TRACING PARTICLE CYCLING IN THE UPPER OCEAN WITH TH-230 AND TH-228 - AN INVESTIGATION IN THE EQUATORIAL PACIFIC ALONG 140-DEGREES-W

Vertical distributions of Th-230 and Th-228 were measured in large-vol ume (2000 to 3000 liter) seawater samples collected with the Multiple Unit In-Situ Large-Volume Filtration System (MVLVFS) and in-situ Fe/Mn -fiber enrichment technique. The samples were from the upper similar t o 800 m of the water column in the central equatorial Pacific along si milar to 140 degrees W during two survey cruises of JGOFS EqPac in Feb ruary/March (Survey I) and August/September (Survey II) 1992. Signific antly lower activities of Th-230 and Th-228 relative to those of their parent isotopes exist in the upper-ocean water column due to their pa rticle-reactive behavior. The activities of these thorium isotopes als o show large temporal variations, in response to surface productivity changes. During Survey I, a period of the El Nino, the Th-230 and Th-2 28 activities were higher than those measured during Survey II (a non- El Nino period), reflecting a depressed ocean productivity and particl e flux under Fl Nino conditions. Vertical profiles of Th-230 show a mi nimum at about 50 to 100 m, suggesting maximal particle scavenging in this depth range. Below this depth interval is a Th-230 chemocline (ra pid increase in Th-230 activities with depth) and a Th-228 activity ma ximum. The occurrence of the Th-230 chemocline and the Th-228 maximum just below the euphotic zone is a manifestation of active particle reg eneration. Based on the extent of radioactive disequilibria between Th -228 and Ra-228 and between Th-230 and U-234, We have estimated vertic al fluxes of the two Th isotopes and of the particle mass. The export particle flux reaches a maximum in the lower portion of the euphotic z one, followed by a rapid decrease resulting from intensive particle re mineralization in the Th-230 chemocline/228Th maximum region. The scav enging rates of Th-228 and Th-230 are not balanced by their in-situ pr oduction and radioactive decay. The imbalance is attributable to their diffusional and advective transport. From the vertical profiles of pa rticle fluxes derived from Th-228 and Th-230, export fluxes of particu late organic carbon (POC) at the base of the euphotic zone are estimat ed to be 0.6-1.3 mmol C m(-2) day(-1) during El Nino, and 1.5-5.0 mmol C m(-2) day(-1) during the non-El Nino period. These values are small compared to the new production, indicating that much of the organic m atter must be removed from the euphotic zone in dissolved form. Deeper at similar to 800 m, the POC fluxes are reduced to 0.14-0.50 mmol C m (-2) day(-1) (El Nino) and 0.5-1.1 mmol C m(-2) day(-1) (non-El Nino).