SHORT RESIDENCE TIME OF COLLOIDS IN THE UPPER OCEAN ESTIMATED FROM U-238 TH-234 DISEQUILIBRIA

RECENT observations of abundant, nonliving, submicrometre particles in the upper ocean1-3 and new measurements of 'dissolved' organic carbon4-7 have fuelled speculation concerning the role of colloidal matter in ocean chemistry and biology. Colloids may act as reactive intermediates in the marine geochemistry of trace metals8-12, and a biologically labile pool of colloidal matter would affect models of ocean carbon cycling13-16. Here we report the use of naturally occurring Th-234 as an in situ tracer to estimate the residence time of colloidal matter in the surface waters near Bermuda. The Th-234 activity of colloidal matter (size range 10,000 nominal molecular weight to 0.2-mu-m) is similar to that of small particles (0.2-53-mu-m). Modelling of our results indicates a mean residence time of colloidal Th-234 with respect to aggregation into small particles of 10 days, which is roughly the same as for small-particle Th-234, yet a factor of approximately 6 less than for the dissolved pool. These results suggest that, more generally, macromolecular colloidal matter has a short residence time and hence a rapid turnover rate in the upper open ocean.