Effectiveness of different pre-treatments in recovering pre-burial isotopic ratios of charred plants.

RATIONALE: Isotopic analysis of archaeological charred plant remains offers useful archaeological information. However, adequate sample pre-treatment protocols may be necessary to provide a contamination-free isotopic signal while limiting sample loss and achieving a high throughput. Under these constraints, research was undertaken to compare the performance of different pre-treatment protocols. METHODS: Charred archaeological plant material was selected for isotopic analysis (δ13 C and δ15 N values) by isotope ratio mass spectrometry from a variety of plant species, time periods and soil conditions. Preservation conditions and the effectiveness of cleaning protocols were assessed through Fourier transform infrared spectroscopy and X-ray fluorescence (XRF) spectrometry. An acid-base-acid protocol, successfully employed in radiocarbon dating, was used to define a contamination-free isotopic reference. Acid-base-acid isotopic measurements were compared with those obtained from untreated material and an acid-only protocol. RESULTS: The isotopic signals of untreated material and the acid-only protocol typically did not differ more than 1‰ from those of the acid-base-acid reference. There were no significant isotopic offsets between acid-base-acid and acid-only or untreated samples. Sample losses in the acid-base-acid protocol were on average 50 ± 17% (maximum = 98.4%). Elemental XRF measurements showed promising results in the detection of more contaminated samples albeit with a high rate of false positives. CONCLUSIONS: For the large range of preservation conditions described in the study, untreated charred plant samples, water cleaned of sediments, provide reliable stable isotope ratios of carbon and nitrogen. The use of pre-treatments may be necessary under different preservation conditions or more conservative measurement uncertainties should be reported.

Fuel remains in archaeological contexts: Experimental and archaeological evidence for recognizing remains in hearths used by Iron Age farmers who lived in peatlands.

In the archaeological record, ash and charred organic material are the only indications of the type of fuel used by ancient societies to feed their fires. This potential source of information may help further understanding of past human behaviour in relation to fuel selection, applied type of fire and function of fires lit in hearths. This study examined ash from reference samples and ash and char samples recovered from an Iron Age peatland site in Vlaardingen, the Netherlands. Local availability and abundance made it possible for the occupants of the site to choose from different fuel resources. Peat and cattle dung were readily available, while wood was less abundant. Reference samples from various locations were collected to investigate the properties of ash. Different types of wood, peat and cattle dung were accordingly ashed and analysed. In total, two techniques were used, that is, chemical analyses (x-ray fluorescence (XRF)) for the determination of the elemental composition and microscopic studies on field images of these ashes, mounted on glass slides, to investigate phytoliths quantitatively (Si and Ca types), siliceous aggregates and spherulites. The properties of the archaeological samples were compared with these results. The archaeological char samples were used for identification and analysed using reflective microscopy to study structure and temperature by reflectance (%Ro). It provided the necessary information to determine the type of fuel used by the Iron Age farmers and obtain more information on the function of the present fire structures. The results are strong indications that Iron Age farmers used all available fuels, that is, peat, cattle dung and wood, to feed their fires. The integrated application of phytolith, geochemical and char analyses has a high potential for the identification of the fuel selected by ancient societies.