Scavenging and Transport of Thorium Radioisotopes in the North Atlantic Ocean

Paul Lerner, Ph.D., 2018
Olivier Marchal, Co-Advisor
Phoebe Lam, Co-Advisor

This thesis focuses on understanding the cycling of thorium as affected by particle concentration
and particle composition in the North Atlantic. This objective is addressed using (i) thorium
radioisotopes and particles measured along the GEOTRACES North Atlantic Transect (GA03)
and (ii) a model for the reversible exchange of thorium with particles. Model parameters are either
estimated by inversion, or prescribed in order to simulate 230Th in a circulation model.
The inversion produces rate parameters of the reversible exchange model that show systematic
variations along GA03. In particular, k1, the apparent first-order rate ”constant” of Th adsorption
onto particles, generally presents maxima in the mesopelagic zone and minima below. Positive
correlations are found between (i) k1 and bulk particle concentration and (ii) k1 and particle composition.
Among the particle phases examined, Mn (oxyhydr)oxides and biogenic particles most
strongly influence k1. I also find that dissolved 230Th data are best represented in the circulation
model by assuming enhanced values of k1 near the seafloor. Collectively, my findings suggest
that spatial variations in Th radioisotope activities observed in the North Atlantic partly reflect
variations in the rate at which Th is removed from the water column.