238U234U230Th232Th systematics and the precise measurement of time over the past 500,000 years

Abstract

We have developed techniques to measure the 23°Th 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 23~U_ 234U-230Th dating and greatly reduce the sample size.We show that 6 × l0 s atoms of Z3°Th can be measured to ± 30%~ (2o) and 2 × 10 l° atoms of 23°Th to ± 2%e. 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 (20) 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.

Neodymium isotope analyses after combined extraction of actinide and lanthanide elements from seawater and deep‐sea coral aragonite

“…Here, we demonstrate accurate Nd isotope results obtained from a combined extraction scheme of Pa, Th, and Nd from seawater and U, Th, and Nd from coralline aragonite. The method is easy to adapt, as it merely requires collecting elution fractions from anion exchange columns set up for separating U-Th-Pa [e.g., Auro et al, 2012;Edwards et al, 1987], which normally would go to waste, and subsequent processing through standard ion exchange chromatography for Nd isotope analyses [e.g., van de Flierdt et al, 2006van de Flierdt et al, , 2012. Our Nd isotope intercalibration results for the combined chemistries are in excellent agreement with results from seawater and coral samples processed for Nd only.…”

“…This is typically achieved by rigorous physical cleaning with a Dremel V R tool and subsequent oxidative and reductive chemical cleaning [e.g., Cheng et al, 2000;Robinson et al, 2005;van de Flierdt et al, 2010]. Sample dissolution was achieved in nitric acid to which a mixed 236 U-229 Th spike was added [Edwards et al, 1987;Hines et al, 2015]. The samples were evaporated, then dissolved in 2 M HCl and 3-5 mg of purified Fe were added as FeCl 3 , followed by addition of ammonium hydroxide to coprecipitate trace metals at pH 5 7-9, whereas alkaline earth metals, and in particular Ca, are not precipitated [e.g., Dulski, 1996].…”

Astronomical forcing of sub-Milankovitch climate oscillations during the late Quaternary

“…Uranium 234 ( 234 U), Thorium ( 230 Th), Radium, Radon, Protactinium, and Polonium form part of this radioactive decay chain, with different decay rates for each parent-daughter pair. For Uranium-Thorium dating that is used on corals, the limit of dating is 500,000 yrs and the minimum is just a few years (Lawrence Edwards et al, 1987;Schwarz, 1989). Measurements are quite precise at the younger end of this scale, whilst there are greater standard errors at the other extreme (Cobb et al, 2008;Schwarz, 1989).…”

“…Individual colonies of coral can live up to >200 yrs, although the average lifespan is ~100 yrs (Cobb et al, 2008). U234 is absorbed directly from seawater and deposited in carbonate secretions in annual growth rings on coral, along with other geochemical properties from its watery environment (Cobb, et al, 2008;Lawrence Edwards et al, 1987). However, thorium is not soluble so that any remaining thorium in the closed system is the result of radioactive decay (Holdaway, 2006).…”

“…Uranium-series dating of uplifted coral terraces in Barbados and New Guinea have provided valuable climatological history, assisting in the establishment of timelines for interglacials, sea-level variations, and their connection with the Milankovitch cycles (Bender et al, 1979;Chappell, 1974;Lawrence Edwards et al, 1987;Lowe and Walker, 1997;Pickett et al, 1989;Schwarcz, 1989). The application of uranium-series dating to calibrating radiocarbon beyond the range of dendrochronological calibration was first explored by Bard et al (1990aBard et al ( , 1990b, using paired uranium-thorium and radiocarbon dating from Barbados coral to allow direct comparison between the two chronometric methods.…”