Role of colloids in upper ocean biogeochemistry in the northeast Pacific Ocean elucidated from 238U-234Th disequilibria

Detailed upper ocean profiles of 234Th is dissolved (<10,000 Daltons), colloidal (10,000 Da-0.45 μm), and particulate (>0.45 μm) forms were measured at a station (46 degrees 45'N, 126 degrees W) in the northeast Pacific Ocean. On average, dissolved, colloidal, and particulate 234Th accounted for ∼ 78, 12 and 10% of the total 234Th in the euphotic zone (0-100 m). A highly positive correlation exists between colloidal 234Th and chlorophyll α; both are characterized by higher concentrations in surface waters, a subsurface maximum at 5 m in the seasonal thermocline, and undetectable levels below the euphotic zone. POC: Chl α ratios in the water column suggest phytoplankton as primary producers of the colloidal material. Scavenging residence times of dissolved, colloidal, and particulate 234Th with respect to their removal processes in the euphotic zone are ∼50, 6, and 8 d. The scavenging rate constant of Th apparently increases with the concentration of colloids. 23 refs., 3 figs., 1 tab