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1.
226Ra data on eleven vertical profiles taken during the GEOSECS program from the Antarctic Ocean and its vicinity in both the Atlantic and the Pacific are presented. Replicate measurements were made on each sample using the Rn-emanation method. The precision (1 σ) based on triplicate analyses averages about ±2.5%. Waters all around the Antarctic continent below 2 km depth appear to exhibit a uniform226Ra concentration of 21.5 ± 1dpm/100kg, except perhaps locally such as the Ross Sea and the Drake Passage where small variations may be present. Higher in the water column, the226Ra contents decrease toward the surface with gradients which vary as a function of the influence exerted by the Antarctic Convergence. Across this oceanic front, a north-to-south increase of226Ra occurs (the increase being the largest near the surface: from 8 to 18 dpm/100 kg), reflecting the combining effect of deep-water upwelling and meridional water mixing. The core layer of the Antarctic Intermediate Water contains about 14 dpm/100 kg of226Ra and that of the Circumpolar Intermediate Water (O2 minimum and local T maximum) about 18 dpm/100 kg. To a first approximation,226Ra covaries with Si in the circumpolar waters.  相似文献   

2.
Based on results obtained during the GEOSECS program the primary features of the distribution of226Ra in the Atlantic Ocean can be defined. Outside the Antarctic no significant variation has been found in the226Ra content of surface waters. Eighty samples yield an average of 7.4 dpm/100 kg (normalized to a salinity of 35.00‰). Deep waters in the central Atlantic have226Ra contents several dpm/100 kg higher than expected from the mixing of Antarctic Bottom Water (21.3 dpm/100 kg) and basal North Atlantic Deep Water (10.3 dpm/100 kg). These excesses correlate well with deficiencies in O2 and excesses in SiO2. The intermediate water226Ra maximum in the South Atlantic is associated with the inflow of low-oxygen Circumpolar Intermediate Water beneath the Antarctic Intermediate Water.  相似文献   

3.
Recent radium measurements from the near-surface Caribbean Sea are presented. The surface horizontal and vertical distributions of226Ra are essentially the same as reported by Szabo et al. (1967) for the early 1960's. The226Ra activity at the surface is relatively uniform across the Caribbean, with an average of8.2±0.4dpm/100kg. The subsurface distribution to ～200 m averages7.8±0.4dpm/100kg and increases slowly below 200 m. reaching ～9.5 dpm/100 kg at 560 m. In contrast to226Ra, the surface concentration of228Ra was much more variable in both time and space. An average increase of 33% was found between 1968 and 1976 in the western Caribbean and during both years an anomalously high228Ra activity was found in the eastern Caribbean. These data support previous hypotheses that water entering the eastern Caribbean has been enriched in228Ra prior to entry and that variable mixing of the Atlantic water masses found to the northeast and southeast of the Lesser Antilles may produce temporal variations in the near-surface228Ra activity. Scatter plots of228Ra vs. salinity and sigma-t indicate that the near-surface vertical distribution of228Ra in the Caribbean Sea is predominantly influenced by advection. Thus228Ra cannot be used to study near-surface vertical mixing rates in this region.  相似文献   

4.
— Urban terrain poses a challenge for modeling air pollutant diffusion. In tropics, because of the dominant low wind speed environment, the importance of understanding the turbulence diffusion is even more critical, and uncertain. The objective of this study is to estimate the vertical eddy diffusivity of an urban, tropical atmosphere in low–wind speeds. Turbulence measurements at 1 Hz were made at 4-m level over an urban terrain with a roughness length of 0.78 m during winter months. Eddy diffusivity is estimated from spectral quantities of the turbulence data involving turbulent kinetic energy (E) and its dissipation rate (?). The spectral information of the vertical velocity fluctuations is used to estimate the vertical length scale which provides information on the eddy diffusivity. In addition, the product of friction velocity and the vertical length scale has been used to non-dimensionalize the eddy diffusivity, which is shown to increase with increasing instability. Using the eddy diffusivity (K) estimates from the E – ? approach, a relation is suggested for the mixing length based eddy diffusivity models of the form: K = c w .[2.5 ? 0.5(z/L)], where z is the measurement height, L is the Obukhov length, and c w has an average value close to 1 for unstable and near 0.5 for stable conditions for the urban terrains.  相似文献   

5.
The228Ra concentrations of the Dead Sea waters range from 0.13 to 1.48 dpm kg−1, two to three orders of magnitude higher than those of ocean waters and lake waters. However, the228Ra/226Ra activity ratios, (0.12–1.29) × 10−2, are in the range reported for the hydrosphere.The surface waters of the Dead Sea are enriched in228Ra by a factor of about three over the near-bottom waters. There is a factor of about two spatial variability in the mid-depth Ra concentrations at the two profile stations. The near-bottom228Ra gradients yield vertical eddy diffusivity coefficient (K) of 2.0 and 0.4 cm2 s−1 at profile locations 1 and 2 respectively. These values are comparable to those measured in oceans and lakes.  相似文献   

6.
Persistent weak temperature stratification characterizes the epilimnion of Lake Opeongo, Ontario, Canada, and reduces the magnitude of turbulent mixing. Throughout July and August 2009, the epilimnion was isothermal for only 34 % of the record, while for 28 % of the record there was at least a 2 °C temperature difference across the 5 m deep epilimnion. During these stratified periods, there were increases in gradient Richardson numbers (Ri g ), and decreases in rates of dissipation of turbulent kinetic energy ( $\varepsilon$ ), the turbulence activity parameter (I = ε/νN 2), an indicator of active mixing, and vertical eddy diffusivity (K z ) inferred from temperature microstructure profiles. During periods of shear induced mixing, values of ε approached 10?6 m2 s?3 and decreased during periods of increasing Ri g . For 0 < Ri g  < 1, average values of I were ~1,000 and values of K z were slightly higher than 10?4 m2 s?1. For Ri g >1, average values of I were ~300 and K z was reduced by one to three orders of magnitude. Mixing during cold fronts occurred over time scales of minutes to hours, which worked to erode diurnal thermoclines. However, during periods of persistent secondary thermoclines, mixing was suppressed throughout the epilimnion.  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.
The flux of226Ra from bottom sediments has been determined from patterns of226Ra/230Th disequilibrium in ten deep-sea cores from the world oceans. Values range from ? 0.0015 dpm/cm2 yr (in the Atlantic) to 0.21 dpm/cm2 yr (in the north equatorial Pacific). The flux is poorly related to sediment type, but is inversely correlated in a non-linear fashion with sediment accumulation rate. There is a direct relationship between the production rate of226Ra near the sediment-water interface (i.e. the integrated230Th activity in the biologically mixed zone) and the226Ra flux. The226Ra concentration in near-bottom water follows the geographic variation in the226Ra flux. The high flux from north equatorial Pacific sediments especially is reflected in the high bottom water226Ra concentrations in that area. The data suggest that both rate of circulation and the magnitude of the radium flux influence the near-bottom226Ra concentration.  相似文献   

9.
The progressive weakening and final disappearance (in 1979) of the long-term meromictic structure of the Dead Sea are clearly reflected in the depth profiles of210Pb and210Po. In 1977/78, prior to overturn, dissolved210Pb (35–50 dpm kg?1) predominated over particulate210Pb (1–2 dpm kg?1) in the oxic upper waters, whereas the reverse was true in the anoxic deep waters (16–20 dpm kg?1 particulate vs. 2–5 dpm kg?1 dissolved). The exact extent of the disequilibrium between210Pb and226Ra is hard to evaluate in the upper oxic layers, because the progressive deepenings resulted in mixing with deep waters. By contrast, one can estimate the residence time of dissolved210Pb in the unperturbed anoxic deepest layers, because these remained isolated, at about 3 years. Following the overturn of 1979, dissolved210Pb exceeded particulate210Pb at all depths. The210Po profiles of the stratified lake resembled in shape those of its grandparent210Pb, but with distinct characteristics of their own in the oxic upper waters where particulate210Po (8–12 dpm kg?1) was greatly in excess over particulate210Pb, while dissolved210Po (25–40 dpm kg?1) was slightly deficient. Immediately following the overturn, dissolved and particulate210Po were similar (about 15 dpm kg?1), at all depths. The destruction of the lake's meromictic structure was accompanied by a reduction of its210Pb inventory, while that of210Po was almost unaffected. Thus, at overturn a transient state was created with the inventory of210Po exceeding that of210Pb.  相似文献   

10.
Three ferromanganese nodules handpicked from the tops of 2500 cm2 area box cores taken from the north equatorial Pacific have been analysed for their U-Th series nuclides.230Thexc concentrations in the surface 1–2 mm of the top side of the nodules indicate growth rates of 1.8–4.6 mm/106 yr. In two of the nodules a significant discontinuity in the230Thexc depth profile has been observed at ～0.3 m.y. ago, suggesting that the nodule growth has been episodic. The concentration profiles of231Paexc (measured via227Th) yield growth rates similar to the230Thexc data. The bottom sides of the nodules display exponential decrease of230Thexc/232Th activity ratio with depth, yielding growth rates of 1.5–3.3 mm/106 yr.The230Thexc and231Paexc concentrations in the outermost layer of the bottom face are significantly lower than in the outermost layer of the top face. Comparison of the extrapolated230Thexc/232Th and230Thexc/231Paexc activity ratios for the top and bottom surfaces yields an “age” of (5?15) × 104 yr for the bottom relative to the top. This “age” most probably represents the time elapsed since the nodules have attained the present orientation.The210Pb concentration in the surface ～0.1 mm of the top side is in large excess over its parent226Ra. Elsewhere in the nodule, up to ～1 mm depth in both top and bottom sides,210Pb is deficient relative to226Ra, probably due to222Rn loss. The absence of210Pbexc below the outermost layer of the top face rules out the possibility of a sampling artifact as the cause of the observed exponentially decreasing230Thexc and231Paexc concentration profiles. The flux of210Pbexc to the nodules ranges between 0.31 and 0.58 dpm/cm2 yr. The exhalation rate of222Rn, estimated from the226Ra-210Pb disequilibrium is ～570 dpm/cm2 yr from the top side and >2000 dpm/cm2 yr from the bottom side.226Ra is deficient in the top side relative to230Th up to ～0.5–1 mm and is in large excess throughout the bottom. The data indicate a net gain of226Ra into the nodule, corresponding to a flux of (24?46) × 10?3 dpm/cm2 yr. On a total area basis the gain of226Ra into the nodules is <20% of the226Ra escaping from the sediments. A similar gain of228Ra into the bottom side of the nodules is reflected by the high228Th/232Th activity ratios observed in the outermost layer in contact with sediments.  相似文献   

11.
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   

12.
The concentrations of222Rn and226Ra in the water column and in the sediments of Santa Barbara and San Nicolas Basins have been measured semi-annually over the last four years. Approximately one-third of excess radon profiles obtained in the water column in these basins can be adequately fit with a one-dimensional eddy diffusion-decay model. Exponential profiles in the center of San Nicolas Basin yield a vertical eddy diffusivity of 26±16 cm2/s and 3.4±1.0 cm2/s for Santa Barbara Basin. The application of a two-dimensional eddy diffusion-decay model to profiles obtained in the center and on the margins of San Nicolas Basin produces a better fit than is found using a one-dimensional vertical eddy diffusivity. The two-dimensional model for San Nicolas Basin predicts a vertical eddy diffusivity of 17 cm2/s and a horizontal eddy diffusivity of 105 cm2/s. These values are in reasonable agreement with those predicted from the vertical buoyancy gradient and the horizontal length scale.The vertically integrated radon excess (standing crop) in the water column of Santa Barbara Basin averages 53±23 atoms/m2 s. This is in good agreement with the flux across the sediment-water interface of 60±15 atoms/m2 s, calculated by measuring radon emanation in the sediments as a function of depth and applying a molecular diffusion-reaction model. Hence, one-dimensional molecular diffusion accurately predicts the flux of radon from the laminated Santa Barbara Basin sediments. In San Nicolas Basin the integrated radon excess in the water column is 376±143 atoms/m2 s, but the diffusive randon flux from San Nicolas Basin sediments averages only 190±53 atoms/m2 s. This descrepancy indicates that a non-diffusive process, probably macrofaunal irrigation, supplies much of the flux of radon from San Nicolas Basin sediments.  相似文献   

13.
226Ra and210Pb were measured in sections and profiles collected in the Weddell Sea during the International Weddell Sea Oceanographic Expedition in 1973. The results can be correlated with the circulation and mixing schemes deduced from hydrographic observations. Along the surface cyclonic gyre the Ra activities are fairly uniform at about 17 dpm/100 kg, quite similar to those of the Circumpolar surface water south of the Antarctic Convergence. The210Pb activities in the northern flank of the gyre, probably influenced by the high210Pb-bearing Circumpolar Deep Water in the north, are as high as 12 dpm/100 kg. At the central gyre and its southern flank, the surface water210Pb activities are about 7 dpm/100 kg. The warmer surface water at the central gyre has a Ra activity of about 19 dpm/100 kg, slightly higher than the colder surface water at the flanks. Thus lower210Pb/226Ra activity ratios are observed in the central gyre, and higher ratios in its flanks. Similar relationships between Ra and Pb are noted in the Weddell Sea Bottom Water (WSBW): lower Pb associated with higher Ra in the center; higher Pb with slightly lower Ra in the flanks.Vertical profiles along the cyclonic gyre show lower Ra and Pb activities in the southwestern Weddell Basin where lower temperature and lower silicate are observed. Similar to Ba, both Ra and Si are non-conservative in the Weddell Sea, with significant input from the bottom sediments and particulate dissolution during subsurface mixing.Each water mass or type in the Weddell Sea is well characterized by its Ra content, but not well by its Pb content. Ra and Si are crudely correlated with a slope of about 7 × 10?4 dpm Ra per μmole of Si. The fact that the WSBW values fall on the slope suggests that the net input rate for Ra (corrected for the decay rate) is proportional to that of Si. The linear extrapolation to zero Si gives a Ra value of 13 dpm/100 kg. These relationships are quite similar to those observed in the Circumpolar waters.  相似文献   

14.
The surface water of the Japan Sea contained226Ra of70 ± 4dpm m−3 which was nearly equal to that of the surface water in the North Pacific. The concentration of226Ra in the Japan Sea deep water below 500 m was151 ± 8dpm m−3, showing a vertically and regionally small variation. This concentration of226Ra in the deep water is unexpectedly high, because the Japan Sea deep water has a higher Δ14 C value by about 50‰ than the Atlantic deep water containing the same226Ra. One of the causes to be considered is larger contribution of226Ra from biogenic particles dissolving in the Japan Sea deep water, but the Japan Sea is not so fertile in comparison to the Bering Sea. The other more plausible cause is the internal ventilation of the Japan Sea water, which means that the residence time of the Japan Sea Proper water is considerably long although the water is vertically mixed fairly well especially in winter. The ventilation may supply some amounts of radiocarbon and oxygen but does not change the inventory of226Ra. The residence times of the Japan Sea deep water and of water within the Japan Sea are calculated by solving simultaneous equations for226Ra and14C with a three-box model to be 300–400 years and 700–1000 years, respectively.  相似文献   

15.
We have conducted RASS (Radio Acoustic Sounding System) observations with the MU (Middle and Upper atmosphere) radar on 28–31 July 1994 at Shigaraki, Japan (34°51′ N, 136°06′ E) and determined the characteristics of turbulence energy dissipation rate (ε) and vertical eddy diffusivity (K) in the troposphere and lower stratosphere. We have first examined the accuracy of the Brunt Väisälä frequency squared (N2) derived from RASS temperature profiles. Then, ε and K are determined by using N2 and the Doppler spectral width of turbulence echoes (σ2turb). We found that the structures of ε and K are sometimes affected by the local variations of N2. We found that sometimes ε became small, even though σ2turb was large, because N2 was small. On the contrary, K was enhanced due to small N2, although σ2turb was small.  相似文献   

16.
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17.
Unsupported226Ra (t12 = 1620years) in marine sediments can provide a basis for measuring rates of accumulation of the order of centimeters per thousand years. The excess radium apparently enters the sediments incorporated in phytoplankton. The sensitivity of the method depends upon the initial value of the unsupported226Ra and of the value of230Th, a parent of226Ra, in the sedimentary components.226Ra dating was applied to a sediment taken from the slope of the San Clemente Basin in the Southern California coastal region. Rates of sedimentation over two half-lives of the nuclide were found to be either 5.2 or 5.3 cm/1000 years depending upon which of two models for the geochronology is used. One model assumes that the230Th brings to the deposit an amount of226Ra in equilibrium with it. The other is based upon the growth of the226Ra from the230Th in the sedimentary components.238+239Pu and210Pb levels in the upper strata indicated sedimentation rates of the order of 100–500 cm/1000 years, i.e. much faster accumulations. We suggest these derived rates are spurious and reflect bioturbative activities of surface-living organisms.  相似文献   

18.
Vertical eddy diffusivities (Kv's) have been estimated at fourteen widely separated locations from fourteen222Rn profiles and two228Ra profiles measured near the ocean floor as part of the Atlantic and Pacific GEOSECS programs. They show an inverse proportionality to the local buoyancy gradient [(g/?)(??pot/?z)] calculated from hydrographic measurements. The negative of the constant of proportionality is the buoyancy flux [?Kv(g/?)(??pot/?z)] which has a mean of ?4 × 10?6 cm2/sec3. Our results suggest that the buoyancy flux varies very little near the ocean floor. Kv's for the interior of the deep Pacific calculated from the relationship Kv = (4 × 10?6cm2/sec3)/[(g/?)(??pot/?z)] agree well with published estimates. Kv's calculated for the pycnocline are one to two orders of magnitude smaller than upper limits estimated from tritium and7Be distributions.Heat fluxes calculated with the model Kv's obtained from the222Rn profiles average 31 μcal cm?2 sec?1 in the Atlantic Ocean and 8 μcal cm?2 sec?1 in the Pacific Ocean.  相似文献   

19.
Thermal diffusivity, k, of three lunar rocks (10049 and 10069; Type A, Apollo 11 and 14311; Apollo 14) and a terrestrial basalt (alkaline olivine basalt, Oki-do?go, Japan) was measured under one atmosphere and in vacuum conditions (10?3 ～ 10?5 mmHg) in the temperature range from 85 to 850°K. The semi-empirical curve of k =A + B/T +CT3 is fitted to the data in each condition. The porosity of rocks strongly affects the thermal diffusivity at low temperature ( T ? 500°K) in vacuum condition. At 150°K, thermal diffusivity of lunar rocks with porosity of 5.5% (10049) and 11% (10069) at one atmosphere is about 1.7 and 3.2 times of that in vacuum, respectively. The difference between the values at one atmosphere and those in vacuum decreases as the temperature increases. Measurements of k should be made at gas pressures at least lower than 10?3 mmHg to estimate the value under lunar surface conditions.  相似文献   

20.
《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   

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