Speleothem growth rate and palaeoclimate.
An initial study of the palaeoclimate signal contained within speleothem growth was
undertaken by' investigating regional variations in speleothem Qrowth frequency. It was
demonstrated that about 500 analyses in such a regional compilation were necessary to
generate a statistically significant curve which did not suffer from sample bias. However, few
such regions are likely to have such a larQe data set. That from north west Europe did provide
a useful palaeoclimate record, giving evidence of multiple interstadial events within isotope
stage 3, and a significantly low level of growth within stage Sa.
An investigation was undertaken into the palaeoclimate signal contained in variations of
speleothem growth rate, based on the theory derived by Dreybrodt (1981) and Buhmann and
Dreybrodt (1985) from calcite precipitation kinetics. It was demonstrated that growth rate
increases with increasing calcium ion concentration, temperature and water flux (drip rate for
stalagmites, water film thickness for flowstones and seasonal variations in water availability
for both speleothems); turbulent flow conditions and cave air pC02, which theoretically affect
growth rate, were demonstrated to be Insi9nificant. If water flux, calcium concentration and
temperature all increase with improving Climate, growth rate increases may reflect climatic
improvement. In particular, it was demonstrated that stalagmites should be most sensitive to
changes in calcium concentrations, temperature, and seasonal shut-off of the water feed,
whilst flowstones would also be sensitive to changes in water film thickness.
Theoretical growth rates were tested for recently forming speleothems in excavated caves
and mines. For these, minimum growth rates were determined by knowing the date of
excavation of the cave or mine, and the growth rate determining variables were measured
over the course of a year. It was demonstrated that the theory accurately predicted growth
rates for both stalagmites and flowstones within the 20' errors based on variations in calcium
ion concentration and water film thickness. However, flowstones generally grew slower than
that predicted by the theory, due to the seasonal shut-off of the water supply feeding these
samples. For flowstones at Kent's Cavem, growth rate was observed to Increase with
Increasing water availability, for stalagmites at Lower Cave, growth rate was shown to
increase with increaSing drip rate.
Assuming a good prediction of growth rate by the theory, applications to Quatemary
speleothems were undertaken to determine past calcium ion concentration, temperature and
water flux. Growth rates were determined by thermal ionisation mass spectrometric uraniumseries
dating. The growth rate of one Holocene sample from Sutherland demonstrated that
variations in growth rate over the last 7 ka did not depend on temperature variations, but
either to changes in calcium concentrations due to vegetation change or a non-linear
response to changes in water flow. Growth rates were also determined for two flowstones
from Yorkshire which had grown over the last 200 ka. However, the very fast growth rates in
these samples prevented a precise record from being obtained.
Mass spectrometric dating also provided a record of the timing of growth commencement and
cessation. This was shown to be more complex than previously considered; in particular the
Holocene growth of the Sutherland stalagmite commenced 5 ka after glacier retreat in the
region, the Yorkshire flowstone from Lancaster Hole had seven growth phases, each for only
1-3 ka, five of which correlated with solar insolation maxima. In contrast, another flowstone
from Stump' Cross in Yorkshire was shown to grow in both interglacial, interstadial and
glacial periods of the last 200 ka.
An investigation was made into the use of 13C/1'C,(t>nM to determine the type of plant
community at the time of speleothem formation, and whether a non-biogenic source of CO2
was present. 13C analyses of the Stump Cross flowstone gave elevated 13C not explicable
by the plant communities present, nor were high enough to have a non-biogenic source.
Further investigations are needed, but this evidence suggests caution in interpreting 13C
records for flowstones.
An annual signal of growth rate and growth rate variability was obtained from ultra-violet
microscopic analysiS of luminescent banding within speleothems. Banding was demonstrated
to be annual by mass spectrometric uranium-series dating, but was only preserved In 10% of
a" samples. Variability of growth rate for the Holocene Sutherland stalagmite was compared
to the theoretical annual variability of growth rate derived from annual variations in the growth
rate determining variables observed today, and a good agreement was observed.
Furthermore, for one period of growth, a 4-5 year period of rapid growth rate was
demonstrated to correlate with the Hekla 3volcanic eruption in Iceland.