Cave atmosphere controls on stalagmite growth rate and palaeoclimate records - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 287

Fig. 5. The calcite deposition rate plotted against height of the stalagmite (top panel). Numbers represent the model
years as in Figure 4. The actual d 18 O that would result from a combination of the d 18 O of the drip water used in the model
and the growth rate (middle panel) as well as the d 18 O obtained (lower panel) if the stalagmite were sampled using
a conventional microdrill (long lines label with 'C', 500 micron integration, 500 micron steps) and a micromill (short
lines labelled with 'm', 100 micron integration, 100 micron step size). The grey shaded area reflects the actual d 18 O
of the precipitated calcite.

than the annually integrated d 18 O record could be
biased toward the d 18 O of the season with the
most deposition. This is illustrated by the deposition
model discussed above, where, although the annual
d 18 O cycle of the percolation water remains invar-
iant through the model (Fig. 4), the d 18 O of the
calcite changes. This effect is particularly strong
once winter calcite deposition ceases and an increas-
ing percentage of calcite reflects summer perco-
lation water d 18 O (Fig. 5). The overall trend is
therefore a slowing in growth concomitant with
an increase in d 18 O heading toward summer rain-
fall values. Microanalytical techniques may only
be able to detect a small fraction of the actual
annual d 18 O signal, but this depends on the resol-
ution of the technique and the growth rate of the
stalagmite. Palaeoclimate records may therefore

underestimate the actual annual range in d 18 O
values (Fig. 5).
Although conservative proxies are only affected
by the timing of calcite deposition relative to the
timing of the geochemical proxy's annual cycle,
non-conservative proxies are directly affected by
either the degassing or more indirectly by growth
rate effects. Carbon isotopes are probably the most
commonly used non-conservative proxies in stalag-
mites, and are directly affected by CO 2 degassing.
Carbon dioxide composed of 12 C is preferentially
degassed over that composed of 13 C, thus increasing
the d 13 C of drip water dissolved inorganic carbon
(DIC). This leads to increased d 13 C of calcite depos-
ited from the water.
It should also be noted that growth inhibitors,
such as certain colloids, organic molecules, and