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Simultaneous concentration and determination of 226Ra, 228Ra in natural waters
... Seawater sample with a volume of 120 L for radium measurement was put into two 60 L PVC buckets by a submerged pump. Seawater was filtered through a PVC pipe packed with 12 g MnO 2 -coated fibers at a flow rate of about 250 mL/min to obtain a high enrichment efficiency of radium (>90%) [Xie et al, 1994]. After the extraction, the MnO 2 -coated fibers were sent back to the onshore laboratory for 226 Ra and 228 Ra analysis. ...
... During the measurements, five internal standards with different δ 18 O values (including -6.91‰, -4.86‰, -2.79‰, -0.23‰, 2.15‰) were synchronously measured after every few samples for data quality assurance. These standards were periodically calibrated against the Vienna and t refers to the same counting time for the sample and background (min) [Xie et al., 1994]. k 226 and η were assessed using a 226 Ra standard with known activities (21.90 Bq). ...
... The emanation efficiency of 222 Rn from MnO 2 -coated fibers could be influenced by the compaction of fibers and the gas transfer conditions. In this study, 12 g of the fibers were used and the water content in the fibers was about 10%, which can effectively avoid the compaction effect of the fibers [Xie et al., 1994]. The gas transfer was achieved using the bubble method under a constant airflow rate and time. ...
Seawater samples for the measurements of ²²⁶Ra, ²²⁸Ra and stable oxygen isotope (δ¹⁸O) were collected from the Bering and Chukchi Seas in the summer of 2014. The fractions of meteoric water (fMW) and sea-ice melted water (fSIM) were estimated based on the mass balance of salinity and δ¹⁸O with a three end-member mixing model. Our results showed that the average fMW increased northward from the Bering Basin to the Canada Basin while the fSIM distributed homogeneously. The lowest fMW and ²²⁸Ra/²²⁶Ra)A.R. values were found in the upper Bering Basin with little terrestrial input. The highest fMW but low ²²⁸Ra/²²⁶Ra)A.R. appeared in the northern Chukchi Sea and the Canada Basin, ascribing to the current-driven accumulation of freshwater and its long residence time. More abundant sea-ice melted water was found on the pack-ice edge, indicating the trap of earlier melted waters by the ice pack. Based upon the linear relationships between ²²⁸Ra/²²⁶Ra)A.R. and fMW in the Bering Shelf and the Chukchi Shelf, the transit time for the Pacific inflow was constrained. The transit time of river water from the Bering Shelf to the Chukchi Shelf was estimated as 0.2−4.4 years with an average of 1.6±1.5 years, while that from the Chukchi Shelf to the Canada Basin was 10.2−13.2 years with an average of 11.8±1.1 years. The spatial variation of the transit time was mainly affected by the current intensity. Our study highlights the importance of in-depth evaluation for the subarctic-arctic exchange.
... In the onshore laboratory, the MnO 2 fibers were unwound and heated in 300 ml 2 mol/l HCl plus 2 ml concentrated NH 2 OH Á HCl solution on a hot plate until the color of the fibers turned whitish. Subsequent analysis involved isolation and b-counting of the short-lived 228 Ac daughter, following the method described in detail by Xie et al. (1994). Although the classic measurement with a spectrometry through decays of its granddaughter ( 228 Th) offers not only high resolution but also the highest efficiency and lowest background of any 228 Racounting method used today, a long wait of B1 a or more is required before the 228 Th granddaughter can grow to a significant level . ...
... Radioactive decay curve of 228 Ac determined by the procedure ofXie et al. (1994). ...
228Ra, 228Th and 234Th in the upper 300 m of the water column were measured to quantify new production and export flux of POC at the intercalibration station in the South China Sea during November 1997. Surprisingly high 228Ra concentrations of about 210 dpm m−3 in the surface mixed layer were observed. The concentration decreased considerably, from >200 dpm m−3 at 25 m to ∼43 dpm m−3 at 200 m, then increased to 87 dpm m−3 at 300 m close to the bottom (460 m), coincident with the typical distribution pattern of 228Ra in open oceans. Concentrations of dissolved and particulate 228Th ranged from ∼45 to ∼27 dpm m−3 and from ∼40 to ∼10 dpm m−3, respectively, and showed a general decrease with increasing depth. A 234Th deficit relative to 238U was also observed in the upper 150 m, as quantified by β-counting. The upward flux of nitrate into the euphotic zone was calculated by the coupled 228Ra-nitrate approach and further converted into a new production of 4.4 mmol C m−2 d−1 based on a Redfield ratio of 6.6 for C : N. The 234Th–238U disequilibrium and the measured ratio of POC to particulate 234Th yielded a POC export flux of 5.7 mmol C m−2 d−1, which is in general agreement with the new production derived from nutrient budgets. POC export based on the 228Th–228Ra disequilibrium was estimated to be 1.7 mmol C m−2 d−1, significantly lower than the derived new production. DOC transport or accumulation, data uncertainties and the different time scales to which the various methods were applied could be reasons for the discrepancy. The vertical fluxes of nitrate and phosphate to the euphotic zone bear a N/P molar ratio of 35, which is significantly higher than the Redfield ratio and suggests P-limitation in the southern South China Sea.
... Three 226 Ra and 228 Ra cartridge standards were made and preprocessed as described in Section 2.3. The recovery ranged from 94% to 99% for 226 Ra and from 100% to 102% for 228 Ra (Table S2), greater than 85% in the study of Xie et al. (1994). We also conducted an experiment to test the recovery of direct transferring the ash to the measurement vial. ...
In the open ocean, radium isotopes are useful tracers of residence time and water-mass mixing. However, limited by the measurement resolution of commonly used gamma counters, the low activity of radium in the open ocean makes it necessary to enrich radium from large volumes of seawater and pretreat radium-enriched carriers prior to measurements. The commonly applied method of radium enrichment and pretreatment, however, has limitations of uneven coating of MnO2 on cartridges, relatively expensive cartridges, time-consuming issues during cartridge-ashing, ash loss during transfer, and changes of gamma counters efficiency caused by different ash weights. To address these issues, in this study we optimized the enrichment and pretreatment of low-activity radium prior to measurements. Firstly, we replaced commonly used acrylic cartridges with cheaper polypropylene cartridges, which took 6 h to be ashed, 42 h shorter than for acrylic cartridges. Secondly, MnO2-coated cartridges were prepared with a circulating hot acidic KMnO4 solution to ensure homogeneous coating. The radium extraction efficiency of this MnO2-coated cartridge was 20%–61% higher than that prepared by directly immersing cartridges in the solution. The radium delayed coincidence counter efficiency for MnO2-coated cartridge was stable with a moisture content of 0.05–1. Lastly, after ashing cartridges, instead of directly transferring the ash to a measurement vial, a mixture of hydroxylamine hydrochloride and hydrochloric acid was used to completely leach the ash for long-lived radium, followed by coprecipitation by BaSO4, to avoid potential loss of ash during transfer and variations in measurement geometry due to different ash weights. And the recovery of long-lived radium pretreatment was 94%–102%, which improved by 11% compared with the common method. In addition, the radium extraction efficiency of the MnO2-coated cartridge varied from 3% to 4% within the in situ pump working flow rate of 4–7 L/min, which fell within the measurement errors.
... To determine 226 Ra activity concentration of the sea ice endmember, sea ice samples were collected at around 691S, 76.31E and melted together under room temperature. The method for 226 Ra measurements is after Xie et al.(1994). Water samples were then concentrated by passing through a column with 12 g MnO 2 -fiber. ...
... The MnO 2 -fiber was taken out of the column after the enrichment process and water held in the fiber was squeezed out gently before it was sealed inside a plastic bag and taken back to the onshore laboratory for 226 Ra measurement. The procedure of MnO 2 -fiber preparation and in situ concentration of Ra from seawater had been discussed in detail by Xie et al. (1994b), and is similar to the pioneer method established by Moore and Reid (1973). ...
During the 13th (1996–1997) and the 19th (2002–2003) Chinese National Antarctica Research Expeditions, we collected 60 discrete surface seawater samples along the cruise from the Chanjiang River (Yangtze) estuary (30°59′S, 122°26′E) through Taiwan Strait, the South China Sea, and the Eastern Indian Ocean to Prydz Bay, Antarctica (69°10′S, 74°30′E), and analyzed them for the 226Ra specific activity. The 226Ra specific activity of the Chanjiang River estuary surface water (3.15 Bq/m3) was found to be the highest among all the surface samples because of the desorption of 226Ra from riverine particles. Between Chanjiang River estuary and 40°S, 226Ra specific activity was found to be relatively uniform with a mean value of 1.07 Bq/m3 (n = 19, SD = 0.14), similar to that of the open ocean. From 40°S to 65°S, 226Ra specific activity increased intensively, then decreased moderately further southwards. Near the Antarctic shore, it increased again, to 2.31 Bq/m3. This distribution was controlled by a combination of deep water upwelling, Southern Ocean fronts, water mixing and the continental 226Ra import. In Prydz Bay and the adjacent sea area, the mean 226Ra activity value was 2.26 Bq/m3 (n = 31, SD = 0.28), with a relatively higher value outside of the bay and low 226Ra activity value in the center of the bay. This was consistent with the topography and hydrological setting of the bay. In addition, we extended the study area northward to the Arctic, by combining the published 226Ra dataset for surface water from the Bering Sea to the Japan Sea. We also discuss the 226Ra distribution of high latitude oceanic surface water and its mechanisms.
... The standard solution for silicate determination was GBW 08647. Full detailed procedure for silicate determination was described in ref. [10]. Analytical uncertainty for reactive silicate measurement was 0.1 μmol/dm 3 . ...
Surface seawater was collected for 226Ra measurement in the North Pacific Subtropical Gyre from July to October, 1999 and October to December, 2003. Combined with
the historical data reported for this sea area, a declined trend of surface 226Ra concentrations was observed since 1960s, indicating the ecosystem shift in response to global warming. On one side, the
enhanced stratification of the upper water column resulting from global warming reduced the 226Ra input from the depth, on the other, the temporal increase of biological production resulting from the climate-related ecosystem
structure change strengthened the 226Ra removal from the surface ocean. Both the physical and biological processes resulted in the decrease of surface 226Ra concentrations in the North Pacific Subtropical Gyre. The temporal trend of surface 226Ra concentrations was consistent with the trends of chlorophyll a, silicate, phosphate and primary production previously reported. This study provided 226Ra evidence for the ecosystem shift under global change.
Submarine groundwater discharge (SGD) is part of the global water cycles, which delivers a large amount of solutes to the biogeochemical cycle and affects the coastal ecological environment. Natural radium isotopes (224Ra and226Ra) were used to estimate the SGD in Jiaozhou Bay and the mass balance models were established. Radium sources including river transport, sediment diffusion, desorption from re-suspended particles, open sea flood tide, municipal wastewater, and SGD are considered. Radium output owning to the radioactive decay and ebb tide are also considered. Our results show the SGD fluxes during September to October in 2011(autumn) and April to May in 2012 (spring) are 15.17 and 6.53×106 m3d-1, respectively. In the same period of time, the input of the dissolved inorganic nitrogen (DIN) from SGD is very close to that from river, yet, the soluble silicate and reactive phosphate vary seasonally. SGD is the major source of the nutrient input of Jiaozhou Bay. © 2014, National Institute of Science Communication and Information Resources (NISCAIR). All rights reserved.
The specific activities of 224Ra, 226Ra and 228Ra were determined by measuring γ-rays of their daughters with HPGe γ spectrometer after preconcentrating Ra isotopes with Mn-fiber adsorption from large volume seawater. The calculating formulas of specific activities of the three nuclides were derived. The methods of sample preparation and spectrum analysis were discussed. The advantages of the method are simple, rapid and three nuclides can be determined simultaneously.
It's very difficult to measure submarine groundwater discharge (SGD) directly, but naturally occurring radium isotopes and radon tracers make it possible to estimate SGD fluxes indirectly. In order to evaluate the residence time of seawater and SGD fluxes in Wuyuan Bay, Xiamen, China, we measured the 224Ra and 226Ra activities in the bay water, open ocean seawater and groundwater, calculated the water parcel ages and average residence time in the Bay based on the differences in half-life between 224Ra and 226Ra, and estimated the SGD fluxes via the mass balance on 224Ra and 226Ra. The water parcel ages range from 0.6 to 2.4 d among thirteen sampling stations, with an average residence time of 1.4 d. The water parcel ages in the bay head relatively get older than those in the bay mouth. The calculated 224Ra and 226Ra fluxes driven by SGD are 5.17×106 Bq/d and 5.28×106 Bq/d, respectively. Both radium fluxes are further converted into SGD fluxes by dividing the activities of groundwater end-member to be 0.21 m3/m2/d (224Ra mass balance model) and 0.23 m3/m2/d (226Ra mass balance model), respectively. The calculated results from two models are so close that their average of 0.22 m3/m2/d can be considered as SGD fluxes in Wuyuan Bay.
Radium diffusion from sediments to overlying water is one of the important sources of radium isotopes in ocean water. A series of laboratory desorption/diffusion experiments were conducted to help elucidate the characteristics of sediments on 224Ra and 226Ra desorption and diffusion, which included 1) 224Ra and 226Ra desorption from sediments in water of different salinities, 2) 224Ra and 226Ra desorption from sediments of different grain sizes in water of the same salinity, 3) 224Ra and 226Ra diffusive fluxes by observing Ra enrichment with time in the overlying water of incubated sediments. The experimental results show that there is an increase in the amount of 224Ra and 226Ra with the increase of salinity from 5 to 30, and Ra desorption is strongest at the salinity of 25. Under the same salt water condition, 224Ra and 226Ra desorption activities of the four grain sizes (2000~1000 μm, 1000~500 μm, 500~250 μm and 250~125 μm) of sediments are very close to each other. When the grain sizes of sediments are larger than 2000μm, the 224Ra and 226Ra desorption activities are slightly higher than those of the sediments of the above-mentioned four grain sizes. When the grain sizes of sediments are less than 125 μm, the 224Ra and 226Ra desorption activities are much larger than those of the above-mentioned five grain sizes. The average diffusive fluxes of 224Ra and 226Ra of sediments from Jiaozhou Bay are 0.85 Bq·m-2·d-1 and 0.022 Bq·m-2·d-1, respectively.
A mass balance model of naturally-occurring short-lived and long-lived radium isotopes ( 224Ra and 226Ra) for the Jiulong River Estuary is developed to estimate the submarine groundwater discharge (SGD) to the estuary. All likely source and sink terms are considered in the model establishment. The source terms for Ra include river discharge, desorption from riverine suspended particles, desorption from resuspended particles, diffusion from subtidal sediments, and input from open sea and SGD. Ra can also be removed through radioactive decay and with ebb tide. The modeling result shows that up to 41. 9% to 56. 9% of the total Ra input can be attributed to the contribution of submarine groundwater in the estuary, and this has to be sustained by a discharge of 1.65 × 10 8 to 1.83 × 10 8 m 3/d in groundwater input. Such groundwater amounts can be four times that of river discharge in the estuary. Assuming that fresh groundwater can account for 10% of the total amount of groundwater, then the model calculated inorganic nutrient fluxes are 4.61 × 10 6 mol/d for dissolved inorganic nitrogen (DIN), 0.22 × 10 6 mol/d for dissolved inorganic phosphate (DIP), and 6.94 × 10 6 mol/d for dissolved inorganic silicon (DSi), respectively. And such inorganic nutrient fluxes are 23%, 28% and 77% of that delivered by river discharge in the estuary, respectively. The result suggests that a considerable amount of nutrients in the Jiulong River Estuary is coming from the contribution of SGD. The latter itself is also significant in relation to the total river discharge in the estuary. Nutrients contributed by SGD could potentially cause environmental concerns of estuary and coastal marine eutrophication. Such an environmental issue must be considered in the future management plan.
Submarine groundwater discharge (SGD), an important part of global water cycle, has recently been a research focus in the field of land-ocean interaction along the coastal zone. Geochemical tracing is a major tool to study submarine groundwater discharge. Taking natural occurring isotope 222Rn as a tracer, the authors built 222Rn mass balance model to estimate groundwater discharge to Jiaozhou Bay, and also evaluated the nutrient fluxes transported via groundwater. River input, dispersion of sediments, and support from parent 226Ra are considered to be the sources of 222Rn mass balance model, whereas radioactive decay, escape from sea-water interface to air and loss in mixing with the low activity water from open ocean are considered to be the sink of the model. Thus, the imbalance of budget from the model is attributed to submarine groundwater discharge. Calculations show that submarine groundwater discharge flux to Jiaozhou Bay was 24.2 L·m-2·d-1 from September to October in 2011, and 7.8 L·m-2·d-1 from April to May in 2014, respectively. The results of the study also indicate that the nutrient fluxes derived from groundwater during the wet season are lower than those transported by the local rivers, whereas the nutrient fluxes derived from groundwater during the dry season are close to those transported by the local rivers and, what is more, soluble reactive phosphate and silicate fluxes from both groundwater and river are very close to each other.
Radium isotopes 226Ra and 228Ra in seawater of the western Yellow Sea were measured by using the Mn-fiber adsorption—HPGe γ spectrum method. The distribution features of the two isotopes have been studied. The activities of 226Ra and 228Ra are 2.72–5.57 Bqm−3 and 7.51–34.3 Bqm−3, respectively. The activities of 226Ra and 228Ra from surface to bottom for each depth profile station are comparable within the experimental error, but the mean activities decrease with distance from the shore. From the distribution data of 228Ra, the horizontal eddy diffusion coefficient was estimated at 29×106 cm2s−1.
228Ra levels in the Yellow Sea and East China Sea were determined using the emanation method. The seawater radium was concentrated
using an Mn-fiber and the 224Ra ingrowth was measured after about half a year when the initial 224Ra in the sample would have decayed. The 224Ra activity in the sample was evaluated using the decay dynamics relationship between parent 228Ra and daughter 228Th. The concentration and distribution feature of 228Ra in the Yellow Sea and East China Sea were studied and the 228Ra concentrations in the surface seawater of the Yellow Sea and the East China Sea were in the range 0.09–15.0Bq/m3 with an average of 6.84Bq/m3 during the summer cruise, and in the range 0.09–16.9Bq/m3 with an average of 6.37Bq/m3 during the winter cruise. The 228Ra distribution in the northern Yellow Sea was different from the southern Yellow Sea and East China Sea. The highest 228Ra activity of surface water was located in the middle of the northern Yellow Sea, but for the southern Yellow Sea and East
China Sea, it decreased with increasing distance from China continent.
Keywords
228Ra-Radon emanation method-Yellow Sea-East China Sea
Seawater was collected from the western Arctic Ocean for measurements of 18O, 226Ra and 228Ra. The fractions of river runoff and sea ice melt-water in water samples were estimated by using δ
18O-S-PO* tracer system. The mean residence time of river water in the Canada Basin was calculated based on the relationship
between 228Ra/226Ra)A.R. and the fractions of river runoff in the shelf and deep ocean. Our results showed that the river runoff fractions in the
Canada Basin were significantly higher than those in the shelf regions, suggesting that the Canada Basin is a major storage
region for Arctic river water. 228Ra activity concentrations in the Chukchi shelf and the Beaufort shelf ranged from 0.16 to 1.22 Bq/m3, lower than those reported for shelves in the low and middle latitudes, indicating the effect of sea ice melt-water. A good
positive linear relationship was observed between 228Ra/226Ra)A.R. and the fraction of river runoff for shelf waters, while the 228Ra/226Ra)A.R. in the Canada Basin was located below this regressive line. The low 228Ra/226Ra)A.R. in the Canada Basin was ascribed to 228Ra decay during shelf waters transporting to the deep ocean. The residence time of 5.0–11.0 a was estimated for the river
water in the Canada Basin, which determined the time response of surface freshening in the North Atlantic to the river runoff
into the Arctic Ocean.
A dispersion model coupled with the Princeton Ocean Model was used to estimate the average residence time of the water in Jiaozhou Bay. The tidal simulation agreed quite well with drift experiments and water elevation observations at the Dagang tide station in the east coast of the bay. In particular, in situ measurements of 228Ra and salinity were carried out to calibrate the dispersion model. The modelled average residence time was about 52 days, ranging from less than 20 days in the deep part near the bay channel, the only passage connecting the bay to the Yellow Sea, to over 100 days in the shallow area in the northwest. The spatial difference of average residence time was controlled by tidal residual currents and the distance to the bay channel. The modelled tidal exchange rate was uneven in the bay, and consistent with 228Ra observations. The temporal evolution of the passive tracer accords with the evolution of the rain fraction after the rainstorm in August 2001.
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