DOI: 10.3334/cdiac/atg.ndp057
title: Carbon-14 Measurements in Atmospheric CO2 from Northern and Southern Hemisphere Sites, 1962-1993 (NDP-057)

description: In the 1960s, thermonuclear bomb tests released significant pulses of radioactive 14C into the atmosphere. This major perturbation allowed scientists to study the dynamics of the global carbon cycle by measuring and observing rates of isotopic exchange. The Radiological Dating Laboratory at the Norwegian Institute of Technology performed 14C measurements in atmospheric CO2 from 1962 to 1993 at a network of ground stations in the Northern and Southern hemispheres. These measurements were supplemented during 1965 with high-altitude (9-12.6 km) air samples collected using aircraft from the Norwegian Air Force. The resulting database, coupled with other 14C data sets, provides a greater understanding of the dynamic carbon reservoir and a crude picture of anomalous sources and sinks at different geographical latitudes. This database is outstanding for its inclusion of early 14C measurements, broad spatial coverage of sampling, consistency of sampling method, and 14C calculation results corrected for isotopic fractionation and radioactive decay. This database replaces previous versions published by the authors and the Radiological Dating Laboratory. Fourteen stations spanning latitudes from Spitsbergen (78° N) to Madagascar (21° S) were used for sampling during the lifetime of the Norwegian program. Some of the stations have data for only a brief period, while others have measurements through 1993. Sampling stations subject to local industrial CO2 contamination were avoided. The sites have sufficient separation to describe the latitudinal distribution of 14C in atmospheric models. The sampling procedure for all the surface (10-2400 m asl) 14C measurements in this database consisted of quantitative absorption of atmospheric CO2 in carbonate-free 0.5 N NaOH solution. The 14C measurements were made in a CO2 proportional counter and calculated (14C) as per mil excess above the normal 14C level defined by the US National Institute of Standards and Technology (NIST). Atmospheric 14C content is finally expressed as 14C, which is the relative deviation of the measured 14C activity from the NIST oxalic acid standard activity, after correction for isotopic fractionation and radioactive decay related to age. The data are organized by sampling station, and each record of the database contains the sampling dates; values for 14C excess (14C) relative to the NIST standard, fractionation 13C (13C) relative to the Pee Dee Belemnite (PDB) standard, and corrected 14C ( 14C) excess; and the standard deviation for 14C. The 14C calculation results presented here are thus corrected for isotopic fractionation and radioactive decay, and constitute the final product of a research effort that has spanned three decades. The 14C station data show a sharp increase in tropospheric radiocarbon levels in the early 1960s and then a decline after the majority of nuclear tests came to an end on August 5, 1963 (Test Ban Treaty). The sharp peaks in tropospheric radiocarbon in the early 1960s are more pronounced in the Northern Hemisphere, reflecting the location of most atomic weapons tests. The measurements show large seasonal variations in the 14C level during the early 1960s mainly as a result of springtime transport of bomb 14C from the stratosphere. During the 1970s, the seasonal variations are smaller and due partly to seasonal variations in CO2 from fossil-fuel emissions. The rate of decrease of atmospheric radiocarbon provides a check on the exchange constants of the atmosphere and ocean. This report and all data it describes are available from the Carbon Dioxide Information Analysis Center (CDIAC) without charge. The Nydal and Lövseth atmospheric 14C database comprises 21 data files totaling 0.2 megabytes in size. The following report describes the sampling methods and analysis. In addition, the report includes a complete discussion of CDIAC's data-processing efforts, the contents and format of the data files, and a reprint of a Nydal and Lövseth journal article.