|Stalagmites as a climate archive|
Fluid inclusions are ubiquitous in many types of minerals, and they keep a memory of their formation conditions. Stalagmites bear an abundance of water fluid inclusions, where the pressure and temperature present in the cave during the stalagmite formation time is kept as a secret. To reveal it, we turn to the measurement of the liquid-vapour homogenisation temperature of the water inclusion; when cooled down to the density maximum of the water inside the inclusion, the water enters a metastable liquid state. Energetically, a two-phase liquid-vapour state is more favorable, but the lack of a nucleation site usually hinders the nucleation of a vapour bubble. A single ultrashort laser pulse is sufficient to transfer the metastable liquid state into the stable two-phase state. Upon heating of the inclusion, the liquid water expands again at the expense of the vapour phase until, at the homogenisation temperature, the vapour bubble collapses and the inclusion goes back into a stable all-liquid state.
The laboratory is equipped with all the tools needed to access this homogenisation temperature. It can be measured easily and is dependant on the formation temperature and the volume of the inclusion. If we are able to measure the volume of the inclusion, we can therefore calculate the formation temperature that equals the cave temperature at the time of the stalagmite formation. This cave temperature can then serve as a measure of the local climate around the cave. Like this, climate records well-resolved both in time and geographically, can be obtained.