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241.
A. Sarkar R. Ramesh S. K. Bhattacharya N. B. Price 《Journal of Earth System Science》2000,109(1):157-169
δ18 O and δ13C of G.sacculifer have been measured in five cores from the northern Indian Ocean. In addition, high resolution analysis (1 to 2 cm) was performed
on one core (SK-20-185) for both δ18O and gd13C in five species of planktonic foraminifera. CaCO3 variation was measured in two cores. The results, presented here, show that
相似文献
– | • the summer monsoon was weaker during 18 ka and was stronger during 9 ka, relative to modern conditions; |
– | • δ13C variations are consistent with independent evidence that shows that during the last glacial maximum (LGM; 18 ka) the upwelling was reduced while during 9 ka it was vigorous; |
– | • calculation of CaCO3 flux shows that the LGM was characterized by low biogenic productivity in the Arabian Sea while during the Holocene productivity increased by ∼65%, as a direct consequence of the changes in upwelling. Similar changes (of lesser magnitude) are also seen in the equatorial Indian Ocean. The amount of terrigenous input into the Arabian Sea doubled during LGM possibly due to the higher erosion rate along the west coast. |
– | • δ18O values indicate that the Arabian Sea was saltier by 1 to 2%o during LGM. The northern part was dominated by evaporation while in the equatorial part there was an increased precipitation. |
242.
海拉尔盆地西部蚀源区岩石提供铀源能力的研究 总被引:1,自引:2,他引:1
利用U-Pb同位素组成演化对海拉尔盆地西部西胡里吐盆地蚀源区火山岩类和克鲁伦凹陷蚀源区花岗岩类岩石的研究表明:蚀源区岩石原始铀含量(U0)较高,火山岩类平均U0为10.061×106,铀变化系数(ΔU)平均为49.57%;花岗岩类平均U0为18.381×106,ΔU平均为80%。这些结果显示了蚀源区岩石能为沉积砂体提供预富集的物质基础,也能提供成矿铀源。岩石的U-Ra平衡系数研究表明,火山岩类存在明显的U-Ra不平衡现象,而花岗岩提供成矿铀源应发生在16000a以前。 相似文献
243.
Tom Magna Uwe Wiechert Timothy L. Grove Alex N. Halliday 《Earth and Planetary Science Letters》2006,250(3-4):428-443
We present lithium (Li) abundances and isotope compositions for a suite of anhydrous olivine tholeiites (HAOTs) and hydrous basalt-andesitic (BA) lavas from the Mt. Shasta and Medicine Lake regions, California. The values of δ7Li vary from + 0.9‰ to + 6.4‰ and correlate inversely with distance from the trench. These data are consistent with continuous isotope fractionation of Li during dehydration of the subducted oceanic lithosphere, an interpretation corroborated by uniformly high pre-eruptive H2O contents in basaltic andesites accompanied by high Li, Rb, Sr, Ba and Pb abundances. The subduction-derived component that was added to these hydrous magmas is shown to be very similar beneath both Mt. Shasta and Medicine Lake volcanoes despite characteristically distinct Li isotope compositions in the magmas themselves. More evolved andesites and dacites from Mt. Shasta have δ7Li from + 2.8 to + 6.9‰ which is identical with the range obtained for HAOTs and BA lavas from Mt. Shasta. Therefore, Li isotopes do not provide evidence for any other crustal component admixed to Mt. Shasta andesites or dacites during magmatic differentiation and magma mixing in the crust. 相似文献
244.
The Marmes Rockshelter archaeological site in southeastern Washington state contains a > 11 kyr stratigraphic record that was excavated in the 1960s but only recently analyzed in detail. We present the results of physical, chemical, and isotopic analyses of archived Marmes sediments from rockshelter, hillslope, and floodplain locations. Multiple lines of evidence including éboulis production, soil chemistry, and δ13C and δ18O signatures in soil organic matter and calcium carbonate suggest that relatively cool, moist conditions 10,600 to 9700 14C yr BP were followed by relatively warm and dry conditions as early as 9000 14C yr BP. Warm and dry conditions extended to the late Holocene, followed by a return to cooler and moister climate. The limited range of δ13C and δ18O values in Marmes paleosols suggests that the magnitude of moisture and temperature shifts was locally buffered in the lower Snake River Canyon but adequate to generate significant changes in sedimentation and soil formation, possibly due to nonlinear geological and pedological processes. These buffered canyon environments were well suited for establishing residential bases associated with foraging and logistical collecting strategies and may have minimized the influence of climate changes in food resource abundance. 相似文献
245.
Sébastien Joannin Frédéric Quillévéré Christophe Lécuyer François Martineau 《Quaternary Research》2007,67(2):264-274
Vegetation inherited from a Pliocene subtropical climate evolved through obliquity oscillations and global cooling leading to modern conditions. An integrated, highly time-resolved record of pollen and stable isotopes (δ18O and δ13C of Globigerina bulloides) was obtained to understand vegetation responses to Early Pleistocene climate changes. Continental and marine responses are compared in the Central Mediterranean region with a particular consideration of environmental changes during anoxic events.Pollen data illustrate vegetation dynamics as follows: [1] development of mesothermic elements (warm and humid conditions); [2] expansion of mid- and high-altitude elements (cooler but still humid conditions); and [3] strengthening of steppe and herb elements (cooler and dry conditions). These successions correlate with precession. δ18O variations recorded by Globigerina bulloides define two cycles (MIS 43-40) related to obliquity. At northern low- to mid-latitudes, the pollen signal records temperature and wetness changes related to precession even during global climate changes induced by obliquity. This may result in unexpected increasing wetness during glacial periods, which has to be considered specific to the Central and Eastern Mediterranean region. Lastly, an analysis of anoxic events reveals that enhanced runoff is indicated by increasing frequency of the riparian trees Liquidambar and Zelkova. 相似文献
246.
Nikolaus Gussone Gerald Langer Markus Geisen Blair A. Steel Ulf Riebesell 《Earth and Planetary Science Letters》2007,260(3-4):505-515
Four species of marine calcifying algae, the coccolithophores Calcidiscus leptoporus, Helicosphaera carteri, Syracosphaera pulchra and Umbilicosphaera foliosa were grown in laboratory cultures under temperatures varying between 14 and 23 °C, and one species, C. leptoporus, under varying [CO32−], ranging from 105 to 219 μmol/kg. Calcium isotope compositions of the coccoliths resemble in both absolute fractionation and temperature sensitivity previous calibrations of marine calcifying species e.g. Emiliania huxleyi (coccolithophores) and Orbulina universa (planktonic foraminifera) as well as inorganically precipitated CaCO3, but also reveal small species specific differences. In contrast to inorganically precipitated calcite, but similar to E. huxleyi and O. universa, the carbonate ion concentration of the medium has no statistically significant influence on the Ca isotope fractionation of C. leptoporus coccoliths; however, combined data of E. huxleyi and C. leptoporus indicate that the observed trends might be related to changes of the calcite saturation state of the medium. Since coccoliths constitute a significant portion of the global oceanic CaCO3 export production, the Ca isotope fractionation in these biogenic structures is important for defining the isotopic composition of the Ca sink of the ocean, one of the key parameters for modelling changes to the marine Ca budget over time. For the present ocean our results are in general agreement with the previously postulated and applied mean value of the oceanic Ca sink (Δsed) of about − 1.3‰, but the observed inter- and intra-species differences point to possible changes in Δsed through earth history, due to changing physico-chemical conditions of the ocean and shifts in floral and faunal assemblages. 相似文献
247.
248.
Theo van Leeuwen Charlotte M. Allen Ade Kadarusman Marlina Elburg J. Michael Palin Muhardjo Suwijanto 《Journal of Asian Earth Sciences》2007,29(5-6):751-777
The Malino Metamorphic Complex (MMC) is located at the western end of the north arm of Sulawesi. It consists of mica schists and gneisses (derived from proximal turbidite and granitoid protoliths), with intercalations of greenschist, amphibolite, marble, and quartzite, forming an E-W elongated dome-like structure bounded on all sides by faults. The age of the MMC is constrained between Devonian and Early Carboniferous. This Paleozoic age, the presence of Archean and Proterozoic inherited zircons, and the isotopic signature of the mica schists and gneisses indicate that the terrane was derived from the New Guinea-Australian margin of Gondwana. Similarities with basement rocks in the Bird’s Head suggests a common origin. Greenschists forming a discontinuous selvage (metamorphic carapace) around the complex were derived from adjacent autochthonous Paleogene formations. The rocks of the MMC show a Barrovian-type progression from greenschist through epidote-amphibolite to amphibolite facies. P–T estimations suggest a depth of burial of up to 27–30 km. K/Ar and 40Ar/39Ar cooling ages of 23–11 Ma, and a 7 Ma age for unconformably overlying volcanic rocks, indicate that the complex was exhumed during the Miocene. Two tectonic scenarios are considered: 1. the continental fragment docked with Sulawesi during the Mesozoic and was exhumed as a metamorphic core complex during the Miocene; 2. it was subducted beneath the north arm during the late Oligocene and then rapidly returned back to the surface. 相似文献
249.
Magmatic History of Somma-Vesuvius on the Basis of New Geochemical and Isotopic Data from a Deep Borehole (Camaldoli della Torre) 总被引:3,自引:0,他引:3
Di Renzo V.; Di Vito M. A.; Arienzo I.; Carandente A.; Civetta L.; D'antonio M.; Giordano F.; Orsi G.; Tonarini S. 《Journal of Petrology》2007,48(4):753-784
A continuous-coring borehole recently drilled at Camaldoli dellaTorre on the southern slopes of SommaVesuvius providesconstraints on the volcanic and magmatic history of the Vesuvianvolcanic area since c. 126 ka BP. The cored sequence includesvolcanic units, defined on stratigraphical, sedimentological,petrological and geochemical grounds, emitted from both localand distal vents. Some of these units are of known age, suchas one Phlegraean pre-Campanian Ignimbrite, Campanian Ignimbrite(39 ka), Neapolitan Yellow Tuff (14· 9 ka) and VesuvianPlinian deposits, which helps to constrain the relative ageof the other units. The main rock types encountered are shoshonite,phonotephrite, latite, trachyte and phonolite. The sequenceincludes, from the base upwards: a thick succession of pyroclasticunits emplaced between 126 and 39 ka, most of them attributedto eruptions that occurred in the Phlegraean area; the CampanianIgnimbrite; the products of a local tuff cone formed between39 ka and the deposition of the products of the earliest activityof the Mt. Somma volcano; the products of the SommaVesuviusvolcano, which include from the base upwards a thick sequenceof lavas, pyroclastic rocks and the products of a local spattercone dated between 3· 7 ka and AD 79. The data obtainedfrom the study of the borehole show that, before the CampanianIgnimbrite eruption, low-energy explosive volcanism took placein the Vesuvian area, whereas mostly high-energy explosive eruptionscharacterized the Campi Flegrei activity. In the Vesuvian area,Campanian Ignimbrite deposition was followed by the eruptionof a local tuff cone and a long repose time, which predatedthe formation of the Mt. Somma edifice. Since 18· 3 ka(Pomici di Base eruption) the activity of SommaVesuviusbecame mostly explosive with rare lava effusions. The shallowestcored deposits belong to the Camaldoli della Torre cone, formedbetween the Pomici di Avellino and Pomici di Pompei eruptions(3· 7 kaAD 79). New geochemical and SrNdPbB-isotopicdata on samples from the drilled core, together with those availablefrom the literature, allow us to further distinguish the volcanicrocks as a function of both their provenance (i.e. Phlegraeanor Vesuvian areas) and age, and to identify different magmaticprocesses acting through time in the Vesuvian mantle source(s)and during magma ascent towards the surface. Isotopically distinctmagmas, rising from a mantle source variably contaminated byslab-derived components, stagnated at mid-crustal depths (810km below sea level) where magmas differentiated and were probablycontaminated. Contamination occurred either with Hercynian continentalcrust, mostly during the oldest stages of Vesuvian activity(from 39 to 16 ka), or with Mesozoic limestone, mostly duringrecent Vesuvian activity. Energy constrained assimilation andfractional crystallization (EC-AFC) modelling results show thatcontamination with Hercynian crust probably occurred duringdifferentiation from shoshonite to latite. Contamination withlimestone, which is not well constrained with the availabledata, might have occurred only during the transition from shoshoniteto tephrite. From the deep reservoir, magmas rosetowards a series of shallow reservoirs, in which they differentiatedfurther, mixed, and fed volcanic activity. KEY WORDS: SommaVesuvius; crustal contamination; source heterogeneity; radiogenic and stable isotopes; magmatic system 相似文献
250.
Zircon and whole-rock Nd-Pb isotopic provenance of Middle and Upper Ordovician siliciclastic rocks, Argentine Precordillera 总被引:1,自引:0,他引:1
JAMES D. GLEASON STANLEY C. FINNEY† SILVIO H. PERALTA‡ GEORGE E. GEHRELS§ KATHLEEN M. MARSAGLIA¶ 《Sedimentology》2007,54(1):107-136
Graptolite‐bearing Middle and Upper Ordovician siliciclastic facies of the Argentine Precordillera fold‐thrust belt record the disintegration of a long‐lived Cambro‐Mid Ordovician carbonate platform into a series of tectonically partitioned basins. A combination of stratigraphic, petrographic, U‐Pb detrital zircon, and Nd‐Pb whole‐rock isotopic data provide evidence for a variety of clastic sediment sources. Four Upper Ordovician quartzo‐lithic sandstones collected in the eastern and central Precordillera yield complex U‐Pb zircon age spectra dominated by 1·05–1·10 Ga zircons, secondary populations of 1·22, 1·30, and 1·46 Ga, rare 2·2 and 1·8 Ga zircons, and a minor population (<2%) of concordant zircons in the 600–700 Ma range. Archaean‐age grains comprise <1% of all zircons analysed from these rocks. In contrast, a feldspathic arenite from the Middle Ordovician Estancia San Isidro Formation of the central Precordillera has two well‐defined peaks at 1·41 and 1·43 Ga, with no grains in the 600–1200 Ma range and none older than 1·70 Ga. The zircon age spectrum in this unit is similar to that of a Middle Cambrian quartz arenite from the La Laja Formation, suggesting that local basement rocks were a regional source of ca 1·4 Ga detrital zircons in the Precordillera Terrane from the Cambrian onwards. The lack of grains younger than 600 Ma in Upper Ordovician units reinforces petrographic data indicating that Ordovician volcanic arc sources did not supply significant material directly to these sedimentary basins. Nd isotopic data (n = 32) for Middle and Upper Ordovician graptolitic shales from six localities define a poorly mixed signal [ɛNd(450 Ma) = −9·6 to −4·5] that becomes more regionally homogenized in Upper Ordovician rocks (−6·2 ± 1·0; TDM = 1·51 ± 0·15 Ga; n = 17), a trend reinforced by the U‐Pb detrital zircon data. It is concluded that proximal, recycled orogenic sources dominated the siliciclastic sediment supply for these basins, consistent with rapid unroofing of the Precordillera Terrane platform succession and basement starting in Mid Ordovician time. Common Pb data for Middle and Upper Ordovician shales from the western and eastern Precordillera (n = 15) provide evidence for a minor (<30%) component that was likely derived from a high‐μ (U/Pb) terrane. 相似文献