Sediment and peat chronologies have been further improved allowing alternative radiometric methods to complement 14C dating. Lacustrine and coastal marine sediments as well as peat deposits in various parts in Scandinavia are studied using 137Cs, 210Pb, 14C and other methods primarily to evaluate the 210Pb but also to extend the 14C chronology. The sampling sites have various sources of input and are characterized by different geochemical, depositional, and post-depositional conditions.

The analysis of rodent middens, principally deposited by packrats (Neotoma sp), has rapidly become the most important paleoecologic and paleoclimatologic tool in the southwestern United States. The recent discovery of rodent middens created by stick-nest rats (Leporillus sp) and rock wallabies (Petrogale sp) in Australia (Green et al, 1983; P S Martin, oral commun, 1984) and by dassie rats (Petromus typicus) in South Africa (L Scott, oral commun, 1984) portends the use of midden analysis in arid regions worldwide. Several recent reviews of southwestern paleoecology (eg, Spaulding et al, 1983) rely heavily on rodent middens for ecologic and climatic reconstructions.

Radiocarbon ages of dendrochronologically-dated wood spanning the last 4500 years were determined at both the Seattle and Belfast laboratories. The combined results are reported in this issue of radiocarbon in two papers, with this paper covering the AD 1950—500 BC interval, and the twin (Pearson & Stuiver, 1986) covering the 500 BC–2500 BC interval.

In modeling the initial 14C activity of ground waters, the δ13C of marine limestone is taken conventionally to vary little about 0‰ PDB. This variation was found to be 6.28‰ in samples taken over intervals from 10-2 to 105m in the Mooney Falls Member of the Redwall Limestone in northern Arizona. Such a variation will cause appreciable variability in the results of all four initial activity models tested. The variability, due primarily to a numerical instability in the models dependent on this parameter, can introduce significant uncertainty into groundwater “age” calculations.

We describe here the general operation of the Oxford Radiocarbon Accelerator Laboratory which has now been dating routinely for 18 months.

This paper is a twin paper to that of Stuiver and Pearson (1986) which covers the time period ad 1950–500 bc. The combined radiocarbon ages of dendrochronologically dated wood presented in this paper covers the time period 500–2500 bc.

The following date list includes archaeologic and geologic samples dated by Gif Radiocarbon Laboratory mostly from 1973 to 1975. Volcanic samples reported here were dated up to 1981. Measurements were made in the same manner as previously reported (R, 1972, v 14, p 280). For undersized samples, a 0.1L CO2 proportional counter was used with 5000-minute standard measurements. Ages listed are conventional 14C ages based on the 5568-year Libby half-life; uncertainties are 1σ statistical standard error. Results are based on 95% of NBS oxalic acid activity. Some dates have been calibrated using the correction curve of Klein et al (1982).

This list contains the results of 14C determinations obtained at the laboratory in 1984. Our laboratory measures 14C activity in the form of methane with a proportional counter built by Heidelberg University and an electronic unit built by Berthold, Benelux Analytical Instruments. The sample counting time is 4000 minutes with printouts at 100 minute intervals. Sample error is reported as one standard deviation. δ13C for mortar is measured by the University of Lyon Laboratory, δ13C was not measured or estimated for other samples.

With Fe powder as a catalyst, CO2 is completely converted to graphite within 90 minutes. The reaction proceeds at 650°C with an excess of H2. The reaction rate is enlarged by forced circulation and by keeping the water vapor pressure very low. The graphite samples obtained, consisting of 5mg of carbon, almost immediately produce stable 12C− currents of the order of 20–30μA in the sputter source of a Van de Graaff accelerator. The currents can be maintained for at least 10 hours and are comparable to those from commercial graphite. No memory effects in the preparation system have been observed.

A “C14” data base has been created for internal use of 14C dates with an Osborne-1 system. It uses a CP/M operating system, Z-80 CPU, and dBASE II software program. The structure meets the users’ requirements of information and easy retrieval. The video display is quite compact, as a record of 32 fields is edited to fill a screen of 22 lines. A standard printout is edited to make room for two records per page and contains all information from these records. Searching may take place in all fields or parts of fields, in interval(s) in one or more fields and in various combinations of the prescribed methods of searching. A field called KEYWORDS is established to ease search operations. “C14” may be adapted to individual requirements and equipment with small adjustments.

The radiocarbon ages of dendrochronologically dated wood samples, each covering 10 years, are reported back to 2500 yr BC. The decadal calibration curve constructed from these data is an extension of the curve previously given for the AD interval (Stuiver, 1982). A major difference with the previous work, however, is the assessment of the error in the radiocarbon age determination. Whereas previously this error was only based on the Poisson counting statistics of the accumulated number of counts for the sample and standards, the current calibration error is based on an estimate of the reproducibility in the radiocarbon activity determination. As a consequence, the uncertainty in the current calibration curve is, on average, 1.6 times that of the AD curve previously given.

A method has been developed at the Arizona AMS facility for production of graphite directly from CO2 by catalytic reduction of CO/H2 mixtures. The method is a modification of procedures discussed by Vogel et al (1984). Currents of 12C− from these targets vary considerably, but have a median value which is 70% of that achieved from spectroscopic graphite. Precisions of ±60 to 100 yr are routinely achieved for 14C ages of 5000-yr-old samples containing one milligram of carbon. The yield of 14C from “blank” targets is 0.6 ± 0.1% of that from modern material.

Samples presented for high-precision 14C dating must satisfy stringent requirements if the 14C determinations are to yield meaningful sharp calendric dates, such as are now possible with the bidecadal high-precision calibration curve. The total carbon content should come from a confined time range 10–20 years (10–20 tree rings in wood or charcoal) appropriate for the bidecadal calibration curve. For accurate calendric dating the relation of these rings to the outer growth rings must be known. Application of the high-precision calibration curve to some archaeologic examples is discussed. It is now up to archaeologists and geoscientists to use this refined chronometric instrument to fullest advantage.

This article reports on archaeologic samples processed from February 1974 through May 1980 at the Illinois State Geological Survey (ISGS) Radiocarbon Dating Laboratory.

High-precision measurement of dendrochronologically dated Irish oak at bi-decade/decade intervals has continued in the Belfast laboratory, extending the 14C data base from ca AD 1840 to 5210 BC. The dendrochronology is now considered absolute (see Belfast dendrochronology this conference) (Brown et al, 1986) and a continuous detailed curve is presented, showing the natural variations in the atmospheric concentration of 14C over >7000 years. Each data point has a precision of <2.5., and some 4500 years have now been compared with Seattle, giving excellent agreement. Discussion of this data base and the justification of the claimed accuracy is given together with a comparison of other chronologies. Some of the advantages and limitations of the above are discussed.