Quantifying bioirrigation using ecological parameters: a stochastic approach†

Irrigation by benthic macrofauna has a major influence on the biogeochemistry and microbial community structure of sediments. Existing quantitative models of bioirrigation rely primarily on chemical, rather than ecological, information and the depth-dependence of bioirrigation intensity is either imposed or constrained through a data fitting procedure. In this study, stochastic simulations of 3D burrow networks are used to calculate mean densities, volumes and wall surface areas of burrows, as well as their variabilities, as a function of sediment depth. Burrow networks of the following model organisms are considered: the polychaete worms Nereis diversicolor and Schizocardium sp., the shrimp Callianassa subterranea, the echiuran worm Maxmuelleria lankesteri, the fiddler crabs Uca minax, U. pugnax and U. pugilator, and the mud crabs Sesarma reticulatum and Eurytium limosum. Consortia of these model organisms are then used to predict burrow networks in a shallow water carbonate sediment at Dry Tortugas, FL, and in two intertidal saltmarsh sites at Sapelo Island, GA. Solute-specific nonlocal bioirrigation coefficients are calculated from the depth-dependent burrow surface areas and the radial diffusive length scale around the burrows. Bioirrigation coefficients for sulfate obtained from network simulations, with the diffusive length scales constrained by sulfate reduction rate profiles, agree with independent estimates of bioirrigation coefficients based on pore water chemistry. Bioirrigation coefficients for O₂ derived from the stochastic model, with the diffusion length scales constrained by O₂ microprofiles measured at the sediment/water interface, are larger than irrigation coefficients based on vertical pore water chemical profiles. This reflects, in part, the rapid attenuation with depth of the O₂ concentration within the burrows, which reduces the driving force for chemical transfer across the burrow walls. Correction for the depletion of O₂ in the burrows results in closer agreement between stochastically-derived and chemically-derived irrigation coefficient profiles.     

Two-dimensional pH distributions and dynamics in bioturbated marine sediments

The seafloor is the site of intense biogeochemical and mineral dissolution-precipitation reactions which generate strong gradients in pH near the sediment-overlying water interface. These gradients are usually measured in one-dimension vertically with depth. Two-dimensional pH distributions in marine sediments were examined at high resolution (65 × 65 μm pixel) and analytical precision over areas of ∼150 to 225 cm² using a newly developed pH planar fluorosensor. Dramatic three-dimensional gradients, complex heterogeneity, and dynamic changes of pH occur in the surficial zone of deposits inhabited by macrofauna. pH can vary by ±2 units horizontally as well as vertically over millimeter scales. pH minima zones often form in association with redoxclines within a few millimeters of inner burrow walls, and become more pronounced with time if burrows remain stable and irrigated for extended periods. Microenvironmental pH minima also form locally around decaying biomass and relict burrow tracks, and dissipate with time (∼5 d). H⁺ concentrations and fluxes in sandy mud show complex acid-base reaction distributions with net H⁺ fluxes around burrows up to ∼12 nmol cm⁻² d⁻¹ and maximum net reaction rates varying between −90 (consumption) to 120 (production) μM d⁻¹ (∼90 nmol cm⁻¹ d⁻¹ burrow length). Acid producing zones that surround irrigated burrows are largely balanced by acid titration zones along inner burrow walls and outer radial boundaries. The geometry and scaling of pH microenvironments are functions of diagenetic reaction rates and three-dimensional transport patterns determined by sediment properties, such as diffusive tortuosity, and by benthic community characteristics such as the abundance, mobility, and size of infauna. Previously, undocumented biogeochemical phenomena such as low pH regions associated with in-filled relict biogenic structures and burrowing tracks are readily demonstrated by two-dimensional and time-dependent images of pH and sedimentary structure.     

The effects of two benthic chamber stirring systems on the diffusive boundary layer,oxygen flux,and passive flow through model macrofauna burrows

Diffusive boundary layers (DBL), sediment oxygen flux, and natural passive flow through model macrofauna burrows were compared in two benthic chambers: one with a conventional rotational stirrer and the other with a two-dimensional flow diffuser system. Oxygen microprobe profiles showed that at similar velocities the mean diffusive boundary layer (DBL) thickness induced by a conventional rotor stirrer (453±118 μm) was not significantly different to that produced by the diffuser system (403±53 μm). The rotor produced twice as much DBL spatial variability (coefficient of variation 27%) as the diffuser (CV 11%). Variability between the rotor system’s DBL transect replicates was also two times greater (average CV of 22%) compared to the diffuser (CV 14%). At equivalent stirring speeds over experimental sediments, mean O₂ consumption rates were also not significantly different between the two systems. The diffuser induced consistently greater (16–37%) passive Bernoullian flow through model macrofauna borrows irrespective of the position of inhalant-exhalent openings. The rotor stimulated anomalous burrow flow regimes over a greater area of the chamber floor (36%) compared to the diffuser (23%). Depending on vent orientation the rotor was shown to reverse (exhalent to inhalant) burrow flow regimes in the central 9% of the chamber floor. This artifact of radial pressure and velocity differentials may have severe implications for tube dwelling infauna that rely on unidirectional flow. The diffuser system more closely mimicked natural two-dimensional water flow over the sediment surface and structures therein and is likely to give more representative results when measuring benthic processes within incubation devices.     

The Influence of Element Diffusion at the Sediment—Water Interface on the Basic Chemical Composition of Overlying Water Mass in Lake Lugu,China

Little work has been done on the influence of seiments on the basic chemical composition of overlying water mass.This paper deals with the vertical profile of the basic constituents such as Ca^ ,K^ ,Na^ ,and HCO3^-,as well as of pH in the overlying water mass and sediment porewater of Lake Lugu-a semi-closed,deep lake in Yunnan Province.The reand sediment porewater of Lake Lugu- a semi-closed,deep lake in Yunnan Province.The results revealed that those basic constituents may diffuse and transport from bottom sediments to overlying water mass through porewater.In the paper are also quantitatively evaluated the diffusive fluxes and the extent of their influence on overlying water mass,indicating that the lake sediment-water interface diffusion plays an important role in controlling the basic chemical composition of water in the whole lake.     

豫西登封地区寒武系第二统朱砂洞组生物扰动构造及其地球化学特征

生物扰动在现代海洋沉积物地球化学循环中起着重要作用,它影响着底栖生物群落特性、有机物分解速率、海水化学性质、沉积物氧化还原性以及营养物循环.豫西登封地区寒武系第二统朱砂洞组碳酸盐岩发育了大量以Thalassinoides主导的扰动构造,对其进行碳和氮同位素测定,可以了解扰动生物对沉积物生物地球化学循环影响.测试结果表明,潜穴充填物和围岩中δ13C_carb值区别显著,不同扰动程度也差异明显,而δ13C_org和δ15N值差异不大,反映出生物对沉积物的扰动改造可以改变沉积物的氧化还原性及孔渗性,这一变化可能与生物扰动引起沉积物孔隙水含氧量增高导致的自生碳酸盐岩比例降低和成岩期白云岩化的增强有关.Thalassinoides造迹生物对沉积物的扰动改造不仅改变了沉积物的原始物理化学信息,促进了海水与沉积物的生物地球化学循环,而且诱导了沉积底质革命,扮演了显生宙早期生态系统工程建造者的角色.      

海洋沉积物—海水界面附近氮、磷、硅的生物地球化学

海洋沉积物中的N和P随沉积物的粒度由粗到细,含量逐渐升高,而Si则降低;积物中N、P、Si的含量还随海区、输入源、季节、动力学过程及生物生产过程不同而变化.控制海洋沉积物-海水界面N、P、Si沉积、释放及循环的因素,包括有机质和溶解氧的浓度、有机质中C、N、P、Si的相对比例、沉积物-海水界面附近的氧化还原环境、生物扰动、温度、水深、pH值、不同形态S的浓度、金属离子以及水动力条件等.一般其综合作用的表现是,沉积物-海水界面之间NH+4、PO3-4和Si(OH)4从沉积物向上覆水扩散转移,而硝酸盐和亚硝酸盐的通量方向相反,通量的大小随着海区的不同差别较大.     

河控三角洲生长的动力和沉积模式

分析河口动力特征,进而揭示其控制下的三角洲前缘挟沙河流床沙载荷(推移质)搬运和沉积机制,是合理构建河控三角洲生长沉积模式的前提.运用流体力学的湍流理论从微观动力过程角度分析失去河床约束条件下依靠惯性作用流动的河水入湖过程中河、湖两类水体的相互作用机制及流动规律,结合水槽物理模拟及前人开展的数值模拟,并借鉴河流泥沙动力学...     

海南红树林湿地可培养硫酸盐还原菌的垂直分布特征研究

利用厌氧微生物分离技术,对深度为1.2 m 的海南红树林湿地沉积物钻孔样品进行了分离培养,共获得11 株 厌氧sulfate-reducing bacteria（SRB） 菌株。经显微观察和16S rDNA序列分析,可归纳为6个属,其中已经报道有芽孢杆菌 属（Bacillus）、弧菌属（Vibrio） 和梭状芽胞杆菌属（Clostridium）,另外3个属分别为伯克霍尔德菌属（Burkholderia）、希瓦氏菌属（Shewanella） 和海杆菌属（Marinobacterium）。不同属的细菌对硫酸盐还原的速率最低为14.71%,最高可达 56.78%,并且以上6属11株菌都能将+6价的硫还原生成-2价硫,并与培养基中的Fe2+结合生成黑色FeS沉淀,而这些无定 形FeS沉淀是生成黄铁矿的前体。红树林湿地SRB种群数量随沉积物深度的增加而降低,结合沉积物的地球化学分析测试 结果表明,表层（0 cm） 水界面的沉积物由于处于氧化-还原界面,氧气的周期性输入在一定程度上抑制了SRB的生长;随着 深度增加（10~40 cm）,充足的有机质、偏中性的pH值以及厌氧环境的增强,使得SRB种类和数量明显增加;而60 cm以下 沉积物中因TOC含量降低,减少了微生物可利用的碳源,pH值明显降低,Na+和Ca2+离子浓度明显增加,这些因素都抑制了 SRB的生长,使得深部沉积物中SRB的种类和数量显著减少。     

Selecting Ore Pass-Finger Raise Configurations in Underground Mines

Material transfer in underground mines often relies on ore and waste pass systems. In mines where ore pass systems transcend multiple production levels, finger raises are used to funnel material into the system. Empirical evidence, from several mines, suggests that the use of finger raises often results in damage in the immediate vicinity of finger raise-ore pass junctions. Of particular concern is damage on the ore pass walls as a result of impact loads generated by material flowing through the fingers on to the ore pass walls. The severity of damage is directly related to the rock mass quality of the excavation walls, material properties of transiting ore and the ore pass-finger raise configuration. This paper examines the influence of different configurations aiming to develop strategies to minimize ore pass wall damage. To these purposes the Particle Flow Code was employed to undertake a series of numerical experiments. This involved dumping a batch of rock fragments, represented by a uniform distribution of disc-shaped particles, into a finger raise and allowed to flow into an ore pass. It has been clearly demonstrated that higher impact loads were generated when the angle of intersection between ore pass and finger raise was 140° to 145°. This configuration results in the most damage. The results of these numerical experiments were collaborated by observations at an underground mine in Canada.     

江浙一带现代海滩的生物扰动构造及其指相意义

底栖生物在沉积物表面及其内部留下的各种活动痕迹——生物扰动构造(Bioturbation)具有指示环境的重要意义,这是由于底栖生物的生存和发展与沉积物的性质和沉积作用过程休戚相关,对沉积环境有极大的依赖性。同时,生物又通过其自身的觅食和摄食活动,反过来影响沉积作用过程,并可破坏和改造沉积物的原生构造。因此,这种原地生成的生物扰动构造如能保存在地层中,它们就可以为沉积环境的判别提供可靠的信息。     

Investigations on different positions of electrodes and their effects on the distribution of Cr at the water sediment interface

In most mining areas, significant concentrations of metals such as Pb, Cu, Cr, Zn, Cd, Fe, Mn, Co, Ni, Hg, Ar, halogenated organic compounds and radionuclides are found. Of those, Cr is one of the well-known heavy metals that forms toxic species. It is necessary to study the mobilization and accumulation of Cr at the sediment water interface in an electric field at varying different positions and conditions of the electrode arrangement. The tests were carried out with a natural sediment containing heavy metals from the river Weisse Elster (Germany). The electrokinetic experiments have been performed in columns filled with sediment using electrodes made of conductive polymers (polyethylene with carbon black) at a maximum current density of 0.5 mA/cm². The experimental results suggest that the mobilization and accumulation of Cr highly depends on chemical factors for e.g., pH value, redox potential, respectively redox status and the content of Fe, A1 and organic matter in the soil or sediment. The sorption of Cr (III, VI) is very high in the pH range > 4.5. As expected, a high mobilization of Cr (III, VI) was seen in the case of the experiments with the anode at the sediment, because the pH value was lower than mentioned above. On the opposite, the best conditions for the Cr (III, VI) immobilization is high pH values (cathode at the sediment).     

Physicochemical and biological influences on sedimentary‐fabric formation in a salinity and oxygen‐restricted semi‐enclosed sea: Gotland Deep,Baltic Sea

Two ca 8000 year long sediment cores from the Gotland Deep, the central sub‐basin of the Baltic Sea, were studied by means of digital images, X‐radiographs and scanning electron microscopy–energy‐dispersive X‐ray mineralogical analysis to gain understanding of the physicochemical and biological influences on sedimentary‐fabric formation in modern and ancient seas with a high flux of organic carbon, and associated oxygen stress and depauperate ichnofauna. Four lithofacies were recognized: (i) sharply laminated mud; (ii) biodeformed mud; (iii) burrow‐mottled mud; and (iv) sedimentation‐event bed. The sharply laminated and burrow‐mottled facies dominate the cores as alternating long intervals, whereas the biodeformed and sedimentation‐event facies occur as thin interbeds within the sharply laminated intervals. The sharply laminated mud comprises alternating diatom‐rich and lithic laminae, with occasional Mn‐carbonate laminae. Lamination discontinuity horizons within the laminites, where the regular lamination is overlain sharply by gently inclined lamination, challenge the traditional view of mud accumulation by settling from suspension, but indicate localized accumulation by particle‐trapping microbial mats and, potentially, by the rapid lateral accretion of mud from bedload transport. The biodeformed interbeds record brief (few years to few decades) oxic–dysoxic conditions that punctuated the anoxic background conditions and permitted sediment‐surface grazing and feeding by a very immature benthic community restricted to the surface mixed tier. The likely biodeformers were meiofauna and nectobenthic pioneers passively imported with currents. The sedimentation‐event interbeds are distal mud turbidites deposited from turbidity currents probably triggered by severe storms on the adjacent coastal areas. The turbidite preservation was favoured by the anoxic background conditions. The long burrow‐mottled intervals are characterized by intensely bioturbated fabrics with discrete Planolites, rare Arenicolites/Polykladichnus and very rare Lockeia trace fossils, as well as bivalve biodeformational structures which represent shallowly penetrating endobenthic feeding and grazing strategies and permanent dwellings. These burrowed intervals represent longer periods (several years to few centuries) of oxic–dysoxic conditions that permitted maturation in the benthos by means of larval settling of opportunistic worm‐like macrofauna and bivalves, resulting in the development of a transition tier. These observations imply more dynamic and oxic depositional conditions in Gotland Deep than previously thought. Comparison to previous zoobenthic studies in the area allowed discussion of the benthic dynamics, and the identification of probable biodeforming and trace‐producing species. Implications for current biofacies and trace‐fossil models are discussed.     

Unusual occurrence of glauconite in a shallow lagoonal environment (Lower Cretaceous, northern Aquitaine Basin, SW France)

Some minerals are considered to be representative of specific natural environments. Among them glauconite is considered to be formed in marine deep platform conditions. However, the term glauconitic is misused to designate green minerals formed in marine outer-shelf environments. X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analyses of individual green materials were carried out leading to the identification of glauconite in the Purbeckian facies. Green glauconite grains predominantly occur as burrow fills and occasionally as faecal pellet replacements. Two depositional environments have been identified from bottom to top of the succession: (1) an argillaceous dolomitic lagoonal sediment formed in saline, shallow water; (2) a marl-limestone alternation deposited in a brackish water estuary. The crystallochemical properties of the glauconites change abruptly. These findings show that glauconite may form in coastal environments and not only in mid-shelf to upper deep water platform environments as classically assumed. Moreover, the glauconite composition changes with the chemical conditions imposed by the local environment.     

Input by Indian rivers into the world oceans

The chemical, sediment and total load carried by the major river basins in India—Ganges, Brahmaputra, Indus (Jhelum), Godavari, Krishna, Narmada, Tapti, Mahnadi and Cauvery have been calculated, based partly on new set of data and partly on existing data. There is a significant amount of chemical load transported by all the Indian rivers, and for global mass transfer calculation, these cannot and should not be ignored. The chemical mass transfer during the monsoon is not surprisingly small, as would be expected for excess discharge and dilution controlled run-off. The sediment mass transfer from non-Himalyan rivers, all within the same range of magnitude, accounts for less than a tenth of that of the Ganges but during the monsoon, except for Cauvery, all the Indian rivers carry a sediment load of greater than 1000 ppm. The total mass transfer from the Indian subcontinent accounts for 6·5 per cent of the global transfer. Except for the Ganges and the Brahmaputra, the erosion rates are similar for all Indian basins, independent of their size and these rates are agreeable with the continental earth average. The Ganges-Brahmaputra basin erosion rates are highest on the continental earth. Based on the average rate of denudation of the Indian subcontinent, the mean elevation of this landmass will be that of the present day mean sea level in 5 million years from now. The average denudation rate of 2·1 cm/100 years is different from the calculated average sedimentation rate of 2·1 cm/100 years is different from the calculated average sedimentation rate of 6·7 cm/100 years in the Bay of Bengal suggesting that an accurate erosion rate in the continent is needed to determine sedimentation rate in the oceans. The chemical and sediment mass transfer rates appear to have a logarithmic linear relationship on a global scale, as against the reported negative logarithmic trend for North America alone.     

The effect of sediment-water exchange on manganese deposition and nodule growth in Jervis Inlet,British Columbia

The processes controlling Mn deposition in the vicinity of a nodule site in Jervis Inlet, a fjord in the British Columbia mainland coast, were studied. The results indicate that Mn, due to reductive remobilization in the subsurface sediments, undergoes intensive exchange across the sediment-water interface and, because of this exchange and the effects of water circulation processes within the inlet, tends to be stripped from shallow water sediments and to be redeposited as an oxide precipitate in the surface sediment of the deepest areas. There it undergoes diagenetic reactions in the subsurface sediments that, as demonstrated by saturation of the pore waters, finally result in its burial as a carbonate phase. The distribution of properties in the vicinity of the Mn nodule site indicates that concretion growth occurs in an area of strong currents and low sedimentation rate where rocks resting on the bottom act as centers to which Mn-oxide particles suspended in the bottom water are apparently being cemented.     

Modelling potential impacts of bottom trawl fisheries on soft sediment biogeochemistry in the North Sea

Bottom trawling causes physical disturbance to sediments particularly in shelf areas. The disturbance due to trawling is most significant in deeper areas with softer sediments where levels of natural disturbance due to wave and tidal action are low. In heavily fished areas, trawls may impact the same area of seabed more than four times per year. A single pass of a beam trawl, the heaviest gear routinely used in shelf sea fisheries, can kill 5–65% of the resident fauna and mix the top few cm of sediment. We expect that sediment community function, carbon mineralisation and biogeochemical fluxes will be strongly affected by trawling activity because the physical effects of trawling are equivalent to those of an extreme bioturbator, and yet, unlike bioturbating macrofauna, trawling does not directly contribute to community metabolism. We used an existing box-model of a generalised soft sediment system to examine the effects of trawling disturbance on carbon mineralisation and chemical concentrations. We contrasted the effects of a natural scenario, where bioturbation is a function of macrobenthos biomass, with an anthropogenic impact scenario where physical disturbance results from trawling rather than the action of bioturbating macrofauna. Simulation results suggest that the effects of low levels of trawling disturbance will be similar to those of natural bioturbators but that high levels of trawling disturbance prevent the modelled system from reaching equilibrium due to large carbon fluxes between oxic and anoxic carbon compartments. The presence of macrobenthos in the natural disturbance scenario allowed sediment chemical storage and fluxes to reach equilibrium. This is because the macrobenthos are important carbon consumers in the system whose presence reduces the magnitude of available carbon fluxes. In soft sediment systems, where the level physical disturbance due to waves and tides is low, model results suggest that intensive trawling disturbance could cause large fluctuations in benthic chemical fluxes and storage.     

pH平面光极在生物扰动存在下水/沉积物体系中的应用

为了解生物扰动存在下水/沉积物体系中pH的分布和变化特征,通过改进传感膜制备方法和图像采集与定量方法,建立了基于平面光极的pH二维分布实时监测系统,并应用于生物扰动存在下淡水/沉积物模拟体系pH时空分布规律的监测。结果表明：与已有方法相比,本方法具有响应pH范围宽（6.2～8.6）、响应快速（响应时间<30 s）、准确性高、可逆性和抗干扰性强、稳定性好等优点,可用于监测有扰动生物存在的水/沉积物模拟体系微环境的pH,直观反映pH的时空变化规律。对模拟体系的监测结果表明：沉积物自身pH时空变化虽较小,但其对上覆水pH有重要影响,上覆水pH存在显著而复杂的变化。体系pH的变化规律是多种机制作用的结果,而生物扰动对这些作用均有影响。     

A comparison of regional geochemical data from lakes and streams in northern Labrador; implications for mixed-media geochemical mapping

Labrador has been covered by reconnaissance-scale geochemical surveys under the National Geochemical Reconnaissance program. Lake sediment and water were the chief sample media, but stream sediment and water were employed in the mountainous terrain of northern Labrador. The main objective of these surveys was mineral resource assessment, but the data are also relevant to geological and environmental studies, and would be most useful to the non-specialist if the data from the two drainage types could be combined to produce unified element distribution maps for the whole region.A comparison of stream and lake data for a 5,700-km² area where both drainage types were sampled suggests that only the pH of the lake and stream waters are directly comparable, showing a common range and similar spatial distribution. Comparing the two types of sediment, most elements show obvious differences in either median content or range or both, indicating that stream and lake sediment are geochemically distinct media, and their element contents cannot be compared directly. The distribution patterns of Cu, Ni and U reflect similar bedrock features in both sediment types. In contrast, Co, F, Fe, Hg, Mn, V and Zn show little or no spatial correlation between stream and lake sediment, but are strongly intercorrelated in the stream sediment data set.The sediments collected from lakes in Labrador represent disturbed column, about 40 cm in length, of organic debris that accumulated over the past several hundred years. Metal accumulation in the sediment is largely through fixation from inflowing surface and groundwater by microorganisms, coprecipitation with hydrous Fe and Mn oxides, sorption by clay minerals and chemical and biochemical processes at and just below the sediment/water interface. The stream sediments in this study were collected from active sediment, and represent principally the mechanical-weathering products of bedrock, with variable amounts of organic matter and hydrous Fe and Mn oxides. Considering the difference in the two sediment types, it is probably not surprising that there is rather limited spatial correlation between the geochemistry of the two sediment types indicating that to a large extent each medium reflects a different facet of the bedrock geochemistry. Only for a few elements should the data sets be merged. The degree of spatial correlation for U, Ni and Cu increases as the data are generalized by averaging into larger blocks, suggesting that the combined data sets will be more successful in defining broad crustal geochemical features rather than local details. The implication of this study for international geochemical mapping is that geochemical patterns for many elements are strongly dependent on the sample medium chosen. Therefore, when it is necessary to change sample media in passing from one terrain type to another, a comparative study must be carried out to determine how the geochemistry of the different sample media compare spatially.     

Geochemical behavior of arsenic in reducing sulfidic sediments of reservoir contaminated by acid mine drainage

The sediment from an acid mine drainage affected reservoir of Guizhou province of China has the iron and arsenic concentration of about 400 and 2.6 g/kg, respectively. Sediment cores were collected, and were used to study the arsenic behavior in the seriously acidified reservoir from the viewpoint of chemical thermodynamics. The limestone neutralization and ferric iron hydrolysis regulated the porewater pH from about 2.9–5.8. The reductive dissolution of As–Fe-rich (hydr)oxides under the mild acidic conditions was the main mechanism for the release of absorbed arsenic into porewater. The maximum concentrations of iron, sulfate and arsenic reached to about 2,800, 9,000 and 1 mg/l, respectively. Arsenic speciation transformation and hydrous ferric oxide (HFO) crystallization enhanced the arsenic mobility in sediment. In addition, the iron sulfide minerals diagenesis could play a role in removing the dissolved arsenic from porewater. The actual distribution of arsenic concentration in porewater was well simulated using the model of surface complexation of arsenic to HFO. Although arsenic concentration in porewater could be above 100 times higher than that of reservoir water, it was not easy to release into the reservoir water through diffusion, because the shallow sediment had relatively strong arsenic adsorption capacity, and new HFO could be generated continuously at the sediment water interface.     

Controls on uranium distribution in lake sediments

Uranium geochemistry has been investigated in three acid lakes located on the Canadian Shield and one circumneutral lake in the Appalachian Region of Eastern Canada. In all Shield lakes, dissolved U concentrations were higher in the porewater than in the overlying water. In one of them, whose hypolimnion is perennially oxic, U released to porewater at depths of Fe remobilization was removed from the porewater at depths of Fe oxyhydroxides precipitation; these similarities in the U and Fe profiles indicate that part of the U becomes associated to Fe oxyhydroxides. The dissolved U and Fe profiles in the other two Shield lakes, whose hypolimnions were anoxic when sampled, did not show any significant recycling of these elements in the vicinity of the sediment-water interface and both elements diffused from the sediment to the overlying water. In contrast, in the Appalachian Lake, dissolved U concentrations were higher in the overlying water than in porewater, strongly decreased at the vicinity of the sediment-water interface and then remained relatively constant with sediment depth. Diagenetic modeling of the porewater U profiles, assuming steady-state, reveals that authigenic U always represented ?3% of the total U concentration in the sediments of all lakes. This observation indicates that diagenetic reactions involving U are not quantitatively important and that most of the U was delivered to the sediments at our study sites as particulate U and not through diffusion across the sediment-water interface, as is seen in continental margin sediments. Comparison of the U:C_org and U:Fe molar ratios in diagenetic material collected across the sediment-water interface with Teflon sheets and in surface sediments (0-0.5 cm) of the lake having a perennially oxic hypolimnion suggest that solid phase U was mainly bound to organic matter originating from the watershed; a strong statistical correlation between sediment non-lithogenic U and C_org in the Appalachian Lake supports this contention. Thermodynamic calculations of saturation states suggest that dissolved U was not removed from porewater through precipitation of UO_2(s), U₃O_7(s) and U₃O_8(s) as previously proposed in the literature.