Short residence time of colloids in the upper ocean estimated from ²³⁸U–²³⁴Th disequilibria

Abstract

RECENT observations of abundant, nonliving, submicrometre particles in the upper ocean^1–3 and new measurements of 'dissolved⁹organic 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 ²³⁴Th as an in situ tracer to estimate the residence time of colloidal matter in the surface waters near Bermuda. The ²³⁴Th activity of colloidal matter (size range 10,000 nominal molecular weight to 0.2 μm) is similar to that of small particles (0.2–53 μm). Modelling of our results indicates a mean residence time of colloidal ²³⁴Th with respect to aggregation into small particles of 10 days, which is roughly the same as for small-particle ²³⁴Th, yet a factor of ∼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.