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——吸收了大约三分之一的人为温室气体排放，从而减缓了大气中二氧化碳水平的增加。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，陆地生物圈在未来可能成为大气碳的来源。因此，对大气二氧化碳水平的预测依赖于我们对陆地生物圈如何应对未来温室气体排放和随之而来的气候变化的机制理解。该项目旨在评估过去2万年的气候变化如何调节热带陆地生物圈中的碳储量。预期结果将更好地从机制上理解气候对陆地生物圈减缓或加剧温室气体排放能力的影响，这是潜在地减轻对社会的影响所迫切需要的。该项目旨在通过伍兹霍尔伙伴关系教育计划（PEP）将年轻科学家纳入前沿研究，该组织旨在为大学大三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为气候变化如何影响并将如何影响热带陆地有机碳循环的时间尺度和动态提供机制理解。基于已有的陆地有机碳动态研究成果，可以认为：(1)热带流域陆地有机碳循环动态主要受水文条件控制；(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缩短，从而向海洋贡献了更年轻的陆地有机碳。本项目将在整个印度洋-太平洋暖池的六个地点，通过重建自末次冰期极大期以来生物圈储层中维管植物来源的有机碳的陆地停留时间，来测试这些假设。在过去的2万年里，这些地点非常好地描述和对比了水文历史——但相似且温和的温度强迫——从而提供了互补的案例研究来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的生态环境，可以很好地模拟其他热带和季风生态系统，在这些生态系统中，预计在不久的将来会发生重大的水文变化。本项目将利用最先进的陆地生物标志物复合特定放射性碳定年技术，重建沉积有机碳库中陆生植被组分的生物圈停留时间。重建的陆地有机碳停留时间的变化将在本研究所选择的相同沉积物岩心数据提供的古气候记录的背景下进行解释。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。这种新的机制理解将广泛适用于全球热带系统，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.