Scavenging of thorium isotopes in the Canada Basin of the Arctic Ocean

Abstract

Investigations on the distribution of thorium isotopes in the water column have been utilized as powerful tracers to investigate the geochemical pathways of particle-reactive material in the ocean. We measured Th ( 228Th, 230Th, 232Th and 234Th) isotopes in samples collected along a horizontal profile from Barrow, AK to the Canada Basin as well as two vertical profiles in the Canada Basin. The suspended particle concentrations were also measured to investigate the role of suspended particulate matter (SPM) in Th scavenging. The SPM inventory is more than an order of magnitude higher than that reported for the Alpha Ridge ice station, indicating that the SPM inventories vary over an order of magnitude in the deep Arctic Basins. The conditional partition coefficient of Th isotopes between suspended particulate matter and dissolved phase generally appears to be higher than those in non-polar regions, and this is attributed to more reactive suspended particulate matter. The residence times of total 230Th are significantly lower than those calculated for the Alpha Ridge station, while higher than those found in the Eurasian Basin and Beaufort Sea in the Canadian Basin. The calculated residence time in all the deep basins of the Arctic Ocean varied between 13±1 and 45±1 years. A reversible scavenging model applied to the distribution of Th isotopes ( 230Th– 228Th in the deep waters and 234Th– 228Th in the upper 100 m) indicates that the forward removal rate constant is higher than the value reported for the Alpha Ridge while the reverse rate constant is comparable to the Alpha Ridge station suggesting faster removal onto particulate matter and slower desorption of Th from the particulate matter.