210Pb and210Po during the destruction of stratification in the Dead Sea

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.     

210Pb/226Ra and 210Po/210Pb disequilibria in seawater and suspended particulate matter

The distribution of²¹⁰Po and²¹⁰Po in dissolved (<0.4 μm) and particulate (>0.4 μm) phases has been measured at ten stations in the tropical and eastern North Atlantic and at two stations in the Pacific. Both radionuclides occur principally in the dissolved phase. Unsupported²¹⁰Pb activities, maintained by flux from the atmosphere, are present in the surface mixed layer and penetrate into the thermocline to depths of about 500 m. Dissolved²¹⁰Po is ordinarily present in the mixed layer at less than equilibrium concentrations, suggesting rapid biological removal of this nuclide. Particulate matter is enriched in²¹⁰Po, with²¹⁰Po/²¹⁰Pb activity ratios greater than 1.0, similar to those reported for phytoplankton. Box-model calculations yield a 2.5-year residence time for²¹⁰Pb and a 0.6-year residence time for²¹⁰Po in the mixed layer. These residence times are considerably longer than the time calculated for turnover of particles in the mixed layer (about 0.1 year). At depths of 100–300 m,²¹⁰Po maxima occur and unsupported²¹⁰Po is frequently present. Calculations indicate that at least 50% of the²¹⁰Po removed from the mixed layer is recycled within the thermocline. Similar calculations for²¹⁰Pb suggest much lower recycling efficiencies.Comparison of the²¹⁰Pb distribution with the reported distribution of²²⁶Ra at nearby GEOSECS stations has confirmed the widespread existence of a²¹⁰Pb/²²⁶Ra disequilibrium in the deep sea. Vertical profiles of particulate²¹⁰Pb were used to test the hypothesis that²¹⁰Pb is removed from deep water by in-situ scavenging. With the exception of one profile taken near the Mid-Atlantic Ridge, significant vertical gradients in particulate²¹⁰Pb concentration were not observed, and it is necessary to invoke exceptionally high particle sinking velocities to account for the inferred²¹⁰Pb flux. It is proposed instead that an additional sink for²¹⁰Pb in the deep sea must be sought. Estimates of the dissolved²¹⁰Pb/²²⁶Ra activity ratio at depths greater than 1000 m range from 0.2 to 0.8 and reveal a systematic increase, in both vertical and horizontal directions, with increasing distance from the sea floor. This observation implies rapid scavenging of²¹⁰Pb at the sediment-water interface and is consistent with a horizontal eddy diffusivity of 3?6 × 10⁷ cm²/sec. The more reactive element Po, on the other hand, shows evidence of rapid in-situ scavenging. In filtered seawater,²¹⁰Po is deficient, on the average, by ca. 10% relative to²¹⁰Pb; a corresponding enrichment is found in the particulate phase. Total inventories of²¹⁰Pb and²¹⁰Po over the entire water column, however, show no significant departure from secular equilibrium.     

210Pb in the Pacific: the GEOSECS measurements of particulate and dissolved concentrations

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.     

226Ra, 210Pb and 210Po disequilibria in the Western North Pacific

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.     

226Ra and210Pb in the Weddell Sea

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.     

210Po and 210Pb distributions in ocean water profiles from the Eastern South Pacific

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.     

210Po and210Pb distributions in the central and eastern Indian Ocean

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.     

210Pb and226Ra distributions in the Circumpolar waters

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.     

Disequilibria betweenRa, Pb and Po in the Arctic Ocean and the implications for chemical modification of the Pacific water inflow

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.     

210Pb/226Ra disequilibrium in the Santa Barbara Basin

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.     

Disequilibria between 210Po and 210Pb in surface waters of the southern South China Sea and their implications

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 K_d 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).     

The distribution of 210Pb and 210Po in the surface waters of the Pacific Ocean

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 τ_Po = 0.6years andτ_Pb = 1.7years. The surface ocean τ_Pb is determined by particulate transport rather than plankton settling. The fact that it is about two orders of magnitude smaller than τ_Pb for the deep ocean implies a sharp change in the adsorptive quality of particles during descent through the water column.     

Geochemical behavior of 210Pb and 210Po in the nearshore waters off western Taiwan

Dissolved and particulate (210)Pb and (210)Po were determined at 15 stations along the coastline off western Taiwan in April 2007. The (210)Pb activities in dissolved and particulate phases fell within a relatively small range of 2.4-5.2 dpm 100 L(-1) and 1.0-3.2 dpm 100 L(-1), respectively. The dissolved and particulate (210)Po activities also fell within a small range of 0.8-3.4 dpm 100 L(-1) and 1.1-2.9 dpm 100 L(-1), respectively. The correlation of the distribution coefficients (K(d)) of (210)Pb and (210)Po with particle concentration in turbid waters are not as evident as in the open ocean. The mass balance calculation shows that the residence times of (210)Pb and (210)Po with respect to particle removal from the nearshore waters ranges from 3 to 15 days and from 14 to 125 days, respectively. The flux of particulate organic carbon was estimated by (210)Po proxy and ranged from 4.8 to 33.7 mmol-C m(-2) d(-1).     

210Pb in GEOSECS water profiles from the North Pacific

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.     

Particulate and soluble210Pb activities in the deep sea

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.     

230Th,226Ra and222Rn in abyssal sediments

A model that predicts the flux of²²²Rn out of deep-sea sediment is presented. The radon is ultimately generated by²³⁰Th which is stripped from the overlying water into the sediment. Data from many authors are compared with the model predictions. It is shown that the continental contribution of ionium is not significant, and that at low sedimentation rates, biological mixing and erosional processes strongly affect the surface concentration of the ionium. Two cores from areas of slow sediment accumulation, one from a manganese nodule region of the central Pacific and one from the Rio Grande Rise in the Atlantic were analyzed at closely spaced intervals for²³⁰Th,²²⁶Ra, and²¹⁰Pb. The Pacific core displayed evidence of biological mixing down to 12 cm and had a sedimentation rate of only 0.04 cm/kyr. The Atlantic core seemed to be mixed to 8 cm and had a sedimentation rate of 0.07 cm/kyr. Both cores had less total excess²³⁰Th than predicted.Radium sediment profiles are generated from the²³⁰Th model. Adsorbed, dissolved, and solid-phase radium is considered. According to the model, diffusional losses of radium are especially important at low sedimentation rates. Any particulate, or excess radium input is ignored in this model. The model fits the two analyzed cores if the fraction of total radium available for adsorption-desorption is about 0.5–0.7, and ifK, the distribution coefficient, is about 1000.Finally, the flux of radon out of the sediments is derived from the model-generated radium profiles. It is shown that the resulting standing crop of²²²Rn in the overlying water may be considered as an added constraint in budgeting²³⁰Th and²²⁶Ra in deep-sea sediments.     

Removal of234Th from a coastal sea: Funka Bay,Japan

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.     

226Ra behavior in the Pee Dee River-Winyah Bay estuary

Concentrations of dissolved²²⁶Ra in Winyah Bay, South Carolina, and in the adjacent Atlantic Ocean are augmented by the desorption of radium from sediments in the low-salinity area of the estuary and diffusion from bottom sediments. Desorption of²²⁶Ra is reflected by lower concentrations in suspended sediments from higher-salinity regions of the estuary. Bottom sediments from the high-salinity region have lower²²⁶Ra/²³⁰Th activity ratios than those from the low-salinity end.The shape of the dissolved²²⁶Ra vs. salinity profile is influenced by the river discharge. During average-discharge conditions, desorption of²²⁶Ra from suspended and bottom sediments increases the dissolved²²⁶Ra concentrations by a factor of 3.5 as the water passes through Winyah Bay. High river discharge produces an initial increase of dissolved²²⁶Ra by a factor of 2 to 3 and apparently reflects only desorption from suspended sediments. By driving the salt wedge down the estuary and reducing the zone of contact of salt water with bottom sediments, the high-flow conditions sharply reduce the flux of²²⁶Ra from bottom sediments.     

Correlation of210Po and210Pb enrichments in the sea-surface microlayer with neuston biomass

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.     

226Ra distribution in the Antarctic Ocean

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.