Real-time in situ measurements of the H/D ratio in water vapour in a limestone cave

The isotopic ratios recorded in speleothems in caves have been used as a measure of paleoclimate. The water on the surface of a growing speleothem contains dissolved C in the form of bicarbonate and O in water. The O incorporated into the speleothem is from surface water that is isotopically determined by the degree of surface evaporation, and has been seen to correlate to monsoonal activity. Cave temperature is likely to be relatively constant and unimportant for the isotopic signature. However, movement of air through the cave, driven by temperature differences between the cave and the external atmosphere could significantly change the isotopic signature and hence complicate the interpretation of any observed signal. We will report real-time measurements of the H/D ratio in water vapour measured in a cave system. The measurements have been made by continuously flowing air through a multi-pass gas cell and measuring the infrared spectrum with a low resolution Fourier Transform InfraRed spectrometer.