Size-fractionated 234Th in continental shelf waters off New England: Implications for the role of colloids in oceanic trace metal scavenging
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Measurements of Th-234 (t(1/2) = 24.1 days) in dissolved, colloidal, and particulate forms have been made to investigate the role of colloids in reactive metal scavenging in the surface waters of Buzzards Bay, over an annual cycle, and in the shelf and slope waters off New England. At-sea sampling involved prefiltering seawater through 0.2 mu m filters followed by cross-flow filtration using a 10,000 nominal molecular weight filter to collect colloidal (10,000 NMW-0.2 mu m) and dissolved (<10,000 NMW) phases. Total Th-234 activities increase with distance from shore, indicative of enhanced scavenging in the particle-rich nearshore 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 particulate, and 93-98% is dissolved. The Th-234 size-distribution exhibits a systematic increase in the association of Th-234 With particulate and, to a lesser extent, colloidal matter with increasing suspended particle concentration (C-p). Moreover, a first-order prediction of the fractionation of Th-234 between the various size classes is demonstrated using measured solid-solution partition coefficients. Box model calculations indicate a mean residence time of colloidal Th-234 With respect to aggregation into particles of 0.3 days in Buzzards Bay, which compares with 2 days for dissolved and 4 days for particulate Th-234. In the offshore surface waters, colloidal and particulate Th-234 residence times are similar to 0.5 days and 2-3 days respectively, compared with 30-85 days for the dissolved phase. The short and relatively invariant residence time of colloidal Th-234 suggests that colloidal aggregation may not be rate-limiting in controlling the scavenging of thorium and, by analogy, other particle-reactive trace metals. An implication of these results is that colloidal Th-234 may be tracing a reactive intermediate in the bacterially mediated decomposition of large, rapidly-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 oceanic waters (C-p similar to 0.01-1 mg 1(-1)).