Particulate and soluble ²¹⁰Pb activities in the deep sea

Abstract

Particulate and soluble, ²¹⁰Pb activities have been measured by filtration of large-volume water samples at two stations in the South Atlantic. Particulate phase ²¹⁰Pb (caught by a 0.4-μm filter) varies from 0.3% of total ²¹⁰Pb in equatorial surface water to 15% in the bottom water. The "absolute activity" of ²¹⁰Pb per unit mass of particulate matter is about 10⁷ times the activity of soluble ²¹⁰Pb per unit mass of water, but because the mass ratio of particulate matter to water is about 10⁻⁸, the particulate phase carries only about 10% of the total activity. In Antarctic surface water the particulate phase carries 40% of the total ²¹⁰Pb activity; the absolute activity of this material is about the same as in other water masses and the higher fraction is due to the much larger concentration of suspended matter in surface water in this region. In the equatorial Atlantic the particulate phase ²¹⁰Pb activity increases with depth, by a factor of 40 from surface to bottom, and by a factor of 4 from the Antarctic Intermediate Water core to the Antarctic Bottom Water. This increase with depth is predicted by our previously proposed particulate scavenging model which indicated a scavenging residence time of 50 years for ²¹⁰Pb in the deep sea. A scavenging experiment showed that red clay sediment removes all the ²¹⁰Pb from seawater in less than a week. The Antarctic particulate profile shows little or no evidence of scavenging in this region, which may be due to the siliceous nature of the particulate phase in circumpolar waters. Our previous observation that the ²¹⁰Pb/²²⁶Ra activity ratio is of the order of 0.5 in the deep water is further confirmed by the two South Atlantic profiles analyzed in the present work.