A coupled adsorption-aggregation model of the POC/Th-234 ratio of marine particles

Thorium has been frequently used as a tracer of particulate carbon removal from the ocean surface mixed layers. Although coagulation is known to be im portant in the process, models used to interpret measurements have been lim ited to simple descriptions of coagulation. We present results from a coupl ed adsorption-aggregation model for Th-234 scavenging. The aggregation comp onent models the particle size spectrum from colloidal particles to particl es of about 100 mu m in size and incorporates details of the physical mecha nisms that bring particles together. We make comparisons between an adsorpt ion model that has a constant mass specific adsorption rate and one that is purely transport limited, the former of which is shown to be unphysical fo r larger particles. The combined model reproduces the observed inverse depe ndence of POC/Th-234 On particle size. Comparison with observational result s is made by simulating a series of filtrations using the model results. Th e filtered values are determined by the maximum of the particle distributio n within the relevant size range, and consequently affect the comparison be tween observed values and model output. This stresses the importance of the particle size spectrum in the scavenging process and suggests that measure ments using more particle size classes should be made to accurately determi ne kinetic rates. (C) 1999 Elsevier Science Ltd. All rights reserved.