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1.
The deployment of particle interceptor traps (PITs) in the three inner basins of the Southern California Bight (Santa Barbara, Santa Monica, and San Pedro) where preserved, laminated, bottom sediments occur, provides a natural calibration between the parameters determined with the PITs and those derived from the historical sedimentary deposits. The accumulation rates and chemical composition of the PIT materials compare favorably with these recently deposited bottom sediments. The ratio of the measured particle collection rate to the sediment accumulation rate for these three basins averaged0.93±0.20. Radionuclide (210Pb,228Th/232Th) and trace element (Fe, Cu, Zn, Cd, Pb) compositions of the PIT materials and surface sediments agree within, generally, 30% in each basin.An application of PITs as a sampling system in an outer basin (San Nicolas) where the sedimentary record has been obscured by bioturbation is also presented to further demonstrate the utility of PITs for spatial and temporal studies of various aspects of particle transport and sediment deposition. Based on the calibration of the PITs in the three inner basins we conclude from the comparison of the radionuclide and trace element compositions between the PIT materials and surface sediments from the San Nicolas Basin that recently deposited sedimentary material has been mixed by bioturbation with older material from the last several hundred years. Excess210Pb and the228Th/232Th activity ratio in the PIT material are 6 and 15 times higher than in the surface sediment. Total Pb and1N HNO3-leachable Pb are 4- and 8-fold higher in the PIT material.  相似文献   

2.
Five near-bottom excess-radon profiles have been measured at the Geosecs-I station in the northeast Pacific, at varying intervals over a period of two and a half years. These profiles vary considerably in both structure and radon concentration, indicating transient characteristics of the bottom water in the deep Pacific. Only two of the five profiles are of the quasi-exponential type (Y. Chung and H. Craig); the stationary vertical eddy diffusivities calculated from these two profiles are 129 cm2/sec in November, 1971, and 22 cm2/sec obtained five months later.A single bottom-radon profile was measured in the western-boundary-current region of the South Pacific at the Geosecs-III station. This profile is also of the quasi-exponential type, with a vertical diffusivity of 32 cm2/sec. Although the radon concentrations of the three quasi-exponential profiles and the calculated diffusivities are quite different, the integrated excess-radon inventory in the water column, and the calculated flux of radon across the sediment—water interface, vary by only a factor of two in the two areas studied.  相似文献   

3.
Samples from the MANOP Santa Barbara Basin sediment trap intercomparison were analyzed for the isotopes of uranium, thorium, radium, lead, and polonium. All of the traps showed approximately the same compositions and isotopic ratios, indicating that they trapped similar materials. The234Th flux via falling particles was very close to the flux predicted from the production and scavenging rates of234Th from the water column. The210Pb content of the trapped particles and the surface sediments were the same, however, the measured flux of210Pb was seven times greater than the predicted flux. Predicted and measured fluxes of228Th and210Po were similarly out of balance. To explain this apparent inconsistency, we suggest (as others have done) that the Santa Barbara Basin is an area where scavenging from the water column is intensified and where sediments deposited initially on the margins may be physically remobilized on a short time scale. These two effects increase the apparent area from which the basin derives the longer-lived isotopes but does not increase significantly the supply of the short-lived234Th.  相似文献   

4.
We present 9 bottom222Rn profiles measured from the western and southern Indian Ocean during the 1977–1978 GEOSECS expedition. These profiles can be grouped into three cypes: one-layer, two-layer, and irregular types. The one-layer profiles with quasi-exponential distributions allow one to estimate the apparent vertical eddy diffusivity,Kv, with a simple model. The two-layer profiles show that there is a benthic boundary layer of the order of 50–100 m in which the excess222Rn distribution shows a vertical gradient much smaller than that of the layer immediately above. Within the boundary layer, the STD potential temperature (θ) and density(σ4) profiles are practically constant, and theKv values are of the order of 1000 cm2/s. The STD profiles for the water column above the boundary layer show gradients of increasing stability, and theKv values are of the order of 100 cm2/s. Modeling of the Rn data in the water column above the boundary layer indicates that there is a transition layer which effectively reduces the penetration of excess Rn from the benthic boundary layer into the upper layer.Sarmiento et al. [10] have shown that the buoyancy gradient or stability is inversely correlated with the apparent vertical eddy diffusivity, and the resulting buoyancy flux is fairly uniform, ranging from 1 to 14 × 10?6 cm2/s3 in the Atlantic and Pacific Oceans. However, Sarmiento et al. [11] show that a much higher buoyancy flux is associated with an intensified flow of the bottom water through a passage. In the Indian Ocean basins, we have found that the buoyancy flux has a comparable range (3–14 × 10?6 cm2/s3), except for a couple of stations where both stability and apparent vertical diffusivity are higher, resulting in a much higher buoyancy flux, probably indicative of rapid bottom water flow.  相似文献   

5.
Radon in the Earth’s crust or soil matrix is free to move only if its atoms find their way into pores or capillaries of the matrix. 222Rn atoms from solid mineral grains get into air, filling pores through emanation process. Then 222Rn enters into the atmosphere from air-filled pores by exhalation process. The estimation of radon flux from soil surface is an important parameter for determining the source term for radon concentration modeling. In the present investigation, radon fluxes and soil-gas radon concentration have been measured along and around the Main Central Thrust (MCT) in Uttarkashi district of Garhwal Himalaya, India, by using Scintillation Radon Monitor (SRM) and RAD7 devices, respectively. The soil radon gas concentration measured by RAD7 with soil probe at the constant depth was found to vary from 12 ± 3 to 2330 ± 48 Bq·m?3 with geometrical mean value of 302 ± 84 Bq·m?3. Th significance of this work is its usefulness from radiation protection point of view.  相似文献   

6.
The vertical distributions of210Pb and226Ra in the Santa Barbara Basin have been measured. The210Pb/226Ra activity ratio is close to unity in surface water, but ranges from 0.2 to 0.6 in deep water with a mean value of 0.3 (d > 250m), suggesting rapid removal of210Pb from the water column. The210Pb concentrations in the particulate phase at different water depths indicate that the removal of210Pb is due to adsorption on settling particles.It is estimated that the particulate210Pb contributes about 50–70% of the total210Pb measured on unfiltered water samples of the Santa Barbara Basin. The fate of210Pb (and Pb) in the water column is thus strongly controlled by the settling particles, which have a mean residence time of one year or less in the basin. Material balance calculation for210Pb in the basin suggests that there is an external source supplying about 70–80% of the210Pb observed in particulate material or sediments. This excess210Pb is most likely provided by particles entering the basin loaded already with210Pb.  相似文献   

7.
Seven vertical profiles of226Ra have been measured along an east-west traverse at about 30°N from San Diego to northwest of Hawaii. These profiles show that there is a distinct core of Ra maximum spreading westward as a tongue in the northeast Pacific deep water. This core starts in the east with 21.1 Ra units (1Ra unit= 10?14g/kg) at 3.9 km depth at about 130°W, and deepens westward to 4.1 km with its Ra reduced to 18.3 units at 150°W. A similar core with some uncertainty due to possible sampling errors extends westward near the bottom at 5.2 km depth from 19.4 Ra units at 150°W to 15.9 units at about 180° longitude. In addition, these profiles appear to be correlated with each other in structure above the cores of Ra maximum. These cores indicate that the Ra input depends locally on the type and composition of sediments and so the flux varies over the ocean bottom. On the basis of a one-dimensional diffusion-decay model, a horizontal diffusion coefficient of 106 cm2/sec has been computed along these cores. Although this value appears to be slightly lower, it is not inconsistent with those derived from other physical methods.  相似文献   

8.
We present the distribution of226Ra in eight vertical profiles from the eastern Pacific. The profiles are located along a meridional trend near 125°W, from 43°S to 29°N. Surface226Ra concentrations are about 7 dpm/100 kg, except for the two stations south of 30°S where the higher values are due to the Antarctic influence. Deep waters show a distinctive south-to-north increase in the226Ra content, from about 26 to 41 dpm/100 kg near the bottom. Unlike in the Atlantic and Antarctic Oceans, the effect of226Ra injection from bottom sediments is clearly discernible in the area. The presence of this primary226Ra can be traced up to at least 1–1.5 km above the ocean floor, making this part of the sea bed among the strongest source regions for the oceanic226Ra. Numerical solutions of a two-dimensional vertical advection-diffusion model applied to the deep (1.2–4 km)226Ra data give the following set of best fits: upwelling velocity(Vz) = 3.5m/yr, vertical eddy diffusivity(Kz) = 0.6cm2/s, horizontal (north-south) eddy diffusivity(Ky) = 1 × 107cm2/s, and water-column regeneration flux of226Ra(J) = 3.3 × 10?5dpmkg?1yr?1 as an upper limit. These parametric values are in general agreement with one-dimensional (vertical) model fits for the Ra-Ba system. However, consideration of226Ra balance leads us to suspect the appropriateness of describing the vertical exchange processes in the eastern Pacific with constantVz and Kz. If future modeling is attempted, it may be preferable to treat the area as a diffusion-dominant mixing regime with depth-dependent diffusivities.  相似文献   

9.
Four sediment traps of radically different design were deployed in the Santa Barbara Basin for approximately 45 days. The measured fluxes ranged from 370 to 774 g m?2 yr?1 for the different designs. These values lie within flux measurements previously determined for the basin. Compared to the 25-year record (920 g m?2 yr?1), however, all fluxes determined in this experiment are somewhat low. Because this experiment was conducted during a general period of high storm activity and runoff, measurement of greater than average flux was expected. It is probable that the higher flux recorded by the sediments results from a significant input of detritus into the basin by near bottom transport.The chemical composition of trapped material was nearly identical in all four trap designs. The deep cone design, however, had a significantly lower Mn content. Since this trap was the only one in which reducing conditions were produced in the sample container, reduction and mobilization of manganese after collection is believed to have occurred.In spite of the very different designs tested, the factor of two agreement in flux determination and the compositional similarity of the material collected is encouraging for future attempts to directly measure the flux of particulates in the ocean.  相似文献   

10.
The near-bottom sedimentation rates were measured by placing cylindrical sediment traps 10 m above the sea floor on each of six moorings deployed between 4100 and 5100 m along a transect across an energetic deep-sea current in the HEBBLE area centered at 40°N, 63°W on the Nova Scotian Rise. Sedimentation rates above the sea floor were monitored with additional traps at 23, 54, 100, 200 and 500 m above the bottom (mab) on the mooring at 4950 m. The total flux at 500 mab for the two-week period, consisting mostly of primary particles from surface water, was 166 mg/m2 day and increased exponentially towards the bottom. The total flux at 10 mab increased down slope from 1160 mg/m2 day at 4158 m where the mean current speed was 8 cm/s to a maximum of 77,300 mg/m2 day at 5022 m where the mean current speed was 32 cm/s, then decreased to 59,400 mg/m2 day at the mooring at 5076 m. The size frequency distributions of large, discrete particles such as foraminifera, diatoms, radiolarians and fecal pellets were quantified in all trap samples to examine whether the large variation in fluxes was due to artifacts such as current velocity or trap tilt. Based on the source, persistence and distribution of these particles, we conclude that the large variations in fluxes across the rise and with distance from the sea floor are due primarily to resuspension and resettling of bottom sediments, with tilt and current effects on trapping having only a secondary effect. The vertical gradients of large-particle fluxes suggest effective vertical eddy diffusivities of 102–104 cm2/s using a two-dimensional model. Horizontal advection and secondary circulation probably play a large role in moving large, rapidly falling (up to 1 cm/s) particles to a height of 50–100 m above the sea floor.  相似文献   

11.
Deep circulation driven by strong vertical mixing in the Timor Basin   总被引：1，自引：1，他引：0  
The importance of deep mixing in driving the deep part of the overturning circulation has been a long debated question at the global scale. Our observations provide an illustration of this process at the Timor Basin scale of ～1000 km. Long-term averaged moored velocity data at the Timor western sill suggest that a deep circulation is present in the Timor Basin. An inflow transport of ～0.15 Sv is observed between 1600 m and the bottom at 1890 m. Since the basin is closed on its eastern side below 1250 m depth, a return flow must be generated above 1600 m with a ～0.15 Sv outflow. The vertical turbulent diffusivity is inferred from a heat and transport balance at the basin scale and from Thorpe scale analysis. Basin averaged vertical diffusivity is as large as 1 × 10?3 m2 s?1. Observations are compared with regional low-resolution numerical simulations, and the deep observed circulation is only recovered when a strong vertical diffusivity resulting from the parameterization of internal tidal mixing is considered. Furthermore, the deep vertical mixing appears to be strongly dependent on the choice of the internal tide mixing parameterization and also on the prescribed value of the mixing efficiency.  相似文献   

12.
Illite clay particles with adsorbed137Cs were added as a submillimeter layer to the surface of silt-clay sediments contained in rectangular Plexiglas cells stored in a temperature-regulated aquarium, in order to trace the effect of the oligochaete, Tubifex tubifex, and the amphipod, Pontoporeia hoyi, on mass redistribution near the sediment-water interface. A well-collimated NaI gamma detector scanned each sediment column (～10 cm deep) at daily or weekly intervals for six months, depicting the time evolution of radioactivity with and without added benthos. In a cell with tubificids (～5 × 104 m?2), which feed below 3 cm and defecate on surface sediments, the labeled layer was buried at a rate of 0.052 ± 0.007 cm/day (20°C). When labeled particles entered the feeding zone,137Cs reappeared in surface sediments creating a bimodal activity profile. In time, the activity tended toward a uniform distribution over the upper 6 cm, decreasing exponentially below to undetectable levels by 9 cm. In a cell with amphipods (～1.6 × 104 m?2) uniform activity developed rapidly (～17 days) down to a well-defined depth (1.5 cm). The mixing of sediments by Pontoporeia is described by a simple quantitative model of eddy diffusive mixing of sediment solids. The value of the diffusion coefficient, 4.4 cm2/yr (7°C) was computed from a least squares fit of theoretical to observed profile broadening over time. In a cell without benthos, small but measurable migration of137Cs indicated an effective molecular diffusion coefficient of 0.02 cm2/yr.  相似文献   

13.
The application of210Pb dating to an inshore marine deposit off the California coast has been successful. The amounts of unsupported210Pb annually accumulated in the Santa Barbara sediments are much greater than those from lake and glacial deposits and than the amount produced through the decay of radium in the overlying water column.  相似文献   

14.
Experiments to measure natural rates of sedimentation and to assess the tolerance of coral species to increased sedimentation were conducted at San Cristobal Reef, Puerto Rico. Sedimentation rates were measured over an 18 month period. Calcareous sediments were applied to colonies of Montastraea annularis, Diploria strigosa, D. clivosa, Acropora palmata and A. cervicornis in different frequencies and in different doses.Mean sedimentation rates from sediment traps were 9.6±2.4 (S.E.) mg cm?2 · day?1 at 10 cm above the bottom and 2.5±0.9 (S.E.) and 2.6±1.2 (S.E.) mg cm?2 · day?1 for two sets of traps at 50 cm above the bottom. Sediment application experiments indicated A. palmata was the least tolerant of the species tested. Although A. cervicornis and D. strigosa colonies were not significantly affected, single applications of 800 mg cm?2 to M. annularis colonies and of 200 mg cm?2 to A. palmata colonies caused death of underlying coral tissue. Algae colonized the smothered portions of these corals.  相似文献   

15.
A14C balance for the Eastern Caribbean deep water indicates the average inflow of Atlantic water into the basin to be 2.3 × 105 m3/sec (±30%), or about 2–4 times the values estimated previously. The balance uses a model representation of the deep-water turnover, and is based on14C concentrations at a station in the Venezuelan Basin which average Δ14C= 89‰ below 800 m depth with a total range of only 9‰, as well as on a14C concentration of the Atlantic inflow of Δ14C= ?71%. as obtained from measurements outside the Antilles Arch. The turnover time of the basin water below 2500 m depth is 55 years, which corresponds to an average upwelling velocity at this depth of about 35 m/year. With such upwelling, the temperature profile below 1800 m (the depth of the sill determining the inflow of new water) requires a vertical eddy diffusivity of about 5 cm2/sec. The oxygen consumption, and silica and CO2 regeneration, rates below 2500 m depth are obtained as ?0.18, + 0.08, and + 0.2 μmole kg?1 yr?1, respectively. The CO2 regeneration has but a negligible effect on the14C balance.  相似文献   

16.
We present contours of the largest horizontal and vertical recorded peak velocities of strong ground motion during the Northridge, California, earthquake. Above the fault, the horizontal peak velocities exceeded 100 cm/s. The vertical velocities were larger than 20 cm/s. We also present contours of peak horizontal and vertical strain factors. Through most of the San Fernando Valley and the Santa Susana Mountains, the horizontal surface strain factor was larger than 10−3. The largest horizontal strain factor computed was for the Rinaldi Receiving Station ∼10−2·2. The corresponding vertical strains were >10−3·25 and 10−13, respectively. Through most of the Los Angeles Basin the horizontal peak surface strain factors were between 10−3·75 and 10−3.  相似文献   

17.
Meteorological rocket soundings, launched between 1968–74 at six locations representative of low, middle, and high latitudes in the northern hemisphere, are employed to determine the vertical, meridional and off-diagonal components of the eddy diffusivity in the northern hemispheric statosphere and lower mesosphere.It is shown that the distribution of the vertical and meridional components of the eddy diffusivity are similar in the northern hemisphere, although the magnitude of the former is 107 smaller than that of the latter; the magnitude of the off-diagonal eddy diffusivity is about 103 smaller than that of the meridional eddy diffusivity. In the troposphere, a maximum eddy diffusivity occurs in the mid-latitude at about 7 km above the mean sea level for both the summer and winter seasons. In the stratosphere, a maximum eddy diffusivity occurs in the mid-latitude at about 33 km in the winter, but no maximum in the summer.Paper presented at the World Meterological Organization Technical Conference on Global Observations of Atmospheric Pollution Relative to Climate, Boulder, Colorado, 20–24 August 1979.  相似文献   

18.
Quantitative determinations of helium and other noble gas concentrations in pore waters of marine sediments permit the calculation of helium fluxes out of the sedimentary column. Values of 1.1 and 6.4 × 104 atoms/cm2 sec were found for two locations in the northeast Pacific. These fluxes are one to two orders of magnitude less than those based on theoretical degassing models of the earth or on measurements of excess helium in the deep ocean water.  相似文献   

19.
《Continental Shelf Research》2007,27(10-11):1477-1500
Constraining the exchange of water from the shoreline to the mid-shelf is necessary for the development of accurate and predictive models of nearshore circulation. Ra isotopes, which emanate from sediments and have a variety of half-lives, may be useful in measuring cross-shelf mixing rates. The distributions of Ra isotopes were measured in transects extending perpendicular from the shoreline at Sunset Beach and Huntington Beach, CA. The average inventory at Sunset Beach was four times greater than at Huntington Beach. Building on previous research on Ra inputs and circulation in San Pedro Bay, a two-dimensional model for surface water Ra was developed to identify the importance of onshore flow and cross-shelf mixing near Huntington Beach. For the mean summertime conditions, the eddy diffusivity (Kh) was 1.4±0.4 m2 s−1, with 8% of the water from Sunset Beach moving down the coast. The remaining water must be low-Ra water that has moved onshore. At time scales greater than a week, the short-lived Ra inventory at Huntington Beach varied by 50%, which reflects changes in the fractions of water moving down-coast and/or in the longshore advection rate. The shoreline Ra concentration varied on time scales of hours, which may be generated by tidal changes in the Ra input at the shoreline and short-period fluctuations in the mixing rate. The low Kh observed in this study in comparison to higher values measured further offshore is evidence that Kh increases with distance offshore. When scale-dependent mixing beyond 455 m offshore is incorporated into the model, the results are consistent with the observed data for 223Ra, 224Ra, and 228Ra. Using the model, the 228Ra input flux to the summertime mixed layer was between 3.4×106 and 4.0×106 atoms s−1 (m shoreline)−1.  相似文献   

20.
Observations are presented of currents, hydrography and turbulence in a jet-type tidally forced fjord in Svalbard. The fjord was ice covered at the time of the experiment in early spring 2004. Turbulence measurements were conducted by both moored instruments within the uppermost 5 m below the ice and a microstructure profiler covering 3–60 m at 75 m depth. Tidal choking at the mouth of the fjord induces a tidal jet advecting relatively warmer water past the measurement site and dominating the variability in hydrography. While there was no strong correlation with the observed hydrography or mixing and the phase of the semidiurnal tidal cycle, the mean structure in dissipation of turbulent kinetic energy, work done under the ice and the mixing in the water column correlated with the current when conditionally sampled for tidal jet events. Observed levels of dissipation of turbulent kinetic energy per unit mass, 1.1×10−7 W kg−1, and eddy diffusivity, 7.3×10−4 m2 s−1, were comparable to direct measurements at other coastal sites and shelves with rough topography and strong forcing. During spring tides, an average upward heat flux of 5 W m−2 in the under-ice boundary layer was observed. Instantaneous (1 h averaged) large heat flux events were correlated with periods of large inflow, hence elevated heat fluxes were associated with the tidal jet and its heat content. Vertical heat fluxes are derived from shear-probe measurements by employing a novel model for eddy diffusivity [Shih et al., 2005. Parameterization of turbulent fluxes and scales using homogeneous sheared stably stratified turbulence simulations. Journal of Fluid Mechanics 525, 193–214]. When compared to the direct heat flux measurements using the eddy correlation method at 5 m below the ice, the upper 4–6 m averaged heat flux estimates from the microstructure profiler agreed with the direct measurements to within 10%. During the experiment water column was stably, but weakly, stratified. Destabilizing buoyancy fluxes recorded close to the ice were absent at 5 m below the ice, and overall, turbulence production was dominated by shear. A scaling for dissipation employing production by both stress and buoyancy [Lombardo and Gregg, 1989. Similarity scaling of viscous and thermal dissipation in a convecting boundary layer. Journal of Geophysical Research 94, 6273–6284] was found to be appropriate for the under-ice boundary layer.  相似文献   

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