Smith Lecture: A Tale of Two Oceans: Unsupported 231Pa/230Th in Glacial and Recent Sediments from the Atlantic and Arctic
Sedimentary inventories of thorium-230 (230Th) and protactinium-231 (231Pa), two long-lived daughters of the radioactive decay of uranium dissolved in seawater, provide important insights into past oceanographic processes. Neither is very soluble and both are removed from the water column by adsorption onto settling particles, resulting in residence times of decades for 230Th and centuries for 231Pa. Throughout most of the global ocean, 230Th is buried on the seafloor at a rate that approximates its production in the overlying water column. Thus the burial ratio of seawater-derived 231Pa/230Th provides evidence for the relative influence of particle rain rates and lateral motions on the eventual removal of 231Pa.
New analyses of deep-sea sediments from the Arctic and Atlantic Oceans combine with previously published data to reveal markedly different behavior in the depth-dependent removal of 231Pa from these basins over the last 20,000 years since the last ice age. In the modern ocean, somewhat surprisingly, 231Pa is being transported away from the deep central Arctic Ocean as well as from the deep North Atlantic. The sedimentary 231Pa/230Th at intermediate depths was lower during the last ice age in both oceans, implying greater lateral export of 231Pa at mid-depth at that time. Yet only the North Atlantic had a compensating increase in 231Pa/230Th burial at deep sites. This suggests that the mid-depth Atlantic at that time was influenced by the shoaling of the Atlantic meridional overturning circulation, while the ocean circulation in the Arctic was characterized by continuing shallow inflow and a persistent deep export through the Fram Strait, despite the likelihood of dramatically different ice age conditions.