Th-234 as a tracer of particle flux and POC export in the northern North Sea during a coccolithophore bloom

A multidisciplinary cruise to the northern North Sea was undertaken during June 1999 to carry out a Lagrangian study of an Emiliania huxleyi bloom. During this experiment, the naturally occurring radionuclide, Th-234, was measured in the water column to estimate particle fluxes and resulting residence times. Simple steady state modelling of changes in Th-234 activity suggests an increase in scavenging efficiency over an 8 day period. The steady state Th-234 flux ranges from 357 to a maximum of 1390 dpm m(-2) d(-1) on the 25th June 1999, with a corresponding particulate organic carbon (POC) flux from 9.5-48 mmol C m(-2) d(-1), estimated from POC/(234) Th ratios on filtered particulate material. Scavenging of the Th-234 by the bloom occurs immediately. The maximum POC fluxes follow the peak in primary production and the maximum coccolithophore cell abundance within a few days. The Th-234 and POC fluxes illustrate the rapid response of export with biological activity. (C) 2002 Elsevier Science Ltd. All rights reserved.