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An investigation of wheat husk phytoliths from a midden deposit at Neolithic Çatalhöyük provided the opportunity to investigate the impact of taphonomic ...
Archaeometry 53, 3 (2011) 631–641

doi: 10.1111/j.1475-4754.2010.00582.x

TAPHONOMIC OBSERVATIONS OF ARCHAEOLOGICAL WHEAT PHYTOLITHS FROM NEOLITHIC ÇATALHÖYÜK, TURKEY, AND THE USE OF CONJOINED PHYTOLITH SIZE AS AN INDICATOR OF WATER AVAILABILITY* L.-M. SHILLITO† BioArCh, Department of Archaeology, University of York, King’s Manor, York YO1 7EP, UK

An investigation of wheat husk phytoliths from a midden deposit at Neolithic Çatalhöyük provided the opportunity to investigate the impact of taphonomic processes on conjoined phytolith size. Wheat husk phytoliths from a possible crop processing deposit were examined using three methods. The results indicate that the size of conjoined forms decreases significantly as a result of laboratory extraction and slide preparation. Observations of the phytolith layer in thin section show some of the depositional and post-depositional processes affecting phytolith size. These results indicate that these taphonomic factors need further investigation before conjoined phytolith size can be used to infer past water availability and agricultural practices. KEYWORDS: PHYTOLITH, WHEAT, CEREALS, IRRIGATION, TAPHONOMY, MICROMORPHOLOGY, TURKEY, ÇATALHÖYÜK

INTRODUCTION

The term ‘phytolith’ refers to any mineral deposited within a plant (Rovner 1971), although in most archaeological studies the term refers to opal or amorphous silica phytoliths (SiO2.nH2O). Silica is present in the soil in an equilibrium between the solid form, such as quartz and feldspar, and the soluble form, monosilicic acid Si(OH)4. Monosilicic acid in ground water is absorbed by plants through their roots, and as the water passes through plants cells the silica is deposited. The site of deposition depends on the species and can occur between cells, within the cell walls or within the cells themselves (Pearsall 1982). The process is controlled both physiologically within designated cells—for example, short cells (Madella et al. 2009)—and environmentally by temperature, soil type and evapotranspiration (Piperno 2006). The deposition of silica is particularly prolific in monocotyledonous plants, and leads to a three-dimensional (3D) replica of the plant cells which remain preserved whilst the organic plant matter decays. Phytoliths can be present either as single- or multicellular forms, the latter sometimes being referred to as silica skeletons or conjoined phytoliths, which are impressions of whole sections of plant tissue. Due to their mineral nature, phytoliths are not susceptible to breakdown by microbes, and do not have to be burnt or waterlogged to be preserved; thus they can be used archaeologically to identify plant remains where charred macrobotanical remains and pollen are scarce or absent. Archaeological studies of phytoliths are diverse, ranging from the identification of materials used in matting and basketry at the Neolithic site of Çatalhöyük (Rosen 2005) and the identifi*Received 4 July 2010; accepted 19 October 2010 †Corresponding author: email [email protected] © University of Oxford, 2011

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cation of crop processing (Harvey and Fuller 2005) to their use as palaeoenvironmental indicators (e.g., Neumann et al. 2009)—although in the latter case it is becoming increasingly apparent that multiple lines of evidence are needed due to the complex taphonomy of phytolith assemblages. An area in which phytoliths show potential is the identification of ancient farming practices such as irrigation. Currently, this relies on indirect evidence through assessing charred weed seed assemblages rather than direct evidence from the cereals themselves (Jones et al. 2005, 2010). However, this can be problematic due to the difficulty in distinguishing between seeds that enter the assemblage from cereal harvesting and those that occur from the burning of animal dung (Miller and Smart 1984; Valamoti 2007). There are also examples of macrobotanical assemblages where wild/weed seeds are largely absent; for example, the Neolithic sites of Makriyalos and Apsalos in Greece (Valamoti 2007). The mode of formation of phytoliths means that they are particularly sensitive to changes in water availability and soil substrate. An experimental study in Israel suggests that the size of conjoined phytoliths can be used to identify ancient irrigation in arid and semi-arid regions (Rosen and Weiner 1994). This study suggests that 10% of phytoliths with 10 or more conjoined cells, or any sample with phytoliths of between 100 and 300 cells is indicative of irrigation. Since this initial study, further experimental work has been carried out on a variety of cereal types in Jordan (Mithen et al. 2008) to investigate the relative impact of water availability and other factors on phytolith formation, which indicates there can be in excess of 600 conjoined cells in irrigated cereal husks. Unlike other plant microfossils, such as pollen, the taphonomy of phytolith deposition is still poorly understood. Despite this problem, conjoined phytolith size has been used in a number of studies to support hypotheses of dryland farming (Ishida et al. 2003; Katz et al. 2007; Roberts and Rosen 2009). A recent study by Jenkins (2009) has highlighted the impact of processing methods on the size of conjoined phytoliths in modern durum wheat samples. Jenkins demonstrated that methods such as acid extraction may completely disaggregate the original cell structure. The methods used to process phytolith samples could, therefore, have significant implications for archaeological samples that have undergone a range of depositional and postdepositional processes in addition to laboratory processing. This paper investigates differences in conjoined phytolith size observed in archaeological wheat phytoliths from the large Neolithic settlement of Çatalhöyük, Turkey. As part of a larger study examining midden formation processes at Çatalhöyük (Shillito et al. 2008), a 2 mm thick layer of wheat phytoliths was observed in a midden, Space 261, in the South Area of the site (Fig. 1). Phytoliths are an abundant component of middens at Çatalhöyük, in diverse contexts such as ash deposits, inclusions in coprolites and animal dung, and from plant remains that have decayed in situ (Shillito et al. 2008), and provide the opportunity to test the reliability of archaeological cereal phytoliths as indicators of farming practice and water availability. Comparisons are made between macroscale observations of the deposit in the field, microscopic observations of the deposit in situ in thin section, and between slides prepared without laboratory extraction (‘smear’ slides) and using an extraction technique commonly used in Near Eastern phytolith studies (Rosen 2005). It is seen that there is a significant difference between the appearance of the phytoliths in situ in thin section and after undergoing laboratory processing, with a significant reduction in phytolith size. In thin section, post-depositional processes are observed that further impact on phytolith size measurements. The implications of this for understanding phytolith taphonomy, and the contribution of phytolith studies to understanding past agricultural practices, is discussed. © University of Oxford, 2011, Archaeometry 53, 3 (2011) 631–641

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Figure 1 A macroscale view of the layer of wheat phytoliths in the Space 261 midden (unit 12519).

METHODS

Sample collection and preparation The wheat phytolith layer was sampled by cutting a micromorphology block directly from the face of the midden section, and securely wrapping with tissue and tape to avoid disturbance. The layer was examined by three different methods. First, a ‘smear’ slide was prepared by scraping a subsample of approximately 1 g from the layer and mounting it directly on to a glass slide. Second, an extraction procedure was used on another subsample of approximately 1 g, using methods described in previous archaeological studies of Near Eastern phytoliths (e.g., Rosen 2005), involving: removal of carbonates with 10% HCl; removal of the clay fraction by settling; and removal of organic matter by heating in a muffle furnace at 500°C for 3 h. Non-phytolith mineral material was removed by centrifuging with sodium polytungstate heavy density solution, calibrated to a specific gravity of 2.3. Separated phytoliths were then rinsed with water and oven dried. Finally, a 150 ¥ 90 mm thin-section slide was prepared by cutting a block from the exposed section face, which was oven dried and impregnated with resin under vacuum, and carefully sliced and ground to a standard geological thickness of 30 mm. Sample observations and counting It has been demonstrated that counting 194 single phytoliths with consistent morphology gives a 23% error margin, which is reduced to 12% for 265 phytoliths (Albert and Weiner 2001). Phytoliths on extracted and smear slides were examined across the entire slide, using a Leica DMLP polarizing microscope at ¥200 and ¥400, with counts being made of all the wheat husks present on the slide. The number of conjoined cells in each husk was counted, including dendritic long cells, short cells and papillae (counts are shown in Table 1). For the smear slide, it should be noted that it is possible that some husks were masked by non-phytolith material. For the thin section, the entire phytolith layer was observed, covering an area of approximately 90 mm ¥ 2 mm. Due to the nature of deposition and the direction of the cut, counts for a number © University of Oxford, 2011, Archaeometry 53, 3 (2011) 631–641

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L.-M. Shillito Table 1 Size ranges and counts

Method

Extraction Smear Thin section

Number of single cells counted in all conjoined husks

Number of conjoined husks present

Size range

Average number cells per husk

Standard deviation

995 1928 3003

29 26 15

6–120 4–520 41–615

34 74 200

28.4 111.8 171.0

of husks were made from the side view, rather than the top view that is seen in extracted and smear slides (Fig. 2). Some top views can be seen in thin section in areas that have been disturbed (Fig. 2 (F)). Thus, counts of conjoined cells in thin section represent a minimum number of conjoined cells, whereas smear and extracted slides represent the total number of conjoined cells observed. Care was taken when focusing on the slides and thin section to distinguish between individual husks and overlapping husks. In cases where this was difficult to distinguish, husks were counted as separate entities. A one-way ANOVA test was calculated on conjoined sizes for each of the three methods, to assess whether the variability in sizes between methods is statistically significant. This test accounts for the variation in total numbers of husks counted for each slide. The null hypothesis was that there is no significant difference in the range of sizes between the three groups of data. RESULTS

The number of cells in the conjoined forms is seen to vary significantly both within and between samples. The result of the ANOVA analysis (Table 2) was highly significant, F(2, 67) = 12.45, P = 0.000025. This indicates that we can reject the null hypothesis of no significant difference, and support the alternative hypothesis that the difference in conjoined phytolith size between the three processing methods is statistically significant. The total number of husks observed by each method and their corresponding sizes can be seen in Figure 3, and the percentage of wheat husks falling into a particular size category is shown in Figure 4. In the extracted sample, the average number of conjoined cells is 34, with a minimum of six and a maximum of 120. Almost half of the conjoined husks observed were between four and 20 conjoined cells, with 16% in the larger size category over 60 conjoined cells, and none above 120 cells. This is contrasted with the smear slide, where the size range is more evenly distributed, with husks between four and 80 conjoined cells contributing a similar percentage of around 20%. The largest size categories comprise 16% of the phytoliths observed, with 6% of these consisting of over 201 conjoined cells. The average number of conjoined cells is 74, with a minimum of four and a maximum of 520. Extracted and smear slide samples show a similar distribution of sizes, which tend to fall under 100 conjoined cells, with one exception from the extracted slide at 120, and four in the smear slide between 112 and 520. The thin-section sample shows a different pattern, with only two husks having significantly fewer than 100 conjoined cells, and approximately half having more than 150 conjoined cells, with an average of 200 and a maximum of 615 conjoined cells. Those husks with fewer than 100 © University of Oxford, 2011, Archaeometry 53, 3 (2011) 631–641

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Figure 2 Microscopic views of wheat husk phytoliths as observed by three different methods: (A) papillae detached from the husk; (B) a dendritic phytolith, partially detached; (C) the broken edge of the wheat husk, where single cells have become detached; (D) two large husk sections in a smear slide; (E) husk in a smear slide; (F) husks in thin section—the arrow shows an area where the husk is in the process of becoming disaggregated from the rest of the plant tissue through post-depositional disturbance.

cells in thin section appear to be broken away from the main conjoined phytolith, as shown in Figure 2. As mentioned earlier, it should be noted that the thin-section counts represent a minimum, as this deposit comprised several overlapping layers of cells and it was impossible to distinguish the underlying layers clearly. © University of Oxford, 2011, Archaeometry 53, 3 (2011) 631–641

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L.-M. Shillito Table 2 Results of ANOVA

Source of variation Between groups Within groups Total

Sum of squares

df

Mean square

F

P-value

Fcrit

276629.7799 744479.9915

2 67

138314.89 11111.64166

12.44775

2.53017E–05

3.133762

1021109.771

69

Figure 3 A comparison of the variation in husk sizes observed by each of the three methods.

Figure 4 The percentage of phytoliths in each size category for each of the three methods.

DISCUSSION

Considering current experimental data on irrigated phytoliths (Rosen and Weiner 1994; Mithen et al. 2008), the archaeological deposit presented here can be interpreted as wheat grown under conditions of high water availability. This matches early field observations at the site of © University of Oxford, 2011, Archaeometry 53, 3 (2011) 631–641

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‘abundant’ large wheat and barley husks that support the hypothesis that cereals were grown on alluvium in local marshlands (Rosen 1998). However, a more recent study has contradicted this, with observations of smaller phytoliths being used to support growing under dryland conditions (Roberts and Rosen 2009). The data presented here, and comparisons with previous studies on experimentally grown wheat (Mithen et al. 2008; Jenkins 2009), suggest that this conflicting interpretation could be a result of taphonomic processes impacting conjoined phytolith size, and this needs to be considered more fully before interpretations can be made about past farming practices and environmental conditions. Depositional and post-depositional processes impacting conjoined phytolith size Observations of phytoliths in thin section have been frequently used to understand formation processes in deposits such as middens, buildings and caves (Matthews 2005; Karkanas 2006; Shillito et al. 2008; Mallol et al. 2010). Petrographic techniques can provide useful information on palaeobotanical remains such as microcharcoal, which can be missed by standard techniques (Goldberg et al. 1994) and it is becoming increasingly recognized that thin-section micromorphology can contribute to the understanding of other microscopic remains and their taphonomy (Matthews 2010). By observing deposits in thin section, disturbance through sampling and processing is avoided. It also provides an opportunity to observe depositional and postdepositional effects on phytoliths. In thin section, a wide range of depositional contexts are observed, each relating to different activities. Each of these activities provides a different depositional pathway through which phytoliths can enter the archaeological record. In the wheat sample studied here, the parallel orientation and lack of mixing indicates that these phytoliths are a result of the in situ decay of plant material, perhaps from an activity such as crop processing. If these had entered the midden in a different way—for example, as redeposited material or in animal dung—the orientation would be random, and the phytoliths would be embedded in a matrix, as is seen in other deposits with cereal and grass husks (Matthews 2005, 2010). Thin-section micromorphology indicates that the first stage of phytolith disaggregation occurs during plant use and deposition. Conjoined phytoliths are a silica impression of organic plant tissue, and breakage of the organic matrix before decay affects what will be seen in the archaeological record. Decayed plant remains consist of large sections of plant tissue, as observed in the wheat sample presented here, compared to charred plant remains, for example in ash deposits, where burning has removed much of the organic supporting matrix (Shillito et al. 2008). Comparison with experimental samples by Jenkins (2009) suggests that what we see archaeologically can be very different from what was originally present. This is followed by post-depositional processes including the decay of plant organic material, sediment compaction, salt crystal formation, disturbance by modern root penetration and faunal burrowing. As the organic remains decay, the supporting matrix for the phytoliths disappears, leaving the fragile silica impression. This is susceptible to further breakage in middens, where further dumping of material and occasional trampling results in compaction of the underlying material. This process has also been observed on floors in buildings at Çatalhöyük, where trampling and sweeping frequently mix and disaggregate plaster fragments and plant remains (Matthews 2010). The crystallization of gypsum is a well-recognized phenomenon in Near Eastern sites, where the evaporation of ground water causes precipitation of gypsum crystals, which mechanically break up sediments and plant remains as the crystals grow. © University of Oxford, 2011, Archaeometry 53, 3 (2011) 631–641

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As well as post-depositional impacts, the size ranges vary significantly with different processing methods. In thin section, representing an undisturbed view of the phytoliths, the husks are very large, in excess of 600 conjoined cells. In the smear slide, the average size is significantly reduced as a result of mechanical breakage of the cells, and in the fully processed sample, the average size is reduced even further, again from mechanical disaggregation from the mixing, sieving and centrifuging of the fragile silica skeletons. The study presented here relates to an in situ deposit of wheat husks in a midden, although there are a huge range of contexts in which phytoliths may be found. Different contexts are subject to different depositional and post-depositional processes, and so each study must be considered separately depending on where the phytoliths are coming from. It is suggested that some contexts, such as building floors, are not suitable for phytolith size analysis, due to intensive trampling and the impossibility of determining the original size of the phytoliths. The impacts of these processes are difficult to quantify without further experimental work.

Advantages and disadvantages of different methods Each of the three methods used has advantages and disadvantages, summarized in Table 3. For agricultural reconstruction and inferring water availability, the best deposits to investigate would be husked grain deposits, non-burnt, or thick lenses, such as that presented here. Relatively pure samples enable observation without extraction, minimizing the possibility of disaggregation. However, even smear slides present difficulties, because of masking with non-phytolith material and the overlapping of multiple phytolith layers, which makes resolving and identifying diagnostic parts difficult. Thin-section observation allows the nature of depositional and postdepositional processes to be observed, and also the original articulation and size of the phytoliths. However, the two-dimensional (2D) nature of the view and overlapping of multiple layers can make it difficult to identify the phytolith types. This problem could potentially be resolved by the application of other methods, such as confocal microscopy and micro-computed tomography (CT) to undisturbed sediments. These techniques enable 3D visualization of deposits, but so far have been little applied in phytolith studies. A recent study of phytolith transport in soil has demonstrated the potential of confocal microscopy in conjunction with image analysis software (Fishkis et al. 2010), whilst micro-CT has been successfully used for the non-destructive imaging of tree rings (Okochi et al. 2007), hominin fossils (Wu and Shepartz 2009) and grains and void spaces within geological specimens (Ketcham 2005). The method of observation therefore depends on the specific research questions. An integration of methods is needed to fully understand the assemblage, with micromorphology providing a means of assessing deposits in situ, and extraction providing the means to quantify assemblages.

Implications for the use of phytoliths as indicators of water availability and irrigation Although experimental studies demonstrate a clear positive relationship between water availability and conjoined phytolith size, this observation cannot be directly applied to archaeological samples that have undergone removal from their original context. In their original study, Rosen and Weiner (1994) noted the possibility of taphonomic processes impacting conjoined phytolith size, but this has not been investigated or taken into consideration in subsequent studies; for example, the suggestion of dryland farming of cereals at Neolithic Çatalhöyük, Turkey (Roberts and Rosen 2009) and Chalcolithic Negev, Israel (Katz et al. 2007). As noted by Jenkins (2009), © University of Oxford, 2011, Archaeometry 53, 3 (2011) 631–641

Advantages

• Allows estimation of wt% of phytoliths in a deposit • Allows estimation of relative percentage contribution of different types to the overall assemblage • 3D views are possible, making identification easier

• Very quick • Allows observation of associated components; e.g., faecal spherulites, ash crystals • Can be used in the field

• View phytoliths in situ • Allows the specific depositional context to be observed • Helps understand taphonomic processes • Preserves the original size of conjoined phytoliths

Method

Phytolith extraction

Smear slide

Thin section

• Time-consuming • 2D view of phytoliths may make identification difficult • Phytoliths may be masked by other material in some deposits

• Disaggregates conjoined phytoliths • Presence of non-phytolith material may make counting and identification difficult • Does not allow wt% of phytoliths to be calculated

• Removes associated material that can help with assemblage interpretation • Disaggregates conjoined phytoliths • Time-consuming, especially for large sample sets

Disadvantages • General questions relating to plant use where conjoined size is not a concern • General questions relating to plant use where distinguishing between micro-contexts is not important • General questions relating to plant use where context can be securely identified at the macroscale • Deposits with a clear context at the macroscale • Deposits consisting largely of phytoliths where conjoined size is not a major concern • Deposits where knowing relative quantities is not important • Complex deposits where depositional pathways are unclear at the macroscale • Questions where conjoined size of phytoliths is important • Questions where identifying the specific context is important

Appropriate application

Finely stratified deposits Latrines and cesspits

Basketry and matting impressions Lenses of phytoliths from in situ decay of plant material

Ash layers Coprolites Lake varves

Examples

Table 3 A summary of the three different methods for observing phytoliths investigated in this study, with suggestions of appropriate applications for each method

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such considerations are vitally important in studies that use conjoined phytolith size to investigate ancient irrigation practices.

CONCLUSIONS

Observations of an archaeological phytolith deposit using three different methods have demonstrated the possible impact of each method on the size of conjoined wheat husk phytoliths. Previous observations of experimental samples have noted similar impacts when comparing dry ashing and acid extraction methods (Jenkins 2009). Each of the three methods investigated here has advantages and disadvantages, and the method used should be chosen based on the questions that are being investigated through phytolith analysis. It is clear that further work needs to be done to quantify the effects of different taphonomic processes before conjoined phytolith size can be reliably used as an environmental proxy. Indeed, the discipline would benefit greatly from further studies on depositional pathways in archaeological contexts. Observations by thin-section micromorphology provide the best indication of the original conjoined phytolith size, and have the added benefit of enabling us to assess depositional and post-depositional impacts on phytolith size, but are perhaps not practical for routine studies. When investigating ancient agricultural practices such as irrigation, it is therefore necessary to use a method that minimizes disaggregation of phytoliths. Deposits should consist of material that has had minimal influence by pre-depositional disaggregation; for example, through burning in ash deposits, or trampling on floors. Further experimental work is necessary to understand the impacts that these processes have on conjoined phytolith size. The observations presented here of wheat husk phytoliths from Çatalhöyük middens demonstrate some of the processes impacting the size of phytoliths in this type of context. In situ observations are essential to understanding the phytolith assemblage when inferring irrigation. It is also suggested that researchers using phytolith analysis need to be more explicit when discussing the taphonomy and limitations of their data, to enable non-specialists to be able to assess and build on conclusions based on this material.

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