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1.
Beryllium isotopes (10Be and9Be) have been measured in suspended particles of < 1 mm size collected by mid-water sediment traps deployed in the eastern Pacific at MANOP sites H (6°32′N, 92°50′W, water depth 3600 m) and M (8°50′N, 104°00′W, 3100 m). For comparison, surface sediments from box cores taken from the two sites were also studied. The concentrations of10Be and9Be in sediment-trap particles are about an order of magnitude smaller than those in the bottom sediments which contain about 8 × 109 and 6 × 1016 atoms g−1 of10Be and9Be, respectively. The sediment trap samples collected from 50 m off the bottom showed significant (26–63%) contributions from resuspended bottom sediments. The10Be/9Be ratio in trap samples varies from 3 to 20 × 10−8. The variation may partly result from varied proportion of authigenic/detrital material. The fluxes of both isotopes exhibit a very strong seasonality. The fluxes of10Be into the traps at about 1500 m are estimated as 9 × 105 and 4 × 105 atoms cm−2 a−1 at sites H and M respectively. These values are to be compared with the fluxes into the sediments of 4–5 × 105 atoms cm−2 a−1 at both locations. Good correlations exist between10Be,9Be and27Al indicating that the primary carrier phase(s) for the beryllium isotopes in the water column may be aluminosilicates.  相似文献   

2.
Oxygen self diffusion rates were determined in quartz samples exchanged with18O-enriched CO2 between 745 and 900°C and various pressures, and the diffusion profiles were measured using an ion microprobe. The activation energy (Q) and preexponential factor (D0) at P(CO2) = P(tot) = 100 bar, for diffusion parallel to the c-axis are 159 ( ± 13) kJ/g atom and 2.10 (+0.75/ −0.55) × 10−8 cm2/s. This rate is approximately 100 times slower than that obtained from hydrothermal experiments and 100 times faster than a previous 1-bar quartz-O2 exchange experiment. The oxygen diffusion rate measured at 0.6 bar, 888°C, and at 900°C in vacuum is in agreement with the previous 1-bar exchange experiments with18O2. The effect of higher CO2 pressures is small. At 900°C, the diffusion rate exchanged with CO2 is = 2.35 × 10−15 cm2/s at 100 bar, 2.24 × 10−15 cm2/s at 3.45 kbar and 8.13 × 10−15 cm2/s at 7.2 kbar.There is probably a diffusing species, other than oxygen, that enhances the oxygen diffusion rate in these quartz-CO2 systems, relative to that occurring at very low pressures or in a vacuum. The effect of this diffusing species, however, is not as strong as that associated with H2O. Preserved oxygen isotope fractionations between coexisting minerals in a slowly cooled, high-grade metamorphic terrane will vary depending upon whether a water-rich phase was present or not. Closure temperatures will be approximately 100°C higher in rocks where no water-rich phase was present during cooling. The measured fractionations between coexisting minerals in metamorphic rocks may potentially be used as a sensor of water presence during retrogression.  相似文献   

3.
Precipitation collected in continuously open containers for about a year at seven sites around the United States was analyzed for10Be,90Sr,210Pb and238U. Based on these data and long-term precipitation,90Sr and210Pb delivery patterns, the stratospheric, tropospheric and recycled10Be components in the collections were estimated and the global10Be production rate was assessed. Single station production rate estimates range from 0.52 × 106 atoms cm−2 yr−1 to 2.64 × 106 atoms cm−2 yr−1. The mean value is 1.21 × 106 atoms cm−2 yr−1 with a standard error of 0.26 × 106 atoms cm−2 yr−1.  相似文献   

4.
The influx of10Be into a globigerinid ooze core (CH72-02) from the eastern North Atlantic has been studied. This core contains a depositional record of the first 11 δ18O stages covering the last 423 ka. It is shown that the marine deposition of10Be is strongly influenced by the sedimentation of clays. Clay particles appear 10 times more efficient than the carbonate component as a carrier in bringing10Be to the bottom sediments. In core CH72-02, the deposition rates of10Be averaged over each oxygen-isotope stage for the past 11 stages show a scatter of ±40% about the mean value of 6.6 × 108 atoms cm−2 ka−1. However, after correction for changes in lithology, the data show that the production rate of10Be over the same period has varied no more than ±25%, and the variations are not systematic in that high or low10Be production appear to be associated with either cold or warm climates. On the time scale of this investigation (intervals of ca. 50 ka over the last 420 ka, with resolutions as fine as 10 ka for portions of the record), it is unlikely that the shielding effect of the solar wind has deviated by more than ±25% or the geomagnetic field intensity has deviated by more than a factor of 1.6 from their long-term averages.  相似文献   

5.
Gas concentrations and isotopic compositions of water have been measured in hydrothermal waters from 13°N on the East Pacific Rise. In the most Mg-depleted samples ( 5 × 10−3 moles/kg) the gas concentrations are: 3–4.5 × 10−5 cm3 STP/kg helium, 0.62–1.24 cm3 STP/kg CH4, 10.80–16.71 × 10−3 moles/kg CO2. The samples contain large quantities (95–126 cm3/kg) of H2 and some carbon monoxide (0.26–0.36 cm3/kg) which result from reaction with the titanium sampling bottles. δ13C in methane and CO2 (−16.6 to −19.5 and −4.1 to −5.5 respectively) indicate temperatures between 475 and 550°C, whereas δ13CCO is compatible with formation by reduction of CO2 on Ti at 350°C close to the sampling temperature.3He/4He are very homogeneous at (7.5 ± 0.1)RA(3He/4He = 1.0 × 10−5) and very similar to already published data as well as CH4/3He ratios between 1.4 and 2.1 × 106.18O and D in water show enrichments from 0.39 to 0.69‰ and from 0.62 to 1.49‰ respectively. These values correspond to W/R ratios of 0.4–7. The distinct18O enrichments indicate that the isotopic composition of the oceans is not completely buffered by the hydrothermal circulations. The3He-enthalpy relationship is discussed in terms of both hydrothermal heat flux and3He mantle flux.  相似文献   

6.
We have developed techniques to measure the230Th abundance in corals by isotope dilution mass spectrometry. This, coupled with our previous development of mass spectrometric techniques for234U and232Th measurement, has allowed us to reduce significantly the analytical errors in238U234U230Th dating and greatly reduce the sample size. We show that 6 × 108 atoms of230Th can be measured to ±30‰ (2σ) and 2 × 1010 atoms of230Th to ±2‰. The time over which useful age data on corals can be obtained ranges from a few years to 500 ky. The uncertainty in age, based on analytical errors, is ±5 y (2σ) for a 180 year old coral (3 g), ±44 y at 8294 years and ±1.1 ky at 123.1 ky (250 mg of coral). We also report232Th concentrations in corals (0.083–1.57 pmol/g) that are more than two orders of magnitude lower than previous values. Ages with high analytical precision were determined for several corals that grew during high sea level stands 120 ky ago. These ages lie specifically within or slightly postdate the Milankovitch insolation high at 128 ky and support the idea that the dominant cause of Pleistocene climate change is Milankovitch forcing.  相似文献   

7.
The groundwaters of the Great Artesian Basin (Australia) have been previously shown to be accumulating in-situ production helium for groundwaters ages < 50 kyr and an external helium flux equivalent to whole crustal production for groundwater ages > 100 kyr [1,2]. New helium isotope measurements show that the observed in-situ production helium (3He/4He 1.6 × 10−8) is isotopically distinct from the crustal degassing helium flux (3He/4He 6.6 × 10−8). Furthermore, the crustal degassing helium isotope ratio is marginally in excess of the whole crustal production ratio (3He/4He= 3.5 × 10−8) and the production ratio in a variety of continental rock types. This suggests that the upper limit on volatile transport across the mantle-crust boundary beneath the (relatively) stable and “complacent” Australian continent can be characterized by a “conductive-diffusive” helium/heat flux ratio of 2.6 × 1064He atoms mW−1 s−1 which is two orders of magnitude less than the “intrusive-volcanic” ratio of 2.9 × 1084He atoms mW−1 s−1 measured at the Galapagos [16]. These results constrain the transcrustal mantle degassing fluxes of4He and40Ar to be much less than the mid-ocean ridge degassing fluxes; which are much less than the degassing of4He and40Ar from continental crust. Thus, the degassing of the Earth's interior is dominated by magmatic processes but the dominant fluxes of4He and40Ar to the atmosphere must come from the continental crust.  相似文献   

8.
Pressure–volume–temperature relations have been measured to 32 GPa and 2073 K for natural magnesite (Mg0.975Fe0.015Mn0.006Ca0.004CO3) using synchrotron X-ray diffraction with a multianvil apparatus at the SPring-8 facility. A least-squares fit of the room-temperature compression data to a third-order Birch–Murnaghan equation of state (EOS) yielded K0 = 97.1 ± 0.5 GPa and K′ = 5.44 ± 0.07, with fixed V0 = 279.55 ± 0.02 Å3. Further analysis of the high-temperature compression data yielded the temperature derivative of the bulk modulus (∂KT/∂T)P = −0.013 ± 0.001 GPa/K and zero-pressure thermal expansion α = a0 + a1T with a0 = 4.03 (7) × 10−5 K−1 and a1 = 0.49 (10) × 10−8 K−2. The Anderson–Grüneisen parameter is estimated to be δT = 3.3. The analysis of axial compressibility and thermal expansivity indicates that the c-axis is over three times more compressible (KTc = 47 ± 1 GPa) than the a-axis (KTc = 157 ± 1 GPa), whereas the thermal expansion of the c-axis (a0 = 6.8 (2) × 10−5 K−1 and a1 = 2.2 (4) × 10−8 K−2) is greater than that of the a-axis (a0 = 2.7 (4) × 10−5 K−1 and a1 = −0.2 (2) × 10−8 K−2). The present thermal EOS enables us to accurately calculate the density of magnesite to the deep mantle conditions. Decarbonation of a subducting oceanic crust containing 2 wt.% magnesite would result in a 0.6% density reduction at 30 GPa and 1273 K. Using the new EOS parameters we performed thermodynamic calculations for magnesite decarbonation reactions at pressures to 20 GPa. We also estimated stability of magnesite-bearing assemblages in the lower mantle.  相似文献   

9.
Measurements of five cosmogenic32Si vertical profiles in Atlantic waters (27°N to 60°S) are presented. The amounts of dissolved SiO2 extracted range from 2 to 54 g; the amounts of water from which SiO2 was extracted range between 540 kg and 270, 000 kg. In additon, SiO2 recovered from four surface particulate composites (64°N to 61°S) were also analyzed for32Si.32Si measurements were made by milking and counting the daughter activity, 32P. The net32P activities range from 0.7 to 6.8 cph; typical errors in measurements of the32P activities are 20–30%.The32Si concentrations vary from 0.6 dpm/106 kg of water in the North Atlantic surface waters to 235 dpm/106 kg at 400 m depth in the circumpolar waters. The vertical profiles of32Si at the five Atlantic stations approximately follow the Si profiles but the depth gradients are different. This would be expected also considering the in-situ release mechanisms due to dissolution and advection/diffusion from the bottom waters. Except for the circumpolar station 89, where the Si and32Si profiles show the effect of marked vertical mixing (nearly depth independent profiles), the profiles show the following features: (1) specific activities of32Si (32Si/SiO2 ratios) are lowest at intermediate depths, and (2) on an average the surface specific activities are higher, by 2–4 times, than the bottom water values. These data are consistent with generation of the highest specific activity32Si waters at the surface, where Si concentrations are lowest and precipitation adds cosmogenic32Si scavenged from the troposphere. Rapid removal of biogenic silica to the water-sediment interface, without much dissolution during transit, leads to the second regime of high32Si specific activities.The32Si inventories in the water column in the latitude belt 27°N-27°S are in the range (1–1.4) × 10−2 dpm32Si/cm2, which is consistent with the expected fallout of cosmogenic32Si. However, the32Si column inventories south of 40°S are higher by a factor of 5–7, whereas the corresponding Si inventories increase by only a factor of 3. This excess32Si in the Southern Ocean cannot be explained by direct fallout from the stratosphere or by melting of Antarctic snow and ice. Instead, this excess is maintained primarily by the southward deep-water transport of32Si dissolved from sinking particulates.  相似文献   

10.
A model of10Be deposition within the subarctic and arctic is developed based on the behavior of90Sr in the troposphere. Measured10Be fluxes, based on analyses of10Be in one year's snow fall (1979–1980) from the Dye-3 site in southwest Greenland and on published data, and predicted10Be fluxes, based on the10Be deposition model, agree. These results indicate that with regard to10Be, the troposphere north of 40°N to 45°N presently behaves as if it is well-mixed and that the average precipitation rate within that reservoir controls in large part the concentration of10Be in Greenland ice. Inversion of the Greenland ice core10Be concentration record with the aid of the model indicates: (1) that the average precipitation rate in the subarctic and arctic was lower than the present rate during the Maunder minimum of solar activity, and higher than the present rate during the Wolf and Sporer solar activity minimums; and (2) that during the Wisconsin-Holocene transition the average precipitation rate in the subarctic and arctic was about one third the present precipitation rate.  相似文献   

11.
Fluid flow from pore pressure measurements off La Palma, Canary Islands   总被引：1，自引：0，他引：1  
In situ subseafloor pore pressure results from the western flank of the island of La Palma, Canary Islands, are presented. The data obtained with a Pop Up Pore Pressure Instrument (PUPPI) provide constraints on the fluid circulation and its causes in a very special context: The sediment piles near an intraplate oceanic island built on the continental rise of the Northwest African Margin. The ambient pore pressures estimated from 2 to 4 days long record are negative in almost all cases with values, at depths of a few meters below sea floor, usually on the order of −10 to −70 Pa. Excess pore pressures develop only in the distal most areas. The permeabilities and compressibilities obtained respectively from the decay of the insertion pressures and the amplitude of the tidally induced pore pressure variations range between 2.5×10−18 and 6.6×10−16 m2 and, 6.2×10−9 and 1.5×10−7 Pa−1. According to these permeabilities fluid flow is estimated to be mostly downward and usually on the range between 0 and −0.3 mm y−1. However, from the excess pore pressure profile a complex pattern of fluid circulation is inferred where horizontal fluid motion cannot be neglected. Horizontal flow is probably controlled by significant contrasts in the permeability of the different layers. The prevailing downward fluid flow is abnormal for a classical passive margin. We thus interpret these results as the superposition to the loss of fluids by sediment compaction (on the continental rise), of a large-scale flow system stimulated by thermal buoyancy (100 km wide) related to the volcanic activity on the island of La Palma.  相似文献   

12.
All previous accounts of the spiral patterns at the Martian poles emphasize that the north polar spiral is centered about the geographic pole, whereas that of the south polar region is off-set by about 4°. This paper demonstrates that the patterns near both poles are centered on topographic highs rather than the spin poles themselves. This is circumstantial evidence in favour of the relatively unexplored mechanism of radial outflow of viscous rock by gravity spreading.The hypothesis developed here is that the spiral patterns are essentially due to crevasse patterns formed perpendicular to flow lines which are perturbed by Coriolis forces. In order to account for a crevasse pattern that has a form concave to the east the angular deflection of an hypothetical ice flow emanating from the topographic high centered about the geographical north pole, must be about 40° or 0.7 radians in a westward direction at 85°N latitude.The polar cap rock has previously been assumed to consist mainly of either frozen carbondioxide or water ice. Corresponding viscosities (at 190 K) allow for the occurrence of radial outflow or gravity driven tectonics at a maximum rate of 1 cm a−1, but the flow pattern remains unaffected by Coriolis forces.The spiral patterns of the Martian poles can be explained if the flowing mass has an occasional effective kinematic viscosity as low as about 7 × 106 m2 s−1, because gravity tectonics will then be deflected by Coriolis forces resulting in appropriately curved flowlines. A tensile fracture pattern, resembling an anticlockwise spiral pattern perpendicular to the clockwise deflected flowlines may subsequently form by local brittle failure.The occasional kinematic viscosity 7 × 106 m2 s−1 would cause flow rates of 0.2 m s−1 along the slopes of the topographic highs. This velocity and the corresponding viscosity is tentatively thought to be possible when thermal and pressure runaway occurs in the polar layered deposits. This would mean glacier surges on the Martian poles are two orders of magnitude faster than those hitherto observed on Earth.  相似文献   

13.
The 1975 sub-terminal activity was characterised by low effusion rates (0.3–0.5 m3 s−1) and the formation of a compound lava field composed of many thousands of flow units. Several boccas were active simultaneously and effusion rates from individual boccas varied from about 10−4 to 0.25 m3s−1. The morphology of lava flows was determined by effusion rate (E): aa flows with well-developed channels and levees formed when E > 2 × 10−3 m3 s−1, small pahoehoe flows formed when 2 × 10−3 m3 s−1 >E > 5 > 10−4 m3 s−1 and pahoehoe toes formed when E < 5 × 10−4 m3 s−1. There was very little variation with time in the effusion temperature, composition or phenocryst content of the lava.New boccas were commonly formed at the fronts of mature lava flows which had either ceased to flow or were moving slowly. These secondary boccas developed when fluid lava in the interior of mature aa flows either found a weakness in the flow front or was exposed by avalanching of the moving flow front. The resulting release of fluid lava was accompanied by either partial drainage of the mature flow or by the formation of a lava tube in the parent flow. The temperature of the lava forming the new bocca decreased with increasing distance from the source bocca (0.035°C m−1). It is demonstrated from the rate of temperature decrease and from theoretical considerations that many of the Etna lavas still contained a substantial proportion of uncooled material in their interior as they came to rest. The formation of secondary boccas is postulated to be one reason why direct measurements of effusion rates tend, in general, to overestimate the total effusion rates of sub-terminal Etna lava fields.  相似文献   

14.
Samples dredged from 2 localities near the crest of the Valu Fa ridge, an active back-arc basin spreading centre in the Lau Basin, consist of highly vesicular lava fragments of andesitic composition. The samples are characterized by rare, euhedral An85 plagioclase phenocrysts in a hypocrystalline groundmass of An60 plagioclase laths, brown glass and rare subhedral clinopyroxene. Samples from within and, to a lesser extent, between the dredge hauls show remarkable isotopic and chemical homogeneity, with: 87Sr/86Sr − 0.70330 ± 2; 143Nd/144Nd − 0.51303 ± 2; 206Pb/204Pb − 18.65 ± 2; 207Pb/204Pb − 15.55 ± 1; 208Pb/204Pb − 38.34 ± 4; Sr − 165 ppm; Rb − 7 ppm; Cs − 0.17 ppm; K − 3300 to 4200 ppm; Ba − 96 ppm; and REE — LREE depleted with 12–18 × chondritic abundances. On Sr-Nd, Pb-Pb and Sr-Pb plots the volcanics lie just within or on the edge of the MORB fields, overlapping with island-arc volcanics from the Marianas and Tonga. Compared with MORB and ocean-island basalts, the samples show alkali-element enrichment relative to REE and higher Cs relative to Rb. The isotopic and geochemical characteristics of the Valu Fa Ridge volcanics clearly indicate a minor, but significant, slab-derived component in the back-arc basin mantle source.  相似文献   

15.
Crustal neon: a striking uniformity   总被引：1，自引：0，他引：1  
By combining data from a diverse suite of crustal fluid samples representing a broad geographical distribution, we have identified a well-defined nucleogenic (crustal) neon component. The neon is produced from (α, n) and (n, α) nuclear interactions involving nuclei of O, Mg, and F [1]. In the limiting case of 20Ne/22Ne = 0, the composition is: 21Ne/22Ne = 0.47 ± 0.01 and 21Ne/4He = (0.46 ± 0.08) × 10−7. A crustal O/F ratio of 110 (atomic) calculated from the 21Ne/22Ne ratio is 4–10 times less than the average crustal O/F ratio. The discrepancy can be accounted for by an enhanced O/F ratio within the 10–40 μm range of the U-Th-generated α-particles.  相似文献   

16.
Water samples were collected from Baffin Bay and surrounding areas in order to evaluate this region as a potential source of Nd from old continental material to Atlantic water. The isotopic data ranged from εNd(0) = −9.0 to −26 with most of the data around εNd(0) = −20 compared with values of North Atlantic Deep Water (NADW) with εNd(0) = −13.5. The concentration of Nd in Baffin Bay waters was as high as 6 × 10−12 g/g compared with 2.5 × 10−12 g/g for NADW. The combination of low εNd and high Nd concentration indicates that Baffin Bay may be a significant source of Nd from very old crustal material. A simple box model was used to evaluate the contribution to the Nd budget of NADW and it was concluded that a substantial fraction of the Nd from ancient crustal sources that is required to maintain the isotopic composition of NADW could be supplied by Baffin Bay outflow.  相似文献   

17.
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18.
Garnets in an amphibolite-facies metasediment from Sulitjelma, North Norway yield precise and concordant SmNd, UPb and RbSr ages that relate directly to the pressure (P) and temperature (T) conditions of mineral growth. Differential mineral reaction between graphitic and non-graphitic layers within this sample preserves a record of theP-T and time (t) history experienced during Barrovian regional metamorphism. Garnets in graphitic layers grew during prograde metamorphism at462 ± 16°C and5.2 ± 0.5 kbar under conditions of lowaH2O, and yield indistinguishable147Sm143Nd and238U206Pb ages of434.1 ± 1.2 Ma and433.9 ± 1.0 Ma, respectively. In contrast, garnet growth in adjacent graphite-free layers did not occur untilP-T conditions of540 ± 18°C and8.0 ± 1.0 kbar were attained, with continued growth in response to minor heating and decompression with final matrix equilibration at544 ± 16°C and7.0 ± 1.0 kbar. The inclusion-free garnet rims in this assemblage record indistinguishable147Sm143Nd and238U206Pb ages of424.6 ± 1.2 Ma and423.4± 1.7 Ma, respectively. These results provide precise estimates for average heating and burial rates during prograde metamorphism of 8.6−4.4+7.5°C Ma−1 and 0.8−0.5+0.9 km Ma−1, respectively. Rb and Sr exchange between coexisting silicates in the graphite-free assemblage continued for some 37 Ma after the “peak” of metamorphism, and require an average cooling rate of about 4.0°C Ma−1 during uplift. These results illustrate a clear relationship between reaction history and the timing of mineral growth and provide definitive constraints on the rates of thermal and tectonic processes accompanying regional metamorphism.  相似文献   

19.
We carried out precise crustal strain observation using a laser strainmeter system at the Rokko-Takao station in Kobe, Japan from 1989 to 1997. The long-term strain record is characterized by remarkable annual changes of the order of 2–3×10−6 and linear strain accumulation of −4.4×10−7/year (in contraction). The annual strain changes are inversely proportional to temperature changes that precede the strain changes by about 1 month. The apparent annual strain changes were mainly caused by refractive-index changes in the light path due to the ambient temperature changes. After eliminating the annual temperature effect, linear strain accumulation is corrected to be −6.3−6.7×10−7/year. Residual strains show the oscillating behavior, in which the oscillating cycle seems to become shorter and shorter as time goes by. During the period, a destructive earthquake of M=7.2 occurred near the Kobe City on 17 January 1995. We investigated the oscillating behavior in secular variations of ground-strains by introducing the deterministic approach of earthquake prediction to search for the “critical point” of the occurrence of an earthquake in the extended power law equation. However, we could not obtain a unique solution to determine eight unknown parameters including the “critical point”. This may be mainly due to lack of data for 4 months from August to November in 1994 by the failure of the laser source before the occurrence of earthquake on 17 January 1995. After removing environmental effects and tidal components, we carefully re-examined strain changes in 7 days and 1 day before the occurrence of the earthquake, but we could not detect anomalous strain changes exceeding 1×10−8 before the earthquake.  相似文献   

20.
Profiles of velocity turbulence in Monterey Canyon, made with a recently developed expendable probe, show the existence of a very turbulent bottom boundary layer. The turbulent flow is up to 170 m thick and has peak microscale shears of 1 m s−1 per meter. The rate of dissipation of kinetic energy, based on the observed shear variance, averaged over the depth of the turbulent boundary layer ranged from 70 to 500 × 10−6W m−3. Temperature measurements indicate that the flow was up canyon at a time of low tide. The upper bound for the vertical eddy viscosity is estimated to be17 × 10−4m2s−1 and for the vertical eddy diffusivity is estimated to be 15 × 10−4m2s−1. The large vertical scale and the intensity of the observed boundary layer suggest that the flow in Monterey Canyon may be important for the renewal and circulation of water over the continental shelf in the bay area.  相似文献   

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