The Response of the Tropical Carbon Cycle to Post-glacial Hydroclimate Variations

在过去的一个世纪里，陆地生物圈--植物和土壤--隔离了大约三分之一的人为温室气体排放，从而减缓了大气二氧化碳水平的上升。然而，陆地生物圈储存的碳量在很大程度上取决于气候，主要是温度和降雨强度，因此陆地生物圈未来可能成为大气的碳源。因此，预测大气二氧化碳水平取决于我们对陆地生物圈将如何应对未来温室气体排放和随之而来的气候变化的机械性理解。该项目旨在评估过去20,000年来气候变化如何调节热带陆地生物圈的碳储存。预期结果将更好地从机制上了解气候对陆地生物圈减少或恶化温室气体排放的能力的影响，这对于潜在地减轻对社会的影响是至关重要的。该项目旨在通过伍兹霍尔伙伴关系教育计划(PEP)将年轻科学家纳入尖端研究，该组织旨在为大学三年级和大四学生提供获得海洋和环境科学实践经验的机会。最后，这项工作将为麻省理工学院/世界卫生组织海洋学联合项目的一名博士生提供培训和教育。该项目的主要目标是从机械上了解气候变化如何影响和将影响热带陆地有机碳循环的时间尺度和动态。根据先前对不同气候制度下陆地有机碳动态的研究结果，人们可以假设：(1)热带流域陆地有机碳循环的动态主要受水文条件的控制；(2)在降水增加的时期，由于土壤有机碳周转加快，陆地有机碳的储存时间减少，从而向海洋提供较年轻的陆地有机碳。该项目将通过重建自上一次冰川盛期以来生物圈储存库中维管植物衍生的有机碳在陆地上的停留时间来检验这些假设，整个印度-太平洋暖池有六个地点。在过去的20,000年里，这些遗址具有非常好的水文历史特征和对比--但相似和温和的温度强迫--因此提供了补充的案例研究，以测试水文气候和陆地碳循环之间的关系。此外，这些地点代表了一系列生态环境，是其他热带和季风生态系统的良好类比，预计这些生态系统在不久的将来将发生重大水文变化。该项目将使用最先进的陆地生物标志物的复合特定放射性碳测年技术重建沉积有机碳库中陆地-植被部分的生物圈停留时间。重建的陆地有机碳停留时间变化将在古气候记录的背景下进行解释，古气候记录是从为本研究选择的相同沉积物岩芯获得的数据提供的。因此，该项目将深入了解气候控制陆地有机碳循环和向热带海洋输送的时间尺度和动态的机制。这一新的机制理解将广泛适用于世界各地的热带系统，从而对全球碳循环具有重要意义。

Over the past century, the terrestrial biosphere - plants and soils - has sequestered about one third of all anthropogenic greenhouse gas (GHG) emissions, thereby mitigating the increase in atmospheric carbon dioxide level. However, the amount of carbon stored by the terrestrial biosphere largely depends upon climate, chiefly temperature and rainfall intensity, such that the terrestrial biosphere could become a source of carbon to the atmosphere in the future. Forecasting atmospheric carbon dioxide levels thus relies on our mechanistic understanding of how the terrestrial biosphere will respond to future GHG emissions and attendant climate change. This project seeks to evaluate how climate changes over the past 20,000 years have modulated the stores of carbon in the tropical terrestrial biosphere. The expected outcome will provide a better mechanistic understanding of the influence of climate on the ability of the terrestrial biosphere to mitigate or worsen GHG emissions, which is critically needed to potentially mitigate the impacts on society. This project is designed to allow for the incorporation of young scientists into cutting-edge research through the Woods Hole Partnership Educational Program (PEP) - an organization designed to provide college juniors and seniors an opportunity to gain practical experience in marine and environmental science. Finally, this work will provide for the training and education one Ph.D. student in the MIT/WHOI Joint Program in Oceanography.The primary objective of this project is to provide a mechanistic understanding of how climate change has affected and will affect the timescales and dynamics of terrestrial organic carbon cycling in the tropics. Based on outcomes of prior studies of terrestrial organic carbon dynamics across climate regimes one can posit that: (1) the dynamics of terrestrial organic carbon cycling in tropical catchments is predominantly controlled by hydrological conditions; and (2) the terrestrial storage time of organic carbon decreases during periods of increased precipitation due to faster soil organic carbon turnover, thereby contributing younger terrestrial organic carbon to the ocean. This project will test these hypotheses through the reconstruction of terrestrial residence times of vascular plant-derived organic carbon in biospheric reservoirs since the last glacial maximum at a set of six sites throughout the Indo-Pacific Warm Pool. Over the last 20,000 years, these sites have exceptionally well characterized and contrasted hydrologic histories - but similar and muted temperature forcing - thus providing complementary case studies to test the relationship between hydroloclimate and terrestrial carbon cycling. In addition, these sites represent a range of ecological settings and serve as good analogs for other tropical and monsoonal ecosystems, where major hydrologic changes are projected for the near future. This project will reconstruct the biospheric residence time of the terrestrial-vegetation component of the sedimentary organic carbon pool using state of the art compound specific radiocarbon dating of terrestrial biomarkers. The reconstructed variations in terrestrial organic carbon residence time will be interpreted in the context of paleoclimate records provided by data obtained from the same sediment cores selected for this study. This project will thus provide insights into the mechanisms by which climate controls the timescale and dynamics of terrestrial organic carbon cycling and delivery to the tropical ocean. This new mechanistic understanding will be widely applicable to tropical systems worldwide, thereby holding significance for the global carbon cycle.