The conventional 14C time scale between 11,500 and 22,000 sidereal years has been calibrated by TIMS U/Th dates for corals. Only a few studies have been made for the time beyond this range. Obtaining samples suitable for numerical dating or estimating the reservoir correction of the 14C dates has been difficult, but we do not have these problems with TIMS U/Th dating of interstadial and interglacial lignite, because reservoir corrections are unnecessary.
[Radiocarbon Volume 40, Numbers 1-2, 1998]