BACTERIAL SCAVENGING OF TH-234 IN SURFACE OCEAN WATERS

Th-234 was determined in the dissolved (< 0.1 mum), bacterial (0.1 to 1.0 mum) and 2 larger particulate pools (1 to 5 mum and > 5 mum) at 5 or 6 depths in the upper 120 m of the water column at a station (31-degrees-51.6' N, 119-degrees-31.9' W) in the Southern California Bight during October 1989 and July 1990. Depth profiles of bacterial abundance and chlorophyll a were also measured. The bacterial fraction had high volume concentration factor (VCF: 1 to 4 x 10(6)) and the highest estimated Th-234/C ratio of all size fractions (5.1 and 5.5 nCi gC-1). The VCF for Th-234 scavenging by bacteria determined in a microcosm experiment (containing a 2 mum filtrate of seawater from Scripps pier and spiked with Th-234) was on the order of 7 x 10(6) Both the bacteria fraction and > 5 mum fraction had Th-234/C ratios comparable to that computed for the colloidal fraction. Sequential transfer of Th-234 (truly dissolved Th-234 --> disperse colloids --> suspended particles --> sinking particles) alone does not explain the observed high Th-234/C ratio in the bacterial and the large particulate fractions. We propose that the abundant and highly Th-234-reactive colloids (which occur bound to the surfaces of free and particle-bound bacteria in addition to their occurrence in the disperse phase) contribute substantially to efficient scavenging of Th-234 by bacteria as well as colonized particles of all sizes. The contribution by bound colloids may vary with the physiological states of bacteria and phytoplankton and their state of aggregation. This unstructured flow model has implications for models which use Th-234/U-238 disequilibrium as an indicator for downward flux of particles in the ocean.