SIZE-FRACTIONATED TH-234 IN CONTINENTAL-SHELF WATERS OFF NEW-ENGLAND - IMPLICATIONS FOR THE ROLE OF COLLOIDS IN OCEANIC TRACE-METAL SCAVENGING

Measurements of Th-234 (t(1/2) = 24.1 days) in dissolved, colloidal, a nd particulate forms have been made to investigate the role of colloid s in reactive metal scavenging in the surface waters of Buzzards Bay, over an annual cycle, and in the shelf and slope waters off New Englan d. At-sea sampling involved prefiltering seawater through 0.2 mu m fil ters followed by cross-flow filtration using a 10,000 nominal molecula r weight filter to collect colloidal (10,000 NMW-0.2 mu m) and dissolv ed (<10,000 NMW) phases. Total Th-234 activities increase with distanc e from shore, indicative of enhanced scavenging in the particle-rich n earshore waters. Clearly seen in Buzzards Bay are seasonal changes in total Th-234, With activities ranging from similar to 0.7 dpm 1(-1) in the winter, preceeding a phytoplankton bloom, to similar to 0.2 dpm 1 (-1) in the summer. Throughout the annual cycle, 2-16% of total Th-234 is colloidal, 22-40% is dissolved, and 45-75% is particulate. In the offshore waters, similar to 1% of total Th-234 is colloidal, 2-6% is p articulate, and 93-98% is dissolved. The Th-234 size-distribution exhi bits a systematic increase in the association of Th-234 With particula te and, to a lesser extent, colloidal matter with increasing suspended particle concentration (C-p). Moreover, a first-order prediction of t he fractionation of Th-234 between the various size classes is demonst rated using measured solid-solution partition coefficients. Box model calculations indicate a mean residence time of colloidal Th-234 With r espect to aggregation into particles of 0.3 days in Buzzards Bay, whic h compares with 2 days for dissolved and 4 days for particulate Th-234 . In the offshore surface waters, colloidal and particulate Th-234 res idence times are similar to 0.5 days and 2-3 days respectively, compar ed with 30-85 days for the dissolved phase. The short and relatively i nvariant residence time of colloidal Th-234 suggests that colloidal ag gregation may not be rate-limiting in controlling the scavenging of th orium and, by analogy, other particle-reactive trace metals. An implic ation of these results is that colloidal Th-234 may be tracing a react ive intermediate in the bacterially mediated decomposition of large, r apidly-sinking biogenic aggregates. Using the size-fractionated Th-234 data, we demonstrate that K-d values for thorium are invariant with C -p and that scavenging rate constants exhibit a first-order dependence on C-p. Thus, ''particle-concentration effects'' are negligible for o ceanic waters (C-p similar to 0.01-1 mg 1(-1)).