Ugo Zoppi

Progress In Radiocarbon Target Preparation At The Antares Ams Centre

We present routine methods of target preparation for radiocarbon analysis at the ANTARES Accelerator Mass Spectrometry (AMS) Centre, as well as recent developments which have decreased our procedural blank level and improved our ability to process small samples containing less than 200 μg of carbon. Routine methods of 14 C sample preparation include sample pretreatment, CO 2 extraction (combustion, hydrolysis and water stripping) and conversion to graphite (graphitization). A new method of cleaning glassware and reagents used in sample processing, by baking them under a stream of oxygen, is described. The results show significant improvements in our procedural blanks. In addition, a new graphitization system dedicated to small samples, using H 2 /Fe reduction of CO 2 , has been commissioned. The technical details of this system, the graphite yield and the level of fractionation of the targets are discussed.

Performance Evaluation of the New AMS System at Accium BioSciences

A new compact accelerator mass spectrometry (AMS) system dedicated to the measurement of radiocarbon has been commissioned at the Accium BioSciences headquarters in Seattle. The entire facility (including ancillary laboratories for the preparation of graphite targets) has been designed to handle samples with a wide range of 14C concentrations. Innnthis paper, we discuss the technical details of the new facility and present performance test results demonstrating state-of-the-artnncapabilities. In particular, modern samples can be readily measured with 0.3% precision and accuracy, machine background levels are consistently in the low 1016 (14C/12C), and chemical background is approximately equivalent to a fraction of modern of 0.004. In addition, when 100-times-modern samples were processed, no increase in background was observed, either during sample processing or during AMS measurement. This corresponds to a dynamic range for 14C analysis of 6 orders of magnitude.

Bomb radiocarbon in tree rings from northern New South Wales, Australia: Implications for dendrochronology, atmospheric transport, and air-sea exchange of CO2

We have analyzed by radiocarbon 27 consecutive single rings, starting from AD 1952, of a preliminarily cross-dated section (DFR 021) of Pinus radiata, which grew in Armidale, northern New South Wales, Australia. The bomb (super 14) C results suggested the possibility of 2 false rings, and, consequently, 2 misidentified rings in the preliminary count for this section. This possibility was supported by a better ring-width correlation between the revised DFR 021 count and other Pinus radiata chronologies in the study region. This indicated that bomb (super 14) C is a useful tool to complement the standard techniques of dendrochronology in tree species where annual rings are not always clearly defined. Our accelerator mass spectrometry (AMS) (super 14) C results for Armidale Pinus radiata, on a corrected timescale, can be compared with previously published atmospheric and oceanic (super 14) C data. The data show interesting features of atmospheric circulation and the regional air-sea exchange of CO (sub 2) for the bomb period. On average, the difference between Delta (super 14) C values for Armidale (30 degrees S) and those for Tasmania (42 degrees S) was negligible, implying a small latitudinal (super 14) C gradient in the Southern Hemisphere. However, small offsets between Armidale and Tasmania were observed for some periods. The variation of these offsets suggests some slight changes in the relative contributions of the 2 excess (super 14) C sources (the northern troposphere and southern stratosphere) to the southern troposphere. In the decay of bomb (super 14) C, atmospheric (super 14) C reached a global equilibrium at the end of the 1960s and decreased exponentially, halving every 16 years. The time for air-sea exchange of CO (sub 2) for southern Pacific mid-latitudes was found to be about 7.5 yr, which was equivalent to a CO (sub 2) flux from the atmosphere to the oceans of 21.5 moles m (super -2) y (super -1) for the 1970s.

Preliminary estimate of the reservoir age in the Lagoon of Venice.

The Lagoon of Venice was formed about 6000 years ago due to the marine transgression associated with the late Pleistocene sea level rise. Already by the time of the Republic of Venice (727-1797 AD) it was recognized that the future of the city and its many historical buildings was strongly correlated with the future of the lagoon itself. During the centuries many engineering projects such as modification of the fluvial systems, construction of coastal barriers, and dredging of navigation channels were carried out to preserve the lagoonal environment. The present-day lagoon is the result of all these processes and covers an area of 540 km (super 2) with an average depth of 0.6 m. A series of radiocarbon age determinations carried out on material obtained from cores collected in the Lagoon of Venice indicate within the sedimentary units the existence of a number of discontinuities and slumping events due to the highly active lagoonal environment. The evaluation of data obtained from a variety of different materials--both terrestrial and marine--allowed us to determine for the first time the marine reservoir effect in the lagoon of Venice. The discussion includes a comparison with other relevant measurements and a possible explanation to the relatively high reservoir age (1200-1300 yr).

MARINE RESERVOIR CORRECTION FOR THE COCOS (KEELING) ISLANDS, INDIAN OCEAN

Known-age corals from the Cocos (Keeling) Islands, Indian Ocean, have been analyzed by accelerator mass spectrometry (AMS) for radiocarbon to determine marine reservoir age corrections. The ΔR value for the Cocos (Keeling)Islands is 66 ± 12 yr based on the analyses undertaken for this study. When our AMS and previously published dates for Cocos nare averaged, they yield a ΔR of 64 ± 15 yr. This is a significant revision of an earlier estimate of the ΔR value for the Cocos (Keeling) Islands of 186 ± 66 yr (Toggweiler et al. 1991). The (revised) lower ΔR for the Cocos (Keeling) Islands is consistent with GEOSECS 14C data for the Indian Ocean, and previously published bomb 14C data for the Red Sea, Gulf of Aden, and Cocos Islands. The revised ΔR is also close to values for the eastern Indian Ocean and adjacent seas. These suggest surface waters that reach the Cocos Islands might be partly derived from the far western Pacific, via the Indonesian throughflow, and nmight not be influenced by the southeast flow from the Arabian Sea.

THE END OF THE CHALCOLITHIC PERIOD IN THE SOUTH JORDAN VALLEY: NEW 14C DETERMINATIONS FROM TELEILAT GHASSUL, JORDAN

This article reports on 12 new accelerator mass spectrometry (AMS) dates from the latest phases of the Chalcolithic period occupation (late 5th millennium cal BC) at Teleilat Ghassul, type site for the south Levantine Ghassulian Chalcolithic culture. The new AMS dates from Teleilat Ghassul favor an amendment to a previous suggestion (Bourke et al. 2001), that all significant occupation at the site had ceased by 4000/3900 cal BC. This end-date should now be amended to 3900/3800 cal BC. Follow-up statistical modelling sourced to published 14C data drawn from a wide selection of south Levantine Chalcolithic period sites (Bourke 2001; Burton and Levy 2001) raises the possibility that Chalcolithic period occupation had ceased at virtually all major centers by 3800/3700 cal BC. This, in turn, suggests that the new data bearing on the end-date for occupation at Teleilat Ghassul may reflect a more widespread horizon of abandonment in the southern Levant.

The chronology of the Ghassulian Chalcolithic period in the southern Levant: New 14C determinations from Teleilat Ghassul, Jordan

This article reports on ten new accelerator mass spectrometry (AMS) dates from the Chalcolithic period (fifth millennium BC) archaeological type-site of Teleilat Ghassul in Jordan. Early radiocarbon assays from the site proved difficult to integrate with current relative chronological formulations. The ten new AMS dates and follow-up enquiries connected with the early assays suggest that the original dates were up to 500 years too early. A necessary reformulation of regional relative chronologies now views the Ghassul sequence falling between Late Neolithic Jericho and the Beersheban Chalcolithic.

RADIOCARBON IN ANNUAL TREE RINGS FROM THAILAND DURING THE PRE-BOMB PERIOD, AD 1938-1954

Annual tree rings from Thailand were analyzed by radiocarbon AMS for AD 1938-1954. The results showed no significant depletion in atmospheric 14C over Thailand during the pre-bomb period, even though the air mass to Thailand during the growing season of tree rings is transported over a potentially significant source of oceanic 14C-depleted CO2, out- gassing in the northern Indian Ocean. When compared with Washington and Chile for different periods from the 17th century to AD 1954, Thailand appears to have the characteristics of Southern Hemisphere 14C. This supports our previous finding that Thailand was strongly influenced by the entrainment of Southern Hemisphere air parcels in the southwest Asian monsoon (Hua et al. 2004). For Thailand, this effect is much stronger than the reduction of atmospheric 14C in association with CO2 out- gassing in the northern Indian Ocean.

The Beginning of the Early Bronze Age in the North Jordan Valley: New 14C Determinations from Pella in Jordan

This article reports on 10 new accelerator mass spectrometry (AMS) radiocarbon dates from early phases of the Early Bronze Age at the long-lived settlement of Pella (modern Tabaqat Fahl) in the north Jordan Valley. The new AMS dates fall between 3400 and 2800 cal BC, and support a recent suggestion that all Chalcolithic period occupation had ceased by 3800/3700 cal BC at the latest (Bourke et al. 2004b). Other recently published Early Bronze Age 14C data strongly supports this revisionist scenario, suggesting that the earliest phase of the Early Bronze Age (EBA I) occupied much of the 4th millennium cal BC (3800/3700 to 3100/3000 cal BC). As this EB I period in the Jordan Valley is generally viewed as the key precursor phase in the development of urbanism (Joffe 1993), this revisionist chronology has potentially radical significance for understanding both the nature and speed of the move from village settlement towards a complex urban lifeway.

Radiocarbon AMS Data Analysis: From Measured Isotopic Ratios to 14C Concentrations

Radiocarbon accelerator mass spectrometry (AMS) measurements are always carried out relative to internationally accepted standards with known 14C activities. The determination of accurate 14C concentrations relies on the fact that standards and unknown samples must be measured under the same conditions. When this is not the case, data reduction is either performed by splitting the collected data set into subsets with consistent measurement conditions or by applying correction factors.nnThis paper introduces a mathematical framework that exploits the intrinsic variability of an AMS system by combining arbitrary measurement parameters into a normalization function. This novel approach allows the en-masse reduction of large data sets by providing individual normalization factors for each data point. Both general features and practicalities necessary fornnits efficient application are discussed.