Abstracts Category : Other

Understanding terrestrial organic carbon export : a time-series approach

by Jordon D Hemingway

Terrestrial organic carbon (OC) erosion, remineralization, transport through river networks, andburial in marine sediments is a major pathway of the global carbon cycle. However, our ability toconstrain these processes and fluxes is largely limited by (i) analytical capability and (ii) temporalsampling resolution.To address issue (i), here I discuss methodological advancements and data analysis techniquesfor the Ramped PyrOx serial oxidation isotope method developed at WHOI. Ramped-temperaturepyrolysis/oxidation coupled with the stable carbon (12C, 13C) and radiocarbon (14C) analysis ofevolved CO2 is a promising tool for understanding and separating complex OC mixtures. Toquantitatively investigate distributions of OC source, reservoir age, and chemical structure containedwithin a single sample, I developed a kinetic model linking RPO-derived activation energy, 13Ccomposition, and radiocarbon content. This tool provides a novel method to fundamentally addressthe unknown relationship between OC remineralization rates and chemical structure in variousenvironmental settings.To address issue (ii), I additionally present results from time-series sample sets collected ontwo end-member systems: the Congo River (Central Africa) and the LiWu River (Taiwan). Forthe Congo River, bulk and plant-wax-lipid 13C compositions indicate that a majority of particulateOC is consistently derived from downstream, C3-dominated rainforest ecosystems. Furthermore,bulk radiocarbon content and microbial lipid molecular distributions are strongly correlated withdischarge, suggesting that pre-aged, swamp-forest-derived soils are preferentially exported whennorthern hemisphere discharge is highest. Combined, these results provide insight into the relationshipbetween hydrological processes and fluvial carbon export.Lastly, I examined the processes controlling carbon source and flux in a set of soils and time-seriesfluvial sediments from the LiWu River catchment located in Taiwan. A comparison between bedrockand soil OC content reveals that soils can contain significantly less carbon than the underlyingbedrock, suggesting that this material is remineralized to CO2 prior to soil formation. Both thepresence of bacterial lipids and a shift toward lower activation energy of 14C-free OC contained insoil saprolite layers indicate that this process is microbially mediated and that microbial respirationof rock-derived OC likely represents a larger geochemical flux than previously thought.The results presented in this thesis therefore provide novel insight into the role of rivers in theglobal carbon cycle as well as their response to environmental perturbations.