Kinetics of thorium and particle cycling along the U.S. GEOTRACES North Atlantic Transect

We find that the adsorption rate constant (k1) generally decreases with depth across the section: broadly, the time scale 1/k1 averages 1.0 yr in the upper 1000 m and (1.4-1.5) yr below. A positive relationship between k1 and particle concentration (P) is found, i.e., k1∝Pb, where b≥1, consistent with the notion that k1 increases with the number of surface sites available for adsorption. The rate constant ratio, K=k1/(k−1+β−1), which measures the collective influence of rate parameters on Th scavenging, averages 0.2 for most stations and most depths. We clarify the conditions under which K/P is equivalent to the distribution coefficient, KD, test that the conditions are met at the stations, and find that K/P decreases with P, in line with a particle concentration effect (dKD/dP<0). In contrast to the influence of colloids as envisioned by the Brownian pumping hypothesis, we provide evidence that the particle concentration effect arises from the joint effect of P on the rate constants for thorium attachment to, and detachment from, particles.