Cave atmosphere controls on stalagmite growth rate and palaeoclimate records - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 286

Fig. 4. The drip water d 18 O (top panel) and temperature (middle panel) for a modelled drip at a hypothetical cave site.
The temperature variability shown controls the P CO 2 of soil air and consequently bedrock dissolution and drip water
[Ca 2þ ], as well as cave atmosphere P CO 2 . These two factors influence growth rate in opposite directions for seasonal flow
sites (Baldini et al. 2008) but because the [Ca 2þ ] is the more important control on calcite deposition rate this controls the
overall growth rate (lower panel). Growth rate is therefore higher during the summer and lower during the winter. The
grey shading indicates times where the model indicates that no calcite deposition is occurring. All calcite deposition
ceases after model year 18 when mean annual temperature drops below 7.6 8C.

(Fig. 4). Drip water [Ca 2þ ] was modelled as reflect-
ing soil temperature using the relationships dis-
cussed in Baldini et al. (2008). The drip water
d 18 O was also modelled as varying annually, with
high values in the summer of 25‰ SMOW and
low values of 29‰ SMOW occurring in the
winter. This annual d 18 O variability was left
unchanged throughout the model.
Calcite growth rates are highest in the summer
due to increased summer drip water [Ca 2þ ] which
overcomes the reduced thermodynamic drive to
degas (due to elevated cave air P CO 2 ). Deposition
rates on the hypothetical stalagmite are lowest in
the winter. Through the course of the model the
gradual reduction of winter temperature eventually

causes winter deposition to cease altogether (once
temperature drops below 8.4 8C). After 18 years
of modelled growth, even summer temperatures
become too cold and no growth occurs at any time
of the year (Fig. 4). This gradual reduction in
growth followed by complete cessation is what is
commonly observed in real samples (e.g. Genty
et al. 2003).

Effects on palaeoclimate proxies

Although cave air P CO 2 does not directly affect
stalagmite d 18 O (except in cases of extreme
degassing), it does affect the seasonality of calcite
deposition; if drip water d 18 O varies seasonally