234U238U and 230Th234U activity ratios in mineral phases of a lateritic weathered zone

A selective phase extraction procedure was developed for the identification of the significant phases of a typical deep soil profile sampled in the vicinity of the Ranger No. 1 uranium ore body, Alligator Rivers region, N.T., Australia. The significant phases were identified as amorphous iron oxide, crystalline iron oxide and a clay/quartz resistate. The distribution of ²³⁸U, ²³⁴U, ²³⁰Th and ²²⁶Ra between the phases was measured. The results indicated that the amorphous iron oxide phase is in adsorption/desorption equilibrium with the ground water. The crystalline iron oxide phase contains a chemical control, the kinetics of which are commensurate with or less than the half-life of ²³⁰Th (7.52 × 10⁴ y). The clay/quartz resistate is enriched in ²³⁸U descendants in such a way that they are not readily accessible to the ground water.     

A study of234U/238U ratios in groundwaters of Taiyuan area,Shanxi Province

Uranium contents and²³⁴U/²³⁸U ratios have been determined on 29 water samples from the Taiyuan area, Shanxi Province. The results show that the same artesian aquifer has similar uranium contents and²³⁴U/²³⁸U activity ratios, and the deeper aquifers have higher A. R. values but lower uranium contents. The A. R. values increase slightly towards groundwater flow in the artesian aquifers dominated by oxidizing ground waters. The Lancun Spring and the famous Jinci Spring belong to two different karst groundwater systems, i.e., the east and west karst groundwater systems. The recharge area of the Lancun Spring should cover the wide limestone outcrops of middle Ordovician in the northeast. The Ordovician fissure-karst ground water to the Jinci Spring is extensively mixed with fissure water in Carboniferous-Jurassic formations and seepage water from the Fenhe River.     

Dissolved Uranium and 234U/238U in the Yamuna and the Chambal Rivers, India

Dissolved uranium concentration and ²³⁴U/²³⁸U activity ratio have been measured in two distinctly different Indian drainage systems: the Yamuna headwaters in the Himalaya and the Chambal river system in the plains to study the weathering and mobility of uranium in these watersheds. The dissolved uranium in the Chambal river system ranges from 0.2 to 1.74 μg L⁻¹ during September (tail end of monsoon), whereas in the Yamuna river system, its concentration varies from 0.1 to 3.18 μg L⁻¹ during October (post-monsoon) and from 0.09 to 3.61 μg L⁻¹ in June (summer). In the Yamuna main stream, uranium is highest at its source and decreases steadily along its course, from 3.18 μg L⁻¹ at Hanuman Chatti to 0.67 μg L⁻¹ at Batamandi, at the base of the Himalaya. This decrease results mainly from mixing of the Yamuna mainstream with its tributaries, which are lower in uranium. The high concentration of uranium at Hanuman Chatti is derived from weathering of the Higher Himalayan Crystalline series (HHC) and associated accessary minerals, which may include uranium-mineralised zones. The ²³⁴U/²³⁸U activity ratios in the samples from the Chambal watershed are in the range of 1.15±0.05 to 1.67±0.04; whereas in the Yamuna the ratios vary from 0.95±0.03 to 1.56±0.07, during post-monsoon and from 0.98±0.01 to 1.30±0.03, during summer. The relatively high ²³⁴U/²³⁸U activity ratios in the Yamuna system are in its tributaries from the lower reaches viz., the Amlawa, Aglar, Bata, Tons and the Giri. It is estimated that ~9×10³ and ~12 × 10³ kg of dissolved uranium are transported annually from the Yamuna at Batamandi and the Chambal at Udi, respectively. This corresponds to uranium weathering rates of 0.9 and 0.09 kg U km⁻² y⁻¹ in the basins of the Yamuna and the Chambal headwaters. This study confirms that uranium weathering rate in the Himalaya is far in excess (by about an order of magnitude) of the global average value of ~0.08 kg U km⁻² y⁻¹.     

Preferential solution of234U from recoil tracks and234U/238U radioactive disequilibrium in natural waters

A mathematical model to calculate the²³⁴U/²³⁸U activity ratio (AR) in an aqueous phase in contact with rock/soil is presented. The model relies on the supply of²³⁸U by dissolution and that of²³⁴U by dissolution and preferential release from radiation damaged regions (recoil tracks). The model predicts that values of²³⁴U/²³⁸U AR>1 in the aqueous phase can be obtained only from weathering “virgin” surfaces. Thus, to account for the observed steady-state supply of²³⁴U excess to the oceans by the preferential leaching model, ‘virgin’ rock/soil surfaces would have to be continually exposed and weathered. The²³⁸U concentration and²³⁴U/²³⁸U AR in continental waters allow us to estimate the exposure rates of “virgin” rock/soil surfaces.     

我国西北地区盐湖中的铀234U/238U比值示踪研究

内蒙古二连盆地、鄂尔多斯盆地典型含铀碳酸型盐湖水、岩两相234U/238U综合分析表明,碳酸型盐湖铀来源于大气降水和潜水对盐湖盆地周围中生代到现代富铀沉积物的溶滤、浸出,具有快速、近源物质来源特点。盐湖卤水和对应沉积物234U/238U值一般为0.8～1.2,湖卤水和潜卤水（晶间卤水）-岩两相中的铀处于沉积平衡状态。早白垩世～上新世含膏盐地层对比研究证实了富铀岩层234U/238U值随铀含量增大而减小,并趋近于1。室内盐湖水蒸发模拟实验发现残余卤水、沉积物234U/238U具有随蒸发程度增大逐渐减小的变化特征。盐湖现代沉积物物相研究发现铀主要以碳酸铀酰和吸附形式赋存在富含有机物和碎屑成分的含盐粘土沉积中,铀在盐类晶体中含量极少,仅存在于封闭水和结晶水中。卤水和沉积物ARu值是盐湖铀源及铀含量水平的指示标志之一。     

洞穴石笋初始234U/238U值变化的古气候记录意义

通过对我国西南地区3个洞穴9根大型石笋272个初始234U/238U数据的分析研究,发现石笋初始234U/238U值长尺度变化与海洋沉积SPECMAP曲线δ18O记录有一定的正相关关系,与北纬25°夏季太阳辐射能量变化曲线呈一定的负相关关系,石笋初始234U/238U值的变化在冰期时波动强烈,而在间冰期波动相对平缓,在间冰期和冰期(间冰阶和冰阶)转化阶段该值呈跳跃状态变化。末次冰期及全新世阶段石笋初始234U/238U变化记录了该时段内的BA暖期和YD突变冷事件;全新世8200 a BP、7200 a BP、5200 a BP、4200 a BP、2800 a BP、1400 a BP 发生的几次较强冷事件在石笋初始234U/238U值都有相应记录,并且和长尺度的变化规律一致,冷事件发生时石笋初始234U/238U值偏重,暖期偏轻。封闭系统形成的洞穴石笋初始234U/238U变化类似于同地区洞穴石笋的δ18O对气候变化的记录特征,在我国西南地区两者与夏季风变化的强弱呈现一种负相关的关系,洞穴石笋初始234U/238U值可以作为一个有用的古气候替代指标来研究古降水的变化。     

Extreme fractionation of 234U238U and 230Th234U in spring waters,sediments, and fossils at the Pomme de Terre Valley,southwestern Missouri

Isotopic fractionation as great as 1600% exists between ²³⁴U and ²³⁸U in spring waters, sediments, and fossils in the Pomme de Terre Valley, southwestern Missouri. The activity ratios of ${}^{234}\text{U}{}^{238}\text{U}$ in five springs range from 7.2 to 16 in water which has been discharged for at least the past 30,000 years. The anomalies in ${}^{234}\text{U}{}^{238}\text{U}$ ratio in deep water have potential usefulness in hydrologic investigations in southern Missouri. Clayey units overlying the spring bog sediments of Trolinger Spring are enriched in ²³⁰Th relative to their parent ²³⁴U by as much as 720%. The results indicate that both preferential displacement via alpha recoil ejection and the preferential emplacement via recoiling and physical entrapment are significant processes that are occurring in the geologic environment.     

234U/238U evidence for local recharge and patterns of ground-water flow in the vicinity of Yucca Mountain,Nevada, USA

Uranium concentrations and ²³⁴U/²³⁸U ratios in saturated-zone and perched ground water were used to investigate hydrologic flow and downgradient dilution and dispersion in the vicinity of Yucca Mountain, a potential high-level radioactive waste disposal site. The U data were obtained by thermal ionization mass spectrometry on more than 280 samples from the Death Valley regional flow system. Large variations in both U concentrations (commonly 0.6–10 μg l⁻¹) and ²³⁴U/²³⁸U activity ratios (commonly 1.5–6) are present on both local and regional scales; however, ground water with ²³⁴U/²³⁸U activity ratios from 7 up to 8.06 is restricted largely to samples from Yucca Mountain. Data from ground water in the Tertiary volcanic and Quaternary alluvial aquifers at and adjacent to Yucca Mountain plot in 3 distinct fields of reciprocal U concentration versus ²³⁴U/²³⁸U activity ratio correlated to different geographic areas. Ground water to the west of Yucca Mountain has large U concentrations and moderate ²³⁴U/²³⁸U whereas ground water to the east in the Fortymile flow system has similar ²³⁴U/²³⁸U, but distinctly smaller U concentrations. Ground water beneath the central part of Yucca Mountain has intermediate U concentrations but distinctive ²³⁴U/²³⁸U activity ratios of about 7–8. Perched water from the lower part of the unsaturated zone at Yucca Mountain has similarly large values of ²³⁴U/²³⁸U. These U data imply that the Tertiary volcanic aquifer beneath the central part of Yucca Mountain is isolated from north-south regional flow. The similarity of ²³⁴U/²³⁸U in both saturated- and unsaturated-zone ground water at Yucca Mountain further indicates that saturated-zone ground water beneath Yucca Mountain is dominated by local recharge rather than regional flow. The distinctive ²³⁴U/²³⁸U signatures also provide a natural tracer of downgradient flow. Elevated ²³⁴U/²³⁸U in ground water from two water-supply wells east of Yucca Mountain are interpreted as the result of induced flow from 40 a of ground-water withdrawal. Elevated ²³⁴U/²³⁸U in a borehole south of Yucca Mountain is interpreted as evidence that natural downgradient flow is more likely to follow southerly paths in the structurally anisotropic Tertiary volcanic aquifer where it becomes diluted by regional flow in the Fortymile system.     

Use of 234U and 238U isotopes to identify fertilizer-derived uranium in the Florida Everglades

Surface water and peat in the northern Everglades have very low natural concentrations of U and are therefore sensitive to the addition of small amounts of U from anthropogenic sources such as fertilizer. Peat samples collected along a nutrient gradient in the northern Everglades have unusually high concentrations of U (>1 μg/g, dry basis) and also have a distinctive ²³⁴U/²³⁸U activity ratio (AR). AR values for U-enriched peat fall in the narrow range of AR values for commercial phosphate fertilizer (1.00±0.05). In contrast, AR values for low-U peat from background sites exceed 1.05. The spatial distribution of anomalous U concentration, and of fertilizer-like AR values in peat, parallel a previously documented pattern of P enrichment. These results strongly suggest that some of the U in nutrient-impacted peatlands is fertilizer-derived. Agricultural drainage water sampled in the northern Everglades has high concentrations of dissolved U (0.3–2.4 μg/l) compared to surface water from background sites (<0.1 μg/l). Measured AR values in drainage water (0.949–0.990) are also permissive of a fertilizer origin for the U and are different from AR values in surface water or peat at background sites (AR>1.05). Synoptic sampling of surface water along drainage canals indicate that Lake Okeechobee, and some drainage from agricultural fields, are sources of dissolved U, whereas wetlands farther downstream act as sinks for U. Historically cultivated agricultural soil has only a marginally elevated (+0.2 μg/g) average concentration of U compared to nearby uncultivated soil and incorporates only 20% of the U from an aqueous solution that was slurried with the soil. In contrast, a similar experiment with fresh Everglades peat indicated uptake of 90% of the added U. These experiments support the proposed removal of U from agricultural fields and concentration of U in downstream peatlands. The methodology of this study can be used to describe the behavior of fertilizer-derived U in other low-U environments.     

A routine method for the determination of the 234U238U ratio in natural water

A simple method to determine the ${}^{234}\text{U}{}^{238}\text{U}$ ratio in water by α-spectrometry is presented. The thiocyanate complex of uranium is extracted from the water by anion exchange. After elution of the U from the resin it is further purified by extraction from nitrate medium into ether. The source for α-spectrometry is prepared by oxalate electrodeposition. The overall yield is 70%.     

234Th238U radioactive disequilibrium in the surface layer of the ocean

²³⁴Th produced from ²³⁸U within sea water was found to be in radioactive disequilibrium with respect to its progenitor nuclide ²³⁸U in the surface layer of the ocean. The median value for ${}^{234}\text{Th}{}^{238}\text{U}$ activity ratio is 0.80 in the upper 200 m layer. A box-model calculation gives a removal residence time of thorium of about 0.38 yr. This suggests that ²³⁴Th is scavenged from the surface layer by the uptake of thorium by biota.     

Spatial and temporal variability in the lower food web of the tidal freshwater Hudson River

The mid Hudson River is a heterotrophic system where allochthonous inputs apparently fuel the largest proportion of secondary production and ecosystem metabolism. We have analyzed a 6-yr dataset collected quarterly at six stations spanning a 150-km reach to assess variability at inter- and intra-annual time scales and regional spatial scales. The major components of the lower food web: bacterial biomass, detrital particulate organic carbon (POC), and dissolved organic carbon (DOC), show surprisingly discordant patterns in temporal and spatial variability. Bacterial abundance shows significant variability at all three scales, but the interannual variability is by far the greatest. DOC concentrations showed greatest variability among years, with intra-annual and spatial variability roughly equal. Freshwater flow is commonly considered a major driving force in river-estuarine variability but simple discharge was not a strong predictor of any component of suspended matter or DOC. For organisms in the Hudson River food web, these multiple scales of variability indicate highly unpredictable food resources in time and space, and these fluctuations may contribute to the variability in higher trophic levels.     

Scavenging and particle residence times determined from 234Th/238U disequilibria in the coastal waters of Mecklenburg Bay

Activities of the naturally occurring radionuclide ²³⁴Th were determined in water samples of Mecklenburg Bay (SW Baltic Sea) using a new Th-specific diatomite adsorption technique followed by liquid scintillation spectrometry. Activities of “dissolved” (operationally defined as Th in the centrifugate) and particulate ²³⁴Th varied in the range of 1.4–6.9 and 0.9–9.3 mBq l⁻¹, respectively. A significant correlation between K_d and SPM concentration was found. From this particle-concentration effect, the “colloidal pumping” model predicts that 98% of the “dissolved” Th is associated with colloids rather than being truly dissolved. Relative to calculated activities of the parent nuclide ²³⁸U in the Bay, the ²³⁴Th data yielded mean ²³⁴Th scavenging residence times in the range of 1.2–9.7 days. Particulate ²³⁴Th activities are inversely correlated to SPM concentrations. Particle residence times ranged from a few days in winter up to 20 days in spring characterized by less intense bottom currents. The hydrodynamic regime is the master variable controlling scavenging of Th and other similarly particle-reactive elements in Mecklenburg Bay.     

Spatial and temporal geochemical variability in basin-related volcanism, northern Israel

Middle Miocene (17-9 Ma) volcanism in northeast Israel migrated from eastern Lower Galilee (Poria, west of the Sea of Galilee) to the southwest (Yizre'el Valley) in association with the development of an extensional basin in that area. The Yizre'el Valley magmas are highly undersaturated in silica (basanites and nephelinites), while those of Poria are alkali-basaltic. Scarce Middle Miocene basalts from the Golan, further to the east, are also alkali-basaltic. Magmas from Kaukab (southeastern Lower Galilee) show a range of compositions from alkali-basalts to basanites. The patterns defined by ratios versus concentrations of incompatible elements in Kaukab basalts (e.g. versus La), as well as the scatter observed in Sr isotope ratios, are interpreted in terms of binary mixing. It is suggested that the Kaukab magmas were derived from veined lithospheric peridotites, melts of the veins and of the peridotites being the two end-members. Accordingly, alkali-basaltic- and basanite-dominated areas are underlain by vein-poor and vein-rich lithosphere, respectively. It is also suggested that melting during the Middle Miocene mainly occurred in response to extension, and that it followed the propagation of the Yizre'el-Galilee Basin from northeast to southwest.     

Analysis of the atmospheric 14C record spanning the past 50,000 years derived from high-precision 230Th/234U/238U, 231Pa/235U and 14C dates on fossil corals

Changes in the geomagnetic field intensity, solar variability, and the internal changes of the carbon cycle are believed to be the three controlling factors of past atmospheric radiocarbon (¹⁴C) concentrations (denoted as Δ¹⁴C). Of these three, it is believed that the field intensity is the dominant factor. We analyze an atmospheric Δ¹⁴C record spanning the past 50,000 years based on previously-published ²³⁰Th/²³⁴U/²³⁸U and ¹⁴C dates of fossil corals from Kiritimati, Barbados, Araki and Santo Islands, and identify the role of the Laschamp geomagnetic field excursion on the long term trend of the Δ¹⁴C record. There is a general consistency between the coral Δ¹⁴C record and the Δ¹⁴C output from carbon cycle models based on the global ¹⁴C production estimates. High-precision, high-accuracy ²³⁰Th/²³⁴U/²³⁸U dates and redundant ²³¹Pa/²³⁵U dates anchor the timing of this Δ¹⁴C record. We propose that a significant fraction of the long-term Δ¹⁴C trend may be due to inaccuracies in the generally accepted ¹⁴C decay constant. The uncertainty in estimating the shape of ¹⁴C beta spectrum below 20 keV leads to one of the greatest errors in decay constant estimates. Once the ¹⁴C half-life is validated via redundant techniques, Δ¹⁴C records will provide a better opportunity to examine the roles of carbon cycle and ¹⁴C production influences.     

石笋初始234 U/238 U值的冰量周期特征及其环境意义 ——以湖北三宝洞为例

石笋氧同位素记录具有明显的2万年周期,其他记录中广泛存在的10万年周期是否在石笋中有所表现目前还鲜有报道。通过对湖北三宝洞20支石笋的铀同位素数据的分析研究发现,石笋初始234U/238U值在序列连续性较好的640.3~299.6 ka B.P.时间段有强烈的10万年周期特征。在间冰期和冰期时,初始234U/238U值分别呈增大和减小状态。初始234U/238U值的10万年周期与全球冰量、黄土磁化率、黄土平均粒度和大气CO₂变化有良好的对应关系。这些对应关系表明全球冰量、大气CO₂对喀斯特区地球化学元素富集和迁移作用有重要影响。石笋氧同位素的显著岁差周期独立于石笋微量元素、高纬冰量和全球温室气体变化,暗示了太阳辐射变化对中低纬水汽环流的直接影响。石笋初始234U/238U与氧同位素、太阳辐射在冰消期时的对应变化支持北半球太阳辐射能量变化对冰期-间冰期旋回的调控作用。     

Spatial and temporal variabilities of hypoxia in the Rappahannock River,Virginia

Hypoxia/anoxia in bottom waters of the Rappahannock River, a tributary estuary of Chesapeake Bay, was observed to persist throughout the summer in the deep basin near the river mouth; periodic reoxygenation of bottom water occurred on the shallower sill at the river mouth. The reoxygenation events were closely related to spring tide mixing. The dissolved oxygen (DO) in surface waters was always near or at the saturation level, while that of bottom waters exhibited a characteristic spatial pattern. The bottom DO decreased upriver from river mouth, reaching a minimum upriver of the deepest point of the river and increasing as the water becaume shallower further upriver. A model was formulated to describe the longitudinal distribution of DO in bottom waters. The model is based on Lagrangian concept—following a water parcel as it travels upriver along the estuarine bottom. The model successfully describes the characteristic distribution of DO and also explains the shifting of the minimum DO location in response to spring-neap cycling. A diagnostic study with the model provided insight into relationships between the bottom DO and the competing factors that contribute to the DO budget of bottom waters. The study reveals that both oxygen demand, either benthic or water column demand, and vertical mixing have a promounced effect on the severity of hypoxia in bottom waters of an estary. However, it is the vertical mixing which controls the longitudinal location of the minimum DO. The strength of gravitational circulation is also shown to affect the occurrence of hypoxia. An estuary with stronger circulation tends to have less chance for hypoxia to occur. The initial DO deficit of bottom water entering an estuary has a strong effect on DO concentration near the river mouth, but its effect diminishes in the upriver direction.     

Spatial and temporal variations of Rb/Sr ratios of the bulk surface sediments in Lake Qinghai

The Rb/Sr ratios of lake sediments have been suggested as indicators of weathering intensity by increasing work. However, the geochemistry of Rb/Sr ratios of lake sediments is variable between different lakes. In this study, we investigated the spatial and temporal patterns of Rb/Sr ratios, as well as those of other major elements in surface sediments of Lake Qinghai. We find that the spatial pattern of Rb/Sr ratios of the bulk sediments correlates well with that of the mass accumulation rate, and those of the terrigenous fractions, e.g., SiO₂, Ti, and Fe. The temporal variations of Rb/Sr ratios also synchronize with those of SiO₂, Ti, and Fe of each individual core. These suggest that Rb/Sr ratios of the surface sediments are closely related to terrigenous input from the catchment. Two out of eight cores show similar trends between Rb/Sr ratios and precipitation indices on decadal scales; however, the other cores do not show such relationship. The result of this study suggests that physical weathering and chemical weathering in Lake Qinghai catchment have opposite influence on Rb/Sr ratios of the bulk sediments, and they compete in dominating the Rb/Sr ratios of lake sediments on different spatial and temporal scales. Therefore, it is necessary to study the geochemistry of Rb/Sr ratio of lake sediments (especially that on short term timescales) particularly before it is used as an indicator of weathering intensity of the catchment.     

Spatial and temporal variability of Holocene temperature in the North Atlantic region

The early Holocene climate of the North Atlantic region was influenced by two boundary conditions that were fundamentally different from the present: the presence of the decaying Laurentide Ice Sheet (LIS) and higher than present summer solar insolation. In order to assess spatial and temporal patterns of Holocene climate evolution across this region, we collated quantitative paleotemperature records at sub-millennial resolution and synthesized their temporal variability using principal components analysis (PCA). The analysis reveals considerable spatial variability, most notably in the time-transgressive expression of the Holocene thermal maximum (HTM). Most of the region, but especially areas peripheral to the Labrador Sea and hence closest to the locus of LIS disintegration, experienced maximum Holocene temperatures that lagged peak summer insolation by 1000-3000 years. Many sites from the northeastern North Atlantic sector, including the Nordic Seas and Scandinavia, either warmed in phase with maximum summer insolation (11,000-9000 years ago) or were less strongly lagged than the Baffin Bay-Labrador Sea region. These spatially complex patterns of Holocene climate development, which are defined by the PCA, resulted from the interplay between final decay of the LIS and solar insolation forcing.