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We present a simple algorithm to model the surface air temperature trends at the middle-high latitudes of the northern and southern hemispheres for the last century. Unlike previous approaches, based on the variation of the solar irradiance only, the algorithm here presented is the sum of one more external influence: the periodic variation of insolation due to the astronomical nutation of the Earth's axis. The model we present predicts the anticorrelated mean surface air temperature trends, measured at middle-high latitudes of the two hemispheres, during the period of low solar irradiance. According to Milankovitch, a change of the Earth's obliquity means a variation of insolation mainly at the middle-high latitudes; this variation takes opposite sign for northern and southern hemispheres. 相似文献
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The Meteora conglomerate is a thick, coarse-grained unit of Oligo-Miocene age in the Meso-Hellenic basin consisting of Gilbert-type delta deposits (wedge-shaped bodies) and deep channel fills. Gilbert-type deltas prograded from the eastern margin basinward, perpendicular to the NW-SE basin axis. Delta bodies are thicker in the distal portion, where the dip of the foresets is lower than in the proximal area. Organized and disorganized conglomerates coexist, suggesting a mixture of mass- and turbulent-flow processes. The mass flow deposits seem to prevail in the uppermost part of the foresets. Channels (up to 25 m deep) are cut into the Gilbert-type delta deposits, and have axes perpendicular to the progradational trend of the deltas. Giant bars displaying accretion of the downstream side formed inside the channels. The channels were probably cut into Gilbert-delta deposits due to uplift of the source area or to a fall in sea-level. Although large cross-sets of both Gilbert delta and channels have been formed by progradation, they can be distinguished by: (i) angle of foreset bed dips, (ii) facies and textures and (iii) vertical sequences. Differences in these features aid in distinguishing Gilbert-type deltas from in-channel bars. 相似文献
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MARCO BRANDANO VIRGILIO FREZZA LAURA TOMASSETTI MARTYN PEDLEY† RUGGERO MATTEUCCI 《Sedimentology》2009,56(4):1138-1158
The Oligocene represents a key interval during which coralline algae became dominant on carbonate ramps and luxuriant coral reefs emerged on a global scale. So far, few studies have considered the impact that these early reefs had on ramp development. Consequently, this study aimed at presenting a high‐resolution analysis of the Attard Member of the Lower Coralline Limestone Formation (Late Oligocene, Malta) in order to decipher the internal and external factors controlling the architecture of a typical Late Oligocene platform. Excellent exposures of the Lower Coralline Limestone Formation occurring along continuous outcrops adjacent to the Victoria Lines Fault reveal in detail the three‐dimensional distribution of the reef‐associated facies. A total of four sedimentary facies have been recognized and are grouped into two depositional environments that correspond to the inner and middle carbonate ramp. The inner ramp was characterized by a very high‐energy, shallow‐water setting, influenced by tide and wave processes. This setting passed downslope into an inner‐ramp depositional environment which was colonized by seagrass and interfingered with adjacent areas containing scattered corals. The middle ramp lithofacies were deposited in the oligophotic zone, the sediments being generated from combined in situ production and sediments swept from the shallower inner ramp by currents. Compositional characteristics and facies distributions of the Attard ramp are more similar to the Miocene ramps than to those of the Eocene. An important factor controlling this similarity may be the expansion of the seagrass colonization within the euphotic zone. This expansion may have commenced in the Late Oligocene and was associated with a concomitant reduction in the aerial extent of the larger benthonic foraminifera facies. Stacking‐pattern analysis shows that the depositional units (parasequences) at the study section are arranged into transgressive–regressive facies cycles. This cyclicity is superimposed on the overall regressive phase recorded by the Attard succession. Furthermore, a minor highstand (correlated with the Ru4/Ch1 sequence) and subsequent minor lowstand (Ch2 sequence) have been recognized. The biota assemblages of the Attard Member suggest that carbonate sedimentation took place in subtropical waters and oligotrophic to slightly mesotrophic conditions. The apparent low capacity of corals to form wave‐resistant reef structures is considered to have been a significant factor affecting substrate stability at this time. The resulting lack of resistant mid‐ramp reef frameworks left this zone exposed to wave and storm activity, thereby encouraging the widespread development of coralline algal associations dominated by rhodoliths. 相似文献
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Uppermost Triassic (Rhaetic) facies, as developed in the Southern Alpine region of Northern Italy, were deposited in a rapidly subsiding, fault-dissected trough (the Lombardy Basin) bounded by carbonate platforms. The main part of the Rhaetic succession consists of 10-m-scale asymmetric cycles, each divided into three parts: a lower shale portion; a central rhythmic portion consisting of repeated marl-limestone couplets, the limestone parts of which thicken upward; and an upper, wholly carbonate unit. A study of the diagenetic history of the series demonstrates that both the major asymmetric cyclicity and the limestone-marl couplets of the central rhythmic member (together constituting a ‘compound’ cyclic form) are fundamentally depositional in nature. It is suggested that this compound cyclicity resulted from the superposition of a low-frequency (approximately 100 000-year periodicity) asymmetric carbonate mud signal with a higher-frequency terrigenous mud signal. Field, petrographic, and geochemical investigations suggest that the basinal carbonate is predominantly allochthonous in origin, having been derived as relatively pure aragonitic mud from adjacent carbonate platforms. It is postulated that the asymmetric carbonate signal was linked to the ecological effects of eustatic fluctuation on platform carbonate systems. Repeated subaerial exposure of subtidal muds in shallow areas indicates that such sea-level variations occurred. A model is presented in which the basinward export of carbonate was negligible in the deepening phase, increased to a maximum during shallowing and was finally halted by the emergence of large platform flats. In contrast, the higher frequency terrigenous mud signal of the basin is thought to have been climatically modulated; fluctuations of a shorter period than those predicted by the Milankovitch theory affected hinterland precipitation and runoff. Particularly rapid subsidence and high depositional rates may have allowed the preservation of this signal. 相似文献
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The oceanic serpentinization of peridotites and the influenceof such an alteration on element cycling during their subductiondewatering are here investigated in a mantle slice (Erro–Tobbioperidotite), first exposed to oceanic serpentinization and laterinvolved in alpine subduction, partial dewatering and formationof a high-pressure olivine + titanian-clinohumite + diopside+ antigorite assemblage in the peridotites and in veins. Previouswork indicates that high-pressure veins include primary brines,representing a residue after crystallization of the vein assemblageand containing recycled oceanic Cl and alkalis. To reconstructthe main changes during oceanic peridotite serpentinizationand subsequent subduction, we analysed samples in profiles fromserpentinized oceanic peridotites to high-pressure serpentinites,and from high-pressure ultramafites to veins. Here we presentresults indicating that the main features of the oceanic serpentinizationare immobility of rare earth elements (REE), considerable waterincrease, local CaO decrease and uptake of trace amounts ofSr, probably as a function of the intensity of alteration. Srentered fine-grained Ca phases associated with serpentine andchlorite. Trace-element analyses of mantle clinopyroxenes andhigh-pressure diopsides (in country ultramafites and veins),highlight the close similarity in the REE compositions of thevarious clinopyroxenes, thereby indicating rock control on thevein fluids and lack of exotic components carried by externallyderived fluids. Presence of appreciable Sr contents in vein-formingdiopside indicates cycling of oceanic Sr in the high-pressurefluid. This, together with the recognition of pre-subductionCl and alkalis in the vein fluid, indicates closed-system behaviourduring eclogitization and internal cycling of exogenic components.Diopside and Ti-clinohumite are the high-pressure minerals actingas repositories for REE and Sr, and for high field strengthelements (HFSE), respectively. The aqueous fluid equilibratedwith such an assemblage is enriched in Cl and alkaline elementsbut strongly depleted in REE and HFSE (less than chondrite abundances).Sr is low [(0·2–1·6) x chondrites], althoughselectively enriched relative to light REE. KEY WORDS: eclogite facies; fluid; trace elements; serpentinite; subduction 相似文献