TERRESTRIAL AND AQUATIC CARBON FATE AND TRANSPORT IN A TEMPERATE FLUVIOKARST WATERSHED

Abstract:

Karst topography has been well recognized to include high velocity, turbulent subterranean fluid pathways that are strongly coupled by fractures and openings to surface streams where water and sediment is exchanged. Recent research has focused on the fate and transport of organic carbon in surface dominated fluvial systems, but we find that little knowledge has been reported with regards to the transport and fate of terrestrially-derived sediment organic carbon within karst subterranean pathways. This study investigates the hypothesis that karst pathways could act as biologically active conveyors of sediment organic carbon that temporarily store sediment, turnover carbon at higher rates than would be considered otherwise, and recharge relatively depleted sediment organic carbon back to the surface stream within the fluvial system. Mixed research methods are applied within a mature karst network in the Bluegrass Region, Kentucky, USA, and methods included high resolution measurements of water and sediment characteristics for surface streams, carbon and stable carbon isotope measurements of transported sediment, and numerical modeling of water and sediment pathways. A modeling framework coupling physical and biogeochemcial processes of transport and decomposition is developed for use in mature fluviokarst watersheds. Results of this study show that phreatic nature of karst conduits results in the subsurface deposition of labile, storm-injected sediment which is subsequently decomposed by heterotrophic bacteria. Results of this study show a 40% depletion of organic content that corresponds with the depletion and outgassing of 0.06 metric tCkm^-2yr^-1 of organic carbon delivered to the subsurface conduit. This result is in contrast to surfacewater dominated streams in the Bluegrass Region for which research has shown that organic carbon can undergo a 50% enrichment from the time it enters the stream until the time it exits.