CHIH-AN HUH

Temporal variation in upper ocean distribution of 234Th between dissolved, colloidal, and particulate forms was observed at an open-ocean station in the northeast Pacific. As a percentage of the total 234Th in the euphotic zone (0–100 m), dissolved, colloidal, and particulate 234Th accounted for 78–91%, 5–12%, and 3–10%, respectively. The concen-tration of colloidal 234Th correlated positively with chlorophyll a and negatively with dissolved 234Th. A prominent feature of the water column profiles of colloidal and particulate 234Th is that the former was always higher than the latter above the permanent pycnocline whereas the opposite was true below the permanent pycnocline. The obser-vations support the perception that the bulk of colloids in the upper ocean are organic-rich macromolecules produced by phytoplankton and they are the immediate phase removing dissolved 234Th from solution. The time integrated mean residence time in the euphotic zone is ~3 months for dissolved 234Th and ~6 days for colloidal and particulate 234Th. The short residence time and organic nature of colloids suggest that, in addition to chemical scavenging of particle-reactive elements, colloids may also be responsible for a significant fraction of carbon export from the upper ocean. Keywords