Method

Fluid inclusion microthermometry is a classical technique to assess mineral formation temperatures based on the liquid-vapor homogenization temperature of the inclusions. Fluid inclusions in stalagmites, however, are typically monophase liquid and spontaneous nucleation of the vapor bubble fails to occur upon cooling due to long-lived metastability of the liquid state of water — a fact that previously made microthermometric analyses of these inclusions impossible. To overcome the metastable phase state, we apply single ultra-short laser pulses to stimulate vapour bubble nucleation. Upon subsequent heating, the liquid water expands while the vapor bubble becomes progressively smaller until it collapses, and the inclusion homogenizes again to the liquid phase. The method is therefore also referred to as nucleation-assisted microthermometry. In order to obtain the correct water density and formation temperature of the stalagmite, we use a thermodynamic model to correct for the effect of surface tension on the measured homogenization temperature (please watch our tutorial video Theoretical background)

Photo:

Yves Krüger

Ideally, one single fluid inclusion would be sufficient to determine the formation temperature of the stalagmite, i.e., the cave temperature at a given point in time. In reality, however, we observe a systematic scatter of temperatures derived from different fluid inclusions within the same growth bands. For this reason, we analyze about 40 fluid inclusions to obtain sound statistics for determining one single cave temperature. The precision of the reconstructed cave temperatures is excellent with standard errors of the mean (2SEM) between 0.2 and 0.5 °C.