Geochronological studies of deep sea sediments by the ionium/thorium method

Abstract

The ionium/thorium method for determining the rates of accumulation of marine sediments has been applied to a group of deposits from the Pacific, Atlantic and Indian Oceans. The thorium isotopes which were accommodated in authigenic minerals (presumably derived from sea water) were obtained through a hot hydrochloric acid leaching of the sample. A model for the mixing of the upper layers of the sediment, by worm-burrowing or near-bottom currents, is proposed and, if valid, allows the depth of mixing to be ascertained from the ionium/thorium profiles. Sediments in the South Pacific had rather uniform and low rates of deposition (0.3–0.6 mm/103 years), while the North Pacific values in general were at least several times higher. These results are in accord with a much greater contribution of sedimentary components from the continents in the northern area. South Atlantic depositional rates were of the same order, millimeters per thousand years, as the North Pacific, while two adjacent Indian Ocean cores gave widely contrasting values. Surface values of the ionium/thorium ratio, and to a lesser extent the hydrochloric acid leachable thorium concentrations, showed a marked geographical dependence which was attributed to the relative amounts of continental run-off an oceanic area receives.