234Th 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, 234Th, was measured in the water column to estimate particle fluxes and resulting residence times. Simple steady state modelling of changes in 234Th activity suggests an increase in scavenging efficiency over an 8day period. The steady state 234Th flux ranges from 357 to a maximum of 1390dpmm−2d−1 on the 25th June 1999, with a corresponding particulate organic carbon (POC) flux from 9.5–48mmol Cm−2d−1, estimated from POC/234Th ratios on filtered particulate material. Scavenging of the 234Th 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 234Th and POC fluxes illustrate the rapid response of export with biological activity.