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.

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.

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.

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.

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.

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.

Introduction

This list includes dates of geological samples measured using a single-channel liquid scintillation 14C counter at the Geochemical and Statistical Laboratory, Tartu, Estonia. Our modern standard is made of benzene enriched in 14C, and its activity is checked with NBS oxalic acid standard sample. Dates are given in conventional radiocarbon years, based on the Libby half-life of 5570 ± 30 years. A.D. 1950 is the reference year. Errors refer only to 1σ standard deviation calculated from count rates involved.

The Radiocarbon Laboratory at the University of Barcelona began its serial measurements in 1985. The following list contains dates obtained between 1985 and 1987. Both archaeological and geological samples were dated.

The measurement technique we use is liquid scintillation counting of benzene. A previous paper (Mestres, García & Rauret 1991) describes sample pretreatment, synthesis and counting protocols and equipment.

This paper includes some determinations of archaeological, art and palaeobotanical samples from Spain and Portugal, obtained at the University of Granada Radiocarbon Dating Laboratory, mostly from 1986 to 1988. Pretreatment of charcoal and wood samples is a standard acid-basic procedure using 8% HC1 and 2% NaOH at boiling temperature. The collagen of bone samples is obtained by the Longin (1971) method.

The method of dating is benzene synthesis and liquid scintillation counting as previously reported (González-Gómez, López-González & Domingo-García 1982; González-Gómez, Sánchez-Sánchez and Domingo-García 1985; González-Gómez, Sánchez-Sánchez and Villafranca-Sánchez 1986, 1987).14C activity was measured in a Packard Tri-Carb Mod 4640 liquid scintillation spectrometer, using 20 ml low 40K counting vials with 5 ml benzene and 10 ml PPO-toluene as scintillator with a background of ca. 9 cpm. Efficiency was approximately 70% using the part of spectrum above the end point of tritium.