Here we present novel cave-analogue experiments directly investigating stable carbon and oxygen isotope fractionation between the major involved species of the carbonate system (HCO3−, CO2, CaCO3 and ...H2O). In these experiments, which were performed under controlled conditions inside a climate box, a thin film of solution flew down an inclined marble or glass plate. After different distances of flow and, thus, residence times on the plate, pH, electrical conductivity, supersaturation with respect to calcite, precipitation rate as well as the δ18O and δ13C values of the dissolved inorganic carbon (DIC) and the precipitated CaCO3 were obtained.
Progressive precipitation of CaCO3 along the plate is accompanied by degassing of CO2 and stable isotope fractionation, and the system is driven out of isotope equilibrium. We observe a strong enrichment of the δ13C values with increasing residence time and a smaller enrichment in δ18O. The temporal evolution of the δ18O and δ13C values of both the DIC and the precipitated CaCO3 can be explained by a Rayleigh fractionation model, but the observed enrichment in δ13C values is much larger than expected based on isotope equilibrium fractionation factors.
Our setup enables to determine the fractionation between CaCO3 and HCO3−, i.e., εCaCO3/HCO3−. Carbon isotope fractionation, 13εCaCO3/HCO3−, is strongly negative for all experiments and much lower than equilibrium isotope fractionation (0–1‰). In addition, 13εCaCO3/HCO3− decreases with increasing residence time on the plate, and thus decreasing supersaturation with respect to calcite. Thus, isotope fractionation depends on precipitation rate and consequently occurs under kinetic conditions. This is in contrast to previous studies, which found no rate-dependence and no or even a positive carbon isotope fractionation between CaCO3 and HCO3−. Oxygen isotope fractionation, 18εCaCO3/HCO3−, is also negative and dependent on precipitation rate. Since no literature values for 18εCaCO3/HCO3− are available, we calculated 18εCaCO3/HCO3− using equilibrium oxygen isotope fractionation factors between water and calcite and water and HCO3−, respectively. At the beginning of the plate, the fractionation is in agreement with the fractionation calculated using fractionation factors determined in cave systems.
The observed fractionation between CaCO3 and water, 1000ln18α, is also in good agreement with the values determined in cave systems and shows a very similar temperature dependence 1000ln18α=16.516±1.267∗103T−26.141±4.356. However, with progressive precipitation of CaCO3 along the plate, the system is forced out of isotope equilibrium with the water, and 1000ln18α increases.
The large, negative, rate-dependent isotope fractionations observed in this study suggest that precipitation of speleothem calcite is strongly kinetically controlled and may, thus, have a large effect on speleothem δ18O and δ13C values. Since these values may erroneously be interpreted as reflecting changes in past temperature, precipitation and/or vegetation density, these results have important implications for paleoclimate reconstructions from speleothems.
Abstract
Surface temperature is a fundamental parameter of Earth’s climate. Its evolution through time is commonly reconstructed using the oxygen isotope and the clumped isotope compositions of ...carbonate archives. However, reaction kinetics involved in the precipitation of carbonates can introduce inaccuracies in the derived temperatures. Here, we show that dual clumped isotope analyses, i.e., simultaneous ∆
47
and ∆
48
measurements on the single carbonate phase, can identify the origin and quantify the extent of these kinetic biases. Our results verify theoretical predictions and evidence that the isotopic disequilibrium commonly observed in speleothems and scleractinian coral skeletons is inherited from the dissolved inorganic carbon pool of their parent solutions. Further, we show that dual clumped isotope thermometry can achieve reliable palaeotemperature reconstructions, devoid of kinetic bias. Analysis of a belemnite rostrum implies that it precipitated near isotopic equilibrium and confirms the warmer-than-present temperatures during the Early Cretaceous at southern high latitudes.
Here we present clumped isotope Δ47 data of cave analogous experiments. We investigate the evolution of the Δ47 values of both the dissolved inorganic carbon (DIC) and the CaCO3 of thin-films flowing ...down in channels along inclined marble and glass plates precipitating CaCO3 along the flow path. With increasing distance of flow and residence time on the plate, we observe large deviations from the initial Δ47 values, along with increasingly over-estimated apparent Δ47- temperatures derived from the precipitated CaCO3. With further increasing residence time on the plates (>250 s) and decreasing supersaturation with respect to calcite, the CaCO3 Δ47 values start to approach the initial equilibrium values again. This highlights the potentially strong effect of disequilibrium isotope effects on clumped isotope temperatures derived from speleothems, in particular because the disequilibrium effect on the Δ47 values is most pronounced in the first few 100 seconds of CaCO3 precipitation.
The experimental Δ47 values are consistent with a recent modeling study by Guo and Zhou (2019). The maximum degree of disequilibrium, however, seems to be more pronounced than predicted by the model for some of the experiments. A potential reason for this observation is that suitable kinetic fractionation factors are so far not available. While the Δ47 values of the CaCO3 decrease along the plate as the model suggests for the Δ47 values of the HCO3−, those of the DIC increase and follow the modelled Δ47 values of the dissolved CO2.
Based on our study, we can neither confirm nor exclude a temperature dependence of the Δ47-δ18O slope as well as a dependence on cave pCO2, which highlights the complexity of this relationship. In addition, the non-linear evolution of Δ47 and δ18O values with time and along the flow path restricts a correction of the Δ47 and δ18O values based on the Δ47-δ18O slope to the initial phase of the isotope evolution. So far, a correction of the kinetic isotope effects is not yet possible, but this might be important in future studies if the disequilibrium slope on a Δ47-δ18O diagram could be precisely determined for the initial phase of isotope evolution.
We present a theoretical derivation of the exchange time, τex, needed to establish isotopic equilibrium between atmospheric CO2 in a cave and HCO3− dissolved in a thin water film covering the surface ...of a speleothem. The result is τex=τredex·HCO3-KH·pCO2cave, where τredex depends on the depth, a, of the water film and on temperature. HCO3- is the concentration of bicarbonate, pCO2cave the partial pressure of CO2, and KH is Henry’s constant. To test the theory we prepared stagnant or flowing thin films of a NaHCO3 solution and exposed them at 20°C to an CO2 containing atmosphere of pCO2 500, 12,500, or 25,000ppmV and defined isotope composition. The δ13C and δ18O values of the DIC in the solution were measured as a function of the exposure time. For stagnant films with depths between 0.06 and 0.2cm the δ13C values exhibit an exponential approach towards isotope equilibrium with the atmospheric CO2 with exchange time, τex. The δ18O values first evolve towards isotopic equilibrium with atmospheric CO2, reach a minimum value and then drift away from the isotopic equilibrium with atmospheric CO2 approaching a steady state caused by isotopic exchange of oxygen with water. The experimental findings are in satisfactory agreement with the theoretical predictions.
To further investigate isotope evolution in cave analogue conditions, a water film containing 5mmol/L of NaHCO3 with a depth of 0.013cm flowing down an inclined borosilicate glass plate was exposed to an atmosphere with pCO2=500ppmV at a temperature of 20°C. The δ13C and δ18O values were measured as a function of flow (exposure) time, t. The isotope compositions in the DIC of the water film decrease linear in time by δDIC(t)=δDIC(0)-(δDIC(0)-δDIC(∞))·t/τex where δDIC(0) is the initial isotope composition of dissolved inorganic carbon (DIC) in the water film and δDIC(∞) its final value. From these data an exchange time τex of ca. 7000s was obtained, in satisfactory agreement with the theoretical predictions. The exchange times can be calculated by τex=τredex·HCO3-KH·pCO2cave, where τredex is given by the theory as function of temperature and the depth, a, of the water film. This way it is possible to obtain exchange times for various conditions of stalagmite growth as they occur in caves.
Rapid degassing of CO2 from a thin film of drip water on the surface of stalagmites is often considered to have a large effect on both speleothem growth and stable isotope values and is offered as an ...explanation for higher δ13C and δ18O values than expected under conditions of stable isotope equilibrium. However, the time constant for degassing of CO2 from the solution only depends on film thickness and the coefficient of molecular diffusion for CO2. Thus, for thin films, the time for degassing of CO2 is much shorter than the time for subsequent equilibration of the dissolved carbon species and precipitation of CaCO3. In this context, degassing of CO2 is always fast.
Here we present three experiments that enable the determination of the time constants for degassing of CO2, τdeg, subsequent equilibration to a new pCO2, τeq, and precipitation of CaCO3, τpr, in a thin film of an H2O–CO2–CaCO3 solution flowing on a calcite surface. The experiments are performed under cave-analogue conditions.
At a temperature of 20°C and for a film thickness of δ≈0.01cm, τdeg≈2s. τeq≈13s and, thus, one order of magnitude larger. Finally, τpr≈400s for δ≈0.01cm, again one order of magnitude larger. The experimentally determined values for τdeg, τeqτpr are in good agreement with the theoretical predictions.
Our results confirm that the chemical evolution of the drip water proceeds in three subsequent major steps. During the first step of degassing of CO2, pH and Ca2+ concentration remain almost constant. During equilibration to the lower pCO2 of the solution, pH increases to about 8 whereas the Ca2+ concentration still remains constant. Finally, during precipitation of calcite, pCO2 remains at its low level and pH decreases slightly.
These results suggest that the drip rate may have an important influence on the stable isotope signals recorded in speleothems. Stalagmites growing beneath drip sites with stable, intermediate drip rates (i.e., in the range of τeq) may be best suited for palaeoclimate reconstruction.
The curvature and slope of speleothem surfaces have been shown to affect the reaction rates in the aqueous carbonate system by altering the thickness of the CaCO3-precipitating solution. However, the ...effects of speleothem geometry and drip rate on the speleothem’s carbon and oxygen isotopic composition have yet to be investigated. Over more strongly sloping surfaces, solutions are thinner and flow faster. The effects of thinner and faster-flowing solutions on the isotopic composition of carbonate minerals precipitated from these solutions are of opposite sense. Thinner solutions enhance rates of CO2 degassing and mineral formation, increasing the degree of isotopic distillation of the dissolved inorganic carbon (DIC) reservoir and leading to larger isotopic fractionation between the carbonate mineral and the initial DIC. Concurrently, faster flow over the steeper surfaces results in shorter residence times of the solutions on the growing speleothem, thereby limiting the degree of isotopic distillation and CaCO3-DIC fractionation. Consequently, predicting the CaCO3-DIC isotopic fractionation as a function of drip rate, surface slope and flow distance is not trivial.
Using an advection-diffusion-reaction model, we tested the sensitivity of the isotopic composition of calcite precipitated along inclined surfaces to the solution discharge (drip) rate and the surface slope. Calcite δ13C and δ18O values correlate well with the degree of prior calcite precipitation (PCP), which is identified as a major determinant of isotopic compositions in speleothems. Our results show that at low PCP, speleothem δ13C and δ18O values may initially decrease relative to calcite-DIC and calcite-water equilibrium due to expression of kinetic isotope effects of mineral precipitation. Upon progressive PCP, δ13C and δ18O values gradually increase due to continuous CO2(aq) formation and degassing. This shift in the isotopic composition of the calcite to lower-than-equilibrium and then higher-than-equilibrium values expands the regime of near-equilibrium compositions during the isotopic evolution. In turn, this may allow quantitative environmental reconstructions, even when the isotopic system is in disequilibrium. Under the simulated conditions, our model predicts maximal potential enrichments of 7 and 3‰ in the calcite δ13C and δ18O values, respectively. In addition, we found a strong dependence of the calcite δ13C and δ18O values on the drip rate and distance of flow, and a weak dependence on the surface slope. In fact, changes in drip rate alone may drive isotopic offsets of several permil, when all other environmental parameters are kept constant. According to our model, higher drip rates and shorter stalactites promote closer-to-equilibrium isotopic compositions of stalagmites, providing a higher signal-to-noise ratio, and minimizing variability that is unrelated to climate.
The equilibrium oxygen isotope fractionation factor between calcite and water (18αcalcite/H2O) is an important quantity in stable isotope geochemistry and allows in principle to infer temperature ...variations from carbonate δ18O if carbonate formation occurred in thermodynamic equilibrium. For this reason, many studies intended to determine the value of the oxygen isotope fractionation factor between calcite and water (18αcalcite/H2O) for a wide range of temperatures using modern cave calcite and the corresponding cave drip water or ancient speleothem carbonate and fluid inclusion samples. However, the picture that emerges from all of these studies indicates that speleothem calcite is not formed in thermodynamic equilibrium but under kinetic conditions, provoking a large variability of determined 18αcalcite/H2O values. Here we present a conceptual framework that can explain the variability of 18αcalcite/H2O values obtained by cave studies. Prior calcite precipitation (PCP) is calcite precipitation before cave drip water is dripping from the cave ceiling and impinges on the surface of a stalagmite or watch glass. Prior to the karst water dripping from the cave ceiling, PCP can occur in the karst above the cave as well as on the cave ceiling, the cave walls and on the surface of stalactites. We argue that PCP leads to increasing the δ18O value of the dissolved HCO3− (δ18OHCO3-), resulting in an oxygen isotope disequilibrium of the δ18OHCO3- values with respect to the δ18O value of water (δ18OH2O). The oxygen isotope disequilibrium between HCO3− and H2O is re-equilibrated by oxygen isotope exchange between H2O and HCO3. Depending on the temperature, the re-equilibration time varies from hours to days and is usually much longer than the residence time of the drip water on stalactites, but much shorter than the time required to percolate through the karst. Therefore, while the oxygen isotope equilibrium between HCO3− and H2O is very likely re-established when PCP occurred in the karst, oxygen isotope disequilibrium conditions between HCO3− and H2O still prevail when PCP occurred inside a cave, e.g., on stalactites. If the oxygen isotope disequilibrium conditions between HCO3− and H2O is not re-established, the precipitated calcite will inherit the elevated δ18O value of the HCO3− and not be in oxygen isotope equilibrium with the corresponding drip water. Consequently, if the 18αcalcite/H2O value is calculated from cave calcite samples affected by PCP, the derived value will be systematically biased.
Trace element to Ca ratios in speleothems have emerged as important proxies that reflect local environmental conditions. However, interpretations of speleothem trace element records can be ...challenging due to various processes. Positive correlations between speleothem Mg/Ca and Sr/Ca have often been interpreted to reflect prior calcite precipitation (PCP), a process potentially modulated by rainfall variability. For quantitative interpretation of PCP, the distribution coefficients for Mg and Sr (DMg and DSr) are required. Here, we use ten cave monitoring calcite and drip water datasets to investigate the influence of temperature and drip water and calcite Mg/Ca and Sr/Ca ratios on speleothem calcite DMg and DSr. The datasets cover a large range of climatic and geological settings resulting in a large range of drip water Mg/Ca ratios. Speleothem calcite DSr shows a positive correlation with the calcite Mg/Ca ratio. Furthermore, DMg shows a clear temperature dependence (DMg = 0.013*e0.035*T).
Previous work proposed that the slope of a trend line through a plot of ln(Sr/Ca) versus ln(Mg/Ca) of a speleothem trace element dataset is between 0.709 and 1.003 if dominated by PCP. However, this only holds true if the initial drip water Mg/Ca and Sr/Ca ratios as well as DSr and DMg are constant for the whole dataset. We use an excel-based PCP model (see Electronic Annex) to assess the potential influence of PCP on drip water and speleothem Mg/Ca and Sr/Ca ratios and simulate different initial drip water Ca, Mg, and Sr concentrations corresponding to limestone, dolostone, and mixed host rock compositions. In the case of enhanced PCP and high Mg/Ca ratios, calcite DSr increases progressively with the mean Mg/Ca ratio of the speleothem time series resulting in steeper slopes of ln(Sr/Ca) versus ln(Mg/Ca) of up to 1.45.
We show that PCP can induce slopes ranging from 0.709 (or even shallower) up to 1.45. This large range suggests that the previously applied criteria to detect PCP in speleothem records were too strict and may lead to unjustified exclusion of PCP as a potential interpretation of speleothem and drip water trace element ratios. Thus, the number of speleothem Mg/Ca and Sr/Ca datasets that potentially reflect past changes in effective rainfall may be larger than previously suggested.
Cryogenic cave carbonates (CCC) represent a specific type of speleothem precipitating from freezing water in caves. Over the past decade, considerable progress has been made concerning cryogenic ...calcite petrography, crystallography and geochemistry. Uncertainties remain, however, as the cave waters from which ancient cryogenic calcites form are not preserved. The present study provides an experimental approach to re-calculate the isotopic composition of the precipitating solution using CCC data. Calcium-rich bicarbonate water prepared by bubbling CO2 gas with a very low δ13C value (∼ −35 ‰) into the water was cooled under controlled laboratory conditions to account for the water chemistry changes during freezing. The experiments yielded cryogenic calcite and vaterite precipitating at temperatures of +1, +0, −0.5, −0.7, −1 and − 2 °C, with complete freezing occurring at −0.7, −1 and − 2 °C and partial freezing at −0.5 °C. The δ18Owater and δ13CDIC values, pH, electrical conductivity and alkalinity were recorded before and after the experiments. Our work documents an increase in pH, suggesting CO2 degassing eventually reaches supersaturation with calcite, leading to CaCO3 precipitation. Calcite precipitation rates are lower at the longer experiment (> 45 days). This is further confirmed by the decreasing calcite saturation index (SIcc) with time. Carbon isotope analyses of the water and carbonates revealed kinetic effects during rapid freezing. A large increase in Δ13CDIC values of the water (up to about 30 ‰) was found between the start and end of the experiments. Reasons may include CO2 degassing with increasing time leading to a markedly 13C-enriched solution. Based on our experimental data, the cryogenic crystal morphotypes are related to the precipitation sequence. Sharp-edged rhombohedra (calcite) precipitate first. Subsequently, spherulitic (vaterite) morphotypes precipitate from almost completely frozen water characterised by a high SIcc. This first experimental work dealing with cryogenic carbonate precipitates and their stable isotope composition sheds light on the origin of these peculiar speleothems and provides constraints to interpret morphologies and isotopic compositions of ancient CCCs.
•Experimentally precipitated cryogenic carbonates unravel the physiochemical conditions that control cryogenic carbonate precipitation.•Cryogenic cave carbonate morphologies (rhombohedral and spherulitic) are influenced by subtle differences in temperature, electrical conductivity and saturation index.•An increase in pH, saturation index, and internal pCO2 of the experimental solution indicate ongoing degassing / precipitation processes.
Speleothem stable carbon isotope (δ13C) records provide important paleoclimate and paleo-environmental information. However, the interpretation of these records in terms of past climate or ...environmental change remains challenging because of various processes affecting the δ13C signals. A process that has only been sparsely discussed so far is carbon isotope exchange between the gaseous CO2 of the cave atmosphere and the dissolved inorganic carbon (DIC) contained in the thin solution film on the speleothem, which may be particularly important for strongly ventilated caves.
Here we present a novel, complete reaction diffusion model describing carbon isotope exchange between gaseous CO2 and the DIC in thin solution films. The model considers all parameters affecting carbon isotope exchange, such as diffusion into, out of and within the film, the chemical reactions occurring within the film as well as the dependence of diffusion and the reaction rates on isotopic mass and temperature. To verify the model, we conducted laboratory experiments under completely controlled, cave-analogue conditions at three different temperatures (10, 20, 30°C). We exposed thin (≈0.1mm) films of a NaHCO3 solution with four different concentrations (1, 2, 5 and 10mmol/l, respectively) to a nitrogen atmosphere containing a specific amount of CO2 (1000 and 3000ppmV). The experimentally observed temporal evolution of the pH and δ13C values of the DIC is in good agreement with the model predictions. The carbon isotope exchange times in our experiments range from ca. 200 to ca. 16,000s and strongly depend on temperature, film thickness, atmospheric pCO2 and the concentration of DIC. For low pCO2 (between 500 and 1000ppmV, as for strongly ventilated caves), our time constants are substantially lower than those derived in a previous study, suggesting a potentially stronger influence of carbon isotope exchange on speleothem δ13C values. However, this process should only have an influence in case of very long drip intervals and slow precipitation rates.
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