Department of Earth System Science

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.