The isotopic ratios O/O and O/O in molecular oxygen and their significance in biogeochemistry

Recently, a new method has been introduced for the estimation of photosynthetic oxygen production from the triple isotope composition (δ¹⁷O and δ¹⁸O) of dissolved O₂ in the ocean and of air O₂ in ice cores. This method is based on the deviations (¹⁷Δ) from mass dependent respiratory fractionation, the major process affecting the isotopic composition of air O₂. To apply this method, the slope in the ¹⁷O/¹⁶O vs. ¹⁸O/¹⁶O relationship used for ¹⁷Δ calculation must be known with high accuracy. Using numerical simulations and closed system experiments, we show how the respiratory slope is manifested in the ¹⁷Δ of O₂ in situations where respiration is the only process affecting oxygen isotopic composition (kinetic slope), and in systems in steady state between photosynthesis and respiration (steady state slope). The slopes of the fractionation line in these two cases are different, and the reasons of this phenomenon are discussed. To determine the kinetic respiratory slope for the dominant O₂ consumers in aquatic systems, we have conducted new experiments using a wide range of organisms and conditions and obtained one universal value (0.5179 ± 0.0006) in ln(δ¹⁷O + 1) vs. ln(δ¹⁸O + 1) plots. It was also shown that the respiratory fractionations under light and dark are identical within experimental error. We discuss various marine situations and conclude that the kinetic slope 0.518 should be used for calculating ¹⁷Δ of dissolved O₂. In contrast, a steady state fractionation slope should be used in global mass balance calculations of triple isotope ratios of O₂ in air records of ice cores.     

Oxygen isotope compositions of phosphate from arvicoline teeth and Quaternary climatic changes, Gigny, French Jura

Oxygen isotope compositions of biogenic phosphates from mammals are widely used as proxies of the isotopic compositions of meteoric waters that are roughly linearly related to the air temperature at high- and mid-latitudes. An oxygen isotope fractionation equation was determined by using present-day European arvicoline (rodents) tooth phosphate: δ¹⁸O_p = 20.98(±0.59) + 0.572(±0.065) δ¹⁸O_w. This fractionation equation was applied to the Late Pleistocene karstic sequence of Gigny, French Jura. Comparison between the oxygen isotope compositions of arvicoline tooth phosphate and Greenland ice core records suggests to reconsider the previously established hypothetical chronology of the sequence. According to the δ¹⁸O value of meteoric water-mean air temperature relationships, the δ¹⁸O value of arvicoline teeth records variations in mean air temperatures that range from 0° to 15°C.     

The triple isotopic composition of oxygen in leaf water

The isotopic composition of atmospheric O₂ depends on the rates of oxygen cycling in photosynthesis, respiration, photochemical reactions in the stratosphere and on δ¹⁷O and δ¹⁸O of ocean and leaf water. While most of the factors affecting δ¹⁷O and δ¹⁸O of air O₂ have been studied extensively in recent years, δ¹⁷O of leaf water—the substrate for all terrestrial photosynthesis—remained unknown. In order to understand the isotopic composition of atmospheric O₂ at present and in fossil air in ice cores, we studied leaf water in field experiments in Israel and in a European survey. We measured the difference in δ¹⁷O and δ¹⁸O between stem and leaf water, which is the result of isotope enrichment during transpiration. We calculated the slopes of the lines linking the isotopic compositions of stem and leaf water. The obtained slopes in ln(δ¹⁷O + 1) vs. ln(δ¹⁸O + 1) plots are characterized by very high precision (∼0.001) despite of relatively large differences between duplicates in both δ¹⁷O and δ¹⁸O (0.02-0.05‰). This is so because the errors in δ¹⁸O and δ¹⁷O are mass-dependent. The slope of the leaf transpiration process varied between 0.5111 ± 0.0013 and 0.5204 ± 0.0005, which is considerably smaller than the slope linking liquid water and vapor at equilibrium (0.529). We further found that the slope of the transpiration process decreases with atmospheric relative humidity (h) as 0.522-0.008 × h, for h in the range 0.3-1. This slope is neither influenced by the plant species, nor by the environmental conditions where plants grow nor does it show strong variations along long leaves.     

Fractionation of oxygen isotopes between mammalian bone-phosphate and environmental drinking water

The δ¹⁸O of mammalian bone-phosphate varies linearly with δ¹⁸O of environmental water, but is not in isotopic equilibrium with that water. This situation is explained by a model of δ¹⁸O in body water in which the important fluxes of exchangeable oxygen through the body are taken into account. Fractionation of oxygen isotopes between body and environmental drinking water is dependent on the rates of drinking and respiration. Isotopic fractionation can be estimated from physiological data and the estimates correlate very well with observed fractionation. Species whose water consumption is large relatively to its energy expenditure is sensitive to isotopic ratio changes in environmental water.     

Calibration of the calcite-water oxygen-isotope geothermometer at Devils Hole, Nevada, a natural laboratory

The δ¹⁸O of ground water (−13.54 ± 0.05 ‰) and inorganically precipitated Holocene vein calcite (+14.56 ± 0.03 ‰) from Devils Hole cave #2 in southcentral Nevada yield an oxygen isotopic fractionation factor between calcite and water at 33.7 °C of 1.02849 ± 0.00013 (1000 ln α_{calcite-water} = 28.09 ± 0.13). Using the commonly accepted value of ∂(α_{calcite-water})/∂T of −0.00020 K⁻¹, this corresponds to a 1000 ln α_{calcite-water} value at 25 °C of 29.80, which differs substantially from the current accepted value of 28.3. Use of previously published oxygen isotopic fractionation factors would yield a calcite precipitation temperature in Devils Hole that is 8 °C lower than the measured ground water temperature. Alternatively, previously published fractionation factors would yield a δ¹⁸O of water, from which the calcite precipitated, that is too negative by 1.5 ‰ using a temperature of 33.7 °C. Several lines of evidence indicate that the geochemical environment of Devils Hole has been remarkably constant for at least 10 ka. Accordingly, a re-evaluation of calcite-water oxygen isotopic fractionation factor may be in order.Assuming the Devils Hole oxygen isotopic value of α_{calcite-water} represents thermodynamic equilibrium, many marine carbonates are precipitated with a δ¹⁸O value that is too low, apparently due to a kinetic isotopic fractionation that preferentially enriches ¹⁶O in the solid carbonate over ¹⁸O, feigning oxygen isotopic equilibrium.     

Oxygen isotope exchange between H2O and CO2 at elevated CO2 pressures: Implications for monitoring of geological CO2 storage

Traditionally, the application of stable isotopes in Carbon Capture and Storage (CCS) projects has focused on δ¹³C values of CO₂ to trace the migration of injected CO₂ in the subsurface. More recently the use of δ¹⁸O values of both CO₂ and reservoir fluids has been proposed as a method for quantifying in situ CO₂ reservoir saturations due to O isotope exchange between CO₂ and H₂O and subsequent changes in δ¹⁸O_H2O values in the presence of high concentrations of CO₂. To verify that O isotope exchange between CO₂ and H₂O reaches equilibrium within days, and that δ¹⁸O_H2O values indeed change predictably due to the presence of CO₂, a laboratory study was conducted during which the isotope composition of H₂O, CO₂, and dissolved inorganic C (DIC) was determined at representative reservoir conditions (50 °C and up to 19 MPa) and varying CO₂ pressures. Conditions typical for the Pembina Cardium CO₂ Monitoring Pilot in Alberta (Canada) were chosen for the experiments. Results obtained showed that δ¹⁸O values of CO₂ were on average 36.4 ± 2.2‰ (1σ, n = 15) higher than those of water at all pressures up to and including reservoir pressure (19 MPa), in excellent agreement with the theoretically predicted isotope enrichment factor of 35.5‰ for the experimental temperatures of 50 °C. By using ¹⁸O enriched water for the experiments it was demonstrated that changes in the δ¹⁸O values of water were predictably related to the fraction of O in the system sourced from CO₂ in excellent agreement with theoretical predictions. Since the fraction of O sourced from CO₂ is related to the total volumetric saturation of CO₂ and water as a fraction of the total volume of the system, it is concluded that changes in δ¹⁸O values of reservoir fluids can be used to calculate reservoir saturations of CO₂ in CCS settings given that the δ¹⁸O values of CO₂ and water are sufficiently distinct.     

Determining the O/O ratio of water using a water-CO2 equilibration method: Application to glacial-interglacial changes in O-excess from the Dome Fuji ice core, Antarctica

Recent studies show that oxygen three isotope measurement (¹⁶O, ¹⁷O, and ¹⁸O) of water provides additional information for investigating the hydrological cycle and paleoclimate. For determining the ¹⁸O/¹⁶O value of water, a conventional CO₂-water equilibration method involves measurement of the ratios of CO₂ isotopologues which were equilibrated with water. However, this long-established technique was not intended to measure the ¹⁷O/¹⁶O ratio, primarily because the historic ion correction scheme does not allow for possible deviations from a fixed (and mass-dependent) relationship between ¹⁷O/¹⁶O and ¹⁸O/¹⁶O isotope ratios. Here, we propose an improved method for obtaining the ¹⁷O/¹⁶O isotope ratio of fresh water by the equilibration method and measurement of the 45/44 CO₂ ion abundance ratio. Equations which we formulated for ¹⁷O/¹⁶O measurement have two features: first, instead of absolute isotope ratio (R), all equations are formulated in δ values, measured by isotope ratio mass spectrometry. Second, we include two “assigned” δ values of water standards in the equations, because the δ¹⁸O are commonly measured against two working standards to normalize the span of the δ scale. This approach clarifies that the contribution from ¹⁷O (¹²C¹⁶O¹⁷O⁺) to the molecular ion current at mass-to-charge ratio m/z 45 signal depends not on the absolute ¹³C/¹²C ratio, but on the relative δ¹³C differences between the working standards and the sample. The pH value of water affects δ¹⁷O estimation because δ¹³C of CO₂ was changed in the water-CO₂ system. We reevaluated this effect using a set of equations, which explicitly includes CO₂ partial pressure effect on pH value. Our new estimation of pH effect is significantly smaller than previously reported value, but it does not alter the main conclusions in the previous study. The method was verified by δ¹⁷O measurements of an international standard reference water (GISP) provided by the IAEA. We applied the method to investigate ¹⁷O-excess of the ice core drilled at the Dome Fuji station, Antarctica. A total of 1320 samples from a 130 m section around Marine Isotope Stage 9.3 (∼330,000 years before present) were measured. The error of a measurement for δ¹⁷O is 0.175‰ and that of ¹⁷O-excess is 184 per meg. Although these analytical uncertainties hampered accurate estimation of the changes in ¹⁷O-excess, the averaged data indicate that ¹⁷O-excess around MIS 9.3 was higher than during the subsequent glacial period. This approach can be applied only to fresh water samples, and additional improvements will be needed to measure samples which contains significant amount of carbonate minerals.     

Biogeochemical implications of the isotopic equilibrium fractionation factor between the oxygen atoms of acetone and water

Carbonyl oxygens of organic molecules undergo isotopic exchange with water during reversible hydration reactions. The equilibrium isotopic fractionation factors between the carbonyl oxygen of acetone and water at 15°, 25°, and 35°C are 1.028, 1.028, and 1.026 respectively. The differences between the δ¹⁸O values of the carbonyl oxygen of acetone and of the water with which it is in equilibrium are similar to the differences that have been observed between the δ¹⁸O values of cellulose and the water used in its synthesis by a variety of aquatic plants and animals. Additionally, the identity of the acetone-water fractionation factors at 15° and 25°C parallels the observation that the difference between the δ¹⁸O values of cellulose and water shows no temperature dependence for individual species of plants grown over the same temperature range. These results are discussed in relation to the proposal that the oxygen isotopic relationship between cellulose and water is established by isotopic exchange occurring during the hydration of carbonyl groups of the intermediates of cellulose synthesis.     

Rare sulfur and triple oxygen isotope geochemistry of volcanogenic sulfate aerosols

We present analyses of stable isotopic ratios ¹⁷O/¹⁶O, ¹⁸O/¹⁶O, ³⁴S/³²S, and ³³S/³²S, ³⁶S/³²S in sulfate leached from volcanic ash of a series of well known, large and small volcanic eruptions. We consider eruptions of Mt. St. Helens (Washington, 1980, ∼1 km³), Mt. Spurr (Alaska, 1953, <1 km³), Gjalp (Iceland, 1996, 1998, <1 km³), Pinatubo (Phillipines, 1991, 10 km³), Bishop tuff (Long Valley, California, 0.76 Ma, 750 km³), Lower Bandelier tuff (Toledo Caldera, New Mexico, 1.61 Ma, 600 km³), and Lava Creek and Huckleberry Ridge tuffs (Yellowstone, Wyoming, 0.64 Ma, 1000 km³ and 2.04 Ma 2500 km³, respectively). This list covers much of the diversity of sizes and the character of silicic volcanic eruptions. Particular emphasis is paid to the Lava Creek tuff for which we present wide geographic sample coverage.This global dataset spans a significant range in δ³⁴S, δ¹⁸O, and Δ¹⁷O of sulfate (29‰, 30‰, and 3.3‰, respectively) with oxygen isotopes recording mass-independent (Δ¹⁷O > 0.2‰) and sulfur isotopes exhibiting mass-dependent behavior. Products of large eruptions account for most of‘ these isotopic ranges. Sulfate with Δ¹⁷O > 0.2‰ is present as 1-10 μm gypsum crystals on distal ash particles and records the isotopic signature of stratospheric photochemical reactions. Sediments that embed ash layers do not contain sulfate or contain little sulfate with Δ¹⁷O near 0‰, suggesting that the observed sulfate in ash is of volcanic origin.Mass-dependent fractionation of sulfur isotopic ratios suggests that sulfate-forming reactions did not involve photolysis of SO₂, like that inferred for pre-2.3 Ga sulfates from Archean sediments or Antarctic ice-core sulfate associated with few dated eruptions. Even though the sulfate sulfur isotopic compositions reflect mass-dependent processes, the products of caldera-forming eruptions display a large δ³⁴S range and exhibit fractionation relationships that do not follow the expected equilibrium slopes of 0.515 and 1.90 for ³³S/³²S vs. ³⁴S/³²S and ³⁶S/³²S vs. ³⁴S/³²S, respectively. The data presented here are consistent with modification of a chemical mass-dependent fractionation of sulfur isotopes in the volcanic plume by either a kinetic gas phase reaction of volcanic SO₂ with OH and/or a Rayleigh processes involving a residual Rayleigh reactant—volcanic SO₂ gas, rather than a Rayleigh product. These results may also imply at least two removal pathways for SO₂ in volcanic plumes.Above-zero Δ¹⁷O values and their positive correlation with δ¹⁸O in sulfate can be explained by oxidation by high-δ¹⁸O and high-Δ¹⁷O compounds such as ozone and radicals such as OH that result from ozone break down. Large caldera-forming eruptions have the highest Δ¹⁷O values, and the largest range of δ¹⁸O, which can be explained by stratospheric reaction with ozone-derived OH radicals. These results suggest that massive eruptions are capable of causing a temporary depletion of the ozone layer. Such depletion may be many times that of the measured 3-8% depletion following 1991 Pinatubo eruption, if the amount of sulfur dioxide released scales with the amount of ozone depletion.     

Stable isotopic and elemental characteristics of recent tufa from a karstic Krka River (south‐east Slovenia): useful environmental proxies?

Tufa samples from 16 consecutive barrages along a 13 km section of the groundwater‐fed Krka River (Slovenia) were analysed for their petrographical, mineralogical, elemental and stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope composition, to establish their relation to current climatic and hydrological conditions. Waters constantly oversaturated with calcite and the steep morphology of the Krka riverbed stimulate rapid CO₂ degassing and subsequent tufa precipitation. The carbon isotope fractionation (Δ¹³C) between dissolved inorganic carbon and tufa in the Krka River evolves towards isotopic equilibrium being controlled by continuous CO₂ degassing and tufa precipitation rate downstream. The Δ¹³C increased from 1·9 to 2·5‰ (VPDB); however, since tufa precipitation rates remain similar downstream, the major controlling factor of carbon isotope exchange is most probably related to the continuous ¹²CO₂ degassing downstream leaving the carbon pool enriched in ¹³C. In the case of oxygen, the isotope fractionation (Δ¹⁸O) was found to be from 1·0 to 2·3‰ (VSMOW) smaller than reported in the literature. The observed discrepancies are due to different precipitation rates of calcite deposits because Krka tufas on cascades grow relatively faster compared to slowly precipitated calcite deposits in cave or stream pools. Due to non‐equilibrium oxygen isotope exchange between Krka tufa and water, the δ¹⁸O proxy showed from 1·2 to 8·2°C higher calculated water temperatures compared to measured water temperatures, demonstrating that δ¹⁸O proxy‐based temperature equations are not reliable for water temperature calculations of fast‐growing tufa on cascades. Because Mg is bound to the terrigenous dolomite fraction in the Krka tufa samples, the Mg/Ca was also found to be an unreliable temperature proxy yielding over up to 20°C higher calculated water temperatures.     

Terrestrial alteration of carbonate in a suite of Antarctic CM chondrites: Evidence from oxygen and carbon isotopes

The oxygen (δ¹⁸O, δ¹⁷O) and carbon (δ¹³C, FMOD¹⁴C-the fraction of modern ¹⁴C) isotopic compositions of carbonate were measured for a set of paired Antarctic CM chondrites (EET 96006, EET 96016, EET 96017, and EET 96019). While the oxygen isotopic compositions do not plot on the terrestrial fractionation line and indicate that a component of the carbonate minerals has an extraterrestrial origin, they also do not fall on the array defined for carbonates by CM falls and are thus consistent with the presence of a terrestrial carbonate component. The δ¹³C and FMOD¹⁴C measurements of carbonate suggest the presence of at least two carbon sources: carbonate derived from atmospheric CO₂ that is inferred to have been produced as a result of silicate weathering reactions and carbonate derived from another carbon source that is either old or non-atmospheric. The relationships between oxygen and carbon isotope data provide additional constraints on the weathering process, and allow the possibility that rock-dominated weathering of the meteorite caused the oxygen isotopic composition of Antarctic water added to the meteorite to evolve away from the terrestrial mass-fractionation array, leading to formation of low temperature terrestrial alteration products that do not lie on the terrestrial fractionation line.     

Modelling δC and δO in the solution layer on stalagmite surfaces

We derive equations describing the evolution of the carbon and oxygen isotope composition of the bicarbonate in a calcite precipitating solution on the surface of a stalagmite using a classical Rayleigh approach. The combined effects of calcite precipitation, degassing of CO₂ and the buffering effect of the water reservoir are taken into account. Whereas δ¹³C shows a progressive increase to a final constant value, δ¹⁸O shows an initial isotopic enrichment, which exponentially decays due to the buffering effect of the water reservoir. The calculated evolution is significantly different for both carbon and oxygen isotopes than derived in a recent paper [Dreybrodt W. (2008) Evolution of the isotopic composition of carbon and oxygen in a calcite precipitating H₂O-CO₂-CaCO₃ solution and the related isotopic composition of calcite in stalagmites. Geochim. Cosmochim. Acta72, 4712-4724.].Furthermore, we discuss the isotopic evolution of the bicarbonate in the solution for long residence times on the stalagmite surface, i.e., for t→∞. The equilibrium isotope ratio of the bicarbonate is then determined by isotopic exchange between the cave atmosphere and the bicarbonate in the solution and can be calculated by equilibrium isotope fractionation. For strongly ventilated caves exchange with the cave atmosphere will result in higher δ¹³C and δ¹⁸O values than those observed in a pure Rayleigh distillation scenario, for sparsely ventilated caves it will result in lower δ¹³C and δ¹⁸O values.     

Modelling fractionation of stable isotopes in stalagmites

High resolution δ¹³C and δ¹⁸O profiles recorded in precisely dated speleothems are widely used proxies for the climate of the past. Both δ¹³C and δ¹⁸O depend on several climate related effects including meteorological processes, processes occurring in the soil zone above the cave and isotope fractionation processes occurring in the solution layer on the stalagmite surface. Here we model the latter using a stalagmite isotope and growth model and determine the relationship between the stable isotope values in speleothem calcite and cave parameters, such as temperature, drip interval, water p_CO2 and a mixing coefficient describing mixing processes between the solution layer and the impinging drop.The evolution of δ¹³C values is modelled as a Rayleigh distillation process and shows a pronounced dependence on the residence time of the solution on the stalagmite surface and the drip interval, respectively. The evolution of δ¹⁸O values, in contrast, is also influenced by buffering reactions between the bicarbonate in the solution and the drip water driving the δ¹⁸O value of the bicarbonate towards the value expected for equilibrium isotope fractionation between drip water and calcite. This attenuates the dependence of the δ¹⁸O values on drip interval. The temperature dependence of δ¹⁸O, however, is more pronounced than for δ¹³C and in a similar range as expected for fractionation under equilibrium conditions.We also investigate the isotopic enrichment of the δ¹³C and δ¹⁸O values along individual growth layers and, thus, the slopes expected for Hendy tests. The results show that a positive Hendy test is only possible if isotope fractionation occurred under disequilibrium conditions. However, a negative Hendy test does not exclude that isotope fractionation occurred under disequilibrium conditions. A more reliable indicator for disequilibrium fractionation is the enrichment of the δ¹³C values along an individual growth layer.     

Carbon and oxygen isotope study of the active water-carbonate system in a karstic Mediterranean cave: Implications for paleoclimate research in semiarid regions

In a semiarid climatic zone, such as the Eastern Mediterranean region, annual rainfall variations and fractionation processes in the epikarst zone exert a profound influence on the isotopic compositions of waters seeping into a cave. Consequently, the isotopic compositions of speleothems depositing from cave waters may show complex variations that need to be understood if they are to be exploited for paleoclimate studies. This is confirmed by a four-year study of the active carbonate-water system in the Soreq cave (Israel). The δ¹⁸O (SMOW) values of cave waters range from −6.3 to −3.5%.. The highest δ¹⁸O values occur at the end of the dry season in waters dripping from stalactites, and reflect evaporation processes in the epikarst zone, whereas the lowest values occur in rapidly dripping (fast-drip) waters at the peak of the rainy seasons. However, even fast-drip waters are about 1.5%. heavier than the rainfall above the cave, which is taken to reflect the mixing of fresh with residual evaporated water in the epikarst zone. δ¹³C (PDB) values of dissolved inorganic carbon (DIC) vary from −15.6 to −5.4%., with fast-drip waters having lower δ¹³C values (mostly −15.6 to −12%.) and higher DIC concentrations relative to pool and stalactite-drip water. The low δ¹³C values of fast-drip waters and their supersaturation with respect to calcium carbonate indicates that the seepage waters have dissolved both soil-CO₂ derived from overlying C₃-type vegetation and marine dolomite host rock.The δ¹⁸O (PDB) values of various types of present-day low-magnesium calcite (LMC) speleothems range from −6.5 to −4.3%. and δ¹³C values from −13 to −5.5%. and are not correlated with speleothem type. An analysis of δ¹⁸O values of present-day calcite rafts and pool waters shows that they form in oxygen isotope equilibrium. Similarly, the measured ranges of δ¹³C and δ¹⁸O values for all types of present-day speleothems are consistent with equilibrium deposition at cave temperatures. The δ¹³C–δ¹⁸O range of contemporary LMC thus reflects the variations in temperatures and isotopic compositions of the presentday cave waters. The 10%. variation in the δ¹³C values in waters can be modeled by a simple Rayleigh calculation of the carbon isotope fractionation accompanying CO₂-degassing and carbonate precipitation. These variations may obscure the differences in the carbon isotopic composition of speleothems that could arise when vegetation cover changes from C₃ to C₄-type plants. This consideration emphasizes that it is necessary to characterize the full range of δ¹³C values associated with contemporaneous speleothems in order to clarify the effects of degassing from those due to differing vegetation types.Isotopic studies of a number of different types of fossil LMC speleothems show many of them to exhibit isotopic trends that are similar to those of present-day LMC, but others show both higher and lower δ¹⁸O ranges. In particular, the higher δ¹⁸O range has been shown by independent age-measurements to be associated with a period of drier conditions. The results of the study thus indicate that it is necessary to work on a well calibrated cave system in semiarid climates and that the fossil speleothem record should be obtained from different types of contemporaneous deposit in order to fully characterize the δ¹⁸O–δ¹³C range representative of any given climatic period.     

Oxygen isotopic fractionation between oxygen of different sites in hydroxyl-bearing silicate minerals

A method has been developed for extraction of hydroxyl oxygen from hydroxyl-bearing silicate minerals for oxygen isotopic analysis.The δO¹⁸ of oxygen of the OH groups is significantly different from that in the rest of the mineral structure. The isotopic fractionation between the two types of sites has the potential to be a sensitive geothermometer.Several δO¹⁸ values were obtained for oxygen of the OH attached to different silicate structures as well as for two muscovite samples with quantitatively estimated different temperatures of formation.The 1000 ln α (mineral-OH) values ranged from 5.2%. for muscovite to about 12.6%. for kaolinite and chlorite.     

An explanation of the effect of seawater carbonate concentration on foraminiferal oxygen isotopes

Stable oxygen isotope ratios of foraminiferal calcite are widely used in paleoceanography to provide a chronology of temperature changes during ocean history. It was recently demonstrated that the stable oxygen isotope ratios in planktonic foraminifera are affected by changes of the seawater chemistry carbonate system: the δ¹⁸O of the foraminiferal calcite decreases with increasing CO₃²⁻ concentration or pH. This paper provides a simple explanation for seawater chemistry dependent stable oxygen isotope variations in the planktonic foraminifera Orbulina universa which is derived from oxygen isotope partitioning during inorganic precipitation. The oxygen isotope fractionation between water and the dissolved carbonate species S = [H₂CO₃] + [HCO₃⁻] + [CO₃²⁻] decreases with increasing pH. Provided that calcium carbonate is formed from a mixture of the carbonate species in proportion to their relative contribution to S, the oxygen isotopic composition of CaCO₃ also decreases with increasing pH. The slope of shell δ¹⁸O vs. [CO₃²⁻] of Orbulina universa observed in culture experiments is −0.0022‰ (μmol kg⁻¹)⁻¹ (Spero et al., 1997), whereas the slope derived from inorganic precipitation is −0.0024‰ (μmol kg⁻¹)⁻. The theory also provides an explanation of the nonequilibrium fractionation effects in synthetic carbonates described by Kim and O’Neil (1997) which can be understood in terms of equilibrium fractionation at different pH. The results presented here emphasize that the oxygen isotope fractionation between calcium carbonate and water does not only depend on the temperature but also on the pH of the solution from which it is formed.     

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.     

Calculation of individual isotope equilibrium constants for geochemical reactions

Theory is derived from the work of Urey (Urey H. C. [1947] The thermodynamic properties of isotopic substances. J. Chem. Soc. 562-581) to calculate equilibrium constants commonly used in geochemical equilibrium and reaction-transport models for reactions of individual isotopic species. Urey showed that equilibrium constants of isotope exchange reactions for molecules that contain two or more atoms of the same element in equivalent positions are related to isotope fractionation factors by α = (K^ex)^1/n, where n is the number of atoms exchanged. This relation is extended to include species containing multiple isotopes, for example ¹³C¹⁶O¹⁸O and ¹H²H¹⁸O. The equilibrium constants of the isotope exchange reactions can be expressed as ratios of individual isotope equilibrium constants for geochemical reactions. Knowledge of the equilibrium constant for the dominant isotopic species can then be used to calculate the individual isotope equilibrium constants.Individual isotope equilibrium constants are calculated for the reaction CO_2g = CO_2aq for all species that can be formed from ¹²C, ¹³C, ¹⁶O, and ¹⁸O; for the reaction between ¹²C¹⁸O_2aq and ¹H₂¹⁸O_l; and among the various ¹H, ²H, ¹⁶O, and ¹⁸O species of H₂O. This is a subset of a larger number of equilibrium constants calculated elsewhere (Thorstenson D. C. and Parkhurst D. L. [2002] Calculation of individual isotope equilibrium constants for implementation in geochemical models. Water-Resources Investigation Report 02-4172. U.S. Geological Survey). Activity coefficients, activity-concentration conventions for the isotopic variants of H₂O in the solvent ¹H₂¹⁶O_l, and salt effects on isotope fractionation have been included in the derivations. The effects of nonideality are small because of the chemical similarity of different isotopic species of the same molecule or ion. The temperature dependence of the individual isotope equilibrium constants can be calculated from the temperature dependence of the fractionation factors.The derivations can be extended to calculation of individual isotope equilibrium constants for ion pairs and equilibrium constants for isotopic species of other chemical elements. The individual isotope approach calculates the same phase isotopic compositions as existing methods, but also provides concentrations of individual species, which are needed in calculations of mass-dependent effects in transport processes. The equilibrium constants derived in this paper are used to calculate the example of gas-water equilibrium for CO₂ in an acidic aqueous solution.     

Intra-annual perturbations of stable isotopes in tufas: Effects of hydrological processes

Tufas, which are freshwater carbonates, are potential archives of terrestrial paleoclimate. Time series of stable isotopic compositions commonly show regular seasonal patterns controlled by temperature-dependent processes, and some perturbation intrinsic to the locality. We examined three tufa-depositing sites in southwestern Japan with similar temperate climates, to understand the origin of local characteristics in the isotopic records. Seasonal change in the oxygen isotope is principally reflected by temperature-dependent fractionation between water and calcite but was perturbed after heavy rainfalls overwhelming the stability of the δ¹⁸O value of the groundwater at one site. Isotopic mass balance indicates an undersaturated and relatively small aquifer at this locality. Water δ¹⁸O values at the other two sites were stable, reflecting a regular seasonal change in the δ¹⁸O value of tufa. Perturbation of the δ¹³C values in tufa is largely due to CO₂ degassing from the stream, which significantly increases the δ¹³C values of dissolved inorganic carbon (DIC). At a site with remarkably high pCO₂ in springwater and a sensitive response of flow rate to rainfall, the amount of CO₂ degassing changed distinctly with flow rate. In contrast, the other two sites having low pCO₂ springwater reflect a regular seasonal pattern of δ¹³C in DIC and tufa specimens.     

Laboratory chalcopyrite oxidation by Acidithiobacillus ferrooxidans: Oxygen and sulfur isotope fractionation

Laboratory experiments were conducted to simulate chalcopyrite oxidation under anaerobic and aerobic conditions in the absence or presence of the bacterium Acidithiobacillus ferrooxidans. Experiments were carried out with 3 different oxygen isotope values of water (δ¹⁸O_H2O) so that approach to equilibrium or steady-state isotope fractionation for different starting conditions could be evaluated. The contribution of dissolved O₂ and water-derived oxygen to dissolved sulfate formed by chalcopyrite oxidation was unambiguously resolved during the aerobic experiments. Aerobic oxidation of chalcopyrite showed 93 ± 1% incorporation of water oxygen into the resulting sulfate during the biological experiments. Anaerobic experiments showed similar percentages of water oxygen incorporation into sulfate, but were more variable. The experiments also allowed determination of sulfate–water oxygen isotope fractionation, ε¹⁸O_SO4–H2O, of ~ 3.8‰ for the anaerobic experiments. Aerobic oxidation produced apparent ε_SO4–H2O values (6.4‰) higher than the anaerobic experiments, possibly due to additional incorporation of dissolved O₂ into sulfate. δ³⁴S_SO4 values are ~ 4‰ lower than the parent sulfide mineral during anaerobic oxidation of chalcopyrite, with no significant difference between abiotic and biological processes. For the aerobic experiments, a small depletion in δ³⁴S_SO4 of ~? 1.5 ± 0.2‰ was observed for the biological experiments. Fewer solids precipitated during oxidation under aerobic conditions than under anaerobic conditions, which may account for the observed differences in sulfur isotope fractionation under these contrasting conditions.