首页|The impact of outgassing of CO2 and prior calcium precipitation to the isotope composition of calcite precipitated on stalagmites. Implications for reconstructing climate information from proxies

The impact of outgassing of CO2 and prior calcium precipitation to the isotope composition of calcite precipitated on stalagmites. Implications for reconstructing climate information from proxies

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  • NSTL
  • Elsevier
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 p(CO2) 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 delta C-13 and delta O-18 that explains the contributions of various parameters such as changes in temperature, changes of p(CO2) 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 delta C-13 and delta O-18 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 delta C-13 and delta O-18 of CaCO3 under various temperatures, p(CO2) in the cave air, degrees of PCP, and concentrations c(Ca)(o) of the water entering the cave.

stalagmitesCO2 outgassingprecipitation of calciteisotope composition C-13 and O-18 of carbonate in water and calciteprior calcite precipitation (PCP)SPELEOTHEM DELTA-O-18 SIGNIFICANCEORBITAL SCALE VARIABILITYGROWTH-RATEKINETIC FRACTIONATIONDISSOLUTION KINETICSSYSTEM CACO3-H2O-CO2FIELD-MEASUREMENTSOXYGEN ISOTOPESDRIP WATERCAVE

Dreybrodt, Wolfgang、Fohlmeister, Jens

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Univ Bremen

Potsdam Inst Climate Impact Res

2022

10.1016/j.chemgeo.2021.120676

Chemical geology

Chemical geology

EISCI
ISSN：0009-2541
年,卷(期)：2022.589
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