Earth

The age of caves

Chemical deposits in caves offer extraordinary  research possibilities to those who are engaged in reconstructing the geological history of the past.
These can, in fact, be easily dated with a method based on the decadence of certain isotopes of the radioactive ‘family’ of 238U.
The latter, in fact, decays into a series of elements: ²³⁴Th, ²³⁴Pa, ²³⁴U, ²³⁰Th, up to ²⁰⁶Pb, which is stable.
Cave speleothemes contain uranium, which substitutes calcium in the crystalline network of calcite, but they do not contain thorium. From the moment the speleotheme is formed,  238U starts decaying, changing into ²³⁰Th. The  measure of the concentration of ²³⁰Th in the calcite is therefore a measure of the time elapsed from its formation. Hence, by measuring the ratio ²³⁰Th/²³⁴U and ²³⁴U/²³⁸U (²³⁴U is another descendent of ²³⁸U) it is possible to obtain the age of a calcite speleotheme.
The U/Th method of dating is very efficient, but only allows the dating of very young calcite, not older than 350,000 years. Using the ²³⁴U/²³⁸U ratio it is possible to extend this limit to 1.5 million years.
Recently it has been discovered that most speleothemes are much older than 1.5 million years and hence other methods are being studied  currently, such as the U/Pb method (that works well on very old deposits) or paleomagnetic methods.
As far as the study of sediments that contain pebbles brought from the outside is concerned,  experiments are being carried out with the so-called cosmogenic isotope method. Cosmic ray radiation (which gives the method its name) produces 10Be, 26Al and other isotopes in addition to the better known 14C in the network of certain minerals (for example, quartz) when they are exposed on the surface .
When sediments are buried beyond the effect of the cosmic rays (for example, in caves deeper than 30 m), the cosmogenic isotopes begin to decay and it is possible to determine, in a way similar to the U/Th  method, the moment of burial, i.e. the age of the deposit, for dates varying between 100,000 to 5 million years.