Other data sources - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 193

Figure 6.6. Relationship between Vostok data and Devil's Hole data based on Devil's Hole
chronology.
6.2 SPELEOTHEMS IN CAVES
A speleothem is a secondary mineral deposit formed in a cave. Speleothems are
typically formed in limestone or dolostone caves. Water seeping through cracks
in a cave's surrounding bedrock may dissolve certain compounds, usually calcite
and aragonite (both calcium carbonate), or gypsum (calcium sulfate). The rate
depends on the amount of carbon dioxide held in solution, on temperature, and
on other factors. When the solution reaches an air-filled cave, a discharge of
carbon dioxide may alter the water's ability to hold these minerals in solution,
causing its solutes to precipitate. Over time, which may span tens of thousands of
years, the accumulation of these precipitates may form speleothems. One typical
form of speleothem is a stalactite which is a pointed pendant hanging from the
cave ceiling. Stalagmites are ground-up counterparts.
Some caves have yielded well-dated low-latitude low-elevation records that
characterize atmospheric moisture earlier in its transit from source regions. An
impressive cave study provided a record of Asian Monsoon precipitation, which
covers most times since the penultimate glacial period, about 160,000 ybp (Yuan
et al. 2004). This cave is located in China at 25 N latitude. Stalagmites with
diameters varying between 12 and 20 cm were collected 100m below the surface,
300 and 500m from the entrance. Stalagmites were subjected to oxygen isotope
analysis and 230 Th dating by thermal ionization. The resultant time series of d 18 O
bore some similarity to the d 18 O data from the GISP2 ice core over the past
70,000 years.