Short-lived radium isotopes on the Scotian Shelf: Unique distribution and tracers of cross-shelf CO2 and nutrient transport

Abstract

Radium (Ra) isotopes have become an established tool for investigating mixing rates on continental shelves, and more recently have been used to quantify the release of dissolved compounds enriched in pore-waters into the water column. We present results from Ra sampling of the Scotian Shelf region of the Canadian northwestern Atlantic Ocean, which reveal cross-shelf Ra distributions that are unique compared to other coastal regions. We explain the observations of lower 224Ra activities near the coast, relatively high activities at large distances offshore (>100km), and gradients in both offshore and onshore directions by referring to regional geomorphology, as well as shelf bathymetry and circulation patterns. Vertical mixing above shallow offshore banks allows for Ra enrichments in offshore surface waters, while horizontal dispersion of this bottom-generated signal can transport Ra off the shelf break in surface waters, and towards the shore beneath the surface mixed layer. On the Scotian Shelf, estimating horizontal diffusivity (KX) using a 1-D Ra diffusion model is limited by the presence of two distinct Ra sources (land and sediment), by three dimensional shelf topography, as well as by complex hydrodynamic conditions. Here, a numerical 2-D diffusion model reproduces Ra distributions on the shelf using both coastal and benthic sources. Horizontal and vertical mixing coefficients are then calculated by minimizing deviations between model output and observations. Onshore gradients in CO2 and nutrient species combined with model-derived KX values can yield onshore carbon and nutrient fluxes in subsurface waters, which in turn supply the CO2 outgassing from the Scotian Shelf. Our results thus provide constraints for cross-shelf transports of carbon and nutrients on the Scotian Shelf in order to guide mass balance or model based budget approaches in future studies.