Radiocarbon Calibration using Speleothems II - Soil carbon-climate interaction

了解大气14C含量的自然变化是放射性碳测年的基本前提。了解过去大气中放射性碳的可变性有助于我们更好地了解气候引起的全球碳循环变化的可变性、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年代测定。根据中国最近从石笋中获得的放射性碳变化，研究全球碳循环变化和宇宙成因核素生产的新机会已经浮出水面。与以前的校准曲线(如IntCal13)的显著偏差，以及各种亚热带洞穴中死碳比例的巨大差异，提出了重新评估土壤中的碳迁移过程以及随后放射性碳在洞穴中的迁移和储存的问题。在我们之前的项目中，我们使用索科特拉(也门)和索菲勒(土耳其)的洞穴标本重建了14C浓度，时间分辨率为数百年，跨度超过30,000年。这些记录表明气候引起的土壤碳循环发生了戏剧性的变化。此外，先前项目关于评估水文对碳循环的影响的基本假设，即在几个世纪的时间尺度上洞穴中的镁/钙比和放射性碳的协方差，已被拒绝。尽管我们之前的研究证实了大气和洞穴14C之间的密切联系，但百年来气候引起的土壤二氧化碳循环急剧变化的原因仍然难以捉摸，也知之甚少。在这个后续项目中，我们现在的目标是深刻地推进我们对气候变化、水文和土壤CO2循环驱动的亚热带洞穴14C变化的机制的理解。这将通过对选定的洞穴和波多黎各拉加洞穴的现代碳酸盐和滴水进行新的观察，并通过开发一个全面的数值模型来模拟各种气候方面(植被、土壤碳循环、降水、同位素分馏、化学瞬变)下的洞穴14C，以最终确定覆盖范围的地球化学和气候过程。在对导致洞穴14C观察的碳循环有了基本的了解后，人们最终可以重新评估洞穴与其他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.