Oxygen isotope fractionation between synthetic aragonite and water: Influence of temperature and Mg concentration

Aragonite was precipitated in the laboratory at 0, 5, 10, 25, and 40 °C to determine the temperature dependence of the equilibrium oxygen isotope fractionation between aragonite and water. Forced CO₂ degassing, passive CO₂ degassing, and constant addition methods were employed to precipitate aragonite from supersaturated solutions, but the resulting aragonite-water oxygen isotope fractionation was independent of the precipitation method. In addition, under the experimental conditions of this study, the effect of precipitation rate on the oxygen isotope fractionation between aragonite and water was almost within the analytical error of ±∼0.13‰ and thus insignificant. Because the presence of Mg²⁺ ions is required to nucleate and precipitate aragonite from Na-Ca-Cl-HCO₃ solutions under these experimental conditions, the influence of the total Mg²⁺ concentration (up to ∼0.9 molal) on the aragonite-water oxygen isotope fractionation was examined at 25 °C. No significant Mg²⁺ ion effect, or oxygen isotope salt effect, was detected up to 100 mmolal total Mg²⁺ but a noticeable isotope salt effect was observed at ∼0.9 molal total Mg²⁺.On the basis of results of the laboratory synthesis experiments, a new expression for the aragonite-water fractionation is proposed over the temperature range of 0-40 °C:

1000lnα_{aragonite-water}=17.88±0.13(10³/T)-31.14±0.46     

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.     

Experimental studies of oxygen isotope fractionation in the carbonic acid system at 15°, 25°, and 40°C

In light of recent studies that show oxygen isotope fractionation in carbonate minerals to be a function of HCO₃⁻ and CO₃²⁻ concentrations, the oxygen isotope fractionation and exchange between water and components of the carbonic acid system (HCO₃⁻, CO₃²⁻, and CO_2(aq)) were investigated at 15°, 25°, and 40°C. To investigate oxygen isotope exchange between HCO₃⁻, CO₃²⁻, and H₂O, NaHCO₃ solutions were prepared and the pH was adjusted over a range of 2 to 12 by the addition of small amounts of HCl or NaOH. After thermal, chemical, and isotopic equilibrium was attained, BaCl₂ was added to the NaHCO₃ solutions. This resulted in immediate BaCO₃ precipitation; thus, recording the isotopic composition of the dissolved inorganic carbon (DIC). Data from experiments at 15°, 25°, and 40°C (1 atm) show that the oxygen isotope fractionation between HCO₃⁻ and H₂O as a function of temperature is governed by the equation:

     

Phosphoric acid fractionation factors for smithsonite and cerussite between 25 and 72°C

The intramolecular kinetic oxygen isotope fractionation between CO₂ and CO₃²⁻ during reaction of phosphoric acid with natural smithsonite (ZnCO₃) and cerussite (PbCO₃) has been determined between 25 and 72°C. While cerussite decomposes in phosphoric acid within a few hours at 25°C, smithsonite reacts very slowly with the acid at 25°C providing yields of CO₂ < 25% after 2 weeks. The low yields result in a low precision for oxygen isotope measurements of the acid-liberated CO₂ (±1.65‰, 1σ, n = 9). The yield and reproducibility of oxygen isotope values of the acid-liberated CO₂ from smithsonite can be improved, the latter to ∼±0.15‰, by increasing the reaction temperature to 50°C for 12 h or to 72°C for 1 h. Our new phosphoric acid fractionation factor for natural cerussite at 25°C deviates significantly from a previously published value on synthetic material. The temperature dependence of the oxygen isotope factionation factor, α between acid-liberated CO₂ and carbonate at 25 to 72°C is given by the following equations

     

Kinetic mechanism of oxygen isotope disequilibrium in precipitated witherite and aragonite at low temperatures: an experimental study

To study what dictates oxygen isotope equilibrium fractionation between inorganic carbonate and water during carbonate precipitation from aqueous solutions, a direct precipitation approach was used to synthesize witherite, and an overgrowth technique was used to synthesize aragonite. The experiments were conducted at 50 and 70°C by one- and two-step approaches, respectively, with a difference in the time of oxygen isotope exchange between dissolved carbonate and water before carbonate precipitation. The two-step approach involved sufficient time to achieve oxygen isotope equilibrium between dissolved carbonate and water, whereas the one-step approach did not. The measured witherite-water fractionations are systematically lower than the aragonite-water fractionations regardless of exchange time between dissolved carbonate and water, pointing to cation effect on oxygen isotope partitioning between the barium and calcium carbonates when precipitating them from the solutions. The two-step approach experiments provide the equilibrium fractionations between the precipitated carbonates and water, whereas the one-step experiments do not. The present experiments show that approaching equilibrium oxygen isotope fractionation between precipitated carbonate and water proceeds via the following two processes:

1.

Oxygen isotope exchange between [CO₃]²⁻ and H₂O:

(1)     

An experimental study of oxygen isotope fractionation between inorganically precipitated aragonite and water at low temperatures

To determine oxygen isotope fractionation between aragonite and water, aragonite was slowly precipitated from Ca(HCO₃)₂ solution at 0 to 50°C in the presence of Mg²⁺ or SO₄²⁻. The phase compositions and morphologies of synthetic minerals were detected by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The effects of aragonite precipitation rate and excess dissolved CO₂ gas in the initial Ca(HCO₃)₂ solution on oxygen isotope fractionation between aragonite and water were investigated. For the CaCO₃ minerals slowly precipitated by the CaCO₃ or NaHCO₃ dissolution method at 0 to 50°C, the XRD and SEM analyses show that the rate of aragonite precipitation increased with temperature. Correspondingly, oxygen isotope fractionations between aragonite and water deviated progressively farther from equilibrium. Additionally, an excess of dissolved CO₂ gas in the initial Ca(HCO₃)₂ solution results in an increase in apparent oxygen isotope fractionations. As a consequence, the experimentally determined oxygen isotope fractionations at 50°C indicate disequilibrium, whereas the relatively lower fractionation values obtained at 0 and 25°C from the solution with less dissolved CO₂ gas and low precipitation rates indicate a closer approach to equilibrium. Combining the lower values at 0 and 25°C with previous data derived from a two-step overgrowth technique at 50 and 70°C, a fractionation equation for the aragonite-water system at 0 to 70°C is obtained as follows:

     

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.     

Mechanisms of equilibrium and kinetic oxygen isotope effects in synthetic aragonite at 25 °C

Aragonite was precipitated in the laboratory at 25 °C in isotopic equilibrium with Na-Ca-Mg-Cl-CO₃ solutions at two different pH values (i.e., pH = ∼8.2 and ∼10.8) by the constant addition method. On the basis of the oxygen isotope composition of the aragonite precipitates, it was demonstrated that the equilibrium aragonite-water fractionation factor is independent of the pH of the parent solution and equal to:

1000lnα_{(aragonite-H2O)}=29.12±0.09     

Reconstruction of seasonal temperature variability in the tropical Pacific Ocean from the shell of the scallop, Comptopallium radula

We investigated the oxygen isotope composition (δ¹⁸O) of shell striae from juvenile Comptopallium radula (Mollusca; Pectinidae) specimens collected live in New Caledonia. Bottom-water temperature and salinity were monitored in-situ throughout the study period. External shell striae form with a 2-day periodicity in this scallop, making it possible to estimate the date of precipitation for each calcite sample collected along a growth transect. The oxygen isotope composition of shell calcite (δ¹⁸O_{shell calcite}) measured at almost weekly resolution on calcite accreted between August 2002 and July 2003 accurately tracks bottom-water temperatures. A new empirical paleotemperature equation for this scallop species relates temperature and δ¹⁸O_{shell calcite}:

t(°C)=20.00(±0.61)-3.66(±0.39)×(δ¹⁸O_{shell calcite VPDB}-δ¹⁸O_{water VSMOW})     

Isotopic composition (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and genesis of carbonates from the famennian manganiferous formation of Pai-Khoi

The results of isotope-geochemical studies of carbonates of different mineral types from manganese and host rocks of the Famennian manganiferous formation of Pai-Khoi are reported. Kutnahorite ores are characterized by δ¹³C values from–6.6 to 1.3‰ and δ¹⁸O from 20.0 to 27.4‰. Rhodonite–rhodochrosite rocks of the Silovayakha ore occurrence have δ¹³C from–5.2 to–2.9 and δ¹⁸O from 25.4 to 24.3‰. Mineralogically similar rocks of the Nadeiyakha ore occurrence show the lighter carbon and oxygen isotopic compositions: δ¹³C from–16.4 to–13.1 and δ¹⁸O from 24.8 to 22.5‰. Similar isotopic compositions were also obtained for rhodochrosite–kutnahorite rocks of this ore occurrence: δ¹³C from–13.0 to–10.4‰ and δ¹⁸O from 24.6 to 21.7‰. Siderorodochrosite ores differ in the lighter oxygen and carbon isotopic compositions: δ¹⁸O from 18.7 to 17.6‰ and δ¹³C from–10.2 to–9.3‰, respectively. In terms of the carbon and oxygen isotopic compositions, host rocks in general correspond to marine sedimentary carbonates. Geological-mineralogical and isotope data indicate that the formation of the manganese carbonates was related to the hydrothermal ore-bearing fluids with the light isotopic composition of oxygen and carbon dissolved in CO₂. The isotopic features indicate an authigenic formation of manganese carbonates under different isotopegeochemical conditions.     

Oxygen isotopic fractionation during inorganic calcite precipitation ― Effects of temperature, precipitation rate and pH

Stable oxygen isotopic fractionation during inorganic calcite precipitation was experimentally studied by spontaneous precipitation at various pH (8.3 < pH < 10.5), precipitation rates (1.8 < log R < 4.4 μmol m^− 2 h^− 1) and temperatures (5, 25, and 40 °C) using the CO₂ diffusion technique.The results show that the apparent stable oxygen isotopic fractionation factor between calcite and water (α_{calcite–water}) is affected by temperature, the pH of the solution, and the precipitation rate of calcite. Isotopic equilibrium is not maintained during spontaneous precipitation from the solution. Under isotopic non-equilibrium conditions, at a constant temperature and precipitation rate, apparent 1000lnα_{calcite–water} decreases with increasing pH of the solution. If the temperature and pH are held constant, apparent 1000lnα_{calcite–water} values decrease with elevated precipitation rates of calcite. At pH = 8.3, oxygen isotopic fractionation between inorganically precipitated calcite and water as a function of the precipitation rate (R) can be described by the expressions

at 5, 25, and 40 °C, respectively.The impact of precipitation rate on 1000lnα_{calcite–water} value in our experiments clearly indicates a kinetic effect on oxygen isotopic fractionation during calcite precipitation from aqueous solution, even if calcite precipitated slowly from aqueous solution at the given temperature range. Our results support Coplen's work [Coplen T. B. (2007) Calibration of the calcite–water oxygen isotope geothermometer at Devils Hole, Nevada, a natural laboratory. Geochim. Cosmochim. Acta 71, 3948–3957], which indicates that the equilibrium oxygen isotopic fractionation factor might be greater than the commonly accepted value.     

An “On-Line” Method for Oxygen Isotope Exchange Between Gas-Phase CO<Subscript>2</Subscript> and Water

An “on-line” mixing system has been developed and evaluated for continuous oxygen isotope exchange between gas-phase CO₂ and liquid water. The system is composed of three basic parts: equipment and materials used to introduce water and gas into a mixing reservoir, the mixing and exchange reservoir, and a vessel used to separate gas and water phases exiting the system. A series of experiments were performed to monitor the isotope exchange process over a range of temperatures (5–40 °C) and CO₂ partial pressures (202–15,200 Pa). Isotopic exchange was evaluated using CO₂ having δ¹⁸O values of 30.4 and 37.8 ‰ and waters of two distinct oxygen isotope compositions (?6.5 to ?5 and 6 to 7.5 ‰). Isotope ratios were determined by isotope ratio mass spectrometry and cavity ring-down spectroscopy. CO₂ did not reach oxygen isotope equilibrium under the conditions described here. However, oxygen isotope exchange rate constants were determined at different temperatures and regressed to yield the expression k (h^?1) = 0.020 × T (°C) + 0.28. Using this expression, the residence time required to reach oxygen isotope equilibrium may be estimated for a given set of environmental conditions (e.g., δ¹⁸O value of water, temperature). System parameters can be modified to achieve a specific δ¹⁸O value for CO₂. Consequently, the exchange system described here has the ability to deliver a constant flow of CO₂ at a desired oxygen isotope composition. This ability is attractive for a variety of applications such as experiments that utilize flow-through reactors and environmental chambers or require static chemical conditions.     

Hydrogen isotope fractionation by Methanothermobacter thermoautotrophicus in coculture and pure culture conditions

We grew a hydrogen-utilizing methanogen, Methanothermobacter thermoautotrophicus strain ΔH, in coculture and pure culture conditions to evaluate the hydrogen isotope fractionation associated with carbonate reduction under low (< several tens of μM; coculture) and high (>6 mM; pure culture) concentrations of H₂ in the headspace. In the cocultures, which were grown at 55 °C with a thermophilic butyrate-oxidizing syntroph, the hydrogen isotopic relationship between methane and water was well represented by the following equation:

δD_CH4=0.725(±0.003)·δD_H2O-275(±3),     

Oxygen and carbon isotopic systematics of aragonite speleothems and water in Furong Cave, Chongqing, China

To understand oxygen and carbon stable isotopic characteristics of aragonite stalagmites and evaluate their applicability to paleoclimate, the isotopic compositions of active and fossil aragonite speleothems and water samples from an in situ multi-year (October 2005-July 2010) monitoring program in Furong Cave located in Chongqing of China have been examined. The observations during October 2005-June 2007 show that the meteoric water is well mixed in the overlying 300-500-m bedrock aquifer, reflected by relatively constant δ¹⁸O, ±0.11-0.14‰ (1σ), of drip waters in the cave, which represents the annual status of rainfall water. Active cave aragonite speleothems are at oxygen isotopic equilibrium with drip water and their δ¹⁸O values capture the surface-water oxygen isotopic signal. Aragonite-to-calcite transformation since the last glaciation is not noticeable in Furong stalagmites. Our multi-year field experiment approves that aragonite stalagmite δ¹⁸O records in this cave are suitable for paleoclimate reconstruction. With high U, 0.5-7.2 ppm, and low Th, 20-1270 ppt, the Furong aragonite stalagmites provide very precise chronology (as good as ±20s yrs (2σ)) of the climatic variations since the last deglaciation. The synchroneity of Chinese stalagmite δ¹⁸O records at the transition into the Bølling-Allerød (t-BA) and the Younger Dryas from Furong, Hulu and Dongge Caves supports the fidelity of the reconstructed East Asian monsoon evolution. However, the Furong record shows that the cold Older Dryas (OD) occurred at 14.0 thousand years ago, agreeing with Greenland ice core δ¹⁸O records but ∼200 yrs younger than that in the Hulu record. The OD age discrepancy between Chinese caves can be attributable to different regionally climatic/environmental conditions or chronological uncertainty of stalagmite proxy records, which is limited by changes in growth rate and subsampling intervals in absolute dating. Seasonal dissolved inorganic carbon δ¹³C variations of 2-3‰ in the drip water and 5-7‰ in the pool and spring waters are likely attributed to variable degrees of CO₂ degassing in winter and summer. The variable δ¹³C values of active deposits from −11‰ to 0‰ could be caused by kinetically mediated CO₂ degassing processes. The complicated nature of pre-deposition kinetic isotopic fractionation processes for carbon isotopes in speleothems at Furong Cave require further study before they can be interpreted in a paleoclimatic or paleoenvironmental context.     

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.     

The transfer of oxygen isotopic signals from precipitation to drip water and modern calcite on the seasonal time scale in Yongxing Cave,central China

Stable isotope data of precipitation (δ¹⁸O_p and deuterium excess), drip water (δ¹⁸O_d), and modern calcite precipitates (δ¹⁸O_c and δ¹³C_c) from Yongxing Cave, central China, are presented, with monthly sampling intervals from June 2013 to September 2016. Moderate correlations between the monthly variation of δ¹⁸O_p values (from ??11.5 to ??0.7‰) and precipitation amount (r = ??0.59, n?=?34, p?<?0.01) and deuterium excess (r?=?0.39, n?=?31, p?<?0.01) imply a combined effect of changes in precipitation amount and atmospheric circulation. At five drip sites, the δ¹⁸O_d values have a much smaller variability (from ??9.1 to ??7.5‰), without seasonal signals, probably a consequence of the mixing in the karst reservoir with a deep aquifer. The mean δ¹⁸O_d value (??8.4‰) for all drip waters is significantly more negative than the mean δ¹⁸O_p value (??6.9‰) weighted by precipitation amount, but close to the wet season (May to September) mean value (??8.3‰), suggesting that a threshold of precipitation amount must be exceeded to provide recharge. Calculation based on the equilibrium fractionation factor indicates that the δ¹⁸O_c values are not in isotopic equilibrium with their corresponding drip waters, with a range of disequilibrium effects from 0.4 to 1.4‰. The δ¹⁸O_c and δ¹³C_c values generally increase progressively away from the locus of precipitation on glass plates. The disequilibrium effects in the cave are likely caused by progressive calcite precipitation and CO₂ degassing related to a high gradient of CO₂ concentration between drip waters and cave air. Our study provides an important reference to interpret δ¹⁸O_c records from the monsoon region of China.     

A general algorithm for the O abundance correction to C/C determinations from CO2 isotopologue measurements, including CO2 characterised by ‘mass-independent’ oxygen isotope distributions

It is widely recognised that a significant limitation to the ultimate precision of carbon stable isotope ratio measurements, as obtained from dual-inlet mass spectrometric measurements of CO₂ isotopologue ion abundances at m/z 44, 45, and 46, is the correction for interference from ¹⁷O-bearing molecular ions. Two long-established, alternative procedures for determining the magnitude of this correction are in widespread use (although only one has IAEA approval); their differences lead to small but potentially significant discrepancies in the magnitude of the resulting correction. Furthermore, neither approach was designed to accommodate oxygen three-isotope distributions which do not conform to terrestrial mass-dependent behaviour. Stratospheric CO₂, for example, contains a strongly ‘mass-independent’ oxygen isotope composition. A new strategy for determining the ¹⁷O-bearing ion correction is presented, for application where the oxygen three-isotope characteristics of the analyte CO₂ are accurately known (or assigned) in terms of the slope λ of the three-isotope fractionation line and the ordinate axis intercept 10³ ln(1 + k) on a 10³ ln(1 + δ¹⁷O) versus 10³ ln(1 + δ¹⁸O) plot. At the heart of the approach is the relationship between ¹⁷R, which is the ¹⁷O/¹⁶O ratio of the sample CO₂, and other assigned or empirically determined parameters needed for the δ¹³C evaluation:

     

Iron isotope fractionation and atom exchange during sorption of ferrous iron to mineral surfaces

The application of stable Fe isotopes as a tracer of the biogeochemical Fe cycle necessitates a mechanistic knowledge of natural fractionation processes. We studied the equilibrium Fe isotope fractionation upon sorption of Fe(II) to aluminum oxide (γ-Al₂O₃), goethite (α-FeOOH), quartz (α-SiO₂), and goethite-loaded quartz in batch experiments, and performed continuous-flow column experiments to study the extent of equilibrium and kinetic Fe isotope fractionation during reactive transport of Fe(II) through pure and goethite-loaded quartz sand. In addition, batch and column experiments were used to quantify the coupled electron transfer-atom exchange between dissolved Fe(II) (Fe(II)_aq) and structural Fe(III) of goethite. All experiments were conducted under strictly anoxic conditions at pH 7.2 in 20 mM MOPS (3-(N-morpholino)-propanesulfonic acid) buffer and 23 °C. Iron isotope ratios were measured by high-resolution MC-ICP-MS. Isotope data were analyzed with isotope fractionation models. In batch systems, we observed significant Fe isotope fractionation upon equilibrium sorption of Fe(II) to all sorbents tested, except for aluminum oxide. The equilibrium enrichment factor, , of the Fe(II)_sorb-Fe(II)_aq couple was 0.85 ± 0.10‰ (±2σ) for quartz and 0.85 ± 0.08‰ (±2σ) for goethite-loaded quartz. In the goethite system, the sorption-induced isotope fractionation was superimposed by atom exchange, leading to a δ^56/54Fe shift in solution towards the isotopic composition of the goethite. Without consideration of atom exchange, the equilibrium enrichment factor was 2.01 ± 0.08‰ (±2σ), but decreased to 0.73 ± 0.24‰ (±2σ) when atom exchange was taken into account. The amount of structural Fe in goethite that equilibrated isotopically with Fe(II)_aq via atom exchange was equivalent to one atomic Fe layer of the mineral surface (∼3% of goethite-Fe). Column experiments showed significant Fe isotope fractionation with δ^56/54Fe(II)_aq spanning a range of 1.00‰ and 1.65‰ for pure and goethite-loaded quartz, respectively. Reactive transport of Fe(II) under non-steady state conditions led to complex, non-monotonous Fe isotope trends that could be explained by a combination of kinetic and equilibrium isotope enrichment factors. Our results demonstrate that in abiotic anoxic systems with near-neutral pH, sorption of Fe(II) to mineral surfaces, even to supposedly non-reactive minerals such as quartz, induces significant Fe isotope fractionation. Therefore we expect Fe isotope signatures in natural systems with changing concentration gradients of Fe(II)_aq to be affected by sorption.     

Hydrochemical and isotope characteristics of spring water and travertine in the Baishuitai area (SW China) and their meaning for paleoenvironmental reconstruction

A method of combining hydrochemical data logging and in situ titrating with measurement of stable carbon and oxygen isotopes was used to reveal the hydrochemical and isotopic characteristics in the Baishuitai travertine scenic area of SW China. It was found that the travertine-forming springs have a very high concentration of calcium and bicarbonate, and accordingly very high CO₂ partial pressures, which are not likely to be produced by biological activity in soil alone. Further analysis of the stable carbon isotopes of the springs shows that the high pressure of CO₂ is mainly related to an endogenic CO₂ source. That means the Baishuitai travertine is endogenic in origin. This is contrast to the commonly accepted saying that the travertine deposition in this study simply is a product of warm and humid conditions in a karst ecological environment. Rapid CO₂ degassing from the water is triggered by the much higher partial pressures in water than that of the surrounding air. Consequently, as the waters flow downstream of the spring the pH increases, the waters become supersaturated with respect to calcite, and travertine is deposited. The preferential release of ¹²CO₂ to the atmosphere results in a progressive increase of travertine ¹³C downstream. This is concluded with a preliminary discussion of variation in travertine-forming water temperatures, according to differences in stable oxygen isotopic compositions of the travertine formed in different epochs at Baishuitai. It was found that the change in water temperature is as high as 13 °C, i.e., from 23 °C at about 2500 years b.p., to 10 °C at present. This may mainly reflect that the effect of geothermal source on water temperature is decreasing. The problems involved in paleoenvironmental reconstruction with endogene travertine are also discussed. They are the impacts of "dead carbon" in radiocarbon dating and the enrichment in ¹³C of travertine by endogenic CO₂ and degassing of CO₂ from water, which has to be considered in paleovegetation reconstruction when using ¹³C data of the endogene carbonate deposits.