Kinetics of calcite precipitation induced by ureolytic bacteria at 10 to 20°C in artificial groundwater

The kinetics of calcite precipitation induced in response to the hydrolysis of urea by Bacillus pasteurii at different temperatures in artificial groundwater (AGW) was investigated. The hydrolysis of urea by B. pasteurii exhibited a temperature dependence with first order rate constants of 0.91 d⁻¹ at 20°C, 0.18 d⁻¹ at 15°C, and 0.09 d⁻¹ at 10°C. At all temperatures, the pH of the AGW increased from 6.5 to 9.3 in less than 1 d. Dissolved Ca²⁺ concentrations decreased in an asymptotic fashion after 1 d at 20°C and 15°C, and 2 d at 10°C. The loss of Ca²⁺ from solution was accompanied by the development of solid phase precipitates that were identified as calcite by X-ray diffraction. The onset of calcite precipitation at each temperature occurred after similar amounts of urea were hydrolyzed, corresponding to 8.0 mM NH₄⁺. Specific rate constants for calcite precipitation and critical saturation state were derived from time course data following a second-order chemical affinity-based rate law. The calcite precipitation rate constants and critical saturation states varied by less than 10% between the temperatures with mean values of 0.16 ± 0.01 μmoles L⁻¹ d⁻¹ and 73 ±3, respectively. The highest calcite precipitation rates (ca. 0.8 mmol L⁻¹ d⁻¹) occurred near the point of critical saturation. While unique time course trajectories of dissolved Ca²⁺ concentrations and saturation state values were observed at different temperatures, calcite precipitation rates all followed the same asymptotic profile decreasing with saturation state regardless of temperature. This emphasizes the fundamental kinetic dependence of calcite precipitation on saturation state, which connects the otherwise dissimilar temporal patterns of calcite precipitation that evolved under the different temperature and biogeochemical regimes of the experiments.     

Comparison of rates of ureolysis between Sporosarcina pasteurii and an indigenous groundwater community under conditions required to precipitate large volumes of calcite

Ureolysis-driven calcite precipitation has potential to seal porosity and fracture networks in rocks thus preventing groundwater flow and contaminant transport. In this study urea hydrolysis and calcite precipitation rates for the model bacterium Sporosarcina pasteurii were compared with those of indigenous groundwater communities under conditions required to precipitate large volumes of calcite (up to 50 g L⁻¹). We conducted microcosm experiments in oxic artificial and anoxic natural groundwaters (collected from the Permo-Triassic sandstone aquifer at Birmingham, UK) that were inoculated with aerobically grown S. pasteurii. The rate constants for urea hydrolysis, k_urea, ranged between 0.06 and 3.29 d⁻¹ and were only affected by inoculum density. Higher Ca²⁺ concentration (50-500 mM Ca²⁺) as well as differences in fO₂ did not inhibit the ureolytic activity of S. pasteurii and did not significantly impact k_urea. These results demonstrate that S. pasteurii has potential to improve calcite precipitation in both oxic and anoxic groundwaters, especially if indigenous communities lack ureolytic activity. Urea hydrolysis by indigenous groundwater communities was investigated in anoxic, natural groundwaters amended with urea and CaCl₂. A notable increase in ureolysis rates was measured only when these communities were stimulated with dilute nutrients (with best results from blackstrap molasses). Furthermore, there was a considerable lag time (12-20 days) before ureolysis and calcite precipitation began. Calculated ureolysis rate constants, k_urea, ranged between 0.03 and 0.05 d⁻¹ and were similar to k_urea values produced by S. pasteurii at low inoculum densities. Overall, this comparative study revealed that the growth of ureolytic microorganisms present within groundwaters can easily be stimulated to enhance rates of urea hydrolysis in the subsurface, and thus can be used to induce calcite precipitation in these environments. The time required for urea hydrolysis to begin is almost instantaneous if an inoculum of S. pasteurii is included, while it may take several weeks for ureolytic groundwater communities to grow and become ureolytically active.     

The role of citrate and phthalate during Co(II) coprecipitation with calcite

The influence of citrate and phthalate on Co coprecipitation with calcite was investigated using a combination of batch experiments, Fourier-transform infra-red (FT-IR) spectroscopy, and thermogravimetric analysis (TGA) over a wide range of precipitation rates. Steady-state growth conditions (at constant [Ca], [Co], DIC, and pH) were generally achieved within 3-5 h, after which Co(II) partitioning into calcite was evaluated. Only minor differences are observed in the partition coefficient (K_d) trends with and without citrate and phthalate as a function of calcite precipitation rate except at very low rates. Slight inhibition of calcite growth is observed in the presence of citrate or phthalate, which can be attributed to adsorption at surface sites. TGA curves for samples coprecipitated with citrate show a significant mass loss between 375 and 550 °C, whereas the weight-loss curves for the Co-phthalate coprecipitates are indistinguishable from those of the organic-free Co coprecipitates. This indicates that citrate is incorporated into calcite during calcite crystallization, whereas phthalate is excluded. FT-IR spectra for the sample with citrate show a broad absorption in the range 3700-3100 cm⁻¹, which is attributable to water molecules coordinated to citrate coprecipitated with calcite. The preferential incorporation of citrate over phthalate likely reflects differences in both aqueous speciation and conformation of the carboxylate groups. This new finding may provide new insight to the factors that control the behavior of macromolecules and their incorporation into the structure of calcium carbonate during biomineralization.     

Experimental study of nickel(II) interaction with calcite: Adsorption and coprecipitation

Partitioning of Ni in calcite, CaCO₃, was evaluated with the aim of collecting data on partition and distribution coefficients and to enhance understanding about the interaction of Ni with the calcite surface and further incorporation into the bulk. This information will aid in the interpretation of geological processes for safety assessment of waste repositories and contamination of groundwater. Coprecipitation experiments were carried out by the constant addition method at 25 °C and pCO₂ = 1 and 10^−3.5 atm. Ni was moderately partitioned from solution into calcite. For dilute solid solutions (X_Ni < 0.001), Ni partition coefficients were estimated to be ∼1 and found to be weakly dependent on calcite precipitation rate in the range of 3-230 nmol m⁻² s⁻¹. Ni molar fraction in the solid is directly proportional to Ni concentration in the solution. The fit of the data to such a model is good evidence that Ni is taken up as a true solid solution, not simply by physical trapping.     

Effect of pH on boron coprecipitation by calcite: further evidence for nonequilibrium partitioning of trace elements

The influence of pH and concentration on boron coprecipitation by calcite were evaluated under near-equilibrium conditions at 25°C. Calcite was precipitated by adding a metastable polymorph (vaterite or aragonite) to a solution of known boron concentration. This method maintains a nearly constant solution composition during the slow conversion of the metastable polymorph to calcite.Boron uptake in calcite was found to be strongly pH-dependent, increasing two orders of magnitude from pH 8.5 to pH 10.5. Boron incorporation into calcites precipitated from vaterite-saturated solutions was five times greater than in calcites precipitated from aragonite-saturated solutions. Ostensibly, these results suggest that the calcite precipitation rates were not low enough to attain equilibrium partitioning of boron into calcite. However, scanning electron micrograph analyses showed that the prevalent crystal forms of calcite generated from aragonite and those generated from vaterite were distinctly different. The different quantities of boron incorporated into these calcites may reflect different crystal growth mechanisms, consistent with face-dependent, nonequilibrium partitioning of trace elements in calcite.At a constant pH of 9.0, boron uptake increased from less than 15 to over 290 mg/kg CaCO₃ as the solution boron concentration was increased from 5 to 100 mg/kg. Our results agree with those of other investigators, despite differences in solution composition, and calcite precipitation techniques used. The agreement between the studies may be because the crystal growth morphology of the calcite was rhombohedral in both cases.     

87Sr/86Sr ratio: a natural tracer to monitor groundwater flow paths during artificial recharge in the Bangkok area, Thailand

The aquifer system in the Thon Buri sedimentary basin below the deltaic flood plain of the Chao Phraya River, central Thailand, has been exploited for public water supply for the capital Bangkok since the early 1920s. Groundwater withdrawal, currently 1.4 million m³/d, has resulted in a maximum decline in hydraulic head of up to 40 m. This has induced land subsidence of as much as 1.7 m (1940–1992) in the eastern suburbs of the metropolis. Artificial injection of purified water within an area-wide network of recharge wells could constitute a remedy to slow the water level depression within the sedimentary basin, and thus the subsidence. This requires a prior shutdown of water withdrawal. The flow paths of the injected water can be traced by changes in the ⁸⁷Sr/⁸⁶Sr ratio of the groundwater and injected water mixture within the three main aquifers in the basin that are used for public supply. The ratios, monitored at five monitoring stations within the cone of depression, have been constant over 3 years. Injection of the calculated cone volume of 5.2?×?10⁹ m³ would take at least 10 years, depending on the injection pressure and the number and position of wells.     

Chemically induced grain boundary migration in calcite: temperature dependence,phenomenology, and possible applications to geologic systems

Chemically induced grain boundary migration (CIGM) is a solid-state reaction mechanism which involves grain boundary migration. Initially straight grain boundaries in calcite bicrystals exhibit CIGM when exposed to a SrCO₃ or BaCO₃-rich melt at temperatures of 680–840° C in a 1-bar CO₂ atmosphere. Under these conditions, CIGM is apparently a much more efficient way to form solid-solutions than lattice diffusion. The maximum observed rate of migration, 1×10^–9 m/s, occurred at about 760° C. Below that temperature, migration was thermally activated, but above it, the process was thermally inhibited. Some portions of the boundary swept through a given crystal volume more than once. In the (Ca, Sr) CO₃ system, the solute concentrations incorporated during a single pass of the boundary were far less than values expected at chemical equilibrium, and departed further from equilibrium at higher temperatures. CIGM may be geologically important when the distances characteristic of grain boundary migration and diffusion are much larger than those characteristic of lattice diffusion. The great similarity between results in ionic calcite and metals systems suggests CIGM is a very general process which may be expected to occur in a large number of geologic systems.     

Freshwater calcite precipitates from in vitro mesocosm flume experiments: a case for biomediation of tufas

Freshwater carbonates (tufas) develop today from the Arctic to the tropics, many being localized about springs and upper water courses. Some Quaternary tufas, especially in the Mediterranean region, extend over tens of square kilometres and exceed 30 m in thickness. Radiometric dating of Holocene deposits shows that many have accumulated at an average rate of 1 mm year^?1. However, local precipitation may be much faster and some Holocene deposits may even have outpaced their tropical marine carbonate counterparts. Recently, the study of active sites has attempted to quantify the precipitation mechanisms which lead to tufa deposition. However, field observation and sampling procedures suffer from the inherent disadvantages of uncontrolled fluctuations in environmental conditions during the study programme. These disadvantages compromise any interpretations, particularly where controls on spar versus micrite precipitation are concerned. Many of these problems have been overcome in the current study by the construction and operation of laboratory mesocosm flumes which simulate the natural conditions (e.g. pH, flow rate, ambient temperature and daylight) in which freshwater carbonate (tufa) is deposited. Three mesocosms were supplied with natural river water from tufa precipitating streams and two mesocosms were supplied with UV‐treated (sterile) river water from the same source. One of the untreated flume mesocosms was linked with a calcium reactor, which replaced calcium ions removed during the precipitation process in order to maintain tufa growth over extended experimental runs. Low‐magnesium calcite precipitates (both rhombic sparite grown from long‐crystallite dendrites and short‐crystallite dendrite triad precursors) and micrite peloids (grown from spherulitic precursors) were precipitated in intimate association with biofilm (extracellular polymeric substances) within the four mesocosms supplied with natural river water. Virtually, no tufa‐like precipitate was obtained from the flumes supplied with UV‐treated river water. A second extended run flume experiment was also carried out for comparison purposes using a calcium hydroxide solution in deionized water. Collectively, these experiments provide convincing evidence confirming that the presence of a microbial biofilm strongly influences the precipitation of carbonates in riverine freshwater settings. In particular, experimental results show that micro‐peloidal micrite and short‐crystallite calcite dendrites are only produced in the presence of microbial extracellular polymeric substances.     

Sr,Cd, Mn and Co distribution coefficients in calcite as a function of calcite precipitation rate

Distribution coefficients, as a function of precipitation rate, were determined for the metals Sr²⁺, Co²⁺, Mn²⁺ and Cd²⁺in calcite. A pH-stat was used to maintain a constant degree of-saturation, and hence precipitation rate, during each coprecipitation run. The precipitation rate was proportional to the degree of supersaturation and the mass of seed crystal introduced. Distribution coefficients (λ) as a function of rate were determined using radioactive isotopes for solutions with saturations $\text{Ω = 1}$ to $\text{Ω = 5.5}$. Strontium distribution coefficients increased with increasing precipitation rate, while Co, Mn and Cd distribution coefficients decreased with increasing precipitation rate. The following rate expressions (at 25°C) were derived: logλ_Sr = 0.249 log R ?1.57logλ_Mn = ?0.266 log R + 1.35logλ_Co = ?0.173 log R + 0.68logλ_Cd = ?0.194 log R + 1.46 where R is the observed precipitation rate in nmoles CaCO₃ per mg seed crystal per min.In separate experiments the uptake of radioactive isotopes was monitored during the recrystallization of calcite seed crystals. Rates of recrystallization were from 100 to 10, 000 times slower than the pH-stat experiments, but yielded distribution coefficients consistent with the above rate expressions.Using gross estimates of biogenic crystal growth rates, aragonite to calcite transformation rates, and the above Sr rate expression, biogenic calcite and diagenetic calcite Sr contents are estimated. These experiments indicate that in addition to solution composition, precipitation rate is a significant factor influencing the trace metal content of naturally occurring calcite.     

Temperature dependence of calcite dissolution kinetics between 1 and 62°C at pH 2.7 to 8.4 in aqueous solutions

The kinetics of calcite dissolution in aqueous KCl-solutions far from equilibrium, between 1 and 62°C in the pH-range 2.7 to 8.4 have been investigated using a rotating disc apparatus. At neutral and alkaline pH in the mixed kinetic regime the empirical apparent activation energy (EAAE) for the surface chemical reaction rate constant is 54 ± 4 kJ mole^?1 for Carrara marble and 46 ± 4 kJ mole^?1 for Iceland spar. Under similar conditions the EAAE of the transport rate constant increases with decreasing temperature, but has a mean value of 27 ± 2 kJ mole^?1. The corresponding diffusion coefficient has a mean EAAE of 37 ± 3 kJ mole^?1 and this high EAAE is consistent with transport dependence on product diffusion in this H⁺-independent regime.In contrast, in acid solutions, where the rate approaches end-member transport control, the EAAE of the diffusion coefficient is 16 kJ mole^?1, also decreasing with increasing temperature. This is compatible with H⁺-diffusion to the surface being rate-controlling.In inhibitor-free natural systems, calcite dissolution kinetics far from equilibrium can be described in terms of three regimes: an H⁺-dependent regime (pH < 4 at 25°C), a transition regime (4 < pH < 5.5 at 25°C) and an H⁺-independent regime (pH > 5.5 at 25°C). At lower temperatures these boundaries move to higher pH values. The presence of inhibitors in natural systems may enhance surface controlled kinetics.     

Dependence of calcite growth rate and Sr partitioning on solution stoichiometry: Non-Kossel crystal growth

Seeded calcite growth experiments were conducted at fixed pH (10.2) and two degrees of supersaturation (Ω = 5, 16), while varying the Ca²⁺ to solution ratio over several orders of magnitude. The calcite growth rate and the incorporation of Sr in the growing crystals strongly depended on the solution stoichiometry. At a constant degree of supersaturation, the growth rate was highest when the solution concentration ratio, r = [Ca²⁺]/[], equaled one, and decreased symmetrically with increasing or decreasing values of r. This behavior is consistent with the kink growth rate theory for non-Kossel crystals, assuming that the frequency factors for attachment to kink sites are the same for the cation and anion. Measured Sr partition coefficients, D_Sr, ranged from 0.02 to 0.12, and correlated positively with the calcite growth rate.     

Ba/Sr,Ca/Sr and 87Sr/86Sr ratios in soil water and groundwater: implications for relative contributions to stream water discharge

Barium/Sr and Ca/Sr ratios have been used to model the relative importance of different sources of stream water. Major and trace element concentrations together with ⁸⁷Sr/⁸⁶Sr ratios were measured in precipitation, soil water, groundwater and stream water in a small (9.4 km²) catchment in northern Sweden. The study catchment is drained by a first order stream and mainly covered with podzolized Quaternary till of granitic composition. It is underlain by a 1.8 Ga granite. A model with mixing equations used in an iterative mode was developed in order to separate the stream water into 3 subsurface components: soil water, shallow groundwater, and deep groundwater. Contributions from precipitation are thus not included in the model. This source may be significant for the stream water generation, but it does not interfere with the calculations of the relative contributions from the subsurface components. The results show that the deep groundwater constitutes between 5 and 20% of the subsurface water discharge into the stream water. The highest values of the deep groundwater fraction occur during base flow. Soil water dominates during snowmelt seasons, whereas during base flow it is the least important fraction. Soil water accounts for 10–100% of the subsurface water discharge into the stream water. Shallow groundwater accounts for up to 80% of the subsurface water discharge with the lowest values at peak discharge during snowmelt seasons and the highest values during base flow. The validity of the model was tested by comparing the measured ⁸⁷Sr/⁸⁶Sr ratios in the stream water with the ⁸⁷Sr/⁸⁶Sr ratios predicted by the model. There was a systematic difference between the measured and modelled ⁸⁷Sr/⁸⁶Sr ratios which suggests that the fraction of soil water is overestimated by the model, especially during spring flood. As a consequence of this overestimation of soil water the amount of shallow groundwater is probably underestimated during this period. However, it is concluded that the differences between measured and predicted values are relatively small, and that element ratios are potentially effective tracers for different subsurface water flowpaths in catchments.     

电阻率法和激发极化法在地下水勘查中的应用

依据提供的物性参数的不同,广泛应用于找水工作的电法手段可分为电阻率法和激发极化法两大类,其中电阻率法提供电阻率参数,主要解决与赋水有关的构造问题(如赋水层位或断裂构造)。方法上有常规电法、电磁法,可根据地区赋水部位的不同选择适当的方法。激发极化法则利用含水层的激发极化效应进一步确定目标层位或构造的赋水性。电阻率法和激发极化法相互配合,在找水工作中相得益彰。     

The combined use of Sr/Sr and carbon and water isotopes to study the hydrochemical interaction between groundwater and lakewater in mantled karst

The hydrochemical interaction between groundwater and lakewater influences the composition of water that percolates downward from the surficial aquifer system through the underlying intermediate confining unit and recharges the Upper Floridan aquifer along highlands in Florida. The ⁸⁷Sr/⁸⁶Sr ratio along with the stable isotopes, D, ¹⁸O, and ¹³C were used as tracers to study the interaction between groundwater, lakewater, and aquifer minerals near Lake Barco, a seepage lake in the mantled karst terrane of northern Florida. Upgradient from the lake, the ⁸⁷Sr/⁸⁶Sr ratio of groundwater decreases with depth (mean values of 0.71004, 0.70890, and 0.70852 for water from the surficial aquifer system, intermediate confining unit, and Upper Floridan aquifer, respectively), resulting from the interaction of dilute oxygenated recharge water with aquifer minerals that are less radiogenic with depth. The concentrations of Sr²⁺ generally increase with depth, and higher concentrations of Sr²⁺ in water from the Upper Floridan aquifer (20–35 μg/L), relative to water from the surficial aquifer system and the intermediate confining unit, result from the dissolution of Sr-bearing calcite and dolomite in the Eocene limestone. Dissolution of calcite [δ¹³C = −1.6 permil (%o)] is also indicated by an enriched δ¹³C_DIC(-8.8 to -11.4% o) in water from the Upper Floridan aquifer, relative to the overlying hydrogeologic units (δ¹³C_DIC < -16%o).

Groundwater downgradient from Lake Barco was enriched in ¹⁸O and D relative to groundwater upgradient from the lake, indicating mixing of lakewater leakage and groundwater. Downgradient from the lake, the ⁸⁷Sr/⁸⁶Sr ratio of groundwater and aquifer material become less radiogenic and the Sr²⁺ concentrations generally increase with depth. However, Sr²⁺ concentrations are substantially less than in upgradient groundwaters at similar depths. The lower Sr²⁺ concentrations result from the influence of anoxic lakewater leakage on the mobility of Sr²⁺ from clays. Based on results from mass-balance modeling, it is probable that cation exchange plays the dominant role in controlling the ⁸⁷Sr/⁸⁶Sr ratio of groundwater, both upgradient and downgradient from Lake Barco. Even though groundwater from the three distinct hydrogeologic units displays considerable variability in Sr concentration and isotopic composition, the dominant processes associated with the mixing of lakewater leakage with groundwater, as well as the effects of mineral-water interaction, can be ascertained by integrating the use of stable and radiogenic isotopic measurements of groundwater, lakewater, and aquifer minerals.     

地下水温度示踪理论与方法研究进展

对地下水温度示踪理论与方法的相关研究做了评述,介绍了当前地质体中温度场与渗流场耦合作用模型、数值模拟技术和渗流参数反演方法,并从温度示踪方法的两个主要应用领域:地表水与地下水交换和工程地下水渗漏探测(以堤坝为例),说明地下水温度示踪的应用研究.在地质体中温度场与渗流场耦合模型方面,裂隙介质、非饱和带、复杂边界条件和非D...     

Using 3H and 14C to constrain the degree of closed-system dissolution of calcite in groundwater

This study uses ³H concentrations, ¹⁴C activities (a¹⁴C), ⁸⁷Sr/⁸⁶Sr ratios, and δ¹³C values to constrain calcite dissolution in groundwater from the Ovens catchment SE Australia. Taken in isolation, the δ¹³C values of dissolved organic C (DIC) and ⁸⁷Sr/⁸⁶Sr ratios in the Ovens groundwater imply that there has been significant calcite dissolution. However, the covariance of ³H and ¹⁴C and the calculated initial ¹⁴C activities (a₀¹⁴C) imply that most groundwater cannot have dissolved more than 20% of ¹⁴C-free calcite under closed-system conditions. Rather, calcite dissolution must have been partially an open-system process allowing ¹³C and ¹⁴C to re-equilibrate with CO₂ in the unsaturated zone. Recognising that open-system calcite dissolution has occurred is important for dating deeper groundwater that is removed from its recharge area in this and other basins. The study is one of the first to use ¹⁴C and ³H to constrain the degree of calcite dissolution and illustrates that it is a valuable tool for assessing geochemical processes in recharge areas.     

The karst collapses induced by environmental changes of the groundwater and their distribution rules in North China

China, whose distribution area of karst region reaches about 3.63 million km², is one of the countries that have the widest karst collapse development. In modern-karst collapses, the karst collapse induced by water environment is an important type in karst collapses of North China. This paper completed a systematic study on the relations between groundwater environmental transformation and karst collapses and their distribution rules in northern China and found out that the karst collapses in northern China are closely related to the change of groundwater environment, which increased along with the declining groundwater levels and increasing hydraulic gradient. Meanwhile, it was also found that in all cases of collapse caused by underground-environmental change of northern China, pumping collapses in urban areas appeared to be the commonest and strongest among modern-karst collapses with 20 regions and 796 mining pits in total, which account for 30.7 and 58% of artificial collapses, respectively, followed by collapses that were induced by mining draining and inrushing water with 33 regions and 282 mining pits, accounting for 20.4%. In addition to the conditions listed above, there were also a few water caving collapse and leakage collapse.     

Climatic dependence of stable carbon and oxygen isotope signals recorded in speleothems: From soil water to speleothem calcite

Understanding the relationship between stable isotope signals recorded in speleothems (δ¹³C and δ¹⁸O) and the isotopic composition of the carbonate species in the soil water is of great importance for their interpretation in terms of past climate variability. Here the evolution of the carbon isotope composition of soil water on its way down to the cave during dissolution of limestone is studied for both closed and open-closed conditions with respect to CO₂.The water entering the cave flows as a thin film towards the drip site. CO₂ degasses from this film within approx. 10 s by molecular diffusion. Subsequently, chemical and isotopic equilibrium is established on a time scale of several 10-100 s. The δ¹³C value of the drip water is mainly determined by the isotopic composition of soil CO₂. The evolution of the δ¹⁸O value of the carbonate species is determined by the long exchange time T_ex, between oxygen in carbonate and water of several 10,000 s. Even if the oxygen of the CO₂ in soil water is in isotopic equilibrium with that of the water, dissolution of limestone delivers oxygen with a different isotopic composition changing the δ¹⁸O value of the carbonate species. Consequently, the δ¹⁸O value of the rainwater will only be reflected in the drip water if it has stayed in the rock for a sufficiently long time.After the water has entered the cave, the carbon and oxygen isotope composition of the drip water may be altered by CO₂-exchange with the cave air. Exchange times, , of about 3000 s are derived. Thus, only drip water, which drips in less than 3000 s onto the stalagmite surface, is suitable to imprint climatic signals into speleothem calcite deposited from it.Precipitation of calcite proceeds with time constants, τ_p, of several 100 s. Different rate constants and equilibrium concentrations for the heavy and light isotopes, respectively, result in isotope fractionation during calcite precipitation. Since T_ex ? τ_p, exchange with the oxygen in the water can be neglected, and the isotopic evolution of carbon and oxygen proceed analogously. For drip intervals T_d < 0.1τ_p the isotopic compositions of both carbon and oxygen in the solution evolve linearly in time. The calcite precipitated at the apex of the stalagmite reflects the isotopic signal of the drip water.For long drip intervals, when calcite is deposited from a stagnant water film, long drip intervals may have a significant effect on the isotopic composition of the DIC. In this case, the isotopic composition of the calcite deposited at the apex must be determined by averaging over the drip interval. Such processes must be considered when speleothems are used as proxies of past climate variability.     

The dependence of bacterial sulfate reduction on sulfate concentration in marine sediments

The effect of dissolved sulfate concentration on the rate of bacterial sulfate reduction in marine sediment from Long Island Sound was examined using a radio-sulfur technique. The experimental results show that the rate is independent of the dissolved sulfate concentration until low levels are reached (<3 mM), and that, when interpreted using a Monod-type rate law, a saturation constant, K_s, of 1.62 ± 0.16 M results. This weak dependence implies that the dissolved sulfate exerts only a limited influence on the rate of sulfate reduction in marine sediments. Given such a weak dependence, dissolved sulfate profiles in marine sediments must resemble profiles generated by models with sulfate independent kinetics. Initially, this would suggest that currently used sulfate-independent diagenetic models are appropriate in modelling sulfate profiles. However, comparison of these models with those containing weak sulfate-dependent kinetic terms shows that there exists considerable disagreement between these models when the parameter grouping $\text{(D}{}_{\mathrm{s}}\text{k)}\text{1}\text{2}\text{/w}$ is larger than ~0.2 and smaller than ~3.0. (Here D_s is the SO^;₄ diffusion coefficient, k the organic matter decay constant and w the sediment burial velocity.) When the currently used models are corrected by employing physically meaningful boundary conditions, this divergence disappears. The modelling results, therefore, confirm the conclusion that any sulfate dependence inherent to the reduction kinetics does not appreciably affect sulfate pore water profiles, and that previous diagenetic studies using strong sulfate dependent models are erroneous.     

河北平原地下水水-岩作用新证据 ——锶同位素示踪演变特征

本文根据河北平原地下水锶同位素比值等测试结果讨论了其演变特征:除个别水点外,地下水锶同位素比值均大于现代海水的均值(0.709073),第四系地下水的锶同位素比值均大于热水点值,且随着埋深的增大、沿迳流途径从补给区到排泄区系统增大,这同锶同位素比值与碳-14年龄呈正相关关系是一致的.本文根据锶同位素比值及锶离子浓度特点将该区地下水分三类进行了分析.在平原中部,呈NE-SW方向,出现了一个锶同位素比值的高值带,大致与子牙河的流向一致,即献县一青县一线.该资料为河北平原第四系地下水演化提供了新的证据.