8 research outputs found
Recommended from our members
Salt effects on stable isotope partitioning and their geochemical implications for geothermal brines
The effects of dissolved salts (NaCI, KCI, MgCl{sub 2}, CaCI{sub 2}, Na{sub 2}SO{sub 4}, MgSO{sub 4}, and their mixtures) on oxygen and hydrogen isotope partitioning between brines and coexisting phases (vapor and calcite) were experimentally determined at 50-350{degree} C and 300{degree} C, respectively. In liquid-vapor equilibration experiments, for all of the salts studied, the hydrogen isotope fractionation factors between the salt solutions and vapor decreased appreciably (up to 20{permille}) compared to pure water-vapor. Except for KCI solutions at 500 C, the oxygen isotope fractionation factors between salt solutions and vapor were higher (up to 4{permille}) than, or very close to, that of pure water. The observed isotope salt effects are all linear with the molalities of the solutions. Mixed salt solutions mimicking natural geothermal brines exhibit salt effects additive of those of individual salts. The isotope exchange experiments of calcite-water at 300{degree}C and 1 kbar yielded a fractionation factor of 5.9+0.3{permille} for pure water and effects of NaCl consistent with those obtained from the liquid-vapor equilibration experiments. The isotope salt effects observed in this study are too large to be ignored, and must be taken into account for isotopic studies of geothermal systems (i.e., estimation of isotope ratios and temperatures of deep-seated geothermal brines)
Tammsaare Park’s lost landmarks of revolution, Soviet-era path layout, and pedestrian use: Tallinn, Estonia
Recommended from our members
Temperature-Dependent Oxygen and Carbon Isotopes Fractionations of Biogenic Siderite
Isotopic compositions of biogenic iron minerals may be used to infer environmental conditions under which bacterial iron reduction occurs. The major goal of this study is to examine temperature-dependent isotope fractionations associated with biogenic siderite (FeCO3). Experiments were performed by using both mesophilic (\u3c35°C) and thermophilic (\u3e45°C) iron-reducing bacteria. In addition, control experiments were performed to examine fractionations under nonbiologic conditions. Temperature-dependent oxygen isotope fractionation occurred between biogenic siderite and water from which the mineral was precipitated. Samples in thermophilic cultures (45–75°C) gave the best linear correlation, which can be described as 103 lnαsid-wt = 2.56 × 106 T−2 (K) + 1.69. This empirical equation agrees with that derived from inorganically precipitated siderite by Carothers et al. (1988) and may be used to approximate equilibrium fractionation. Carbon isotope fractionation between biogenic siderite and CO2, based on limited data, also varied with temperature and was consistent with the inorganically precipitated siderite of Carothers et al. (1988). These results indicate that temperature is a controlling factor for isotopic variations in biogenic minerals examined in this study. The temperature-dependent fractionations under laboratory conditions, however, could be complicated by other factors including incubation time and concentration of bicarbonate. Early precipitated siderite at 120-mM initial bicarbonate tended to be enriched in 18O. Siderite formed at \u3c30 mM of bicarbonate tended to be depleted in 18O. Other\u3evariables, such as isotopic compositions of water, types of bacterial species, or bacterial growth rates, had little effect on the fractionation. In addition, siderite formed in abiotic controls had similar oxygen isotopic compositions as those of biogenic siderite at the same temperature, suggesting that microbial fractionations cannot be distinguished from abiotic fractionations under conditions examined here
Low 13C-13C abundances in abiotic ethane
Distinguishing biotic compounds from abiotic ones is important in resource geology, biogeochemistry, and the search for life in the universe. Stable isotopes have traditionally been used to discriminate the origins of organic materials, with particular focus on hydrocarbons. However, despite extensive efforts, unequivocal distinction of abiotic hydrocarbons remains challenging. Recent development of clumped-isotope analysis provides more robust information because it is independent of the stable isotopic composition of the starting material. Here, we report data from a 13C-13C clumped-isotope analysis of ethane and demonstrate that the abiotically-synthesized ethane shows distinctively low 13C-13C abundances compared to thermogenic ethane. A collision frequency model predicts the observed low 13C-13C abundances (anti-clumping) in ethane produced from methyl radical recombination. In contrast, thermogenic ethane presumably exhibits near stochastic 13C-13C distribution inherited from the biological precursor, which undergoes C-C bond cleavage/recombination during metabolism. Further, we find an exceptionally high 13C-13C signature in ethane remaining after microbial oxidation. In summary, the approach distinguishes between thermogenic, microbially altered, and abiotic hydrocarbons. The 13C-13C signature can provide an important step forward for discrimination of the origin of organic molecules on Earth and in extra-terrestrial environments