Efficacy of 230Th normalization in sediments from the Juan de Fuca Ridge, northeast Pacific Ocean

Th-230 normalization is an indispensable method for reconstructing sedimentation rates and mass fluxes over time, but the validity of this approach has generated considerable debate in the paleoceanographic community. Th-230 systematics have been challenged with regards to grain size bias, sediment composition (CaCO3), water column advection, and other processes. In this study, we investigate the consequences of these effects on Th-230 normalization from a suite of six cores on the Juan de Fuca Ridge. The proximity of these cores (< 30 km) suggests that they should receive the same particle rain rate of sediment, but the steep bathymetry of the ridge leads to substantial sediment redistribution and variable carbonate preservation, both of which may limit the usage of Th-230 in this region. Despite anticipated complications, 230Th normalization effectively reconstructs nearly identical particle rain rates from all six cores, which are summarily unrelated to the total sedimentation rates as calculated from the age models. Instead the total sedimentation rates are controlled almost entirely by sediment focusing and winnowing, which are highly variable even over the short spatial scales investigated in this study. Furthermore, no feedbacks on Th-230 systematics were detected as a consequence of sediment focusing, coarse fraction variability, or calcium carbonate content, supporting the robustness of the Th-230 normalization technique. (C) 2016 Elsevier Ltd. All rights reserved.