Using 230Th in Marine Sediments to Reconstruct the Late Quaternary History of Sea Level

Abstract

We propose a method to determine past sea levels based on the record of 230Th accumulating in seafloor sediments. The decay of uranium in seawater produces this nuclide, which then adsorbs rapidly onto particles settling to the seafloor. Since the concentration of uranium is similar at all depths in the water column, the flux of 230Th to the seafloor at any location is proportional to water column height. Consequently, changes in sea level must cause changes in these fluxes. At shallow depths (<1000 m), fluctuations of sea level cause changes in the expected 230Th flux that are large enough to produce measurable changes in the 230Th content of the sediment. We have developed two approaches to determine past sea levels from the content of 230Th in sediments: (1) in a single core, the ratio of the past and present fluxes of 230Th to a site on the seafloor is a measure of the ratio of the past and present depths of the site below sea level and (2) in two or more cores from a region where the vertical input of sediment is similar at all depths in the water column, the concentrations of 230Th in synchronous sediment samples are linearly related to depth and indicate past sea level as the depth where the concentrations extrapolate to zero. The latter approach requires fewer assumptions and allows sea level to be estimated at discrete points in time. The methodology, assumptions, advantages and disadvantages of the first approach is illustrated using preliminary data from a core recovered from the margin of Little Bahama Bank.