Comment on “On the importance of opal, carbonate, and lithogenic clays in scavenging and fractionating 230Th, 231Pa and 10Be in the ocean” by S. Luo and T.-L. Ku

Luo and Ku have reanalyzed part of the data set we published in 2002 to address the question of the differential scavenging of thorium, protactinium and beryllium by different particulate phases (organic carbon, lithogenic material, opal and calcium carbonate). They conclude upon reanalysis that the data are consistent with their earlier proposal, based on sediment separation experiments, that lithogenic material possesses an affinity for Th some 500 times greater than do biogenic phases. While we welcome new analyses and fresh interpretations of our data set we have to disagree with their conclusion that the partition coefficient (Kd) for adsorption of Th onto lithogenic particles (Kd(Th)_L) is on the order of 2.3×108, and far greater than the Kd for adsorption of Th onto calcium carbonate (Kd(Th)_C). Here we show that such a high value for Kd(Th)_L is inconsistent with a large body of observations. These observations suggest Kd(Th)_L is closer to 1×107, consistent with our previous study.