231Pa / 230Th fractionation by ocean transport, biogenic particle flux and particle type

231Pa and 230Th are removed from the water column by a process of reversible scavenging which quickly removes 230Th to the sediment. 231Pa is less efficiently scavenged onto particles than 230Th and is therefore more effectively transported via advection and diffusion before it reaches the ocean sediment. This study combines particle fields (dust, opal, CaCO3, POC) derived from observations with the Bern3D intermediate complexity ocean model and an equilibrium-scavenging model for isotopes. The equilibrium partition coefficient for particulate versus dissolved isotope activity is varied with particle type. The model can explain many of the features of the global 231Pa and 230Th distribution. The success of such a simple model at representing the global pattern of 231Pa / 230Th activity ratio supports the use of this proxy in paleoceanographic studies. We use the model to address the controversy concerning which particle types are dominant in fractionating 231Pa / 230Th in the ocean. The lithogenic (dust) flux is found to be unimportant for 231Pa / 230Th fractionation- the ocean fractionation of 231Pa / 230Th is dominated by the distribution of the CaCO3 and opal flux. We also confirm that opal is a weak scavenger of 230Th.