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. Vertical 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. From the 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).