Degassing of CO2 and precipitation of calcite to the surface of stalagmites can strongly impact isotope signals imprinted into the calcite of these speleothems. Here, we show that in all the variety ...of conditions occurring in nature only two distinct types of degassing exist. First, when a thin film of calcareous solution comes in contact to cave air, which has a lower pCO2 value than that of the aqueous CO2 in the water, molecular CO2 escapes by physical diffusion in several seconds. In a next step lasting several ten seconds, pH and DIC in the solution achieve chemical equilibrium with respect to the CO2 in the cave atmosphere. This solution becomes supersaturated with respect to calcite. During precipitation for each unit CaCO3 deposited one molecule of CO2 is generated and escapes from the solution. This precipitation driven degassing is active during precipitation only. We show that all variations of out gassing proposed in the literature are either diffusive outgassing or precipitation driven degassing and that diffusive outgassing has no influence on the isotope composition of the HCO3− pool and consequently on that of calcite. Its isotope imprint is determined solely by precipitation driven degassing in contrast to most explanations in the literature. We present a theoretical model of δ13C and δ18O that explains the contributions of various parameters such as changes in temperature, changes of pCO2 in the cave atmosphere, and changes in the drip intervals to the isotope composition of calcite precipitated to the apex of the stalagmite. We use this model to calculate quantitatively changes of δ13C and δ18O observed in field experiments (Carlson et al., 2020) in agreement to their experimental data. We also apply our model to prior calcite precipitation (PCP) in the field as reported by Mickler et al. (2019). We discuss how PCP influences isotope composition signals. In summary, we present a transparent method based on few commonly accepted equations that allows calculation of the isotope composition δ13C and δ18O of CaCO3 under various temperatures, pCO2 in the cave air, degrees of PCP, and concentrations cCao of the water entering the cave.
•Degassing of CO2 from thin water films during the formation of stalagmites.•Only two types of degassing: one by diffusion, one due to deposition of calcite.•No influence of diffusion degassing to isotope compositions of the HCO3_ pool.•Calculation of isotope composition of 13C and 18O in calcite precipitated.•Prior calcium precipitation (PCP) adds isotope signal to the calcite deposited.
The magnitudes, variations, locations and mechanisms responsible for the global atmospheric CO2 sink are uncertain and under continuing debate. Previous studies have focused mainly on the sinks in ...the oceans, and soil and vegetation on the continents. Here, we show, based on theoretical calculations and field monitoring evidence, that there is an important but previously underestimated sink for atmospheric CO2 as DIC-dissolved inorganic carbon that results from the combined action of carbonate dissolution, the global water cycle and the photosynthetic uptake of DIC by aquatic organisms in ocean and land. The sink constitutes up to 0.8242Pg C/a, amounting to 29.4% of the terrestrial CO2 sink, or 10.4% of the total anthropogenic CO2 emission. 0.244 PgC/a are transferred to the sea via continental rivers and 0.2278Pg C/a by meteoric precipitation over the seas. 0.119Pg C/a is released back to the atmosphere again, and 0.2334Pg C/a is stored in the continental aquatic ecosystem. Therefore, the net sink is estimated as 0.7052Pg C/a. This sink may increase with an intensification of the global water cycle as a consequence of global warming, rising anthropogenic emissions of CO2 and carbonate dust in atmosphere, and afforestation, which increases the soil pCO2 and thus the carbonate dissolution. Fertilization with the elements N, P, C, Fe, Zn, and Si increases the organic matter storage/burial by aquatic organisms and thus decreases the CO2 return to the atmosphere. Based on the ensemble mean projection of global warming for the year 2100 by IPCC, it is estimated that the atmospheric CO2 sink will increase by 21%, or about 0.18Pg C/a. However, the uncertainty in the estimation of this sink needs further exploration.
Kinetic fractionation of18O and clumped isotopes13C18O in calcite precipitated to speleothems in cave environments renders the paleo-climatic interpretation of these proxies difficult. Therefore a ...better understanding of the processes generating the isotope imprint is needed. A heuristic approach is taken to interpret recent data of the fractionations ... in a cave analogue experiment of calcite precipitation (Hansen et al. 2019) that shows a dependence on experimental precipitation rates, F. An expression, ..., is derived that is based on uni-directional irreversible precipitation and is valid for large F when the forward rate of precipitation dominates the backward rate of dissolution. In that derivation it is assumed that the kinetic constants of precipitation rates are different for the different isotopologues and that this is also true for their equilibrium concentrations ceq with respect to calcite. The constant, £, is expressed by the kinetic fractionation ... where α denote the rate constants of precipitation for the rare and abundant isotopologues. The second constant, λ, is determined by the differing equilibrium concentrations of HCO3- isotopologues with respect to calcite and the pCO2 in the surrounding atmosphere. Fitting this expression to the experimental data one obtains the parameters ε and λ for different temperatures. Regarding these results the temporal evolution of18δCaCO3 (t) and Δ47(t) is discussed for the experimental conditions and for cave environments. This has implications to the application of 1000lnα18CaCO3-H2O as a paleo-thermometer. It shows the reason why so many differing calibrations have been reported. These results analogously can be applied also to clumped isotopes13C18O and the calibration of the Δ47-thermometer with regard to speleothem calcite. In summary, a better understanding of the problems arising in the search for generally valid calibrations of18O and Δ47 paleothermometers is presented.
It is widely accepted that chemical weathering of Ca–silicate rocks could potentially control long-term climate change by providing feedback interaction with atmospheric CO₂ drawdown by means of ...precipitation of carbonate, and that in contrast weathering of carbonate rocks has not an equivalent impact because all of the CO₂ consumed in the weathering process is returned to the atmosphere by the comparatively rapid precipitation of carbonates in the oceans. Here, it is shown that the rapid kinetics of carbonate dissolution and the importance of small amounts of carbonate minerals in controlling the dissolved inorganic C (DIC) of silicate watersheds, coupled with aquatic photosynthetic uptake of the weathering-related DIC and burial of some of the resulting organic C, suggest that the atmospheric CO₂ sink from carbonate weathering may previously have been underestimated by a factor of about 3, amounting to 0.477Pg C/a. This indicates that the contribution of silicate weathering to the atmospheric CO₂ sink may be only 6%, while the other 94% is by carbonate weathering. Therefore, the atmospheric CO₂ sink by carbonate weathering might be significant in controlling both the short-term and long-term climate changes. This questions the traditional point of view that only chemical weathering of Ca–silicate rocks potentially controls long-term climate change.
Time series of stable carbon and oxygen isotope values in stalagmite calcite are important paleoclimate proxies. However, the isotope signal resulting from climatic variability is superimposed by ...isotope imprints resulting from physical and chemical processes during precipitation of speleothem calcite, which are independent of climate. Precipitation of calcite causes enrichment of the heavy isotopes for both oxygen and carbon in the DIC that depends on drip time, the calcium concentration in the drip water, but also on its temperature. In conclusion the work of Hansen et al. supports the prior theory of Dreybrodt et al., 2016, and provides experimental evidence that the exchange times are predicted correctly by both approaches if the combined pH dependent rate constant k is used in both models.
When calcite is precipitated from a water film on top of a stalagmite to its surface, the carbonate in the solution and consequently also the carbonate in the calcite deposited becomes enriched in ...the heavy isotopes 18O and 13C (Hansen et al. 2016; Dreybrodt & Scholz 2011; Romanov et al. 2008). This isotope signal is added to the isotope imprint resulting from climate variability. Therefore a physical model of the evolution of the isotope composition of carbonate in a water film, either flowing down the surface of the stalagmite at high drip rates or stagnant, when the drip rate is low, is necessary to discriminate the climate signal from the signal resulting from physical processes in the cave. Currently two models are proposed. In the first one (Scholz et al. 2009) one assumes that the isotope evolution can be described by a Rayleigh distillation process (Mook 2000) where during the entire process of precipitation the fractionation factor for carbon or oxygen, respectively remains constant. In another model one assumes that precipitation is a uni-directional non-equilibrium process, where the constants in the rate equations are slightly different for the light and the heavy isotope (Dreybrodt 2008; Dreybrodt & Scholz 2011). Results from these models have been compared for various scenarios by Scholz et al. (2009), Dreybrodt and Scholz (2011), Dreybrodt and Deininger (2014) and Dreybrodt and Romanov (2016). Differences of opinion clearly exist in the research community regarding which of these models best represents reality. In this letter I discuss the meaning of the Rayleigh equation for precipitation of calcite under equilibrium conditions and for precipitation of calcite governed by uni-directional rate equations.
In this comment it is shown that analysis of the fractionation constants of the DIC describing the enrichment of 13C and 18O along the flow path of a thin water layer flowing down an inclined marble ...plate precipitating calcite in analogy to precipitation to speleothems, enables one to decide whether the Rayleigh-Distillation-model or the kinetic model is valid. Appropriate analysis of εtot values for carbon and oxygen as reported by Hansen et al. (2019) reveals the validity of the kinetic model and allows determining the constants εkin and λ. This is in contrast to findings of Hansen and coauthors and gives a deeper understanding of the meaning of kinetic fractionation during precipitation of calcite.
•Isotope fractionation 18O and 13C in DIC during precipitation of calcite.•Classical Rayleigh-Distillation equation is not applicable.•A kinetic model must regard differing rate constants and equilibrium concentrations.•This model describes experiments by use of two kinetic fractionation constants.
Understanding the relationship between stable isotope signals recorded in speleothems (δ
13C and δ
18O) 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 CO
2.
The water entering the cave flows as a thin film towards the drip site. CO
2 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 δ
13C value of the drip water is mainly determined by the isotopic composition of soil CO
2. The evolution of the δ
18O 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 CO
2 in soil water is in isotopic equilibrium with that of the water, dissolution of limestone delivers oxygen with a different isotopic composition changing the δ
18O value of the carbonate species. Consequently, the δ
18O 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 CO
2-exchange with the cave air. Exchange times,
τ
ex
CO
2
, 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
<
0.1
τ
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.
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