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Understanding how stalagmites grow under changing climate conditions is of great significance for their application as a paleoclimate archive. In this study, we present a shape modeling approach to stalagmite growth by combining three existing models accounting for climate variables, karst water chemistry, and speleothem deposition. The combined model requires only four input parameters: calcium concentration of the water drop, drip interval, cave temperature, and cave carbon dioxide (CO2) concentration. Using the output of the coupled atmosphere–ocean–land surface model MPI-ESM1.2 and the CaveCalc model for speleothem chemistry, we simulated stalagmite growth at Sofular Cave, Northern Turkey, (in the last 25 kyr) and compared the results to those of the existing So-1 stalagmite from the same cave. This approach allows simulating, completely independent of measured boundary conditions, a stalagmite geometry that follows the trend of the experimental data for the growth rate, with input parameters within the respective error ranges. When testing the sensitivity of the individual model parameters, the model suggests that the stalagmite radius mainly depends on the drip interval, whereas the growth rate is driven by the calcium concentration of the water drop. The model is also capable of showing some basic phenomena, like a decrease in growth rate (as observed in the real stalagmite), as CO2 concentration in the cave increases. The coupling of input parameters for the model to climate models represents the first attempt to understand an important climate archive in its shape and isotope content and opens the possibility for a new inverse approach to paleoclimate variables and model constraints.

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.

In this study, the dead carbon fraction (DCF) variations in stalagmite M1-5 from Socotra Island in the western Arabian Sea were investigated through a new set of high-precision U-series and radiocarbon (14C) dates. The data reveal an extreme case of very high and also climate-dependent DCF. For M1-5, an average DCF of 56.2±3.4 % is observed between 27 and 18 kyr BP. Such high DCF values indicate a high influence of aged soil organic matter (SOM) and nearly completely closed-system carbonate dissolution conditions. Towards the end of the last glacial period, decreasing Mg∕Ca ratios suggest an increase in precipitation which caused a marked change in the soil carbon cycling as indicated by sharply decreasing DCF. This is in contrast to the relation of soil infiltration and DCF as seen in stalagmites from temperate zones. For Socotra Island, which is influenced by the East African–Indian monsoon, we propose that more humid conditions and enhanced net infiltration after the Last Glacial Maximum (LGM) led to dense vegetation and thus lowered the DCF by increasing 14CO2 input into the soil zone. At the onset of the Younger Dryas (YD) a sudden change in DCF towards much higher, and extremely variable, values is observed. Our study highlights the dramatic variability of soil carbon cycling processes and vegetation feedback on Socotra Island manifested in stalagmite DCF on both long-term trends and sub-centennial timescales, thus providing evidence for climate influence on stalagmite radiocarbon. This is of particular relevance for speleothem studies that aim to reconstruct past atmospheric 14C (e.g., for the purposes of 14C calibration), as these would rely on largely climate-independent soil carbon cycling above the cave.