Following Harwell Measurements VIII (Walker, Williams & Otlet 1990), this is the final series of lists of English archaeological dates commissioned for measurement by the Historic Buildings and Monuments Commission (HBMC) for England within prescribed contractual periods. This list, containing 127 dates, refers to the period April 1988 to March 1990, and results are reported irrespective of whether the associated projects are completed or ongoing.

Atmospheric 14C concentrations vary with time and latitude. These variations, measured directly on atmospheric samples, or in independently-dated organic material such as tree rings, supply data essential for the calibration of dynamic models of the global carbon cycle. Short variations in the production rate of atmospheric 14C are strongly attenuated in the relatively large atmospheric CO2 reservoir. In-situ production of 14C should be negligible for ages up to 80 ka bp. Background problems in AMS dating are more likely attributable to contamination of very small samples.

The Radiological Dating Laboratory in Trondheim relatively often dates samples with ages >30 ka BP. Contaminated background materials are known to affect the accuracy of very old dates. We have found, by measurements of different materials, that such contamination is small when using our conventional gas proportional counting (GPC) system. We have also studied contamination levels of our target preparation for 14C accelerator mass spectrometry (AMS) dating in Uppsala. A significant lower background is obtained for Icelandic double spar than for marbles, probably due to a crystal structure of the double spar that is more insensitive to contaminating processes. The background for combusted samples is at the same level as for samples of double spar, indicating that additional 14C contamination due to combustion is negligible. Dates obtained on interstadial samples (T >30 ka bp) by both GPC and AMS agree well.

This list contains the results of 14C determinations that we obtained in 1988 and 1989. δ13C was measured by the Free University, Brussels. Our dating techniques have been described previously (Dauchot-Dehon & Heylen 1971; Dauchot-Dehon, Van Strydonck & Heylen 1986).

We discuss the nature of the twofold record of cosmogenic nuclides in ice sheets, of nuclei produced in the atmosphere, and of nuclei produced in situ due to interactions of cosmic-ray particles with oxygen nuclei in ice. We show that a wealth of geophysical information, in principle, can be derived from a suitable combination of nuclides in ice deposited at different latitudes. Such knowledge includes temporal changes in the cosmic-ray flux, in the geomagnetic field and in climate. The rate of deposition of cosmogenic atmospheric nuclei in ice depends on the global cosmic-ray flux and a host of climatic factors. The global cosmic-ray flux, in turn, depends on the level of solar activity, and of the geomagnetic dipole field. Thus, the task of deconvolution of the record of cosmogenic nuclides is difficult, but can be facilitated by considering the recently discovered record of in-situ-produced cosmogenic 14C, whose production rate at high latitudes is independent of the geomagnetic dipole field (Lal 1992b). We also present a brief review of work done to date and new prospects for deciphering geophysical records using ice sheets.

The following is a partial list of samples of archaeological interest processed between February 1981 and October 1985 at the Illinois State Geological Survey (ISGS) Radiocarbon Dating Laboratory. The list contains samples from west-central Illinois that were related to projects conducted by current or former researchers at the Center for American Archeology (CAA) (formerly Foundation for Illinois Archaeology) and Northwestern University, Department of Anthropology, or, as noted, by colleagues from other institutions. Although some of the samples reported here came from non-cultural contexts and are primarily of geological significance, all were from or related to archaeological investigations.

I describe a simple, rapid and inexpensive method for converting CO2 samples into AMS target graphite. The technique is applicable for both small- and large-scale production facilities. With some modification, the method is suitable for use with very small samples.

We conducted a study of relative gas composition changes of CO2, CO and CH4 during the formation of graphite targets using different temperatures, catalysts and methods. Reduction with H2 increases the reaction rate without compromising the quality of the AMS target produced. Methane is produced at virtually any temperature, and the amount produced is greater at very low temperatures. The reduction of CO to graphite is very slow when H2 is not included in the reaction.

We report here, in part, dates measured on samples from Italy since the construction of a new 14C lab in 1990 for research in the fields of geology, paleomorphology and the biogeochemical cycle of carbon. We use liquid scintillation counting (LSC), following sample conversion to benzene through the four classical chemical steps: 1) combustion or hydrolysis to CO2; 2) production of Li2C2; 3) hydrolysis to C2H2; 4) trimerization of acetylene to C6H6 with specific catalysts. The vacuum line for benzene synthesis has been designed following a comparative review of the literature (Tamers 1960; Noakes et al. 1963; Scharpenseel & Pietig 1970; Harkness & Wilson 1973; Polach, Gower & Fraser 1972; Gupta & Polach 1985), and the valuable suggestions of many colleagues from well-established labs abroad. The compact vacuum line is mostly modular with “O”-ring-joined components, and is equipped with greaseless stopcocks. We use commercial two-stage rotary pumps, pressure transducers and gauges; an electrovalve coupled with a pressure control unit supplies the purified oxygen for sample combustion while holding constant, just higher than 1 atm, the gas pressure inside the combustion tube.

We find no evidence for an anomalously intense 11-yr cycle in Δ14C at high latitudes during the period, AD 1870–1885, as reported by Fan et al. (1983, 1986). However, there does appear to be a regional effect within the MacKenzie River region (67°N, 130°W), with atmospheric 14C depressed by relative to the Olympic Peninsula. Such an effect would require only 5% of CO2 in the air mass to have been derived from 5% 14C-depleted soil gas CO2. This small but apparently significant regional effect could be caused by accumulation of CO2 within the frozen earth followed by outgassing during the spring thaw. The short growing season would enhance the effect by allowing insufficient time for global atmospheric equilibration.

Graphite is the most common type of target for 14C accelerator mass spectrometry (AMS). It is readily produced by catalytic reduction of CO2, but the presence of a small amount of impurities (e.g., sulfur compounds) may retard the reaction. We have tested some techniques to find a method that reduces the impurity content of CO2 produced by combustion of organic material. We found that using water during combustion reduces the average time for graphite conversion of CO2 from organic matter from >3 h to ca. 2 h. This is the time for graphite production from CO2 obtained by acid hydrolysis of calcite. Measurements of known-age and background samples show that this combustion method neither changes the isotopic ratios nor introduces any additional background.

The following list consists of dates of archaeological samples, obtained by gas-proportional counting of ethane, between 1985 and the end of 1991. During this time a close cooperation has been established with the Israel Antiquities Authority, which now has an archaeologist (D.S.) participating in the operation of the lab and serving as a liaison officer with the archaeological community.

Analysis of the Vostok ice-core record of 10Be (Raisbeck et al. 1987) suggests that the sharply resolved increases in 10Be at 35 ka (kyr) and 60 ka are due to cosmic-ray (CR) increases. As an alternate to long-term solar modulation or strong decreases in the Earth's magnetic field, supernova (SN) forcing is qualitatively consistent with the generation of a forward-reverse shock ensemble from a spherical blast wave of age very approximately at 75 ka. This age agrees with Davelaar, Bleeker and Deerenberg's (1980) identification of 75 ka for the age of a North Polar Spur SN remnant. Confirmation would be the first geochemical detection of supernova forcing of spallogenic and perhaps cosmogenic isotope production in the atmosphere. The three 10Be increases can be satisfied by a modification of the Sonett, Morfill, and Jokipii (1987) model. This consists of 2 or 3 shock waves from a single SN event, which includes the first stage in the expansion, leading to a forward shock, S1+, and a pair of reverse waves, S1− and S2−. One reverse wave arises from the spherical expansion, itself, and the other is a reflected wave from a remnant precursor shell boundary from a more ancient SN. The model requires the solar system to be immersed in the ‘bubble’ of the earlier post-SN evolution, possibly affecting estimates of heliospheric boundary distance. However more recent analysis of Camp Century ice core data discloses only the 35 ka 10Be peak. This recent result compounds the difficulty of constructing a completely consistent model for the source of the Vostok spikes. This paper is written in the spirit of suggesting only one of possibly several different models, even within the subclass of SN models.

Many factors influence the preparation and quality of graphite targets for 14C accelerator mass spectrometry (AMS). We identified four factors (sample size, H2 pressure, catalyst temperature and pretreatment time) as potentially critical, and investigated their effects on two particular characteristics: the integrated rates of CO2 reduction (to graphite) and methane production. We used a 2-level fractional factorial experimental design and determined chemical reduction yield rates through manometry and partial pressure monitoring of residual gases by mass spectrometry.

Chemical reduction yield rates ranged from 0.2% to 6.2% per hour. With respect to their influence on percent yield rate, the factors we studied were ordered as: sample size > level of hydrogen > pretreatment of the catalyst. The temperature of the catalyst, and the sample size × hydrogen (2-factor) interaction, were only marginally influential. Other interactions did not appear to be significantly important. We estimated uncertainty in the order of influence and magnitudes of the effects by the Monte Carlo method of error propagation.

We observed significant methane production in only one experiment, which suggests that methane originates from indigenous carbon in untreated iron catalyst only in the presence of hydrogen and only at thermodynamically favorable temperatures. This exploratory investigation indicates that factorial design techniques are a useful means to investigate multivariate effects on the preparation and quality of AMS graphite targets.

We describe our method of determining solar cosmic-ray flux and spectrum in the past, based on the comparison of different cosmogenic isotopes. For the period, AD 1781–1950, we have detected several intervals with a high probability of powerful solar flares.

This paper includes determinations of archaeological, geological, palaeobotanical and other samples from Spain, Portugal and Bolivia, measured at the University of Granada Radiocarbon Dating Laboratory, mostly from 1989 to 1990. Pretreatment of charcoal and wood samples is a standard acid-basic procedure using 8% HCl and 2% NaOH at boiling temperature. We obtain collagen from bone samples using the Longin (1971) method.

A study of the solar proton event data between 1954 and 1986 indicates that the large fluence events at the Earth are usually associated with a sequence of solar activity and related geomagnetic storms. This association appears to be useful to infer the occurrence of major fluence proton events extending back to 1934, albeit in a non-homogeneous manner. We discuss the possibility of identifying major solar proton events prior to 1934, using geomagnetic records as a proxy.

Procedures and equipment used in the University of Wisconsin laboratory have been described previously (Bender, Bryson & Baerreis 1965; Steventon & Kutzbach 1986). Except as otherwise indicated, wood, charcoal and peat samples are pretreated with dilute NaOH-Na4P2O7 and dilute HCl before conversion to counting gas methane; when noted, marls and lake cores are treated with acid only.

The application of AMS to the detection of 14C makes possible a new class of sensitive experiments in molecular biology. Such experiments inherently produce large numbers of samples for the determination of biological variability in molecular interactions. The samples vary in 14C concentration over many orders of magnitude. We added TiH2 to aid the reduction of CO2 by zinc in a sealed tube to reproducibly make graphite without sample cross-contamination. The CO2 is transferred from a combustion tube to the reaction tube through a disposable plastic manifold. The sealed tubes are heated to a single-reaction temperature in a muffle furnace. The process is complete within 5 h. Bulk isotopic fractionation in the finished graphite is less than 0.5%.

We present here radiocarbon analyses made since our previous list (Srdoč et al. 1989). Sample pretreatment, combustion and counting techniques are essentially the same as described previously (Srdoč, Breyer & Sliepčević 1971), supplemented by new techniques for groundwater processing (Srdoč et al. 1979). The calculation of ages follows conventional protocol (Stuiver & Polach 1977). These ages can be converted from the 5570-year half-life to the chronometrically more correct half-life of 5730 years by multiplying by the factor, 1.029. Ages and standard deviations of all samples are adjusted for stable isotope fractionation according to the recommendations in Stuiver and Polach (1977), except for groundwater, calcareous deposits and aquatic plants. The δ13C values of the latter reflect the environmental conditions during their formation or growth rather than fractionation (Mook 1976; Krajcar Bronić et al. 1986; Marčenko et al. 1989). Thus, any percent modern (pMC) correction based on δ13C values is meaningless or even misleading. Sample descriptions are prepared in collaboration with collectors and submitters. Calibrated ages were calculated using the program of Stuiver and Reimer (1987).