Modeling the particle flux effect on distribution of Th-230 in the equatorial Pacific

The normalization of sediment accumulation rates by Th-230 is increasingly used to constrain particle rain rates because Th-230 is produced at a known rate in the water column and removed relatively quickly to the sediment. Several recent contributions have questioned this approach and suggested that the preferential removal of Th-230 in areas of high particle flux, the "particle flux effect,'' introduces a systematic bias in the Th-230 normalization technique. We use a circulation model that includes a description of particle scavenging to show that the particle flux effect cannot explain observations of high Th-230 accumulation in equatorial Pacific sediments (relative to Th-230 production in the water column). We further consider the possible variation of particle rain rates over time. We find only a minimal sensitivity in the particle flux effect due to increases in the particle rain rate at the equatorial Pacific by a factor of up to 10. This situation exists because the residence time of Th-230 in the water column is too short to permit significant lateral transport with reasonable isopycnal mixing coefficients. We conclude that the increased rates of Th-230 accumulation found in equatorial Pacific sediments deposited during the Last Glacial Maximum cannot have been caused by the particle flux effect.