Th230/U238and U234/U238ages of Pleistocene high sea level stand

Abstract

Unrecrystallized fossil corals occurring in their growth positions between 2 and 9 meters above sea level at many locations in the Pacific and Indian oceans have been dated by the Th230/U238 and U234/U238 methods. Where possible, recent corals were also collected and analyzed for their uranium and thorium isotopes. The uranium contents of the corals were determined fluorimetrically; the U234/U238 ratios and the thorium concentrations were ascertained by alpha spectrometry. The Th230/U238 ages of the fossil corals range from 90,000±20,000 to 160,000±40,000 years and the U234/U238 ages from 80,000±50,000 to 180,000±60,000 years. Absence of Th230 in the recent corals and absence of Th232 in both the recent and the fossil corals confirm the assumption that the Th230 found in the fossil corals resulted solely from radioactive decay of its parent uranium. Control samples of pre-Pleistocene material showed radioactive equilibrium between the various members in the U238 decay chain. Both the internal consistency of the ages, within experimental error, and the agreement between the Th230/U236 and U234/U238 ages, strongly support the general validity of these ages. The similarity of the Th230/U238 ages of Pleistocene coral reefs to those of like elevations above sea level in many localities suggests a eustatic sea level stand higher than now at about 120,000±20,000 years ago, possibly during an interglacial stage of the Pleistocene.