Carbon isotopes in stalagmites and drip water : Tracers of soil processes

Abstract

Stalagmites in caves are new climate archives recording meteorological parameters and processes occurring in the soil above caves. Due to advances in mass spectroscopy (TIMS and ICPMS) stalagmites can be dated reliably by the 230Th/U-method. Carbon isotopes, recorded in stalagmites, are of interest because they depend on climate influenced soil processes above caves. In this thesis 14C and 13C in speleothem environments were used as tracers to investigate soil processes for the present day situation and during the Holocene. The present day situation is studied by using monthly collected drip water samples from two caves (Ernesto cave in Trentino, Italy, and Bunker cave in Sauerland, Germany), which were analysed for their carbon isotope content. To interpret the isotopic composition a drip water model including various modes of limestone dissolution with respect to carbon isotopes was developed for the first time. The modelled carbon isotope composition of the drip water agrees well with the measurements. The annual trend in the carbon isotopes, observed in the drip water samples, can be attributed to changes in the water supply in the soil for Ernesto cave and to changes of the soil air carbon isotopic composition for Bunker cave. The information about the soil-cave-systems obtained in the investigation of the present day situation was applied to interpret the carbon isotopes of Holocene stalagmites of both caves. An inverse modelling method was developed to determine the soil CO2 content from measured carbon isotope pairs (14C, 13C). The results indicate that the soil CO2 content increased during the late Holocene in the soil above Ernesto cave due to a rising vegetation density. The Bunker cave stalagmite reveals a constant soil pCO2 in the past.