THE MOBILITY OF LONG-LIVED RADIOISOTOPES AND THEIR BURIAL IN THE MARINE ENVIRONMENT

Marine sediments record chemical signals that reflect past environmental conditions. It is important to establish how these signals are created and whether they may be altered over time so that they can be useful for reconstructing ocean history. Measurements of uranium isotopes are used as a novel proxy for sedimentary diagenetic processes (Chapter 2). Because 234U can be ejected from mineral lattice during the decay of 238U, it creates a pool of U in porewater that is potentially mobilized and then deposited elsewhere in the core. We found that alpha-recoiled 234U is sensitive to differences in sediment redox condition and will diffuse from core segments of low reducing capacity and reprecipitate at core segments deposited during glacial-interglacial transitions where organic matter delivery was higher. While our data is consistent with this model, calculations suggest there is also an additional source of excess 234U at deglacial segments that is unaccounted for. Dissolved water column measurements of 230Th and 231Pa were taken in the open North Pacific (Chapter 3) along the 152° meridian and at the northern Mid-Atlantic Ridge (Chapter 4). These elements are prone to strong scavenging by marine particles, although, because of their differing solubilities, they are distributed by marine processes to different degrees. In the Pacific, 230Th and 231Pa were used to investigate the primary processes driving particle scavenging including productivity fronts, hydrothermal activity, and benthic resuspension. Strong regional contrasts in scavenging intensity were evaluated for potential impact to 230Th as a paleoceanographic proxy of burial flux. It was found that hydrothermally influenced waters significantly impacted the expected 230Th transport between the Equatorial Particle Veil and subtropical gyres, which creates potential uncertainties in 230Th reconstructions over areas of the equatorial Pacific. This is one example of recent research showing how the distal effects of hydrothermal scavenging are potentially significant throughout basin-scales. For the Mid-Atlantic, we employ a novel strategy to predict background scavenging as a reference to infer the degree of scavenging by hydrothermal plumes. 230Th and 231Pa can be explained by dispersal of hydrothermal particles and were dependent on the timescale of dispersal vs. mixing with seawater