Radiocarbon in Stalagmites: Indicator of Climate Variability and Key to Atmospheric Radiocarbon Reconstruction

Abstract

Due to their ability to record climate change over large periods of time, stalagmites remain a primary focus of paleoclimatology. Utilizing high-precision age determination methods such as U-series dating, it is possible to measure climate-induced variations of the geochemical composition of speleothems over the time of their growth periods. This dissertation focuses on the investigation of radiocarbon (14C) concentration in several stalagmites. To this end, a new setup for the chemical preparation of carbonate samples for 14C measurements was planned and successfully put into operation. The incorporation of 14C into stalagmites is dependent on various climate-related processes in the soil and in the karst host rock above the cave. Using stable isotope ratios and trace element concentrations, this was investigated in two case studies with high-resolution 14C measurements. Reduced 14C concentration was observed in a stalagmite originating from Moomi Cave on Socotra Island which is indicative of aged soil organic matter influencing the stalagmite formation. It was demonstrated that a combination of higher soil humidity and denser vegetation towards the end of the last glacial period caused higher stalagmite 14C concentration. In a second study on a stalagmite from Sofular Cave in Turkey, the 14C signature allowed for the observation of various aspects of soil carbon dynamics from the last glacial period through into the Holocene. Moreover, the record suggests an increase of atmospheric 14C concentration coincident with the geomagnetic minimum approximately 41000 years ago. This may contribute to the ongoing effort to improve radiocarbon calibration datasets, on which the 14C dating method is based. Lastly, a summary of various stalagmite studies conducted at the Institute of Environmental Physics and a review of the relevant processes of stalagmite 14C incorporation is presented.