Bacterial scavenging of 234Th in surface ocean waters

Abstract

234Th was determined in the dissolved (< 0.1 µm), bacterial (0.1 to 1.0 µm) and 2 larger particulate pools (1 to 5 µm and > 5 µm) at 5 or 6 depths in the upper 120 m of the water column at a station (31°51.6' N, 119° 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 times 10-6) and the highest estimated 234Th/C ratio of all size fractions (5.1 and 5.5 nCi gC-1). The VCF for 234Th scavenging by bacteria determined in a microcosm experiment (containing a 2 µm filtrate of seawater from Scripps pier and spiked with 234Th) was on the order of 7 times 10-6. Both the bacteria fraction and gt 5 µm fraction had 234Th/C ratios comparable to that computed for the colloidal fraction. Sequential transfer of 234Th (truly dissolved 234Th--> disperse colloids--> suspended particles--> sinking particles) alone does not explain the observed high 234Th/C ratio in the bacterial and the large particulate fractions. We propose that the abundant and highly 234Th-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 234Th 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 234Th/238U disequilibrium as an indicator for downward flux of particles in the ocean.