A coupled adsorption–aggregation model of the POC/234Th 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 important in the process, models used to interpret measurements have been limited to simple descriptions of coagulation. We present results from a coupled adsorption–aggregation model for 234Th scavenging. The aggregation component models the particle size spectrum from colloidal particles to particles of about 100 μm in size and incorporates details of the physical mechanisms that bring particles together. We make comparisons between an adsorption 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 for larger particles. The combined model reproduces the observed inverse dependence of POC/234Th on particle size. Comparison with observational results is made by simulating a series of filtrations using the model results. The filtered values are determined by the maximum of the particle distribution within the relevant size range, and consequently affect the comparison between observed values and model output. This stresses the importance of the particle size spectrum in the scavenging process and suggests that measurements using more particle size classes should be made to accurately determine kinetic rates.