Tracing particle cycling in the upper ocean with 230Th and 228Th: An investigation in the equatorial Pacific along 140°W

Vertical distributions of 230Th and 228Th were measured in large-volume (2000 to 3000 liter) seawater samples collected with the Multiple Unit In-Situ Large-Volume Filtration System (MULVFS) and in-situ Fe/Mn-fiber enrichment technique. The samples were from the upper ∼800 m of the water column in the central equatorial Pacific along ∼140°W during two survey cruises of JGOFS EqPac in February/March (Survey 1) and August/September (Survey II) 1992. Significantly lower activities of 230Th and 228Th 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 Niño, the 230Th and 228Th activities were higher than those measured during Survey II (a non-El Niño period), reflecting a depressed ocean productivity and particle flux under El Niño conditions. al profiles of 230Th show a minimum at about 50 to 100 m, suggesting maximal particle scavenging in this depth range. Below this depth interval is a 230Th chemocline (rapid increase in 230Th activities with depth) and a 228Th activity maximum. The occurrence of the 230Th chemocline and the 228Th maximum just below the euphotic zone is a manifestation of active particle regeneration. Based on the extent of radioactive disequilibria between 228Th and 228Ra and between 230Th and 234U, 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 230YTrh chemocline/228Th maximum region. The scavenging rates of 228Th and 230Th are not balanced by their in-situ production and radioactive decay. The imbalance is attributable to their diffusional and advective transport. he vertical profiles of particle fluxes derived from 228Th and 230Th, export fluxes of particulate organic carbon (POC) at the base of the euphotic zone are estimated to be 0.6–1.3 mmol Cm−2 day−1 during El Niño, and 1.5–5.0 mmol Cm−2 day−1 during the non-El Niño 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 ∼800 m, the POC fluxes are reduced to 0.14–0.50 mmol Cm−2 day−1 (El Niño) and 0.5–1.1 mmol Cm−2 day−1 (non-El Niño).