We have developed techniques to measure the 230Th abundance in corals by isotope dilution mass spectrometry. This, coupled with our previous development of mass spectrometric techniques for 234U and 232Th measurement, has allowed us to reduce significantly the analytical errors in 238U234U230Th dating and greatly reduce the sample size. We show that 6 × 108 atoms of 230Th can be measured to ±30‰ (2σ) and 2 × 1010 atoms of 230Th to ±2‰. The time over which useful age data on corals can be obtained ranges from a few years to ∼ 500 ky. The uncertainty in age, based on analytical errors, is ±5 y (2σ) for a 180 year old coral (3 g), ±44 y at 8294 years and ±1.1 ky at 123.1 ky (250 mg of coral). We also report 232Th concentrations in corals (0.083-1.57 pmol/g) that are more than two orders of magnitude lower than previous values. Ages with high analytical precision were determined for several corals that grew during high sea level stands ∼ 120 ky ago. These ages lie specifically within or slightly postdate the Milankovitch insolation high at 128 ky and support the idea that the dominant cause of Pleistocene climate change is Milankovitch forcing.
Bibliographical noteFunding Information:
editor, K.K. Turekian are thanked for constructive criticisms of the manuscript. Philosophical and technical advice from D.A. Papanastassiou is gratefully acknowledged. Funding for this work was specifically denied by the National Science Foundation. This work was possible through the support of the John D. MacArthur Fund using equipment supported by NASA grant No. NAG 9-43. Division Contribution No. 4371 (557).