Speleothems to infer past temperatures
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Speleotherms – geological formations created by mineral deposits that accumulate over time
in natural caves. Stalactites, Stalagmites, flowstone, columns, drapery, straws
Are important natural archives of past hydroclimate change
◦ can be accurate and precisely dated with U-Th methods
◦ contain multiple geochemical proxies within growth layers
◦ are widely distributed throughout karst regions
◦ often grow uninterrupted for thousands of years
Oxygen (δ18O) and carbon (δ13C) stable isotopes are used to track variation in rainfall
temperature, precipitation, and vegetation changes over the past ~500,000 years.
The Mg/Ca proxy has likewise been used as a moisture indicator, although its reliability as a
palaeohygrometer can be affected by cave ventilation during dry seasons.
Similar to tree rings - Variations in precipitation alter the width of speleothem rings: closed
rings indicates little rainfall, wider spacing indicates heavier rainfall, and denser rings
indicate higher moisture.
Drip rate counting and trace element analysis of the water drops record short-term climate
variations, such as El Niño–Southern Oscillation (ENSO) climate events.
The δ13C values of speleothem carbonates are locally controlled by biogenic soil
productivity associated with the vegetation type (C4- or C3-type) and density, which
regulates the soil CO2 content. Furthermore, it can reflect the availability of CO2 in the soil
during the dissolution of limestone, which is a function of the water level in the karst and
thus of the local precipitation amounts.
By combining several stalagmites from the same cave or region, scientists can reconstruct
long-duration, high-resolution, and well-dated paleoclimate records spanning multiple
glacial–interglacial cycles
A recent global compilation of speleothem δ18O and δ13C records, the Speleothem Isotope
Synthesis and Analyses database (SISALv2), includes 691 speleothem records from 294
cave sites (Comas-Bru et al. 2020) spanning the majority of the world’s major karst regions
(Chen et al. 2017) in the low-to-mid latitudes
There are many environmental, aesthetic, ethical, and legal factors to consider when
sampling stalagmites in caves, and care should be taken to collect previously broken
samples whenever possible to minimize impact on the cave environment.
◦ Speleotherms most likely to yield results should be selected
◦ Once collected, stalagmites are typically cut in half lengthwise, polished, and digitally
imaged prior to subsampling for geochemical analyses.
◦ Oxygen and carbon isotope ratios are normally measured by isotope ratio mass
spectrometry analysis of calcite powders, though secondary ionization mass
spectrometry (SIMS) has proven a promising technique for ultrahigh-resolution in situ
analyses