Speaker: James Baldini
Abstract
Title: ‘Drip composition and stalagmite growth rates’
Outline: Stalagmites are rapidly becoming one of the most important types of
terrestrial climate archives available. Understanding stalagmite growth is critical for
the robust interpretation of stalagmite-based palaeoclimate records. The dependency
of stalagmite growth on local environmental conditions was appreciated very early on
in the history of speleothem science, but even now many interpretations do not
consider potential biases induced by seasonally variable growth rates. Additionally,
relatively short-lived changes in drip composition can result in difficult to detect
micro-hiatuses that may affect both chronologies and climate proxy data. Therefore,
thorough site monitoring is important for the identification of any seasonal intervals
of non-deposition, or for evaluating if micro-hiatuses arising from changed
environmental conditions may have affected geochemical records from the monitored
site in the past.
This talk will discuss the variables that control stalagmite growth, and how several of
these are ultimately driven by climate. The controls on stalagmite growth are now
reasonably well understood, and include drip water [Ca2+], drip rate, and the thickness
of the thin film of water from which the carbonate precipitates. Contrary to popular
belief, drip rate is usually not the most important factor in determining stalagmite
growth rate. Instead, the growth rate of stalagmites is largely driven by the difference
between drip water [Ca2+] and the amount of [Ca2+] that would exist in the drip water
if the dissolved CO2 in the drip water were in equilibrium with cave atmosphere
pCO2. The initial drip water [Ca2+] is largely controlled by the degree of bedrock
dissolution, and therefore an important control on stalagmite growth rate is the
difference between the pCO2 of the soil or ‘ground air’ reservoir with which the drip
water has equilibrated with (which determines the amount of bedrock dissolved) and
the pCO2 of the cave air; the larger the difference between the two the more rapidly
CO2 degasses from the drip water and the more rapidly a stalagmite would grow.
Cave air pCO2 often varies seasonally and is generally, but not always, controlled by
temperature-driven density differences in cave air versus outside air. At temperate
mid-latitude cave sites, summer cave air pCO2 values are often elevated compared to
winter values due to reduced ventilation, which may lead to reduced growth rates
during the summer and a bias toward winter drip water isotope ratios for some drip
types. Other characteristics of the drip water are important for both stalagmite growth
and the nature of the climate signal within the stalagmite, and other factors
influencing drip water chemistry will be presented briefly.
A few relevant articles:
1
Baker, A. et al. Testing theoretically predicted stalagmite growth rates with
Recent annually laminated samples: implications for past stalagmite
deposition. Geochim. Cosmochim. Acta 62, 393-404 (1998).
2
3
4
5
Baldini, J. U. L. et al. Very high-frequency and seasonal cave atmosphere
PCO2 variability: Implications for stalagmite growth and oxygen isotope-based
paleoclimate records. Earth Planet. Sc. Lett. 272, 118-129 (2008).
Sherwin, C. M. & Baldini, J. U. L. Cave air and hydrological controls on prior
calcite precipitation and stalagmite growth rates: Implications for
palaeoclimate reconstructions using speleothems. Geochim. Cosmochim. Acta
75, 3915-3929 (2011).
Dreybrodt, W. Chemical kinetics, speleothem growth and climate. Boreas 28,
347-356 (1999).
Baker, A. J. et al. Reconstructing modern stalagmite growth from cave
monitoring, local meteorology, and experimental measurements of dripwater
films. Earth Planet Sc. Lett. 392, 239-249 (2014).