Accelerator Mass Spectrometry and Bayesian Data Analysis

Accelerator mass spectrometry (AMS) is an ultrasensitive technique for measuring the concentration of a single isotope. The electric and magnetic fields of an electrostatic accelerator system are used to filter out other isotopes from the ion beam. The high velocity means that molecules can be destroyed and removed from the measurement background. As a result, concentrations down to one atom in 10^16 atoms are measurable. This thesis describes the construction of the new AMS system in the Accelerator Laboratory of the University of Helsinki. The system is described in detail along with the relevant ion optics. System performance and some of the 14C measurements done with the system are described. In a second part of the thesis, a novel statistical model for the analysis of AMS data is presented. Bayesian methods are used in order to make the best use of the available information. In the new model, instrumental drift is modelled with a continuous first-order autoregressive process. This enables rigorous normalization to standards measured at different times. The Poisson statistical nature of a 14C measurement is also taken into account properly, so that uncertainty estimates are much more stable. It is shown that, overall, the new model improves both the accuracy and the precision of AMS measurements. In particular, the results can be improved for samples with very low 14C concentrations or measured only a few times.

(Table 1) Summary of accelerator mass spectrometer (AMS) radiocarbon ages in ODP Sites 169-1033 and 169-1034

Continuous coring in Saanich Inlet (Ocean Drilling Program, ODP Leg 169S), British Columbia, Canada, yielded a detailed record of Late Quaternary climate, oceanography, marine productivity, and terrestrial vegetation. Two sites (1033 and 1034) were drilled to maximum depths of 105 and 118 m, recovering sediments ranging in age from 13,300 to less than 300 14C yr. Earliest sediments consist of dense, largely massive, gray glaciomarine muds with dropstones and sand and silt laminae deposited during the waning stages of glaciation. Deposition of organic-rich olive gray sediments began in the fjord about 12,000 14C yr ago, under well-oxygenated conditions as reflected by the presence of bioturbation and a diverse infaunal bivalve community. At about 10,500 14C yr, a massive, gray unit, 40-50 cm thick, was emplaced in a very short span of time. The unit is marked by a sharp lower contact, a gradational upper contact and an abundance of reworked Tertiary microfossils. It has been interpreted as resulting from massive flood events caused by the collapse of glacial dams in the Fraser Valley of mainland British Columbia. Progressively greater anoxia in bottom waters of Saanich Inlet began about 7000 14C yr ago. This is reflected in the preservation of varved sediments consisting of diatomaceous spring-summer laminae and terrigenous winter laminae. Correlation of the sediments was based on: marked lithologic changes, the presence of massive intervals (reflecting localized sediment gravity flow events), the Mazama Ash, occasional thin gray laminae (indicative of abnormal flood events in nearby watersheds), varve counts between marker horizons, and 71 accelerator mass spectrometry (AMS) radiocarbon dates.

Wet and dry deposition of 129I in Seville (Spain) measured by accelerator mass spectrometry

Iodine-129 (Full-size image (<1 K)) concentrations have been determined by accelerator mass spectrometry in rainwater samples taken at Seville (southwestern Spain) in 1996 and 1997. This technique allows a reduction in the detection limits for this radionuclide in comparison to radiometric counting and other mass spectrometric methods such as ICP-MS. Typical 129I concentrations range from 4.7×107129I atoms/l (19.2%) to 4.97×109129I atoms/l (5.9%), while 129I depositions are normally in the order of 108–1010 atoms/m2 d. These values agree well with other results obtained for recent rainwater samples collected in Europe. Apart from these, the relationship between 129I deposition and some atmospheric factors has been analyzed, showing the importance of the precipitation rate and the concentration of suspended matter in it.

The earliest archaeological maize (Zea mays L.) from highland Mexico: New accelerator mass spectrometry dates and their implications

Accelerator mass spectrometry age determinations of maize cobs (Zea mays L.) from Guilá Naquitz Cave in Oaxaca, Mexico, produced dates of 5,400 carbon-14 years before the present (about 6,250 calendar years ago), making those cobs the oldest in the Americas. Macrofossils and phytoliths characteristic of wild and domesticated Zea fruits are absent from older strata from the site, although Zea pollen has previously been identified from those levels. These results, together with the modern geographical distribution of wild Zea mays, suggest that the cultural practices that led to Zea domestication probably occurred elsewhere in Mexico. Guilá Naquitz Cave has now yielded the earliest macrofossil evidence for the domestication of two major American crop plants, squash (Cucurbita pepo) and maize.

Attomole level protein sequencing by Edman degradation coupled with accelerator mass spectrometry

Edman degradation remains the primary method for determining the sequence of proteins. In this study, accelerator mass spectrometry was used to determine the N-terminal sequence of glutathione S-transferase at the attomole level with zeptomole precision using a tracer of 14C. The transgenic transferase was labeled by growing transformed Escherichia coli on [14C]glucose and purified by microaffinity chromatography. An internal standard of peptides on a solid phase synthesized to release approximately equal amounts of all known amino acids with each cycle were found to increase yield of gas phase sequencing reactions and subsequent semimicrobore HPLC as did a lactoglobulin carrier. This method is applicable to the sequencing of proteins from cell culture and illustrates a path to more general methods for determining N-terminal sequences with high sensitivity.

Long-term mass balance of perennial firn and ice in an Alpine cave (Austria):Constraints from radiocarbon-dated wood fragments

Hundsalm ice cave located at 1520 m altitude in a karst region of western Austria contains up to 7-m-thick deposits of snow, firn and congelation ice. Wood fragments exposed in the lower parts of an ice and firn wall were radiocarbon accelerator mass spectrometry (AMS) dated. Although the local stratigraphy is complex, the 19 individual dates - the largest currently available radiocarbon dataset for an Alpine ice cave - allow to place constraints on the accumulation and ablation history of the cave ice. Most of the cave was either ice free or contained only a small firn and ice body during the 'Roman Warm Period'; dates of three wood fragments mark the onset of firn and ice build-up in the 6th and 7th century ad. In the central part of the cave, the oldest samples date back to the 13th century and record ice growth coeval with the onset of the 'Little Ice Age'. The majority of the ice and firn deposit, albeit compromised by a disturbed stratigraphy, appears to have been formed during the subsequent centuries, supported by wood samples from the 15th to the 17th century. The oldest wood remains found so far inside the ice is from the end of the Bronze Age and implies that local relics of prehistoric ice may be preserved in this cave. The wood record from Hundsalm ice cave shows parallels to the Alpine glacier history of the last three millennia, for example, the lack of preserved wood remains during periods of known glacier minima, and underscores the potential of firn and ice in karst cavities as a long-term palaeoclimate archive, which has been degrading at an alarming rate in recent years. © The Author(s) 2013.

New radiocarbon accelerator dates on artefacts from the early Mesolithic site of Star Carr, North Yorkshire

For the well-known early Mesolithic site of Star Carr, dating of organic artefacts by accelerator mass spectrometry (AMS) has been hampered by treatment of bone and antler recovered during the original excavations with preservatives. Some, untreated, artefacts were, however, collected after Clark's excavation in 1950. Four of these artefacts were AMS dated in 1995, but two of the dates were significantly younger than the others, and were questionable due to their low collagen yields. These suspect samples have now been re-analysed, demonstrating that all four artefacts are of similar date. The significance of these dates for the chronology of Star Carr is discussed.