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-volume (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 to 800 m of the water column in the central equatorial Pacific along similar to 140 degrees W during two survey cruises of JGOFS EqPac in February/March (Survey I) and August/September (Survey II) 1992. Significantly 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 particle-reactive behavior. The activities of these thorium isotopes also show large temporal variations, in response to surface productivity changes. During Survey I, a period of the El Nino, the Th-230 and Th-228 activities were higher than those measured during Survey II (a non-El Nino period), reflecting a depressed ocean productivity and particle flux under Fl Nino conditions.Vertical profiles of Th-230 show a minimum at about 50 to 100 m, suggesting maximal particle scavenging in this depth range. Below this depth interval is a Th-230 chemocline (rapid increase in Th-230 activities with depth) and a Th-228 activity maximum. The occurrence of the Th-230 chemocline and the Th-228 maximum just below the euphotic zone is a manifestation of active particle regeneration. Based on the extent of radioactive disequilibria between Th-228 and Ra-228 and between Th-230 and U-234, We have estimated vertical 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 zone, followed by a rapid decrease resulting from intensive particle remineralization in the Th-230 chemocline/228Th maximum region. The scavenging rates of Th-228 and Th-230 are not balanced by their in-situ production and radioactive decay. The imbalance is attributable to their diffusional and advective transport.From the vertical profiles of particle fluxes derived from Th-228 and Th-230, export fluxes of particulate organic carbon (POC) at the base of the euphotic zone are estimated 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 matter 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).