Radiocarbon: A chronological tool for the recent past

The past few hundred years have seen large fluctuations in atmospheric C-14 concentration. In part, these have been the result of natural factors, including the climatic changes of the Little Ice Age, and the Sporer and Maunder solar activity minima. In addition, however, changes in human activity since the middle of the 19th century have released C-14-free CO2 to the atmosphere. Moreover, between c. 1955 and c. 1963, atmospheric nuclear weapon testing resulted in a dramatic increase in the concentration of C-14 in the atmosphere. This was followed by a significant decrease in atmospheric C-14 as restrictions on nuclear weapon testing began to take effect and as rapid exchange occurred between the atmosphere and other carbon reservoirs. The large fluctuations in atmospheric C-14 that occurred prior to 1955 mean that a single radiocarbon date may yield an imprecise calibrated age consisting of several possible age ranges. This difficulty may be overcome by obtaining a series of C-14 dates from a sequence and either wiggle-matching these dates to a radiocarbon calibration curve or using additional information on dated materials and their surrounding environment to narrow the calibrated age ranges associated with each C-14 date. For the period since 1955 (the bomb-pulse period), significant differences in atmospheric C-14 levels between consecutive years offer the possibility of dating recent samples with a resolution of from one to a few years. These approaches to dating the recent past are illustrated using examples from peats, lake and salt marsh sediments, tree rings, marine organisms and speleothems. (C) 2009 Elsevier Ltd. All rights reserved.