Tracers of chemical scavenging in the ocean: boundary effects and large-scale chemical fractionation

There is evidence that chemical scavenging is important in governing the behaviour of many heavy metals and other trace substances in the ocean. The rate at which scavenging occurs can be estimated from measurements of radioactive imbalance in seawater. The distribution of lead 210 indicates intensified uptake at boundaries, especially at the ocean margins. This has been confirmed by sediment traps and measurements of lead 210 accumulations in the sediment column. A similar phenomenon of intensified scavenging at ocean margins has been observed for protactinium 231. Studies show that its delivery to the sediments is mainly caused by the local flux of sinking particles. The two ways for removal by scavenging - one with a strong horizontal component due to the intensified uptake at the margins and the other with a strong vertical component due to the particle flux can be followed by comparing levels of thorium 230 and protactinium 231. Model calculations indicate that horizontal mixing rate and particle flux are also important in the partitioning of reactive chemical substances in sediments. Changes in time in the partitioning can be recorded as changes in the thorium 230/palladium 231 ratio in a sediment column. (U.K.)