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210Pb and 210Po in the equatorial Pacific and the Bering Sea: The effects of biological productivity and boundary scavenging
Abstract
New data are presented for210Po and210Pb concentrations in seawater of the equatorial Pacific and the Bering Sea. Their implications are discussed in terms of productivity and boundary scavenging. The210Pb concentrations in the equatorial waters are high at the surface (0.10.15 d.p.m. 1−1, approximately twice the226Ra activity) with significant210Po deficits in the upper 1000 m water column ranging from 2.6 to 4.3 d.p.m. cm−2. Clearly210Po is preferentially removed relative to210Pb from the surface waters. Along the Tokyo-San Diego-Panama transect in the midNorth Pacific, the removal rate of210Po is correlated with the chlorophyll-a content in the surface waters, suggesting that scavenging of210Po to particles is virtually controlled by phytoplankton in the open ocean. In the deep water,210 Pb is always deficient relative to226Ra. Box-model210Pb residence times, based on mean 210Pb/226Ra activity ratios, are approximately 90 years at the open-ocean equatorial sites. These are significantly shorter than those of the North Pacific gyre (greater than 200 years). The short residence times can be ascribed to the intensified scavenging of210Pb because of the high particle flux regime of the equatorial zone. The deep-sea210Pb residence time becomes significantly shorter as the western topographic boundary is approached, and is only 8 years in the Bismarck Sea. This tendency cannot be ascribed to the difference in the particle flux in the equatorial zone. It seems that210Pb is actively taken up from the deep water by sediments at or near the bottom interface presumably in association with manganese redox cycling. The Bering Sea data support this mechanism. Compilation of deep-sea210Pb residence times available in the literature and the new data presented here suggests that the lateral transport of210Pb via isopycnal mixing followed by scavenging at the sediment-water interface is a major control on the210Pb-226Ra disequilibrium in the deep sea. The surface ocean productivity may be of secondary importance in this context.
... Naturally occurring 210 Po is a β − -decay product of 210 Pb via short-lived 210 Bi (T 1/2 = 5.0 days), and 210 Pb is produced throughout 226 Ra decay via several short-lived isotopes ( 222 Rn, 218 Po, 214 Pb, etc.). 210 Po mainly comes from in-situ decay of 210 Pb in seawater, and 210 Pb has three sources: atmospheric deposition, terrestrial runoff, and in situ production via 226 Ra decay (Bacon et al., 1976;Moore and Smith, 1986;Nozaki et al., 1997;Wei et al., 2011;Kaste and Baskaran, 2012). ...
... Some previous studies have shown a secular equilibrium is reached between 210 Pb and 210 Po in water deeper than 1000 m, such as the North Atlantic (Bacon et al., 1976), the South Pacific (Turekian and Nozaki, 1980), and the Indian Ocean (Chung and Finkel, 1988). However, a large deficiency of 210 Po has been found in deep waters of some seas, such as the East China Sea and the Philippine Sea (Nozaki et al., 1990), the equatorial Pacific and the Bering Sea (Nozaki et al., 1997), the Sargasso Sea (Kim and Church, 2001), and the South China Sea (Chung and Wu, 2005;Hong et al., 2013). In this study, most of the 210 Po in deep water around Prydz Bay are in equilibrium with 210 Pb, but there is an excess or deficiency of 210 Po at certain depths at some sites (Figure 4). ...
... Therefore, the 210 Po in CDW is more likely to be supplied by local source. In the process of remineralization of POM, both 210 Po and 210 Pb are released into the dissolved phase, but the priority of 210 Po can lead to an excess of 210 Po over 210 Pb, because more 210 Po is bound to organic matter (Shimmield et al., 1995;Nozaki et al., 1997;Wei et al., 2011). Therefore, the remineralization of particulate organic matter is most likely to be responsible for the high 210 Po and its excess in the CDW. ...
Due to the remoteness and difficulty of sampling, the ²¹⁰Po and ²¹⁰Pb data are scarce in the Southern Ocean. Here, the activity concentrations of ²¹⁰Po and ²¹⁰Pb around Prydz Bay in austral summer were determined to understand their spatial variation and evaluate the dynamics of particle organic matter (POM). The activity concentrations of dissolved ²¹⁰Po (D²¹⁰Po) and ²¹⁰Pb (D²¹⁰Pb) range from 0.47 to 3.20 Bq⋅m–3 and from 1.15 to 2.97 Bq⋅m–3, respectively, with the lower values in the shelf. The particulate ²¹⁰Po (P²¹⁰Po) and ²¹⁰Pb (P²¹⁰Pb) are lower in the open ocean and increase to the coastal waters, among which the circumpolar deep water (CDW) is the lowest. The activity concentration of total ²¹⁰Pb (T²¹⁰Pb) ranges from 1.26 Bq⋅m–3 to 3.16 Bq⋅m–3, with a higher value in CDW, which is ascribed to radiogenic production from ²²⁶Ra and subsequent lateral transport. Occasionally a high value of T²¹⁰Po occurs in deep water (>3.00 Bq⋅m–3), which may be caused by the remineralization of POM. The disequilibria between T²¹⁰Po and T²¹⁰Pb appears throughout the water column at most stations. The average T²¹⁰Po/T²¹⁰Pb)A.R. in the euphotic zone is 0.66, reflecting the effect of strong particle scavenging. There is a good positive correlation between the solid-liquid ratio of ²¹⁰Po and POC, while ²¹⁰Pb does not, indicating that particulate organic matter regulates the biogeochemical cycle of ²¹⁰Po around Prydz Bay. Based on the ²¹⁰Po/²¹⁰Pb disequilibria, the export flux of POC in the water column is estimated to be 0.8–31.9 mmol m–2 d–1, with the higher values in the shelf.
... The observed disequilibrium between and .3 years), as well as 210 Pb and Radium-226 ( 226 Ra, half-life: 1600 years), in marine system has been exploited to investigate several biogeochemical processes that include deep ocean scavenging of Pb, settling velocity of sinking particles, adsorption-desorption rate constants between particles and solution, export fluxes of particulate organic carbon, biogenic silica, and remineralization of sinking particulate matter (Craig et al., 1973;Bacon et al., 1976;Somayajulu and Craig, 1976;Nozaki et al., 1990Nozaki et al., , 1997Friedrich and Rutgers van der Loeff, 2002;Henderson and Maier-Reimer, 2002;Cochran and Masqué, 2003;Murray et al., 2005;Verdeny et al., 2009;Rigaud et al., 2015). The activity ratios of 210 Po/ 210 Pb and 210 Pb/ 226 Ra in the water column that differ widely in different zones (near sediment-water and air-sea interfaces) can be utilized to quantify the rates of processes that cause such disequilibria. ...
... From the vertical profiles of ( 210 Po/ 210 Pb) T and ( 210 Pb T / 226 Ra) AR from all deep-water stations ( Fig. 2d and e), the following general observations are made: i) in the upper ∼100 m, ( 210 Po/ 210 Pb) T ARs are < 1.0; ii) between ∼100 and ∼300 m, the ( 210 Po/ 210 Pb) T ARs are > 1.0; iii) at depths > 300 to the sediment-water interface, the ( 210 Po/ 210 Pb) T ARs are variable, with no discernable trend; iv) the vertical profiles of 210 Pb T / 226 Ra AR in the deep-water stations generally show 210 Pb T / 226 Ra AR < 1.0 at depths > 500 m; v) the 210 Pb T / 226 Ra AR > 1.0 from the air-sea interface down to ∼300 m, due to atmospheric source for 210 Pb; and vi) although the farthest station from the coast (ST-36) is expected to receive the lowest atmospheric depositional flux of 210 Pb, this is the most oligotrophic station with the lowest scavenging intensity and hence 210 Pb T / 226 Ra AR is higher (Table 5a). Overall vertical profiles of 210 Po and 210 Pb are similar to other published results from the Equatorial and South Pacific Nozaki et al., 1997), North Pacific (Nozaki and Tsunogai, 1976;Cochran et al., 1990), Indian Ocean (Cochran et al., 1983) and Atlantic Ocean (Bacon et al., 1976;Cochran et al., 1990;Rigaud et al., 2015). However, there are variations in the distribution of these nuclides in different basins, depending on the concentration and composition of particulate matter, rates of upwelling, diapcynal and isopycnal mixing of water, etc. ...
... This is evidenced by the observed highest inventories of dissolved nitrate, phosphate and silicate coinciding with the highest inventory of 210 Fig. 6a). Although it has been shown that there is a correlation between Chl. a and activity of 210 Po at discrete layers (Nozaki et al., 1997), in this study we show stronger inverse correlation between the inventories of 210 Po T and inventories of nutrients, including PO 4 3-. Higher nutrients in the surface waters promote higher biological productivity leading to larger removal of 210 Po by biogenic particulate matter which in turn results in lower inventory of 210 Po T . ...
The contrasting geochemical behavior of two long-lived progeny of ²²²Rn (daughter product of ²²⁶Ra), ²¹⁰Po and ²¹⁰Pb, provide valuable insights on the extent of recycling of biogenic particulate matter and their preferential removal from the water column. We collected and analyzed 135 water samples from six vertical profiles for ²¹⁰Po and ²¹⁰Pb as well as a suite of aerosol samples for ²¹⁰Pb along the US GEOTRACES East Pacific Zonal Transect (GP16). Calculated atmospheric depositional flux of ²¹⁰Pb from the measured ²¹⁰Pb activity in aerosols exhibits an overall decrease from east to west along the cruise track. The inventories of ²¹⁰Po and ²¹⁰Pb and residence times in key zones that include biotic (upper 60 and 300 m), sediment-water (bottom 300 m) and volcanogenic (bottom 300 m at the East Pacific Rise (EPR) station) were calculated and compared. A comparison of inventories of ²¹⁰Po and ²¹⁰Pb in 500-m layers (maximum penetration depth of atmospherically-delivered ²¹⁰Pb was reported to be upper < 500 m) for all stations indicates clearly discernable disequilibrium between total (= particulate + dissolved) ²¹⁰Po and total ²¹⁰Pb, with ²¹⁰Po/²¹⁰Pb activity ratios less than 0.95 or greater than 1.05, in 63% (26 out of 41) of the 500-m layers. The observed large variations in the ²¹⁰Po/²¹⁰Pb activity ratio (AR) in 500 m layers (range: 0.79–1.32) of the whole water column is by far the largest disequilibrium in open ocean water reported when integrated over 500 m depths. However, the whole water-column ²¹⁰Po/²¹⁰Pb AR, calculated from the inventory of total ²¹⁰Po and total ²¹⁰Pb, varied between 0.97 ± 0.02 and 1.06 ± 0.02 (except in the EPR site, with 1.09 ± 0.02), indicating at or near-equilibrium for the whole water column in all five deep water stations. This contrasts with earlier claims of gross disequilibrium between ²¹⁰Po and ²¹⁰Pb in water depths >1000 m and we contend that most of these observations do not take in to account the variable extent of redistribution of ²¹⁰Po and ²¹⁰Pb in the water column due to remineralization of biogenic particulate matter and preferential scavenging of ²¹⁰Po over ²¹⁰Pb. We observed intense scavenging of ²¹⁰Po and ²¹⁰Pb in the bottom 300 m of the EPR site (attributed to discharge of high amounts of dissolved and colloidal Fe and Mn from the hydrothermal vent) and lower scavenging in the upper and bottom 300 m at the most oligotrophic station (ST-36), indicating that the scavenging intensities in bottom waters are likely coupled to the surface waters.
... To evaluate the degree of deviation of the secular equilibrium, the SiO 2 − 226 Ra correlation, 226 Ra (dpm 100 L −1 ) = 5.15 + 0.14 SiO 2 (µmol L −1 ), which is based on the data determined at station PA-11 by Nozaki and Yamamoto (2001) (Fig. 1 (Bacon et al., 1976), the average 210 Pb t / 226 Ra ratio decreases dramatically in the thermocline, then maintains a constant value of 0.5 below 1000 m. Compared to the values summarized in the compilation of Nozaki et al. (1997), the 210 Pb t / 226 Ra ratio is higher than that found in the Bay of Bengal (Cochran et al., 1983;Sarin et al., 1994), the Bismarck Sea (Nozaki et al., 1997), and the Sea of Japan (Nozaki et al., 1973) but lower than in the deep water of the North Pacific (Craig et al., 1973;Nozaki et al., 1980) and the North Atlantic (Bacon et al., 1976). Minor differences of the ratio seem to exist among cruises, however, the variation is too small to reveals apparent temporal variation. ...
... To evaluate the degree of deviation of the secular equilibrium, the SiO 2 − 226 Ra correlation, 226 Ra (dpm 100 L −1 ) = 5.15 + 0.14 SiO 2 (µmol L −1 ), which is based on the data determined at station PA-11 by Nozaki and Yamamoto (2001) (Fig. 1 (Bacon et al., 1976), the average 210 Pb t / 226 Ra ratio decreases dramatically in the thermocline, then maintains a constant value of 0.5 below 1000 m. Compared to the values summarized in the compilation of Nozaki et al. (1997), the 210 Pb t / 226 Ra ratio is higher than that found in the Bay of Bengal (Cochran et al., 1983;Sarin et al., 1994), the Bismarck Sea (Nozaki et al., 1997), and the Sea of Japan (Nozaki et al., 1973) but lower than in the deep water of the North Pacific (Craig et al., 1973;Nozaki et al., 1980) and the North Atlantic (Bacon et al., 1976). Minor differences of the ratio seem to exist among cruises, however, the variation is too small to reveals apparent temporal variation. ...
... Recent results of an inter-calibration project also showed a significant deficiency of 210 Po relative to 210 Pb in the deep water, with a 210 Po / 210 Pb ratio of 0.74 ± 0.06 at 2000 m at BATS in the North Atlantic and 0.80 ± 0.11 at 3000 m at the baseline station in the North Pacific, respectively (Church et al., 2012). It should be noted that, contrary to the 210 Po deficiencies in these environments, the 210 Po t / 210 Pb t ratio is commonly close to unity in the deep layer (> 1000 m) of the ocean (Bacon et al., 1976(Bacon et al., , 1988Cochran et al., 1983;Nozaki et al., 1997). ...
Vertical distributions of dissolved and particulate ²¹⁰Pb and ²¹⁰Po in the water column at the SouthEast Asian Time-series Study (SEATS, 18°00' N and 116°00' E) station in the northern South China Sea were determined from 4 cruises between January 2007 and June 2008. A large deficiency of ²¹⁰Pb, 379 ± 43 × 10³ dpm m⁻², from the secular equilibrium was found within the 3500 m water column. On the other hand, a smaller deficiency of ²¹⁰Po, 100 ± 21 × 10³ dpm m⁻², relative to ²¹⁰Pb was found in the water column. Time-series data showed insignificant temporal variability of the ²¹⁰Pb and ²¹⁰Po profiles. To balance these deficiencies, the removal fluxes for ²¹⁰Pb and ²¹⁰Po via particle settling range from 45 to 51 dpm m⁻² d⁻¹ and from 481 to 567 dpm m⁻² d⁻¹, respectively, are expected at 3500 m. The ²¹⁰Pb removal flux is comparable with, whereas the ²¹⁰Po removal flux is much higher than, the flux directly measured by moored sediment traps. The discrepancy between the modeled ²¹⁰Po flux and the measured flux suggests that sporadic events that enhance ²¹⁰Po removal via sinking ballast may occur in the water column at the site.
... Both 210 Pb and 210 Po are particle-reactive radionuclides, resulting in their quick removal from seawater. Owing to the much shorter resident timescale compared to the time required for reaching equilibrium, the deficit of 210 Po to 210 Pb is ubiquitous in the upper ocean (Bacon et al., 1976;Nozaki et al., 1997Nozaki et al., , 1998Kim, 2001). Furthermore, the extent of the deficit of 210 Po (over that expected from 210 Pb decay) is expected to quantitatively relate to the particle removal rate from the upper ocean. ...
... Although the 210 Po- 210 Pb pair has been increasingly used in recent years, specific particle type and functionalities that are being tracked by this proxy are still unclear. For example, field observations revealed close relationships between 210 Po and particulate organic matter (Murray et al., 2005;Stewart et al., 2007b) or Chl-a (Nozaki et al., 1997). Laboratory experiments also reported significant influence of selected organic http://dx.doi.org/10.1016/j.epsl.2015.05.007 0012-821X/© 2015 Elsevier B.V. All rights reserved. ...
... These results provided valuable insights into the discriminative geochemical behavior between 210 Pb and 210 Po in marine environments though both 210 Pb and 210 Po are particle-reactive nuclides (e.g., Masqué et al., 2002;Yang et al., 2011bYang et al., , 2011c. In previous field studies, a positive correlation between the removal rate of 210 Po and chlorophyll-a (Nozaki et al., 1997(Nozaki et al., , 1998 and negative correlation between particulate 210 Po activity and fluorescence (Masqué et al., 2002) were observed, indicating that either phytoplankton or biogenic particulate organic matter are probably responsible for the scavenging of 210 Po. In addition, Kim (2001) and Kim and Church (2001) reported the influence of nitrogen-fixation on the partition of 210 Po between particulate and dissolved phases. ...
Improved applications of 210Po, 210Pb and 7Be as geochemical proxies require more detailed understanding of their interactions with particles. Here, laboratory sorption experiments were carried out to examine the adsorption of 210Po, 210Pb and 7Be and their fractionation on inorganic nanoparticles, including SiO2, CaCO3, Al2O3, TiO2 and Fe2O3, in the presence or absence of macromolecular organic compounds (MOCs) that include humic acids (HA), acid polysaccharides (APS) and proteins (BSA), in natural seawater. Results showed that nanoparticle sorption was not greatly enhanced over that of microparticles as would be expected from their much higher specific surface areas, likely indicating their aggregation in seawater. It was found that synergistic interactions between inorganic nanoparticles, MOCs, and radionuclides determined the sorption, although their adsorption was particle composition-dependent. MOCs enhanced the sorption of selected nuclides on most nanoparticles. On average, in the presence of MOCs, partition coefficients ( ) of 210Po, 210Pb, and 7Be on nanoparticles increased 2.9-, 5.0- and 5.9-fold, respectively. The effect of MOCs could be explained for 210Po and 210Pb from their different values on inorganic nanoparticles. In addition, fractionation effects between 210Po and 210Pb (or between 210Pb and 7Be) could be quantified from their relative values on end-member sorbent components. Applications of both 210Po–210Pb and 7Be–210Pb pairs as particle dynamics tracers could be more quantitative when the nature of the organic coatings is taken into account.
... Manuscript submitted to JGR:Biogeosciences Deriving a meaningful 'average' dPb trend for estuaries is inevitably a challenging exercise given the uneven distribution of studied sites with a bias towards northern Europe and North America, and the likelihood that the majority of sites reported in the literature have been strongly affected by historical anthropogenic Pb deposition. Both within estuaries and more generally across aquatic environments, scavenging of dPb onto particle surfaces is however a key feature of dPb distribution (Brügmann et al., 1985;Nozaki et al., 1997). Scavenging is driven both by cellular uptake or absorption onto cell walls, and by adsorption and co-precipitation of dPb onto Fe and Mn oxides (Nozaki et al., 1997;Windom et al., 1985). ...
... Both within estuaries and more generally across aquatic environments, scavenging of dPb onto particle surfaces is however a key feature of dPb distribution (Brügmann et al., 1985;Nozaki et al., 1997). Scavenging is driven both by cellular uptake or absorption onto cell walls, and by adsorption and co-precipitation of dPb onto Fe and Mn oxides (Nozaki et al., 1997;Windom et al., 1985). A high fraction of labile particulate Pb is bound to Fe/Mn oxides in some freshwater systems (Tessier et al., 1980) and dPb concentrations can thus be tightly coupled to dissolved Fe concentrations (Erel et al., 1991;Erel & Morgan, 1992). ...
Higher than expected concentrations of dissolved lead (dPb) have been consistently observed along glaciated coastlines and it is widely hypothesized that there is a net release of dPb from glacier‐derived sediments. Here we further corroborate that dPb concentrations in diverse locations around west Greenland (3.2–252 pM) and the Western Antarctic Peninsula (7.7–107 pM) appear to be generally higher than can be explained by addition of dPb from glacier‐derived freshwater. The distribution of dPb across the salinity gradient is unlike any other commonly studied trace element (e.g., Fe, Co, Ni, Cu, Mn, and Al) implying a dynamic, reversible exchange between dissolved and labile particulate Pb. Incubating a selection of glacier‐derived particles from SW Greenland (Ameralik and Nuup Kangerlua) and Svalbard (Kongsfjorden), with a range of labile particulate Pb (LpPb) content (11–113 nmol g⁻¹), the equivalent of 2–46% LpPb was released as dPb within 24 hr of addition to Atlantic seawater. Over longer time periods, the majority of this dPb was typically readsorbed. Sediment loading was the dominant factor influencing the net release of dPb into seawater, with a pronounced decline in net dPb release efficiency when sediment load increased from 20 to 500 mg L⁻¹. Yet temperature also had some effect with 68 ± 22% higher dPb release at 11°C compared to 4°C. Future regional changes in dPb dynamics may therefore be more sensitive to short‐term suspended sediment dynamics, and potentially temperature changes, than to changing interannual runoff volume.
... In general, 210 Po is expected to be in equilibrium with 210 Pb in the deep ocean due to the absence of a net downward 210 Po flux, weak particle dynamics, and its long residence times. Equilibrium between 210 Po and 210 Pb has been observed in deep waters of the open ocean, such as in the North Atlantic (Bacon et al., 1976), the Equatorial Pacific (Nozaki et al., 1997), and the eastern Pacific (Turekian and Nozaki, 1980). However, 210 Po deficiencies relative to 210 Pb in deep waters have also been observed in the Aleutian Basin (Hu et al., 2014), the Philippine Sea (Nozaki et al., 1990) and the Sargasso Sea (Kim, 2001), which were attributed to the lateral transport of particles, plumes of methane, and consumption by bacteria or cyanobacteria and the subsequent transfer of 210 Po to higher trophic levels in the food web, respectively. ...
... Therefore, the seasonal fluctuations of atmospheric 210 Po deposition is ignored in this study during the estimation of 210 Po export flux. The residence time of dissolved (τ DPo ), particulate (τ PPo ), and total (the sum of dissolved and particulate phases; τ TPo ) 210 Po in the designated depths (0-100 m and 100-1000 m) with respect to its scavenging and removal by total particulate matter can be calculated using Eq. 4 (Bacon et al., 1976;Shimmield et al., 1995;Nozaki et al., 1997;Yang et al., 2009): ...
... Distribution of 210 Po and 210 Pb in marine and freshwater systems have aided in examining their partitioning between dissolved and particulate phases as well as particle dynamics over the past several decades (Cochran and Masque, 2003;Chai and Urban, 2004;Rutgers van der Loeff and Geibert, 2008;Mudbidre et al., 2014;Rigaud et al., 2015). For example, the disequilibrium between 210 Po and 210 Pb has been utilized to estimate the dissolved-particulate exchange rates (e.g., scavenging rates, Nozaki et al., 1997;Murray et al., J o u r n a l P r e -p r o o f powerful tracer. Therefore, the sorption behavior of 210 Bi to naturally-occurring minerals (e.g., kaolinite, smectite) needs to be investigated. ...
... Details on the calculation of POC export under steady and non-steady state conditions are summarized inVerdeny et al. (2009). Note that the present study as well as earlier studies(Nozaki et al., 1997;Murray et al., 2005; Yang et al., 2013 Yang et al., , 2015 showed that the 210 Po/ 210 Pb activity ratio in the particulate phase varied widely. Therefore, the use of 210 Po/ 210 Pb ratio as a tracer for particle dynamics requires that we consider the composition of the particulate matter. ...
The long-lived progeny of radon-222, lead-210 (210Pb), polonium-210 (210Po) and bismuth-210 (210Bi), serve as a tool in the study of marine biogeochemical processes. Here, we report our investigation on the partitioning of 210Pb, 210Po and 210Bi (207Bi) between seawater and solids (twelve commonly-occurring mineral particles) under different values of pH, salinity and contact time. The Kd values of 210Pb and 207Bi for MnO2 varied widely between the synthetic and natural minerals, and thus the suitability of using synthetic material for their partitioning studies in marine system needs to be re-assessed. The Kd values of 210Pb in clay minerals were found to be in the order: smectite > illite > kaolinite. The sorption efficiencies (=the particulate activity divided by the sum of particulate and dissolved activity) of 210Pb, 207Bi and 210Po on illite were above 81% when the contact time was >2 h. These radionuclides were more soluble when pH was ≤5, but show more particle-reactive nature for pH of6 to 8. The fractionation factor between 210Po and 210Pb (FPo/Pb) varied with salinity for different mineral components while the FBi/Pb was less affected by mineral components, salinity and pH. The in-situ experimental results showed that the FBi/Pb increased with increase in Chl-a concentration. Considering the much shorter half-life of 210Bi (5.0 d) and the high Kd values established in this study, we propose that 210Bi/210Pb activity ratio could potentially be used to estimate the particle export from phytoplankton bloom on time scale much shorter than 234Th or 210Po.
... However, in the Arctic, the inputs of 210 Po to the ocean surface due to sea-ice melt could lead to greater 210 Po/ 210 Pb ratios resulting from the ingrowth of 210 Po from 210 Pb decay in sea ice (range: 0.4-1.0 in sea-ice cores; Masqué et al., 2007;Roca-Martí et al., 2016). Then, a preferential removal of 210 Po over 210 Pb by particle export in surface waters (Nozaki et al., 1997) may explain the non-significant enrichment of 210 Po in the mixed layer. ...
... A deficiency of 210 Po in the ocean interior has also been observed in other oceanic regions (e.g. Kim and Church, 2001;Nozaki et al., 1997Nozaki et al., , 1990Sarin et al., 1994;Thomson and Turekian, 1976), but its origin is still unclear. Hence, while this study cannot elucidate the reasons for these deficits, the interpretation of the 210 Po/ 210 Pb pair at depth remains uncertain until further investigations are made. ...
Pb and ²¹⁰Po are naturally occurring radionuclides that are commonly used as a proxy for particle and carbon export. In this study, the distribution of the ²¹⁰Po/²¹⁰Pb pair was investigated in the water column of the Barents, Kara and Laptev Seas and the Nansen, Amundsen and Makarov Basins in order to understand the particle dynamics in the Arctic Ocean during the 2007 sea-ice minimum (August–September). Minimum activities of total ²¹⁰Pb and ²¹⁰Po were found in the upper and lower haloclines (approx. 60–130 m), which are partly attributed to particle scavenging over the shelves, boundary current transport and subsequent advection of the water with low ²¹⁰Pb and ²¹⁰Po activities into the central Arctic. Widespread and substantial (> 50%) deficits of ²¹⁰Po with respect to ²¹⁰Pb were detected from surface waters to 200 m on the shelves, but also in the basins. This was particularly important in the Makarov Basin where, despite very low chlorophyll-a levels, estimates of annual new primary production were three times higher than in the Eurasian Basin. In the Nansen, Amundsen and Makarov Basins, estimates of annual new primary production correlated with the deficits of ²¹⁰Po in the upper 200 m of the water column, suggesting that in situ production and subsequent export of biogenic material were the mechanisms that controlled the removal of ²¹⁰Po in the central Arctic. Unlike ²¹⁰Po, ²³⁴Th deficits measured during the same expedition were found to be very small and not significant below 25 m in the basins (Cai et al., 2010), which indicates, given the shorter half-life of ²³⁴Th, that particle export fluxes in the central Arctic would have been higher before July–August in 2007 than later in the season.
... On the other hand, 210 Po is more of a biological removal and is actively involved in the biogeochemical cycle compared to 210 Pb (Tsunogai and Nozaki, 1971). The removal of 210 Pb is probably by the scavenging of particulate matter from the sea surface into the deep sea and absorption via sediment-water interface interaction (Nozaki et al., 1997). ...
... Chlorophyll-a content in water column has been utilized as an indicator of assessing abundance of phytoplankton biomass (Nozaki et al., 1997). The simultaneous measurements of chlorophyll-a, 210 Po and 210 Pb in water column were performed in this study with purpose to study the biogeochemistry behavior of these well known as particles reactive nuclides at shallow water region. ...
210Po and 210Pb activities were determined in dissolved and particulate phases in order to understand the behavior of both natural radionuclides in shallow water regions such as Mersing Estuary. Strong statistical correlations between the distribution coefficient values of polonium and lead in dissolved phases with SPM proved that the natural nuclides of polonium and lead have a high affinity to suspended particle materials in the water column, where the SPM acts as a carrier to transport and remove natural isotopes of polonium and lead from their geochemical behavior. However a low statistical correlation (r=0.414) found between chlorophyll-a with an activity ratio of 210Po/210Pb and SPM implies that the enrichment of 210Po was not associated with the abundance of chlorophyll-a. But a strong correlation between soluble reactive phosphorus (SRP) and 210Po in the dissolved phase proved that the contribution of phosphate element as catalysis increasing the activity levels of 210Po at coastal waters.
... This value was obviously lower than that in the Bering Sea, which varied from 0.38 to 3.29 Bq/m 3 with an average of 1.42 Bq/m 3 (our unpublished data) and most of the global oceans (0.56-5.17 Bq/m 3 ) (Nozaki and Tsunogai, 1976;Thomson and Turekian, 1976;Chung, 1981;Chung and Craig, 1983;Nozaki et al., 1997;Kim, 2001;Yang et al., 2009); however, it was slightly higher than D 210 Pb (0.1-0.95 Bq/m 3 ) in the Alpha Ridge reported by Moore and Smith (1986). ...
... The residence times of D 210 Pb in the shelf ranged from 1.06 to 2.28 a (Table 3), comparable to those reported in the Arctic Ocean by Smith et al. (2003) and Lepore et al. (2009). They were lower than those in the Atlantic (2.5 a) (Bacon et al., 1976;Sarin et al., 1999), equatorial Pacific (2.4-8 a) (Nozaki et al., 1997;Murray et al., 2005), northwestern Mediterranean Sea (3 a) (Masqué et al., 2002), and Prydz Bay in Antarctica (1.44-3.79 a, averaging 2.20 a) (Yang, 2005). ...
Pb-210 was measured during the 3rd Chinese National Arctic Research Expedition cruise to investigate its spatial pattern in the western Arctic Ocean, as well as its relation with the thermocline in the Canadian Basin. The specific activities varied from 0.04 to 2.72, < 0.013 to 4.37, and 0.1 to 4.85 Bq/m(3) for dissolved, particulate, and bulk Pb-210, respectively, corresponding to respective averages of 0.65, 0.43, and 1.08 Bq/m(3). In the Canadian Basin, the minimum Pb-210 activities occurred in the thermocline, which was characterized by low temperature of -1.52A degrees C and salinity of 33.1. Combining the spatial distribution of Pb-210 and hydrographical characteristics in the western Arctic Ocean, this scenario was ascribed to the effective scavenging of Pb-210 when the Pacific water flowed across the Chukchi Shelf. Quantitatively, this interpretation was supported by both the shorter residence times and higher scavenging efficiencies (SE) of dissolved Pb-210 over the Chukchi Shelf. The highest SE values were observed in the Herald Shoal and bottom waters over the slope.
... Similar to 7 Be, both 210 Pb and 210 Po are easily adsorbed by atmospheric particles of sub-micron scale, and then reach the Earth's surface via dry and wet deposition and are finally buried in sediments (Peirson et al., 1966;Baskaran, 2011). In the absence of 210 Po from sources other than the radioactive decay of atmospheric 222 Rn-derived 210 Pb, the 210 Po/ 210 Pb activity ratio (AR) usually can be utilized to determine the age of aerosols, particles, snow, melt pond, ice core, etc. (Moore et al., 1973;Robbins, 1978;Nozaki et al., 1997;Marley et al., 2000;Baskaran and Santschi, 2002;Huang, 2012;Du et al., 2016;Baskaran and Krupp, 2021). ...
The natural radioisotopes ⁷Be, ²¹⁰Pb, and ²¹⁰Po, with different half-lives, are all particle-reactive and serve as natural tracers to study sources and transportation of sediments, sedimentation rates, and sediment chronology. Atmospheric deposition of these radioisotopes is the premise and foundation of their tracing application. The Maowei Sea is a semi-closed bay along the Beibu Gulf, which is an important gulf in the northwest of the South China Sea, but the atmospheric deposition of the abovementioned radioisotopes has not been systematically reported along the coast. In this research paper, the atmospheric depositional fluxes of ⁷Be, ²¹⁰Pb, and ²¹⁰Po were observed over a period from June 2018 to December 2021 on the coast of the Maowei Sea. The annual atmospheric depositional fluxes (Bq m⁻² yr⁻¹) of ⁷Be, ²¹⁰Pb, and ²¹⁰Po on the coast of the Maowei Sea were 496.80, 201.72, and 58.08, respectively. The distributions for ⁷Be and ²¹⁰Pb depositional fluxes during a whole year (years 2019 and 2021) followed a bimodal pattern, with one peak from February to April and another peak from August to October, while the distribution for ²¹⁰Po depositional flux showed only one peak during the year of 2019 and another during 2021. The deposition flux and activity of ⁷Be showed a strong positive correlation with the deposition flux and activity of ²¹⁰Pb, respectively; the deposition flux and activity of ²¹⁰Po also showed positive correlations with the deposition fluxes and activities of ²¹⁰Pb and ⁷Be, respectively, indicating a similarly scavenging behavior from the atmosphere. A Pearson correlation matrix was used to illustrate the factors influencing the atmospheric depositions and found that precipitation, air quality index (AQI), and PM (both PM2.5 and PM10) were the major factors that influenced the deposition of these three radionuclides. Precipitation had significant positive correlations with the deposition fluxes of all three radionuclides, indicating that, for these radionuclides, rainfall was the main scavenging way from the atmosphere. The observations for specific single rainfall events and their air mass backward trajectory analyses showed that the air masses movement during the rainfall may be another important factor that impacted the depositional fluxes for ⁷Be, ²¹⁰Pb, and ²¹⁰Po.
... The focusing factors along the four hadal trench axes were derived by applying the same pelagic data and generally expressed elevated values, ranging from 1.3 (A2) to 3.5 (K6) and with an average of 2.3 ± 1.1 (Table 3). We cannot exclude different deficient 210 Pb inventories in waters above the respective trench regions due to intensified 210 Pb scavenging along the trench boundaries (e.g., Nozaki et al., 1997), effects of seasonal and local variabilities of primary productivity (Cochran, 1992) (Table 3). However, these effects are expected to be minor and the t-test for the focusing factors between abyssal plain group (A7, K7, and M1) and hadal trench groups (A10, A2, A3, A4, A5, A6, KK1, KK4, KK9, KK11, K4, K6, and M2) resulted in p-value of 0.06, supporting the concept of intensified focusing along trench axes. ...
Hadal trenches are considered to act as depocenters for organic material, although pathways for the material transport and deposition rates are poorly constrained. Here we assess focusing, deposition and accumulation of material and organic carbon in four hadal trench systems underlying different surface ocean productivities; the eutrophic Atacama and Kuril‐Kamchatka trenches, the mesotrophic Kermadec trench and the oligotrophic Mariana Trench. The study is based on the distributions of naturally occurring ²¹⁰Pbex, ¹³⁷Cs and total organic carbon from recovered sediment cores and by applying previously quantified benthic mineralization rates. Periods of steady deposition and discreet mass‐wasting deposits were identified from the profiles and the latter were associated with historic recorded seismic events in the respective regions. During periods without mass wasting, the estimated focusing factors along trench axes were elevated, suggesting more or less continuous downslope focusing of material toward the interior of the trenches. The estimated organic carbon deposition rates during these periods exhibited extensive site‐specific variability, but were generally similar to values encountered at much shallower settings such as continental slopes and margins. Organic carbon deposition rates during periods of steady deposition were not mirrored by surface ocean productivity, but appeared confounded by local bathymetry. The inclusion of deposition mediated by mass‐wasting events enhanced the sediment and organic carbon accumulations for the past ∼150 years by up to a factor of ∼4. Thus, due to intensified downslope material focusing and infrequent mass‐wasting events, hadal trenches are important sites for deposition and sequestration of organic carbon in the deep sea.
... Sorption experiments have confirmed that 210 Po can be incorporated into proteins (Yang et al., 2015). Such behavior helps account for observed links between chlorophyll α (as a proxy for living organic matter), productivity, POC concentration, and the rates of Po scavenging and regeneration from sinking particles (Choi et al., 2014;Nozaki et al., 1997). Polonium is classed with the Group 16 metalloids such as selenium and tellurium and may share biogeochemical behavior with them. ...
Deposition of aerosols to the surface ocean is an important factor affecting primary production in the surface ocean. However, the sources and fluxes of aerosols and associated trace elements remain poorly defined. Aerosol ²¹⁰Pb, ²¹⁰Po, and ⁷Be data were collected on US GEOTRACES cruise GP15 (Pacific Meridional Transect, 152°W; 2018). ²¹⁰Pb fluxes are low close to the Alaskan margin, increase to a maximum at ∼43°N, then decrease to lower values. There is good agreement between ²¹⁰Pb fluxes and long‐term land‐based fluxes during the SEAREX program (1970–1980s), as well as between GP15 and GP16 (East Pacific Zonal Transect, 12°S; 2013) at adjacent stations. A normalized fraction f(⁷Be, ²¹⁰Pb) is used to discern aerosols with upper (high f) versus lower (low f) troposphere sources. Alaskan/North Pacific aerosols show significant continental influence while equatorial/South Pacific aerosols are supplied to the marine boundary layer from the upper troposphere. Lithogenic trace elements Al and Ti show inverse correlations with f(⁷Be, ²¹⁰Pb), supporting a continental boundary layer provenance while anthropogenic Pb shows no clear relationship with f(⁷Be, ²¹⁰Pb). All but four samples have ²¹⁰Po/²¹⁰Pb activity ratios <0.2 suggesting short aerosol residence time. Among the four samples (²¹⁰Po/²¹⁰Pb = 0.42–0.88), two suggest an upper troposphere source and longer aerosol residence time while the remaining two cannot be explained by long aerosol residence time nor a significant component of dust. We hypothesize that enrichments of ²¹⁰Po in them are linked to Po enrichments in the sea surface microlayer, possibly through Po speciation as a dissolved organic or dimethyl polonide species.
... It has also been recurrently used as a tracer for particle cycling and export processes in ocean surfaces, which are known to play a critical role as biogeochemical components of the ocean's carbon balance system (Murray et al. 2005). This research on particle dynamics is usually done by applying the formula of radioactive de ciency of 210 Pb relative to its parent 226 Ra to estimate scavenging rates in the deep sea (Craig et al. 1973) and subsequently to quantify boundary scavenging at the edges of ocean basins (Nozaki et al. 1997). Recently, as the main progeny of 222 Rn, 210 Pb has been extensively used as powerful tracers to understand atmospheric processes such as for sources of continental dust and transport time scales of air masses (Baskaran 2011). ...
Radiochemical separation techniques were applied to measure ²¹⁰ Pb radioactivity in the samples to determine spatial distribution of total ²¹⁰ Pb radionuclide in surface sediments and water column for the main waters on the Sunda Shelf. The average ²¹⁰ Pb radioactivity level recorded from the Malacca Strait (MS) is significantly higher than the southern South China Sea (sSCS); this is attributed to the supply of input from the Andaman Sea in its northern areas, and massive terrestrial processes from bordering land masses. The results reveal that the decreasing trend of total ²¹⁰ Pb in Peninsular Malaysia’s marine sediment starts with the north maritime zone of Malacca Strait (64.28 ± 5.97 Bq/kg) > south maritime zone of Malacca Strait (50.88 ± 6.15 Bq/kg) > east maritime zone of sSCS (41.01 ± 3.07 Bq/kg) > south maritime zone of sSCS (40.78 ± 3.16 Bq/kg). The Kelantan and Pahang Deltas have been identified as two of the main anthropogenic sources of input for total ²¹⁰ Pb in the sSCS while atmospheric transboundary mobilization affecting total ²¹⁰ Pb in water column. In the Malacca Strait, however, the distribution of total ²¹⁰ Pb might be influenced mainly by in situ processes of the strait’s seafloor and sources origin of sediment.
... In the particle-poor deep ocean, radioactive equilibrium between 210 Pb and 210 Po is anticipated because of the long scavenging residence times and short half-life of 210 Po (e.g., Bacon et al., 1976;Cochran et al., 1983). Yet, large deficits of 210 Po have been observed in the mesopelagic (∼ 100-1000 m) and bathypelagic (>1000 m) zones in different regions of the world ocean, including the North Atlantic (e.g., Kim and Church, 2001;Hong et al., 2013;Rigaud et al., 2015), the North, Equatorial and South Pacific (e.g., Thomson and Turekian, 1976;Nozaki et al., 1990Nozaki et al., , 1997Chung and Wu, 2005;Hu et al., 2014), the Arctic Ocean (e.g., Smith et al., 2003;Roca-Martí et al., 2018), and the Southern Ocean (e.g., Friedrich and Rutgers van der Loeff, 2002). Disequilibrium in deep waters has been commonly attributed to the scavenging of 210 Po by particles from the local upper water column or from the shelves due to high biological productivity (Hu et al., 2014;Rigaud et al., 2015;Ma et al., 2017). ...
The disequilibrium between lead-210 (²¹⁰Pb) and polonium-210 (²¹⁰Po) is increasingly used in oceanography to quantify particulate organic carbon (POC) export from the upper ocean. This proxy is based on the deficits of ²¹⁰Po typically observed in the upper water column due to the preferential removal of ²¹⁰Po relative to ²¹⁰Pb by sinking particles. Yet, a number of studies have reported unexpected large ²¹⁰Po deficits in the deep ocean indicating scavenging of ²¹⁰Po despite its radioactive mean life of ∼ 200 days. Two precipitation methods, Fe(OH)3 and Co-APDC, are typically used to concentrate Pb and Po from seawater samples, and deep ²¹⁰Po deficits raise the question whether this feature is biogeochemically consistent or there is a methodological issue. Here, we present a compilation of ²¹⁰Pb and ²¹⁰Po studies that suggests that ²¹⁰Po deficits at depths >300 m are more often observed in studies where Fe(OH)3 is used to precipitate Pb and Po from seawater, than in those using Co-APDC (in 68 versus 33% of the profiles analyzed for each method, respectively). In order to test whether ²¹⁰Po/²¹⁰Pb disequilibrium can be partly related to a methodological artifact, we directly compared the total activities of ²¹⁰Pb and ²¹⁰Po in four duplicate ocean depth-profiles determined by using Fe(OH)3 and Co-APDC on unfiltered seawater samples. While both methods produced the same ²¹⁰Pb activities, results from the Co-APDC method showed equilibrium between ²¹⁰Pb and ²¹⁰Po below 100 m, whereas the Fe(OH)3 method resulted in activities of ²¹⁰Po significantly lower than ²¹⁰Pb throughout the entire water column. These results show that ²¹⁰Po deficits in deep waters, but also in the upper ocean, may be greater when calculated using a commonly used Fe(OH)3 protocol. This finding has potential implications for the use of the ²¹⁰Po/²¹⁰Pb pair as a tracer of particle export in the oceans because ²¹⁰Po (and thus POC) fluxes calculated using Fe(OH)3 on unfiltered seawater samples may be overestimated. Recommendations for future research are provided based on the possible reasons for the discrepancy in ²¹⁰Po activities between both analytical methods.
... In general, the residence time increased with distance offshore and the higher productive area had lower residence time, which can be attributed to more scavenging. It has been shown that lower 210 Po residence times correspond to higher chlorophyll and productivity in the surface water (Nozaki et al., 1997;Murray et al., 2005). Overall, the mean residence time for 210 Po is 2.18 ± 0.47 y, which is in the range of previously reported studies from varying oceanic settings such as the Mediterranean Sea (Masque et al., 2002), BATS , and the river dominated South China Sea (Wei et al., 2014). ...
An important pathway for CO 2 sinking in the ocean is via the "biological pump", driven by the production of particulate organic carbon (POC) in the surface ocean and its subsequent export to the deeper ocean via vertical settling. Here we examine the vertical fluxes of POC in the continental slope of the northern Gulf of Mexico by utilizing the short-lived radionuclide pair 210 Po-210 Pb. The total activities of 210 Po and 210 Pb ranged from 2.5 ± 0.35 to 16.52 ± 1.98 dpm 100 L-1 and from 3.47 ± 0.45 to 24.49 ± 3.42 dpm 100 L-1 respectively with the exception of higher activities of 210 Po (87.36 ± 11.38 dpm 100 L-1) and 210 Pb (32.25 ± 4.35 dpm 100 L-1) from the near bottom water in the hypersaline anoxic Orca basin. In the upper 150 m of the water column, the inventories of 210 Pb were consistently greater than those of 210 Po while the total overall water column inventories of 210 Po and 210 Pb were found to be near equilibrium. The 210 Po fluxes varied between 9.8 ± 6.34 dpm m-2 d-1 and 30.7 ± 12.06 dpm m-2 d-1 with corresponding POC fluxes between 8.6 mg C m-2 d-1 and 37.65 mg C m-2 d-1. The fluxes showed a general decreasing trend further offshore, closely following a similar trend in net primary productivity across the slope. The export efficiency ratios "e-ratios" of the biological pump over this slope region was found to vary between 0.027 and 0.11 averaging at 0.05 ± 0.01, which is low. The 210 Po based and the model-based e-ratio estimates were different but within the range of previous estimates.
... The 210 Po concentration showed seasonal fluctuations (Uddin et al., 2015a between 0.48 and 0.69 mBq L −1 during the months of low phytoplankton productivity, and between 0.29 and 0.37 mBq L −1 during the phytoplankton blooms (Fig. 21). These 210 Po concentrations in northwestern Gulf waters are quite consistent over the years and are comparable to other marine areas, e.g., Baltic Sea (Skwarzec and Bojanowski, 1988), parts of the Pacific Ocean (Nozaki et al., 1997), Gulf of Maine, USA (Fleer and Bacon, 1984), Southern Atlantic (Shannon et al., 1970) and the NW Mediterranean Sea (Masque et al., 2002). ...
The region around the Gulf is moving toward a nuclear energy option with the first nuclear power plant now operational in Bushehr, Iran. Others are soon to be commissioned in Abu Dhabi and in Saudi Arabia. For this reason, radiological safety is becoming a prime concern in the region. This review compiles published data on radionuclide concentrations in seawater, sediment, and biota that have been analyzed in the Gulf countries, along with spatial distribution patterns to enable a synoptic view of the available datasets. The seawater concentrations of 3 H, 210 Po, 210 Pb, 137 Cs, and 90 Sr varied between 130 and 146, 0.48-0.68, 0.75-0.89, 1.25-1.38, 0.57-0.78 mBq L −1 , respectively. The 226 Ra concentration in seawater varied between 0.26 and 3.82 Bq L −1. Extremely high 40 K concentrations between 132 and 149 Bq L −1 have been reported from the Iranian coast compared to 8.9-9.3 Bq L −1 from the western side of the Gulf. Concentrations of 40 K, total 210 Pb, 137 Cs, 90 Sr, 226 Ra, 228 Ra, 238 U, 235 U, 234 U, 239+240 Pu, and 238 Pu were determined in sediment and ranged between 353 and 445, 23.6-44.3, 1.0-3.1, 4.8-5.29, 17.3-20.5, 15-16.4, 28.7-31.4, 1.26-1.30, 29.7-30.0, 0.045-0.21 and 0.028-0.03 Bq kg −1 dry weight, respectively. Significantly higher 137 Cs values have been reported from the Iranian coast compared to the western coast of the Gulf. Whole fish concentrations of 40 K, 226 Ra, 224 Ra, 228 Ra, 137 Cs, 210 Po and 90 Sr ranged between 230 and 447, 0.7-7.3, <0.5-6.6, <0.5-15.80, <0.17, 0.88-4.26 and 1.86-5.34 Bq kg −1 dry weight, respectively. 210 Po was found to be highly concentrated in several marine organisms with the highest 210 Po concentration found in the clam Marcia marmorata (193.5-215.6 Bq kg −1 dry weight). The review highlights the overall paucity of data and inconsistencies in the measurement of radionuclides throughout the Gulf region. Further, since the region is moving toward nuclear energy to meet its increasing energy demand, and coupled with the environmental effects from offshore oil exploration and the heavy impact of climate change, there is a pressing need to undertake a comprehensive marine radioactivity monitoring and assessment effort by conducting a joint cruise in the Gulf with participation of all the adjoining countries. Several recommendations on sampling marine matrixes in the Gulf are given with the aim of improving comparability of radionuclide data from the various studies undertaken in the Gulf region.
... 226 Ra,210 Pb,and 210 Po are used to estimate the rates at which reactive elements are removed from seawater by adsorption onto particles, to determine residence times of lead, other pollutants and materials in the oceans, and to assess redistribution of pollutants from the atmosphere in estuarine systems (Wu and Boyle, 1997;Zaborska et al., 2007;Matthews et al., 2007;Fowler, 2011;Mikalauskiene et al., 2018). 210 Po/ 210 Pb activity ratio measurements have been used to study biological productivity and boundary scavenging (Nozaki et al., 1976(Nozaki et al., , 1997Smith and Ellis, 1995). 222 Rn and its progenies, 210 Pb, 210 Bi, and 210 Po, are very useful tracers in quantifying several atmospheric processes such as circulation, mixing, stability, transport, and vertical movement of air masses, removal rate constants, and the residence time of aerosols, chemical behavior of analog species and washout ratios, and deposition velocities of aerosols (Peck and Smith, 2000;Papastefanou, 2006;Yamamoto et al., 2006;Baskaran, 2011;Persson and Holm, 2011). ...
Liquid scintillation counting (LSC) is a major method for the measurement of beta-emitting and some alpha-emitters. It has been applied in many aspects such as the monitoring of environmental radioactivity, the behavior of radionuclides in the environment, and the characterization of nuclear waste for the decommissioning of nuclear facilities. Radionuclides are also widely applied to the investigation of environmental processes. However, the level of radionuclides in the environment is normally very low, and the resolution of LSC alone is not sufficiently high to identify and quantitatively measure different radionuclides. Low background LSC and chemical separation and purification are needed. This chapter aims to present the principles of LSC measurement techniques for the above purposes and focus on the practical issues in the LSC determination of environmental radionuclides, as well as the status of the present research progress in these aspects. The principles of low-level counting using LSC including the methods for reducing the background in the LSC instrument and practical issues in sample preparation are presented. The principles and applications for the measurement of alpha-emitters using LSC with the alpha/beta discrimination features are presented. The main application of triple-to-double coincidence ratio–based LSC in normal and Cerenkov counting models are also described. Most of the effort of this chapter focuses on the application of LSC in the determination of different anthropogenic and naturally occurring radionuclides, including tritium, ¹⁴C, ⁵⁵Fe, ⁶³Ni, 89,90Sr, ⁹⁰Y, ⁹⁹Tc, ²⁴¹Pu, ³⁶Cl, ⁴¹Ca, ¹²⁹I, ²¹⁰Po, ²¹⁰Pb, isotopes of uranium, thorium, radium, and radon. Besides the LSC measurement methods, the detailed and practical chemical separation of these radionuclides from different sample matrices, as well as the sample preparation methods for LSC measurement are discussed based on the open publications and the experiences of the authors.
... Though 210 Po and 210 Pb are particleactive, there is difference between them during adsorbing onto particles. 210 Po is easier to be absorbed onto biogenic particles than 210 Pb and get involved into the biogeochemical cycling (Nozaki et al., 1997;Fisher et al., 1987;Fowler, 2011;Yang et al., 2013Yang et al., , 2015. In general, the disequilibrium between 210 Po and 210 Pb is prevalent in the biogeochemical cycling processes. ...
210Po and 210Pb are increasingly used to constrain particle dynamics in the open oceans, however they are less used in coastal waters. Here, distributions and partitions of 210Po and 210Pb were examined in the Taiwan Strait, as well as their application to quantify particle sinking. Activity concentrations of dissolved 210Po and 210Pb (<0.6 μm) ranged from 1.21 to 7.63 dpm/(100 L) and from 1.07 to 6.33 dpm/(100 L), respectively. Activity concentrations of particulate 210Po and 210Pb varied from 1.96 to 36.74 dpm/(100 L) and from 3.11 to 38.06 dpm/(100 L). Overall, particulate 210Po and 210Pb accounted for the majority of the bulk 210Po and 210Pb. 210Po either in dissolved or particulate phases showed similar spatial patterns to 210Pb, indicating similar mechanisms for controlling the distributions of 210Po and 210Pb in the Taiwan Strait. The different fractionation coefficients indicated that particles in the Zhemin Coastal Current (ZCC) inclined to absorb 210Po prior to 210Pb while they showed an opposite effect in the Taiwan Warm Current (TWC). Based on the disequilibria between 210Po and 210Pb, the sinking fluxes of total particulate matter (TPM) were estimated to range from–0.22 to 3.84 g/(m2·d), showing an overall comparable spatial distribution to previous reported sediment accumulation rates. However, our sinking fluxes were lower than the sedimentation rates, indicating a sediment resuspension in winter and horizontal transport of particulate matter from the Taiwan Strait to the East China Sea.
... In the Canada Basin, the upper 50 m had F Po/ Pb > 1 and followed a similar pattern as that observed in the interior stations. Previous studies have shown a positive correlation between the removal rate of 210 Po and the concentration of chlorophyll-a (Nozaki et al., 1997(Nozaki et al., , 1998 and a negative correlation between particulate 210 Po activity and fluorescence (Masqué et al., 2002), suggesting that phytoplankton and POM contribute to the scavenging of 210 Po. F Po/ Pb values < 1 have been reported for North Atlantic surface water (Tang et al., 2017;Bacon et al., 1988). ...
... Thus, radionuclides can be used to investigate the cycling of TEs that have similar geochemical behavior or similar sources/ sinks, and can provide a time constraint on the biogeochemical processes of these TEs in the oceans. For example, Th and polonium (Po) are particle-reactive in seawater and can be useful for quantifying scavenging rates of carbon and TEs onto particles in the water column (e.g., Buesseler, 1998;Nozaki et al., 1997). In contrast, Th daughters, i.e., radium Chapter 1 -Introduction 2 (Ra) isotopes, do not interact strongly with particles in seawater, and can therefore be suitable tracers for ocean mixing processes (Ku and Luo, 2008), and powerful tools to investigate exchange rates between the ocean boundaries Dulaiova et al., 2009;Charette et al., 2016). ...
Trace elements (TEs) play crucial roles in regulating ocean processes including marine biogeochemical cycles, and are therefore vital to support marine life. Understanding the biogeochemical cycling of TEs requires knowledge of their sources/ sinks and transport in the oceans. Radium isotopes are suitable tools to study inputs of elements from the continental margins, as they are produced by the decay of thorium isotopes in sediments, and are soluble in seawater. Therefore, in this Ph.D. thesis, I used Ra isotopes to determine boundary TE fluxes from two diverse environments that constrain the major TE sources, including shelf sediments in an Arctic shelf region and in an eastern boundary system off the western African coast, as well as rivers such as the River Congo. First, the distributions of the dissolved and total dissolvable TEs (Cd, Fe, Ni, Cu, Zn, Pb, Mn, and Co) were investigated in the Chukchi Sea and 228Ra isotope was applied as tracer of benthic TE inputs. The results show that elevated benthic TE inputs on the Chukchi shelf provided suitable conditions for phytoplankton blooms. The 228Ra activities in spring appear to be 2-fold higher than in summer, which was a surprising observation and requires further investigation. Next, 228Ra was used to investigate the influence of the Congo River on surface ocean TE concentrations in the South Atlantic Ocean. The results show that the Congo River plume constitutes a large and unexpected input of TEs (Fe, Mn, and Co) into the South Atlantic. Finally, Ra isotopes were used to investigate a number of key ocean boundary processes in the Southeast Atlantic Ocean. Upwelling in the Benguela region is visible in the Ra distributions, and elevated Ra isotopes, Fe (II) and Si concentrations were observed in the Benguela oxygen minimum zone, possibly due to inputs from the reducing shelf sediments, or submarine groundwater discharge along the Namibian shelf.
... They explained 210 Pb distribution by dust deposition from the atmosphere and scavenging in the water column, estimating the residence time of Pb in surface water to be 1.7 years in the North Pacific subtropical gyre. Then, Nozaki et al. (1997) evaluated the residence time of 210 Pb in the Pacific Deep Water (PDW) to be >200 years in the central gyre and 50-100 years at the margins. For stable Pb, Schaule and Patterson (1981) studied the concentration of total dissolvable Pb (tdPb) in seawater between Hawaii and California, revealing the first reliable vertical profile of stable Pb in 1976. ...
Aluminum (Al), manganese (Mn), cobalt (Co), and lead (Pb) are key trace elements in seawater and thus significant in chemical oceanography research. However, although all of these elements are highly scavenged in the ocean, only a few studies focus on the intercomparison of their distributions. Here, we report the basin-scale and full-depth sectional distributions of these elements observed during three GEOTRACES Japan cruises in the North Pacific. We confirmed that a surface maximum of the dissolved (d) species is not a common feature for the four elements and that the d species have the lowest concentrations in the Pacific Deep Water (PDW) as compared to other oceans. The elements showed different speciations and distributions. The fraction of labile particulate (lp) species was calculated as the difference between the total dissolvable (td) species and d species. The lpM/tdM ratio, where M refers to an element, is highest for Al, at 0.66 ± 0.31 (average ± sd, n = 489), and lowest for Pb, at 0.02 ± 0.08 (n = 575). Further, the distribution of each element is uniquely related to ocean circulation. The tdAl concentration is high in the Equatorial Under Current (EUC), the North Equatorial Current (NEC), and the Lower Circumpolar Deep Water (LCDW). Manganese is supplied from reductive sources such as sediments on the continental shelves around the northern boundary. Cobalt is concentrated in the North Pacific Intermediate Water (NPIW) and in the Equatorial Pacific Intermediate Water (EqPIW) owing to the combined effects of supply from the continental shelves, biogeochemical cycling, and scavenging. Lead shows a subsurface maximum centered at ∼35°N and ∼200 m depth, implying an association with the formation of the Subtropical Mode Water (SMW) and the Central Mode Water (CMW). Although the subsurface Pb maximum in the Atlantic has diminished over the last three decades owing to the ban on leaded gasoline use, it has been sustained in the North Pacific through the growth of other anthropogenic sources in Asia and Russia. We propose that the enrichment factor of dM, defined as EF(dM) = (dM/dAl) seawater /(M/Al) upper crust , where (M/Al) upper crust is the molar ratio in upper crustal abundance, can be a good parameter for the sources. The median is 1.3 × 10 ² (n = 436) for EF(dMn), 3.2 × 10 ² (n = 430) for EF(dCo), and 1.2 × 10 ³ (n = 413) for EF(dPb). The EF(dPb) found in this study is on the same order of magnitude as the EF values for aerosols found in the literature, suggesting that the deposition of aerosols is a major source for dPb. Because EF(dMn) and EF(dCo) are ten to hundred times higher than the EF for aerosols, sources other than the aerosol deposition are more significant contributors to the concentrations of Mn and Co.
... Generally, excess 210 Pb relative to its 226 Ra precursor is observed in surface waters, and this excess is thought to come from the deposition of 210 Pb-bearing aerosols resulting from the decay of atmospheric 222 Rn transported from the continents and from riverine inputs (mainly for the marginal seas). However, a much lower 210 Pb activity compared to dissolved 226 Ra has been reported in the deeper waters due to the scavenging of 210 Pb by suspended particulate matter (SPM) (Craig et al., 1973;Bacon et al., 1976;Nozaki et al., 1997;Hong et al., 2013;Marsan et al., 2014;Rigaud et al., 2015). 210 Po, the grand-daughter of 210 Pb, is generally deficient relative to 210 Pb in the upper water column (mainly <~200 m) due to both its preferential removal by biogenic particulate matter and the overall short residence time of particulate matter in the surface waters, and the scavenging rate for 210 Po has been reported to vary with biomass and primary production (Shannon et al., 1970;Turekian et al., 1974). ...
... REE transportation processes in the atmosphere begin from terrestrial areas especially those of soil origins, dust, aeolian processes, smoke and ions. In the biogenic phase, REE mixes with smoke and demonstrates complexation behaviour with suspended particles, volatile ions (Behera, 2013) and natural radionuclide (Nozaki et al., 1997) in the atmosphere. However lithogenic REEs originate from soil, dust, aerosol and aeolian processes (Tsunogai et al., 1985). ...
The aim of this work is to review the distribution and behavior of rare earth elements (REE) in the southern South China Sea region especially in the atmosphere, land, seawater, river, surface sediment and in suspended particulate matters (SPM). The South China Sea is a strategic ocean basin with currently undergoing vast development. China has been the largest REE producer in the world since 2010 producing about 90% of the world's REE supplies followed by the United States of America, Australia and ASEAN countries. In the southern South China Sea, the geological texture plays an important role for the abundance of REE. REE is widely distributed throughout northern South China Sea continental shelf within the boundaries of several ASEAN zone countries especially Malaysia, Singapore, Brunei, Indonesia, west Philippines, Thailand and Vietnam. REE is used as a tracer and proxy in geochemistry and marine sedimentation disciplines as an indicator of the origins and patterns of sediment and soil and in both dissolved and particulate phases. It is extremely important to understand the chemical properties and behavior of REEs so as to be able to clarify the natural bioavailability and scavenging and absorption processes, complexes and precipitation during biogeochemical reactions. REE is transported and accumulated during the natural weathering process on land and moves to rivers and oceans, affecting the distribution and deposits of elements. There is no specific or systematic REE cycle model developed in the world. REE research was carried out and documented by Piper and Henderson in the early 1980s. This manuscript will address the relation and effect of REEs on the marine ecosystem and ecology, as well as its impact on human civilization especially in the realms of politics, the environment, society and economy. As a conclusion the distribution of REE as reviewed in past research is presented in the model REEsurface sediment > REEorganism > REEtotal suspended solid > REEPM10 > REEriver > REEseawater.
... Various particulate components have different affinity with 210 Po and 210 Pb [Yang et al., 2013[Yang et al., , 2015b. It is generally accepted that organic matter usually has stronger affinity for 210 Po comparing with 210 Pb [Stewart et al., 2007], while inorganic particulate components seem to show little difference in adsorbing them [Nozaki et al., 1997[Nozaki et al., , 1998]. Such a difference is usually quantified by the fractionation factor (i.e., F value) [Guo et al., 2002], defined as the ratio of partition coefficient between two nuclides (i.e., K d value, calculated by particulate activity divided by dissolved activity and TPM content). ...
The 210Po-210Pb pair is increasingly used as a proxy of quantifying organic carbon export from the euphotic zone. However, disequilibria between 210Po and 210Pb in mesopelagic water have been poorly studied. Here we present unusual deficiencies of 210Po with respect to 210Pb in mesopelagic water (200-1000 m) in the South China Sea (SCS). The total particulate matter (TPM) increased by up to 32% in the mesopelagic layer comparing with the euphotic zone. The total 210Po/210Pb ratio varied from 0.41 to 0.98 with an average of 0.72±0.19, showing an enhanced removal of 210Po in mesopelagic water. On average, particulate 210Po and 210Pb increased by 23% and 32% at the slope stations respectively. These results indicated that the 210Po deficits result from lateral transport, probably via benthic nepheloid layer. Based on the deficiency of 210Po, the residence times of particulate 210Po were estimated to range from 0.11 to 0.25 yr (avg. 0.17±0.07 yr), allowing re-suspended sediment to disperse over a long-range. The export fluxes of 210Po varied from 68 to 121 dpm m−2 d−1 with an average of 96±27 dpm m−2 d−1, which was 6 times that out of the euphotic zone. Using the 210Po deficits, the export fluxes of TPM out of the mesopelagic layer were quantified to vary from 4.19 to 10.20 g m−2 d−1, revealing a large amount of particles from the shelf to the SCS basin. This study suggests that 210Po-210Pb could be an effective tracer of tracking particle cycling in mesopelagic water.
... 210 Pb in coastal seawater is derived mainly from atmospheric deposition (Baskaran 2011), and this source usually shows little inter-annual variation on the western Pacific coasts (Huh et al. 2006;Wang et al. 2014). 210 Pb in coastal seawater is usually scavenged into local sediments because coastal waters have very high particle concentrations, which efficiently remove atmosphere-derived 210 Pb and even 210 Pb transported from open seawater, i.e. boundary scavenged-210 Pb (Nozaki et al. 1997;Huh and Su 1999). Hence, the flux of 210 Pb at a specific site in coastal seas would, to a large extent, meet the precondition of the CRS model rather than other 210 Pb-chronology models (i.e. ...
In the last 150 years, China transformed its economy from a feudal system (pre-1911) to a modern market economy. During this time, policy-driven rapid development created a series of ecological and environmental problems, especially in the coastal economic regions. Although the synthetic effects are certainly identifiable, the specific effects of different policies in different developing periods have been difficult to disentangle. Here we show that the footprints of Chinese policies in the last 150 years, and their different influences on coastal environments, were archived in the temporal dynamics of sedimentation in Jiaozhou Bay. Before 1935, natural processes predominantly controlled the sedimentation in Jiaozhou Bay. After the introduction of modern economic policies in 1935, the sediment mass accumulation rate (MAR) was overwhelmingly driven by the sea areas of the bay as a result of policy-driven anthropogenic activities (i.e. saltern development, mariculture, and land reclamation). An increasing MAR at the early stage of each policy, followed by a decrease at the late stage was observed. Land reclamation in the 1990s led to a much quicker increase of MAR than the earlier saltern development. Our results demonstrated that marine exploitation, instead of natural processes, are currently regulating the ecological environments of Chinese bays.
... As for 230 Th, we attribute the 231 Pa xs depletion to an intense boundary scavenging in the water flowing along the eastern escarpment of the plateau, possibly due to particle re-suspension and/or nepheloid layers in the deep and bottom waters (Venchiarutti et al., 2008). Indeed, it has been shown (notably for 210 Pb) that particles supplied from the seafloor with nepheloid layers can have a strong impact on the radionuclide scavenging in the deep ocean (Nozaki et al., 1997;Okubo et al., 2007;Turnewitsch et al., 2008). However, the exact processes yielding this bottom scavenging remain to be determined. ...
Dissolved and particulate excess <sup>230</sup>Th and <sup>231</sup>Pa concentrations (noted <sup>230</sup>Th<sub>xs</sub> and <sup>231</sup>Pa<sub>xs</sub> respectively) and <sup>231</sup>Pa<sub>xs</sub>/<sup>230</sup>Th<sub>xs</sub> activity ratios were investigated on and out of the Kerguelen plateau (Southern Ocean) in the framework of the Kerguelen Ocean and Plateau compared Study project in order to better understand the influence of particle flux and particle chemistry and advection on the scavenging of <sup>231</sup>Pa.
In the wake of Kerguelen, particulate <sup>231</sup>Pa<sub>xs</sub> is relatively abundant compared to its content in the dissolved phase. This, together with the low fractionation observed between <sup>230</sup>Th and <sup>231</sup>Pa ( F <sub>Th/Pa</sub> ranging from 0.06 ± 0.01 to 1.6 ± 0.2) reflects the domination of the biogenic silica in the particle pool.
Along the eastern escarpment of the Kerguelen plateau, the strong <sup>231</sup>Pa<sub>xs</sub> horizontal gradient in the deep waters highlights the intense removal of <sup>231</sup>Pa at depth, as already observed for <sup>230</sup>Th<sub>xs</sub>. This local boundary scavenging was attributed to re-suspension of opal-rich particles by nepheloid layers, resulting in fractionation factors F <sub>Th/Pa</sub> ≤ 1 along the Kerguelen plateau slope. Therefore, both the composition (biogenic opal) and the flux (intense along the margin) of particles control the scavenging of the two radionuclides in the Kerguelen wake.
The modelling of <sup>231</sup>Pa distribution with an advection-scavenging model demonstrates that lateral advection of open ocean water on the Kerguelen plateau could supply most of the <sup>231</sup>Pa, which is then efficiently scavenged on the highly productive plateau, as previously proposed for <sup>230</sup>Th<sub>xs</sub>. It stresses that lateral advection can play a significant role in the overall budget of particle reactive trace elements in a coastal-open ocean system.
... Generally, excess 210 Pb relative to its 226 Ra precursor is observed in surface waters, and this excess is thought to come from the deposition of 210 Pb-bearing aerosols resulting from the decay of atmospheric 222 Rn transported from the continents and from riverine inputs (mainly for the marginal seas). However, a much lower 210 Pb activity compared to dissolved 226 Ra has been reported in the deeper waters due to the scavenging of 210 Pb by suspended particulate matter (SPM) (Craig et al., 1973;Bacon et al., 1976;Nozaki et al., 1997;Hong et al., 2013;Marsan et al., 2014;Rigaud et al., 2015). 210 Po, the grand-daughter of 210 Pb, is generally deficient relative to 210 Pb in the upper water column (mainly <~200 m) due to both its preferential removal by biogenic particulate matter and the overall short residence time of particulate matter in the surface waters, and the scavenging rate for 210 Po has been reported to vary with biomass and primary production (Shannon et al., 1970;Turekian et al., 1974). ...
... Ra-isotopes for groundwater discharge into the coastal ocean [1], 210 Pb sediment chronology [2] 238 U-234 Th for export fluxes of particles from the upper ocean [3] or 231 Pa-230 Th for the ocean circulation [4]. Specifically, the disequilibrium between the radioactive pairs 234 Th-238 U (T 1/2 ¼24 days) and 210 Pb-210 Po (T 1/2 ¼138 days) have been widely used in the marine environment to study scavenging rates [5,6], carbon export fluxes [7] and more recently e.g. [8][9][10] carbon export efficiency [10,11]. ...
Measurement of radionuclides in marine samples, specifically radioactive pairs disequilibrium, has gained interest lately due to their ability to trace cutting edge biogeochemical processes. In this context, we developed a fast, direct method for determining 210Pb and 210Po water through the use of ultra low-level liquid scintillation counting and alpha-particle spectrometry respectively and through Eichrom Sr resins for the Po-Pb separation. For 210Pb analysis, the method uses stable lead as a yield tracer measured by a robust ICP-MS technique, and 210Po is determined through self-deposition using the conventional 209Po yield tracer. The improvements of the method over other techniques are: a) the analysis can be completed within 6 days, simplifying other methods, b) very low limits of detection have been achieved − 0.12 and 0.005 mBq L−1 for 210Pb and 210Po, respectively - and c) most of the method could be carried out in on-board analysis. We applied the method to different aqueous samples and we analysed its application to marine samples. We determined 210Pb and 210Po in the dissolved fraction of Mediterranean Sea water and an estuary at the South-West of Spain. We found that it can be successfully employed to marine samples but we recommend to i) use a minimum of 20 L water to measure the 210Pb in the dissolved phase by LSC and lower volumes to measure total concentrations; ii) wait for 210Pb and 210Bi secular equilibrium and measure the total spectrum to minimize the limit of detection and improve accuracy.
... Similar to 234 Th: 238 U pair, they would be in secular equilibrium in the ocean in the absence of sinking particles. However, a deficiency of 210 Po relative to 210 Pb is often observed through the upper few hundred meters of the water column, as a result of particle removal (Bacon et al., 1976;Nozaki et al., 1997). This disequilibrium is predominantly due to the difference in Po and Pb biogeochemistry. ...
The northern Gulf of Mexico (NGOM) is one of the well-studied areas of global ocean, yet direct estimates of upper ocean particulate organic carbon (POC) fluxes from this region are limited. The present work reports vertical fluxes of POC from the oligotrophic region of NGOM utilizing short-lived radionuclide pairs 234Th/238U and 210Po/210Pb. In spite of the difference in time scale both 210Po and 234Th based estimates are in reasonable agreement with sinking POC fluxes, caught in sediment traps. POC flux estimates ranged between 22–41 mg C m− 2 day− 1 at 150 m and 6–40 mg C m− 2 day− 1 at 250 m. The average export efficiency at base of euphotic zone (Ez) was found to be 0.07 ± 0.03 while the export ratio (T100) at 100 m below euphotic zone was found to be 0.66 ± 0.18 indicating that most of the attenuation of NPP in this region is set in the surface layer (low Ez ratio) and there is relatively little flux attenuation in the subsurface (high T100) which is typical for oligotrophic settings. Satellite based export efficiencies predicted by the Laws and Dunne models are on average found to be two times higher than the in situ observations while estimates from the Henson model were found to be similar or lower. This is consistent with the observation on a global scale where we find export estimates from in situ data to be consistently lower than those predicted by the Laws export model for the temperature range of 20–25 °C. The discrepancy between modelled estimates and in situ measurements of POC fluxes highlights the fact that global empirical models of satellite based POC fluxes that only consider temperature are overly simple and may need further refinement for ocean biome specific scaling to accurately predict export fluxes.
... There is a lot of information on the distribution of Po between both particulate and dissolved phases in marine systems (e.g. Carvahlo, 1997 or Nozaki et al., 1997), and in unperturbed freshwaters. So, Chai and Urban (2004) have found that 210 Po has a strong tendency to be bound to particulate matter in the water column by surface adsorption or biological uptake. ...
In this chapter, the dynamics of radionuclides in the environment are studied. Thus, some general principles of radioecology and interesting parameters are introduced to study the transfer of radionuclides between the different environmental compartments. The levels and behavior of natural and man-made radionuclides in the environment are also reviewed in order to provide the reader with a general view on the presence of radioactivity in Nature.
The ²¹⁰Po/²¹⁰Pb disequilibrium was attempted to reveal the small-scale particle dynamics in the eastern tropical North Pacific. Seawater samples in the full water column were collected from three sites in the Tehuantepec bowl near the East Pacific Ridge for determination of dissolved and particulate ²¹⁰Po and ²¹⁰Pb. Our results show that TPo/TPb activity ratios in the full water column at the three sites are less than 1, with an average of 0.56, indicating that the total ²¹⁰Po in the oligotrophic sea is significantly deficient. The activity ratios of DPo/DPb in the dissolved phase are less than 1, while those in the particulate phase are greater than 1 (except for the bottom 300 m), indicating fractionation between ²¹⁰Po and ²¹⁰Pb in the scavenging process. A negative linear relationship between ²¹⁰Po deficit and silicate proves that biological activities are responsible for ²¹⁰Po deficiency in the upper 200 m. However, the deficit of ²¹⁰Po in the bottom 300 m may be caused by the horizontal transport of the hydrothermal plume. After correcting the horizontal contribution, the removal rates of ²¹⁰Po for the 200–1,500 m and the bottom 300 m layers increased by 7.5–21 and 26.1–29.5%, respectively. Correspondingly, the variation range of the residence time of a total ²¹⁰Po became smaller. Our calculations suggest that horizontal transport is acting as a stabilizer for small-scale variation in the ²¹⁰Po deficit in the eastern tropical North Pacific. Our study highlights the need to pay more attention to the small-scale variation of ²¹⁰Po deficit when applying ²¹⁰Po/²¹⁰Pb disequilibria to trace biogeochemical processes, and the mechanism responsible for this variation deserves further study.
Dissolved and particulate ²¹⁰Po and ²¹⁰Pb were measured to reveal the particle dynamics around Prydz Bay in the austral summer of 2015. The spatial variations of ²¹⁰Po activity concentration and ²¹⁰Po/²¹⁰Pb disequilibria are mainly controlled by ²¹⁰Po absorption and particle scavenging, and particulate organic matter (POM) remineralization. Different water masses around Prydz Bay show different characteristics of ²¹⁰Po/²¹⁰Pb disequilibria. The activity ratio of total ²¹⁰Po to total ²¹⁰Pb (expressed as TPoTPbAR) in Antarctic Surface Water, Winter residual Water, Shelf Water and Thermocline Water are all less than 1.0, indicating that biological absorption and particle adsorption preferentially scavenge ²¹⁰Po from these waters. Circumpolar Deep Water shows the highest TPoTPbARcharacteristics, and the total ²¹⁰Po is in equilibrium or excess to the total ²¹⁰Pb, which reflects the effect of POM remineralization. The TPoTPbARvalue in the Antarctic Bottom Water is less than 1.0, and it falls between the ratios of Shelf Water and Circumpolar Deep Water, which proves that the formation of Antarctic Bottom Water has an impact on the ²¹⁰Po/²¹⁰Pb disequilibria. fretained is defined to represent the proportion of ²¹⁰Po scavenging from the dissolved phase to the particulate phase but suspended in seawater; we found that there is a linear relationship between fretained and particulate organic carbon (POC), silicic acid and phosphate concentrations, indicating that the balance between input (POM remineralization) and removal (absorption and particle adsorption) determines the disequilibrium between ²¹⁰Po and ²¹⁰Pb. Based on the ²¹⁰Po/²¹⁰Pb disequilibria, the POC export from the euphotic zone is estimated to be 4.11-47.82 mmol C m⁻² d⁻¹, showing large spatial variability. The POC export on the shelf is generally higher than that on the slope and basin, which corresponds to the spatial variations of phytoplankton biomass and primary productivity.
Significance
Anthropogenic lead (Pb) is widespread and far reaching in the environment. However, it was thought that western Arctic Ocean seawater was pristine based on low dissolved Pb and proxy data. By measuring Pb isotopes on seawater with extremely low concentrations, this study shows that anthropogenic Pb is pervasive in western Arctic Ocean seawater, and much of the dissolved Pb is from remobilization of previously deposited aerosols from the high-Pb emission period of the 20th century. Thus, historic Pb pollution still impacts Arctic seawater, and accelerated melting of permafrost and ice and increased coastal erosion may enhance this remobilization. This study also demonstrates that dissolved Pb isotopes are a sensitive tracer of contaminant and particulate sources in Arctic seawater.
La presente investigación tuvo como objetivo principal la cuantificación de Ra-226, Ra-228 y Pb-210 en el Río Rímac, principal fuente de abastecimiento de agua a la población de
Lima. Las muestras de agua fueron recolectadas durante un período de seis meses, desde abril a setiembre del 2000, en 16 estaciones de muestreo ubicadas estratégicamente a lo largo del cauce del Río Rímac. El método analítico utilizado consiste en la separación de radio y plomo contenidos en las muestras de agua utilizando portadores de bario y plomo, y que después de algunas etapas de precipitación y redisolución, el radio es coprecipitado como Ba(Ra)SO4 y el plomo precipitado como PbCrO4. La determinación de Ra-226 fue realizada mediante la medida alfa total de precipitado de Ba(Ra)SO4, y la determinación del Ra-228 y Pb-210 por la medida beta total de los precipitados de Ba(Ra)SO4 y PbCrO4 respectivamente. Las mediciones se realizaron en un contador proporcional de flujo de gas de bajo fondo. Los resultados de las concentraciones de la actividad varían desde 0,0021 a 0,1754 Bq/L para el Ra-226; desde 0,0025 a 0,2355 Bq/L para el Ra-228 y desde 0,0026 a 0,0528 Bq/L para el Pb-210. En ninguno de los casos se supera la concentración máxima admisible para el agua de consumo humano. En comparación con otros ríos en el mundo, el Río Rimac presenta concentraciones semejantes.
During the North Atlantic Bloom Experiment (NABE) of the Joint Global Ocean Flux Study (JGOFS), water column sampling for particulate and dissolved ²¹⁰Po and ²¹⁰Pb was performed four times (26 April and 4, 20, 30 May 1989) during a month-long Lagrangian time-series occupation of the NABE site, as well as one-time samplings at stations during transit to and from the site. There are few prior studies documenting short-term changes in ²¹⁰Po and ²¹⁰Pb profiles over the course of a phytoplankton bloom, and we interpret the profiles in terms of the classical “steady-state” (SS) approach used in most studies, as well as by using a non-steady state approach suggested by the temporal evolution of the profiles. Changes in ²¹⁰Po profiles during a bloom are expectable as this radionuclide is scavenged and exported. During NABE, ²¹⁰Pb profiles also displayed non-steady state, with significant increases in upper water column inventory occurring midway through the experiment. Export of ²¹⁰Po from the upper 150 m using the classic “steady-state” model shows increases from 0.5 ± 8.5 dpm m⁻² d⁻¹ to 68.2 ± 4.2 dpm m⁻² d⁻¹ over the ∼one-month occupation. Application of a non-steady state model, including changes in both ²¹⁰Pb and ²¹⁰Po profiles, gives higher ²¹⁰Po export fluxes. Detailed depth profiles of particulate organic carbon (>0.8 μm) and particulate ²¹⁰Po (>0.4 μm) are available from the 20 and 30 May samplings and show maxima in POC/Po at ∼37 m. Applying the POC/²¹⁰Po ratios at 150 m to the “steady state” ²¹⁰Po fluxes yields POC export from the upper 150 m of 8.2 ± 1.5 mmol m− 2 d⁻¹ on 20 May and 6.0 ± 1.6 mmol C m⁻² d⁻¹ on 30 May. The non-steady state model applied to the interval 20 to 30 May yields POC export of 24.3 mmol C m⁻² d⁻¹. The non-steady state (NSS) ²¹⁰Po-derived POC fluxes are comparable to, but somewhat less than, those estimated previously from ²³⁴Th/²³⁸U disequilibrium for the same time interval (37.3 and 45.0 mmol m⁻² d⁻¹, depending on the POC/Th ratio used). In comparison, POC fluxes measured with a floating sediment trap deployed at 150 m from 20 to 30 May were 11.6 mmol m⁻² d⁻¹. These results suggest that non-steady state Po-derived POC fluxes during the NABE agree well with those derived from ²³⁴Th/²³⁸U disequilibrium and agree with sediment trap fluxes within a factor of ∼2. However, unlike the ²³⁴Th-POC flux proxy, non-steady stage changes in profiles of ²¹⁰Pb, the precursor of ²¹⁰Po, must be considered.
This paper presents the results of radioecological monitoring study of natural radionuclide ²¹⁰Po in 11 lakes located in different regions of the Crimean peninsula. These investigations of the Crimean salt lakes were conducted for the first time in the history. The main objectives of this work were: to determine the features of the ²¹⁰Ро behavior in the salt lakes ecosystems, as well as calculation of the doses received by the lakes hydrobionts from α-radiation of absorbed ²¹⁰Po. Concentrations of ²¹⁰Po in the water, suspended matter, the bottom sediments and biota were determined by radiochemical processing and α-spectroscopy measurements. The concentrations of dissolved ²¹⁰Po in the water of investigated lakes were in 0.9–327.1 times higher than in the Black Sea closest regions. The highest concentrations of ²¹⁰Po in water were determined in the lakes of the Kerchenskaya group. These lakes are located on the territory of Crimea where oil is produced. The ²¹⁰Ро activity concentrations in the bottom sediments from Crimean salt lakes were comparable with those of the Black Sea coastal zone. Concentration ratio (CR) of polonium in suspended matter ranged from 10 to 10⁴ for different lakes. A significant trend in a decrease of CR values of ²¹⁰Ро for suspended matter with increasing water salinity was revealed. High levels of ²¹⁰Po accumulation were noted for adult crustacean Artemia spp. (typical inhabitant of the Crimean saline lakes). The CR of ²¹⁰Po for adult Artemia spp. reached 10⁵ while the CR of this radionuclide by their cysts was significantly lower. The absorbed doses from ²¹⁰Po α-radiation calculated for adult Artemia spp. were more than 60 times lower than the permissible dose rate for biota (IAEA, 1992).The obtained results will be used to identify the biogeochemical peculiarities in behavior of the main dose-formative radionuclide ²¹⁰Po, in the water ecosystems with different salinity, including water reservoirs poorly studied in the radioecological aspect and having extreme condition for the existence of lots of species of hydrobionts, such as hypersaline Crimean lakes.
In this study we present dissolved lead (Pb) distributions from samples collected during the GEOTRACES Canadian transect in 2015. Dissolved Pb showed an increase in concentration from the Canada Basin towards Baffin Bay and the Labrador Sea. Canada Basin was the most isolated region from anthropogenic Pb inputs with low background concentrations (1.4-6.2 pmol kg˗1) across most of the basin, albeit with higher Pb features from the advection of Pacific derived waters and sea ice meltwater were present. Likewise, high Pb peaks (13.5-26.6 pmol kg˗1) along narrow isopycnal surfaces (σθ 27.4-27.6) in Baffin Bay were attributed to the advection of North Atlantic waters spreading a high Pb signature into the bay, contrasting with the low values (2.5-7.4 pmol kg˗1) present in the mixed layer, Arctic and Baffin Bay Deep waters. The Labrador Sea, largely influenced by the recirculation of North Atlantic waters, had the highest Pb concentrations (~17-34 pmol kg˗1) measured in this study. Finally, the Canadian Arctic Archipelago (CAA) represents a transition environment influenced by Arctic waters imprinting a low Pb signature in the western CAA and the southern side of Parry Channel, while Baffin Bay waters prevailed in eastern CAA recirculating westward along the northern side of Parry Channel and progressively losing their relatively high Pb signature.
This study focuses on creating baseline for 238U, 235U, 234U, 210Pb, 210Po and 40K concentrations in the northern
Arabian Gulf. The respective concentration ranges were 0.047–0.050, 0.00186–0.00198, 0.054–0.057,
0.00085–0.00092, 0.00051–0.00062 and 18.6–19.1 Bq l−1. These results suggest that the levels are generally
comparable to other marine waters in the northern hemisphere. There were no hot spots observed from oil and
gas industry. These data will serve as a baseline to gauge possible future inputs of TENORMs in the northern
Gulf. A positive and linear correlation was observed between 238,234U, 40K isotopes and seawater salinity. The
results also suggest significant fractionation between 210Po and 210Pb, attributed to rapid removal of 210Po by
biota compared to 210Pb. The mean residence time for 210Po in the study area was 371 days. The 234U/238U and
238U/235U activity ratios in seawater samples vary between 1.14–1.15, and 0.038–0.040. The 234U/238U and
235U/238U ratio is similar to the expected composition of seawater (1.148 ± 0.002) and 0.0462.
We report the continued lead (Pb) contamination of the Northwest Pacific Ocean in 2002 and present the first comprehensive Pb isotope dataset for that region. In the upper ocean, a Pb concentration maxima (64 - 113 pmol kg(-1)) extended throughout the entire North Pacific Subtropical Gyre (NPSG). We determined most of the Pb in this feature was from industrial emissions by many nations in the 1980s and 1990s, with the largest contributions from leaded gasoline emissions. In contrast, the deep water (> 1000 m) Pb concentrations were lower (6 - 37 pmol kg(-1)), and constituted a mix of background (natural) Pb and anthropogenic Pb inputs from preceding decades. Deep water below the Western Subarctic Gyre (WSAG) contained more industrial Pb than below the NPSG, which was attributed to a calculated 60-fold greater flux of particulate Pb to abyssal waters near the Asian continent. Assuming Pb isotope compositions in the North Pacific Ocean were homogenous prior to large-scale 20th century anthropogenic inputs, this evidence suggests a relatively faster change in Pb isotope ratios of North Pacific deep water below the WSAG versus the NPSG.
The activities of 228Th, 230Th, 234Th, 226Ra, 228Ra, 210Po, and 210Pb have been reported in the East Sea (Japan Sea) over the past few decades. The activities of thorium isotopes over the entire depth of the East Sea ranged from 0.4 to 8.3 dpm 100 L-1 for 228Th, 0.06 to 0.37 dpm 1000 L-1 for 230Th, and 0.4 to 2.1 dpm L-1 for 234Th. Thorium isotopes in the East Sea were used for determining the collection efficiency of sediment traps (using 230Th) and the annual vertical flux of particulate organic carbon (using 234Th). The activities of radium isotopes over the entire depth of the East Sea ranged from 0.3 to 11 dpm 100 L-1 for 228Ra and 6.4 to 16 dpm 100 L-1 for 226Ra. Radium isotopes in the East Sea were used for determining the vertical eddy diffusion coefficient (using 228Ra) and the residence time of deep water (using 226Ra). The activities of 210Pb over the entire depth ranged from 4.4 to 27 dpm 100 L-1, and the activities for 210Po over the entire depth ranged from 1.5 to 11 dpm 100 L-1. The 210Pb and 210Po in the East Sea were used for estimating the residence time of trace elements in the dissolved, colloid, and particulate phases. In summary, U-series radioisotopes have provided useful information on various physical and biogeochemical processes in the East Sea. © Springer International Publishing Switzerland 2016. All rights reserved.
This study reports the concentrations of important tracers of the marine environment, Po-210 and Pb-210, in different matrices from field samples collected at Central Adriatic Sea (Italy), a non-contaminated marine ecosystem. Po-210 concentration appears to decrease with increasing distance from the coast and a significant difference in Po-210 concentrations in seawater samples at two different depths is not appreciable. Po-210 and Pb-210 present the same trend in the different periods of the year. Po-210 and Pb-210 present high concentration factors and high Kd values: in particular Po-210 shows values of 2.4 10(4) in plankton, 3.2 10(5) in the particulate fraction and 1.1 10(5) in sediment; Pb-210 shows values of 1.3 10(4) in plankton, 1.8 10(4) in the particulate fraction and 4.9 10(4) in sediment. The Po-210/Pb-210 ratio results < 1 in sea water and >1 in the particulate fraction, sediment and plankton due to a significant fractionation occurring between the two radionuclides during their removal from solution to particle and due to their different biogeochemical cycling pathways in the marine environment. The noticeable accumulation of Po-210 in the food chain is not accompanied by an identical accumulation of Pb-210. Due to its biomagnification in the marine food chain, Po-210 provides the larges radiation dose to any marine organism under natural conditions. The data reported provide reference values for the Central Adriatic (Mediterranean Sea) coastal environment.
This study reports the concentrations of important tracers of the marine environment210Po and 210 Pb, in different matrices from field samples collected at Central Adriatic Sea (Italy), a non-contaminated marine ecosystem. 210Po concentration appears to decrease with increasing distance from the coast and a significant difference in 210Po concentrations in seawater samples at two different depths is not appreciable. 210 Po and 210Pb present the same trend in the different periods of the year. 210Po and 210Pb present high concentration factors and high Kd values: in particular 210Po shows values of 2.4 104 in plankton, 3.2 105 in the particulate fraction and 1.1 105 in sediment; 210Pb shows values of 1.3 104 in plankton, 1.8 104 in the particulate fraction and 4.9 104 in sediment. The 210Po/210Pb ratio - < 1 in sea water and >1 in the particulate fraction, sediment and plankton due to a significant fractionation occurring between the two radionuclides during their removal from solution to particle and due to their different biogeochemical cycling pathways in the marine environment. The noticeable accumulation of 210Po in the food chain is not accompanied by an identical accumulation of 210Pb. Due to its biomagnification in the marine food chain210Po provides the larges radiation dose to any marine organism under natural conditions. The data reported provide reference values for the Central Adriatic (Mediterranean Sea) coastal environment.
Different kinds of water cover more than two thirds of the Earth’s surface. This resource is extremely important for human life: water is used for direct consumption, it is used in the production of food, it is used for many industrial activities, etc. Thus radioactivity present in water can reach humans and the environment through many different mechanisms.
Documented is an intercalibration (IC) exercise for both Po-210 and Pb-210 in seawater aliquots distributed between up to eight international laboratories that followed individual protocols. Dissolved and particulate samples were provided by GEOTRACES during two IC cruises at baseline stations in the North Atlantic and North Pacific oceans. Included were surface and/or deep dissolved and particulate samples at each site, plus complete profiles analyzed by the laboratory of the lead author. An unspecified solid phase standard was also distributed with Po-210 and Pb-210 in secular equilibrium to confirm spike calibrations. The Po-210 activities reported n = 8) for the standard were very similar with a relative standard deviation (RSD) of 3.6% and mean value indistinguishable from the certified value, confirming accurate calibration of Po spikes. For seawater samples, the agreement was strongly dependent for both nuclides on the activity of the samples. The agreement was relatively good for dissolved seawater samples (RSD = 9% to 29%, n = 4), moderate for the particulate samples (RSD = 12% to 80%, n = 8), and poor for particulate dip blanks (RSD = 50% to 200%, n = 8). Noted is the higher apparent affinity of Po-210 versus Pb-210 for polysulphone filter material. Some lack of reproducibility between labs may have been caused by unspecified differences in individual lab protocols and calculations. A minimum sample activity of 0.1 dpm for both nuclides is recommended for an adequate reproducible sample activity. It is suggested that a consistent set of procedures and calculations be used to optimize future Po-210 and Pb-210 analyses in seawater samples.
The export flux of particulate organic carbon from the euphotic zone in the central equatorial Pacific was measured using an approach that utilizes 234Th and organic carbon analyses on water column and drifting sediment trap samples. This study was conducted as part of the U.S. Joint Global Ocean Flux Study (U.S. JGOFS) EqPac process study from 12°N to 12°S at 140°W. Samples were collected during the Survey I (February–March 1992) and Survey II (August–September 1992) cruises. The accuracy of drifting sediment traps was evaluated by comparing the measured flux of 234Th with the flux calculated from the deficiency of 234Th relative to 238U in the water column. Calculated 234Th fluxes were corrected for the effects of horizontal and vertical advection. The uncertainties on these 234Th fluxes averaged 39% for Survey I and 20% for Survey II. Comparison of measured and calculated 234Th fluxes revealed evidence for overtrapping, especially in the shallow traps (≤ 100 m). Measured and calculated 234Th fluxes agreed to within 50% for traps at 150–250 m. Good correlation was obtained between measured fluxes of organic carbon and 234Th except for some shallow samples high in organic carbon, suggesting that 234Th was a good tracer for organic carbon. The flux of particulate organic carbon (POC) was calculated as the product of the calculated flux of 234Th times the organic carbon/234Th ratio in trap samples. Assuming that the organic carbon/234Th ratio in trap samples was representative of sinking particles, we used an average value for the organic carbon/234Th ratio for each station. The variability in the station-averaged POC/234Th ratio ranged from 10% to 30%. The POC fluxes calculated using our combined 234Th-trap approach ranged from 1 to 6 mmol C m−2 day−1 during Survey I, and from 2 to 30 mmol C m−2 day−1 during Survey II. The average uncertainty for the POC fluxes was ±60%. Primary and new production integrated to the depth of the 0.1 % light level varied by factors of 2–3 for Survey I and Survey II, respectively. The export of particulate organic carbon from the euphotic zone also increased by a factor of 3. The corresponding e-ratios (POC export/primary production) ranged from 0.03 to 0.11 for Survey I, and 0.04 to 0.23 for Survey II. Annual average regional rates (10°N–10°S; 90°W–180°E) of new (0.47 Gt C year−1) and particulate export (0.42 Gt C year−1) production were in good agreement, suggesting that, on an annual basis, significant export of DOC need not be invoked to balance new and export production in this region.
This is the first detailed study on the distribution of lead-210 in the Japan Sea water. The content of lead-210 ranged from 9.3±2.1 dph/l in the surface water to 3.4+-0.8 dph/l in the deep water—a quite low content as compared to that in the deep water of the North Pacific. Vertical profiles show that the content of lead-210 abruptly decreases below the seasonal thermocline (10–20 m in depth) and nearly uniform in the deep water. It is suggested that a significant amount of air-borne lead-210 deposited over the Japan Sea is transported along with the Tsushima Current to the open ocean. The budget of lead-210 is calculated by using a simple box-model and the mean residence time of lead-210 in the Japan Sea is estimated to be 15 yr.
The following topics are discussed: measurement of the radon-222 flux to the atmosphere; measurement of the lead-210 atmospheric flux; and the mean residence time of aerosols. A total global model is proposed for radon-222 and its daughters.
Vertical profiles of dissolved and particulate *"Pb and "'PO were determined at two stations in the Black Sea in June 1988. Vertical fluxes of *'?'b and "?o were also measured in the upper 150 m, using floating sediment traps. The fractionation of *"Pb between dissolved and particulate phases in the Black Sea is strongly influenced by the redox conditions in the water column. Dissolved *"Pb dominates in the oxic zone, while particulate *"Pb is the major form in the deep sulfide-rich anoxic zone. The distribution of *"Pb across the suboxic zone appears to be mainly controlled by redox cycling of manganese and iron. In the sulfide-rich layer copr~ipi~tion of lead with iron sulfide is probably the dominant scavenging mechanism. A simple scavenging model was used to calculate the residence times of dissolved and particulate *"Pb in the oxic, suboxic, and anoxic zones. The residence times of dissolved "'Pb relative to scavenging by particles are OS-l, 2-3, and 3.5 years in the oxic, suboxic, and anoxic layers, respectively. The corre-sponding residence times of particulate "'Pb relative to particle removal processes in the same layers are 0.1, 1 S-2.5, and 8.5 years, respectively. A particle settling velocity of about 40 m y -' was derived from the *"Pb/*"Ra disequilibrium in the deep Black Sea. The relatively short residence times of *"Pb support the hypothesis that anoxic basins are important sites for boundary scavenging of "'Pb. The *"PO profiles indicate that biological rather than inorganic particles are the major carrier phases for polonium in the Black sea. Dissolved "'PO is deficient relative to dissolved "'Pb in the euphotic zone, suggesting preferential uptake of "'PO over 2'oPb by particles residing in that layer. The residence time of dissolved *"PO, with respect to scavenging by particles in the euphotic zone, is about 200 days. Below the mid-depth of the suboxic zone, "'PO is in excess relative to "*Pb, and is thought to originate from shelf and slope sediments. Based on the magnitude of distribution coefficients (&), the relative partitioning of lead, polonium, and thorium to particles found in the oxic and anoxic layers of the Black Sea are PO > Th > Pb and PO = Pb > Th, respectively. The dependency of KD on particle concentration suggests that colloidal phases may be important for the scavenging of these radionuclides. INTRODUff ION THE GEOCHEMICAL DISTRIBUTIONS Of trace metals have been predicted and shown to vary from oxic to sulfidic environ-ments, depending on their electronic configumtion and re-sulting class A vs. transition metal, vs. class B behavior (EMERSON et al., 1983; JACOBS and EMERSON, 1982). These intrinsic chemical affinities should also apply to the reactivity of metals with particle surfaces. Class A metals are expected to react more strongly with oxide than sulfide particles. Class B metals should react strongly with sulfides. Of elements nor-mally used to study scavenging from seawater, thorium (234Th) should be a tracer for class A behavior while lead (2'oPb), and to a lesser extent polonium (*"PO), should tend to display class B behavior. "'PO may also be a tracer for nutrient like behavior (KAIXO, 1993) . The Black Sea is an ideal location to study how the dis-tribution and fluxes of 234Th 2'oPb and 2'oPo are influenced by oxic and sulfidic enviroiments: The anoxic interface in the Black Sea has been well defined (MURRAY et al. consists of an oxic surface layer, a sub-oxic interface layer and anoxic, sulfide rich deep water (CODISPOTI et al., 199 1). The distribution and fluxes of 234Th have been presented by WEI and MURRAY (1991). Here we present the dissolved and particulate water column profiles and drifting sediment trap fluxes for "?b and *"PO collected during the 1988 Black Sea Expedition.
Decreased consumption of leaded gasoline in the United States over the
past two decades has drastically altered the flux and isotopic
composition of Pb entering the western North Atlantic from the
atmosphere. Here we exploit the resulting temporal changes in the
distribution and isotopic composition of oceanic Pb to investigate
interactions between dissolved and particulate Pb in the oceanic water
column. Measurements of dissolved Pb isotopic composition on samples
collected in 1987 in the upper water column near Bermuda demonstrate
that surface water 206Pb/207Pb decreased from
˜1.203 to ˜1.192 since 1983 and that a measurable change
propagated to 300-500 m since the 1984 profile of Shen and Boyle (1988).
The first accurate measurements of suspended particulate Pb in an open
ocean profile show concentrations of 1-3 pmol/L, equal to 2-4% of total
Pb. Vertical profiles of (1) the stable lead isotopic composition and
(2) the ratio of total Pb to 210Pb in suspended particles
closely track contemporaneous depth variations in these ratios for
dissolved Pb throughout the upper 2000 m of the Sargasso Sea near
Bermuda. Thus suspended particles reach isotopic equilibrium with
ambient sea water Pb on a time scale which is shorter than their
residence time with respect to vertical removal, in agreement with
equilibrium scavenging hypotheses based on interpretations of Th isotope
distributions. A simple flux model suggests that the effect of deep
ocean scavenging processes on the flux and isotopic composition of Pb
sinking on large particles was minor throughout the preanthropogenic and
most of the anthropogenic era but has become important as surface inputs
decrease to pre-leaded gasoline levels and may exceed the contribution
of surface-derived Pb flux in the next decade.
Disequilibrium between 210Po and 210Pb and between
210Pb and 226Ra 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 and
210Pb 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 to
226Ra 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 of 210Pb 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
(where 210Pb/ 226Ra activity ratios as low as 0.16
are observed) and near the Mid-Indian Ridge. The data suggest that
removal of 210Pb at oceanic boundaries, coupled with eddy
diffusion along isopycnals, can explain gradients in 210Pb
near the boundary. Application of a simple model including isopycnal
diffusion, chemical removal, production and radioactive decay produces
fits the observed 210Pb/ 226Ra gradients for eddy
diffusion coeffients of ˜ 10 7 cm 2/s. High
productivity in surface waters of the Bay of Bengal makes this region a
sink for reactive nuclides in the northern Indian Ocean.
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.
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.
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.
The distribution of Pb/sup 2//sup 1//sup 0/ which enters the oceans
subsequent to its production ln the atmosphere by Rn/sup 2//sup 2//sup 2/ decay,
shows an increase with depth in sea water. By using a simplified two-layer model
of the ocean, a residence time of Pb in the upper mixed layer of less than 2
years is derived. It is suggested that the marine biosphere is responsible for
the conveyance of Pb from surface to deeper waters. The distribution of Pb/sup 2/
/sup 1//sup 0/ in the Colorado River indicated a rapld removal along the path
from its origin in the feed waters to the reservoir at Jake Mead. (auth)
Three vertical water profiles of ²¹°Pb and ²¹°Po have been measured in the East China and Philippine seas. All the profiles show a large ²¹°Po deficiency of â¼ 8 dpm/cm² relative to ²¹°Pb in the top â¼ 1,000 m of the water column. Based on the ²¹°Po deficiency, the steady state flux for ²¹°Po removal from the surface water is estimated to be 14 dpm cmâ»Â²/yr. The ²¹°Po/²¹°Pb ratio of sinking particles will be too high unless an additional source of ²¹°Pb into the surface is accounted for. Thus the large ²¹°Po deficit is probably caused by the increased atmospheric input of ²¹°Pb without any significant ²¹°Po, which is focused in the Kuroshio region by isopycnal transport, and the preferential scavenging of ²¹°Po relative to ²¹°Pb by settling particles. The transient model calculations indicate that the model can account for the observed high excess ²¹°Pb inventory and large ²¹°Po deficiency in the water column if focusing of atmospherically derived ²¹°Pb in the Kuroshio water is a factor of 2-3 more than the local input. Such lateral redistribution by the western North Pacific gyre circulation is not inconsistent with the deeper penetration and the high water column inventories of anthropogenic substances observed in this region.
A method is outlined for the determination of lead-210 and polonium-210 in sea water by solvent extraction, followed by electrodeposition and alpha counting. Data are given for the lead-210 and polonium-210 content of twenty nine samples collected at a depth of 20 m in the sea around the Cape of Good Hope during March 1969. The mean activities of lead-210 and polonium-210 in these sea water samples were 38 × 10 -15 c/1 and 20 × 10 -15 c/1 respectively. Real variations in the concentrations of these nuclides were found to exist, and these could in part be ascribed to different water masses and current systems. The lead-210 and polonium-210 contents of eleven Zooplankton and three phytoplankton samples collected during the same period were also determined. Mean values for lead-210 and polonium-210 in Zooplankton were 33 pc/kg wet weight and 399 pc/kg wet weight respectively. A correlation between these nuclides in Zooplankton was found to exist, the lead-210 activity being on the average one twelfth of the polonium-210 activity. An attempt has been made to evaluate the biogeochemical balance of lead-210 and polonium-210 in the marine environment. Using the data presented, and making certain stated assumptions, the removal times of these nuclides from the upper mixed layer have been calculated. These times were found to be about 5 yr for lead-210 and 0.6 yr for polonium-210.
The distributions of Th isotopes in both suspended particulate matter and seawater were measureed in the western North Pacific and Japan Sea. Suspended particulate matter was collected simultaneously on four 142-mm-diameter Nuclepore filters (1.0 mum in pore size) by using an in situ deep-sea pumping system. Particle associations were found to be 6-40% for 228Th, 9-52% for 230Th, and 9-72% for 232Th with mean values of 26-32% for all three isotopes. In order to explain the observed Th isotope distributions in the Pacific, a reversible exchange model was developed that includes two classes (fine and large) of particle population in the ocean. The fine particles are defined as those collected by filtration and comprising most of the standing crop of particles in seawater with no significant settling flux. The large particles, on the other hand, are defined as those collected via sediment traps and represent a negligible standing stock in the water but play a predominant role in the vertical flux. The model calculation suggests that the transfer of Th from solution to fine particles is controlled by an adsorption/desorption process which depends upon the chemical reactivity of elements. Aggregation of fine particles to fast sinking large particles was estimated to occur throughout the water column on a time scale of one to five months. The large aggregates seem to disintegrate on a time scale of less than a few days, suggesting their extremely fragile nature.
The 226Ra and 210Pb concentrations in the Gulf of
California and eastern North Pacific seawaters appear to be governed by
the extents of upwelling and by the consequential biological
productivities. The residence times of 226Ra and
210Pb in the highly productive Gulf of California surface
waters are calculated to be a few months and less than a month,
respectively, on the basis of upwelling rates.
210Po, 210Pb, 228Ra, and 226Ra were measured in surface sea waters along the 1989–1990 global traverse of the oceans using the new R.V. Hakuho-Maru. Where the traverse intersects other expedition routes, the data are generally confirmatory. In the high-productivity regimes like the Red Sea, and the Arabian Sea 210Po is removed from the mixed layer at much faster rates than 210Pb. This fractionation occurs during scavenging presumably because 210Po is strongly sorbed by organic particles, whereas 210Pb is more likely associated with inorganic detritus. The 210Po/210Pb activity ratios leaving the mixed layer by particulate transport can be estimated from the steady state balance of 210Pb and 210Po in the surface waters for different oceanic regions, and are compared with those in the literature obtained by sediment-trap experiments. Although this comparison appears to merge, there exist some inconsistencies, which may be attributable to one of the following possibilities: (1) the model-derived atmospheric 210Pb flux is overestimated for the North Pacific and the North Atlantic, or (2) the sediment-trap data do not represent the real 210Po/210Pb ratio in the vertical particulate flux due to some experimental artifacts, such as incomplete trapping and size fractionation. Further careful studies of sediment trapping including seasonal variation are needed to resolve this issue.Our Ra data confirmed that strong sources for 228Ra exist in the Bay of Bengal and the Southeast Asian continental shelf zone, whereas the Mediterranean and the Red Sea, though they are surrounded by land, are not effective sources of 228Ra in the surface water.
Vertical distributions of 230Th and 228Th were measured in large-volume (2000 to 3000 liter) seawater samples collected with the Multiple Unit In-Situ Large-Volume Filtration System (MULVFS) and in-situ Fe/Mn-fiber enrichment technique. The samples were from the upper ∼800 m of the water column in the central equatorial Pacific along ∼140°W during two survey cruises of JGOFS EqPac in February/March (Survey 1) and August/September (Survey II) 1992. Significantly lower activities of 230Th and 228Th relative to those of their parent isotopes exist in the upper-ocean water column due to their particle-reactive behavior. The activities of these thorium isotopes also show large temporal variations, in response to surface productivity changes. During Survey I, a period of the El Niño, the 230Th and 228Th activities were higher than those measured during Survey II (a non-El Niño period), reflecting a depressed ocean productivity and particle flux under El Niño conditions.
Diel variations in beam attenuation profiles in the upper water column were observed during Joint Global Ocean Flux Study (JGOFS) programs in the North Atlantic (North Atlantic Bloom Experiment-NABE) and Equatorial Pacific (EqPac). Beam attenuation due to particles in surface waters increased during the day by as much as 70% and decreased at night in both studies. In the equatorial Pacific the magnitude of the variations was larger during cool non-El Niño conditions (October, 1992) than during El Niño conditions (March/April 1992). However, the percent increase from the mean beam attenuation value for each time period was similar. The daytime increases result primarily from primary production, with possible contributions from changing optical effects of living cells. Nocturnal decreases in beam attenuation, reflecting particle loss, could be caused by grazing, remineralization, and the production of large particles (aggregates, fecal pellets) with subsequent settling. The nocturnal decreases also were associated with increases in the surface mixed layer depth and increases in nutrient concentrations. Part of these diel changes could result from mixing upward particle-depleted, nutrient-enriched water from below the daytime mixed layer.Since many biological and chemical species have strong gradients in the upper 100 m, regular oscillations in the depth of mixing can be an important forcing function for vertical exchange in surface waters. When it occurs, this “mixed-layer pump” is important in sustaining new primary production and in removing particles from surface waters, and should be included in models of oceanic surface mixing.
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.
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 {ballot box} model residence times of 400 years northeast of Hawaii decreasing to 100 years at the most marginal stations.
A total of 29 vertical Ra profiles has been measured from the Pacific as part of the GEOSECS program. These profiles are located on an east-west section along ~30°N, and a north-south section, close to the western boundary of the major basins in the western Pacific. Profiles from the northeast Pacific show a deep Ra maximum, with an excess concentration relative to the potential temperature and salinity. This maximum extends westward in the direction with decreasing Ra content, and finally vanishes completely in the northwest Pacific near Japan. Ra profiles along the western boundary show a mid-depth maximum around 3 km and a near-bottom minimum due to southward intrusion of the high-Ra Pacific Deep Water and a northward spreading of the low-Ra Antarctic Bottom Water. The contrast between the maximum and the minimum intensifies toward the south, where the benthic front has clearly separated these two water masses. Ra is thus a useful tracer for the studies of oceanic mixing and circulation in the Pacific.
226Ra, 210Pb and 210Po were measured in oceanic profiles at two stations near the Bonin and Kurile trenches. 210Po is depleted by 50% on average relative to 210Pb in the surface water. In the deep water, 210Pb is about 25% deficient relative to 226Ra. 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.
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.
The distribution of ²³â´Th, ²³°Th, and ²²â¸Th between dissolved and particulate forms was determined in 17 seawater samples from the Guatamala and Panama basins. Sampling was carried out in situ with battery-powered, submersible pumping systems in which the seawater first passed through a Nuclepore filter (1.0-..mu..m pore size) and then through a cartridge packed with nitex netting that was impregnated with MnOâ to scavenge the dissolved Th isotopes. Natural ²³â´Th was used as the tracer for monitoring the efficiency of scavenging. For all three isotopes, most of the activity was found in the dissolved form. On the average 4% of the ²³â´Th, 15% of the ²²â¸Th, and 17% of the ²³°Th occurred in the particulate form. On the average 4% of the ²³â´Th, 15% of the ²²â¸Th, and 17% of the ²³°Th occurred in the particulate form, though the percentages were found to be strongly dependent on particle concentration. These distributions are not consistent with chemical scavenging models that assume irreversible uptake of Th on particle surfaces. the results can be explained, however, if continuous exchange of Th isotopes between seawater and the particle surfaces is assumed. Vertical profiles of both particulate and dissolved ²³°Th show increasing concentrations with depth, as required by the assumption of reversible exchange. Some of the dissolved ²³°Th profiles, however, show a reversal of this trend near the bottom, indicating accelerated scavenging near the water/sediment interface. Kinetics of both adsorption and desorption can be examined if at least two Th isotopes are measured in the same samples. Results show that reaction times are short (a few months) compared to the residence time of suspended matter in the deep ocean (several years) indicating that particles suspended in the deep sea are close to equilibrium with respect to exchange of metals at their surfaces.
Profiles of dissolved and particulate 234Th were determined at several stations within the Cali- fornia Current. Modeling of the disequilibria between the 234Th and 23aU within the surface waters provides for estimates of the residence time of dissolved thorium with respect to particle scavenging (TP varies from 6 to 50 days), the particle residence time (TP varies from 2 to 20 days), and the particulate 234Th flux exiting the surface layer. The model-derived, first-order scavenging rate constant for dissolved thorium is observed to be proportional to the rate of primary production. Particle residence times seem to be governed by the rate of zooplankton grazing and the types of zooplankton present. Model-derived particulate 234Th fluxes are in good agreement with direct measurements by sediment traps.
Four vertical 210Pb profiles obtained within 250 m above bottom in the deep western North Pacific showed no significant variation with depth, irrespective of marked increase of excess 222Rn relative to 226Ra near the seafloor. Consequently, the apparent box-model residence time based on 210Pb/222Rn activity ratio sharply decreases from ~ 100 years in ocean interior to < 10 years near the bottom. However, the 210Pb data can also be explained using a vertical mixing and first-order scavenging model without any enhanced scavenging near the sediment-water interface. This model calculation indicates that bottom excess 222Rn can hardly influence 210Pb in the water and the short residence time derived using box-model is an artifact and does not reflect chemical reactivity of Pb. This also implies that the deficiency of 210Pb as a result of removal by scavenging is best estimated based on 226Ra and not on 222Rn. The 210Pb scavenging at the sediment-water interface that has been inferred from the oceanic distributions of 210Pb appears difficult to be detected from bottom 210Pb profile measurements.
210Pb and 226Ra profiles have been measured at five GEOSECS stations in the Circumpolar region. These profiles show that 226Ra is quite uniformly distributed throughout the Circumpolar region, with slightly lower activities in surface waters, while 210Pb varies with depth as well as location or area. There is a subsurface 210Pb maximum which matches the oxygen minimum in depth and roughly correlates with the temperature and salinity maxima. This 210Pb 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 to 226Ra, the activity of 210Pb is deficient in the entire water column of the Circumpolar waters. The deficiency increases from the depth of the 210Pb maximum toward the bottom, and the 210Pb/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 than 226Ra in this region.
The deficiency of /sup 210/Pb relative to /sup 226/Ra in the world's deep ocean is well documented, and the overall residence time of /sup 210/Pb has been calculated to be about 15 to 100 years. It has been assumed, generally, that the removal mechanism is one of in situ adsorption on settling particles, but Bacon et al. (1976) suggested that a boundary scavenging process with diffusive and advective fluxes of /sup 210/Pb from the interior ocean may be a significant factor. In this contribution researchers examine some two-dimensional models of /sup 210/Pb in a section across the North Atlantic Ocean at 15N. Researchers show that satisfactory fits to the /sup 210/Pb distribution may be obtained with a model that includes both boundary scavenging and in situ adsorption with a constant adsorption rate constant or with a model that has no boundary scavenging but allows the in situ adsorption rate constant to be a function of distance from the boundary. Researchers have calculated the interior and boundary fluxes of /sup 210/Pb required of each model and suggest that the boundary fluxes are of a magnitude to be compatible with Fe and Mn redox cycling in slope and shelf sediments but that the variations in the in situ adsorption flux are difficult to explain in terms of the known distributions of fine particles. Researchers suggest that the boundary scavenging process is a function of the flux of Fe/sup +2/ and Mn/sup +2/ from anoxic sediments accompanied by rapid oxidation and precipitation in the boundary mixed layers.The boundary fluxes calculated for model amount to 2 to 8 dpm cm/sup -2/ yr/sup -1/ for slope and shelf sediments but only about 0.04 dpm cm/sup -2/ yr/sup -1/ for the deep ocean bottom boundary. The bottom boundary flux in the deep ocean is small and would not be detectable above the large particle flux of about 0l7 dpm cm/sup -2/ yr/sup -1/ contributed by biological removal from the ocean surface.
>/sup 210/Pb and /sup 210/Po in sea water of the Pacific. were
determined and applied to make clear the behavior of lead in the surface water of
the ocean. The concentration of /sup 210/Pb in the surfuce water was much
smaller than the expected value, if it were soluble in seawater. Its
geographical distribution showed a latitudinal variation that presented higher
values in both mid-latitudes of the Pacific and lower values in the tropical
Pacific and the southern South Pacific. These facts suggest that lead is
settling with the velocity of (1-5) x 10/sup -4/ cm/sec in the surface layer.
However, it is very unlikely that the lead exists as pure solid phase of lead
oxide. The lead probably forms an eutectic solid phase with other host metals
such as manganese oxide. The ratios of /sup 210/Po to /sup 210/Pb in surface
water were larger than those in meteoric precipitation, but most of thera were
smaller than unity. (auth) Data are presented as a basis of evaluating the
increase in activity of waters in the vicinity of Jaslovske Bohunice due to
startup of the nuclear power plant. The monitoring of radioactivity of waters on
the territory of western Slovakia conducted between 1968 and l970 showed that the
average annual values were constant and that annual fluctuations were due to an
increased fallout and to meteorological conditions. (INIS)
Sediment trap experiments have been carried out at sites in the Sargasso Sea (S/sub 2/) and in the Atlantic off Barbados (E) to determine the mass flux and chemical composition of material sinking to the sea floor. At the S/sub 2/ site, the mass flux increases with depth, at the E site the flux is constant. Chemical analyses show that K, Ti, Al, La, V, Co and /sup 232/Th are derived largely from terrigenous materials whereas Ca, Sr, Mg, Si, Ba, /sup 220/Ra, U and I are carried by biogenic particles. The reactive elements Mn, Cu, Fe, Sc, and /sup 230/ /sup 234/Th, show increasing ratios to Al with depth due to scavenging from the water column. A mean particle settling velocity of 21 m day/sup -1/ and scavenging residence times ranging from 22 years for Th to 500 years for Cu have been calculated. The flux of aluminosilicates increases with depth, and this cannot be attributed to changes in trap efficiency. Thus horizontal transport of material must be invoked.
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.
A method is described for the determination of Th isotopes (232Th,230Th,228Th and227Th) in seawater through analysis of Th adsorbed on MnO2-impregnated fiber that has been moored in the deep sea for up to 10 months. Since the MnO2-fiber adsorbs Th from seawater at a constant rate, natural234Th can be used as a yield monitor by making a correction for its decay during the period of deployment. The results obtained by the method showed good reproducibility and accuracy. The method has the advantage over the chemical coprecipitation method that the time and labor for sampling and processing a large-volume of seawater is reduced.
During October 1990 in the western equatorial Pacific, from 5°S to 5°N along 155°E, the isothermal and isopycnal lines (near 150 m) domed upwards, surface temperatures were high (29.5–30.5°C), surface salinites were low (33.8–34.2), SST anomalies were <0.5°C between 120°E and 160°E, and the westward flowing SEC was well developed. These conditions are typical of non-ENSO conditions. An eastward flowing surface current was observed that was probably the result of a westerly wind burst. Between 5°S and 5°N, the barrier layer was much thicker south of the equator and the thickness of the barrier layer varied from 88 m at 2°S to 6 m at 1°N. Nitrate and nitrite were not detected in surface waters, but phosphate and silicate were present at concentrations of about 0.15 μM and 1.5 μM, respectively. Nitrate was always present at the deep chlorophyll maximum (DCM), which varied in depth from about 74 to 96 m.
210Pb and 226Ra profiles have been measured in the North Atlantic and North Pacific Deep Water. The 210Pb activity is 25% to 80% of that of 226Ra, averages about 50% in each profile, and is lowest in the bottom water. This deficiency of 210Pb relative to 226Ra shows that 210Pb is rapidly and continually scavenged from deep water, probably by adsorption on particulate material sinking from the surface. A method is developed for simultaneous application of the vertical diffusion-advection model to 226Ra and 210Pb deep-water profiles, and the in-situ source terms for both isotopes are obtained as a function of the parametric upwelling velocity. For a positive source term for 226Ra in deep water, the parametric velocity is limited to a small range (3–12 m/y) and the deep-Pacific residence time of lead is 54 yr, a value two orders of magnitude smaller than residence times estimated from stable lead concentrations. The model calculations show that an in-situ scavenging process, acting throughout the water column, is required to remove the Pb. Box model calculations yield a similar short residence time for lead in North Atlantic Deep Water, indicating that radioactive disequilibrium for 210Pb, due to fast scavenging, is a general phenomenon through the deep sea.
Laboratory experiments were conducted to determine the efficiency with which improved manganese-oxide-coated acrylic fibers extract radium, thorium, and actinium from seawater. Tests were made using surface seawater spiked with227Ac,227Th, and223Ra. For sample volumes of approximately 30 liters and flow rates up to 0.5 liters per minute, radium and actinium are removed quantitatively. Approximately 80–95% of the thorium is removed under these same conditions.
Seven GEOSECS stations in the western Indian Ocean were measured for dissolved210Po, and five of these were also measured for particulate210Po. Dissolved phase210Po is lower than210Pb in the deep water north of Madagascar, with values in the Arabian Sea being lower by about 4 dpm/100 kg (∼ 30%) for almost the entire water column. South of Madagascar,210Po is essentially in equilibrium with210Pb in the deep water, but with210Po excess in subsurface water south of the Crozet basin. In the benthic boundary layer,210Po is consistently lower than210Pb for all the stations.In the particulate phase,210Po shows a general increase with depth and is highest in the benthic boundary layer. In general,210Po is lower than210Pb in the particulate phase, but at the one Somali basin station where data are available, deep water210Po is systematically higher than210Pb. A mid-depth maximum of particulate210Pb is generally associated with that of dissolved210Pb.The partition of210Po between the particulate and dissolved phases is not significantly different from that of210Pb. The activities of the two nuclides in the particulate phase are generally less than 10% of those in the dissolved phase. The range is from 1% to 60%. High particulate and low dissolved activities are observed in the deep water, especially in the benthic boundary layer.
The extent of radioactive disequilibrium between234Th-238U,210Po-210Pb and210Pb-226Ra has been measured in four vertical profiles from the northeastern Arabian Sea, a region characterized by oxygen deficient and denitrification layers at intermediate depths. The most common feature of the data is that the dissolved concentrations of the daughter nuclides are considerably less than those of their parents, exceptions being the near equilibrium or excess concentrations of210Po (relative to210Pb) at ∼100m and that of210Pb relative to226Ra in surface waters. These results suggest that scavenging of dissolved234Th,210Po and210Pb occurs throughout the water column in the northeastern Arabian Sea. In the mixed layer, the scavenging residence time of Th is ∼1–2months and that of Po and Pb is ∼4–8months. At depths greater than ∼300m, the mean dissolved [210Po/210Pb] and [210Pb/226Ra] are ∼0.75 and ∼0.4, respectively,representing some of the most pronounced disequilibria observed in the deep sea. The particle reactivity of the measured nuclides increases as Th > Po > Pb.The238U concentration and the extent of234Th-238U disequilibrium are similar in the oxic surface layers and su☐ic intermediate waters, indicating that U and Th distributions are not significantly affected by oxygen depletion and denitrification processes. On the contrary, both210Po-210Pb and210Pb-226Ra disequilibria show enhanced scavenging of210Po and210Pb at stations that are characterized by intense denitrification.Modelling of dissolved210Po profile in the surface ∼100m shows its potential to yield eddy diffusion coefficients in surface waters and to place constraints on the new production rates. Results obtained in this study, although encouraging, stress the need for further work to confirm some of the assumptions of the model.
In order to determine the areal extent and distribution of 231Pa sinks in the Pacific Ocean, we have analyzed 34 deep-sea surface sediment samples from widely distributed locations for their Pa, Th and U isotopes. While the 230Thxs content varies by more than a factor often, the 231Paxs content is confined in a relatively narrow range with a mean value of 2.8 ± 1.4 dpm/g. There is a positive correlation between 230Thxs and 231Paxs and, using the regression line it is possible to calculate the () activity ratios in samples for which only 230Th data are available. We then constructed a map of () distribution. Using () < 11 (the production ratio in the water column) as a criterion for identifying preferential 231Pa sinks, we have identified an area covering 30–40% of the Pacific Ocean. This area includes most of the Pacific margins, the northwest Pacific, the Antarctic Ocean and perhaps the mid-ocean ridge regions, and is nearly coincident with that of high sediment accumulation rates: this suggests that the paniculate flux is important for the removal of 231Pa from the ocean.Two models for the removal of 231Pa produced in the open ocean are: 1.(1) lateral transport into the vertical flux in regions of high particulate flux (Damaster, 1979; Andersonet al, 1983a,b) or2.(2) uptake at the sediment-water boundary: the second model is more likely to be correct. To distinguish unequivocally between the two proposed models, we would need sediment trap data for the western North Pacific.
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.
The upper ocean 234Th activity distribution at 77 stations was measured between 12°N and 10°S, and 95°W and 170°W in the spring and autumn of 1992. A regional scavenging model was used to estimate vertical export of particulate 234Th. Given the relatively high upwelling rates in this region, particularly at equatorial latitudes near 140°W, it was necessary to include upwelling of 234Th in our model in order to quantify particulate export. Using this export flux and the measured organic C or N to 234Th ratio on particles, one can empirically determine POC and PON fluxes for this region. The estimated particulate organic C flux varies spatially and temporally within this region, ranging from 1 to 7 mmol C m−2 day−1, with enhanced export occurring over the equator. Fluxes are also enhanced along 95°W coincident with a low temperature/high nutrient peak at 4°S. Along 140°W, particulate organic C export from the upper 100 m is on the order of 2 mmol C m−2 day−1 at latitudes beyond 4°N and 4°S, with an equatorial peak of 3–5 mmol C m−2 day−1 in both spring and fall. These results suggest that a relatively small per cent of the total production is exported locally on sinking particles (particle export/primary production <5–10%). This finding of low particle export is relatively insensitive to the chosen upwelling rates or particulate organic ratios. Given the measured ratio, particulate N fluxes from the upper 100 m would be 6 times lower than for POC.
Four vertical profiles of230Th and228Th were determined using large volume water samples in the western North Pacific. An almost linear increase of230Th with depth was observed for all of the profiles for which the unidirectional first order scavenging model was difficult to explain. We developed a model which included a dissolved-particulate transformation as well as parameters of the scavenging model. Application of the model to the vertical distributions of total and the GEOSECS particulate Th isotopes (230Th and234Th) yielded the residence time of dissolved Th with respect to adsorption to particles and the turnover time of particulate Th to be 235 days and 57 days, respectively. The Th isotopes appeared to be carried down the water column by fine particles with a mean settling velocity of 1 m/day which continually release Th into sea water as well as pick up Th from the water along their journey to the bottom.For228Th, a large excess over232Th was observed throughout the water column with pronounced high concentrations in surface and bottom waters, suggesting that the228Th was derived from228Ra diffused out of sediments. The vertical distributions of228Th seemed to be significantly influenced by lateral mixing along isopycnals.
Distributions of 234Th were determined in three particle-size classes ( > 53, 1–53 and 0.7–1.0 μm) and in filtered seawater during each of the two time-series cruises of the U.S. JGOFS Process Study in the equatorial Pacific. Four vertical profiles were measured on the equator at 140°W from the sea surface to 400 m depth between 24 March and 9 April 1992 (Time-series I) and again between 3 and 18 October 1992 (Time-series II). In addition, both organic and inorganic carbon were measured in each of the particle fractions. The results were used with a one-dimensional model, which includes the equatorial upwelling, to estimate the flux of particulate carbon sinking out of the surface layer. The flux of particulate organic carbon (POC) at the base of the euphotic zone (0.1 % light level, 120 m depth) was estimated to average 1.9 mmol m−2 day−1 during El Niño (Time-series I) and 2.4 mmol m−2 day−1 during the cold period that followed (Time-series II). These values amount to only ∼ 2% of the primary production measured during each of the same periods and are insufficient to balance the new production, estimated previously to be ∼ 17% of primary production. These results are consistent with the hypothesis that the major part of the new production is removed from the region by advection in the form of dissolved organic matter. The POC flux profile indicates a net remineralization below the 1 % light level (80 m depth) such that the flux reaching 200 m depth has been reduced by ∼ 55%, giving a remineralization length scale of ∼ 155 m. For particulate inorganic (carbonate) carbon the flux at 200 m averaged 0.54 mmol m−2 day−1 during Time-series I and 0.71 mmol m−2 day−1 during Time-series II, very similar to the fluxes reported in deep sediment traps deployed at the same time. Estimates of the average large-particle sinking velocity give values < 10 m day−1 in the upper part of the euphotic zone, show a sharp increase near the base of the euphotic zone and level off to values of 30–60 m day−1 at 200 m depth.
We present evidence for strong hydrothermal activity in the eastern Manus Basin (depth: 1700–2100 m), the existence of large scale triple-layered buoyant plumes at depths of ∼1100 m (“shallow plume”), ∼ 1700 m (“deep plume”), and ∼1400 m (“middle plume” with less extent than the other two plumes) that were revealed from water column anomalies of CH4, Mn, Al and pH observed in November to December 1990. Judging from the horizontal distribution of these parameters, the deep plume seems to originate from two distinct hydrothermal sites (eastern and western sites) in the research area, the eastern site being visually ascertained with deep-tow observations at the same time. The CH4/Mn ratio (mol mol−1) of the deep plume (0.02–0.05) is the lowest yet observed in hydrothermal plumes. The order of magnitude difference of CH4/Mn ratios between the shallow plume and the deep plume suggests that different kinds of fluid-rock interaction occurred to make the hydrothermal end members for the deep and shallow plumes. The shallow plume, which had an areal extent of more than 50 km, may be an episodic “megaplume”, because it was not recognized in the previous CH4 profiles in 1986, and because it has a similar CH4/Mn ratio as the megaplume observed in the North Fiji Basin. We found that the eastern deep plume is characterized by enormously high aluminium concentrations (0.6– 1.5 μmol kg−1), pH anomalies (∼0.1) and high Al/Mn ratios (10–17). The endmember fluid for the eastern deep plume may have an unusually low pH value to dissolve this much aluminum during fluid-rock interaction, or this plume may originate from an eruption-influenced fluid.
Adsorption (surface complexation) has long been considered to be the dominant process involved in the oceanic scavenging of many trace metals. Much of what we know about metal removal in the ocean (i.e. rate and extent) is based on measurements of U and Th decay series isotopes. However, the scavenging equations developed from radioactive parent-daughter relationships presume no specific metal removal process and cannot be directly used to verify a particular one.In this paper we examine and compare the phenomenological model of adsorption and oceanic scavenging observations. The formalisms of surface coordination and colloid chemistry are linked to the mechanism-free observations of oceanic trace metal scavenging by the strong similarities in the description of the reaction rates and the influence of particle concentration on those rates and the equilibrium distributions. The correspondence between laboratory sorption data and field scavenging observations as well as the consistency of the hypothesis over a wide range of environmental systems successfully link oceanic trace metal scavenging with surface coordination and colloid aggregation reactions. The merging of descriptions of surface and colloid chemistry and field observations of scavenging provide a framework for interpreting field data and understanding how master variables (e.g. reaction rate, particle concentration, or particle flux) influence metal removal from the oceans.
Vertical and horizontal distributions of dissolved and suspended particulate Fe and Mn, and vertical fluxes of these metals (obtained with sediment traps) were determined throughout the Pacific Ocean. In general, dissolved Fe is low in surface and deep waters (0.1 to 0.7 nmol/kg), with maxima associated with the intermediate depth oxygen minimum zone (2.0 to 6.6 nmol/kg). Vertical distributions of dissolved Mn are similar to previous reports, exhibiting a surface maximum, a subsurface minimum, a Mn maximum layer coincident with the oxygen minimum zone, and lowest values in deep waters.Near-shore removal processes are more intense for dissolved Fe than for dissolved Mn. Dissolved Mn in the surface mixed layer remains elevated much farther offshore than dissolved Fe. Elevated near-surface dissolved Mn concentrations occur in the North Pacific Equatorial Current, suggesting transport from the eastern boundary. Near-surface mixed-layer dissolved Mn concentrations are higher in the North Pacific gyre reflecting enhanced northern hemisphere aeolian sources.Residence time estimates for the settling of refractory paniculate Fe and Mn from the upper water column are 62–220 days (Fe), and 105–235 days (Mn). In contrast, residence times for the scavenging of dissolved Fe and Mn are 2–13 years (Fe) and 3–74 years (Mn). Scavenging residence times for dissolved Mn based on horizontal mixing in the surface mixed layer of the northeast Pacific are 0.4 years (nearshore) to 19 years (1000 km offshore).There is no evidence for in situ Fe redox dissolution within sub-oxic waters in the eastern tropical North Pacific. Dissolved Fe appeared to be controlled by dissolution from sub-oxic sediments, with oxidative scavenging in the water column or upper sediment layers. However, in situ Mn dissolution within the oxygen minimum zone was evident.
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.
A simple model that accounts for the formation of the Mn maximum in the oxygen minimum is presented here. In this model, Mn is proposed to cycle in a constant proportion to carbon, as do nitrogen and phosphorous. Superimposed on the Mn-carbon cycle is the removal of Mn(II) via scavenging onto sinking particles and transport by vertical diffusion. Scavenging is assumed to follow the rate law observed in the laboratory for Mn(II) oxidation. Manganese (II) concentrations were calculated with the model at stations in the Pacific and Atlantic Oceans and compared with measurements of dissolved Mn. All parameters in the model were based on laboratory measurements or field observations. The model reproduced Mn(II) maxima of the correct concentration and at the correct depth. This agreement was observed at a range of oxygen concentrations. The calculations demonstrate that the Mn maximum can form because of a reduction in the pseudo-first order scavenging rate constant (k′) within the oxygen minimum. The value of k′ will decrease in regions of the water column with low oxygen and pH (k′ = k0 [O2] OH−2). These regions will accumulate higher dissolved Mn(II) concentrations before the rate of Mn(II) removal, k′ [Mn(II)], equals the input from remineralization of POC and a steady state is reached. An additional source of Mn, such as flux from continental margin sediments or dissolution of Mn oxides, is not necessary to account for formation of the Mn maximum.