Monitoring present day climatic conditions in tropical caves using an Environmental Data Acquisition System (EDAS)

Abstract

This paper presents data from automatic stations which have been installed for monitoring climatic parameters in caves in two areas of Brazil. These devices, initially developed at the Royal Observatory of Belgium to monitor environmental parameters in geophysical observatories, were adapted in our study to operate under tropical cave conditions and to measure temperature, atmospheric pressure and drip rate of stalactites. Similar devices were installed at the surface near to the caves to measure air temperature, atmospheric pressure and rainfall. The results reveal that the drip rate at the tip of stalactites is related to the effective rainfall (water excess). The stable drip regime observed during the dry season seems to be reproducible from one year to the other and could be related to the infiltration of water which has a long residence time in the aquifer. Regular pressure oscillations, with amplitude ranging between 1 and 2 mb, are observed in both of the monitored caves. Spectral analysis of the data suggests that these oscillations are linked to the diurnal and semi-diurnal solar tides (S1 and S2). In one cave, very small temperature variations (0.02–0.05 °C) are also observed with a similar diurnal and semi-diurnal pattern, and we argue that the generating process of the thermal components of the S1 and S2 frequencies is a mixture of thermal convection produced by the surface meteorological variations and of an adiabatic induction of the S2 atmospheric pressure modulation. A very large annual thermal amplitude (13 °C) is observed in the other cave; this is a great motivation to study the stable isotope geochemistry of its speleothems as they probably have recorded past temperature fluctuations linked to paleoclimate variations in this area of south-western Brazil.