Radiocarbon Calibration using Speleothems II - Soil carbon-climate interaction

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Understanding natural variations of the atmospheric 14C content is an essential prerequisite for radiocarbon dating. The knowledge of past atmospheric radiocarbon variability than allows for improvements in our understanding of the variability of the climate induced global carbon cycle changes, the Earth's magnetic field, the cycling of carbon through the ocean and land biosphere, as well as the precise dating of climatic events. According to recent radiocarbon variations obtained in stalagmites from China, novel opportunities to study global carbon cycle changes and the production of cosmogenic nuclides have come to light. Significant deviations from previous calibration curves (e.g., IntCal13) and large differences in the proportion of dead carbon in various subtropical speleothems raise the issue of reassessment of carbon transport processes in soils and the subsequent transport and storage of radiocarbon in speleothems. In our previous project, we reconstructed the 14C concentration using speleothems from Socotra (Yemen) and Sofular (Turkey) with a time resolution in cases of some hundred years and spanning more than 30,000 years. These records demonstrate dramatic climate-induced changes in the soil carbon cycle. Furthermore, a basic assumption of the previous project to evaluate hydrological influences on the carbon cycle, i.e. covariance of Mg/Ca ratio and radiocarbon in speleothems on time scales of centuries, has been rejected. Although a close association between atmosphere and speleothem 14C was validated in our previous study, the reasons for dramatic centennial climate-induced changes in soil CO2 cycling remain elusive and poorly understood. In this follow-up project, we now aim to profoundly advance our understanding of the mechanisms that lead to variations of 14C in subtropical speleothems driven by climate change, hydrology, and soil CO2 cycling. This will be done through new observations on selected speleothems and modern carbonates and drip water from Larga cave in Puerto Rico, and through the development of a comprehensive numerical model to simulate speleothems 14C under various climate aspects (vegetation, soil carbon cycling, precipitation, isotope fractionation, chemical transient change) to ultimately determine the covering geochemical and climate processes. Armed with a fundamental understanding of the carbon cycle leading to speleothems 14C observations, one can ultimately reevaluate the importance and significance of differences between speleothems and other 14C archives.