Diagenetic effects on the distribution of uranium in live and Holocene corals from the Gulf of Aqaba     

Boaz Lazar  Rivka Enmar  Miriam Bar-Matthews  Mordechai Stein 《Geochimica et cosmochimica acta》2004,68(22):4583-4593

We investigated the effects of diagenetic alteration (dissolution, secondary aragonite precipitation and pore filling) on the distribution of U in live and Holocene coral skeletons. For this, we drilled into large Porites lutea coral-heads growing in the Nature Reserve Reef (NRR), northern Gulf of Aqaba, a site close to the Marine Biology Laboratory, Elat, Israel, and sampled the core material and porewater from the drill-hole. In addition, we sampled Holocene corals and beachrock aragonite cements from a pit opened in a reef buried under the laboratory grounds. We measured the concentration and isotopic composition of U in the coral skeletal aragonite, aragonite cements, coral porewater and open NRR and Gulf of Aqaba waters.Uranium concentration in secondary aragonite filling the skeletal pores is significantly higher than in primary biogenic aragonite (17.3 ± 0.6 compared to 11.9 ± 0.3 nmol · g⁻¹, respectively). This concentration difference reflects the closed system incorporation of uranyl tri-carbonate into biogenic aragonite with a U/Ca bulk distribution coefficient (K_D) of unity, versus the open system incorporation into secondary aragonite with K_D of 2.4. The implication of this result is that continuous precipitation of secondary aragonite over ∼1000 yr of reef submergence would reduce the coral porosity by 5% and can produce an apparent lowering of the calculated U/Ca - SST by ∼1°C and apparent age rejuvenation effect of 7%, with no measurable effect on the calculated initial U isotopic composition.All modern and some Holocene corals (with and without aragonite cement) from Elat yielded uniform δ²³⁴U = 144 ± 5, similar to the Gulf of Aqaba and modern ocean values. Elevated δ²³⁴U values of ∼180 were measured only in mid-Holocene corals (∼5000 yr) from the buried reef. The values can reflect the interaction of the coral skeleton with ²³⁴U-enriched ground-seawater that washes the adjacent granitic basement rocks.We conclude that pore filling by secondary aragonite during reef submergence can produce small but measurable effects on the U/Ca thermometry and the U-Th ages. This emphasizes the critical importance of using pristine corals where the original mineralogy and porosity are preserved in paleooceanographic tracing and dating.  相似文献   

Surface ruptures induced by the devastating 1068 AD earthquake in the southern Arava valley, Dead Sea Rift, Israel   总被引：2，自引：1，他引：2  

E. Zilberman  R. Amit  N. Porat  Y. Enzel  U. Avner 《Tectonophysics》2005,408(1-4):79

The Elat fault (a segment of the Dead Sea Transform) runs along the southern Arava valley (part of the Dead Sea Rift, Israel) forming a complex fault zone that displays a time-dependent seismic behaviour. Paleoseismic evidence shows that this fault zone has generated at least 15 earthquakes of magnitude larger than M 6 during the late Pleistocene and the Holocene. However, at present the Elat fault is one of the quietest segments of the Dead Sea Transform, lacking even microsesimicity. The last event detected in the southern Arava valley occurred in the Avrona playa and was strong enough to have deformed the playa and to change it from a closed basin with internal drainage into an open basin draining to the south.Paleoseismological, geophysical and archaeological evidences indicate that this event was the historical devastating earthquake, which occurred in 1068 AD in the eastern Mediterranean region. According to the present study this event was strong enough to rupture the surface, reactivate at least two fault branches of the Elat fault and vertically displace the surface and an early Islamic irrigation system by at least 1 m. In addition, the playa area was uplifted between 2.5 and 3 m along the eastern part of the Elat fault shear zone. Such values are compatible with an earthquake magnitude ranging between M 6.6 and 7. Since the average recurrence interval of strong earthquakes during the Holocene along the Elat fault is about 1.2 ± 0.3 ky and the last earthquake occurred more about 1000 years ago, the possibility of a very strong earthquake in this area in the future should be seriously considered in assessing seismic hazards.  相似文献   

Stable sulfur isotope partitioning during simulated petroleum formation as determined by hydrous pyrolysis of Ghareb Limestone, Israel     

Alon Amrani  Zeev Aizenshtat 《Geochimica et cosmochimica acta》2005,69(22):5317-5331

Hydrous pyrolysis experiments at 200 to 365°C were carried out on a thermally immature organic-rich limestone containing Type-IIS kerogen from the Ghareb Limestone in North Negev, Israel. This work focuses on the thermal behavior of both organic and inorganic sulfur species and the partitioning of their stable sulfur isotopes among organic and inorganic phases generated during hydrous pyrolyses. Most of the sulfur in the rock (85%) is organic sulfur. The most dominant sulfur transformation is cleavage of organic-bound sulfur to form H₂S_(gas). Up to 70% of this organic sulfur is released as H₂S_(gas) that is isotopically lighter than the sulfur in the kerogen. Organic sulfur is enriched by up to 2‰ in ³⁴S during thermal maturation compared with the initial δ³⁴S values. The δ³⁴S values of the three main organic fractions (kerogen, bitumen and expelled oil) are within 1‰ of one another. No thermochemical sulfate reduction or sulfate formation was observed during the experiments. The early released sulfur reacted with available iron to form secondary pyrite and is the most ³⁴S depleted phase, which is 21‰ lighter than the bulk organic sulfur. The large isotopic fractionation for the early formed H₂S is a result of the system not being in equilibrium. As partial pressure of H₂S_(gas) increases, retro reactions with the organic sulfur in the closed system may cause isotope exchange and isotopic homogenization. Part of the δ³⁴S-enriched secondary pyrite decomposes above 300°C resulting in a corresponding decrease in the δ³⁴S of the remaining pyrite. These results are relevant to interpreting thermal maturation processes and their effect on kerogen-oil-H₂S-pyrite correlations. In particular, the use of pyrite-kerogen δ³⁴S relations in reconstructing diagenetic conditions of thermally mature rocks is questionable because formation of secondary pyrite during thermal maturation can mask the isotopic signature and quantity of the original diagenetic pyrite. The main transformations of kerogen to bitumen and bitumen to oil can be recorded by using both sulfur content and δ³⁴S of each phase including the H₂S_(gas). H₂S generated in association with oil should be isotopically lighter or similar to oil. It is concluded that small isotopic differentiation obtained between organic and inorganic sulfur species suggests closed-system conditions. Conversely, open-system conditions may cause significant isotopic discrimination between the oil and its source kerogen. The magnitude of this discrimination is suggested to be highly dependent on the availability of iron in a source rock resulting in secondary formation of pyrite.  相似文献   

Cold dark matter variant cosmological models — I. Simulations and preliminary comparisons     

Michael A. K. Gross  Rachel S. Somerville  Joel R. Primack  Jon Holtzman  & Anatoly Klypin 《Monthly notices of the Royal Astronomical Society》1998,301(1):81-94

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O-Sr isotopic variations in Miocene granitoids from the Aegean: evidence for an origin by combined assimilation and fractional crystallization     

Rainer Altherr  Friedhelm Henjes-Kunst  Alan Matthews  Hans Friedrichsen  Bent Tauber Hansen 《Contributions to Mineralogy and Petrology》1988,100(4):528-541

Sr, O, and D/H isotopic compositions have been analyzed in Miocene metaluminous to slightly peraluminous (I-type) granitoids of the central Aegean. Individual plutonic complexes display significant variations in their ¹⁸O and initial⁸⁷Sr/⁸⁶Sr compositions.D and ¹⁸O compositions of minerals and whole-rocks are mostly in the magmatic range. Some samples from Naxos and Mykonos/Delos show low D and ¹⁸O values characteristic of meteoric-water-hydrothermal interaction, but as a whole the changes in ¹⁸O and Sr isotopic compositions as a result of hydrothermal alteration were slight, even in instances where marked alteration is petrographically observable. Consequently, the bulk-rock variations of ¹⁸O from 8.1 to 12.0 and of⁸⁷Sr/⁸⁶Sr from 0.70438 to 0.71450 may be regarded as primary and indicative of the conditions of their evolution. Heterogeneous isotopic compositions observed in the individual plutons of Serifos, Ikaria, Samos and Kos may be caused by the multiple intrusion of chemically and isotopically distinct magma pulses, with high viscosities and relatively rapid consolidation in most cases preventing complete homogenization. The granitoids of Serifos, Ikaria and Kos display weak correlations between the initial⁸⁷Sr/⁸⁶Sr and ¹⁸O and 1/Sr. The granitoid province shows a positive correlation between⁸⁷Sr/⁸⁶Sr and ¹⁸O and a non-linear relationship between⁸⁷Sr/⁸⁶Sr and 1/Sr, whereby 1/Sr increases more rapidly than the isotopic ratio as the degree of fractionation of the rocks increases. It is argued that assimilation of older continental material by mantle-derived arc magmas with combined fractionation (AFC) is the most plausible model to explain the chemical and isotopic characteristics of the granitoids and the geological situation in which rock-types trend from granodiorites in the (south)west, near the inferred Oligocene-Miocene suture, to granites in the center and monzonites in the (north)east of the province.  相似文献   

Climatic change as a topic in the classical Greek and Roman literature     

J. Neumann 《Climatic change》1985,7(4):441-454

A search was made of the classical Greek and Roman literature for references to climatic change, irrespective whether facts of observation or views. It was found that several scholars/scientists of the classical antiquity made pronouncements on the subject and their statements are either summarized or quoted verbatim in this paper. From the Greek literature we quote Plato, Aristotle and Theophrastus; Herodotus is also quoted for an indirect reference to the topic. From the Roman literature we cite the agricultural writer Columella and the scientist Pliny the Elder; actually, Columella quotes the distinguished writers on husbandry, the Sasernas, father and son.Aristotle's statement is relevant to a degree to the Mycenaean drought (~ 1200 B.C.) problem. The Columella-Saserna pronouncement relates to the astronomical theory of climatic change, while Theophrastus discusses man-made climatic changes.Appendix I gives a statistics of storms, as gleaned from the classical literature, covering approximately the months April–October. Further, it lists some pointers to the relative coolness of the era 850/800–400/300 B.C. in some areas of the Mediterranean. Appendix II offers a few examples for the usefulness of the ancient literature from the point of view of environmental data or events.Now visiting with the Department of Meteorology, The University of Helsinki, Helsinki, Finland.  相似文献   

Subsurface countercurrent in the Bosporus already known to 6th century A. D. Byzantine fishermen     

Prof. Dr. Manfred Korfmann  Prof. Dr. Jehuda Neumann 《Ocean Dynamics》1985,38(4):189-190

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Kinetics and mechanisms of the reaction of zoisite to anorthite under hydrothermal conditions: reaction phenomenology away from the equilibrium region     

Alan Matthews 《Contributions to Mineralogy and Petrology》1985,89(2-3):110-121

The kinetics and mechanisms of the dehydration reactions of zoisite have been studied at 635°–792° C, 1–2 kbar. The equilibrium reaction does not occur and is replaced by metastable reactions involving the formation of gehlenite and a calcium tri-octahedral mica, instead of corundum: zoisite anorthite+grossular+gehlenite +calcium 3T mica+H₂O.The experimental data can be interpreted by zero-order equations dX _An/dt=k (X _An=fractional extent of reaction, t=time, k=zero-order rate constant). These relations hold for variations in P, T, A _initial(the initial surface area of zoisite) and also in the presence of seed crystals, which enhance the reaction rate. No induction period is evident and only at advanced stages of reaction are sharp decreases observed in the rate, which are attributed to physical affects (shrinking particles, armouring). SEM studies show that dissolution of zoisite is anisotropic, occurring preferentially parallel to the crystallographic b-axis, with the result that a characteristic sawtooth etch structure develops. Garnet grows as euhedral crystals located in cavities or on teeth of dissolving zoisite, whereas anorthite forms as clusters of coalescing grains which spread over and enclose the zoisite. In seeded runs, garnet growth initiates on both seeds and on zoisite surfaces whereas anorthite growth is more closely tied to the seeds, resulting in the development of clusters of smaller grains.The experimental evidence favours dissolution and nucleation-controlled growth as rate-determining processes. The preservation of zero-order kinetics in the face of shrinking particles is attributed to the anisotropic dissolution mechanism, which effectively preserves a constant reaction interface. The rate effects of nucleation appear to accord with the classic model in which growth of crystal layers is initiated by the formation of coherent nuclei.The temperature dependence of rate constants reflects both thermally activated Arrhenius-type behaviour and the rate-depressing influences of approach to equilibrium. Similarly pressure affects on reaction rate can be interpreted in terms of competition between rate enhancement due to pressure increase and rate-depression accompanying the approach to equilibrium. Although the equilibrium-approach effects accord with current treatments of reaction kinetics, a problem exists in deriving an exact relation coupling dissolution and nucleation rate control. Consequently an overall-reaction rate equation, such as that of Fisher and Lasaga (1981), is only partially successful in interpreting the temperature dependence of rates. The data suggest that the surface reaction equation of Wood and Walther (1983) only applies when nuclei are present.  相似文献   

On the evolution of central stars of planetary nebulae     

Raphael Z. Yahel 《Astrophysics and Space Science》1977,51(1):135-152

The evolution of nuclei of planetary nebulae has been calculated from the end of the ejection stage that produces the nebulae to the white, dwarf stage. The structure of the central star is in agreement with the general picture of Finzi (1973) about the mass ejection from the progenitors of planetary nebulae. It has been found that in order to obtain evolutionary track consistent with the Harman-Seaton track (O'Dell, 1968) one has to assume that the masses of the nuclei stars are less than 0.7M . The calculated evolutionary time scale of the central stars of planetary nebulae is 2×10⁴ yr. This time scale is negatively correlated with the stellar mass: the heavier the stellar mass, the shorter the evolutionary time scale.  相似文献   

Analysis of log-probability plots of recent Atlantic sediments and its analogy with simulated mixtures     

AMOS BEIN  EYTAN SASS 《Sedimentology》1978,25(4):575-581

The log-probability plots of some shallow to deep Atlantic sediments are analysed and compared with simulated mixtures of log-normal populations. The plots consist characteristically of two smoothly connected segments which display different slopes. The point of maximum curvature (PMC) between the two segments is shown to reflect the granulometric parameters (mean and sorting) and quantitative proportions of two mixed overlapping populations. This understanding is contrary to the concept which relies heavily on the break points of cumulative plots as indicating truncation of grain size distributions and therefore reflecting hydrodynamic processes. The particular behaviour of the cumulative plots from the Atlantic indicates fining of the two component populations with depth and distance from the shore, accompanied by an increasing dominance of the finer population. This behaviour is compatible with sorting processes which operate in sediments transported through individual grain movements, rather than through mass transportation or turbidity currents.  相似文献