共查询到20条相似文献，搜索用时 15 毫秒
1.
Hisashi Narita Ryota Abe Kazuhisa Tate Young-Ill Kim Koh Harada Shizuo Tsunogai 《Journal of Oceanography》2003,59(5):739-750
We report the role of particle composition and lateral particle movement that influences the oceanic distribution of 231Pa and 230Th. Settling particles were collected during sediment trap experiments. These and surface sediments were obtained from five
stations along 38 to 44°N in the northwestern North Pacific. The high total mass flux and seasonal variations in the marginal
area of the western North Pacific are controlled by the supply of lithogenic materials and primary productivity. The high
content of the lithogenic material in the settling particles in this area contributes to 230Thex fluxes that exceed the local rate of supply. The lithogenic materials are important as a carrier of 230Th and contribute to the fractionation between 230Th and 231Pa in the ocean, as the 231Paex/230Thex ratio in the settling particles decreases with increasing 232Th concentration. The 231Paex/230Thex ratio in the settling particles collected in the abyssal basin decreases with water depth, which indicates that lateral transport
of the lithogenic particles from the marginal area and/or shallower depth plays an important role in determining the 231Paex/230Thex ratio in a population of settling particles and remineralization. This indicates that lateral redistribution of particles
and sediment focusing influence the 231Paex/230Thex ratios in surface sediments. Thus, the observations reported here mean that the use of the sediment 231Paex/230Thex ratio as a paleoproductivity proxy will be problematic in the northwestern North Pacific.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
3.
东太平洋多金属结核中放射性核素的不破坏γ谱分析 总被引：2，自引：0，他引：2
用HPGeγ谱仪不破坏地直接测定了采自东太平洋的20个多金属结核样品,样品中可探测到的核素有238U,230Th,226Ra,210Pb,228Ra,228Th,235U,227Ac(或231Pa)和40K.对样品顶侧和底侧分别面对探测器的计数率和计数率比进行了分析,获得了其所蕴含的多金属结核生长、运动、放射性核素来源及其与环境关系的信息.提出了判断多金属结核顶、底侧的新方法.并根据顶、底侧230Th计数率比推测多金属结核不同的滚动时间. 相似文献
4.
Non-destructive 7 spectrum analyses of 20 polymetallic nodules from the eastern Pacificwere carried out. Numerous nuclides, such as 238U,230Th,226Ra,210Pb,228Ra,228Th,235U,227Ac (or 231 Pa) and 40K were detected. The count rates of the nuclides in the top or bottom side of nodules facing detector were measured and the ratio R of the count rates of nuclides in the top and the bottom sides was obtained. From counts and ratios, some useful information relating to the growth and movement of the nodules, the source of nuclide and relationship between those and environment can be gotten. A new method for clear distinction between the top and bottom sides of the nodule based on the R value of 226Ra or 210Pb was developed. In addition, one can infer the turnover of nodules according to the R value of 230Th. 相似文献
5.
A record of radionuclide fluxes at a deep marginal sea of the Northwest Pacific Ocean (39°40′N 132°24′ E, Japan Basin, East
Sea/Sea of Japan) was obtained from analysis of a 1-year continuous collection of sediment-trap samples. The trap was placed
at a depth of 2800 m, 500 m above the sea floor, and the samples were recovered at the end of one year. Concentrations of
238U, 234U, 232Th, 230Th and 228Th were measured in the trapped material. All of the radionuclide fluxes showed seasonal variations that were in phase with
the variations in total particle flux, which were shown in earlier work to be closely tied to the primary production in the
surface water. The formation of authigenic 238U appears to be less than in other open ocean regimes. The residence time of particulate thorium isotopes were of the order
of a year, resulting in an average settling rate of a meter per day. Comparison of the measured 230Thex flux with that of the theoretical production in the overlying water column yielded about 40% surplus of 230Th, indicating that lateral advection contributes 230Th to this sampling site. 相似文献
6.
Using the moored MnO2-fiber method, we have obtained 38 determinations of Th and Pa isotope concentrations from 18 sites along the margin of the western North Pacific near Japan, from water depths of 1,330 to 5,873 m. From our data, we are able to show that (1)228Ra and227Ac are being supplied to the seawater from the slope sediments of Honshu, Japan, (2) our230Th and231Pa concentrations match those obtained byin situ pumping with a MnO2-fiber adsorber in the Japan and Izu-Ogasawara trenches but are significantly higher than those from the Panama and Guatemala basins, and (3) our232Th concentrations show a similar systematic decrease with depth as do those of trace metals like Mn, Al, Te and Bi whose concentrations are strongly controlled by particulate matter scavenging.In contrast, our data fail to show (1) that enhanced removal of230Th and231Pa by scavenging from the water column is taking place near the western margins and (2) that231Pa is being removed in preference to230Th from the water column to the marginal sediments. This is probably due to rapid mixing of the deep waters as compared to the scavenging rates of230Th and231Pa in the water column and at the sediment/seawater interface. 相似文献
7.
Dissolved and particulate samples were collected to study the distribution of thorium isotopes (234Th, 232Th and 230Th) in the water column of the Indian sector of the Southern Ocean (from 42°S to 47°S and from 60°E to 66°E, north of the Polar Front) during Austral summer 1999. Vertical profiles of excess 230Th (230Thxs) increases linearly with depth in surface water (0–100 m) and a model was applied to estimate a residence time relative to the thorium scavenging (τscav). Low τscav in the Polar Front Zone (PFZ) are found, compared to those estimated in the Subtropical Front Zone (STZ). Changes in particle composition between the PFZ and STZ could influence the 230Thxs scavenging efficiency and explain this difference. An innovative coupling between 234Th and 230Thxs was then used to simultaneously constrain the settling velocities of small (0.6–60 μm) and large (above 60 μm) particles. Although the different hydrological and biogeochemical regimes visited during the ANTARES IV cruise did not explain the spatial variation of sinking velocity estimates, our results indicate that less particles may reach the seafloor north (60 ± 2 m d− 1, station 8) than south of the Agulhas Return Current (119 ± 23 and 130 ± 5 m d− 1 at stations 3 and 7, respectively). This information is essential for understanding particle transport and by extension, carbon export. In the deep water column, the 230Thxs concentrations did not increase linearly with depth, probably due to lateral transport of North Atlantic Deep Water (NADW) from the Atlantic to the Indian sector, which renews the deep waters and decreases the 230Thxs concentrations. A specific 230Thxs transport model is applied in the deep water column and allows us to assess a “travel time” of NADW ranging from 2 to 15 years. 相似文献
8.
An 8.5 m long apparently undisturbed core from a hilltop on the crest of the East Pacific Rise has uranium and thorium isotope distributions that are very unusual. The core is very poor in 232Th, and very rich in U, particularly at the 500-cm level, where a value of about 150 ppm is reached. At the same depth the 230Thxs reaches very large negative values. These facts could be accounted for if one assumes that solutions rich in U and poor in Th had been postdepositionally injected into the sediments about 90,000–110,000 years ago. The top of the sediment received much of its U from seawater, judging from the ratio. Possibly carbonate rich solutions were the carriers of the injected uranium. 相似文献
9.
Norikazu Kinoshita Yuko Sato Takeyasu Yamagata Hisao Nagai Akihiko Yokoyama Takashi Nakanishi 《Journal of Oceanography》2007,63(5):813-820
In order to estimate the deposition rate of extraterrestrial material onto a manganese crust in a search for supernova debris,
we analyzed the contents of 10Be, 230Th, 231Pa, and 239,240Pu in a sample of manganese crust collected from the North Pacific Ocean. On the basis of the depth profile of 10Be, the growth rate of the manganese crust was determined to be 2.3 mm Myr−1. The uptake rates of 10Be, 230Th, and 231Pa onto the manganese crust were estimated to be 0.22–0.44%, 0.11–0.73%, and 1.4–4.5%, respectively, as compared to the deposition
rates onto the deep-sea sediments near the sampling station, while that for 239,240Pu was 0.14% as compared to the total inventory of seawater and sediment column. Assuming that sinking particles represent
0.11–4.5% of the uptake rates, the deposition rate of extraterrestrial material onto the manganese crust was estimated to
be 2–800 μg cm−2Myr−1 according to the uptake of 10Be onto the manganese crust. Further, our estimate is similar to the value of 9–90 μg cm− 2Myr−1 obtained using the integrated global production rate of 10Be and the deposition rate of 10Be onto the manganese crust. 相似文献
10.
Manganese nodules of the Clarion–Clipperton Fracture Zone (CCFZ) in the NE Pacific Ocean are highly enriched in Ni, Cu, Co, Mo and rare-earth elements, and thus may be the subject of future mining operations. Elucidating the depositional and biogeochemical processes that contribute to nodule formation, as well as the respective redox environment, in both water column and sediment, supports our ability to locate future nodule deposits and to evaluate the potential ecological and environmental effects of future deep-sea mining. For these purposes we studied the local hydrodynamics and pore-water geochemistry with respect to the nodule coverage at four sites in the eastern CCFZ. Furthermore, we carried out selective leaching experiments at these sites in order to assess the potential mobility of Mn in the solid phase, and compared them with the spatial variations in sedimentation rates. We found that the oxygen penetration depth is 180–300 cm at all four sites, while reduction of Mn and NO3− is only significant below the oxygen penetration depth at sites with small or no nodules on the sediment surface. At the site without nodules, potential microbial respiration rates, determined by incubation experiments using 14C-labeled acetate, are slightly higher than at sites with nodules. Leaching experiments showed that surface sediments covered with big or medium-sized nodules are enriched in mobilizable Mn. Our deep oxygen measurements and pore-water data suggest that hydrogenetic and oxic-diagenetic processes control the present-day nodule growth at these sites, since free manganese from deeper sediments is unable to reach the sediment surface. We propose that the observed strong lateral contrasts in nodule size and abundance are sensitive to sedimentation rates, which in turn, are controlled by small-scale variations in seafloor topography and bottom-water current intensity. 相似文献
11.
12.
A method is described for the determination of Th isotopes (232Th,230Th,228Th and227Th) in seawater through analysis of Th adsorbed on MnO2-impregnated fiber that has been moored in the deep sea for up to 10 months. Since the MnO2-fiber adsorbs Th from seawater at a constant rate, natural234Th can be used as a yield monitor by making a correction for its decay during the period of deployment. The results obtained by the method showed good reproducibility and accuracy. The method has the advantage over the chemical coprecipitation method that the time and labor for sampling and processing a large-volume of seawater is reduced. 相似文献
13.
G.P. Glasby 《Marine Chemistry》1973,1(2):105-125
The rare-earth distribution has been determined in five manganese nodules from a range of marine environments. Manganese nodules from Loch Fyne, Scotland, and the Gulf of Aden are characterized by a low absolute abundance of the rare-earth elements and a low Ce/La ratio compared with deep-sea nodules. This difference is interpreted in terms of the diagenetic remobilization of manganese in the high sedimentation regimes leading to the effective fractionation of manganese from the rare-earth elements and the resultant depletion of the rare earths in nodules from these environments. The mechanism of incorporation of the rare-earth elements into manganese nodules is thought to be either the direct adsorption of these elements from seawater or the scavenging of the elements by colloidal iron oxide prior to their incorporation into the authigenic phase of nodules. There appears to be no evidence for the surface transfer of these elements from inorganic detritus into the authigenic phase of nodules as suggested by previous authors. The distribution of U and Th in the nodules is controlled by factors similar to those controlling the rare-earth abundance in nodules. The higher U/Th ratios in nodules from Loch Fyne and the Gulf of Aden is considered to be due to the preferential reduction of UO2(CO3)34? to some lower-valency uranium species such as U3O8 in nodules from less oxidizing environments. 相似文献
14.
Dietrich Heye 《Progress in Oceanography》1978,7(5-6)
Twenty-four manganese nodules from the surface of the sea floor and fifteen buried nodules were studied. With three exceptions, the nodules were collected from the area covered by Valdivia Cruise VA 04 some 1200 nautical miles southeast of Hawaii.Age determinations were made using the ionium method. In order to get a true reproduction of the activity distribution in the nodules, they were cut in half and placed for one month on nuclear emulsion plates to determine the α-activity of the ionium and its daughter products. Special methods of counting the α-tracks resolution to depth intervals of 0.125 mm. For the first time it was possible to resolve zones of rapid growth (impulse growth) with growth rates, s > 50 mm/106 yr and interruptions in growth. With few exceptions the average rate of growth of all nodules was surprisingly uniform at 4–9 mm/10 yr. No growth could be recognized radioactively in the buried nodules. One exceptional nodule has had recent impulse growth and, in the material formed, the ionium is not yet in equilibrium with its daughter products. Individual layers in one nodule from the Indian Ocean could be dated and an average time interval of t = 2600±400 yr was necessary to form one layer. The alternation between iron and manganese-rich parts of the nodules was made visible by colour differences resulting from special treatment of cut surfaces with HCl vapour. The zones of slow growth of one nodule are relatively enriched in iron.Earlier attempts to find paleomagnetic reversals in manganese nodules have been continued. Despite considerable improvement in areal resolution, reversals were not detected in the nodules studied. Comparisons of the surface structure, microstructure in section and the radiometric dating show that there are erosion surfaces and growth surfaces on the outer surfaces of the manganese nodules. The formation of cracks in the nodules was studied in particular. The model of age-dependent nodule shrinkage and cracking surprisingly indicates that the nodules break after exceeding a certain age and/or size. Consequently, the breaking apart of manganese nodules is a continuous process not of catastrophic or discontinuous origin. The microstructure of the nodules exhibits differences in the mechanism of accretion and accretion rate of material, shortly referred to as accretion form. Thus non-directional growth inside the nodules as well as a directional growth may be observed. Those nodules with large accretion forms have grown faster than smaller ones. Consequently, parallel layers indicate slow growth. The upper surfaces of the nodules, protruding into the bottom water appear to be more prone to growth disturbances than the lower surfaces, immersed in the sediment. Features of some nodules show, that as they develop, they neither turned nor rolled. Yet unknown is the mechanism that keeps the nodules at the surface during continuous sedimentation. All in all, the nodules remain the objects of their own distinctive problems. The hope of using them as a kind of history book still seems to be very remote. 相似文献
15.
The concentration of thorium isotopes and the activity ratios of230Th/232Th and228Th/232Th in sea water collected in the Kuroshio region, the mixing area of Oyashio and Kuroshio, the Japan Sea and the East China
Sea in the western North Pacific were determined. Thorium isotopes were analyzed by α-ray spectrometry after separating them
with an anion exchange resin. The average content of thorium (232Th) of 2.2×10−9 g/l was obtained in the open Pacific waters. The ratio of230Th/232Th is in accord with that of the top layer of the sediment in the same area. The high values of228Th/232Th ratio up to 36 were observed in sea water. The excess228Th in sea water may be due to the migration of228Ra through the water-sediment interface. Thorium content in suspended matter was 10 to 20% of the total thorium content in
the Pacific water. 相似文献
16.
The contents of plutonium isotopes (239Pu and238Pu), thorium isotopes (232Th,230Th and228Th) and protactinium-231 in sea water collected in the North Pacific, the East China Sea and the Japan Sea were determined. These nuclides were sequentially analyzed byα-ray spectrometry after separating them mainly with solvent extraction technique. The contents of239Pu in surface sea water ranged from 0.6 to 1.6 pCi/10001,238Pu/239Pu activity ratios being 0.2～0.7. The228Th/232Th activity ratios for the North Pacific waters varied between 7.6 and 30, whereas the sample from the East China Sea showed the very high value, 65. The contents of231Pa are less than 6 percent of that in equilibrium with its parent235U. Furthermore, the analysis of plutonium isotopes in recent coral from Yoron Island was carried out and it was confirmed that plutonium isotopes have concentrated in recent coral with the concentration factor of about 1～2×103. 相似文献
17.
Each about 400 l of seawater sample was collected in the northern North Pacific and filtered through a membrane filter. Four radioisotopes of thorium,232Th,230Th,228Th and234Th, were determined for the two FractionsF (filtrate) andP (particles on the filter). In the percentages of FractionP in the subsurface water,230Th was significantly larger than other 3 isotopes, and232Th was significantly smaller than other 3 isotopes. The former finding can be explained by the slower rates in the reversible change between the FractionsF andP. The latter one, however, cannot be explained if thorium isotopes in the FractionF are truly dissolved with the same chemical form. This suggests that major part of the FractionF of232Th is not identical with those of other radiogenic thorium isotopes, and it should not be composed of simple dissolved ions. The removal of radiogenic234Th was related to the biological activity, but there was a deviation, between the FractionP and radioactivity deficiency of234Th in their vertical profiles. The deviation was similar to that between the chlorophylla and phaeo-pigments contents including their maximum depths. 相似文献
18.
J. C. Scholten M. M. Rutgers van der Loeff A. Michel 《Deep Sea Research Part II: Topical Studies in Oceanography》1995,42(6)
Distribution of 230Th and 231Pa in the water column was measured in the Eurasian basins of the Arctic Ocean, i.e. the Makarov, Amundsen and Nansen basins. The profiles obtained were compared to one previously published from the Alpha Ridge (Bacon et al., Earth and Planetary Science Letters, 95 (1989) 15–22. The Amundsen and Nansen basins are similar in their 23OTh and 231Pa distribution, whereas in the Makarov Basin and at the Alpha Ridge these radionuclides are comparatively enriched in the deep basin. This may be due to low particle flux in conjunction with low water mass renewal rates. The 230Th concentration in water depth < 1000 m in the Makarov Basin is relatively low compared with the Alpha Ridge, which may be caused by an exchange of water masses between the Amundsen and Makarov basins.In the Eurasian Basin Deep Water, 230Th profiles can be explained both by a scavenging model and by a mixing-scavenging model. Enhanced particle load caused by resuspension may explain the increased scavenging in the Eurasian Basin Bottom Water. The differences in scavenging between the Norwegian-Greenland Sea and the Eurasian basins at present are not large enough to cause significant fractionation between 230Th and 231Pa. 相似文献
19.
Seasonal deposition and reworking at the sediment-water interface on the northwestern Iberian margin
Seabed distributions of 234Th excess (Thxs) were determined in the upper centimetres of 38 sediment cores from the north-western Iberian Margin, sampled from 41–44°N and from 9–12°E during five OMEX II cruises. Three main areas, a northern, and at 42°38 and 42°N, were investigated during representative seasons (winter, spring and summer). Low 234Thxs activities in summer 1998 (18–252 Bq per kg) were similar to those measured in summer 1997. In winter 234Th also showed moderate excess. The highest values were observed in spring with surface 234Thxs values up to 402 Bq kg−1. Maximum penetration depths of 234Thxs ranged from a few mm to 3 cm. 234Thxs activities always showed a smooth decrease with depth, without any evidence of non-local mixing. Thus particle mixing on a short time scale can be described as an eddy diffusive process, and bioturbation rates, calculated on this basis, range from 0.02 to 3.07 cm2 per year. Data (activities, inventories, bioturbation rates) are discussed in order to relate the observed surface and down-core variations to spatial and seasonal trends. Using 234Thxs data in sediment as a substitute for sediment trap estimates, particle fluxes were calculated from 234Thxs inventories. The range of 234Th-derived particle fluxes for the north-western Iberian Margin is 16–1418 mg.m−2.d−1. Mean values indicate a gradual decrease of mass fluxes from the shelf to the open ocean. On a 100-day scale, the northern area (43–44°N) represents a low sedimentation regime. Further south, around 42°–43°N, particle inputs are more important. On the middle slope, around 1000 to 2000 m depth, high inventories and bioturbation rates indicate enhanced, and probably organic-rich, particle fluxes to the seafloor, particularly in spring. 相似文献