Radiocarbon Calibration using Speleothems II - Soil carbon-climate interaction

了解大气中14C含量的自然变化是放射性碳测年的必要前提。对过去大气放射性碳变异性的了解使我们能够更好地理解气候变化引起的全球碳循环变化、地球磁场、碳在海洋和陆地生物圈中的循环以及气候事件的精确定年。根据最近在中国石笋中获得的放射性碳变化，研究全球碳循环变化和宇宙核素产生的新机会已经出现。与以往校正曲线（如IntCal13）的显著偏差以及不同亚热带土壤中死碳比例的巨大差异，提出了重新评估土壤中碳运输过程以及随后土壤中放射性碳的运输和储存的问题。在我们之前的项目中，我们利用索科特拉（也门）和索富拉尔（土耳其）的洞穴化石重建了14C浓度，时间分辨率在几百年的情况下，跨越了3万多年。这些记录证明了气候引起的土壤碳循环的剧烈变化。此外，先前评估水文对碳循环影响的项目的一个基本假设，即洞穴中Mg/Ca比率和放射性碳在世纪时间尺度上的协方差，已被否定。虽然我们之前的研究证实了大气和洞穴碳- 14之间的密切联系，但百年来气候引起的土壤二氧化碳循环剧烈变化的原因仍然难以捉摸，而且人们对其知之甚少。在这个后续项目中，我们现在的目标是深入了解气候变化、水文和土壤二氧化碳循环驱动的亚热带洞穴中14C变化的机制。这将通过对选定的洞穴、现代碳酸盐和波多黎各Larga洞穴的滴水进行新的观测，并通过开发一个综合数值模型来模拟洞穴14C在各种气候方面（植被、土壤碳循环、降水、同位素分馏、化学瞬态变化），最终确定覆盖的地球化学和气候过程。有了对导致洞穴主题14C观测的碳循环的基本理解，人们最终可以重新评估洞穴主题与其他14C档案之间差异的重要性和意义。

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.