  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 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 d13C and
d18O 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 d13C and
d18O 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. ...
