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71.
GRAHAME J. LARSON DANIEL E. LAWSON EDWARD B. EVENSON ÓSKAR KNUDSEN RICHARD B. ALLEY MANTHA S. PHANIKUMAR 《Boreas: An International Journal of Quaternary Research》2010,39(3):457-470
Larson, G.J., Lawson, D.E., Evenson, E.B., Knudsen, Ó., Alley, R.B. & Phanikumar, M.S. 2010: Origin of stratified basal ice in outlet glaciers of Vatnajökull and Öræfajökull, Iceland. Boreas, Vol. 39, pp. 457–470. 10.1111/j.1502‐3885.2009.00134.x. ISSN 0300‐9483. During the period 2000–2005, we collected samples of englacial ice, vent water, frazil/anchor ice and stratified basal ice from warm‐based outlet glaciers of Vatnajökull and Öræfajökull, Iceland, and analysed them for 3H, 18O and D. Results of 3H analyses show that the stratified basal ice contains 3H from atmospheric thermonuclear testing and is younger than the englacial ice. Results of the 18O and D analyses show that frazil/anchor and stratified basal ice are both enriched by an average of 2.4‰ in 18O and 11‰ in D relative to vent water. These values are consistent with fractionation during partial freezing of supercooled subglacial water in an open system, one in which the remaining water is continuously removed and replenished by water of similar composition. The isotopic data and field observations do not support either a regelation or a thermal ad‐freeze‐on origin for the stratified basal ice. 相似文献
72.
73.
强对流过程对大尺度模式洋面通量加强的参数化研究与进展 总被引:1,自引:0,他引:1
由于地球表面热通量直接决定着大气环流运动,在大气数值模式中如何准确描述地球表面热通量就变处得十分重要.洋面是地球表面最广泛分布的下垫面,它的表面热通量描述尤为被重视.尤其是西赤道太平洋暖池、东赤道太平洋冷舌、ITCZ及信风带等关键区域的洋面热通量的准确估计更是非常重要,这些区域的地表通量变化通常被认为是全球气候变化即ENSO现象的最重要的信号[1].众所周知,中尺度降水对流在以上关键海域频繁发生.中尺度降水对流系统往往有较强的下沉气流(downdraft)[2],这种下沉气流在中尺度降水对流系统与大尺度过程的相互作用中伴演着重要角色. 相似文献
74.
The sedimentology of a Recent carbonate mound is investigated to further our understanding of mound building communities, surface and subsurface mound sediments, and the overall sediment budget of mounds. Nine sedimentary facies of the surface of Tavernier mound, Florida Keys are described. These sediments are composed of Neogoniolithon, Halimeda, Porites, mollusc and foraminiferal grains, and lime mud. Muds rich in aragonite and high magnesian calcite show little mineralogical variation over the mound surface. Geochemical evidence suggests that the mud is mainly formed from breakdown of codiacean algae and Thalassia blade epibionts. Production rates of the facies are established from in situ growth rate experiments and standing-crop surveys. Annual calcium carbonate production is c. 500gm-2, intermediate between reef and other bay and lagoonal environment production rates in the Caribbean. The internal structure of the mound, studied from piston cores and sediment probes, indicates that seven facies can be identified. Five of these can be related to the present-day facies, and occur in the upper part of the mound (gravel-mound stage). The remaining two facies, characterized by molluscs and aragonite-rich muds, occur in the lower part of the mound (mud-mound stage), and are most similar to facies from typical Florida Bay mud mounds. Mangrove peats within the mound indicate former intertidal areas and C14 dates from these peats provide a time framework for mound sedimentation. The mound appears to have formed because of an initial valley in the Pleistocene surface which accumulated mud in a shallow embayment during the Holocene transgression. A sediment budget for the mound is presented which compares production rates from present-day facies with subsurface sediment masses. During the mud mound stage production rates were similar to accumulation rates and the mound was similar to the present-day mounds of Florida Bay. During the gravel mound stage (3400 yr BP-present day), conditions were more normal marine and the establishment of Porites and Neogoniolithon on the mound increased production rates 10% over accumulation rates. This excess sediment is thought to be transported off the mound to the surrounding seabed. Models are proposed which divide carbonate mounds on the basis of internal versus external sediment supply. Comparisons are made with other Recent and ancient mounds. Similarities exist between the roles of the biotic components of late Palaeozoic mounds but major differences are found when structures and early diagenesis are compared. 相似文献
75.
Utilisant une méthode rapide et précise, l'eau combinée (H2 O+) a été dosée dans les standrds géochimiques de l'ANRT et du CRPG. 相似文献
76.
Clay mineral distribution patterns as influenced by depositional processes in the Southeastern Levantine Sea 总被引:1,自引:0,他引:1
Analysis of the clay minerals in stratigraphically defined cores (23,000 years BP to the present) in the southeastern Levantine-Nile Cone sector of the eastern Mediterranean indicates that depositional processes are very significant in determining the distribution of clay assemblages. The interplay of long-distance transport by water mass circulation, downslope mass gravity transport and wind dispersal is recorded by the clay assemblages in each of the stratigraphic layers. The temporal variations and spatial distribution of smectite, the dominant clay mineral of the River Nile, can be related closely to downslope gravity-related processes and to deposition from water mass flow. Increased kaolinite, in part of wind-blown origin from North Africa, correlates with areas receiving low terrigenous input but influenced by enhanced suspended sediment transport. Illite and chlorite distributions are most closely associated with a northern Levantine provenance and dispersal by the circulation of eastern Mediterranean water masses. Climatically induced changes may have altered the clay minerals in the region and minor diagenetic changes may have occurred, but these factors do not fully explain observed vertical clay mineral changes in the Late Quaternary. We conclude that palaeoclimatic interpretations based on vertical clay mineralogical changes at single core localities should be approached cautiously in small ocean basins such as the Mediterranean. 相似文献
77.
Sediment in tectonically active, topographically restricted settings of the western Hellenic Arc, eastern Mediterranean, consists primarily of clayey silt and silty clay. Failure of metastable sediment temporarily stored on relatively steep slopes is triggered by earthquake tremors and eustatic oscillations. Redeposition of these materials by gravitative transport has resulted in markedly different lithofacies from site to site. Most piston cores include three Late Quaternary stratigraphic units that can be correlated with sections in other parts of the eastern Mediterranean; numerous radiocarbon-age determinations enhance the correlation. Seven fine-grained sediment types are identified in cores from eight distinct depositional environments. Some muds are closely related to specific environments (slump and debris flow deposits on slope and high-relief environments), or to time (well laminated mud during the latest Pleistocene-mid-Holocene), or to both (uniform and faintly laminated muds restricted to trench basins). Turbiditic and hemipelagic muds are common throughout the study area. Mud distribution patterns correlate closely with calculated sedimentation rates. We propose two depositional models for these sediments. The first emphasizes downslope transformations resulting in progressively reduced flow concentration during transport: from slump and debris flow–>turbidity current–>low density turbidity current or turbid layer mechanisms. The distal end-member deposits settling from low concentration flows are thick, rapidly emplaced, fine-grained uniform muds closely associated with faintly laminated muds. These were ponded in flat trench basin-plains. Planktonic and terrigenous fractions in the turbiditic, finely laminated and uniform muds record mixing of materials of gravitative and suspension origin during redeposition. This sequence prevails under conditions of minimal stratification of water masses, as characterized by the present Mediterranean. In the second model developed for conditions of well-developed water mass stratification, well laminated rather than uniform mud prevails as the end product of low concentration flows. These very finely laminated and graded muds record particle-by-particle settling from detached turbid layers concentrated along density interfaces; they include material from turbid layers complemented by the normal ‘rain’ of pelagic material. Stratification barriers resulted in region-wide distribution of such deposits, in both slope and trench environments. 相似文献
78.
79.
TORMEY DANIEL R.; FREY FREDERICK A.; LOPEZ-ESCOBAR LEOPOLDO 《Journal of Petrology》1995,36(2):265-298
Along strike of the Quaternary magmatic arc in the SouthernVolcanic Zone of the Andes, there is a south to north increasein crustal thickness, and the lavas define systematic geochemicaltrends which have been attributed to variations in the proportionsand compositions of mantle-and crustal-derived components. Realisticinterpretations of these regional geochemical trends requiresan understanding of the sources and processes that control lavacompositions at individual volcanoes. Because it is in an importantgeophysical and geochemical transition zone, we studied theAzufrePlanchonPeteroa volcanic complex, a nestedgroup of three volcanoes <055 m.y. in age located at 3515'Sin the Southern Volcanic Zone of the Andes. North of this complexat 3335S the continental crust is thick, basalts areabsent, and there is abundant evidence for crustal componentsin the evolved lavas, but south of 37S, where the crust isrelatively thin, basaltic lavas are abundant and the contributionof continental crust to the lavas is less obvious. In additionto its location, this volcanic complex is important becausethere is a diversity of lava compositions, and it is the northernmostexposure of recent basaltic volcanism on the volcanic front.Therefore, the lavas of this complex can be used to identifythe relative roles of mantle, lower-crustal and upper-crustalsources and processes at a single location. Volcan Azufre is the oldest and largest volcano of the complex;it is a multi-cycle, bimodal, basaltic andesitedacitestratovolcano. Volcan Planchon is the northernmost basalt-bearingvolcano along the volcanic front of the Southern Andes, andVolcan Peteroa, the youngest volcano of the complex, has eruptedmixed magmas of andesitic and dacitic composition. Most basalticandesite lavas at Azufre and Planchon are related by a plagioclase-poor,anhydrous mineral fractionating assemblage. High-alumina basaltis produced from a tholeiitic parent by an 48 kbar fractionatingassemblage. During this moderatepressure crystallization, themagmas also incorporated a crustal component with high La/Yband high abundances of Rb, Cs and Th. Based on the chemicalcharacteristics of the added component and the inferred depthof crystallization, the crustal source may have been garnetgranulite derived from solidified arc magmas in the lower tomiddle continental crust. At Planchon, the role of crustal assimilationhas increased with decreasing eruption age probably becausecrustal temperatures have increased during continued volcanism.Azufre dacite lavas formed at low pressures by fractionationof a plagioclase-rich assemblage. These dacite lavas containan upper-crustal component, probably derived in part from limestone,with high values of 87Sr/86Sr and 18O/16O. Thus two depths (upperand lower crust) of crystallization and associated crustal assimilationare evident in PlanchonAzufre lavas. Peteroa, the focusof recent volcanism, consists of calc-alkaline andesite anddacite eruptive products whose textures and compositions indicatean important role for magma mixing. Therefore, the volcanismevolved from a tholeiitic system of basalt and subordinate dacite(PlanchonAzufre) to a calc-alkaline system with abundantmixed lavas of intermediate composition (Peteroa). In additionto crustal thickness, two important parameters which controlledthe diversity of lava composition in this complex are magmasupply rate from the mantle and crustal temperature. Both parametersvaried with time, and they must be considered in broader interpretationsof along-strike geochemical trends. KEY WORDS: arc magmas; Andes; Peteroa; Planchan; geochemistry
*Corresponding author. Present address: ENTRIX, Inc., 4II North Central Avenue, Glendale, CA 91203, USA 相似文献
80.
Estimating regional carbon exchange in New England and Quebec by combining atmospheric, ground-based and satellite data 总被引:1,自引:0,他引:1
DANIEL M. MATROSS ARLYN ANDREWS MAHADEVAN PATHMATHEVAN CHRISTOPH GERBIG JOHN C. LIN STEVEN C. WOFSY BRUCE C. DAUBE ELAINE W. GOTTLIEB VICTORIA Y. CHOW JOHN T. LEE CONGLONG ZHAO PETER S. BAKWIN J. WILLIAM MUNGER DAVID Y. HOLLINGER 《Tellus. Series B, Chemical and physical meteorology》2006,58(5):344-358