The characterization of late Holocene climates in northern Australia has, in the past, been based on local investigations. This examination of the chenier record of northern Australia indicates that there has been a statistically significant regional change in conditions between 1600–2800 years bp, possibly a period of relative aridity. Support for this conclusion may be found in the vegetation record from the Atherton Tableland where numerical comparisons of dryland fossil and modern pollen spectra suggest that rainfall may have been up to 50% higher during the period 7000 to 3000 bp.

Results of the 14C measurements in atmospheric CO2 in the first half of 1986 are presented. CO2 samples were systematically collected in Krakow in two-week cycles and, after conversion to benzene, measured in a liquid scintillation spectrometer. 14C activity and 13C/12C ratio are reported as δ14C and δ13CPDB, respectively. For about three weeks after April 26, 1986 (the Chernobyl accident) an increase of ∼9% above the normal level for Krakow was observed. A rough estimate of the 14C release to the lower atmosphere during the accident gave a value 900 Ci, which is ∼1.8 × 10−5 of the total activity released to the atmosphere.

The levels and sources of the measurement background in an AMS 14C dating system have been studied in detail. The relative contributions to the total background from combustion, graphitization, storage, handling, and from the accelerator were determined by measuring the C concentrations in samples of anthracite coal ranging in size from 15μg to 20mg. The results show that, for the present system, the uncertainty in the background is greater than that due to measurement precision alone for very old or for very small samples. While samples containing 100μg of carbon can yield useful 14C dates throughout the Holocene, 200 to 500μg are required for dating late Pleistocene materials. With the identification of the procedures that introduce contamination, the level and uncertainty of the total system background should both be reducible to the point that 100μg of carbon would be sufficient for dating most materials.

An assessment of the contamination contribution of various sample preparation procedures used at the Isotrace Radiocarbon Facility, University of Toronto, is described. Samples of geologic material, millions of years old, or samples derived therefrom, were tested because these would presumably contain only dead carbon. Results showed, however, that 14C contamination could be detected in several samples, complicating the contamination assessment. Best estimates of the contamination contribution from sample preparation were reported as: cracking: <0.17% modern, acetylene synthesis: <0.25% modern, combustion: <0.39% modern, and handling: <0.54% modern. These estimates were reported as upper limits because they likely represented 14C derived from two sources: sample preparation and the sample itself.

A technique for 14C measurement of small volume (0.5L) oceanic water samples by Accelerator Mass Spectrometry (AMS) is described. Samples were taken from a CTD/rosette system used for standard hydrographic work. After CO2 extraction and target preparation, the samples were measured at the Zürich tandem accelerator facility. On the basis of 14C data from samples collected on a station in the northern Weddell Sea, the precision of the measurements is estimated to ca ±8‰. The error in the present AMS results is dominated by the statistical error in 14C detection. From results of duplicate targets, it is concluded that a precision of ±5° can be reached. The 14C data are discussed in relation to the Weddell Sea hydrography.

Most of the 14C measurements reported here were made between October 1985 and October 1986. Equipment, measurement, and treatment of samples are as reported previously (R, 1968, v 10, p 36–37; 1976, v 18, p 290; 1980, v 22, p 1045; 1986, v 28, no. 3, p 1111).

This date list contains the results of 14C determinations of archaeologic samples from Spain and Portugal obtained at the Laboratory mostly from 1983 to July 1986. Preparation and measurements were made in the same manner as previously reported (R, 1982, v 24, no. 2, p 217–221; R, 1985, v 27, no. 3, p 610–615; R, v 28, no. 3, p 1200–1205).

Measurements have continued with the same proportional counter system, pretreatment procedure, methane preparation and measurement, and calculation, as described previously (R, 1970, v 12, no. 1, p 298–318). Uncertainties quoted are single standard deviations. No 13C/12C ratios were measured. Sample descriptions have been prepared in cooperation with submitters. Some dates have been calibrated using the correction tables of Klein et al (1982) and are reported as “cal” ages in the comment.

Procedures and equipment used in the University of Wisconsin laboratory have been described in previous date lists. Except as otherwise indicated, wood, charcoal, and peat samples are pretreated with dilute NaOH—NA4P2O7 and dilute H3PO4 before conversion to the counting gas methane; when noted, marls and lake cores are treated with acid only. Very calcareous materials are treated with HCL instead of H3PO4. Pretreatment of bone varies with the condition of the bone sample; solid bone with little deterioration is first cleaned manually and ultrasonically. The bone is treated with 8% HCL for 15 minutes, then dilute NaOH—Na4P2O7 for 3 hours at room temperature, washed until neutral, and the collagen extracted according to Longin (1971). Charred bone is treated with dilute HCL, NaOH—Na4P2O7, and then dilute HCL again.

Samples of inshore marine shell species (various taxa, see description below) were collected from controlled excavation of ceramic-bearing strata of two archaeologic sites in the Manu'a Island group, American Samoa. Located on the closely adjacent islands of Ta'u and Ofu (14° 14’ 30” S, 169° 30’ 40” E and 14° 10’ 55” S, 169° 39’ 0” E, respectively), these sites represent human occupation along shorelines undergoing a parallel depositional sequence of calcareous sand dune development and concomitant seaward progradation. Our primary objective was to obtain an initial age estimate for prehistoric ceramics from eastern Samoa. On stylistic and technologic criteria, the ceramics recovered from our excavations can be classified as thick-coarse Polynesian Plainware. Based on previous studies in Western Samoa, Polynesian Plainware represents a terminal phase of prehistoric pottery manufacture in the Samoan Islands, believed to date from ca 200 bc to ad 300 (Green & Davidson, 1974).