The focus of this research is to quantify transformation rates between dissolved, colloidal, small particle, and large particle size classes in the surface and deep waters of the Atlantic basin. The approach is to use highly sensitive thermal ionization mass spectrometry and radiochemical procedures to determine 234Th, 228Th, and 230Th in the various size classes. The colloidal phase, traditionally collected along with the filtrate, will be sampled in-situ using a cross-flow filtration technique. Field sampling methods will involve the collection of large particles using filters (33um) and the direct separation and collection of colloidal (5,000 NMW-0.2 um) and dissolved (<5,000 NMW) fractions using in-situ cross-flow filtration. These data will allow for a separation of the processes of sorption onto particle surfaces from the aggregation/disaggregation of colloids and suspended particles. In particular, the mass contribution of colloidal Th and the potentially important role colloids play as reactive intermediates in oceanic particle cycling and metal scavenging will be assessed.
