Extent Of Isotope Fractionation Research Articles

Residual gas in extensive stratified Miocene Izura carbonate concretions exhibiting thermogenic origin and isotopic fractionation associated with carbonate precipitation

A major carbonate-cemented strata and carbonate concretions intercalated in the Early Miocene Kokozura Formation are common in the Izura area along the Pacific coast, in north Ibaraki, Japan. Chemotrophic mollusk fossils occur frequently with the Izura carbonate concretions. Significant quantities of archaeal biomarkers such as pentamethylicosane, crocetane, and squalane are detected in the carbonate concretions. Residual gas from pulverizing the carbonate concretion contains mostly CO2 with subordinate amounts of CH4, C2H6, and C3H8. The CH4/(C2H6+C3H8) ratio and the δ13C values of CH4 and carbonate indicate the Izura carbonate concretions originate from thermogenic methane. The extent of isotopic fractionation during formation of the carbonate concretions is determined by a novel dataset of δ13C values of CO2 in the residual gas and carbonates. Largescale leakage of thermogenic methane due to the deformation, fissuring, and destruction of the paleo-Izura petroleum deposit produced the vast Izura carbonate concretions. This deformation is associated with the tectonic separation of Japanese Islands from the paleo-Eurasian continent in the late Early Miocene.

Carbon Stable Isotope Fractionation of Sulfamethoxazole during Biodegradation by Microbacterium sp. Strain BR1 and upon Direct Photolysis.

Carbon isotope fractionation of sulfamethoxazole (SMX) during biodegradation by Microbacterium sp. strain BR1 (ipso-hydroxylation) and upon direct photolysis was investigated. Carbon isotope signatures (δ(13)C) of SMX were measured by LC-IRMS (liquid chromatography coupled to isotope ratio mass spectrometry). A new LC-IRMS method for the SMX metabolite, 3-amino-5-methylisoxazole (3A5MI), was established. Carbon isotope enrichment factors for SMX (ε(C)) were -0.6 ± 0.1‰ for biodegradation and -2.0 ± 0.1‰ and -3.0 ± 0.2‰ for direct photolysis, at pH 7.4 and pH 5, respectively. The corresponding apparent kinetic isotope effects (AKIE) for ipso-hydroxylation were 1.006 ± 0.001; these fall in the same range as AKIE in previously studied hydroxylation reactions. The differences in SMX and 3A5MI fractionation upon biotic and abiotic degradation suggest that compound specific stable isotope analysis (CSIA) is a suitable method to distinguish SMX reaction pathways. In addition, the study revealed that the extent of isotope fractionation during SMX photolytic cleavage is pH-dependent.

Chemical weathering processes in the Great Artesian Basin: Evidence from lithium and silicon isotopes

Variations in lithium and silicon isotope ratios in groundwaters of the Great Artesian Basin in Australia, and the causes of these variations, have been explored. The chemistries of Li and Si in groundwater are influenced by the dissolution of primary phases, the formation of secondary minerals, and the reaction of solid phases with dissolved constituents, while isotopic variations are generated by uptake into clays, which preferentially incorporate the light isotopes. The lithium isotopic composition (expressed as δLi7) of the groundwaters ranges from +9 to +16‰, and clearly reflects changes in aquifer conditions. Reaction-transport modelling indicates that changes in Li concentrations are principally controlled by the ratio of the weathering rate of primary minerals to the precipitation rate of secondary minerals, whereas δLi7 is affected by the extent of isotope fractionation during secondary mineral formation (which is dependent on mineralogy). The patterns of groundwater Si concentrations and δSi30 values versus flow distance suggest that Si is at steady state in the aquifer. The δSi30 value of most of the groundwater samples is close to −1‰, which is significantly lower than the δSi30 value of the reservoir rocks (∼0‰). Since precipitation of clays preferentially removes the light Si isotopes from solution, the most plausible explanation for these low groundwater δSi30 values is addition of Si by dissolution of isotopically light secondary minerals. These data, together with model calculations, show that Li isotopes are extremely sensitive to changes in the chemical and physical conditions in the aquifer, whereas Si is not. Importantly, the model suggests that even in large aquifers with long fluid residence times, where steady-state would be expected to be reached, the concentrations and isotopic fractionation of trace elements are not controlled by Li adsorption. The model developed here provides a basis for using Li isotopes measured in groundwaters and surface waters to constrain weathering processes.

Stable carbon isotope fractionation between substrates and products of Methanosarcina barkeri

Stable carbon isotope ratios are an important tool for understanding methanogenesis in the environment. When applied to biological methanogenesis, interpretation of carbon isotope ratios requires a thorough understanding of how the availability of different substrates affects the eventual δ 13C of methane, biomass and lipids. Methanosarcina barkeri was grown on four substrates: methanol, trimethylamine (TMA), acetate, and H 2/CO 2, under variable conditions in which the substrate was either present in excess or limited in availability. The extent of isotopic fractionation between the carbon substrate and the products of M. barkeri was dependent on the substrate type and availability. Growth on unlimited substrate resulted in a range of observed isotope fractionation, with growth on methanol yielding methane, biomass and lipids most depleted in 13C relative to substrate, and growth on acetate yielding the least depleted products. Autotrophic fractionations were intermediate. Substrate-limited growth afforded smaller depletions in 13C on all substrates. There were large differences in the δ 13C among the M. barkeri lipids produced within each experiment, with the notable exception of growth on acetate. The 13C content of lipids was generally well correlated with that of biomass, with archaeol showing the strongest relationship. The 13C content of individual lipids varied with substrate availability in some cases, but did not show patterns that could be used to identify the growth substrate of methanogens in natural environments.

Evaluation of Gas Chromatographic Isotope Fractionation and Process Contamination by Carbon in Compound-Specific Radiocarbon Analysis

The relevance of both modern and fossil carbon contamination as well as isotope fractionation during preparative gas chromatography for compound-specific radiocarbon analysis (CSRA) was evaluated. Two independent laboratories investigated the influence of modern carbon contamination in the sample cleanup procedure and preparative capillary gas chromatography (pcGC) of a radiocarbon-dead 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169) reference. The isolated samples were analyzed for their 14C/12C ratio by accelerator mass spectrometry. Sample Delta14C values of -996 +/- 20 and -985 +/- 20 per thousand agreed with a Delta14C of -995 +/- 20 per thousand for the unprocessed PCB 169, suggesting that no significant contamination by nonfossil carbon was introduced during the sample preparation process at either laboratory. A reference compound containing a modern 14C/12C ratio (vanillin) was employed to evaluate process contamination from fossil C. No negative bias due to fossil C was observed (sample Delta14C value of 165 +/- 20 per thousand agreed with Delta14C of 155 +/- 12 per thousand for the unprocessed vanillin). The extent of isotopic fractionation that can be induced during pcGC was evaluated by partially collecting the vanillin model compound of modern 14C/12C abundance. A significant change in the delta13C and delta14C values was observed when only parts of the eluting peak were collected (delta13C values ranged from -15.75 to -49.91 per thousand and delta14C values from -82.4 to +4.71 per thousand). Delta14C values, which are normalized to a delta13C of -25 per thousand, did not deviate significantly (-58.9 to -5.8 per thousand, considering the uncertainty of approximately +/-20 per thousand). This means that normalization of radiocarbon results to a delta13C of -25 per thousand, normally performed to remove effects of environmental isotope fractionation on 14C-based age determinations, also cor-rects sufficiently for putative isotopic fractionation that may occur during pcGC isolation of individual compounds for CSRA.

Effect of iron type on kinetics and carbon isotopic enrichment of chlorinated ethylenes during abiotic reduction on Fe(0).

Four samples of two commercially available iron brands used as substrate for iron permeable reactive barriers (PRBs) were tested for suitability for remediation of perchloroethylene (PCE), trichloroethylene (TCE), cis-dichloroethylene (cDCE) and vinyl chloride (VC). Kinetic studies indicate that rates of reaction are enhanced for cDCE and VC on Connelly iron (2.8 x 10(-4) to 6.9 x 10(-4) L/m2/hr and 2.0 x 10(-4) to 9.0 x 10(-4) L/m2/hr, for cDCE and VC, respectively) vs. Peerless iron (3.1 x 10(-5) to 4.6 x 10(-5) L/m2/hr and 2.4 x 10(-5) to 4.1 x 10(-5) L/m2/hr, for cDCE and VC, respectively). Carbon isotopic analyses of the residual chlorinated ethylene (CE) during degradation indicate significant fractionation occurs during reductive dechlorination, with, for example, up to 70% enrichment in carbon isotopic values observed when VC is more than 99% degraded. Comparison of fractionation factors (epsilon) indicates significant differences in carbon isotopic fractionation for different iron types and for different CEs. For the lower CEs (cDCE and VC) in particular, both slower reaction rates and larger fractionation are observed for degradation on Peerless vs. Connelly iron. This is the first study to establish a correlation between the rate of abiotic degradation on Fe(0) and the extent of isotopic fractionation, and the first to confirm consistent differences in these two parameters as a function of iron type. The possibility that these differences in kinetics and carbon isotopic fractionation for cDCE and VC are related to differences in branching ratios between competing hydrogenolysis and beta-elimination reactions during reductive dechlorination on the iron surfaces is discussed.

Isotope fractionation by natural populations of sulfate-reducing bacteria

Isotope fractionation during sulfate reduction was explored for natural populations of sulfate-reducing bacteria. High fractionations of 30‰ to 40‰ were produced when the natural population metabolized with indigenous organic substrate at environmental temperatures of 15°C to 25°C. Fractionations were unaffected by changes in sulfate concentration between 2 mM and 28 mM. After the natural substrate was exhausted, the sulfate-reducing bacterial population metabolized, in turn, with acetate, ethanol, and lactate. The high fractionations encountered with natural substrate were only reproduced when the amended substrate was supplied at concentrations limiting the activity of the sulfate-reducing population. Higher, nonlimiting concentrations of amended substrate produced lower fractionations of 16‰ to 21% at 25°C. The natural sulfate-reducing population, therefore, probably experienced substrate limitation while utilizing the natural substrate. At the low temperature of 5°C fractionations with amended substrate ranged from 8‰ to 14‰ and were lower than expected based on the normal relationship between rates of sulfate reduction and the extent of isotope fractionation. The processes likely acting to control the magnitude of isotope fractionation are discussed.

The laser microprobe: A technique for extracting carbon, nitrogen, and oxygen from solid samples for isotopic measurements

The laser microprobe extraction technique has been adapted for the determination of concentrations and stable isotopic compositions of carbon, nitrogen, and oxygen. The power from a focused laser beam is used to selectively pyrolyse, or combust, specific areas of a solid sample of interest, which is located within a vacuum extraction line. Gases released from the sample are collected, purified, and then admitted to an appropriate mass spectrometer as either molecular nitrogen (for δ15N measurements) or carbon dioxide (for δ13C and δ18O measurements). The minimum amounts of gas that can be measured Isotopically are less than a nanomole. A number of samples have been analysed in order to evaluate the efficacy of the laser microprobe including two carbonate minerals (calcite and siderite), graphite, and titanium nitride. Initial studies on the distribution of nitrogen in an iron meteorite (Uwet) and a carbonaceous chondrite (Murchison) have been undertaken. Nitrogen in Uwet was found to be concentrated in the phosphide mineral schreibersite rather than in the kamacite. Laser extractions of the dark matrix of Murchison reveal 620 to 790 pprn of nitrogen with a δ15N = +38.5 to +44.7‰ (in agreement with stepped heating extractions of bulk Murchison) while much lower concentrations, down to 80 ppm (and δ15N = +12 to +60‰), were found in the light‐coloured high‐temperature inclusions. For carbon‐containing phases, there appears to be an isotope fractionation associated with the formation of CO2 and CO, However, preliminary results suggest that the extent of isotope fractionation may be reproducible, allowing suitable corrections to be applied to the raw data.