Distribution of 226Ra and 210Pb in the mixed layer of the western equatorial Pacific Ocean

Abstract

The residence time of particulate and dissolved ²¹⁰Pb in the upper layer of the western equatorial Pacific Ocean is examined. Activities of dissolved ²²⁶Ra, dissolved and particulate ²¹⁰Pb, and particulate ²¹⁰Po were determined to a depth of 300 m in a series of depth profiles collected along a transect across the equator at 155˚E in November 1993. Total ²¹⁰Pb in the surface water decreased from 2·7 Bq m^-3 at 10˚N to 1·8 Bq m^-3 at 10˚S. Dissolved ²¹⁰Pb generally decreased with depth but showed subsurface (100–150 m) maxima at 10˚N and 5˚N. The nutrient concentrations at 300 m were highest at these stations, suggesting some degree of upwelling. Calculations indicate that the residence times of dissolved (<0·45 µm) and particulate (>0·45 µm) ²¹⁰Pb in the top 300 m were 4·6–9·6 years and 0·15–0·29 year respectively.