Sampling strategies and extraction techniques

In document Agriculture in Tongan Prehistory: An Archaeobotanical Perspective (Page 62-66)

Research on sampling and extraction techniques has continually forced revision of the methods used to extract starch. These differ technically when researchers are dealing with residues on artefacts or in sediments, but issues of scale influence both. By nature micro-analyses such as microbotanical studies represent high resolution palimpsests of events occurring in the past, but it can be difficult to interpret the spatial and temporal scale of these events. It is therefore essential that the choice of a sampling strategy is related to a specific research question, and the

45 connection between the microbotanical remains and the processes that created the archaeological record is made explicit by comparing samples. It is this comparison that ensures that a sample represents more than just micro-scale data. Here, discussion focuses on the sampling and extraction of sediments, as this relates to the methodological approach employed on Tongatapu in the current research.

Many microbotanical studies target multiple microfossils, and an appropriate sampling strategy incorporates taking samples from a combination of archaeological features and background environmental deposits in swamps or trenches. Samples need to be of an appropriate size, or multiple samples should be taken from the same contexts, due to problems stemming from the use of combined multiple microfossil extraction techniques (Coil et al. 2003; Korstanje 2003; Torrence 2006b). Pearsall (2000) provides a number of different guidelines for sampling strategies that target either individual excavated contexts such as house floors, or sub- sampling multiple contexts from stratigraphic profiles. Often project research questions dictate the selection of one of these over the other, but an excellent example of integrating both these techniques was published by Coil (2003) in a study targeting changes in agricultural practices and ecology as part of the Kahikinui Archaeological Project in Hawaii. The combination of archaeological and palaeoecological data from individual features, trenches in agricultural landscapes, and sediment cores enabled discussion of landscape use and change within temporal and geographic scales.

Where studies focus on site function, sampling tends to target individual features or contexts of interest within a defined boundary. In the Pacific, Horrocks (and Barber 2005; Horrocks and Bedford 2005, 2010; Horrocks, Smith, Nichol, Shane and Jackman 2008; Horrocks, Smith, Nichol and Wallace 2008; Horrocks and Wosniak 2008; Horrocks et al. 2004, 2012) has sampled a range of agricultural features such as ditches, stone walls and terraces for preserved starch, as well as the surrounding landscape. Balme and Beck (2002) sampled residues on artefacts and sediments to discuss inter-site activity areas within a rock shelter at Petzeks Cave in NSW, Australia. Similarly, sampling of sediments from prehistoric features has been integrated into archaeological projects carried out in Asia and the New World, particularly in locations where macrobotanical preservation is low. This analysis has led to the identification of functional variation within and between sites (Laurence 2013; Messner, Dickau and Harbison 2008), which has been extrapolated further to discus mobility, and the nature and chronology of plant domestication (Barton 2005; Holst, Moreno and Piperno 2007; Messner, Dickau and Harbison 2008; Perry et al. 2007). These examples demonstrate how well-dated and carefully sampled individual contexts can provide information upon site function.

Stratigraphic column sampling, often within test pits or excavation trenches, provides a means of establishing plant use over time. The technique is borrowed from palaeoenvironmental

46 research (Pearsall 2000), and involves evenly spaced or continuous samples taken from the exposed section of excavation units (Coil 2003; Lentfer and Therin 2006; Therin et al.1999; Torrence 2006b). Sampling generally proceeds from the base of the freshly scraped section and works towards the top, with samples carefully placed into labelled zip lock bags using cleaned sampling tools to avoid cross-contamination (Lentfer and Therin 2006:153). Others prefer to sample in the laboratory environment and use box monoliths to collect sediments in situ (Reitz and Shackley 2012). Sampling using paleoenvironmental coring techniques is also common where information is sought upon background environmental and landscape change (Coil 2003; Horrocks et al. 2011; Therin et al.1999) and is validated by studies of modern environmental variation within surface samples (Lentfer et al. 2002; Lenfter and Therin 2006).

Sample sizes often vary as it is difficult to gauge the density of starch preserved within sediments unless a pilot study has been conducted (Torrence 2006b). Most researchers recommend that more sediment is taken than the standard 1-5g that will usually be processed in each batch, with standard sample sizes of up to 100g taken from each context or stratigraphic level (Fullagar et al. 1998:51-2). This enables replication of extraction and identification processes to confirm results, as well as multiple samples to be taken for the extraction of other organic material aside from starch such as phytoliths and pollen.

Once these samples have been collected in the field, they are sub-sampled for starch extraction in the laboratory using one of a number of methodologies. Steps generally involve sample preparation, disaggregation and deflocculation to break up sediments, the removal of unwanted particles, slide mounting and viewing; however, there are a number of ways in which each of these steps can be carried out (Coil et al. 2003; Torrence 2006b). Experimental studies have demonstrated that many chemicals used during standard paleoenvironmental laboratory protocols can damage starch, and therefore alternatives must be sought (Crowther 2009; Torrence 2006b). Crowther (2009) conducted a range of experiments to test a number of chemicals and processing techniques that had been used previously to extract starch from sediments and charred residues. These experiments showed that nitric acid, hydrogen peroxide and heavy liquid in the form of sodium polytungstate (Na6 (H2W12O40)(SPT) had some

damaging effects on starch granules. Instead, Crowther (2009:82-83) recommended a protocol using simple chemical disaggregation using weak sodium hydroxide in conjunction with mild agitation and limited sonication within an ultrasonic bath.

Similarly, Torrence and Therin (in Torrence 2006b) tested the effects of Calgon, caesium chloride (CsCl), and sodium polytungstate— two chemicals used for heavy liquid

flotation on native starch granules. Samples were monitored at regular intervals up to one week by sub-sampling. The results of these experiments contrasted with Crowther’s (2009) in that sodium hydroxide did not have any corrosive effect on granules, nor did this chemical affect

47 starch quantities when sub-samples were compared over time (Torrence and Therin, in Torrence 2006b:156-7). Caesium chloride was found to have a deleterious effect, but this peaked after five hours of exposure and starch quantities subsequently stabilised. Overall, the 5% dilution of Calgon was found to be the most damaging chemical, with only 59% of starch remaining after five hours of exposure. Torrence and Therin (in Torrence 2006b:157) concluded that other means of deflocculation be explored in the future. The difference between these results for sodium polytungstate may be explained by another study on taphonomy in the laboratory by Korstanje (2003), where Zinc Iodide (ZnI2) was assessed as another potential means of heavy

liquid separation. In this experiment, the use of the chemical was constant, while other variables such as humidity and temperature varied. Korstanje (2003:116) concluded that Zinc Iodide could only be considered destructive to starch when combined with humidity and heat during slide preparation and scanning.

Heat is a variable that has been debated by starch analysts with regard to starch extraction protocols. As discussed earlier, starch gelatinises or melts when exposed to heat and moisture, and so these environments are mostly avoided in starch extraction protocols. However, some protocols suggest that sediments are dried prior to processing to ensure that weights of sediment remain consistent within samples, and also to reduce excessive moisture that can affect the concentration of chemicals (Lentfer and Therin 2006:160). It is argued that heat should be kept at a maximum of 40ºC, when a longer period of exposure is required. This temperature limit also applies to slide drying, where it is recommended that slides are either dried slowly at very low temperatures on a hotplate (Lentfer, pers comm. 2008), or covered at room temperature (Coil et al. 2003; Kortanje 2003).

Variation within processing protocols also reflects the type and contents of the sediments that are being processed. Some chemicals such as hydrogen peroxide are useful oxidising chemicals to disaggregate sediments high in clay or organic content, or charred residues (Crowther 2009; Lentfer and Therin 2006). Calgon is also often used as a deflocculant to separate clay particles. Where these are not required, many protocols simply use heavy liquid flotation and sieving to separate starch from sediments (Atchison and Fullagar 1998; Barton et al. 1998; Fullagar et al. 1998; Messner 2011; Therin et al. 1999; Therin 1994). Many researchers are also quick to point out that techniques are often only in early stages of development and so protocols are only broad outlines of principles rather than definitive methodologies (Fullagar et al. 1998; Korstanje 2003; Therin et al. 1999). Protocols for starch extraction must constantly be updated in light of new data about starch modification that is produced both in the food sciences and archaeobotany. Microwave extraction is a new technique that is currently being tested, especially where fast results are required (Parr 2002, 2006), and involves the pressurization and digestion of organic material within the microwave before sieving to capture microfossils.


Chapter 4

Reviewing Parenchyma

The analysis of archaeological parenchyma (or charred vegetative storage tissues) has been under-researched within archaeobotany due to a prevailing view that these remains do not preserve in tropical conditions. Where these remains are preserved in archaeological contexts, they can be extracted through a combination of excavation and flotation practices. When analysed, these data can then provide information upon the diet and subsistence practices of prehistoric cultures that utilise root and tuber crops, such as those in the Pacific. Additionally, the presence of charred parenchyma can be used to infer inter- and intra-site function involving food processing and cooking areas, and also the timing and geographic range of crop dispersal and population migration. Hather (2000) in his seminal guide suggests compiling a comparative collection of relevant domestic and wild species, in the form of both histological thin sections and experimentally charred samples that should be analysed using Scanning Electron Microscopy (SEM).

In document Agriculture in Tongan Prehistory: An Archaeobotanical Perspective (Page 62-66)