THE USE OF RADIUM ISOTOPES TO INVESTIGATE BOUNDARY EXCHANGES IN COASTAL OCEAN SYSTEMS

Abstract

Coastal seas are of major economic and social importance for the majority of the human population, and are amongst the most biologically and geochemically active areas of the biosphere. Coastal systems are also highly dynamic and complex areas of the global oceans, due in part to the extensive interactions that occur at the system boundaries. Carbon and nutrient fluxes across these boundaries at the seafloor, the atmosphere, the land, and the shelf-break, fuel intense biological activity and high spatial and temporal variability characteristic of coastal seas. Despite their importance, the magnitudes of dissolved material exchanges across these boundaries represent a significant area of uncertainty in coastal ocean biogeochemistry. This research utilizes naturally-occurring radium isotope tracers (224Ra, 223Ra, 228Ra), which have well defined land and seafloor sources, to quantify cross-boundary fluxes in various coastal regions of the North Atlantic ocean. This thesis presents flux measurements in a series of coastal regimes, throughout which a clear progression can be seen in the overall scope of the research. The initial study utilizes vertical radium profiles and a 1-D diffusion model to quantify benthic fluxes of carbon, nutrients and oxygen at a time-series station in a small enclosed basin (Bedford Basin, Nova Scotia). This work is then extended to transects of radium measurements across an open continental shelf (Scotian Shelf, northwestern Atlantic) which are combined with 2-D numerical simulations to yield estimates of cross-shelf carbon and nutrient transport from the adjacent deep ocean. Finally, a comprehensive basin-wide radium survey in a semi-enclosed shelf sea of the northeastern Atlantic (the North Sea) is used to provide detailed assessment of sediment-water column exchanges across the region and land-ocean interaction at the European continental coastline. Despite differences in the geometry and spatial scale of each region, significant boundary exchange in all the regimes provides the opportunity to quantify important carbon inputs using radium-based techniques. Overall, this research provides a greater understanding of both the magnitude and biogeochemical impact of boundary exchanges in coastal waters, while also encouraging and guiding future radium studies through novel methodology and detailed results.