katerloch exerts the primary control and the results suggest a secondary (amplifying/attenuating) influence of the drip rate. Drip rate, however, might be the controlling parameter for lamina development at cave sites experiencing only small seasonal cave air exchange. Importantly, the seasonally variable composition of drip water does not reflect the seasonal cycle of processes in the soil zone, but results from exchange with the cave atmosphere. The alternating porous and dense calcite fabric is the expression of a variable degree of lateral coalescence of smaller crystallites forming large columnar crystals. The white, porous laminae represent partial coalescence and form during the warm season: low calcite δ13C values are linked to low δ13C values of cave air and drip water during that time. This observation corresponds to times of reduced cave ventilation, high pCO2 of cave air, low drip water pH, lower calcite supersaturation and typically high drip rates. In contrast, the translucent, dense laminae represent more or less complete lateral coalescence (inclusion‐free) during the cold season (high calcite, drip water and cave air δ13C values), i.e. times of enhanced cave ventilation, low cave air pCO2, increased drip water pH, relatively high calcite supersaturation and typically low drip rates. In essence, the relative development of the two lamina types reflects changes in the seasonality of external air temperature and precipitation, with a strong control of the winter air temperature on the intensity of cave‐air exchange. Thick translucent, dense laminae are favoured by long, cold and wet winters and such conditions may be related closely to the North Atlantic Oscillation mode (weak westerlies) and enhanced Mediterranean cyclone activity during the cold season. Studies of speleothem lamination can thus help to better understand (and quantify) the role of seasonality changes, for example, during rapid climate events. 相似文献
1