A new tool for palaeoclimate reconstruction: Noble gas temperatures from fluid inclusions in speleothems

Abstract

The aim of our research is to apply the absolute palaeothermometer provided by dissolved noble gases in water to the reliably dated high-resolution archive of speleothems. Here we report on methods used to extract and quantify water and noble gases from fluid inclusions in speleothems and the first successful derivation of noble gas temperatures (NGTs) from this archive. The water amount was determined manometrically, whereas the low noble gas amounts were measured by mass spectrometry, applying rigorous blank control. Gas was extracted by crushing either in a steel cylinder or in a copper tube with little heating, which both yielded acceptably low blanks. Extraction in the steel cylinder is more efficient as a finer grain size distribution can be achieved. A major problem is the often high contribution of noble gases from air inclusions, masking the temperature information present in the noble gases dissolved in water-filled inclusions. However, NGTs with an uncertainty in the range of 1 °C can be derived from suitable stalagmites with low air/water ratios in the inclusions. Such favourable conditions were found to be present in stalagmites of milky white appearance. Suitable samples were collected from the stalagmites BU-U, BU-1 and a soda straw from the Bunker Cave in North-West Germany. Inverse modeling of measured Ne, Ar, Kr, and Xe concentrations leads to a mean temperature of (2.9 ± 0.7) °C for all BU-U sub-samples with an age between 10 800 and 11 700 years and of (7.1 ± 0.8) °C for a 1300 years old sample from BU-1. Compared to the modern mean annual air temperature at the investigated site of about 9.5 °C, the calculated temperatures appear rather low but not unreasonable, as the ages correspond to comparatively cool periods around the Younger Dryas/Preboreal transition and the Early Middle Ages, respectively. Moreover, the indicated temperature difference of about 4 °C between the early and late Holocene periods is in agreement with expectations.