Masayo Minami

The HVEE Tandetron AMS system at Nagoya University

Abstract In 1996/97, we installed a second-generation AMS system (model 4130-AMS) manufactured by HVEE, B.V., The Netherlands. Although we encountered a lot of problems during the installation and tuning up of the machine, our new system has now reached good stability and reproducibility in performing 14 C / 12 C and 13 C / 12 C measurements. According to the reproducibility tests conducted in January 1999, where six graphite targets prepared from HOxII standard material were measured, the standard deviation for 14 C / 12 C reproducibility is around 1.6 ‰ (comparable with counting statistics) and that for 13 C / 12 C is 0.28 ‰ . The background level of the AMS system was measured using a pure graphite (commercial graphite rod). It lies between 50 and 55 ka BP. We are still on the way to reduce the background. However, we are now prepared to start routine measurements of unknown-age samples.

Application of 14C Wiggle-Matching to Support Dendrochronological Analysis in Japan

14C wiggle-matching was applied to two wood samples closely related to geological and archaeological events with associated dendrochronological dates, to demonstrate the accuracy of 14C dating with accelerator mass spectrometry (AMS). Wiggle-matching on charred wood with bark, excavated from a pyroclastic mud-flow deposited by the huge 10th Century eruption of the Baitoushan Volcano, revealed the eruption age as cal A.D. 935 +8/−5 with 95% confidence. This date is consistent with the eruption age of A.D. 912 to A.D. 972 estimated by dendrochronology on two wooden boards that had clear stratigraphical connections to the B-Tm tephra deposit in Japan, an ash fall layer formed by the eruption of the Baitoushan Volcano. The date is also consistent with an A.D. 937–938 date estimated by the analysis of varved sediments from Lake Ogawarako in Aomori prefecture. The other wooden board collected from the Mawaki archaeological site in Ishikawa prefecture was wiggle-matched as 783 +13/−11 cal B.C. with 95% confidence, which is consistent with the dates of 830 cal B.C. to 759 cal B.C. obtained for seven wooden poles from the same wooden structures as the wooden board. These results are highly encouraging for obtaining accurate dates on wood when dendrochronology cannot be used.

Difference in radiocarbon ages of carbonized material from the inner and outer surfaces of pottery from a wetland archaeological site.

AMS (Accelerator Mass Spectrometry) radiocarbon dates for eight potsherds from a single piece of pottery from a wetland archaeological site indicated that charred material from the inner pottery surfaces (5052 ± 12 BP; N = 5) is about 90 14C years older than that from the outer surfaces (4961 ± 22 BP; N = 7). We considered three possible causes of this difference: the old wood effect, reservoir effects, and diagenesis. We concluded that differences in the radiocarbon ages between materials from the inner and outer surfaces of the same pot were caused either by the freshwater reservoir effect or by diagenesis. Moreover, we found that the radiocarbon ages of carbonized material on outer surfaces (soot) of pottery from other wetland archaeological sites were the same as the ages of material on inner surfaces (charred food) of the same pot within error, suggesting absence of freshwater reservoir effect or diagenesis.

AMS radiocarbon age for fossil bone by XAD-2 chromatography method

Abstract The XAD-2 chromatography method was examined for its ability to efficiently eliminate exogenous organic matter from fossil bones and to improve the accuracy of radiocarbon ( 14 C ) dating and stable isotope determinations on bone proteins. The fossil bones used in the experiment were animal fossil bones collected from the Awazu submarine archaeological site, Shiga, Japan. For comparison, the gelatin-extraction method was also applied to the same samples. It was found that the gelatin-extraction method is sufficient for 14 C dating on well-preserved bones, but insufficient on poorly preserved bones, containing less than 1% extractable gelatin. The XAD-2 resin is useful for the clean up of proteins especially from poorly preserved bones. The carbon stable isotope fractionation of around 1‰ by XAD-2 treatment on modern collagen standards was larger than reported previously. The isotopic variation by sequential extraction of bones probably originates from changes in the amino acid composition and seems to be less sensitive to the indication of the removal of organic contamination.

Early post-mortem formation of carbonate concretions around tusk-shells over week-month timescales.

Carbonate concretions occur in sedimentary rocks of widely varying geological ages throughout the world. Many of these concretions are isolated spheres, centered on fossils. The formation of such concretions has been variously explained by diffusion of inorganic carbon and organic matter in buried marine sediments. However, details of the syn-depositional chemical processes by which the isolated spherical shape developed and the associated carbon sources are little known. Here we present evidence that spherical carbonate concretions (diameters φ : 14 ~ 37 mm) around tusk-shells (Fissidentalium spp.) were formed within weeks or months following death of the organism by the seepage of fatty acid from decaying soft body tissues. Characteristic concentrations of carbonate around the mouth of a tusk-shell reveal very rapid formation during the decay of organic matter from the tusk-shell. Available observations and geochemical evidence have enabled us to construct a ‘Diffusion-growth rate cross-plot’ that can be used to estimate the growth rate of all kinds of isolated spherical carbonate concretions identified in marine formations. Results shown here suggest that isolated spherical concretions that are not associated with fossils might also be formed from carbon sourced in the decaying soft body tissues of non-skeletal organisms with otherwise low preservation potential.

Ultrafiltration Pretreatment for 14C Dating of Fossil Bones from Archaeological Sites in Japan

To study the effect of ultrafiltration on the radiocarbon ages of relatively poorly preserved bones in Japan, we analyzed the 14C dates of high-molecular-weight (HMW) gelatin samples and compared them with those of other extracted organic fractions, unfiltered gelatin samples extracted from NaOH-treated or NaOH-untreated collagen, and XAD-purified hydrolysates of animal fossil bones (~4600 BP; gelatin yield of 2–4%) from the Awazu underwater archaeological site, Shiga, Japan. NaOH-treated, unfiltered gelatins and XAD-purified hydrolysates showed statistically similar 14C ages to those of HMW gelatins. The 14C ages of the HMW gelatins were the oldest and similar to those of wood collected from the same layer as the bones, and the NaOH-treated, unfiltered gelatins gave 14C ages within the acceptable margins of error; therefore, ultrafiltration was effective for accurate 14C dating, while NaOH-treated gelatin without ultrafiltration was also sufficient to obtain accurate 14C dates on the animal bones. The 14C ages of human skeletons (~750 BP; gelatin yield of 2–11%) from 5 individuals excavated from an archaeological site in Yuigahama, Kamakura, Japan, showed statistically the same 14C ages as NaOHtreated, unfiltered gelatins and HMW gelatins within the margins of error, although HMW gelatins were likely to give slightly older ages than unfiltered gelatin with a yield of less than ~3%. These results indicate that unfiltered gelatins extracted from fossil bones of gelatin yield more than ~3% can produce accurate 14C ages without the need for ultrafiltration. Ten bone fragments from 3 humans showed the same 14C ages for each individual, suggesting that any bone part from an individual can be used to obtain a representative age. The 14C ages of tooth enamels of 2 individuals were 35 and 70 yr older than their bone ages. Death dates obtained from these age gaps agreed with those determined by morphology.

Strontium Isotope Analysis using Resonance Ionization Mass Spectrometry for Determination of Bone Origin

We have considered the applicability of strontium isotope analysis, using Laser Ablation-assisted Resonance Ionization Mass Spectrometry (LA-RIMS), to the determination of bone origin. We separated the uncertainty into two categories: ion counting statistics, and short-term or uncontrollable shifts of experimental conditions; only the former decreases with increasing signal intensity. Additionally, we confirmed that long-term shifts of isotope ratio in LA-RIMS can be cancelled by an internal correction method. We preliminarily estimated the precision of our technique to be 0.7% (1σ) for 105 laser shots; the accuracy is also confirmed within the present uncertainty through the comparison with a reference value.

An extraction system to measure carbon-14 terrestrial ages of meteorites with a Tandetron AMS at Nagoya University.

We have constructed a system to extract carbon from meteorites using a vacuum-tight RF melting method in order to study radiocarbon activities in meteorites. The extraction system was examined using iron standards of known carbon content. The carbon extraction efficiencies and (super 14) C ages of the iron standards by this method were compared with the results obtained previously by our older melting system and a wet oxidation method. Higher collection efficiencies of about 90% for the iron samples of relatively high carbon content were achieved by the new system. The efficiency of extracting a small amount of carbon is also near 90% after improving the extraction procedure. The (super 14) C ages of the iron standards were compared to the ages by the wet method. The results indicate that contamination by modern carbon is negligible in the system. Furthermore, terrestrial (super 14) C ages of two Antarctic meteorites, Y-75102 and ALH-77294, from the Yamato and Allan Hills ice fields, respectively, were determined. The age of Y-75102 is estimated 4.0+ or -1.0 ka, and the age of ALH-77294 is 19.5+ or -1.2 ka. The (super 14) C ages on the meteorites roughly agree with the literature value. However, further study is needed in improvement on reducing a background value and of complete fusion of a meteorite in the extraction system.

Generalized conditions of spherical carbonate concretion formation around decaying organic matter in early diagenesis

Isolated spherical carbonate concretions observed in marine sediments are fascinating natural objet trouve because of their rounded shapes and distinct sharp boundaries. They occur in varied matrices and often contain well preserved fossils. The formation process of such concretions has been explained by diffusion and rapid syn-depositional reactions with organic solutes and other pore water constituents. However, the rates, conditions and formation process of syngenetic spherical concretions are still not fully clear. Based on the examination of different kinds of spherical concretions from several locations in Japan, a diffusion based growth diagram was applied to define the generalized growth conditions of spherical concretions formed around decaying organic matter. All analytical data imply that the spherical concretions formed very rapidly, at least three to four orders of magnitude faster than previously estimated timescales. The values indicate that spherical concretions are preferentially grown within clay- to silt-grade marine sediments deposited in relatively deep (a few tens of metres) environments dominated by diffusive solute transport, very early in diagenesis.

Changes of Chemical Structure and Composition of Charcoal by Radiocarbon Pretreatments: Decontamination by ABA and ABOx Treatments

Charcoal is widely used for radiocarbon dating in archaeological and paleoenvironmental studies. Reliable 14 C dating requires appropriate chemical treatment to remove postdeposition contamination from the charcoal samples. This study assesses two pretreatments: acid-base-acid (ABA) and acid-base-oxidation with stepped combustion (ABOx-SC). In addition to 14 C, the effects of the treatments on the chemical structure and composition of charcoal were studied using Fourier transform infrared spectroscopy (FTIR) and C/H/O elemental analysis. Samples of pine wood charred in the laboratory at 270, 300, 400, 500, and 600°C, and environmental samples of charred pine wood from pyroclastic flow deposits in southern Kyushu, Japan, were tested. The laboratory-charred samples showed that NaOH treatment removed highly hydrophilic organic components derived from endogenous and exogenous organic materials in the samples and that oxidation treatment caused the oxidative degradation of molecules in samples starting from its edges. The ABA-treated environmental charcoal yielded younger 14 C dates than the ABOx-treated samples, probably owing to the effects of remaining organic contaminants bound to the edges of the aromatic molecular structures produced by the original pyrolysis. Meanwhile, it was found that ABA-SC treatment can reduce contaminants as effectively as ABOx-SC treatment. This implies that the stepped combustion (SC), not the chemical oxidation, is the key to reduce contaminant residue left after ABA and ABOx treatments. The results in this study indicate that the investigation of the structural and compositional changes of charcoal during its pretreatment is useful for assessment of the reliability of the 14 C ages.