F. Braadbaart

Stable carbon isotope changes during artificial charring of propagules

Abstract Charred organic remains are ubiquitous in the archaeological and fossil record and are often used to interpret past environments and climate. This study focuses on the physical and chemical alteration that takes place during heating (i.e. charring). Modifications to the internal and external morphology were noted alongside the change in molecular and stable carbon isotope signature. Molecular analyses were undertaken using direct temperature resolved mass spectrometry and the stable carbon isotopes determined using isotope ratio mass spectrometry. The results of this study document a general enrichment in 13 C/ 12 C composition of charred material which could reflect the changes observed in both the molecular composition and the relative proportions of the molecules formed. These results indicate that spurious results might be inferred when comparing the stable carbon isotope signature of charred/charcoalified material with uncharred organic matter

Nanodiamonds and wildfire evidence in the Usselo horizon postdate the Allerød-Younger Dryas boundary

The controversial Younger Dryas impact hypothesis suggests that at the onset of the Younger Dryas an extraterrestrial impact over North America caused a global catastrophe. The main evidence for this impact—after the other markers proved to be neither reproducible nor consistent with an impact—is the alleged occurrence of several nanodiamond polymorphs, including the proposed presence of lonsdaleite, a shock polymorph of diamond. We examined the Usselo soil horizon at Geldrop-Aalsterhut (The Netherlands), which formed during the Allerød/Early Younger Dryas and would have captured such impact material. Our accelerator mass spectrometry radiocarbon dates of 14 individual charcoal particles are internally consistent and show that wildfires occurred well after the proposed impact. In addition we present evidence for the occurrence of cubic diamond in glass-like carbon. No lonsdaleite was found. The relation of the cubic nanodiamonds to glass-like carbon, which is produced during wildfires, suggests that these nanodiamonds might have formed after, rather than at the onset of, the Younger Dryas. Our analysis thus provides no support for the Younger Dryas impact hypothesis.

Changes in Mass and Dimensions of Sunflower (Helianthus annuus L.) Achenes and Seeds Due to Carbonization

When analyzing sunflower (Helianthus annuus L.) remains, which are often carbonized, archaeobotanists commonly differentiate between wild and domesticated achenes and seeds based on the measured length (L) and width (W) or the calculated index L*W. Carbonization reduces the dimensions. To compensate for these reductions, archaeobotanists use a single correction factor proposed by Richard Yarnell (1978) for all cases. The use of a single correction factor can bias the reconstructed dimensions as carbonization is a highly variable process. The current study determines the relationship between carbonization and the dimensions of length and width. Measurements established that a decrease of 2.5-22.5% in achene length and 10-29% in achene width can occur, depending on temperature, heating rate, and variety. For seeds, temperature is of most importance, and shrinkage ranges from 0-27% for the length and from 0-20% for the width. These ranges make the use of a single correction factor problematic. A method is developed in which reflectance (an optical property applied in coal technology to determine coal rank) is used to measure the carbonization temperature, and in turn the shrinkage can be calculated. Subsequently, correction factors are calculated to reconstruct the original length and width. When applied to an assemblage of carbonized sunflower achenes, the newly developed method shows that the Yarnell single correction factor may bias the dimensions towards classifications of “wild” or “ruderal” forms of sunflower

Charred olive stones: experimental and archaeological evidence for recognizing olive processing residues used as fuel

Abstract After extracting oil from olives a residue is left usually referred to as the olive oil processing residue (OPR). This study explores the way in which ancient societies may have used OPR as fuel for fires to generate heat and the various issues that are related to the residues of this fuel. After drying, the high heating value and structure of OPR makes it an excellent and efficient fuel. Upgrading OPR further, through thermal conversion or charring, provides an even more efficient fuel (COPR), with a hotter and smoke free flame, a higher heating value and which is lighter in mass and thus easier to transport. After a fire is extinguished two types of remains of the fuel are left i.e. char and ash. Analyses on both remains, recovered from archaeological deposits, could be used as a source of information on fuel utilization. Laboratory experiments on charred modern OPR and stones show that by measuring their reflectance and analyzing their structure under reflected light microscopy, OPR and COPR can be distinguished in the charred material recovered from three archaeological sites in Greece and Syria. Based on these investigations it is suggested that on the three sites COPR was used as fuel. Ash, sampled together with the char, provides the possibility of investigating if other types of fuel were used, apart from OPR or COPR. On the investigated sites no ash was collected, but the analysis of the modern OPR showed that the properties of its ash could be used to distinguish it from other types of fuel. Ash from modern OPR and olive stones showed the presence of phytoliths. The often discussed issue related to the sharpness and smoothness of the edges of charred fragmented olive stones was investigated. The results showed that this is not a reliable criterion for recognizing olive oil production. It is recommended that in addition to the identification of the botanical material more properties of the remains of fuels should be analysed. To prevent destroying and losing char and ash as a result of excavation activities such as flotation and sieving, special measures have to be taken. The results show that analysing char and ash may provide valuable information on the (pyro)technology practised in ancient societies.

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.