M.-J. Nadeau

The Leibniz-Labor AMS facility at the Christian-Albrechts University, Kiel, Germany

The AMS facility of the Leibniz-Labor fur Altersbestimmung und Isotopenforschung of the Christian-Albrechts Universitat is based on a 3 MV Tandetron from High Voltage Engineering Europa (HVEE) with a single cesium sputter ion source and a separator/recombinator for simultaneous injecton of the three isotopic carbon beams. The AMS system is similar to those at the Woods Hole Oceanographic Institution, USA, and the University of Groningen, The Netherland, but it has some new features based on experience at these two facilities. These include improved vacuum seals, beam diagnostics, X-ray and background suppression as well as a more reliable system control through a PLC-unit with a serial line to the main system computer. The open system design of the beam optics allows significant horizontal and vertical movement of the ion beams without loss to the walls of the system. This leads to plateaus in the response of the isotope beams and ratios to changing values of various ion optical elements. Combined with highly stable power supplies, this gives reproducible measurements. The acceptance tests, e.g., showed that Poisson counting statistics at 0.15% and 0.22% respectively, determined the statistical uncertainty in the 14C12C ratios measured for the individual samples of two test series. Strong discrimination of unwanted ions results in low background count rates in the detector, equivalent to an apparent age of 75000 years at present, in spite of the open architecture. Routine measurements since late January 1996 (to late May 1996) have dated 127 unknown samples, mostly foraminifera. The prototype of the carbonate to CO2 conversion system and the graphite system used for the measurements are also described.

Complexity of soil organic matter: AMS 14C analysis of soil lipid fractions and individual compounds

Radiocarbon measurements of different lipid fractions and individual compounds, isolated from soil samples collected on 2 different agricultural long-term study sites, located in the rural area of Rotthalmunster (Germany) and in the city of Halle/Saale (Germany), were analyzed to obtain information about sources and the stability of soil organic matter (SOM). Different lipid compound classes were isolated by automated solvent extraction and subsequent medium-pressure liq- uid chromatography. Generally, 14C contents of lipid compound classes from topsoil samples of maize plots at Rotthalmunster are close to the modern atmospheric 14C content. Lower 14C values of aliphatic and aromatic hydrocarbons isolated from neu- tral lipids suggest a contribution of old carbon to these fractions. In contrast, 14C values of bulk soil (52 pMC) as well as iso- lated lipid classes from Halle are highly depleted. This can be attributed to a significant contribution of fossil carbon at this site. Extremely low 14C contents of aromatic (7 pMC) and aliphatic hydrocarbons (19 pMC) reflect the admixture of fossil hydrocarbons at the Halle site. Individual phospholipid fatty acids (PLFA), which are used as a proxy for viable microbial bio- mass, were isolated by preparative capillary gas chromatography (PCGC) from topsoils at Rotthalmunster and Halle. PLFA 14C values are close to atmospheric 14C values and, thus, indicate a clear microbial preference for relatively young SOM. At Rotthalmunster, the 14C concentration of short-chain unsaturated PLFAs is not significantly different from that of the atmo- sphere, while the saturated PLFAs show a contribution of sub-recent SOM extending over the last decades. At Halle, up to 14% fossil carbon is incorporated in PLFAs n-C17:0 and cy-C18:0, which suggests the use of fossil carbon by soil microor- ganisms. Moreover, it can be concluded that the 14C age of soil carbon is not indicative of its stability.

Experimental Study on the Origin of Cremated Bone Apatite Carbon

Bones that have undergone burning at high temperatures (i.e. cremation) no longer contain organic carbon. Lanting et al. (2001) proposed that some of the original structural carbonate, formed during bioapatite formation, survives. This view is based on paired radiocarbon dating of cremated bone apatite and contemporary charcoal. However, stable carbon isotope composition of carbonate in cremated bones is consistently light compared to the untreated material and is closer to the δ13C values seen in C3 plant material. This raises the question of the origin of carbonate carbon in cremated bone apatite. That is, does the isotope signal reflect an exchange of carbon with the local cremation atmosphere and thus with carbon from the burning fuel, or is it caused by isotopic fractionation during cremation? To study the changes in carbon isotopes (14C, 13C) of bone apatite during burning up to 800 °C, a modern bovine bone was exposed to a continuous flow of an artificial atmosphere (basically a high-purity O2/N2 gas mix) under defined conditions (temperature, gas composition). To simulate the influence of the fuel carbon available under real cremation conditions, fossil CO2 was added at different concentrations. To yield cremated bone apatite properties similar to archaeological cremated bones, in terms of crystallographic criteria, water vapor had to be added to the atmosphere in the oven. Infrared vibrational spectra reveal large increases in crystal size and loss of carbonate upon cremation. The isotope results indicate an effective carbon exchange between bone apatite carbonate and CO2 in the combustion gases depending on temperature and CO2 concentration. 14C dates on archaeological cremated bone apatite may thus suffer from an old-wood effect. Paired 13C and 14C values indicate that in addition to this exchange, isotope fractionation between CO2 and carbonate, and admixture of carbon from other sources such as possibly collagen or atmospheric CO2, may play a role in determining the final composition of the apatite carbonate.