Role of biopolymers as major carrier phases of Th, Pa, Pb, Po, and Be radionuclides in settling particles from the Atlantic Ocean

• Organic components in particles are important carrier phases for radionuclides.
• Binding mechanisms of different radionuclides are postulated.
• Hydroxamate siderophores are postulated as important in the binding of Pa and Po.
• Hydroxamate siderophores might be important components of recalcitrant carbon.

The concentrations of potential organic (e.g., proteins, polysaccharides, uronic acids, hydroquinones, hydroxamate- and catechol-type siderophores) and inorganic (Fe, Mn, Si, and CaCO3) carrier phases for radionuclides (234Th, 233Pa, 210Po, 210Pb and 7Be) and their particle–water partition coefficients (Kd) were determined for particles collected by sediment traps deployed at the Oceanic Flux Program (OFP) site off Bermuda (500, 1500 and 3200 m). The purpose was to better understand the mechanisms that control the chemical composition of sinking particles as well as the scavenging and fractionation behavior of those five radionuclides. Different components contributed differently to the scavenging of different radionuclides at the three depths. Chemical considerations (e.g., ionic potential, ionization energy, multifunctional group structures), as well as factor analysis (FA) and correlations of logKd values with chemical parameters, indicate that hydroxamate siderophores are major classes of biopolymers that have a role in binding Po and Pa. MnO2 and FeO2, whose presence is closely related to that of hydroxamate siderophores (HS), are also involved in binding of Pa and Po. The carbonate and biogenic silica phases are identified to be important in predicting removal and fractionation of Th and Be in the ocean.