Ligand extraction of rare earth elements from aquifer sediments: Implications for rare earth element complexation with organic matter in natural waters

The ability of organic matter as well as carbonate ions to extract rare earth elements (REEs) from sandy sediments of a Coastal Plain aquifer was investigated for unpurified organic matter from different sources (i.e., Mississippi River natural organic matter, Aldrich humic acid, Nordic aquatic fulvic acid, Suwannee River fulvic acid, and Suwannee River natural organic matter) and for extraction solutions containing weak (i.e., CH₃COO⁻) or strong (i.e., ) ligands. The experimental results indicate that, in the absence of strong REE complexing ligands in solution, the amount of REEs released from the sand is small and the fractionation pattern of the released REEs appears to be controlled by the surface stability constants for REE sorption with Fe(III) oxides/oxyhydroxides. In the presence of strong solution complexing ligands, however, the amount and the fractionation pattern of the released REEs reflect the strength and variation of the stability constants of the dominant aqueous REE species across the REE series. The varying amount of REEs extracted by the different organic matter employed in the experiments indicates that organic matter from different sources has different complexing capacity for REEs. However, the fractionation pattern of REEs extracted by the various organic matter used in our experiments is remarkable consistent, being independent of the source and the concentration of organic matter used, as well as solution pH. Because natural aquifer sand and unpurified organic matter were used in our experiments, our experimental conditions are more broadly similar to natural systems than many previous laboratory experiments of REE-humic complexation that employed purified humic substances. Our results suggest that the REE loading effect on REE-humic complexation is negligible in natural waters as more abundant metal cations (e.g., Fe, Al) out-compete REEs for strong binding sites on organic matter. More specifically, our results indicate that REE complexation with organic matter in natural waters is dominated by REE binding to weak sites on dissolved organic matter, which subsequently leads to a middle REE (MREE: Sm-Ho)-enriched fractionation pattern. The experiments also indicate that carbonate ions may effectively compete with fulvic acid in binding with dissolved REEs, but cannot out compete humic acids for REEs. Therefore, in natural waters where low molecular weight (LMW) dissolved organic carbon (DOC) is the predominant form of DOC (e.g., lower Mississippi River water), REEs occur as “truly” dissolved species by complexing with carbonate ions as well as FA, resulting in heavy REE (HREE: Er-Lu)-enriched shale-normalized fractionation patterns. Whereas, in natural terrestrial waters where REE speciation is dominated by organic complexes with high molecular weight DOC (e.g., “colloidal” HA), only MREE-enriched fractionation patterns will be observed because the more abundant, weak sites preferentially complex MREEs relative to HREEs and light REEs (LREEs: La-Nd).     

Increased concentrations of dissolved trace metals and organic carbon during snowmelt in rivers of the alaskan arctic

Arctic rivers typically transport more than half of their annual amounts of water and suspended sediments during spring floods. In this study, the Sagavanirktok, Kuparuk and Colville rivers in the Alaskan Arctic were sampled during the spring floods of 2001 to determine levels of total suspended solids (TSS) and dissolved and particulate metals and organic carbon. Concentrations of dissolved organic carbon (DOC) increased from 167 to 742 μmol/L during peak discharge in the Sagavanirktok River, at about the same time that river flow increased to maximum levels. Concentrations of dissolved Cu, Pb, Zn and Fe in the Sagavanirktok River followed trends observed for DOC with 3- to 25-fold higher levels at peak flow than during off-peak discharge. Similar patterns were found for the Kuparuk and Colville rivers, where average concentrations of dissolved trace metals and DOC were even higher. These observations are linked to a large pulse of DOC and dissolved metals incorporated into snowmelt from thawing ponds and upper soil layers. In contrast with Cu, Fe, Pb and Zn, concentrations of dissolved Ba did not increase in response to increased discharge of water, TSS and DOC. Concentrations of particulate Cu, Fe, Pb and Zn were more uniform than observed for their respective dissolved species and correlated well with the Al content of the suspended particles. However, concentrations of particulate Al were poorly correlated with particulate organic carbon. Results from this study show that >80% of the suspended sediment and more than one-third of the annual inputs of dissolved Cu, Fe, Pb, Zn and DOC were carried to the coastal Beaufort Sea in 3 and 12 d, respectively, by the Kuparuk and Sagavanirktok rivers.     

Submarine groundwater discharge is an important net source of light and middle REEs to coastal waters of the Indian River Lagoon, Florida, USA

Porewater (i.e., groundwater) samples were collected from multi-level piezometers across the freshwater-saltwater seepage face within the Indian River Lagoon subterranean estuary along Florida’s (USA) Atlantic coast for analysis of the rare earth elements (REE). Surface water samples for REE analysis were also collected from the water column of the Indian River Lagoon as well as two local rivers (Eau Gallie River, Crane Creek) that flow into the lagoon within the study area. Concentrations of REEs in porewaters from the subterranean estuary are 10-100 times higher than typical seawater values (e.g., Nd ranges from 217 to 2409 pmol kg⁻¹), with submarine groundwater discharge (SGD) at the freshwater-saltwater seepage face exhibiting the highest REE concentrations. The elevated REE concentrations for SGD at the seepage face are too high to be the result of simple, binary mixing between a seawater end-member and local terrestrial SGD. Instead, the high REE concentrations indicate that geochemical reactions occurring within the subterranean estuary contribute substantially to the REE cycle. A simple mass balance model is used to investigate the cycling of REEs in the Indian River Lagoon and its underlying subterranean estuary. Mass balance modeling reveals that the Indian River Lagoon is approximately at steady-state with respect to the REE fluxes into and out of the lagoon. However, the subterranean estuary is not at steady-state with respect to the REE fluxes. Specifically, the model suggests that the SGD Nd flux, for example, exported from the subterranean estuary to the overlying lagoon waters exceeds the combined input to the subterranean estuary from terrestrial SGD and recirculating marine SGD by, on average, ∼100 mmol day⁻¹. The mass balance model also reveals that the subterranean estuary is a net source of light REEs (LREE) and middle REEs (MREE) to the overlying lagoon waters, but acts as a sink for the heavy REEs (HREE). Geochemical modeling and statistical analysis further suggests that this fractionation occurs, in part, due to the coupling between REE cycling and iron redox cycling within the Indian River Lagoon subterranean estuary. The net SGD flux of Nd to the Indian River Lagoon is ∼7-fold larger than the local effective river flux to these coastal waters. This previously unrecognized source of Nd to the coastal ocean could conceivably be important to the global oceanic Nd budget, and help to resolve the oceanic “Nd paradox” by accounting for a substantial fraction of the hypothesized missing Nd flux to the ocean.     

Geochemistry of the Rare Earth Elements in Natural Terrestrial Waters：A Review of What Is Currently Knows

The range of observed chemical compositions of natural terrestrial waters varies greatly especially when compared to the essentially constant global composition of the oceans.The concentrations of the REEs in natural terrestrial waters also exhibit more variation than what was reported in seawater,In terrestrial waters ,pH values span the range from acid up to alkaline,In addition,terrestrial waters can range from very dilute waters through to highly concentrated brines.The REE concentrations and their behavior in natural terrestrial waters reflect these compositional ranges,Chemical weathering of rocks represents the source of the REEs to natural terrestrial waters and ,consequently,the REE signature of rocks can impart their REE signature to associated waters,In addition,Because of the typical low solubilities of the REEs both surface and solution complexation can be important in fractionating REEs in aqueous solution.Both of these processes are important in all natural terrestrial waters,however,their relative importance varies as a function of the overall solution composition,In alkaline waters,for example,Solution complexation of the REEs with carbonate ions appears to control their aqueous distributions whereas in acid waters,the REE signature of the labile fraction of the REEs is readily leached from the rocks.In circumneutral pH waters,both processes appear to be important and their relative significance has not yet been determined.     

Rare earth elements in pore waters of marine sediments

The rare earth elements (REEs) were measured in pore waters of the upper ∼25 cm of sediment from one site off Peru and three sites on the California margin. The pore water REE concentrations are higher than sea water and show systematic down core variations in both concentration and normalized pattern. From these analyses and from comparison to other chemical species measured (dissolved Fe, Mn, Ba, oxygen, nitrate, phosphate), it is suggested that pore water REEs can be grouped into three categories: those that are from an Fe-source, those that are from a POC-source, and cerium oxide. REEs from the Fe-source appear where anoxia is reached; they have a distinctive “middle-REE (MREE) enriched” pattern. The concentrations in this source are so elevated that they dominate REE trends in the Fe-oxide reduction zone. The net result of flux from the POC-source is relative enrichment of heavy-REEs (HREEs) over light-REEs (LREEs), reflecting remineralizing POC and complexation with DOC. A common “linear” REE pattern, seen in both oxic and anoxic sediments, is associated with this POC-source, as well as a “HREE enriched” pattern that is seen in surficial sediments at the Peru site. Overall, the pore water results indicate that Mn-oxides are not an important carrier of REEs in the oceans.A REE biogeochemical model is presented which attempts to reconcile REE behavior in the water and sediment columns of the oceans. The model proposes that POC, Fe-oxide and Ce-oxide sources can explain the REE concentration profiles and relative abundance patterns in environments ranging from oxic sea water to anoxic pore water. The model is also consistent with our observation that the “Ce-anomaly” of pore water does not exceed unity under any redox condition.     

Mechanisms for chemostatic behavior in catchments: Implications for CO2 consumption by mineral weathering

Concentrations of weathering products in streams often show relatively little variation compared to changes in discharge, both at event and annual scales. In this study, several hypothesized mechanisms for this “chemostatic behavior” were evaluated, and the potential for those mechanisms to influence relations between climate, weathering fluxes, and CO₂ consumption via mineral weathering was assessed. Data from Loch Vale, an alpine catchment in the Colorado Rocky Mountains, indicates that cation exchange and seasonal precipitation and dissolution of amorphous or poorly crystalline aluminosilicates are important processes that help regulate solute concentrations in the stream; however, those processes have no direct effect on CO₂ consumption in catchments. Hydrograph separation analyses indicate that old water stored in the subsurface over the winter accounts for about one-quarter of annual streamflow, and almost one-half of annual fluxes of Na and SiO₂ in the stream; thus, flushing of old water by new water (snowmelt) is an important component of chemostatic behavior. Hydrologic flushing of subsurface materials further induces chemostatic behavior by reducing mineral saturation indices and increasing reactive mineral surface area, which stimulate mineral weathering rates. CO₂ consumption by carbonic acid mediated mineral weathering was quantified using mass-balance calculations; results indicated that silicate mineral weathering was responsible for approximately two-thirds of annual CO₂ consumption, and carbonate weathering was responsible for the remaining one-third. CO₂ consumption was strongly dependent on annual precipitation and temperature; these relations were captured in a simple statistical model that accounted for 71% of the annual variation in CO₂ consumption via mineral weathering in Loch Vale.     

Adsorption of rare earth elements onto Carrizo sand: Experimental investigations and modeling with surface complexation

Rare earth element (REE) adsorption onto sand from a well characterized aquifer, the Carrizo Sand aquifer of Texas, has been investigated in the laboratory using a batch method. The aim was to improve our understanding of REE adsorption behavior across the REE series and to develop a surface complexation model for the REEs, which can be applied to real aquifer-groundwater systems. Our batch experiments show that REE adsorption onto Carrizo sand increases with increasing atomic number across the REE series. For each REE, adsorption increases with increasing pH, such that when pH >6.0, >98% of each REE is adsorbed onto Carrizo sand for all experimental solutions, including when actual groundwaters from the Carrizo Sand aquifer are used in the experiments. Rare earth element adsorption was not sensitive to ionic strength and total initial REE concentrations in our batch experiments. It is possible that the differences in experimental ionic strength conditions (i.e., 0.002-0.01 M NaCl) chosen were insufficient to affect REE adsorption behavior. However, cation competition (e.g., Ca, Mg, and Zn) did affect REE adsorption onto Carrizo sand, especially for light rare earth elements (LREEs) at low pH. Rare earth element adsorption onto Carrizo sand can be successfully modeled using a generalized two-layer surface complexation model. Our model calculations suggest that REE complexation with strong surface sites of Carrizo sand exceeds the stability of the aqueous complexes LnOH²⁺, LnSO₄⁺, and LnCO₃⁺, but not that of Ln(CO₃)₂^- or LnPO₄^o in Carrizo groundwaters. Thus, at low pH (<7.3), where major inorganic ligands did not effectively compete with surface sites for dissolved REEs, free metal ion (Ln³⁺) adsorption was sufficient to describe REE adsorption behavior. However, at higher pH (>7.3) where solution complexation of the dissolved REEs was strong, REEs were adsorbed not only as free metal ion (Ln³⁺) but also as aqueous complexes (e.g., as Ln(CO₃)₂^- in Carrizo groundwaters). Because heavy rare earth elements (HREEs) were preferentially adsorbed onto Carrizo sand compared to LREEs, original HREE-enriched fractionation patterns in Carrizo groundwaters from the recharge area flattened along the groundwater flow path in the Carrizo Sand aquifer due to adsorption of free- and solution-complexed REEs.     

The aquatic chemistry of rare earth elements in rivers and estuaries

Laboratory experiments were carried out to determine how pH, colloids and salinity control the fractionation of rare earth elements (REEs) in river and estuarine waters. By using natural waters as the reaction media (river water from the Connecticut, Hudson and Mississippi Rivers) geochemical reactions can be studied in isolation from the large temporal and spatial variability inherent in river and estuarine chemistry. Experiments, field studies and chemical models form a consistent picture whereby REE fractionation is controlled by surface/solution reactions. The concentration and fractionation of REEs dissolved in river waters are highly pH dependent. Higher pH results in lower concentrations and more fractionated composition relative to the crustal abundance. With increasing pH the order of REE adsorption onto river particle surfaces is LREEs > MREEs > HREEs. With decreasing pH, REEs are released from surfaces in the same order. Within the dissolved (<0.22 µm) pool of river waters, Fe-organic colloids are major carriers of REEs. Filtration through filters and ultrafilters with progressively finer pore sizes results in filtrates which are lower in absolute concentrations and more fractionated. The order of fractionation with respect to shale, HREEs > MREEs > LREEs, is most pronounced in the solution pool, defined here as <5K and <50K ultrafiltrates. Colloidal particles have shale-like REE compositions and are highly LREE enriched relative to the REE composition of the dissolved and solution pools. The addition of sea water to river water causes the coagulation of colloidal REEs within the dissolved pool. Fractionation accompanies coagulation with the order of sea water-induced removal being LREEs > MREEs > HREEs. While the large scale removal of dissolved river REEs in estuaries is well established, the release of dissolved REEs off river particles is a less studied process. Laboratory experiments show that there is both release and fractionation of REEs when river particles are leached with seawater. The order of sea water-induced release of dissolved REE(III) (LREEs > MREEs > HREEs) from Connecticut River particles is the same as that associated with lowering the pH and the same as that associated with colloidal particles. River waters, stripped of their colloidal particles by coagulation in estuaries, have highly evolved REE composition. That is, the solution pool of REEs in river waters are strongly HREE-enriched and are fractionated to the same extent as that of Atlantic surface seawater. This strengthens the conclusions of previous studies that the evolved REE composition of sea water is coupled to chemical weathering on the continents and reactions in estuaries. Moreover, the release of dissolved Nd from river particles to sea water may help to reconcile the incompatibility between the long oceanic residence times of Nd (7100 yr) and the inter-ocean variations of the Nd isotopic composition of sea water. Using new data on dissolved and particle phases of the Amazon and Mississippi Rivers, a comparison of field and laboratory experiments highlights key features of REE fractionation in major river systems. The dissolved pool of both rivers is highly fractionated (HREE enriched) with respect to the REE composition of their suspended particles. In addition, the dissolved pool of the Mississippi River has a large negative Ce-anomaly suggesting in-situ oxidation of Ce(III). One intriguing feature is the well developed maximum in the middle REE sector of the shale normalized patterns for the dissolved pool of Amazon River water. This feature might reflect competition between surface adsorption and solution complexation with carbonate and phosphate anions.     

鄱阳湖流域赣江北支水体和沉积物中稀土元素的含量和分异特征

稀土的开发和广泛应用使得人们倍加关注其在环境中的分布及其环境地球化学行为。赣江作为鄱阳湖流域五大入湖河之一,发源于稀土资源富集的赣南地区,而其下游水体及周边地下水中稀土元素的含量和分异特征目前尚不完全清楚。以赣江北支水体及沉积物为研究对象,开展了稀土元素地球化学研究。结果表明,赣江北支水体中稀土元素总量在地表水中为230~1 146 ng/L(均值458.85 ng/L),地下水中为284~1 498 ng/L(均值634.94 ng/L),沉积物中稀土元素总量为177.9~270.7 mg/kg(均值226.99 mg/kg)。PHREEQC模拟计算表明,水体中的稀土元素主要以碳酸根络合物(REEC0₃⁺)的形式存在。地表水和地下水总体上均表现为重稀土元素相较于轻、中稀土元素富集,沉积物未表现出明显的富集特性;水体具有Ce、Eu负异常特点,而沉积物表现为Ce正异常和Eu负异常,指示氧化还原环境和水岩相互作用对稀土元素在水-沉积物系统中迁移转化的影响。地下水中稀土元素的含量沿流向具有上升趋势,而水体中重稀土元素的富集程度不断减弱,同时碳酸根络合物(REEC0₃⁺)的占比不断降低,反映水体中稀土元素的含量受到pH、胶体吸附、络合作用以及地下水-地表水相互作用的影响。水体中重稀土元素的富集受到碳酸根络合反应的影响,Ce、Eu负异常与Ce氧化沉淀和母岩特性相关。Gd异常值表明,研究区中下游水体中的Gd元素受到人为输入的影响。     

Controls of fluid chemistry and complexation on rare-earth element contents of anhydrite from the Pacmanus subseafloor hydrothermal system,Manus Basin,Papua New Guinea

Ocean Drilling Program (ODP) leg 193 successfully drilled four deep holes (126 to 386 m) into basement underlying the active dacite-hosted Pacmanus hydrothermal field in the eastern Manus Basin. Anhydrite is abundant in the drill core material, filling veins and vesicles, cementing breccias, and occasionally replacing igneous material. We report rare-earth element (REE) contents of anhydrite from a site of diffuse venting (Site 1188) which show extreme variability, in terms of both absolute concentrations (e.g., 0.08–28.3 ppm Nd) and pattern shape (La_N/Sm_N=0.08–3.78, Sm_N/Yb_N=0.48–23.1, Eu/Eu*=0.59–6.1). The range of REE patterns in anhydrite includes enrichments in the middle and heavy REEs and variable Eu anomalies. The patterns differ markedly from those of anhydrite recovered during ODP Leg 158 from the TAG hydrothermal system at the Mid-Atlantic Ridge which display uniform LREE-enriched patterns with positive Eu anomalies, very similar to TAG vent fluid patterns. As the system is active, the host-rock composition is uniform, and the anhydrite veins appear to relate to the same hydrothermal stage, we can rule out predominant host-rock and transport control. Instead, we propose that the variation in REE content reflects waxing and waning input of magmatic volatiles (HF, SO₂) and variable complexation of REEs in the fluids. REE speciation calculations suggest that increased fluoride and possibly sulfate concentrations at Pacmanus may affect REE complexation in fluids, whereas at TAG only chloride and hydroxide complexes play a significant role. The majority of the anhydrites do not show positive Eu anomalies, suggesting that the fluids were more oxidizing than in typical mid-ocean ridge hydrothermal systems. We use other hydrothermal fluids from the Manus Basin (Vienna Woods and Desmos), which bracket the Pacmanus fluids in terms of acidity and ligand concentrations, to examine the dependence of REE complexation on fluid composition. Geochemical modeling reveals that under the prevailing conditions at Pacmanus (pH~3.5, T=250–300 °C), Eu oxidation state and the relative importance of fluoride versus chloride complexing are very sensitive to small variations in oxygen fugacity, temperature, and pH. Patterns with extreme mid-REE enrichment may reflect speciation effects (free-ion abundance) coupled with crystal chemical control. We conclude that the great variability in REE concentrations and pattern shape is likely due to variable fluid composition and REE complexation in the fluids. Editorial handling: L. Meinert     

Speciation of rare earth elements in natural terrestrial waters: assessing the role of dissolved organic matter from the modeling approach

Humic Ion-Binding Model V, which focuses on metal complexation with humic and fulvic acids, was modified to assess the role of dissolved natural organic matter in the speciation of rare earth elements (REEs) in natural terrestrial waters. Intrinsic equilibrium constants for cation-proton exchange with humic substances (i.e., pK_MHA for type A sites, consisting mainly of carboxylic acids), required by the model for each REE, were initially estimated using linear free-energy relationships between the first hydrolysis constants and stability constants for REE metal complexation with lactic and acetic acid. pK_MHA values were further refined by comparison of calculated Model V “fits” to published data sets describing complexation of Eu, Tb, and Dy with humic substances. A subroutine that allows for the simultaneous evaluation of REE complexation with inorganic ligands (e.g., Cl⁻, F⁻, OH⁻, SO₄²⁻, CO₃²⁻, PO₄³⁻), incorporating recently determined stability constants for REE complexes with these ligands, was also linked to Model V. Humic Ion-Binding Model V’s ability to predict REE speciation with natural organic matter in natural waters was evaluated by comparing model results to “speciation” data determined previously with ultrafiltration techniques (i.e., organic acid-rich waters of the Nsimi-Zoetele catchment, Cameroon; dilute, circumneutral-pH waters of the Tamagawa River, Japan, and the Kalix River, northern Sweden). The model predictions compare well with the ultrafiltration studies, especially for the heavy REEs in circumneutral-pH river waters. Subsequent application of the model to world average river water predicts that organic matter complexes are the dominant form of dissolved REEs in bulk river waters draining the continents. Holding major solute, minor solute, and REE concentrations of world average river water constant while varying pH, the model suggests that organic matter complexes would dominate La, Eu, and Lu speciation within the pH ranges of 5.4 to 7.9, 4.8 to 7.3, and 4.9 to 6.9, respectively. For acidic waters, the model predicts that the free metal ion (Ln³⁺) and sulfate complexes (LnSO₄⁺) dominate, whereas in alkaline waters, carbonate complexes (LnCO₃⁺ + Ln[CO₃]₂⁻) are predicted to out-compete humic substances for dissolved REEs. Application of the modified Model V to a “model” groundwater suggests that natural organic matter complexes of REEs are insignificant. However, groundwaters with higher dissolved organic carbon concentrations than the “model” groundwater (i.e., >0.7 mg/L) would exhibit greater fractions of each REE complexed with organic matter. Sensitively analysis indicates that increasing ionic strength can weaken humate-REE interactions, and increasing the concentration of competitive cations such as Fe(III) and Al can lead to a decrease in the amount of REEs bound to dissolved organic matter.     

Determination of rare earth elements,thorium and uranium in seaweed samples on the coast in Niigata Prefecture by inductively coupled plasma mass spectrometry

The determination of rare earth elements (REEs), Th and U in seaweeds of various species and in ambient surface seawater were performed by inductively coupled plasma mass spectrometry. The seaweeds and the seawater samples were collected at 10 different coastal areas of Niigata Prefecture in Japan twice a year (spring and autumn) in 2004 and 2005. Results showed that concentration of REEs in seaweed species from Niigata Prefecture were about 10³ times higher than those in seawater, and the enrichment factors of REEs were larger in heavy REE than light REE. A significant concentration difference of REEs, Th and U was found among species even in the same phylum. Undaria pinnatifida had the highest concentration of U. Considering that U. pinnatifida has low concentrations of REEs among typical brown algae, different REEs and U uptake mechanisms are suggested. The concentration of REEs, Th and U did not vary remarkably between sampling points and/or seasons.     

辽东湾东南部海域柱状沉积物稀土元素地球化学特征与物源识别

对辽东湾东南部海域LDC30孔沉积物稀土元素（REE）、粒度等指标进行了分析,研究了其沉积物中稀土元素组成特征及其控制因素,并对其物质来源进行了探讨。结果表明,LDC30孔沉积物ΣREE平均值为149.49 μg/g,略高于黄海和东海,但是低于渤海和南海,并且低于全球沉积物ΣREE的平均值。研究区沉积物REE配分模式表现为明显的轻稀土富集、重稀土相对亏损;δEu的平均值为0.71,为中度亏损,δCe无异常;轻稀土与重稀土之间的分异作用较强,且轻重稀土内部分异明显。根据沉积物REE垂向变化特征,可将LDC30岩芯以51cm为界划分为两段,上段（0~51 cm）ΣREE含量随着深度的减小而呈增加的趋势,下段（51~99 cm）ΣREE含量在垂向上没有明显的波动变化,并且上段较下段稀土分异明显。δCe整体上比较稳定,但是在上段（0~51 cm）呈现下降的趋势。δEu垂向上也相对稳定的趋势。该孔沉积物REE参数与粒度之间无明显的相关性,REE组成不受粒度的控制,但重矿物对REE的组成和分布状况有重要的影响。LDC30孔沉积物物质来源比较稳定,并且具有强烈的陆源特征,其沉积物主要来源于辽东湾北部河流（大辽河、小凌河、双台子河等）,同时辽东湾西部河流滦河可能对LDC30孔上段沉积物有一定贡献。     

Rare earth elements (REE) of dissolved and suspended loads in the Xijiang River, South China

The Xijiang River is the main channel of the Zhujiang (Pearl River), the second largest river in China in terms of water discharge, and flows through one of the largest carbonate provinces in the world. The rare earth element (REE) concentrations of the dissolved load and the suspended particulate matter (SPM) load were measured in the Xijiang River system during the high-flow season. The low dissolved REE concentration in the Xijiang River is attributed to the interaction of high pH and low DOC concentration. The PAAS-normalized REE patterns for the dissolved load show some common features: negative Ce anomaly, progressively heavy REE (HREE) enrichment relative to light REE (LREE). Similar to the world’s major rivers the absolute concentration of the dissolved REE in the Xijiang River are mainly pH controlled. The degree of REE partitioning between the dissolved load and SPM load is also strongly pH dependent. The negative Ce anomaly is progressively developed with increasing pH, being consistent with the oxidation of Ce (III) to Ce (IV) in the alkaline river waters, and the lack of Ce anomalies in several DOC-rich waters is presumably due to both Ce (III) and Ce (IV) being strongly bound by organic matter. The PAAS-normalized REE patterns for the dissolved load and the SPM load in rivers draining the carbonate rock area exhibit middle REE (MREE) enrichment and a distinct maximum at Eu, indicating the preferential dissolution of phosphatic minerals during weathering of host lithologies. Compared to the Xijiang River waters, the MREE enrichment with a maximum at Eu disappeared and light REE were more depleted in the South China Sea (SCS) waters, suggesting that the REE sourced from the Xijiang River must be further fractionated and modified on entering the SCS. The river fluxes of individual dissolved REE introduced by the Xijiang River into the SCS vary from 0.04 to 4.36 × 10⁴ mol a⁻¹.     

天山南坡黄水沟与清水河寒区流域极端水文事件对气候变化的响应

冰川、积雪和冻土变化产生的水文效应对下游水资源供给具有重要影响,近几十年来新疆区域洪水呈显著加重趋势,尤其是南疆区域洪水明显加剧. 以天山南坡黄水沟与清水河寒区流域为研究区域,通过分析水文站极端水文事件,结合流域上游山区巴伦台气象站资料,研究了高寒山地流域在气候变化背景下极端水文过程出现时间、年最大和最小径流的响应特征. 结果表明：1986年是水文过程的突变点,从1986年开始随着降水、气温的增加,河流径流量呈增加趋势;最大年径流出现时间从6月中下旬推迟到7月下旬;最大径流和最小径流与年径流量呈正相关关系,最大径流与夏季降水关系密切,而最小年径流与冬春季的气温关系密切. 随着1986年以来的气温升高,冻土退化产生的水文效应使冬季径流增加明显,也使年最小径流明显增大;1986年以来降水变化决定着年径流量增加,使年最大径流集中出现在夏季且量级增大. 总体来讲,20世纪80年代中期以后山区河流年极端洪峰量增大,洪水量增多,年际间变化幅度明显增大,从而对下游造成更严重的灾害. 因此,加强气候变化对寒区流域水资源和洪水灾害的影响评估,使科学技术在减灾方面发挥主导作用.     

Dissolved silicate dynamics of the Rhode River watershed and estuary

We continuously measured dissolved silicate concentrations and fluxes discharged from various Rhode River subwatersheds for a period of 14 yr from 1984 to 1998 and for 15 mo in 1971–1972. We also measured dissolved silicate concentrations along a transect from the head of the tide in Rhode River estuary to Chesapeake Bay. The average concentration of dissolved silicate discharged from the Rhode River watershed was 10.8 mg Si l^?1. There were consistent and significant differences in silicate concentrations discharged over time and space among subwatersheds. Mean annual silicate flux from the watershed was 26.6 kg Si ha^?1 and 93% of this occurred during the winter and spring seasons. There were large interannual variations in silicate flux, due primarily to differences in precipitation and water discharge, rather than silicate concentration. Land use had little or no effect on silicate flux from various subwatersheds. Silicate concentrations discharged from a subset of subwatersheds in 1995–1996 were 25% to 35% lower than in a period with similar precipitation in 1971–1972. Mean annual concentrations of silicate discharged from nine subwatersheds have been declining about 1.5% yr^?1 or by 0.21–0.26 mg Si l^?1 yr^?1 over the last 25 yr. Despite high average silicate fluxes from the watershed, at times the Rhode River estuary developed low dissolved silicate concentrations, which could have been limiting to the growth of diatoms. Examples were in the spring after a winter with low watershed discharge (as low as 0.019 mg Si l^?1 in 1995) and after protracted drought (as low as 0.041 mg Si l^?1 in 1993).     

Comparison of the behaviour of rare earth elements in surface waters,overburden groundwaters and bedrock groundwaters in two granitoidic settings,Eastern Sweden

This work, which was done within the Swedish nuclear waste management program, was carried out in order to increase the understanding of the mobility and fate of rare earth elements (REEs) in natural boreal waters in granitoidic terrain. Two areas were studied, Forsmark and Simpevarp, one of which will be selected as a site for spent nuclear fuel. The highest REE concentrations were found in the overburden groundwaters, in Simpevarp in particular (median ∑REE 52 μg/L), but also in Forsmark (median ∑REE 6.7 μg/L). The fractionation patterns in these waters were characterised by light REE (LREE) enrichment and negative Ce and Eu anomalies. In contrast, the surface waters had relatively low REE concentrations. They were characterised either by an increase in relative concentrations throughout the lanthanide series (Forsmark which has a carbonate-rich till) or flat patterns (Simpevarp with carbonate-poor till), and had negative Ce and Eu anomalies. In the bedrock groundwaters, the concentrations and fractionation patterns of REEs were entirely different from those in the overburden groundwaters. The median La concentrations were low (just above 0.1 μg/L in both areas), only in a few samples were the concentrations of several REEs (and in a couple of rare cases all REEs) above the detection limit, and there was an increase in the relative concentrations throughout the lanthanide series. In contrast to these large spatial variations, the temporal trends were characterised by small (or non existent) variations in REE-fractionation patterns but rather large variations in concentrations. The Visual MINTEQ speciation calculations predicted that all REEs in all waters were closely associated with dissolved organic matter, and not with carbonate. In the hydrochemical data for the overburden groundwater in particular, there was however a strong indication of association with inorganic colloids, which were not included in the speciation model. Overall the results showed that within a typical boreal granitoidic setting, overburden groundwaters are enriched in REEs, organic complexes are much more important than carbonate complexes, there is little evidence of significant mixing of REEs between different water types (surface, overburden, bedrock) and spatial variations are more extensive than temporal ones.     

Seasonal and annual variations in the organic matter contributed by the St Lawrence River to the Gulf of St. Lawrence

The St. Lawrence River discharges a substantial volume of water (405 km³/a) containing suspended (SPM; 3.42 × 10⁶t) and dissolved (68.0 × 10⁶t) materials to the Gulf of St. Lawrence. The total load contains organic carbon in paniculate (POC; 3–14% of SPM), and dissolved (DOC; 3.76 ± 0.63 mg/l) form. The concentration of POC (and particulate organic nitrogen) is positively correlated with discharge (increased during the spring flood and the fall enhancement of flow), but concentration of DOC is not so simply related to discharge. In consequence, the total organic carbon (POC + DOC) load is relatively invariant, and increased annually by only 2–3% despite a progressive increase of 8% in discharge over the years of this study. Seasonal differences in the composition of the particulate organic matter (POM) are interpreted as reflecting dominant contributions from within-river production in summer and from terrestrial sources in spring and fall. In years when the annual discharge was greater than average, a higher proportion of the POM was terrigenous. The organic matter in surface sediments of the estuary to which the river discharges is predominantly of terrestrial provenance.     

Abundance,distribution, and fluxes of dissolved organic carbon (DOC) in four small sub-tropical rivers of the Tampa Bay Estuary (Florida,USA)

The delivery of organic carbon (C) from rivers to the coastal ocean via estuaries is recognized as an important component of the global C budget however, smaller river systems are often overlooked and modern flux estimates are not very different from historical estimates. Here, the seasonal (wet vs. dry) concentration and fluxes of dissolved organic C (DOC) were measured in five small sub-tropical rivers that drain into the Tampa Bay (FL, USA) estuary. DOC distributions were highly variable among riverine, mesohaline, and marine end-member samples in all river catchments and no significant differences were observed among or between DOC concentrations with respect to river catchment, season, or year of sampling. In general, DOC mixed non-conservatively during the wet seasons, and conservatively during the dry seasons, with the estuarine reaches of each river serving as a sink of DOC. Fluxes were strongly tied to discharge irrespective of season, and the estuaries removed 15–65% of DOC prior to export to coastal Bay and Gulf of Mexico waters. DOC concentrations were similar to others reported for low-elevation sub-tropical rivers, and a combination of elevation, residence time, and climate appear to control the abundance and variability of DOC in sub-tropical vs. tropical river systems. The characterization of DOC in small, sub-tropical rivers, which share characteristics with both their temperate and tropical counterparts, is critical for quantitatively constraining the importance of these systems in local-to-regional scale ocean C budgets. In addition to geomorphic properties, the role of past, present, and future land cover and other environmental change in small coastal rivers also exerts control on the quantity and flux of DOC in these systems.     

Controls on the geochemistry of rare earth elements along a groundwater flow path in the Carrizo Sand aquifer,Texas, USA

Groundwater samples were collected along a groundwater flow path in the Carrizo Sand aquifer in south Texas, USA. Field measurements that included pH, specific conductivity, temperature, dissolved oxygen (DO), oxidation–reduction potentials (Eh in mV), alkalinity, iron speciation, and H₂S concentrations were also conducted on site. The geochemistry (i.e., concentrations, shale-normalized patterns, and speciation) of dissolved rare element elements (REEs) in the Carrizo groundwaters are described as a function of distance along a flow path. Eh and other redox indicators (i.e., DO, Fe speciation, H₂S, U, and Re) indicate that redox conditions change along the flow path in the Carrizo Sand aquifer. Within the region of the aquifer proximal to the recharge zone, groundwaters exhibit both highly oxidizing and localized mildly reducing conditions. However, from roughly 10 km to the discharge zone, groundwaters are reducing and exhibit a progressive decrease in redox conditions. Dissolved REE geochemical behavior exhibits regular variations along the groundwater flow path in the Carrizo Sand aquifer. The changes in REE concentrations, shale-normalized patterns, and speciation indicate that REEs are not conservative tracers. With flow down-gradient, redox conditions, pH and solution composite, and adsorption modify groundwater REE concentrations, fractionation patterns, and speciation.