Scott Doney continues MIT's Marine Chemistry course (12.742, Fall 2006) with a session on how organic matter sinks and transforms as it moves through the water column. He covers remineralization processes that break down particulate matter at depth, then turns to the radionuclides thorium and protactinium as tracers scientists use to measure particle fluxes and export rates from the surface ocean. The lecture also addresses dissolved organic matter, its composition, sources, and how it cycles through marine systems over different timescales. Doney works through the chemical and physical mechanisms governing these processes, connecting isotope geochemistry to broader questions about the ocean's carbon cycle. Delivered as part of a graduate-level MIT OpenCourseWare series, the session runs 84 minutes and assumes familiarity with prior lectures on ocean particle dynamics.