Mass Spectrometric ¹⁴C and U-Th Measurements in Coral

Abstract

ABSTRACT. We discuss U-Th and 14C measurements in coral. Samples with U-Th dates in excess of 50 ka BP were chosen for study. Some bulk samples from this group have measurable 14C dates, which range from 30 ka to 43 ka BP. These can be explained by 0.5-2.5% contamination by modern carbon. This small amount of contamination can produce significant offsets in 14C dates of coral samples older than -10 ka. It may be undetectable in X-ray powder diffraction patterns. We describe a sample pretreatment that removes the modern carbon by selective dissolution and produces accurate 14C dates.

Paired 14C and 230Th/U Dating of Surface Corals from the Marquesas and Vanuatu (Sub-Equatorial Pacific) in the 3000 to 15,000 Cal Yr Interval

“…Although the mean and standard deviation obtained for the corals from the Marquesas and Vanuatu (0.21 ± 0.05 pMC: apparent age: 49,350 ± 1900 BP) are slightly higher than those obtained for 1 coral (Irene 30) from Mururoa (0.16 ± 0.02 pMC; apparent age: 51,800 ± 1050 BP), they are nevertheless statistically indistinguishable from one another. The mean and standard deviation for the old 14 C-depleted corals using this cleaning procedure are 0.20 ± 0.05 pMC (apparent age 50,100 ± 2070 BP) and are similar to slightly lower than those previously measured (Bard et al 1990b;Burr et al 1992;Yokoyama et al 2000). The AIEA-C1 marble, currently used to assess the full procedural blank of the AMS 14 C dating method of our laboratory, has a value of 0.08 ± 0.02 pMC (n = 7; apparent age 57,280 ± 2000 BP).…”

Radiocarbon Results from a 13-Kyr BP Coral from the Huon Peninsula, Papua New Guinea

“…All of the samples were subjected to the selective dissolution technique described by Burr et al (1992). A piece of the core was analyzed using X-ray powder diffraction to ensure that it was free of secondary calcite.…”

Comment on “Radiocarbon calibration curve spanning 0 to 50,000 years BP based on paired 230Th/234U/238U and 14C dates on pristine corals” by R.G. Fairbanks et al. (Quaternary Science Reviews 24 (2005) 1781–1796)☆☆doi of original article: 10.1016/j.quascirev.2005.04.007 and “Extending the radiocarbon calibration beyond 26,000 years before present using fossil corals” by T.-C. Chiu et al. (Quaternary Science Reviews 24 (2005) 1797–1808)☆☆☆☆doi of original article: 10.1016/j.quascirev.2005.04.002

“…Extensive studies by Burr et al (1992) show quite the opposite; however, indicating that in many cases partial dissolution of the coral removes recent contamination. Indeed, Burr et al (1992) and Yokoyama et al (2000) have done careful stepwise experiments of coral leaching for which both leachates and residues were dated.…”

“…They claim, in effect, to be the first to properly identify the issue that secondary calcite can bias 14 C ages of corals -however, this is in fact a well-known and long-standing problem (e.g. Chappell and Polach (1972); Burr et al (1992)). However, it should be clarified that this problem mainly affects samples that did not stay below sea level, thereby exposing them to alteration by fresh water.…”