Recent experimental work has suggested that the relative fractionation of 14C to 13C may differ from the accepted value of b = 2. In order to explore the implications of this possibility, the standard formulae for correcting radiocarbon dates for fractionation effects are rederived, but without making any of the usual assumptions or approximations. A generalized dating equation is derived (where ASN and AON are normalized sample and standard activities, β is a factor which reflects changes in atmospheric 13C and 14C content, {RST (o)/RST}b accounts for post-depositional changes in sample 13C ratio, and tcal is calendar age in years before ad 1950. The errors in calculated ages which might arise from different b values are estimated and shown to be small relative to other dating uncertainties. The effect of b ≠ 2 may be important in the calibration of radiocarbon dates using tree-ring samples of known age. A theoretical analysis suggests that b ≠ 2 effects may result in a correlation between age anomaly (ie, the difference between radiocarbon age and calendar age) and sample 13C data. However, an analysis of published data reveals no meaningful correlation. This result, while not eliminating the possibility that b ≠ 2, highlights its unimportance even in this high-precision application of radiocarbon dating.

Routine radiocarbon analyses were last reported for the Laboratory of Isotope Geochemistry at the University of Arizona in 1971 (Haynes, Grey, and Long, 1971), and a special date list on packrat middens appeared in 1978 (Mead, Thompson, and Long, 1978). This list presents results obtained from our gas proportional counting facility before its major renovation and before the addition of a liquid scintillation counting system. The characteristics of these new systems will be described in the next date list.

Samples in this paper are a continuation of the Florida geologic samples, reported earlier (R, 1978, v 20, p 436–440). Each specimen analyzed in this list was a carbonate shell. All dates are from single shells as opposed to means of multiple shell samples.

The results presented in this date list are from the same sampling program previously reported (R, 1978, v 20, p 436–440). Each sample reported herein was geologic in origin and, in most instances, represents a single shell date.

This date list includes most of the archaeologic and geologic samples dated in this laboratory since publication of our last date list (R, 1978, v 20, p 210–233), as well as some samples dated previously which lacked adequate sample information.

The results presented in this list have been obtained by benzene synthesis and liquid scintillation counting: ages are calculated using the 14C half-life of 5570 yr with 0.95 activity of NBS oxalic acid as modern standard, with no correction. The given precision is obtained from a one-standard-deviation criterion.

Results reported here are for samples of geologic or geographic context completed during the period 1975 to 1978.

The Menlo Park radiocarbon laboratory of the US Geological Survey began operation in January 1976 in a new building with a counting room 9.8m below the ground surface. Shielding above the counters consists of 30cm lead and 8.2m earth. The ionizing cosmic ray flux at the counters is 0.28 cpm/cm2. Background levels obtained thus far are summarized in Table 1.

Measurements have continued with the same proportional counter system, pretreatment procedure, methane preparation and measurement, and calculation, as described previously (R, 1970, v 12, p 298–318). Uncertainties quoted are single standard deviations originating from standard, sample, background counting rates, and half-life. No 13C/12C ratios were measured. Sample descriptions have been prepared in cooperation with submitters.