The IntCal09 and Marine09 radiocarbon calibration curves have been revised utilizing newly available and updated data sets from ¹⁴C measurements on tree rings, plant macrofossils, speleothems, corals, and foraminifera. The calibration curves were derived from the data using the random walk model (RWM) used to generate IntCal09 and Marine09, which has been revised to account for additional uncertainties and error structures. The new curves were ratified at the 21st International Radiocarbon conference in July 2012 and are available as Supplemental Material at www.radiocarbon.org. The database can be accessed at http://intcal.qub.ac.uk/intcal13/.

Dating Carbon: Radiocarbon dating is the best way to determine the age of samples that contain carbon and that are younger than ∼50,000 years, the limit of precision for the method. There are several factors that complicate such age determinations, however, some of the most important of which include variability of the ¹⁴ C production in the atmosphere (which affects organic samples whose radiocarbon inventories are derived from atmospheric CO ₂ ), surface ocean reservoir effects (which affect marine samples that acquire their radiocarbon signatures from seawater), and variable dead carbon fraction effects (which affect speleothems that derive their carbon from groundwaters). Bronk Ramsey et al. (p. 370 ; see the Perspective by Reimer ) avoid the need to make such assumptions, reporting the ¹⁴ C results of sediments from Lake Suigetsu, Japan. Analysis of terrestrial plant macrofossils in annually layered datable sediments yielded a direct record of atmospheric radiocarbon for the entire measurable interval up to 52.8 thousand years ago.

The majority of the early crops grown in Europe had their origins in south-west Asia, and were part of a package of domestic plants and animals that were introduced by the first farmers. Broomcorn millet, however, offers a very different narrative, being domesticated first in China, but present in Eastern Europe apparently as early as the sixth millennium BC. Might this be evidence of long-distance contact between east and west, long before there is any other evidence for such connections? Or is the existing chronology faulty in some way? To resolve that question, 10 grains of broomcorn millet were directly dated by AMS, taking advantage of the increasing ability to date smaller and smaller samples. These showed that the millet grains were significantly younger than the contexts in which they had been found, and that the hypothesis of an early transmission of the crop from east to west could not be sustained. The importance of direct dating of crop remains such as these is underlined.

Calibration is a core element of radiocarbon dating and is undergoing rapid development on a number of different fronts. This is most obvious in the area of ¹⁴C archives suitable for calibration purposes, which are now demonstrating much greater coherence over the earlier age range of the technique. Of particular significance to this end is the development of purely terrestrial archives such as those from the Lake Suigetsu sedimentary profile and Kauri tree rings from New Zealand, in addition to the groundwater records from speleothems. Equally important, however, is the development of statistical tools that can be used with, and help develop, such calibration data. In the context of sedimentary deposition, age-depth modeling provides a very useful way to analyze series of measurements from cores, with or without the presence of additional varve information. New methods are under development, making use of model averaging, that generate more robust age models. In addition, all calibration requires a coherent approach to outliers, for both single samples and where entire data sets might be offset relative to the calibration curve. This paper looks at current developments in these areas.