Cave atmosphere controls on stalagmite growth rate and palaeoclimate records - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 278

Cave atmosphere controls on stalagmite growth rate and
palaeoclimate records

JAMES U. L. BALDINI
Department of Earth Sciences, University of Durham, Durham DH1 3LE, UK
(e-mail: [e-mail])

Abstract: Cave atmosphere P CO 2 partially controls calcite deposition on stalagmites by changing
the thermodynamic drive of drip water to deposit calcite. The dissolved carbon dioxide contained
in karstic percolation water is generally controlled by the soil P CO 2 , and this CO 2 will degas in any
void spaces with a lower P CO 2 , including caves. If void space P CO 2 is higher than the P CO 2 of the
water, dissolution may occur. Measured cave air P CO 2 ranges of several caves in different climate
regimes suggest that soil temperature is a major control on cave air P CO 2 , but that the observed
trend deviates from the modelled trend when soil carbon dioxide production is moisture-limited.
Calcite deposition models illustrate how soil and cave air P CO 2 can influence stalagmite growth
rates, and demonstrate how gradual temperature changes can skew the geochemical proxy signal
in stalagmites in favour of certain seasons and eventually can result in total cessation of growth.

Speleothems, such as stalagmites, are critically
important archives of climate, particularly in low-
to mid-latitude terrestrial locations where glacial
ice core palaeoclimatology is not possible.
Additionally, records from stalagmites are dateable
using high-precision U-series dating and can conse-
quently assist in the dating of previously identified
but poorly constrained climatic features (Wang
et al. 2001; Sp¨ tl & Mangini 2002; Genty et al.
2003), thus helping to determine correlations
between different records. However, calcite
growth rates may vary both on long and short time-
scales, consequently complicating the interpretation
of stalagmite proxy records. Also, some stalagmites
may be fed by drips that experience seasonal, or
intermittent, undersaturation with respect to
calcite, resulting in either the cessation of calcite
deposition or actual dissolution of previously depos-
ited calcite. Stalagmite climate records sampled at
multiannual resolution integrate the annual climate
signal and therefore potentially bias the climate
signal towards the season with the highest depo-
sition rate. Microanalytical techniques are allowing
the creation of increasingly better resolved records,
but it is critical that high frequency variability in the
parameters that control calcite growth are known in
order to maximise the value of these important
proxy records.
Although the basic controls on calcite deposition
are well quantified, their spatiotemporal variabi-
lity in caves is currently not well constrained.
Drip rate, drip water temperature, soil temperature,
soil moisture conditions, soil and cave air carbon
dioxide partial pressure (P CO 2 ), the Ca 2þ concen-
tration of drip water, and the thickness of the water
film over the stalagmite all control calcite precipi-
tation (Buhmann & Dreybrodt 1985a, b; Dreybrodt

1999), and although the behaviour of many of these
is fairly well-constrained, variability in cave air
P CO 2 is not. Recent studies have demonstrated that
intra and interannual variability in stalagmite
growth and geochemical climate proxies contained
within can be ascribed to rapid and seasonal shifts
in cave air P CO 2 (Frisia et al. 2000; Sp¨ tl et al.
2005; Banner et al. 2007; Baldini et al. 2008;
Mattey et al. 2008).
Soil P CO 2 is the primary control on dissolved CO 2
in groundwater in both open and closed system con-
ditions (White 1988). Closed system conditions
occur when percolation water loses contact with
the CO 2 source, thus restricting the amount of lime-
stone dissolution that can occur. Conversely, open
system conditions are characterised by continued
contact with the CO 2 source, thus potentially gener-
ating much higher groundwater alkalinity values.
However, in practice, most cave percolation waters
are likely to have experienced both open and closed
system conditions. Generally, soil P CO 2 is substan-
tially higher (typically 0.1-10.0% atm) than atmos-
pheric values (c. 0.0385% atm) (Troester & White
1984; White 1988), and is largely responsible for
the total dissolved CO 2 contained in vadose water.
Carbonate dissolution occurs until the dissolved
carbon dioxide is completely consumed. The system
then remains at equilibrium until the water reaches
air-filled voids with lower P CO 2 than that dissolved
in the water (reflecting soil air P CO 2 ), at which point
dissolved CO 2 degassing occurs, followed by calcite
precipitation. Therefore, a reduction in cave atmos-
phere P CO 2 would increase stalagmite growth rates,
assuming all other variables remain unchanged.
Previous studies have applied stalagmite growth
rate directly as a palaeoclimate proxy (Polyak &
Asmerom 2001; Baldini et al. 2002; Sp¨ tl et al.