  The equilibrium oxygen isotope fractionation factor between calcite and water
(18{\alpha}calcite/H2O) is an important quantity in stable isotope geochemistry
and allows in principle to infer temperature variations from carbonate
{\delta}18O 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 (18{\alpha}calcite/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 18{\alpha}calcite/H2O
values. Here we present a conceptual framework that can explain the variability
of 18{\alpha}calcite/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
{\delta}18O value of the dissolved HCO3- ({\delta}18OHCO3-), resulting in an
oxygen isotope disequilibrium of the {\delta}18OHCO3- values with respect to
the {\delta}18O value of water ({\delta}18OH2O). 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 ...
