Comparison of 234Th/238U and mass balance models for estimating metal removal fluxes in the Gulf of Maine and Scotian Shelf

Abstract

Two independent methods were used to estimate particle export fluxes of metals in the Gulf of Maine and Scotian Shelf. The first method used measurements of particulate trace metals (Cd, Cu, Fe and Pb) and metal/234Th ratios on large (>53µm), rapidly sinking, particles together with prior estimates of 234Th/238U disequilibrium to quantify the particle export flux of these metals. The second method adopted a mass balance approach whereby measurements of the distribution of dissolved (<0.4µm) and fine particulate (0.4–53µm) trace metals (Cd, Cu, Fe, Pb, Mn, Ni and Zn) were combined with rates of water volume transport to determine advective metal fluxes in the Gulf of Maine and Scotian Shelf. Estimates were also made of rates of external supply of metals from atmospheric, riverine and municipal sewage sources. Imbalances between total input fluxes to the Gulf of Maine and Scotian Shelf and the removal fluxes by offshore advection were used to estimate metal export fluxes to the sediments. The mass balance model export fluxes are generally within, or close to, the range of values determined independently from the 234Th/238U method, indicating that metal transport through shelf/slope regimes can be reasonably characterized using relatively limited data sets. Specifically, these results indicate that ~40–60% of the Pb and Zn inputs to the Gulf of Maine and Scotian Shelf are scavenged and exported to the sediments, whereas only ~5–15% of the Ni and Cu inputs are removed by particle export, as would be anticipated based on their respective particle affinities.