224,223Ra和137Cs在吕宋海峡及周边海域的分布以及其对“彩虹”台风的响应

本文报道2015年9月和2016年5月期间天然放射性核素²²⁴Ra和²²³Ra在吕宋海峡及周边海域表层和垂向水体的分布特征。为理解日本福岛核事故的影响,本文亦分析研究区域内人工放射性核素¹³⁷Cs的分布特征。结果表明,^224,223Ra和¹³⁷Cs比活度水平均处于我国南海海洋天然放射性本底变化范围之内。²²⁴Ra在吕宋海峡以西南海北部海域比活度较高,在吕宋海峡以东菲律宾海域比活度较低。¹³⁷Cs没有明显的分布趋势。基于三站位（LS3,LS5和LS8）²²⁴Ra、¹³⁷Cs以及温盐的垂向分布特征,本文揭示²²⁴Ra和¹³⁷Cs在热带表层水、次表层水和中-深层水中比活度水平和梯度变化的差异特征。彩虹台风事件扭转了整个吕宋海峡及周边海域的海流循环过程。大量以低水平²²⁴Ra为特征的西太平洋海水涌入南海,降低水体²²⁴Ra比活度水平。但是,西太平洋和南海北部海域水体¹³⁷Cs比活度水平没有明显差异,台风导致的海流变化对水体¹³⁷Cs比活度没有明显影响。     

Short-lived radium isotopes in the Hawaiian margin: Evidence for large fluid fluxes through the Puna Ridge

We measured significant activities of short-lived radium isotopes, ²²³Ra (half-life = 11 days) and ²²⁴Ra (half-life = 3.7 days), around the margins of the Hawaiian Islands to water depths of 3500 m. These measurements suggest fluid inputs from the basalt to the surrounding ocean. In general ²²³Ra activities were considerably greater than ²²⁴Ra in spite of the expected higher production rate of ²²⁴Ra activity in basalt. The ²²³Ra was not supported by dissolved ²²⁷Ac. The highest enrichments of ²²³Ra were measured over the Puna Ridge (2100 m depth) east of Hawaii. Here ²²³Ra activities reached 19 dpm/m³, similar to activities measured near sites of active submarine groundwater discharge in the South Atlantic Bight. To explain the high activities of ²²³Ra unaccompanied by ²²⁴Ra, we postulate that thermally-driven circulation of seawater through the Puna Ridge deposits ²³¹Pa on basalt surfaces. With time the ²³¹Pa produces ²²⁷Ac and ²²³Ra; and ²²³Ra desorbs into the circulating fluids. These fluids then transport ²²³Ra into the overlying ocean. Based on the inventory of ²²³Ra above the Puna Ridge, we estimate the flow of fluids through the ridge to be on the order of 20–60 cm³ cm^− 2 day^− 1. In less than 1000 years the incoming seawater could provide enough ²³¹Pa to basalt surfaces to balance the inventory of ²²³Ra above the ridge if only 8% of the ²²³Ra was transported to the overlying water. These observations on the flanks of a volcanically-active ocean island have significant implications for quantifying fluid fluxes from the flanks of the mid-ocean ridge system. By mapping ²²³Ra inventories in the ocean above ridge flanks and measuring the activity of ²²³Ra in the emerging fluids, the fluid flux can be obtained.     

Radium and radon radioisotopes in regional groundwater, intertidal groundwater, and seawater in the Adelaide Coastal Waters Study area: Implications for the evaluation of submarine groundwater discharge

The input of groundwater-borne nutrients to Adelaide's (South Australia) coastal zone is not well known but could contribute to the ongoing decline of seagrass in the area. As a component of the Adelaide Coastal Waters Study (ACWS), the potential for using the radium quartet (²²³Ra, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra) and ²²²Rn to evaluate submarine groundwater discharge (SGD) was evaluated. Potential isotopic signatures for SGD were assessed by sampling groundwater from three regional aquifers potentially contributing SGD to the ACWS area. In addition, intertidal groundwater was sampled at two sand beach sites. In general, the regional groundwaters were enriched in long-lived Ra isotopes (²²⁶Ra and ²²⁸Ra) and in ²²²Rn relative to intertidal groundwater. Radium activity (but not ²²²Rn activity) was positively correlated to salinity in groundwater from one of the regional aquifers and in intertidal groundwater. Radium isotope ratios (²²³Ra/²²⁶Ra, ²²⁴Ra/²²⁶Ra and ²²⁸Ra/²²⁶Ra) were less variable than individual Ra isotope activities within potential SGD sources. Recirculated seawater (estimated from the intertidal groundwater samples with seawater-like salinities) also had distinctly higher Ra isotope ratios than the regional groundwaters. The activities for all radioisotopes were relatively low in seawater. The activity of the short-lived ²²³Ra and ²²⁴Ra were highest at the shoreline and declined exponentially with distance offshore. In contrast, ²²⁸Ra and ²²⁶Ra activities had a weak linear declining trend with distance offshore. Rn-222 activity was at or near background in all seawater samples. The pattern of enrichment in short-lived Ra isotopes and the lack of ²²²Rn in seawater suggest that seawater recirculation is the main contributor to SGD in the ACWS area. Preliminary modeling of the offshore flux of ²²⁸Ra and ²²⁶Ra suggest that the SGD flux to the ACWS area ranges between 0.2 and 3 · 10^− 3 m³ (m of shoreline)^− 1 s^− 1.     

The release of dissolved actinium to the ocean: A global comparison of different end-members

The measurement of short-lived ²²³Ra often involves a second measurement for supported activities, which represents ²²⁷Ac in the sample. Here we exploit this fact, presenting a set of 284 values on the oceanic distribution of ²²⁷Ac, which was collected when analyzing water samples for short-lived radium isotopes by the radium delayed coincidence counting system. The present work compiles ²²⁷Ac data from coastal regions all over the northern hemisphere, including values from ground water, from estuaries and lagoons, and from marine end-members. Deep-sea samples from a continental slope off Puerto Rico and from an active vent site near Hawaii complete the overview of ²²⁷Ac near its potential sources.The average ²²⁷Ac activities of nearshore marine end-members range from 0.4 dpm m^− 3 at the Gulf of Mexico to 3.0 dpm m^− 3 in the coastal waters of the Korean Strait. In analogy to ²²⁸Ra, we find the extension of adjacent shelf regions to play a substantial role for ²²⁷Ac activities, although less pronounced than for radium, due to its weaker shelf source. Based on previously published values, we calculate an open ocean ²²⁷Ac inventory of 1.35 * 10¹⁸ dpm ²²⁷Ac_ex in the ocean, which corresponds to 37 moles, or 8.4 kg. This implies a flux of 127 dpm m⁻² y^− 1 from the deep-sea floor. For the shelf regions, we obtain a global inventory of ²²⁷Ac of 4.5 * 10¹⁵ dpm, which cannot be converted directly into a flux value, as the regional loss term of ²²⁷Ac to the open ocean would have to be included.Ac has so far been considered to behave similarly to Ra in the marine environment, with the exception of a strong Ac source in the deep-sea due to ²³¹Pa_ex. Here, we present evidence of geochemical differences between Ac, which is retained in a warm vent system, and Ra, which is readily released [Moore, W.S., Ussler, W. and Paull, C.K., 2008-this issue. Short-lived radium isotopes in the Hawaiian margin: Evidence for large fluid fluxes through the Puna Ridge. Marine Chemistry]. Another potential mechanism of producing deviations in ²²⁷Ac/²²⁸Ra and daughter isotope ratios from the expected production value of lithogenic material is observed at reducing environments, where enrichment in uranium may occur. The presented data here may serve as a reference for including ²²⁷Ac in circulation models, and the overview provides values for some end-members that contribute to the global Ac distribution.     

Uncertainties in the preparation of Ra Mn fiber standards

Delayed coincidence counters (RaDeCC), used for measuring ²²³Ra and ²²⁴Ra preconcentrated from water onto MnO₂-impregnated acrylic fiber (“Mn-fiber”), require a standard Mn-fiber column that has a precisely known activity of ²²⁴Ra for calibration. This may be done by adding an aged ²²⁸Th standard solution to adsorb both ²²⁸Th and its daughter ²²⁴Ra quantitatively onto a Mn fiber. We used both seawater and deionized water (DIW) for testing the adsorption efficiency of Th and Ra onto Mn fibers. Our experimental results show that more than 50% of thorium (²³²Th and ²²⁸Th) breaks through the Mn-fiber column when DIW is used as a medium. However, near quantitative recoveries are obtained if filtered (0.45 μm) seawater is used to prepare the standard. In the case of pure DIW, the pH (initial pH  5.3) rises to > 10 after passing through the column while seawater (initial pH  7.8) changes to  7.2. Thus, the lack of thorium adsorption in DIW may be attributed to this huge increase of pH and the consequent formation of Th(OH)₄ and polyhydroxyl colloids. Based on these observations, we recommend that one should use either artificial seawater or natural seawater (which has negligible ²²⁴Ra and ²²⁸Th) as a loading solution after 0.45 μm filtration. In addition, the thorium adsorption efficiency should be confirmed either by thorium analysis of the effluent solution or long-term monitoring of the supported ²²⁴Ra on the Mn fiber using the RaDeCC. Similar cautions are likely necessary for making ²²³Ra standards by adsorption of ²²⁷Ac onto Mn fibers.     

Fifteen years experience in measuring Ra and Ra by delayed-coincidence counting

The RaDeCC™ system has proved to be a robust method of measuring ²²³Ra and ²²⁴Ra extracted from natural waters. Samples ranging in size from < 1 to > 1000 L are first concentrated onto Mn-fiber. The Mn-fiber is partially dried and placed into a circulation system with helium as the carrier gas. Alpha decays of the radon daughters of ²²³Ra and ²²⁴Ra are recorded in a scintillation cell. These data are used to activate electronic gates in the delayed coincidence system, which separate events due to ²²³Ra from those due to ²²⁴Ra. The system combines a high detection efficiency with low background to accurately measure ²²³Ra and ²²⁴Ra at concentrations on the order of 5 atoms per liter. Since this system was first deployed, numerous improvements in electronics and technique have occurred. In this paper I present tests we have conducted to confirm certain assumptions and improve the overall technique of sample collection and measurement.     

TIMS measurements of Ra and Ra in the Gulf of Lion, an attempt to quantify submarine groundwater discharge

Submarine groundwater discharge (SGD) is now recognized as an important pathway for water and chemical species fluxes to the coastal ocean. In order to determinate SGD to the Gulf of Lion (France), we measured the activities of ²²⁶Ra and ²²⁸Ra by thermal ionization mass spectrometry (TIMS) in coastal waters and in the deep aquifer waters of the Rhone deltaic plain after pre-concentration of radium by MnO₂. Compared to conventional counting techniques, TIMS requires lower quantities of water for the analyses, and leads to higher analytical precision. Radium isotopes were thus measured on 0.25–2 L water samples containing as little as 20 fg of ²²⁶Ra and 0.2–0.4 fg of ²²⁸Ra with precision equal to 2%. We demonstrate that coastal surface waters samples are enriched in ²²⁶Ra and ²²⁸Ra compared to the samples further offshore. The high precision radium measurements display a small but significant ²²⁶Ra and ²²⁸Ra enrichment within a strip of circa 30 km from the coast. Radium activities decrease beyond this region, entrained in the northern current along the shelf break or controlled by eddy diffusion. The radium excess in the first 30 km cannot be accounted for by the river nor by the early diagenesis. The primary source of the radium enrichment must therefore be ascribed to the discharge of submarine groundwater. Using a mass-balance model, we estimated the advective fluxes of ²²⁶Ra and ²²⁸Ra through SGD to be 5.2 × 10¹⁰ and 21 × 10¹⁰ dpm/d respectively. The ²²⁶Ra activities measured in the groundwater from the Rhone deltaic plain aquifer are comparable to those from other coastal groundwater studies throughout the world. By contrast, ²²⁸Ra activities are higher by up to one order of magnitude. Taking those groundwater radium activities as typical of the submarine groundwater end-member, a minimum volume of 0.24–4.5 × 10¹⁰ l/d is required to support the excess radium isotopes on the inner shelf. This has to be compared with the average rivers water runoff of 15.4 × 10¹⁰ l/d during the study period (1.6 to 29% of the river flow).     

Progress in the determination of Ac in sea water

²²⁷Ac is a naturally occurring radioisotope with a unique combination of properties that make it suitable for the determination of deep ocean mixing and upwelling rates. Here, we present a method for the determination of ²²⁷Ac in sea water on sample sizes of 20–80 L. The measurement is based on co-precipitation of ²²⁷ Ac with MnO₂, followed by chemical isolation of actinium in the presence of an artificial Ac isotope. Actinium is then electrodeposited onto silver discs. In two alpha-spectrometric counting periods, first the artificial ²²⁵Ac isotope is counted, then after > 100 days five daughters of ²²⁷Ac. The first counting period gives a total yield for the procedure, integrating chemical recovery and detector efficiency. The total yield was found here to be on average 15 ± 5%, the chemical yield on average about 50%. The counting of five decay products of ²²⁷Ac in the second period makes the method particularly sensitive. Using appropriate decay corrections, the initial ²²⁷Ac activity can be determined to better than 10% relative error for concentrations < 10,000 atoms/L. We compare data acquired by the new method to a data set from in-situ pumps, from a parallel sampling campaign in the Eastern Weddell Gyre, and we can show excellent agreement. Repeated determinations of ²²⁷Ac in a uranium reference material (UREM-11) demonstrate the accuracy of the method.     

Helium isotopes of seawater in the Japan Sea

We have collected fifty-five seawater samples at seven stations at various depths in the Yamato and Japan Basins of the Japan Sea and measured their helium isotopic ratios. The ³He/⁴He ratios vary from 0.997 R_atm to 1.085 R_atm where R_atm is the atmospheric ratio. The maximum ³He excesses about 8%, are observed at mid-depth (1000 m), and these values are significantly lower than those observed in deep Pacific waters. This implies that mantle-derived helium in deep Pacific water cannot enter the Japan Sea since it is an almost landlocked marginal sea. The observed 8% excess ³He may be attributable to the decay product of tritium. Slightly higher ³He/⁴He ratios in the Bottom Water were observed in the Yamato Basin than in the Japan Basin. The ventilation ages of seawater shallower than 1000 m are calculated as about 5 to 20 years, which is consistent with the CFC ages reported in the literature. There is a positive correlation between the apparent oxygen utilization and ³H-³He ages. The estimated oxygen utilization rate from the correlation in a layer between 500 m and 1000 m is about 3 μmol/kg/yr, which is similar to that in the eastern subtropical North Atlantic.     

Th and Pa isotopes in the waters of the western margin of the pacific near Japan: Evidence for release of228Ra and227Ac from slope sediments

Using the moored MnO₂-fiber method, we have obtained 38 determinations of Th and Pa isotope concentrations from 18 sites along the margin of the western North Pacific near Japan, from water depths of 1,330 to 5,873 m. From our data, we are able to show that (1)²²⁸Ra and²²⁷Ac are being supplied to the seawater from the slope sediments of Honshu, Japan, (2) our²³⁰Th and²³¹Pa concentrations match those obtained byin situ pumping with a MnO₂-fiber adsorber in the Japan and Izu-Ogasawara trenches but are significantly higher than those from the Panama and Guatemala basins, and (3) our²³²Th concentrations show a similar systematic decrease with depth as do those of trace metals like Mn, Al, Te and Bi whose concentrations are strongly controlled by particulate matter scavenging.In contrast, our data fail to show (1) that enhanced removal of²³⁰Th and²³¹Pa by scavenging from the water column is taking place near the western margins and (2) that²³¹Pa is being removed in preference to²³⁰Th from the water column to the marginal sediments. This is probably due to rapid mixing of the deep waters as compared to the scavenging rates of²³⁰Th and²³¹Pa in the water column and at the sediment/seawater interface.     

白令海峡水团来源的镭同位素示踪

对白令海峡64.3°N纬向断面镭同位素的研究表明,水体中226Ra比活度、228Ra比活度和228Ra/226Ra)A.R.存在明显的纬向变化,反映出太平洋与北冰洋水体交换的多种路径.根据温度、盐度和镭同位素的水平与垂直分布,太平洋水进入北冰洋的路径可能主要有3支,分别为白令海峡西侧的阿拉德水、白令海峡东侧的阿拉斯加沿...     

The oxygen isotope composition of water masses within the North Sea

Based on measurements of the ¹⁸O isotope composition of 247 samples collected over a 3-year period we have assessed the oxygen isotope composition of water masses in the North Sea. This is the first δ¹⁸O data set that covers the entire North Sea basin. The waters lie on a mixing line: δ¹⁸O (‰_VSMOW) = −9.300 + 0.274(S) with North Atlantic sub-polar mode water (SPMW) and surface waters, and Baltic Sea water representing the saline and freshwater end members respectively. Patterns exhibited in surface and bottom water δ¹⁸O distributions are representative of the general circulation of the North Sea. Oxygen-18 enriched waters from the North Atlantic enter the North Sea between Scotland and Norway and to a lesser extent through the English Channel. In contrast, oxygen-18 depleted waters mainly inflow from the Baltic Sea, the rivers Rhine and Elbe, and to a lesser degree, the Norwegian Fjords and other river sources. Locally the δ¹⁸O–salinity relationship will be controlled by the isotopic composition of the freshwater inputs. However, the range of local freshwater compositions around the North Sea basin is too narrow to characterise the relative contributions of individual sources to the overall seawater composition. This dataset provides important information for a number of related disciplines including biogeochemical research and oceanographic studies.     

Comparison of techniques for pre-concentrating radium from seawater

In the framework of the KEOPS project (KErguelen: compared study of the Ocean and the Plateau in Surface water), we aimed to provide information on the water mass pathways and vertical mixing on the Kerguelen Plateau, Southern Ocean, based on ²²⁸Ra profiles. Because ²²⁸Ra activities are extremely low in this area (~ 0.1 dpm/100 kg or ~ 2.10^− 18 g kg^− 1), the filtration of large volumes of seawater was required in order to be able to detect it with minimal uncertainty. This challenging study was an opportunity for us to test and compare methods aimed at removing efficiently radium isotopes from seawater. We used Mn-fiber that retains radium and that allows the measurement of all four radium isotopes (²²⁶Ra, ²²⁸Ra, ²²³Ra, ²²⁴Ra). First, we used Niskin bottles or the ship's seawater intake to collect large volumes of seawater that were passed onto Mn-fiber in the laboratory. Second, we filled cartridges with Mn-fiber that we placed in tandem on in situ pumps. Finally, we fixed nylon nets filled with Mn-fiber on the frame of in situ pumps to allow the passive filtration of seawater during the pump deployment.Yields of radium fixation on the cartridges filled with Mn-fiber and placed on in situ pumps are ca. 30% when combining the two cartridges. Because large volumes of seawater can be filtered with these pumps, this yields to effective volumes of 177–280 kg (that is, higher than that recovered from fourteen 12-l Niskin bottles). Finally, the effective volume of seawater that passed through Mn-fiber placed in nylon nets and deployed during 4 h ranged between 125 and 364 kg. Consequently, the two techniques that separate Ra isotopes in situ are good alternatives for pre-concentrating radium from seawater. They can save ship-time by avoiding repeated CTD casts to obtain the large volumes of seawater. This is especially true when in situ pumps are deployed to collect suspended particles. However, both methods only provide ²²⁸Ra/²²⁶Ra ratios. The determination of the ²²⁸Ra specific activity is obtained by multiplying this ratio by the ²²⁶Ra activity measured in a discrete sample collected at the same water depth.     

福岛核事故泄漏入海的137Cs长期输运的数值研究

The Fukushima nuclear accident in 2011 released large amounts of radionuclides, including ~(137)Cs, into the Pacific Ocean. A quasi-global ocean radioactive transport model with horizontal grid spacing of 0.5°×0.5° and 21 vertical layers was thereafter established to study the long-term transport of the Fukushima-derived ~(137)Cs in the ocean.The simulation shows that the plume of ~(137)Cs would be rapidly transported eastward alongside the Kuroshio Current and its extensions. Contaminated waters with concentrations lower than 2 Bq/m3 would reach the west coast of North America 4 or 5 years after the accident. The ~(137)Cs tends to be carried, despite its very low concentration, into the Indian and South Pacific Oceans by 2016 via various branches of ocean currents.Meanwhile, the ~(137)Cs concentrations in the western part of the North Pacific Ocean decrease rapidly with time. Up to now the highly contaminated waters have remained in the upper 400 m, showing no evidence of significant penetration to deeper layers.     

High-precision measurement of oceanic 226Ra

A system capable of oceanic ²²⁶Ra measurements with a precision of ±1% is described, which represents an improvement of approximately a factor of three over existing techniques. ²²²Rn grown-in from ²²⁶Ra decay in 14-l seawater samples is quantitatively transferred to, and measured in, proportional gas counters. Errors other than counting statistics are estimated not to exceed ±0.5%, which is consistent with repeated ²²⁶Ra measurements on the same samples. A NE Atlantic ²²⁶Ra depth profile (2000–5000 m) is reported as an example. It is found that with the precision reported here, certain hitherto unresolved features of the ²²⁶Ra distribution in deep water become apparent.     

δ15N of PON and Nitrate as a Clue to the Origin and Transformation of Nitrogen in the Subarctic North Pacific and Its Marginal Sea

Nitrogen isotope compositions of particulate organic matter and nitrate were analyzed for seawater sampled at five stations at the Alaskan Gyre, Western Subarctic Gyre and East China Sea, focusing on the samples from the surface to 5000 m water to characterize the nitrogen cycling in the subarctic North Pacific Ocean and its marginal sea. The ¹⁵N of particulate organic matter showed little agreement with a conceptual closed model that interprets isotopic variation as being caused by isotope discrimination on nitrate utilization. The ¹⁵N and ¹³C of particulate organic matter varied with the water depth. A correlation between isotope compositions and C/N elemental ratio was found generally at all stations, although some irregular data were also found in deep layers. We developed a hypothetical nitrogen balance model based on N₂ fixation and denitrification in seawater and attempted to apply it to distinguish nutrient cycling using both ¹⁵N-NO₃ ^– and N* variation in seawater. This model was applied to the observed data set of ¹⁵N-NO₃ ^– and N* in the North Pacific water and estimated the ¹⁵N-NO₃ ^– of primordial nitrate in the North Pacific deep water as 4.8. The North Pacific intermediate water for all stations showed similar ¹⁵N-NO₃ ^– and N* values of 6 and –3 µmol/kg, respectively, suggesting a similar nitrogen biogeochemistry. In the East China Sea, analysis showed evidence of water exchange with the North Pacific intermediate water but a significant influence of nitrogen from the river runoff was found in depths shallower than 400 m.     

Seasonal variation in the Ra/Ra ratio of coastal water within the Sea of Japan: Implications for the origin and circulation patterns of the Tsushima Coastal Branch Current

In the current study, low-background γ-spectrometry was employed to determine the ²²⁸Ra/²²⁶Ra activity ratio and ¹³⁷Cs activity of 84 coastal water samples collected at six sites along the main island of Japan (Honshu Island) within the Sea of Japan, including the Tsushima Strait, and two other representative sites on Honshu Island (a Pacific shore and the Tsugaru Strait) at 1-month intervals in 2006.The ²²⁸Ra/²²⁶Ra ratio of coastal waters in the Sea of Japan exhibited similar patterns of seasonal variation, with minimum values during early summer (²²⁸Ra/²²⁶Ra = 0.6–0.8), maximum values during autumn (²²⁸Ra/²²⁶Ra = 1.5–3), and a time lag in their temporal changes ( 2.5 months and over  1300 km distance). However, the 2 other sites represented no clear periodic variation.In contrast to the positive correlation between ¹³⁷Cs activity (0.6–1.7 mBq/L) and salinity (15–35), the ²²⁸Ra/²²⁶Ra ratio of coastal water samples from the Sea of Japan was not observed to correlate with salinity, and the increase in the ²²⁸Ra/²²⁶Ra ratio was not as marked (0.5–1; May–June 2004 and 2005) during the migration along Honshu Island. The input of land-derived water and/or the diffusion of radium from coastal sediments is unlikely to have affected the wide seasonal variation in the ²²⁸Ra/²²⁶Ra ratio observed in these water samples.The seasonal variation in the ²²⁸Ra/²²⁶Ra ratio recorded for the coastal waters of the Sea of Japan is considered to be mainly controlled by the remarkable changes in the mixing ratio of the ²²⁸Ra-poor Kuroshio and the ²²⁸Ra-rich continental shelf waters within the East China Sea (ECS). After passing through the Tsushima Strait, this water mass moves northeast along the coastline of the Sea of Japan as the Tsushima Coastal Branch Current (TCBC).     

228Ra, 226Ra, 224Ra and 223Ra in potential sources and sinks of land-derived material in the German Bight of the North Sea: implications for the use of radium as a tracer

Activities of the naturally occurring radium nuclides ²²⁸Ra, ²²⁶Ra, ²²⁴Ra and ²²³Ra were determined in waters of the open German Bight and adjacent nearshore areas in the North Sea, in order to explore the potential use of radium isotopes as natural tracers of land–ocean interaction in an environment characterised by extensive tidal flats, as well as riverine and groundwater influx. Data collected at various tidal phases from the Weser Estuary (²²⁸Ra: 46.3 ± 4.6; ²²⁶Ra: 17.1 ± 1.1; ²²⁴Ra: 26.1 ± 8.2 to 36.5 ± 6.1; ²²³Ra: 1.8 ± 0.1 to 4.0 ± 0.4), tidal flats near Sahlenburg (²²⁸Ra: 39.3 ± 3.8 to 46.0 ± 4.5; ²²⁶Ra: 15.5 ± 1.5 to 16.5 ± 1.7; ²²⁴Ra: 34.3 ± 2.2 to 85.3 ± 6.3; ²²³Ra: 3.6 ± 0.5 to 8.0 ± 1.2), freshwater seeps on tidal flats near Sahlenburg (²²⁸Ra: 42.1 ± 4.1; ²²⁶Ra: 21.3 ± 2.2; ²²⁴Ra: 5.1 ± 0.9; ²²³Ra: 2.6 ± 1.3) and also in permanently inundated parts of the North Sea (²²⁸Ra: 23.0 ± 2.3 to 28.2 ± 2.8; ²²⁶Ra: 8.2 ± 0.8 to 11.8 ± 1.2; ²²⁴Ra: 3.1 ± 1.0 to 10.1 ± 0.9; ²²³Ra: 0.1 ± 0.02 to 0.9 ± 0.05; units: disintegrations per minute per 100 kg water sample) reveal that, except for the fresh groundwater, the potential end-members of nearshore water mass mixing have quite similar radium signatures, excluding a simple discrimination between the sources. However, the decreasing activities of the short-lived ²²⁴Ra and ²²³Ra isotopes recorded towards the island of Helgoland in the central German Bight show a potential to constrain fluxes of land-derived material to the open North Sea. The largest source for all radium isotopes is generally found on the vast tidal flats and in the Weser Estuary. Future work could meaningfully combine this so-called radium quartet approach with investigations of radon activity. Indeed, preliminary data from a tidal flat site with fresh groundwater seepage reveal a ²²²Rn signal that is clearly lower in seawater.     

The fluorescence of dissolved organic matter in seawater

A total of 28 vertical profiles of seawater fluorescence was measured in the Sargasso Sea, the Straits of Florida, the Southern California Borderlands, and the central Pacific Ocean. In all cases, surface seawater fluorescence was low as a result of photochemical bleaching which occurs on the timescale of hours. Fluorescence of deep water was 2–2.5 times higher than that of surface waters, and was constant, implying a long residence time for fluorescent organic matter, possibly of the order of thousands of years. Fluorescence correlates well with nutrients (NO₃⁻, PO₄³⁻) in mid-depth waters (100–1000 m) in the Sargasso Sea and the central North Pacific, consistent with results in the central Pacific and the coastal seas of Japan. This suggests that regeneration or formation of fluorescent materials accompanies the oxidation and remineralization of settling organic particles.The various sources and sinks of fluorescent organic matter in the global oceans are assessed. The major sources are particles and in situ formation; rivers, rain, diffusion from sediments, and release from organisms are minor sources. The major sink is photochemical bleaching.     

Helium Isotopes in Pacific Waters from Adjacent Region of Honshu, Japan

We have measured helium isotopic ratios of thirty-seven Pacific water samples from various depths collected in adjacent regions of Honshu, Japan. The ³He/⁴He ratios vary significantly from 0.989 R _atm to 1.208 R _atm where R _atm is the atmospheric ratio of 1.39 × 10⁻⁶. The mid-depth (750–1500 m) profile of ³He/⁴He ratios at ST-1 located Northwestern Pacific Ocean east of Japan (Off Joban; 37°00′ N, 142°40′ E) is significantly different from that at ST-2 of the Northern Philippine Sea south of Japan (Nankai Trough; 33°07′ N, 139°59′ E), suggesting that these waters were separated by a topographic barrier, the Izu-Ogasawara Ridge. Taking ³He/⁴He data of the Geosecs expeditions in the western North Pacific, an extensive plume of 15% excess ³He relative to the air may be traced at ST-1 over 12,000 kilometers to the northwest of the East Pacific Rise where the mantle helium may originate. The 20% excess found at ST-2 may be attributable to the additional source of the subduction-type mantle helium in the Okinawa Trough. A 15% excess of ³He has also been discovered at a depth of about 1000∼1500 m at ST-3 adjacent to Miyakejima Island (33°57′ N, 139°22′ E) and ST-4 of Sagami Bay (35°00′ N, 139°22′ E). It is confirmed that mid-depth all over the western North Pacific water is affected by the mantle helium with a high ³He/⁴He ratio. This revised version was published online in July 2006 with corrections to the Cover Date.