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