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Archaeobotany

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INDEX
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Introductory notes
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Main approaches in archaeobotany
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Methodological aspects
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Preservation of fossil plant remains
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Recovery of plant remains
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Dating of samples
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Stable isotopes in archaeobotanical remains
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Main steps in the estimation of cereal yield in prehistoric agriculture
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Main steps in the estimation of annual precipitation from wood charcoal
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References
 
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Introductory notes

In the process of excavation of archaeological sites one comes across plant matter of one kind or another. Most archaeological plant remains are found in a charred state and are often incomplete for standard systemathic studies. In the past, these botanical remains received very scant attention. Although some early studies date from the end of the XIXth century, the interest on plant remains did not began to generalise until the works of Helbaek (Helbaek and Schultze 1981) in the Near East. Since then, and due to its particular relevance for the origin of Old World agriculture, the Near East is still the region more extensively studied from archaeobotany (see e.g. (Bar-Yosef and Kislev 1986; Hillman 1984; Renfrew 1973; Van Zeist and Casparie 1984; Willcox 1995). In the Iberian Peninsula, the earliest archaeobotanical works are those from (Siret and Siret 1890), but the great impulse to the discipline came from (Téllez and Ciferri 1954) and, in the 60’s and 70’s, from M. Hopf (Zohary and Hopf 1988). In the 80’s, the extensive work of (Buxó 1997) helped to consolidate archaeobotanical studies in the area, that are now well developed around the Iberian Peninsula (Allué 2002; Alonso 1999; Peña-Chocarro 1992; Ros 1997).

 

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Main approaches in archaeobotany

At the beginning, the interest of archaeobotany was centred on the origin of cultivated plants, from a genetic point of view, as well as on the processes associated to plant domestication and agriculture. Later, as the number of data available increased and new techniques became available, the initial interest on taxonomy per se derived into the study of the complex interactions between human communities and plants. This change in direction was evidenced through the foundation of the International Work Group of Palaeoethnobotany (IWGP) in 1968, which has served as a productive discussion forum for European archaeobotanists. Nowadays, the interest of archaeobotanists includes both the inference of plant exploitation strategies in the past and the reconstruction of palaeovegetation. For the latter, it is of special relevance a subdiscipline of archaeobotany, the anthracology or charcoal analysis, which is devoted to the study of wood remains. This approach has been well developed in Europe through the work of Vernet and his numerous co-workers and disciples (Bazile et al. 1977; Ros 1997; Vernet and Thiebault 1987). Nevertheless, the use of charcoal analysis to reconstruct vegetation is rather controversial, as it attempts to derive vegetation from material that has been collected by human groups, thus assuming that this would reflect plant communities. However, charcoal remains from archaeological contexts reflect the interaction between vegetation and their use by human groups. Some charcoal studies are indeed focused on the socio-economic (e.g. fuel exploitation, collection strategies) implications of this kind of remains (Piqué 1998; Willcox 1999). Thus, plant macroremains might provide also useful information from a palaeoenvironmental point of view, but the interpretation of results require a careful consideration of other factors determining the composition of plant assemblages.

 

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Methodological aspects

Preservation of fossil plant remains

In temperate climates, the most common way of preservation of plant remains is carbonisation. This is the case of wood charcoal, either derived from occasional fires or from the use of wood as fuel. Seeds and grains can also be burnt during different phases of food preparation (e.g. toasting), or included within animal dung used as fuel (Buxó 1997; Van Zeist and Casparie 1984). When plant materials are combusted under poor oxygen availability, they become carbonised, which prevents decomposition of plant remains by fungal or bacterial decay. However, as carbonisation increases, plant remains become more brittle, and thus the final preservation requires the ability to survive subsequent physical constraints during burial and recovery (Colledge 2001; Wright 2003). Plant remains can be also preserved in the form of mineralised tissues. In some cases, plant material may be preserved in an uncharred, dry, or saturated state. In very arid and desert climates, like ancient Egypt or coastal Peru, dried plant parts may last for millennia. Even in temperate climates, plants may be well preserved in dry caves and rockshelters. Saturated sites (bogs, swamps, lakes, etc) may also preserve fragile plant materials indefinitely, and this has been profited, for example, to build long tree-ring chronologies in Northern and Central Europe (McCarroll and Loader 2004).

 

Recovery of plant remains

 

Fig. VII Examples of the main steps in the recovery of archaeobotanical remains. On the left, (top) partial view of the exacavated strata in Tell Halula, and (bottom) detail of charcoal remains included in the sediment. On the right, (top) extraction from the sediment of charcoal and charred seeds by flotation, and (bottom) archaeological grains of wheat and barley. Original pictures from J.L. Araus and R. Buxó.

Until relatively recently, most plant remains recovered from archaeological context were detected by chance. If a large quantity of charred seeds, or a big piece of wood, caught the eye of the archaeologist, this material would be gathered up and saved, otherwise not. However, small charred seeds and plant parts are not easily detected by the naked eye and, as a consequence, specialized means are necessary to obtain such remains. The most common method by which these smaller macrobotanical plant fragments are concentrated and recovered is through the technique known as flotation (see Fig. VII). Many different flotation systems have been devised, but all of them depend upon the same basic principle: if archaeological sediment is released into a container filled with water, then the sediment sinks and the charred plant remains float. There are several techniques of flotation (Buxó 1997): some techniques are multiple-operator, machine-assisted assembly lines, whereas others are simple, one-person bucket or barrel operations. It is sufficient to note here, however, that all of these systems are aimed at retrieving charred macrobotanical plant remains, and that the main purpose is the same for all: to concentrate and collect charcoal that is dispersed throughout archaeological deposits.

Although the use of flotation has found worldwide acceptance, not all archaeologists employ it as a technique to retrieve plant remains. In some areas, archaeological sediments may not be amenable to flotation. Even in cases where flotation is not physically possible, however, attempts are often made to recover plant remains. Dry or water sieving of sediments, for example, is routinely used in some European countries to obtain plant remains. On the other hand, the recovery and study of archaeobotanical remains is time-consuming and expensive and thus many archaeological projects simply do not have the necessary time and money. Fortunately, the interest on archaeobotanical studies is going beyond traditional archaeology, thus making accessible alternative sources for funding.

Dating of samples

The dating of archaeobotanical remains is not a single issue. The radiocarbon dating method, based on the rate of decay of the unstable isotope 14C, is today the most widely applied dating technique for the late Pleistocene and Holocene periods (Taylor and Aitken 1997). 14C is formed in the upper atmosphere through the effect of cosmic ray neutrons upon 14N. It is rapidly oxidised and transformed into 14CO2, and subsequently fixed by plants. As soon as the plant dies, it ceases 14CO2 uptake, and 14C content in plant tissue decays progressively. In 1949, Libby, Anderson and Arnold were the first to measure the rate of this decay. They found that after 5568 years, half the 14C in the original sample will have decayed and after another 5568 years, half of that remaining material will have decayed, and so on. The half-life is the name given to this value, which Libby measured at 5568±30 years. Currently, this value is considered to be 5730±40 years (“Cambridge” half-life). The initial solid carbon method developed by Libby and his collaborators was replaced with the Gas counting method in the 1950's. Liquid scintillation counting (LSC), utilising benzene, acetylene, ethanol, methanol etc, was developed at about the same time. Today the vast majority of radiocarbon laboratories use these two methods of radiocarbon dating. Their main handicap is the amount of sample required: even using small sample capabilities, about 100 mg are required for moderate precision datings. Of major recent interest is the development of the Accelerator Mass Spectrometry method of direct C14 isotope counting, which allows milligram sized samples to be dated. Radiocarbon dating should be calibrated externally to account for past changes in atmospheric 14C composition, and the final accuracy varies from several hundred years to a few decades, depending on the period considered (Jull 2005; Stuiver et al. 1998). On the other hand, the elevated cost of these analyses strongly limits the amount of samples to be dated. Consequently, most plant macroremains are assigned an age according to the dating obtained from other charcoal, seeds or bones from the same (or the nearest) stratigraphic unit. In some cases, a combination of stratigraphic and archaeological methods (e.g. based on ceramics and other artefacts) can provide more accurate dating than radiocarbon analyses. Dating of a given stratigraphic unit from wood charcoal has an additional uncertainty, as a given wood fragment might be older than the sediment in which is included. Thus, seeds and other short-living plant remains are often preferred for dating.

 

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Stable isotopes in archaeobotanical remains

Stable isotopes in plant tissues reflect the environmental conditions in which they were developed. Charred archaeobotanical remains are indeed a subproduct of plant tissues, derived form their partial combustion, and thus may preserve some kind of environmental signal in their isotopic composition. De Niro and coworkers (DeNiro and Hastorf 1985; Marino and DeNiro 1987) were the first to propose the analyses of d13C and d15N in archaeobotanical remains, with the aim to provide reference values for dietary studies. They performed experimental carbonisations of several types of seeds, and assayed different treatments to remove soil contaminations, such as carbonates or humic acids, finding little change in d13C due to carbonisation and chemical treatments, being greater for d15N. Such an extensive work, along with the increasing physiological knowledge about d13C variability in grain crops (Condon et al. 1987; Farquhar et al. 1982; Farquhar 1984), provided the necessary background for the first attempts to apply d13C in archaeological grains to infer the water status of ancient crops (Araus et al. 1997a; Araus et al. 1999a; Araus and Buxó 1993; Ferrio et al. 2005a; Ferrio et al. 2006b). In these works, Araus and his colleagues developed quantitative models to estimate past water inputs in winter cereals (wheat and barley) through the analysis of d13C in grains. Following a similar approach, and based on the close relationship between d13C (as indicator of water status) and cereal yield (Araus et al. 1998; Condon et al. 1987; Voltas et al. 1998), they used d13C of archaeological grains to infer ancient crop yields (Araus et al. 1999b; Araus et al. 2001; Araus et al. 2003).

From the point of view of climatic reconstruction, the analysis of crop remains has the disadvantage they may reflect either climatic or agronomic changes affecting crop water status (Araus et al. 1997b). However, provided an independent estimation of climate for the archaeological sites studied, this “handicap” might become an useful tool to reconstruct ancient agronomic practices. In this context, the use of charcoal remains of forest species appears to be the best alternative. The only studies available on the d13C in ancient wood charcoal are those from (February and Van der Merwe 1992) and (Vernet et al. 1996). Mostly based on the comparison with other palaeoenvironmental evidences, both concluded that past changes in the d13C of charcoal could be related to changes in water availability, as occurred in intact wood (see tree-rings). However, they did not studied the potential effect of carbonisation on wood d13C, which, according to previous works, might be considerable (Florit 2001; Jones and Chaloner 1991; Ferrio et al., 2006a). Consequently, prior to the application of stable isotopes in wood charcoal for palaeoenvironmetal studies, it is necessary to assess the effects of carbonisation under different conditions on the original environmental signal of wood.

Main steps in the estimation of cereal yield in prehistoric agriculture

Kernels recovered by flotation were washed in HCl, rinsed with distilled water, dried and finely ground. Samples of 0.7-0.9 mg were combusted in an elemental analyser and carbon isotope composition (d13C) was determined by mass spectrometry. Carbon isotope discrimination (Δ) was then calculated to account for past changes in atmospheric d13C (dair). The relationship between D of grains and yield was established on present-day cereals growing under Mediterranean conditions (durum wheat for this figure) using published agronomic data. The equation relating D and yield was then used to infer yield from the D of fossil grains. Yield estimations were further corrected from the variation in harvest index (multiplying by a factor of 0.5) by using agronomical information on different wheat cultivars released from 1860 to 1990 and grown together in the Mediterranean conditions of Western Australia. A further correction due to the differences in atmospheric CO2 concentration between the present-day (ca. 350 ppm) and the beginning of agriculture (ca. 275 ppm) was established by multiplying the estimated yield by 0.6 (Araus et al. 1999b; Araus et al. 2001; Araus et al. 2003).

Main steps in the estimation of  annual precipitation from wood charcoal

Charcoals recovered by flotation were washed in HCl, rinsed with distilled water, dried and finely ground. Samples of 0.7-0.9 mg were combusted in an elemental analyser and carbon isotope composition (d13Craw) was determined by mass spectrometry. The effect of carbonisation on d13Craw was assessed through experimental charring of present wood samples. From this experiment, we found that d13Craw (d13C in charcoal) could be corrected to get an estimate of d13C before charring (d13Ccor), using carbon content (%C) as an indicator of the degree of carbonisation. Carbon isotope discrimination (Δ) was then calculated to account for past changes in atmospheric d13C (dair). The relationship between D of wood (Aleppo pine for this figure) and annual precipitation was established on present-day wood samples (both intact and carbonised, once corrected). The equation relating D and rainfall was then used to infer annual precipitation from the D of fossil charcoal (Ferrio et al., 2006a).