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1.
Profiles of226Ra and dissolved210Pb have been measured at several stations in the Red Sea. At one station in the central Red Sea an expanded profile was measured including226Ra and dissolved and particulate210Pb and210Po. These profiles show several distinct features: (1)226Ra displays a mid-depth maximum of about 13 dpm/100 kg at about 500 m; (2) dissolved210Pb concentrations are uniformly low at about 2 dpm/100 kg with little lateral or vertical variation; (3) the surface-water210Pb excess which is commonly observed in low-latitude open ocean regions is entirely lacking; (4)210Pb and210Po activities are essentially identical to each other in both particulate and dissolved phases although210Po activities appear somewhat lower; (5) about 20% of the210Pb and210Po in the water column residues on particulate matter.Assuming the atmospheric210Pb flux to be in the dissolved form and at the lower level of the normal range i.e. 0.5 dpm/cm2 yr, the residence time of the dissolved Pb is about 1.5 years. However, if the same atmospheric flux is entirely in particulate form, then the residence time of the dissolved Pb is about 5 years. The residence time of Pb in the particulate phase is less than 0.4 years if all the Pb is removed only by sinking particles.  相似文献   

2.
Disequilibrium between210Po and210Pb and between210Pb and226Ra has been mapped in the eastern and central Indian Ocean based on stations from Legs 3 and 4 of the GEOSECS Indian Ocean expedition.210Po/210Pb activity ratios are less than 1.0 in the surface mixed layer and indicate a residence time for Po of 0.6 years.210Po and210Pb are generally in radioactive equilibrium elsewhere in the water column except at depths of 100–500 m, where Po may be returned to solution after removal from the surface water, and in samples taken near the bottom at a few stations.210Pb excesses relative to226Ra are observed in the surface water but these excesses are not as pronounced as in the North Pacific and North Atlantic. The difference is attributable to a lower flux of210Pb from the atmosphere to the Indian Ocean. Below the main thermocline,210Pb activities increase with depth to a broad maximum before decreasing to lower values near the bottom. Departures from this pattern are especially evident at stations taken in the Bay of Bengal (where210Pb/226Ra activity ratios as low as 0.16 are observed) and near the Mid-Indian Ridge. The data suggest that removal of210Pb at oceanic boundaries, coupled with eddy diffusion along isopycnals, can explain gradients in210Pb near the boundary. Application of a simple model including isopycnal diffusion, chemical removal, production and radioactive decay produces fits the observed210Pb/226Ra gradients for eddy diffusion coeffients of ～ 107 cm2/s. High productivity in surface waters of the Bay of Bengal makes this region a sink for reactive nuclides in the northern Indian Ocean.  相似文献   

3.
Two ocean profiles from the Peru Basin from regions with different surface productivities were analyzed for total210Pb and201Po to evaluate the influence of particulates in the water column on their distribution. Comparison with a published226Ra profile for the region was made. The profile closest to the coast, where upwelling and productivity are high, shows depletion of210Pb relative to226Ra at all depths, with particularly marked excursions from radioactive equilibrium at the surface and in the bottom water.210Po appears to be deficient relative to210Pb at depth as well. Mean residence times in the deep water, relative to particulate removal from the water column to the sediments, of about 100 years for210Pb and about two years for210Po are indicated. The profile northwest of the upwelling region shows the226Ra210Pb210Po system close to equilibrium at all depths to 1500 m (except for the effect of atmospheric210Pb input seen at the surface.  相似文献   

4.
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   

5.
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   

6.
226Ra,210Pb and210Po were measured in oceanic profiles at two stations near the Bonin and Kurile trenches.210Po is depleted by 50% on average relative to210Pb in the surface water. In the deep water,210Pb is about 25% deficient relative to226Ra. Based on the deficiency,210Pb residence time with respect to removal by particulate matter was estimated to be less than 96 years in the deep water.210Pb deficiency in the bottom water was significantly greater than that of the adjacent deep water, indicating more effective removal near or at the bottom interface.210Pb,210Po and Th appear to have similar overall rate constants of particulate removal throughout the water column.  相似文献   

7.
210Pb and226Ra profiles have been measured at five GEOSECS stations in the Circumpolar region. These profiles show that226Ra is quite uniformly distributed throughout the Circumpolar region, with slightly lower activities in surface waters, while210Pb varies with depth as well as location or area. There is a subsurface210Pb maximum which matches the oxygen minimum in depth and roughly correlates with the temperature and salinity maxima. This210Pb maximum has its highest concentrations in the Atlantic sector and appears to originate near the South Sandwich Islands northeast of the Weddell Sea. Concentrations in this maximum decrease toward the Indian Ocean sector and then become fairly constant along the easterly Circumpolar Current.Relative to226Ra, the activity of210Pb is deficient in the entire water column of the Circumpolar waters. The deficiency increases from the depth of the210Pb maximum toward the bottom, and the210Pb/226Ra activity ratio is lowest in the Antarctic Bottom Water, indicating a rapid removal of Pb by particulate scavenging in the bottom layer and/or a short mean residence time of the Antarctic Bottom Water in the Circumpolar region.226Ra is essentially linearly correlated with silica and barium in the Circumpolar waters. However, close examination of the vertical profiles reveals that Ba and Si are more variable than226Ra in this region.  相似文献   

8.
We report here on particulate and dissolved210Pb profiles at 16 stations, and on total210Pb profiles at 3 stations, all occupied during the Pacific GEOSECS expedition. Comparison with measurements at Yale on GEOSECS library samples indicates that during separation of particulate lead from dissolved lead, our filtered water samples suffered some loss of210Pb in the filtration system; this effect appears to have reduced the dissolved210Pb activities by ～ 20% in stations where the water was filtered. However, for these first Pacific data on the210Pb distribution between the two phases, this effect does not significantly interfere with our recognition of the major features of both particulate and dissolved210Pb distributions.The dissolved210Pb profiles in general vary geographically, following the226Ra profiles. In deep water,226Ra increases northward and eastward from the southwest Pacific, from ～ 22dpm/100kg, to over 40 dpm/100 kg in the northeast Pacific. Our dissolved210Pb profiles show a similar increase in deep water, varying from about 10 to 20 dpm/100 kg along this line, and are commonly characterized by a mid-depth maximum. This210Pb maximum reflects the mid-depth226Ra maximum of the Pacific Deep Water observed along the western boundary current.In surface water at low latitudes there is a significant210Pb flux from the atmosphere, which produces a210Pb/226Ra activity ratio generally greater than unity. This flux penetrates as deep as 600 m, as indicated by an “induced”210Pb minimum caused by the surface maximum. The surface water210Pb excess decreases toward high southern latitudes and vanishes in the Circumpolar region.The particulate210Pb profiles show a general increase with depth, from ～ 0.3dpm/100kg in subsurface water to ～ 1.5dpm/100kg in bottom water, with or without a mid-depth maximum that reflects the226Ra or dissolved210Pb maximum. The particulate210Pb normally comprises about 2% of the total210Pb in subsurface water, and this fraction increases to about 10% near the bottom. As the filtration loss is not taken into account, the fraction of particulate210Pb quoted here is an upper limit. Since the particulate matter concentrations are quite uniform in the water column below a few hundred meters, the210Pb activity of the particulate matter also increases with depth. The particulate matter has a210Pb concentration of ～ 100dpm/g in subsurface water, but the concentration increases to ～ 500dpm/g or more toward the bottom. This indicates that there is a cumulative adsorption of Pb onto the suspended particles as they are sinking through the water column.  相似文献   

9.
Ten GEOSECS profiles from the North Pacific have been analyzed for210Pb. GEOSECS226Ra data on the same profiles are used to calculate210Pb excess or deficiency relative to secular equilibrium. The resultant profiles are divisible into a thermocline zone (<2000m) showing an expected decrease with depth, a mid-water zone of about 2000 m showing small constant deficiencies with a zone of increasing deficiency to a bottom zone of about 1000 m having the highest deficiency virtually invariant with depth. The exponentially decreasing portion in the thermocline yields a “diffusion” coefficient of 3 cm2/s. The mid-water deficiencies yield ? model residence times of 400 years northeast of Hawaii decreasing to 100 years at the most marginal stations.  相似文献   

10.
Particulate and soluble,210Pb activities have been measured by filtration of large-volume water samples at two stations in the South Atlantic. Particulate phase210Pb (caught by a 0.4-μm filter) varies from 0.3% of total210Pb in equatorial surface water to 15% in the bottom water. The “absolute activity” of210Pb per unit mass of particulate matter is about 107 times the activity of soluble210Pb per unit mass of water, but because the mass ratio of particulate matter to water is about 10?8, the particulate phase carries only about 10% of the total activity. In Antarctic surface water the particulate phase carries 40% of the total210Pb activity; the absolute activity of this material is about the same as in other water masses and the higher fraction is due to the much larger concentration of suspended matter in surface water in this region.In the equatorial Atlantic the particulate phase210Pb activity increases with depth, by a factor of 40 from surface to bottom, and by a factor of 4 from the Antarctic Intermediate Water core to the Antarctic Bottom Water. This increase with depth is predicted by our previously proposed particulate scavenging model which indicated a scavenging residence time of 50 years for210Pb in the deep sea. A scavenging experiment showed that red clay sediment removes all the210Pb from seawater in less than a week. The Antarctic particulate profile shows little or no evidence of scavenging in this region, which may be due to the siliceous nature of the particulate phase in circumpolar waters. Our previous observation that the210Pb/226Ra activity ratio is of the order of 0.5 in the deep water is further confirmed by the two South Atlantic profiles analyzed in the present work.  相似文献   

11.
During reoccupation of the GEOSECS-I test station in May, 1979, more than eighty 30-liter Niskin samples were collected in profile, many as replicates, for210Pb intercomparison measurements by the WHOI, SIO and Yale groups. In addition to the inter-laboratory comparisons, the SIO group also carried out extensive experiments to test the effect of sample scavenging method. Pb equilibration time (storage effect), and filtration process on the measured210Pb results.The intercomparison measurements indicate that there is a general agreement between the various sets of data. The sample set which allows a direct comparison at the same depth was available in most cases only between two of the three groups. The direct paired comparison shows that (1) the WHOI data are systematically 3% lower than the SIO data; (2) there are no systematic differences observed between the SIO and Yale data although the scatter is rather large; (3) the Yale data are systematically higher than the WHOI data by about 8%.The SIO experiments show that (1) the two scavenging methods employed (Fe(OH)3 and Co-APDC co-precipitation) yield identical210Pb results; (2) variation of Pb carrier equilibration time or of storage time has no discernible effect; (3) the filtration apparatus and procedure employed at this station do not result in210Pb loss or contamination.The210Pb profile structure and absolute concentration measured earlier at the same location (GOGO-II test station and GEOSECS station 347) agree with those of station 500 within 10%. The present profile shows a minimum210Pb concentration around 500 m depth, marking the penetration depth of the flux of excess210Pb from the atmosphere. There is a mild mid-depth maximum around 2500–3000 m. The210Pb/226Ra activity ratio decreases monotonically from about 1 at the210Pb minimum to about 0.5 near the bottom. The particulate210Pb profile shows a systematic increase from the subsurface water to the bottom water by a factor of 5. This feature has been observed in many GEOSECS particulate210Pb profiles.  相似文献   

12.
Measurements have been made of226Ra and both dissolved and particulate forms of210Pb and210Po in a vertical profile at 85°50′N, 108°50′W in the Arctic Ocean.In the upper water column226Ra shows a concentration maximum that is coincident with one in the nutrients, silicate, phosphate, and nitrate, while at the same depth, dissolved and particulate210Pb and210Po all show minimum concentrations. It is suggested that the concentration maxima are partly due to sources of the respective elements in the continental shelf sediments, the shelf waters being subsequently advected into the Arctic Ocean basins. The210Pb and210Po minima have similarly been explained by interaction between the shelf sediments and overlying waters. An estimate is made of the possible contributions of shelf sediments to the layer of silica-rich water which covers the Canada Basin at a depth of 100–150 m.Residence times have been calculated for dissolved210Pb and210Po at various depths in the water column. Surface water residence times of dissolved and particulate forms of these radionuclides are longer than in surface Atlantic waters, probably due to lower biological activity in the surface waters of the Canada Basin. An estimatee has been made of the average sinking velocity of particulate material.  相似文献   

13.
Activities of the naturally occurring radionuclides, 210Pb and 210Po, were measured in both dissolved (<0.45 μm) and particulate (>0.45 μm) phases from surface waters of the southern South China Sea. The average activity of particulate 210Pb, 0.23 Bq/m3 (n=23), accounted for about 12% of the total 210Pb, which corresponds with values of open oceans. Particulate 210Po, with an average activity of 0.43 Bq/m3, accounted for about 40% of the total 210Po, which was much higher than those of open and eutrophic oceans. The residence times of total 210Po and 210Pb in surface waters estimated from an irreversible steady-state model were 0.82 a and 1.16 a, respectively. The consistently high fractionation factor calculated either by scavenging rate constants (5.42) or Kd values (6.69) suggested that a significant fractionation occurred between 210Po and 210Pb during their removal from solution to particles and that the two radionuclides had different biogeochemical cycling pathways in the oligotrophic South China Sea. Furthermore, our results indicated that there exist different fractionation mechanisms between 210Po and 210Pb in different marine environments: in eutrophic ocean, plankton detritus and fecal pellets are the main carrier of 210Po and 210Pb, by which 210Po and 210Pb have been scavenged and removed; while in oligotrophic ocean, microbes could become the main carrier of 210Po and fractionate 210Po and 210Pb significantly as a result of scarce plankton detritus and fecal pellets. These results suggest the use of 210Po to trace marine biogeochemical processes relating to microbial activities and the cycling of sulfur group elements (S, Se, Te and Po).  相似文献   

14.
Samples of the surface microlayer, of bulk seawater from 20-cm depth and of the neustonic organisms inhabiting the top 5 cm of the sea were collected at regular intervals over a period of 17 months at a site 3 km off Monaco and analysed for the naturally occurring radionuclides210Po and210Pb. Enrichment of210Po in the microlayer compared with the bulk seawater was always observed, and the degree of enrichment was found to be correlated significantly with the neuston biomass per unit volume. Enrichment of210Pb in the microlayer was also observed, but only under the higher neuston biomass conditions. The210Po:210Pb ratio was always higher in the microlayer than in the bulk seawater. Additional information was obtained from210Po measurements made on the bulk seawater in which the neuston had been collected and in which it had stood for periods of 2 to 4 h. These showed that the neuston lost210Po to the water at a rate of about 1 pCi g?1 dry biomass h?1. A significant flux of210Po from bulk seawater to the surface microlayer, and thence possibly to the atmosphere, is estimated. This flux is mediated by the biota, and will vary seasonally with the planktonic biomass. Under high biomass conditions a similar flux for210Pb may also be significant. An association of210Po with the organic cycle at the top of the sea, and with marine bacteria in particular, is suggested.  相似文献   

15.
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   

16.
By modelling the observed distribution of210Pb and210Po in surface waters of the Pacific, residence times relative to particulate removal are determined. For the center of the North Pacific gyre these are τPo = 0.6years andτPb = 1.7years. The surface ocean τPb is determined by particulate transport rather than plankton settling. The fact that it is about two orders of magnitude smaller than τPb for the deep ocean implies a sharp change in the adsorptive quality of particles during descent through the water column.  相似文献   

17.
The concentrations of214Pb (half-life=26.4minutes) and22Rn (half-life=3.84days) have been measured in deep groundwaters of Gujarat, India. The results show that the abundance of214Pb in the water is only ～25% of that expected from its production through the radioactive decay of dissolved222Rn. This deficiency if modelled in terms of a first-order removal, yields a residence time of ～10 minutes for214Pb in these waters. The estimated residence time for214Pb is the shortest observed for any nuclide in natural water systems and suggests that reactive nuclides lead like could be removed from aqueous phases to adjoining solid surfaces on extremely short time scales. Results of laboratory experiments using the212Pb-224Ra pair are compatible with the observed fast removal of214Pb from groundwaters.Re-evaluation of234Th residence times in these waters using a model with a recoil flux of234Th into aqueous phase, the same as that of222Rn, yields values in the range of 23 to<176 minutes, very similar to that of214Pb. This “concordancy” in the residence times seems to suggest that the geochemical behaviour of234Th and214Pb in these waters is quite similar.  相似文献   

18.
In 1977 and 1981 a hydrothermal plume was detected at the East Pacific Rise (EPR) near 8°45′N and at MANOP Site M, 25 km east of the EPR, by anomalous222Rn and Mn concentrations. In 1981, samples were also taken for210Pb,210Po and226Ra analyses to determine if enhanced scavenging of these elements occurred in the plume. At both the ridge crest and at Site M, the 210Pb/226Ra ratios range from 0.09 to 0.35, which are among the lowest values ever measured. It appears that removal of the210Pb is occurring by processes operating at or near the seafloor. There is also significant 210Po/210Pb disequilibrium at both locations, which appears to increase away from the seafloor towards the plume (as indicated by elevated Mn concentrations). An in-situ water column scavenging process is suggested by correlation of [210Pb-210Po] and Mn concentrations in the plume. The residence time of the210Po is only about 1 year, which is close to its residence time in surface waters but quite short compared to typical deep sea values. Thus rapid scavenging of the polonium in the plume and relatively rapid settling of particles from the plume is suggested.  相似文献   

19.
An experiment was designed to assess the relative importance of sediment accumulation and bioturbation in determining the vertical distribution of nuclides in estuarine sediments. A diver-collected core, 120 cm long, was raised from central Long Island Sound and analyzed down its length for:210Pb and226Ra;239, 240Pu; and Mn, Zn, Cu, and Pb. Sampling for chemical analysis was guided by X-radiography of the core. Excess210Pb (relative to226Ra) is roughly homogeneous in the top 2–4 cm of the core, then decreases quasi-exponentially to zero at (or above) 15 cm.239, 240Pu and excess Zn, Cu, and Pb, relative to background values at greater depths in the core, are distributed like excess210Pb in the top 10–15 cm. The absence of Mn enrichment at the top of the core, in contrast to other cores raised from this station, suggests that 1–3 cm of sediment was lost by erosion at the site of this core sometime prior to sampling. Below 15 cm excess210Pb and excess Zn, Cu, and Pb are found only in the bulk sample from 25 to 30 cm and in clearly identifiable burrow fillings dissected from 70 cm and 115 cm depth. Infilling of large burrows, excavated and then abandoned by crustaceans, is therefore a mechanism for transfer of surficial material to depth in these sediments.The bioturbation rate in the top several centimeters at this station has been determined previously using234Th (24-day half-life). The distribution of239, 240Pu can be used to estimate a bioturbation rate for the underlying layer (to ～10 cm depth); this rate is found to be 1–3% of the maximum mixing rate for the top 2–3 cm. Using these two mixing rates in a composite-layer, mixing + sedimentation model, the distribution of excess210Pb in the top 15 cm was used to constrain the sediment accumulation rate, ω. While the apparent rate of sediment accumulation (assuming no mixing below 2–4 cm) is 0.11 cm/yr, the model requires ω < 0.05 cm/yr. Thus in an area of slow sediment accumulation, a low rate of bioturbation below the surficial zone of rapid mixing causes an increase of at least a factor of two in apparent accumulation rate.  相似文献   

20.
In Funka Bay of Hokkaido, Japan, seawater, suspended matter and settling matter were collected once every month in the summer of 1974. These samples were analyzed for234Th, a short-lived daughter of dissolved238U. A pronounced disequilibrium between234Th and238U, and a highly variable concentration of234Th were found. Positive correlation, however, exist among the deficiency of234Th relative to238U in seawater, the concentration of particulate234Th, the fraction of particulate234Th to total234Th in seawater, the total dry weight of suspended matter, and the primary productivity during the month previous to sampling. The specific activity of234Th for the settling particles (620 ± 170 dpm/g) was nearly equal to that for suspended particles (720 ± 600 dpm/g) but much greater than that for plankton (47 ± 24 dpm/g). These facts suggest that suspended particles are somehow closely related to the removal of heavy metals from seawater, in spite of the negligibly small settling flux of suspended matter. The residence time of thorium in Funka Bay (mean depth: 60 m) is found to be about 60 days, which is nearly equal to those of210Pb and210Po.  相似文献   

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