Using manganese-impregnated fiber extraction and high-efficiency gamma counting techniques, we measured the distribution of 228Ra and 226Ra in surface waters near the coast of Japan and in the western North Pacific. There is no evidence in our data that any significant amount of 228Ra is added to open ocean surface waters from the coastal waters around Tokyo Bay. High 228Ra concentrations (> 10 dpm/103 kg), were observed along the Kuroshio Current as compared to < 2.5 dpm/103 kg between 10° and 30°N of the central gyre, and hence the major source of 228Ra in the surface water is likely to be the East Asian continental shelf zones. A simple one-dimensional eddy diffusion and advection model is used to explain the observed decrease of 228Ra from coast to the open ocean. The model results indicate two mixing regimes across the Kuroshio Current System with apparent eddy diffusion coefficients of Ky = 4 × 105 cm2 s−1 at distance y < 200 km from the coast, and Ky = 4 × 107 cm2 s−1 at y > 200 km. Along 40°N where an eastward flow of the ‘Kuroshio Extension’ prevails, an advective flow of > 0.1 knot is consistent with the observation of nearly constant 228Ra along the track.The geographical distribution pattern of 228Ra is clearly different from that of atmospherically derived 210Pb. Thus the 228Ra in surface water serves as a useful tracer that accompanies fluvially and coastally derived elements during their subsequent lateral transport toward the central gyre. 相似文献
A mesoscale iron-fertilization experiment was carried out in the western subarctic Pacific during summer 2001. The iron-patch was traced for 14 days after the fertilization, and the abundance and behavior of mesozooplankton were compared with those outside of the patch. The phytoplankton biomass in the patch rapidly increased to over 15 times the initial level by the later half of the observation period, and was composed of large-sized (>10 mm), centric diatoms. Dominant zooplankton species in the upper 200-m depth were large copepods: Neocalanus plumchrus, Neocalanus cristatus, Eucalanus bungii and Metridia pacifica. Mesozoplankton biomass as well as species composition did not change significantly in the patch over the observation period. Furthermore, no changes of vertical distribution or diel vertical migration were observed for any species or stages of mesozooplankton throughout the observation period. However, the abundance of the first copepodite stages of N. plumchrus and E. bungii increased several fold in the patch after the diatom bloom formation compared to the densities outside the patch. The increases of both species are considered to be due to lowered mortality during the egg and nauplius stages. Spawning of N. plumchrus takes place at depth using lipid storage, while spawning of E. bungii takes place in the surface layer supported by grazing. These facts suggest that the relative importance of nauplii in the diets of the large copepods was decreased in the patch by the diatom bloom. Gut-pigment contents of dominant copepods in the patch increased 4–18 times, and the maximum values were observed during the bloom peak. However, the grazing impact on phytoplankton was low throughout the experiment, especially during the bloom period (<6% of the primary production). 相似文献
Grazing experiments and production estimation based on life-history analysis of Neocalanus copepods (N. cristatus, N.plumchrus and N.flemingeri) were carried out in the Oyashio region to understand the carbon flows associated with the interzonal migrating copepods. These copepods, and also Eucalanus bungii, fed on nano- and micro-sized organisms non-selectively throughout the season. However, diatoms were the dominant food resource until May and organisms, such as ciliates were the major resource after May. Daily growth rate was estimated from the Ikeda–Motoda, Huntley–Lopez and Hirst–Sheader models. Since the growth rates were considered to be overestimates for the Huntley–Lopez model and underestimates for the other two models, we applied the weight-specific growth rates previously reported for these species in the Bering Shelf. Surface biomass of Neocalanus increased rapidly in June during the appearance of C5, and a successive increase of overwintering stock was evident in the deeper layer. The deep biomass decreased gradually from September to May during the dormant and reproduction period. N. cristatus has the largest annual mean biomass (2.3 gC m−2), followed by N. plumchrus (1.1) and N. flemingeri (0.4). Daily production rate of Neocalanus varied from 0.4 to 363.4 mgC m−2 day−1, to which N. cristatus was the largest contributor. Annual production was estimated as 11.5 gC m−2 year−1 for N. cristatus, 5.7 for N. plumchrus and 2.1 for N. flemingeri, yielding annual P/B ratio of 5 for each species. The annual production of Neocalanus accounted for 13.2% of the primary production in the Oyashio region. Their fecal pellets were estimated to account for 14.9% (0.7 gC m−2 year−1) of sinking flux of organic carbon at 1000-m depth. Moreover, their export flux by ontogenetic vertical migration, which is not measured by sediment trap observations, is estimated to be 91.5% (4.3 gC m−2 year−1) of carbon flux of sinking particles at 1000-m depth. These results suggest the important role of interzonal migrating copepods in the export flux of carbon. 相似文献
It is known that there is a front-like structure at the mixed layer depth (MLD) distribution in the subtropical gyre, which
is called the MLD front, and is associated with the formation region of mode water. In the present article, the generation
mechanism of the MLD front is studied using an idealized ocean general circulation model with no seasonal forcing. First,
it is shown that the MLD front occurs along a curve where ug·∇Ts = 0 is satisfied (ug is the upper ocean geostrophic velocity vector, Ts is the sea surface temperature and ∇ is the horizontal gradient operator). In other words, the front is the boundary between
the subduction region (ug·∇Ts > 0) and the region where subduction does not occur (ug·∇Ts < 0). Second, we have investigated subduction of low potential vorticity water at the MLD front, which has been pointed out
by past studies. Since ug·∇Ts = 0 at the MLD front, the water particles do not cross the outcrop at the MLD front. The water that is subducted at the MLD
front has come from the deep mixed layer region where the sea surface temperature is higher than that at the MLD front. The
temperature of the water in the deep mixed layer region decreases as it is advected eastward, attains its minimum at the MLD
front where ug·∇Ts = 0, and then subducts under the warmer surface layer. Since the deep mixed layer water subducts beneath a thin stratified
surface layer, maintaining its thickness, the mixed layer depth changes abruptly at the subduction location. 相似文献
We conducted hydrothermal flow-through experiments at 430 °C and 31 MPa to investigate the mechanism of silica precipitation on granite under crustal conditions. Two experiments were performed using different input solutions: a single-component Si solution, and a multi-component solution with minor Al, Na, and K. The degree of supersaturation with respect to quartz, Ω = CSi/CSi,Qtz,eq, where CSi and CSi,Qtz,eq indicate Si concentration in solutions and the solubility of quartz within water, respectively, decreased from 3-3.5 to <1.1 along the flow path. A variety of silica minerals formed during the experiments (opal-A, opal-C, chalcedony, and quartz), and their occurrences and modal abundances changed in response to Ω and the presence of additives in the solution.For near-equilibrium solutions (Ω < ∼1.2), silica precipitation occurred in a simple way in both experiments, being restricted to overgrowths on pre-existing quartz surfaces in the granite. At higher saturation levels (Ω > ∼1.2), silica minerals were deposited on other surfaces in addition to quartz. In the single-component experiment, the dominant silica minerals changed in the order of opal-A → opal-C → quartz with decreasing Si concentration along the flow path. In contrast, in the multi-component experiment, quartz and minor chalcedony formed throughout the entire reaction vessel. This finding indicates that impurities (Na, K, and Al) in the solutions inhibited the precipitation of opal and enhanced the direct nucleation of quartz. The systematic appearance of metastable silica minerals were examined by nucleation processes and macroscopic precipitation kinetics. Our experimental results indicate that different precipitation mechanisms yield contrasting textures, which in turn suggests that vein textures can be used as indicators of solution chemistry within the fracture. 相似文献
In order to understand the effects of a landfill operation on groundwater flow behavior, a 2D horizontal groundwater simulation
model was carried out. The model saved the memory of computer and time consumption, comparing it with the 3D groundwater flow
model. However, the greatest difficulty is the assignment of the collecting pipe boundary at the study site. Therefore, a
2D vertical model was applied to calculate the change of the groundwater table above the collecting pipe. This paper focuses
on examining the validation of the assignment of the collecting pipe boundary by applying the results of the 2D vertical model.
The 2D horizontal model was coupled with the recharge model to solve the partial differential equation of groundwater flow.
The finite difference method and iterative successive over relaxation were applied. The drainage volume of leachate collection
was summed up in the whole landfill site and compared with the average volume of treated wastewater. The study demonstrated
that the results of the 2D vertical model validated and can be applied to the 2D horizontal groundwater flow simulation. 相似文献
The concentrations of helium and carbon in fluorite associated with Cretaceous to Neogene (90–13 Ma) granitic magmatism in the Japanese arc have been measured. Concentrations of Li, U, Th and Gd were measured to correct for secondary generated 3He. The CO2/3He of fluorites are almost uniform (1.5×1010–4×1010) and in fair agreement with the range of present island arc volcanic gases. The calculated mantle C contribution in the Mesozoic subduction zone appear to have been identical to the present one (7–19%) indicating that the C flux from the mantle in supra-subduction zone environments has remained fairly constant during the past 70 million years. 相似文献
Two sediment cores (pilot gravity and piston) were obtained from the bottom of the Izu-Ogasawara Trench at 9750 m and analyzed for various elements and radioisotopes. The results showed a history of complex and frequent turbidite deposition: In the gravity core, eight layers rich in manganese were observed, of which five are enriched in Cu and Co as well. The other three are also enriched in Mo but no other heavy metals, suggesting the presence of at least two mechanisms of formation. Trapping of iron manganese micronodules can account for the enrichment of Mn, Cu and Co. The other three layers rich in Mn and Mo appear to be formed by a post-depositional diagenetic process of Mn mobilization and redeposition in the sediment column. A strong correlation between 226Ra and Cu in the gravity core suggests that the 226Ra was also carried into the bottom of the trench in turbidites in association with Mn micronodules. Little excess of 210Pb over 226Ra was found at the top but the excess was significant at mid-depths from 30 to 70 cm, indicating that those sediments were deposited within the last 200 y.
In the piston core there is a sharp discontinuity of chemical and radiochemical composition around a depth of 250 cm. Below that depth the sediments appear to be dominated by materials derived from terrestrial sources, as compared with those in the upper layer which are of contemporary marine origin. 226Ra is deficient relative to 230Th throughout the sediment column down to 6 m. This finding is consistent with the finding that the average rate of sediment accumulation is 1–2 orders of magnitude faster than that in the western North Pacific abyssal plain, suggesting the convergence of materials into the bottom of the trench. 相似文献