In terms of both quenching and, in particular, counting efficiency, scintillation counters employing burst counting circuitry are sensitive to changes in scintillant concentration. Our research demonstrates that there are plateau regions where neither parameter is significantly influenced by moderate changes in a cocktail containing butyl - PBD and bis - MSB. One of these, consisting of butyl - PBD 2.8mg.g-1 and bis - MSB 3mg.g-1 of benzene is recommended for these counters, as it enables open window counting efficiencies of ca 90% while exhibiting optimal resistance to quenching. A method for accurate dispensing of such small weights is also described.

AMS 14C measurements on pteropod shells from eastern North Atlantic deep-sea cores reveal distinct periods of aragonite preservation during the last 16,000 years. Most preservation spikes coincide with documented periods of climatic change on a scale of 2 × 101 to 2 × 103 years.

I discuss here the basis of a comparison of methods for radiocarbon dating and introduce a new index for the relative merit of a system, factor of counting capacity, that is generally more appropriate than the commonly used factor of merit. The merit of a dating system cannot be based on a single figure but other factors must also be considered. A comparison of the gas proportional, liquid scintillation and accelerator mass spectrometry technique is presented that for gas proportional counters is based on a multi-detector system rather than a single detector.

The dating of palimpsest marine sediments using broken shell fragments (shell hash) is considered to be a necessary but unreliable technique because of the mixed age of the fragments. An analysis of geological mixing models and radiocarbon data on shell hash from sandy sediments on the southeast Australian coast and shelf are used to examine the possibility for simulating the depositional processes, and thus, to better understand the age structure of the deposits.

Detailed stable oxygen isotope analyses coupled with AMS 14C measurements on an eastern Mediterranean sapropel S1 sequence indicate that adverse bottom conditions persisted for ca 8000 years. AMS dates on additional sequences show that complete bottom anoxia lasted for 300-800 years. The S1 event is not synchronous throughout the eastern Mediterranean, but started earlier in the deeper parts of the basin.

Radiocarbon was measured in dissolved inorganic carbon (DIC) and living organisms collected off the west coast of Baja California in October 1980. Samples from three locations were examined. Δ14C of DIC at the southernmost station was higher than those further north, which reflects reduced upwelling in the southern region. Crabs and anchovies had Δ14C values significantly lower than surface DIC Δ14C, indicating the incorporation of ‘older', sediment-derived carbon sources from their diets. Comparisons are made between our DIC Δ14C measurements and those obtained during other cruises and at a coastal site, from 1959 through 1987. Two distinct time histories of DIC Δ14C are apparent for the post-bomb period: 1) a lower Δ14C curve for sites close to the coast influenced by enhanced coastal upwelling; and 2) a higher Δ14C curve for sites further offshore within the California Current.

In 1983, the University of Barcelona, the Institut d'Estudis Catalans and the Department of Culture of the Generalitat de Catalunya agreed to establish a laboratory for radiocarbon measurement at the Analytical Chemistry Department of the University of Barcelona. The liquid scintillation technique was chosen for radiocarbon counting. The counting laboratory is in the basement of a six-story building.

High count rates of lithium ions in an AMS measurement system for 14C were traced to the lithium content in the quartz tubes used in the graphitization of the samples. The lithium contamination was nearly eliminated by the use of borosilicate glass reaction tubes at a lower reaction temperature. The ion beam current and the measurement precision of the isotope ratio were not affected.

Between the Alaska Range to the south and the Brooks Range to the north, the Yukon River and its tributaries form an extensive network of waterways leading through the lowlands of interior Alaska deep into the North American continent (Fig 1). Despite the extremely cold winters of this arctic and subarctic landscape, much of the region remained unglaciated during the last 50,000 years. The central Alaskan lowlands are at the west end of the “ice-free corridor,” thought by most prehistorians to be the pathway to the Americas for Asian hunter-gatherers crossing the continental Beringian “landbridge.” Until recently, relatively little was known of the early human prehistory of Alaska's interior. Growing interest in the timing, nature and paleoecological context of the initial peopling of the Americas has prompted excavation at a number of early sites in central Alaska and the adjacent Yukon (see Powers & Hamilton 1978; West 1981; Fagan 1987: 119-134; Hamilton 1989; Powers & Hoffecker 1989).

It is well known that 14C dating of fossil bone with seriously depleted protein levels, or bone that has been consolidated with preservatives, can produce erroneous results. In the tropics, warm and moist soil conditions lead to constant reworking of organic matter and add to the danger of bone contamination. Because of this, 14C dating of preservative-impregnated bone from such areas has rarely been successful. We report here a set of AMS dates on both unconsolidated animal bone and polyvinyl acetate/polyvinyl alcohol (PVA/PV-OH) impregnated human burials from the Maya site of Cuello, Belize. The steps needed to purify the samples are described, together with details on the use of qualitative infra-red (IR) spectra as a means of assessing sample purity.

The following list consists of dates, obtained by liquid scintillation counting of benzene, for archaeologic samples mostly measured between June 1987 and October 1989.

Charcoal and grain samples were pretreated with 1M HCl followed by washing in water and, where considered necessary, with dilute alkali for the removal of humic acids. Wood samples were treated either in the same way, or, where large enough, were reduced to cellulose by the action of chlorine dioxide produced in situ. All antler and bone samples were treated with cold dilute acid. The term ‘collagen’ is used throughout to mean the acid insoluble organic fraction produced by this treatment. Peat samples were treated with dilute acid and alkali to separate the humin and humic acid fractions, which were dated individually.

A suite of 23 14C age determinations, from a well-stratified rockshelter and from 3 pollen cores on Mangaia Island is reported. The rockshelter has yielded significant evidence for avifaunal extinctions during the period cal. A.D. 1000-1600. The Lake Tiriara pollen cores span a period from ca. 6500 cal. b.p. to the present, and palynological analysis of the TIR 1 core indicates major anthropogenic disturbance on the island's vegetation after ca. 1600 cal. B.P. These sites, and the radiocarbon ages associated with them, provide the first chronologically secure evidence for human impacts on the island ecosystems of the southern Cook Islands.

We present here dates for archaeological and geological samples. The dates are based on τ1/2 = 5568 years, using 1950 as the base year. The modern standard was 95% of activity of NBS oxalic acid.

Samples were pretreated, prepared by methane synthesis and counted in a proportional counter. The detailed procedure has been described earlier (Agrawal, Gupta & Kusumgar 1971). We reported minor changes in previous date lists. Quoted errors are based on counting statistics alone. For samples younger than 10,000 years, the error is 1 standard deviation, and for older samples, 2 standard deviations. The dates are not corrected for 13C fractionation. Samples have been arranged alphabetically according to the name of the site.

We converted to CO2 samples of organic materials, Quaternary carbonate rocks and carbonates extracted from groundwater. We measured 14C radioactivity in a proportional counter with an effective volume of 723ml filled to 0.28MPa. A mechanical box with an inside cylinder of a plastic scintillator, 4cm thick, arranged in anticoincidence provides the shielding. We present a review of radiocarbon ages of Quaternary and archaeological samples.