Efficiency of biological pump estimated by using the short-lived Th isotopes

This article reviews the recent studies on the export flux of particulate organic carbon (POC) from the euphotic zone to the deep sea using 234Th(half-life, 24.1 days) and 228Th(half-life, 1.9 years) as natural tracers. The POC export flux estimated by the 234Th method generally has a large error, propagated from uncertainties in the estimates of 234Th flux and POC/234Th rations in sinking particulate matter. In relatively low particle flux regime, where the surface water 234Th is close to secular radioactive equilibrium with 238U, reducing errors in the 234Th measurements becomes of primary importance and hence the analytical method should be chosen based not on wherever possible in such cases. It is also clear that more efforts are needed to establish a relation between the rate of POC removal and 234Th flux. Nonetheless, the natural variations of POC flux with location, season and depth, are large enough to make the 234Th-based POC export fluxes to be significant in our understanding of the mechanism of 'biological pump'. Much of the ocean, like the equatorial Pacific and the oligotrophic regions were the sea conditions are stable, ThE defined as a ratio of 234Th-based POC export to primary production (Buesseler, 1998) is low at 2-10%, suggesting that POC is efficiently recycled within the euphotic zone. However, during bloom conditions at high latitudes and at mid-latitudes in spring, ThE approaches to ∼50%. Such high POC export flux most often occurs in food webs with primary producers dominated by large phytoplankton, particularly diatoms. (author)