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重要先决条件。对过去大气放射性碳变化的了解可以提高我们对气候变化引起的全球碳循环变化、地球磁场、海洋和陆地生物圈碳循环以及气候事件精确年代的理解。根据最近在中国石笋中获得的放射性碳变化，为研究全球碳循环变化和宇宙成因核素的产生提供了新的机会。与之前的校准曲线（例如 IntCal13）的显着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碳迁移过程以及随后洞穴中放射性碳的迁移和储存的问题。在我们之前的项目中，我们使用索科特拉岛（也门）和索富拉尔（土耳其）的洞穴沉积物重建了 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巨大的变化。此外，先前项目评估水文对碳循环影响的基本假设，即几个世纪时间尺度上洞穴中镁/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与洞穴植物 14C 之间的密切联系，但百年以来气候引起的土壤二氧化碳循环发生巨大变化的原因仍然难以捉摸且知之甚少。在这个后续项目中，我们现在的目标是深入推进我们对气候变化、水文学和土壤二氧化碳循环驱动的亚热带洞穴中 14C 变化的机制的理解。这将通过对选定的洞穴沉积物和现代碳酸盐以及来自波多黎各拉尔加洞穴的滴水进行新的观测来完成，并通过开发综合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、化学瞬态变化）下的洞穴沉积物14C，以最终确定覆盖的地球化学和气候过程。有了对导致洞穴 14C 观测的碳循环的基本了解，人们最终可以重新评估洞穴和其他 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.