A global synthesis of speleothem radiocarbon data – is it a sensitive proxy for past ecosystem processes?

Speleothems, secondary cave carbonate precipitates, can serve as tools to reconstruct past terrestrial ecosystem processes, particularly related to soil and vegetation. Radiocarbon, often in conjunction with other geochemical proxies, has been increasingly used for this purpose, as the speleothem reservoir effect retains useful information on local ecosystem conditions. On the other hand, speleothems are also of interest to the radiocarbon community as they can be dated very precisely with the U-Th method, which may allow the reconstruction of atmospheric radiocarbon levels in time if the reservoir effect remained constant.Over the past decades, a growing number of speleothem radiocarbon records have been generated from vastly different climate zones and ecosystem types. While much progress has been made in the interpretation of these records, a unified and global view of the factors driving variability in speleothem radiocarbon is still lacking. We compiled a global dataset of averaged speleothem radiocarbon measurements, and provide a critical evaluation of the applicability of the radiocarbon reservoir effect as a proxy for past ecosystem conditions. We compare our dataset to geographic (latitude, elevation), climatic (temperature and precipitation), and ecosystem and geological parameters (soil and bedrock thickness, soil age, vegetation type, and land cover). Our preliminary results show that it is difficult to extract a strong globally relevant driving factor for the mean absolute value in the reservoir effect at the investigated cave sites, and highlights the importance of detailed reporting of local conditions.To provide insight into the amplitude range of processes affecting published speleothem radiocarbon records we perform a series of numerical forward modeling experiments. We test how the effects of changing soil age, soil pCO2, carbonate dissolution regime, and pyrite oxidation affect carbon isotopes in stalagmites.Together, the global synthesis and modeling experiments provide us with the first global overview of how cave site parameters and climate and ecosystem processes affect speleothem radiocarbon records, and allow us to assess the sensitivity of this proxy as a tool for past ecosystem conditions.