Particle/Solution Partitioning of Thorium Isotopes in Framvaren Fjord: Insights into Sorption Kinetics in a Super-Anoxic Environment

Abstract

Water samples were collected in a detailed depth profile (32 samples) of Framvaren Fjord (water depth: 183 m) and examined for dissolved ( 0.22 µm) phase 234Th (t1/2 = 24.1 days) concentrations. The particulate thorium fraction is relatively constant (50–60% of total) in the oxic surface waters of Framvaren Fjord. Below the oxic-anoxic interface (approximately 18 m), the particulate thorium fraction increases dramatically, reaching a maximum (96%) at 24 m, coincident with the maximum in total suspended matter concentrations (TSM). In this environment, the incorporation of thorium into the particulate phase seems to be more closely tied to the presence of bacteria than to the formation of particulate Fe and Mn oxyhydroxide phases. Distribution coefficients (Kd) for thorium in surface waters are 106–107 L/kg, typical for low particulate marine environments. Below the TSM maxima, Kd values are much lower (104–105 L/kg) and remain consistently low in the anoxic bottom waters. These are among the lowest Kd values for thorium ever observed for a marine environment. Equilibrium and kinetic approaches to quantifying the flux of thorium into the particulate phase indicate that, in the anoxic bottom waters of Framvaren Fjord, thorium sorption reactions become less dominant relative to reverse reactions such as desorption and particulate remineralization.