X-ray and electron microprobe investigation of the speleothems from Godarville tunnel

A calcite deposit (stalagmite) from the Godarville tunnel (Belgium) was investigated by the use of x-ray and electron microprobes in order to detect any seasonal variations present in the chemical data and to make a comparison with meteorological data. The uniqueness of this speleothern system is that it is human constructed, controlled from the chronological point of view and with complete temperature and hydrological documentation. The Godarville tunnel stalagmites started to form since the time of closing down of the tunnel in 1960 and continued until the collection of the samples in December 1992. The growth of the annual laminae was observed on the cross-sections of several stalagmites and attributed to consecutive years and even seasons. The laminar structure, ascribed to seasonal variations, is very clear and formed by a set of alternating bright, porous and dark, compact calcite layers. This structure gave rise to the optical profiles of the annual variability that were compared with the x-ray synchrotron-based microprobe chemical profile data obtained at LURE (Orsay University, France). Using this method, several profiles were measured: Ca as the main element, Sr as a very important minor component and Fe, Zn, Pb and Br as trace elements. The results display an impressive correlation between the spatial distributions of Sr and Br with the optical linear profile of the sample, demonstrating that these elements also have a seasonal variation in the stalagmite layers. The Fe, Zn and Pb concentration profiles were characterized by sudden and random increases in contents (up to 20 times over background levels) whose origin is still uncertain. The results were supplemented by similar linear scan measurements made by the electron microprobe. The results show that Sr concentrates in the bright layers whereas the Mg spatial distribution is less correlated with the optical linear scan. A strontium-rich layer is formed during the autumn/winter periods. It is consistent with the ways of inclusion of that element in calcite, with the temperature-dependent and hydrological aspects. Copyright (c) 2005 John Wiley & Sons, Ltd.