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1.
Rare earth elements in the pore waters of reducing nearshore sediments   总被引：4，自引：0，他引：4  
The REE are mobile during early diagenesis in reducing nearshore sediments of Buzzards Bay leading to greatly enhanced concentrations in pore waters, e.g. 815 pmol kg−1 Nd and 1910 pmol kg−1 Ce within 30 cm of the sediment-seawater interface, about 10–50 times local seawater values. Two principal diagenetic reactions have been identified. Preferential Ce enrichment (positive Ce anomalies) and preferential heavy REE enrichment (light REE removal) in the pore waters is associated with redox cycling of Fe and Mn within the upper few centimeters of the sediment. Release of REE, without fractionation, from sediments and addition to pore waters occurs deeper within the sediment column. The impact on the bulk sediment chemistry is undetectable but the porewater gradients imply that there are significant dissolved REE fluxes, both internal to the sediment system and across the sediment-seawater interface.  相似文献   

2.
Analyses have been made of the REE contents of a suite of hydrogenous δMnO2-rich ferromanganese encrustations obtained from a variety of depths (1000–4700 m) on the Line Islands Archipelago. The crusts form a coherent sample group, the REE contents of which are distinctly higher than those of diagenetic nodules. Crusts from 1 to 2 km depth exhibit higher Mn/Fe ratios and are approximately 50% REE depleted with respect to crusts below ～ 2 km. Furthermore, the shale-normalised patterns of the REE fall into the same depth-related categories. Crusts from depths greater than 2000–2500 m exhibit slight heavy REE depletion relative to intermediate REE whilst crusts from above that level exhibit more fractionated, heavy-REE-enriched patterns.To explain these depth-related processes, it is proposed that Mn and Fe oxides exhibit distinct behaviour with respect to REE scavenging. Between ～ 1 and 2 km, higher Mn/Fe ratios in the crusts are considered to be caused by an enhanced supply of Mn2+ —by diffusion and advection from nearshore sediments—which is adsorbed by and occludes existing oxide flocs. At these depths, oxides therefore tend to exhibit predominantly Mn oxide adsorptive properties. Consideration of the REE patterns shows that Fe oxides must be enriched overall in REE relative to Mn oxides and exhibit a flat shale-normalised REE pattern, whilst Mn oxide exhibits a heavy-REE-enriched pattern.Comparison of dissolved REE depth profiles with the crust data enables a qualitative appraisal of the application of scavenging models to the oxide-REE system. The data are not simply described by either equilibrium or irreversible adsorption models alone. It is concluded that whilst oxides may play a general role in creating the LREE depleted seawater pattern, they have little effect on individual vertical profiles, except close to points where fresh oxides precipitate.  相似文献   

3.
Analysis of a suite of ferromanganese nodules and crusts from the dredge Y73-3-22D in the Bauer Deep (13°40′S, 102°08′W) shows distinct differences between the nodules and crusts. Ultra-slow-scan X-ray diffraction shows that the nodules are more enriched in todorokite while the crusts are more enriched in δ MnO2. Both have phillipsite and smectite as accessory minerals as well as minor amounts of apatite, barite, and quartz. Chemical analyses show that the nodules also have higher abundances of Mn, Ni, Cu, Zn, and Ba, while crusts are more enriched in Fe, Co, and Ca. We suggest that normal authigenic precipitation of ferromanganese oxyhydroxides from seawater controls the mineralogy and chemistry of the crusts, while nodule mineralogy and chemistry are governed by small-scale diagenetic reactions in the sediment. Todorokite may form because iron in the Fe-Mn oxyhydroxide material dispersed in the sediment reacts with amorphous silica to form iron-rich smectites. The remaining oxyhydroxide material recrystallizes as todorokite.  相似文献   

4.
In China, most Precambrian banded iron formations(BIFs) are situated in the North China Craton. The Yuanjiacun iron deposit, located in the Lüliang area, is arguably the most representative Superior-type BIF. This iron deposit is coherent with the sedimentary rock succession of the Yuanjiacun Formation in the lower Lüliang Group, and was interpreted to be deposited at 2.3–2.1 Ga, based on ages of overlying and underlying volcanic strata. This age overlaps with the time range of the Great Oxidation Event(GOE, 2.4–2.2 Ga). The Yuanjiacun BIF consists mainly of subhedral-xenomorphic magnetite and quartz and rarely other minerals with a lower degree of metamorphism, from greenschist to lower amphibolite facies. The geochemical characteristics of this BIF are similar to those of Superior-type BIFs. Prominent positive La, Y, and Eu anomalies normalized by the Post Archean Australian Shale(PAAS) indicate that the primary chemical precipitate is a result of solutions that represent mixtures of seawater and high-T hydrothermal fluids. The contamination from crustal detritus found is negligible based on low abundances of Al2O3 and TiO2(0.5%) and of trace elements such as Th, Hf, Zr, and Sc(1.5 ppm), as well as the lack of co-variations between Al2O3 and TiO2. In particular, the Yuanjiacun BIF samples do not display significant negative Ce anomalies like those of the Archean iron formations, but rather, the Yuanjiacun BIF samples exhibit prominent positive Ce anomalies, low Y/Ho ratios, and high light to heavy REE((Pr/Yb)SN) ratios, which are essentially consistent with the late Paleoproterozoic(2.0 Ga) BIFs around the world. These characteristics of the Yuanjiacun BIF samples imply that the ancient ocean(2.3–2.1 Ga) was redox-stratified from oxic shallow water to deeper anoxic water. The specific redox conditions of the ancient ocean may be related to the GOE, which gave rise to the oxidation of Ce and Mn in the upper water, and to the presence of a Mn oxide shuttle in the ocean, resulting in varying REE patterns due to the precipitation and dissolution of this Mn oxide shuttle under different redox states. Therefore, the Yuanjiacun BIF appears to have formed near the redoxcline and lower-level reduced marine water.  相似文献   

5.
High precision trace element data obtained by inductively coupled plasma mass spectrometry and Sr–Nd isotope analyses are presented for mafic volcanic rocks from Gough Island, South Atlantic. The new data reveal negative Ce anomalies, with Ce/Ce? values in Gough lavas extending down to ～ 0.92. Ce is only fractionated from other rare earth elements (REE) due to formation of Ce4+ under oxidizing conditions of near-surface environments while other REE remain trivalent. Ce anomalies in convergent margin magmas have been shown to indicate a contribution of a subducted sediment component. In contrast, Ce anomalies in intra-plate basalts have been attributed to weathering processes, but can be excluded here based on element–element systematics indicating magmatic trends rather than weathering-induced element mobility. Shallow-level contamination by local marine sediments with negative Ce anomaly inherited from seawater can be excluded because Gough lavas with increasingly negative Ce anomalies do not trend towards low Ce/Pb ratios characterizing such sediments. Rather, it is argued that the negative Ce anomalies in Gough Island lavas are consistent with variable amounts of a sediment component in the mantle plume source. Mixtures between estimates of subducting sediment columns with negative Ce anomaly and mantle capable of giving rise to Gough Island magmas without Ce anomalies reproduce the Gough compositional array with the exception of highly fluid-mobile elements. The calculated trace element composition of the deeply recycled sediment in the Gough plume source is depleted in fluid-mobile elements such as Ba and Pb relative to the composition of some present-day subducting sediments. This loss is attributed to the dehydration or flushing of sediment in the subduction factory, consistent with constraints from arc magmas.  相似文献   

6.
In situ REE concentrations of various dolomites from Tarim Basin were obtained by LA-ICP-MS analysis, and the data were normalized to standard seawater (Seawater Normalized=SWN). Most of the samples have a ΣREE range of less than 20 ppm. All samples show similar REESWN distributions with heavy REE depletion, and positive Ce anomaly, which indicates that they have the same dolomitization fluids (seawater). According to the origin and diagenetic process of dolomite, two types of dolomite are determined and described as follows: 1) syndepositional dolomite, with the highest REE concentrations (more than 20 ppm), the cores of which are more enriched in REE compared with their cortexes, indicating that they underwent the dolomitization of calcareous sediments by hypersaline and subsequent diagenesis decreased the REE content of the cortex because of the low REE concentration of the diagenetic fluids; 2) diagenetic dolomite, which can be subdivided into four groups. (1) burial dolomite which has higher REE concentrations than limestone, but lower than syndepositional dolomite. This shows that pore fluids with high salinity dolomitized the pre-existing limestone; (2) void filling dolomite which has the similar REE patterns with the matrix dolomite. In addition, the Eu anomaly is not obvious, suggesting that the dolomitization fluids originated from the diagenetic fluids; (3) recrystallized dolomite, whose REE concentration was obviously decreased, indicating that the REE concentration was decreased during the recrystallization processes; and (4) hydrothermal altered void-filling dolomite, which has the lowest REE concentration, but obvious positive Eu anomaly, reflecting its hydrothermal activity related origin. Thus, the diverse REE signatures, which were recorded in different dolomites, retain the information of their formation conditions and subsequent diagenetic processes. In situ REE analysis of dolomite is an effective probe into the origin and diagenetic process of dolomite. Supported by National Basic Research Program of China (Grant No. 2005CB422103)  相似文献   

7.
Two diagenetic manganese nodules from the Peru Basin were investigated by thermal ionization mass spectrometry and high resolution alpha spectrometry for uranium and thorium. The TIMS concentrations for nodule 62KD (63KG) vary as follows: 0.12–1.01 ppb (0.06–0.59) 230Th, 0.51–1.98 ppm (0.43–1.40) 232Th, 0.13–0.80 ppb (0.09–0.49) 234U, and 1.95–13.47 ppm (1.66–8.24) 238U. Both nodules have average growth rates of 110 mm per million years. However, from the variations of excess 230Th with depth we estimate partial accumulation rates which range from 50 to 400 mm per million years. The δ234U dating method cannot be applied due to remobilization of U from the sediment and subsequent incorporation into the nodules' crystal lattice, reflected by decay corrected δ234U values far above the ocean water value. Sections of fast nodule growth are related to those layers having high Mn/Fe ratios (up to 200) and higher densities. As a possible explanation we develop a scenario that describes similar glacial/interglacial trends in both nodules as a record of regional changes of sediment and/or deep water chemistry.  相似文献   

8.
Clay fractions in the non-calcareous surface sediments from the eastern Pacific were analyzed for clay minerals, REE and 143Nd/144Nd. Montmorillonite/illite ratio (M/I ratio), total REE contents ((REE), LREE/HREE ratio and cerium anomaly (бCe) may effectively indicate the genesis of clay minerals. Clay fractions with M/I ratio >1, бCe (0.85, (REE (400 μg/g, LREE/HREE ratio (4, and REE patterns similar to those of pelagic sediments are terrigenous and autogenetic mixed clay fractions and contain more autogenetic montmorillonite. Clay fractions with M/I ratio <1, бCe=0.86 to 1.5, ΣREE=200 to 350 μg/g, LREE/HREE ratio (6 and REE distribution patterns similar to that of China loess are identified as terrigenous clay fraction. The 143Nd/144Nd ratios or (э)Nd values of clay fractions inherit the features of terrigenous sources of clay minerals. Clay fractions are divided into 4 types according to (э)Nd values. Terrigenous clay minerals of type I with the (э)Nd values of -8 to -6 originate mainly from North American fluvial deposits. Those of type II with the (э)Nd values of -9 to -7 are mainly from the East Asia and North American fluvial deposits. Those of type III with (э)Nd values of -6 to -3 could come from the central and eastern Pacific volcanic islands. Those of type IV with (э)Nd values of -13 to -12 may be from East Asia eolian. The terrigenous and autogenetic mixed clay fractions show patchy distributions, indicating that there are volcanic or hot-spot activities in the eastern Pacific plate, while the terrigenous clay fractions cover a large part of the study area, proving that the terrigenous clay minerals are dominant in the eastern Pacific.  相似文献   

9.
Secular variations in the Pb isotopic composition of a mixed hydrogenous-hydrothermal ferromanganese crust from the Bauer Basin in the eastern Equatorial Pacific provide clear evidence for changes in hydrothermal contributions during the past 7 Myr. The nearby Galapagos Rise spreading center provided a strong hydrothermal flux prior to 6.5 Ma. After 6.5 Ma, the Pb became stepwise more radiogenic and more similar to Equatorial Pacific seawater, reflecting the westward shift of spreading to the presently active East Pacific Rise (EPR). A second, previously unrecognized enhanced hydrothermal period occurred between 4.4 and 2.9 Ma, which reflects either off-axis hydrothermal activity in the Bauer Basin or a late-stage pulse of hydrothermal Pb from the then active, but waning Galapagos Rise spreading center.Hafnium isotope time-series of the same mixed hydrogenous-hydrothermal crust show invariant values over the past 7 Myr. Hafnium isotope ratios, as well as Nd isotope ratios obtained for this crust, are identical to that of hydrogenous Equatorial Pacific deep water crusts and clearly indicate that hydrothermal Hf, similar to Nd, does not travel far from submarine vents. Therefore, we suggest that hydrothermal Hf fluxes do not contribute significantly to the global marine Hf budget.  相似文献   

10.
Major, trace and rare earth elements were measured in 27 samples of the Middle to Late Permian limestones from the Tieqiao section located on the marginal zone of an isolated platform (Laibin, South China). Shale-normalized REE+Y patterns of all samples show notably negative Ce anomalies (0.21–0.66, average 0.33), slightly positive Gd anomalies (1.08–1.30, average 1.20), and positive Y anomalies with superchondritic Y/Ho ratios (36–91, average 55), which are consistent with those of modern shallow seawater. Their relative LREEs enrichment with higher NdSN/YbSN ratios (0.58–1.80) than those of modern shallow seawater (0.21–0.50) suggests complicated sources of REEs for all samples. Compared with geochemical features of sediments influenced by terrigenous particles and hydrothermal fluids, it is concluded that ambient shallow seawater was the primary source of REEs in these limestones. Comparing the indicators of REE+Y elements (ΣREE, NdSN/YbSN, Ce/Ce*, Gd/Gd*, Eu/Eu* and Y/Ho) in limestones with those in bedded cherts from the Tieqiao section, we consider that limestone and bedded chert have similar sources of REE+Y elements: ambient shallow seawater with more or less hydrothermal fluids. In addition, there is a completely negative correlation between CaCO3 and SiO2 contents in limestones and bedded cherts. These results imply that precipitation of CaCO3 was inhibited by that of SiO2 which was derived mainly from hydrothermal fluid, especially in bedded cherts from the Tieqiao section.  相似文献   

11.
The distribution and anomalies of rare earth elements(REEs) of granitic regolith were studied in Inner Mongolia and Hainan Island, China. One profile showed slight REE enrichment of an upper layer and no obvious light REE/heavy REE(LREE/HREE) fractionation(La_N/Yb_N of 0.9). The second profile was significantly enriched in REEs and enriched in LREEs in the upper portion(La_N/Yb_N1.8). Eu, Ce, and Gd anomalies of the two profiles are different. Slightly negative Eu, Ce, and Gd anomalies in NMG-3-1 indicate slow dissolution of primary minerals and little secondary products; in contrast, a positive Eu anomaly in HN-2 suggests the vegetation cycle may contribute to soil. The Ce anomaly of HN-2 reflects oxidation of Ce and coprecipitation by Fe-and Mn-oxides and organic matter. Correlation between Ce and Gd anomalies in HN-2 suggests Ce and Gd are both influenced by redoxreduction.  相似文献   

12.
The mineralogical and geochemical characteristics of Fe-oxyhydroxide samples from one dredge station (long. 103°54.48′W, lat. 12°42.30′N, water depth 2655 m) on the East Pacific Rise near lat 13°N were analyzed by XRD, ICP-AES, and ICP-MS. Most Fe-oxyhydroxides are amorphous, with a few sphalerite microlites. In comparison with Fe-oxyhydroxides from other fields, the variable ranges in the chemical composition of Fe-oxyhydroxide samples are very narrow; their Fe, Si, and Mn contents were 39.90%, 8.92%, and 1.59%, respectively; they have high Cu (0.88%–1.85%) and Co (65×10−6−704×10−6) contents, and contain Co+Cu+Zn+Ni> 1.01%. The trace-element (As, Co, Ni, Cu, Zn, Ba, Sr) and major-element (Fe, Ca, Al, Mg) contents of these samples are in the range of hydrothermal sulfide from the East Pacific Rise near 13°N, reflecting that this type of Fe-oxyhydroxide constitutes a secondary oxidation product of hydrothermal sulfide. The Fe-oxyhydroxide samples from one dredge station on the East Pacific Rise near 13°N are lower in ΣREE (5.44×10−6–17.01×10−6), with a distinct negative Ce anomaly (0.12–0.28). The Fe-oxyhydroxide samples have similar chondrite-normalized rare-earth-element (REE) patterns to that of seawater, and they are very different from the REE composition characteristics of hydrothermal plume particles and hydrothermal fluids, showing that the REEs of Fe-oxyhydroxide are a major constituent of seawater and that the Fe-oxyhydroxides can become a sink of REE from seawater. The quick settling of hydrothermal plume particles resulted in the lower REE content and higher Mn content of these Fe-oxyhydroxides, which are captured in part of the V and P from seawater by adsorption. The Fe-oxyhydroxides from one dredge station on the East Pacific Rise near 13°N were formed by secondary oxidation in a low temperature, oxygenated environment. In comparison with the elemental (Zn, Cd, Pb, Fe, Co, Cu) average content of hydrothermal sulfide samples from the East Pacific Rise near 13°N, the Zn, Cd, and Pb contents of the Fe-oxyhydroxides are lower, and their Fe, Co, and Cu contents are higher. Supported in part by the Pilot Project of Knowledge Innovation Project, Chinese Academy of Sciences (Grant No. KZCX3-SW-223), and the Special Foundation for the Tenth Five Plan of COMRA (Grant No. DY105-01-03-1)  相似文献   

13.
Early diagenetic manganese nodules from the northeast Pacific nodule belt and from the southeast Pacific (Peru Basin) show primary growth features of dendritic microtextures consisting of alternating laminae of crystalline 10-Åmanganate (A1 substance) and amorphous material which is composed of an intimate mixture of ferric hydroxide, silicate, and δ-MnO2 (A2 substance). The formation of rhythmic sequences of A1 and A2 microlayers is explained by physico-chemical changes in the peneliquid sediment layer and in the microenvironment of the accreting nodule surface: (a) upward diffusion of Mn2+ in the interstitial water as a result of decay of organic matter and Mn mobilization; (b) oxidation of Mn2+ and formation of 10-Åmanganate in the upper part of the peneliquid sediment layer, leading to pH depression in the microenvironment of the nodule surface which decreases the mobility of silicate, resulting in formation of A2 layers; (c) restoration of pH and renewed precipitation of 10-Åmanganate.Based on data of 171 bulk analyses and on electron microprobe investigations, interelement relationships between Co, Mn, and Fe are pointed out. Fe and Co show a significant positive correlation (r = 0.84), while Mn and Co are poorly negatively correlated (r = ?0.21), assuming linear regression. The enrichment of Co within the amorphous A2 phase is attributed to specific surface adsorption and subsequent oxidation of Co2+ to Co3+ in the strong electric field of Si4+. Robust complexes of Co(III) and ≡ FeH2SiO4? prevent most of the Co from being available for the 10-Åmanganate precipitation. Concerning the relation between Mn and Fe and Co respectively, the highest correlation coefficients are obtained using an inverse logarithmic regression. Under early diagenetic conditions, the concentration and precipitation of Mn2+ in the interstitial water depends on the redox gradient which is controlled by the amount of decomposing organic matter. The Nernst equation describes the relationship as a reciprocal logarithmic function. However, the quantities of colloidal ferric hydroxide and of dissolved silicate are not affected by variations of the redox potential. These different characteristics in the precipitation of Mn and the Fe- and Si-rich colloidal phase may cause the significantly negative logarithmic correlation of Mn versus Fe and Co, respectively.  相似文献   

14.
Simulation outputs were used to contrast the distinct evolution patterns between two types of El Niño. The modeled isotherm depth anomalies closely matched satellite sea surface height anomalies. Results for the El Niño Modoki (central Pacific El Niño) corresponded well with previous studies which suggested that thermocline variations in the equatorial Pacific contain an east–west oscillation. The eastern Pacific El Niño experienced an additional north–south seesaw oscillation between approximately 15° N and 15° S. The wind stress curl pattern over the west-central Pacific was responsible for the unusual manifestation of the eastern Pacific El Niño. The reason why the 1982/1983 El Niño was followed by a normal state whereas a La Niña phase developed from the 1997/1998 El Niño is also discussed. In 1997/1998, the Intertropical Convergence Zone (ITCZ) retreated faster and easterly trade winds appeared immediately after the mature El Niño, cooling the sea surface temperature in the equatorial Pacific and generating the La Niña event. The slow retreat of the ITCZ in 1982/1983 terminated the warm event at a much slower rate and ultimately resulted in a normal phase.  相似文献   

15.
The organic matter-rich strata of the Yurtusi Formation of Lower Cambrian are wildly spread and steady in the Tarim Basin, in which cherts are developed, companied with phosphorite. Al/(Al+Fe+Mn) ratio and Si/(Si+Al+Fe) ratio of cherts range from 0.0023 to 0.0046 and 0.965 to 0.98, respectively, suggesting that cherts are formed in submarine hydrothermal activity and far away from terrestrial. Trace elements, such as As, Hg, Pb, Zn, Cu, Co, P, V, Ba, etc., are markedly rich in organic matter-rich sedimentary rocks of which Ba/Sr ratios are between 21.2 and 158.1. Compared with modern hydrothermal sediments, their Ba/Sr ratios are similar, it indicates that the organic matter-rich strata have the geochemical feature of submarine hydrothermal sediments. The total content of rare earth elements (ЕREE) in cherts is from 8.81 to 56.682 μg/g, on average 31.41 μg/g and the ΕREE of cherts is between continental margin chert's and abyssal chert's. The LREE/HREE ratio of cherts varies from 1.01 to 3.56, which reveals the characteristics of hydrothermal sediments. There is positive correlation between the total organic content (TOC) and Ba/Sr ratio which is geochemical index of submarine hydrothermal activity, which indicates that the submarine hydrothermal activity obviously influences the richness of organic matter in the Yurtusi Formation.  相似文献   

16.
This study focuses on the trace and rare earth elements(REE) geochemistry of the Nkporo and Ekenkpon Shales of the Calabar Flank.The main aim is to infer their depositional environment and the degree of their metal enrichment.The shale samples were analyzed using inductively coupled plasma mass spectrometry.The results indicated that the mean concentrations of K,Na,and Fe in Nkporo and Ekenkpon Shales are 1.45,0.4,and 4.17 wt%,and 1.11,0.44,and 5.42 wt%;respectively.The Nkporo Shale is enriched with the following trace elements;PMn Sr Ba Zn Ce Rb Zr VCr Ni and depleted in the following trace elements;Ta GeSb Bi Cd Ag Te In Hg.While the Ekenkpon Shale is enriched with the following trace elements;P Mn Ba Sr VCe Zr Rb Cr Zn Ni and depleted in;Sb Ge Bi Ag Ce Te InHg.The concentration of redox-sensitive elements such as V,Ni,Mo,U,Cu,Cr,Re,Cd,Sb,Ti,Mn,and their ratio V/Mo and U/Mo in the black and grey shale samples show different patterns.The REE obtained from the Nkporo and Ekenkpon Shales were PAAS normalized.The Nkporo Shale showed a slightly flat light rare-earth element(LREE),middle rare-earth element(MREE),and heavy rare earth element(HREE) pattern enrichment.Ce/Ce*ranges from 0.95 to 1.09 in Nkporo Shale and 0.67 to 1.40 in Ekenkpon Shale.The Ekenkpon Shale showed a slight LREE,MREE enrichment,and depleted HREE patterns.The Mn contents and U/Mo ratio in Nkporo and Ekenkpon Shales suggests a poor oxygen transitional environment.The V/Mo and V/(V+Ni) ratios indicated that the Nkporo shales were deposited in an anoxic to suboxic conditions and Ekenkpon shales were also deposited under an anoxic to suboxic conditions.The V/Ni ratio indicated that the organic matter in the Nkporo shale is terrigenous while that of the Ekenkpon shales are both terrigenous and marine in origin.  相似文献   

17.
The middle sector of the Yarlung Zangbo suture zone stretches over 200 km long from Ngamring through Geding to Rinbung, roughly along Yarlung Zangbo River valley (Fig. 1). This belt resulted from the closure of the Tethyan ocean and the collision be- tween Indian plate and Lhasa block[1―8]. Lots of works demonstrated that rifting of the Tethyan basin in southern Tibet started from Triassic time. Initial oce- anic crust appeared in the Late Jurassic, and then ex- perienced a rapid sprea…  相似文献   

18.
The circulation and zonal wind anomalies in the lower troposphere over the equatorial western Pacific and their roles in the developing and decaying processes of the 1982–1983, 1986 –1987, 1991–1992 and 1997–1998 El Ni?o events and the occurrence of La Ni?a events are analyzed by using the observed data in this paper. The results show that before the developing stage of these El Ni?o events, there were cyclonic circulation anomalies in the lower troposphere over the tropical western Pacific, and the anomalies brought the westerly anomalies over the Indonesia and the tropical western Pacific. However, when the El Ni?o events developed to their mature phase, there were anticyclonic circulation anomalies in the lower troposphere over the tropical western Pacific, and the anomalies made the easterly anomalies appear over the tropical western Pacific. A simple, dynamical model of tropical ocean is used to calculate the response of the equatorial oceanic waves to the observed anomalies of wind stress near the sea surface of the equatorial Pacific during the 1997/98 ENSO cycle, which was the strongest one in the 20th century. It is shown that the zonal wind stress anomalies have an important dynamical effect on the devel-opment and decay of this El Ni?o event and the occurrence of the following La Ni?a event.  相似文献   

19.
The Aegean volcanic arc formed in response to northeasterly subduction of the Mediterranean sea floor beneath the Aegean Sea. The active arc lies over 250 km from the Hellenic Trench in a region which has suffered considerable extension and subsidence since the mid-Tertiary. Suites of samples from the different volcanic centres making up the arc have been studied geochemically in order to assess lateral variations and to constrain the contribution of crustal contamination and sediment subduction in their petrogenesis.Lavas from all the major volcanic centres exhibit typical calc-alkaline major-element characteristics, and show enrichment in light REE and LIL elements but low contents of HFS elements. The enrichment in light REE is greater in the eastern (Nisyros, Kos) and western (Milos, Poros, Methana, Aegina) sectors of the arc (Cen/Ybn=4) than in the central Santorini sector (Cen/Ybn=2). All lavas have significant negative Eu anomalies and many have slight negative Ce anomalies. Less coherence is observed in the abundances and ratios of the other LIL elements, compared with the REE, along the island chain.Whereas the effects of crystal fractionation are evident in the trace-element patterns of lavas from individual islands, and are particularly well marked for Santorini, it is clear that there are consistent differences in trace-element abundances and ratios in the lavas of the various islands which reflect compositional differences in the mantle source and/or in melting conditions. Lavas from the eastern and western sectors have much higher levels of Ba and Sr but relatively lower Th, K and Rb than those from Santorini. Although some geochemical features could be explained through involvement of a component of subducted sediment in the source regions of the volcanoes, other element abundances and ratios indicate that this component must be very small. Detailed consideration of the inter-island geochemical variations suggests a complex make-up of the underlying lithosphere, resulting from a long history of subduction. In the region of Santorini, where crustal stretching is greatest, the underlying asthenosphere may be involved in magma production.  相似文献   

20.
The stratigraphical sequences composed of chert and basalt were found in the Daxinshan area of Simao and the Manbie area of Jinghong, southwestern Yunnan. The Middle Permian to ealiest Late Permian radiolarians, such as Follicucullus and Pseudoalbaillella, have been identified from the chert. The chert from the Manbie area of Jinghong is characterized by high SiO2content (over 92%), large ratios of MnO/TiO2 (2.15) and low ratios of Al/(Al+Fe+Mn) (≤0.1) and Ce/Ce*(0.4), which indicate that the chert was deposited in pelagic basin. The chert from the Daxinshan area of Simao, however, is characterized by low SiO2 content, low ratios of MnO/TiO2 (0.27) and high ratios of Al/(Al+Fe+Mn) (0.49) and Ce/Ce*(0.88), which imply that the chert was deposited in continental margin basin. The basalts from the both areas belong to tholeiite series, and the chemical compositions of their major, rare earth and trace elements show the characteristics of MORB. These results evidence that there are volcanic rocks and chert sequences representing pelagic basin and oceanic basin near continent. These sequences and the formerly reported island-arc volcanic rock sequences imply that the Daxinshan Formation in the Lancangjiang belt represents a sedimentary assemblage formed in active continental margin basin.  相似文献   

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