The effects of growth phase and salinity on the hydrogen isotopic composition of alkenones produced by coastal haptophyte algae

The isotopic fractionation of hydrogen during the biosynthesis of alkenones produced by marine haptophyte algae has been shown to depend on salinity and, as such, the hydrogen isotopic composition of alkenones is emerging as a palaeosalinity proxy. The relationship between fractionation and salinity has previously only been determined during exponential growth, whilst it is not yet known in which growth phases natural haptophyte populations predominantly exist. We have therefore determined the relationship between the fractionation factor, alpha alkenones-water, and salinity for C37 alkenones produced in different growth phases of batch cultures of the major alkenone-producing coastal haptophytes Isochrysis galbana (strain CCMP 1323) and Chrysotila lamellosa (strain CCMP 1307) over a range in salinity from ca. 10 to ca. 35. alpha alkenones-water was similar in both species, ranging over 0.841-0.900 for I. galbana and 0.838-0.865 for C. lamellosa. A strong (0.85 <= R**2 <= 0.97; p < 0.0001) relationship between salinity and fractionation factor was observed in both species at all growth phases investigated. This suggests that alkenone dD has the potential to be used as a salinity proxy in coastal areas where haptophyte communities are dominated by these coastal species. However, there was a marked difference in the sensitivity of alpha alkenones-water to salinity between different growth phases: in the exponential growth phase of I. galbana, alpha alkenones-water increased by 0.0019 per salinity unit (S 1), but was less sensitive at 0.0010 S 1 and 0.0008 S 1 during the stationary and decline phases, respectively. Similarly, in C. lamellosa alpha alkenones-water increased by 0.0010 S 1 in the early stationary phase and by 0.0008 S 1 during the late stationary phase. Assuming the shift in sensitivity of alpha alkenones-water to salinity observed at the end of exponential growth in I. galbana is similar in other alkenone-producing species, the predominant growth phase of natural populations of haptophytes will affect the sensitivity of the alkenone salinity proxy. The proxy is likely to be most sensitive to salinity when alkenones are produced in a state similar to exponential growth.

Barium isotope fractionation during witherite (BaCO3) dissolution, precipitation and at equilibrium

This study examines the behavior of Ba isotope fractionation between witherite and fluid during mineral dissolution, precipitation and at chemical equilibrium. Experiments were performed in batch reactors at 25 oC in 10-2 M NaCl solution where the pH was adjusted by continuous bubbling of a water saturated gas phase of CO2 or atmospheric air. During witherite dissolution no Ba isotope fractionation was observed between solid and fluid. In contrast, during witherite precipitation, caused by a pH increase, a preferential uptake of the lighter 134Ba isotopomer in the solid phase was observed. In this case, the isotope fractionation factor αwitherite-fluid is calculated to be 0.99993 ± 0.00004 (or Δ137/134Bawitherite-fluid ≈ -0.07 ± 0.04 ‰, 2sd). The most interesting feature of this study, however, is that after the attainment of chemical equilibrium, the Ba isotope composition of the aqueous phase is progressively becoming lighter, indicating a continuous exchange of Ba2+ ions between witherite and fluid. Mass balance calculations indicate that the detachment of Ba from the solid is not only restricted to the outer surface layer of the solid, but affects several (~7 unit cells) subsurface layers of the crystal. This observation comes in excellent agreement with the concept of a dynamic system at chemical equilibrium in a mineral-fluid system, denoting that the time required for the achievement of isotopic equilibrium in the witherite-fluid system is longer compared to that observed for chemical equilibrium. Overall, these results indicate that the isotopic composition of Ba bearing carbonates in natural environments may be altered due to changes in fluid composition without a net dissolution/precipitation to be observed.

Experimental data of CO2, CO3, Omega calcite saturation, pH values, alpha and epsilon fractionation factors, and Dd18O calctite-water

The oxygen isotopic composition (d18O) of calcium carbonate of planktonic calcifying organisms is a key tool for reconstructing both past seawater temperature and salinity. The calibration of paloeceanographic proxies relies in general on empirical relationships derived from field experiments on extant species. Laboratory experiments have more often than not revealed that variables other than the target parameter influence the proxy signal, which makes proxy calibration a challenging task. Understanding these secondary or "vital" effects is crucial for increasing proxy accuracy. We present data from laboratory experiments showing that oxygen isotope fractionation during calcification in the coccolithophore Calcidiscus leptoporus and the calcareous dinoflagellate Thoracosphaera heimii is dependent on carbonate chemistry of seawater in addition to its dependence on temperature. A similar result has previously been reported for planktonic foraminifera, supporting the idea that the [CO3]2- effect on d18O is universal for unicellular calcifying planktonic organisms. The slopes of the d18O/[CO3]2- relationships range between -0.0243 per mil/(µmol/kg) (calcareous dinoflagellate T. heimii) and the previously published -0.0022 per mil/(µmol/kg) (non-symbiotic planktonic foramifera Orbulina universa), while C. leptoporus has a slope of -0.0048 per mil/(µmol/kg). We present a simple conceptual model, based on the contribution of d18O-enriched [HCO3]- to the [CO3]2- pool in the calcifying vesicle, which can explain the [CO3]2- effect on d18O for the different unicellular calcifiers. This approach provides a new insight into biological fractionation in calcifying organisms. The large range in d18O/[CO3]2- slopes should possibly be explored as a means for paleoreconstruction of surface [CO3]2-, particularly through comparison of the response in ecologically similar planktonic organisms.

Experiment: The effect of temperature, salinity and growth rate on the stable hydrogen isotopic composition of long chain alkenones produced by Emiliania huxleyi and Gephyrocapsa oceanica

Two haptophyte algae, Emiliania huxleyi and Gephyrocapsa oceanica, were cultured at different temperatures and salinities to investigate the impact of these factors on the hydrogen isotopic composition of long chain alkenones synthesized by these algae. Results showed that alkenones synthesized by G. oceanica were on average depleted in D by 30 compared to those of E. huxleyi when grown under similar temperature and salinity conditions. The fractionation factor, alpha alkenones-H2O, ranged from 0.760 to 0.815 for E. huxleyi and from 0.741 to 0.788 for G. oceanica. There was no significant correlation of alpha alkenones-H2O with temperature but a positive linear correlation was observed between alpha alkenones-H2O and salinity with ~3 change in fractionation per salinity unit and a negative correlation between alpha alkenones-H2O and growth rate. This suggests that both salinity and growth rate can have a substantial impact on the stable hydrogen isotopic composition of long chain alkenones in natural environments.

Sputtering by multiply-charged ions, and preferential sputtering of isotopic mixtures

Collector-type experiments have been conducted to investigate two different aspects of sputtering induced by keV ions. The first study looked for possible ejection mechanisms related to the primary charge state of the projectile. Targets of CsI and LiNbO_3 were bombarded with 48 keV Ar^(q+), and a Au target was bombarded with 60 keV Ar^(q+), for q = 4, 8, and 11. The collectors were analyzed using heavy-ion Rutherford backscattering spectroscopy to determine the differential angular sputtering yields; these and the corresponding total yields were examined for variations as a function of projectile charge state. For the Au target, no significant changes were seen, but for the insulating targets slight (~10%) enhancements were observed in the total yields as the projectile charge state was increased from 4+ to 11+.

In the second investigation, artificial ^(92)Mo/^(100)Mo targets were bombarded with 5 and 10 keV beams of Ar^+ and Xe^+ to study the isotopic fractionation of sputtered neutrals as a function of emission angle and projectile fluence. Using secondary ion mass spectroscopy to measure the isotope ratio on the collectors, material ejected into normal directions at low bombarding fluences (~ 10^(15) ions cm^(-2)) was found to be enriched in the light isotope by as much as ~70‰ compared to steady state. Similar results were found for secondary Mo ions sputtered by 14.5 keV O^-. For low-fluence 5 keV Xe^+ bombardment, the light-isotope enrichment at oblique angles was ~20‰ less than the corresponding enrichment in the normal direction. No angular dependence could be resolved for 5 keV Ar^+ projectiles at the lowest fluence. The above fractionation decreased to steady-state values after bombarding fluences of a few times 10^(16) ions cm^(-2) , with the angular dependence becoming more pronounced. The fractionation and total sputtering yield were found to be strongly correlated, indicating that the above effects may have been related to the presence of a modified target surface layer. The observed effects are consistent with other secondary ion measurements and multiple-interaction computer simulations, and are considerably larger than predicted by existing analytic theory.

Experimental study on the fractionation of yttrium from holmium during the coprecipitation with calcium carbonates in seawater solutions

The partitioning of Y and Ho between CaCO3 (calcite and aragonite respectively) and seawater was experimentally investigated at 25 degrees C and I atm. Both Y and Ho were observed to be strongly partitioned into the overgrowths of calcite or aragonite. Their partition coefficients, D-Y and D-Ho, were determined to be similar to 520-1400 and similar to 700-1900 in calcite, similar to 1200-2400 and similar to 2400-4300 in aragonite, respectively. Y fractionates from Ho during the coprecipitation with either calcite or aragonite. Within our experimental conditions, the fractionation factor, k = D-Y/D-Ho, was determined to be similar to 0.62-0.77 in calcite and similar to 0.50-0.57 in aragonite, respectively. The aqueous complexation of Y and Ho, which is a function of solution chemistry, probably plays an important role in both the partitioning and the fractionation. Further analyses suggest that the difference in covalency between Y and Ho associated with changes in their coordination environments is the determinant factor to the Y-Ho fractionation in the H2CO3-CaCO3 System.

Pretreatment Method for Nitrogen Isotopic Analysis of Nitrate in Seawater

A suitable method for the pretreutment of dissolved nitrate samples in seawaters for nitrogen isotopic analysis was established. First, the seawater samples were processed by removing nitrite and amonium. Then Devard's alloy was added in sample for conversion of dissolved nitrate to ammonium. The sample was distilled, and then the ammonium condensate was collected with zeolite. after distillation, the collected condensate was filtered and prepared for determining nitropic values. Some tests of the method were conducted. The distillation condition, the influence of salinity on nitrogen isotopic analysis, absorption of ammonium onto zeolite and an improved method on a large volume of seawater were discussed in this study. The results showed that the distillation step had an average recovery of (104.9 +/- 4.2) % (n = 6) when distillating every 300 mL aliquot of the sample under a strong alkaline condition with 0.5 g devard's alloy and a distillation time of 30 min. The nitrogen isotopic fractionation decreased markedly when salinity was increased from 0% to 0.5%; further increase(1% - 3.5%) showed little effect. The adsorption rate of ammonium onto zeolite had a high yield of (95.96 +/- 1.08) % (n = 6) in average. An improved collection method was used to process a large volume of seawater with several distillations, and had good effect on analysis. The method had been applied to analyze water samples collected from Changjiang estuary. The analytical results indicate that the method is suitable for delta N-15 analysis of dissolved nitrate in seawaters. The present method could provide valuable information about the source and cycle mechanism of dissolved nitrogen in estuary waters.

Stable carbon and nitrogen isotopic compositions of high molecular weight dissolved organic matter from four US estuaries

High molecular weight dissolved organic matter (HMW-DOM) represents an important component of dissolved organic carbon (DOC) in seawater and fresh-waters. In this paper, we report measurements of stable carbon (delta(13)C) isotopic compositions in total lipid, total hydrolyzable amino acid (THAA), total carbohydrate (TCHO) and acid-insoluble "uncharacterized" organic fractions separated from fourteen HMW-DOM samples collected from four U.S. estuaries. In addition, C/N ratio, delta(13) C and stable nitrogen (delta(15)N) isotopic compositions were also measured for the bulk HMW-DOM samples. Our results indicate that TCHO and THAA are the dominant organic compound classes, contributing 33-46% and 13-20% of the organic carbon in HMW-DOM while total lipid accounts for only <2% of the organic carbon in the samples. In all samples. a significant fraction (35-49%) of HMW-DOM was included in the acid-insoluble fraction. Distinct differences in isotopic compositions exist among bulk samples, the compound classes and the acid-insoluble fractions. Values of delta(13)C and delta(15)N measured for bulk HMW-DOM varied from -22.1 to -30.1parts per thousand and 2.8 to 8.9parts per thousand, respectively and varied among the four estuaries studied as well. Among the Compound classes, TCHO was more enriched in C-13 (delta(13)C = -18.5 to -22.8parts per thousand) compared with THAA (delta(13)C = -20.0 to -29.6parts per thousand) and total lipid (delta(13)C = -25.7 to -30.7parts per thousand). The acid-insoluble organic fractions, in general, had depleted C-13 values (delta(13)C = -23.0 to -34.4parts per thousand). Our results indicate that the observed differences in both delta(13)C and delta(15)N were mainly due to the differences in sources of organic matter and nitrogen inputs to these estuaries in addition to the microbial processes responsible for isotopic fractionation among the compound classes. Both terrestrial sources and local sewage inputs contribute significantly to the HMW-DOM pool in the estuaries studied and thus had a strong influence on its isotopic signatures. Copyright (C) 2004 Elsevier Ltd.

富硫流体的成因与硫酸盐还原作用研究

Origins of H_2S, thiols, thiophenes in natural gases and sulphur-enriched oils are complicated and thus some debates exist on them. The post-doctoral research is based upon oil- and gas-field data. Cases for study include Triassic Jianglingjiang Formation natural gases, Wolonghe Field, Sichuan Basin, Paleozoic oils and bitumen, Central Tarim, gases reserviored nearby Carboniferious - Ordovician unconformity, Hetianhe Field, Tarim Basin and sulphur-enriched oils in Tertiary reserviors in Jinxian Sag, Bohai Bay Basin. We have carried out analyses on the oils and gases for chemistry, δ~(13)C, δ~(34)S, and molecular composition of biomarkers, analyzed authigenetic pyrite forδ~(34)S, formation water for chemistry and δD and δ~(18)O along with petroleum system and burial history analyses, The aims are to assess the origins of the H2S and authigenetic pyrite, to discuss the possibility of reduced sulphur incorporation into hydrocarbons and to determine the mechanisms of hydrocarbon secondary alteration in the above four cases by comparison. The research shows that the reduced sulphur in the four cases is the result of thermochemical and biological sulphate reduction., TSR and BSR, respectively. No evidence indicates an origin of decomposition of organic matter or mantle - derived H2S in the cases. Elevated H_2S contents (up to 32%) in the Triassic Jialingjiang Formation are considered to result from TSR while relatively low H_2S (up to 2000ppm) in the Hetianhe Field resulted from BSR. However, it is not the case for the Central Tarim where relatively low H2S but abundant authigenetic pyrite occurr. Part of the H_2S in the Central Tarim reservoirs has reacted with iron released from clay minerals to precipitate pyrite. Thus, reduced sulphur δ~(34)S and reservoir temperatures rather than the H2S amount are reliable parameters to distinguish between TSR and BSR. TSR in Sichuan Basin Triassic Jialingjiang Formation and Central Tarim Paleozoic reservoirs are showed to take place at more than 125℃. the H2S and authigenetic pyrite have δ~(34)S close to parent anhydrite. In contrast, BSR in the reservoirs near the Carboniferous - Ordovician unconformity in the Hetianhe Field and in the Tertiary in the Jinxian Sag took place at temperatures less than 80℃with sulphide δ~(34)S as light as -24.9‰ and -12.5‰, anhydrite δ~(34)S as heavy as +26‰and +3 5-+40‰, respectively. Chemistry and isotopic composition of the natural gases change as the result of sulphate reduction. It has been observed that relative composition of light hydrocarbon gases is changed along with a rise in H_2S and CO_2. TSR in the Triassic Jialingjiang Formation and BSR in the Hetianhe Field result in a greater degree of preferential depletion of methane than larger molecular hydrocarbon gases. As TSR or BSR proceeds, hydrocarbon gases evolved to heavier carbon isotope as the result of kinetic isotopic fractionation, i.e., selective anaerobic oxidation of ~(12)C. Using the model of residual methane (Whiticar, 1999) to describe the relationship among the proportion of methane oxidation, isotopic shift and fraction factor, about 30% methane is calculated to have been oxidized during BSR in the western part of the Hetianhe Field. From the above, it can be concluded that in the area where H_2S is abundant, empiricalδ~(13)C -Ro relationships do not work. Sulphate reduction results in a rise in sulphur content, gravity and viscosity of an oil as well as changes in δ~(13)C and δ~(34)S. On special conditions, the reduced sulphur from sulphates might be incorporated into oils, i.e., the increasing sulphur is derived from secondarily reduced sulphur. A positive correlative relationship exists between sulphur content and δ~(34)S in the oils in Paleozoic reservoirs in Central Tarim, indicating that enhanced sulphur is ~(34)S-enriched, originated from TSR. The Jinxian oil with the highest sulphur content has the lightest δ~(34)S, suggesting part of the sulphur in the oil is ~(34)S-depleted, originated from BSR. In the Jinxian oil with increasing sulphur content, asphaltenes shows higher content and more negative δ~(13)C, and saturates shows evidence of biodegradetion and a decreasing content and a positive δ~(13)C shift. Thus, asphaltenes have δ~(13)C values closer to saturates. All the above indicate that the reduced sulphur has been incorporated into biodegradated saturates to generate new asphaltenes with relatively light δ~(13)C of saturates. Thiols and thiophenes in natural gases in the Triassic Jialingjiang Formation may result from reaction of H_2S with hydrocarbon. In the Jialingjiang Formation hydrocarbon gases are dominated by methane thus have a high dryness coefficient and thiols are showed to be positively related to H_2S content, suggesting that the thiols may result from H_2S reaction with short chain hydrocarbons. In contrast, high thiophenes occur in wet gases in Jurassic reservoirs with their source rock from sulphur - depleted type I kerogen, indicating that thiophenes may be a product of reaction of H2S with longer chain hydrocarbons.

中-新元古代海洋的硫和碳同位素的演化

Data on seawater carbon isotope in the Mesoproterozoic and Neoproterozoic is abundant. However, the sulfur isotopic age curve of seawater sulfates determined through the analysis of sulfur isotopic composition of marine evaporite is uncertain in the Mesoproterozoic and Neoproterozoic since evaporites are generally rare in Precambrian. The Mesoproterozoic and Neoproterozoic Carbonate Formations preserve not only the carbon isotopic records, but also the sulfur isotopic records of coeval seawater in the Huabei Platform and the Yangtze Platform, China. Sulfur isotopic composition can be determined by the extraction of trace sulfate from carbonate samples. Successive measurements of sulfur and carbon isotopic compositions of carbonate samples from the Mesoproterozoic and Neoproterozoic strata in the Huabei Platform and the Yangtze Platform was accomplished through the extracting of trace sulfate from carbonates. Sulfur and carbon isotopic compositions of coeval seawater were obtained from analytical results of sulfur and carbon isotopes of the same sample without diagenetic alteration. The high-resolution age curve of sulfur isotope given in this paper may reflect the trend of variations in sulfur isotope composition of seawater sulfates during the Mesoproterozoic and Neoproterozoic. It can be correlated with the characteristics of variation in age curve of carbon isotope of coeval seawater carbonates. The δ34S values of seawater varied from +10.3-37.0‰ during the Mesoproterozoic, which took on oscillated variation on the whole. The δ34S values took on high values in the Mesoproterozoic Chuanlinggou stage, Tuanshanzi stage Tieling stage and in Neoproterozoic Jing'eryu stage. The average of those was about +30‰. The sulfates have low δ34S values in the Mesoproterozoic Yangzhuang stage and Hongshuizhuang stage, The average of those was all lower than +20‰. There occured large-amplitude changs in δ34S values of seawater during the Mesoproterozoic. Large-amplitude oscillate of 534S values occured in the intervals of 1600~1400Ma and 1300~1200Ma. The δ13C values of seawater are mostly negative in Changcheng stage of late Paleoproterozoic, -0 ± 1‰ range in Jixian stage of Mesoproterozoic , and the positive 2±2‰ commonly in early Neoproterozoic Jing'eryu stage. From 1000 Ma to 900 Ma, about 108 years interval of oceanic 513C record is shortage. At the end of Paleoproterozoic (1700 - 1600 Ma), the oceanic 813C values change from -3‰ to 0‰, but strongly oscillate near 1600 Ma. Two larger variations of seawater 513C values occur in the Mesoproterozoic: one is a cycle of about 4%o happens at ca. 1400 Ma; another is rise from >2‰ to>5‰ at ca. 1250 Ma and then become stable at the near 1000 Ma. There appears a large positive excursion over +20‰ in 534S value of ancient seawater sulfates in the early Doushantuo stage. Simultaneously, 8 C values of ancient seawater occur a positive excursion reaching 10‰. These allow δ4S values and 513C values to reach high values of+51.7‰ and +6.9‰, respectively. The range of variation in 834S values of seawater is relatively narrow and 513C values are quite high in the middle Doushantuo stage. Then, δ34S values of seawater become oscillating, the same happens in δ13C values. Negative excursions in 834S values and 813C values occur simultaneously at the end of the Doushantuo stage, and the minimum of δ34S values and δ13C values dropped to -11.3‰ and -5.7‰, respectively. The ancient seawater in the Dengying stage has high δS values and δ13C values. Most of the δ34S values of the trace sulfate samples varied between +23.6‰ and +37.9‰ except two boundaries of the Dengying Formation, and the S13C values of the carbonate samples of the Dengying Formation varied between +0.5‰ and +5.0‰. There appeared large negative excursion in 834S values and δ13C values of ancient seawater at the bounder of Precambrian-Cambrian. The isotopic characteristics of sulfur and carbon implicated that the organic productivity and isotopic fractionation caused by biology were low and the palaeoceanic environment was quite unstable during the Mesoproterozoic. The increase and subsequent oscillation of seawater δ13C value occurred from 1700 to 1600 Ma and near 1300 Ma may be responsible to the two global tectonic events happened at coeval time. The characteristics of variation in sulfur and carbon isotopes of ancient seawater imply strong changes in oceanic environment, which became beneficial to inhabitation and propagation of organism. The organic production and the burial rate of organic carbon once reached a quite high level during the Doushantuo stage. However, the state of environment became unstable that means the global climate and the environment possibly were fluctuating and reiterating after the global glaciation. The negative excursions of S34S values and δ13C values occurring at the end of the Doushantuo stage represent a global event, which might be relative to the oxidation of deep seawater. The isotopic characteristics of sulfur and carbon implicated that there were a high organic productivity and a high burial rate of organic carbon in the Dengying stage. It is obvious that the palaeoceanic environment in Dengying stage was stable corresponding and beneficial for biology to inhabit and propagate except for the two boundaries. The tendency of sulfur and carbon isotopic variations maybe resulted from the gradual oxygenation of ocean environment during the Dengying stage. It has been reported that the secular variations of the sulfur isotopic compositions in seawater was negative correlated with that of carbon isotopic compositions. However, our results show that it is not the case. They were negatively correlated in some intervals and positively in some other intervals of the Mesoproterozoic and Neoproterozoic. The difference in correlation may be associated with the changes in conditions of redox in oceanic environment, e.g. sharp change of the oxidation-reduction interface. The strong changes in global environment may induce the abnormality to occur in the biogeo chemical S and C cycles in the ocean and accordingly sharp Variations in isotopic composition of seawater sulfur and carbon during the Mesoproterozoic and Neoproterozoic. Simultaneously, the global tectonism caused large changes of 87Sr/86Sr ratios. The leading factor that causes the variation in isotopic composition is different in the different intervals of the Mesoproterozoic and Neoproterozoic. Thus, there may exist different models of the biogeochemical S and C cycles in the ocean during the Mesoproterozoic and Neoproterozoic.

Large carbon isotope fractionation associated with oxidation of methyl halides by methylotrophic bacteria

The largest biological fractionations of stable carbon isotopes observed in nature occur during production of methane by methanogenic archaea. These fractionations result in substantial (as much as 70) shifts in 13C relative to the initial substrate. We now report that a stable carbon isotopic fractionation of comparable magnitude (up to 70) occurs during oxidation of methyl halides by methylotrophic bacteria. We have demonstrated biological fractionation with whole cells of three methylotrophs (strain IMB-1, strain CC495, and strain MB2) and, to a lesser extent, with the purified cobalamin-dependent methyltransferase enzyme obtained from strain CC495. Thus, the genetic similarities recently reported between methylotrophs, and methanogens with respect to their pathways for C1-unit metabolism are also reflected in the carbon isotopic fractionations achieved by these organisms. We found that only part of the observed fractionation of carbon isotopes could be accounted for by the activity of the corrinoid methyltransferase enzyme, suggesting fractionation by enzymes further along the degradation pathway. These observations are of potential biogeochemical significance in the application of stable carbon isotope ratios to constrain the tropospheric budgets for the ozone-depleting halocarbons, methyl bromide and methyl chloride.

Microbial perchlorate reduction: A precise laboratory determination of the chlorine isotope fractionation and its possible biochemical basis

Perchlorate-reducing bacteria fractionate chlorine stable isotopes giving a powerful approach to monitor the extent of microbial consumption of perchlorate in contaminated sites undergoing remediation or natural perchlorate containing sites. This study reports the full experimental data and methodology used to re-evaluate the chlorine isotope fractionation of perchlorate reduction in duplicate culture experiments of Azospira suillum strain PS at 37 degrees C (Delta Cl-37(Cr)--ClO4-) previously reported, without a supporting data set by Coleman et al. [Coleman, M.L., Ader, M., Chaudhuri, S., Coates,J.D., 2003. Microbial Isotopic Fractionation of Perchlorate Chlorine. Appl. Environ. Microbiol. 69, 4997-5000] in a reconnaissance study, with the goal of increasing the accuracy and precision of the isotopic fractionation determination. The method fully described here for the first time, allows the determination of a higher precision Delta Cl-37(Cl)--ClO4- value, either from accumulated chloride content and isotopic composition or from the residual perchlorate content and isotopic composition. The result sets agree perfectly, within error, giving average Delta Cl-37(Cl)--ClO4- = -14.94 +/- 0.15%omicron. Complementary use of chloride and perchlorate data allowed the identification and rejection of poor quality data by applying mass and isotopic balance checks. This precise Delta Cl-37(Cl)--ClO4-, value can serve as a reference point for comparison with future in situ or microcosm studies but we also note its similarity to the theoretical equilibrium isotopic fractionation between a hypothetical chlorine species of redox state +6 and perchlorate at 37 degrees C and suggest that the first electron transfer during perchlorate reduction may occur at isotopic equilibrium between art enzyme-bound chlorine and perchlorate. (C) 2008 Elsevier B.V. All rights reserved.

The effect of aqueous diffusion on the fractionation of chlorine and bromine stable isotopes

Diffusive isotopic fractionation factors are important in order to understand natural processes and have practical application in radioactive waste storage and carbon dioxide sequestration. We determined the isotope fractionation factors and the effective diffusion coefficients of chloride and bromide ions during aqueous diffusion in polyacrylamide gel. Diffusion was determined as functions of temperature, time and concentration. The effect of temperature is relatively large on the diffusion coefficient (D) but only small on isotope fractionation. For chlorine, the ratio, D-35cl/D-37cl varied from 1.00128 +/- 0.00017 (1 sigma) at 2 degrees C to 1.00192 +/- 0.00015 at 80 degrees C. For bromine, D-79Br/D-81Br varied from 1.00098 +/- 0.00009 at 2 degrees C to 1.0064 +/- 0.00013 at 21 degrees C and 1.00078 +/- 0.00018 (1 sigma) at 80 degrees C. There were no significant effects on the isotope fractionation due to concentration. The lack of sensitivity of the diffusive isotope fractionation to anything at the most common temperatures (0 to 30 C) makes it particularly valuable for application to understanding processes in geological environments and an important natural tracer in order to understand fluid transport processes. (C) 2009 Elsevier Ltd. All rights reserved.