^(238)U-^(234)U-^(230)Th-^(232)Th systematics and the precise measurement of time over the past 500,000 years

Abstract

We have developed techniques to measure the ^(230)Th abundance in corals by isotope dilution mass spectrometry. This, coupled with our previous development of mass spectrometric techniques for ^(234)U and ^(232)Th measurement, has allowed us to reduce significantly the analytical errors in ^(238)U-^(234)-^(230)Th dating and greatly reduce the sample size. We show that 6 × 10^8 atoms of ^(230)Th can be measured to ±30‰ (2σ) and 2 × 10^(10) atoms of ^(230)Th 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 ^(232)Th 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.