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Taphonomy and phytoliths: A user manual
Quaternary International 275 (2012) 76e83
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Taphonomy and phytoliths: A user manual Marco Madellaa, *, Carla Lancelottib a b
ICREA e Department of Archaeology and Anthropology, Institució Milà i Fontanals, Spanish National Research Council (CSIC), C/Egipciaques 15, 08001 Barcelona, Spain Department of Archaeology and Anthropology, Institució Milà i Fontanals and CCHS, Spanish National Research Council (CSIC), C/Egipciaques 15, 08001 Barcelona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 16 September 2011
Taphonomy is commonly described as “the study of the environmental conditions affecting the preservation of animal or plant remains”. This broad definition is usually understood to refer to postdepositional processes. One of the main purposes of taphonomic studies is the understanding of possible biases in the fossil record. In this respect, different forms of pre- and post-depositional taphonomy can affect phytolith assemblages that scientists retrieve from sediments and soils. These can be both natural- and human-induced. This paper analyses the forms of possible biases during- and post-depositional, as well as during sampling, recovery and study and proposes some methodological adjustments as to reduce the possibility of errors as well as to check the representativeness of the assemblages retrieved. It is shown that control is possible when human action is involved in the form of attentiveness in sampling and recovery strategies. Pragmatic and statistic methods are proposed so to understand the impact of pre- and post-depositional taphonomic processes on the final phytolithic assemblage. With an increase of stratigraphic control and simple calculation of representativeness it is possible to achieve a confident level of standardisation so to reduce biases at minimum and obtain reliable results with phytolith analysis. Ó 2011 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction The main purpose of taphonomic studies is the understanding of possible biases in the fossil record, being this understood in a biological, mineralogical/geological or archaeological perspective. Taphonomy (from the Greek sάfo2 meaning burial and nόmo2 meaning law) is commonly described as the study of the environmental conditions affecting the preservation of animal or plant remains (e.g. Efremov, 1940), and of artefacts. This broad definition is usually understood to refer to post-depositional processes. However, to comprehend taphonomy in both an anthropic and a non-anthropic setting one needs to understand what happened to the subject of study before it was deposited and how its level of significance might have changed before entering the deposit. Different forms of pre- and post-depositional taphonomy can affect phytolith assemblages that are retrieved from sediments and soils. These taphonomical aspects can be natural- and/or humaninduced. The present paper highlights and discusses the forms of possible transformations (biases) that can enter during- and postdepositional, as well as in the course of sampling, recovery and
* Corresponding author. Fax: þ34 934 430 071. E-mail address: [email protected] (M. Madella). 1040-6182/$ e see front matter Ó 2011 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2011.09.008
study of the assemblage. Furthermore, some methodological adjustments are proposed as to reduce the possibility of inaccuracy as well as to check the representativeness of the retrieved assemblages.
2. Non-anthropic systems 2.1. Significance of phytolith assemblages Phytoliths, owing to their inherent characteristics (e.g. taxonomical significance, production related to physiological and environmental conditions, resistance to decay, ubiquity, etc.), are good indicators of past vegetation cover and environmental conditions (e.g. Blinnikov et al., 2002; Delhon et al., 2003; Strömberg, 2004; Iriarte, 2006). Phytoliths are incorporated in the sediments through a set of processes (Dodd and Stanton, 1990; Osterrieth et al., 2009) that comprise: (1) necrolysis, which refers to decomposition and disaggregation of the plant at the time of death; (2) biostratinomy, that relates to all the processes that take place after the plant death but before the phytoliths burial; (3) fossil diagenesis, which encompass the cumulative effects of the physical, chemical and biological processes that may alter or destroy the phytolith fossil (buried) record (Fig. 1).
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Fig. 1. Theoretical diagram of the depositional and post-depositional processes of phytoliths in natural deposits (from Osterrieth et al., 2009).
2.2. Living plant, necrolysis and biostratinomy (pre-depositional) Phytoliths are produced in and between the cells of living plants. Two primary mechanisms, genetically and environmentally determined, control the production of these bodies (Madella et al., 2009). Certain cells are primed (genetically determined) to deposit opal silica due to their role in the plant tissues (e.g. hairs, short cell of the grass epidermis, etc.) while others are filled by opal silica because of high evapotranspiration (e.g. bulliform cells or stomata cells in the grass leaf). The absorbed aqueous silicic acid [Si(OH)4] is moved through the plant along the transpiration stream (Raven, 2003) and its polymerisation can take place through several processes (Kaufman et al., 1985; Perry, 1989). Silica bodies are made of porous opal-A (SiO2$nH2O) (Mann et al., 1983) and their formation might not be complete, especially if the plant dies before the end of its life cycle. Observation under the microscope of fresh plant tissues from grasses (Gramineae) shows that phytoliths are present in the epidermal cells at different stages of formation (i.e. different degrees of silicification). Some of these silica bodies are therefore incomplete or display patterns that, in phytoliths from sediments or soils, could be interpreted as originating from taphonomy (Fig. 2). Cabanes et al. (2011) observed these pseudo-taphonomised phytoliths in dissolution experiments using fresh assemblages. Once the plant dies, and necrolysis sets in, phytoliths are released in the environment due to the decomposition of the organic matter. At this point the silica bodies are exposed to transport that could mechanically damage their bodies (this transport can also happen at the stage of dead plant but while phytoliths are protected within the tissues the damage should be negligible). Transport can happen by wind and/or by water (Fig. 3) and the result of it can be the breakage of phytolith finest appendices and weak points (e.g. the narrower part of a bilobate), the chipping of the main body and/or abrasion. The extreme
Fig. 2. Partial silicification of the epidermal cells from the grasses Setaria pumila and Urochloa ramosa. The silicification of the phytolith bodies is incomplete, both in the bilobates (short cells) of S. pumila and in the long cells of U. ramosa, showing patterns that might be interpreted as post-depositional dissolution. Photographs M. Madella.
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Fig. 3. Simplified line drawing of phytolith transport by wind. Depending on the strength of the wind and the weight and shape of the silica body, transport can happen according to surface creep, saltation and/or suspension with different intensity of damage to the phytolith body. Similar transport patterns will be in water.
consequence of transport could be the complete modification of the silica body characteristics (especially in the case of strong abrasion) that results in an impossibility of correct identification. In general, phytoliths are not transported over long distances because they are relatively “heavy” particles (as opposed to pollen, for instance) but there are situations in which transport can be an important factor in the making of an assemblage. This is the case of dry environments with sparse vegetation and strong winds (e.g. deserts, loess plateaus) or environment with heavy precipitations and important runoff. Finally, some dissolution may occur at this stage in environments with abundant water availability such as wet tropical areas (Alexandre et al., 1997). However, dissolution normally affects phytoliths as a post-depositional process and, as such, it will be discussed in the next paragraph.
The degree of phytolith translocation related to water seeping through the soils column has been open to debate. Whilst some authors considered this a significant process (Alexandre et al., 1997, 1999; Hart and Humphreys, 1997; Hart, 2003; Humphreys et al., 2003), others see phytoliths to be almost stationary (e.g. Fisher et al., 1995). Fishkis et al. (2010) demonstrated that downward movements of phytoliths (both for bioturbation and water seeping) could be an important process in soils (4 cm/year) and that interpretation on past vegetation based on fossil phytoliths must take into consideration these movements. Also, the transport rate of
2.3. Pedogenesis and fossil diagenesis (post-depositional) Once phytoliths are incorporated in to the soil/sediment system (post-deposition) they are subjected to pedogenesis (soil formation) and fossil diagenesis (rock formation). Soil formation results from the combined effect of physical, chemical, biological, and anthropogenic processes on the soil parent material. These processes involve additions, losses, transformations (physicochemical) and translocations of material that compose the soil. Diagenesis gradually transforms sediments in a rock by chemical and/or structural modifications of the original components. Another process to which phytoliths are subjected after deposition is bioturbation. The translocation of phytolith due to bioturbation has been broadly acknowledged (Hart and Humphreys, 1997; Runge, 1999; Humphreys et al., 2003; Farmer, 2005). Extreme bioturbation can give origin to two clearly different (in chronology and composition) phytolith pools existing side by side in the same soil horizon. It is the case, for instance, of tropical and sub-tropical soils with the presence of termites, such as the case illustrate in Fig. 4 where the oxisol developed under an Araucaria forest (Machado, southern Minas Gerais State, Brazil; Calegari et al., in press) shows channels filled with darker (more organic) A and A2 horizon constituents all the way down to a depth of 200 cm.
Fig. 4. This close up photograph shows the abundance of termite channels in a humic oxisol from Minas Gerais in Brazil. Abundant parts of the overlying A horizon have been vertically translocated for more than 100 cm in the soil profile.
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phytoliths can vary importantly depending on the soil’s physical characteristics (Fig. 5; Fishkis et al., 2010). Dissolution (chemical attack) during pedogenesis and diagenesis can lead to a substantial change in the phytolith pool, depending on pH, water availability and temperature. It has been shown that in forest environments an important part of the phytolith pool is quickly dissolved and absorbed by the plants and phytoliths contribution to the flux of dissolved silica is 3 times higher than that of the mineral component (Bartoli, 1983; Alexandre et al., 1997) (Fig. 6). The importance of the silicon reservoir and the recycling of the phytolith fraction by the vegetation are becoming clear (e.g. Meunier et al., 1999; Meunier and Colin, 2001). Dissolution rates for phytoliths seems to be similar and independent from the specific surface area of the morphotype (Fraysse et al., 2009) (Fig. 7) and experiments suggest that the major release of Si happens during litter degradation (Fraysse et al., 2009). Finally, phytoliths can be removed or added to a specific assemblage due to erosion and/or re-deposition. This scenario can be rather important in certain condition such as the ones of loess/ palaeosol formation (see Osterrieth et al., 2009) where phytoliths can be transported and re-deposited over long distances. 3. Anthropic systems 3.1. Significance of phytolith assemblages The same properties that make phytoliths an interesting microfossil in environmental studies are also fundamental in anthropic systems. In this case, the taxonomical and anatomical significance are paramount for their use in archaeology (see Piperno, 2006 for a general review). Phytoliths are incorporated in archaeological sediments through a set of processes similar to those that acts on natural environments but in this case the input that defines the phytolith assemblage is the one originating from the anthropic activities (Fig. 8). These multiform anthropic activities (e. g., exploitation of plant materials, crop processing, animal stabling, architectural structures, etc.) produce phytolith assemblages that are normally much larger than those produced from the natural vegetation or the phytolith soil bank. The processes of phytoliths incorporation, however, follow the same path: (1)
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necrolysis (decomposition and disaggregation of the plant); (2) biostratinomy (all the processes that take place after the plant death but before the phytoliths’ burial); (3) fossil diagenesis (the cumulative effects of the physical, chemical and biological processes that may alter or destroy the phytolith fossil e buriede record) (Fig. 8). 3.2. Plant material, necrolysis and biostratinomy (pre-depositional) Depending on the type of activities carried out by the human group or individual, the natural input of phytoliths can be more or less important in the constitution of the fossil assemblage. Indeed, a temporary hunteregatherer campsite would have (in general) a much weaker anthropic phytolith input than that of a tell from an agrarian society. Plant material will be arriving at the human frequentation spot because of the multiform use of plant material for food, utensils, construction, fuel as well as ornaments, just to list a few. All these human activities will leave behind plant organic matter that can undergo the same processes of transport highlighted in the previous section. In anthropic settings, however, there is also the action of the same people that might intervene in moving around the plant material. For example, cleaning can spread and dislocate the plant residues discarded on a floor, the majority of which could be retrieved from a completely different context (e.g. a midden or a fireplace) from where they originated. 3.3. Pedogenesis and fossil diagenesis (post-depositional) The effects of pedogenesis and diagenesis in deposits of anthropic origin are very similar to the ones already discussed for non-anthropic sediments. However, human actions can further intervene in the movement of the original deposits, sometime with drastic effects in which sediments are recycled or displaced (e.g. the use of older anthropic sediments for the construction of new structures). Human activities can also affect preservation (pre- or postdeposition) because of, for instance, increased trampling from people and/or animals or semi-industrial activities (e.g. use of high temperature furnaces). From all this, it is clear that anthropic systems are much more complex than the natural ones and
Fig. 5. Distribution of recovered phytoliths (%) with soil depth for two European soils. Concentrations are higher in the upper layers but the percentage of recovery shows that there is a rate of transport of ca. 4 cm/year in both soils (For more discussion see Fishkis et al., 2010). Error bars indicate standard deviations (n ¼ 3) (modified from Fishkis et al., 2010).
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Fig. 6. Biogeochemical cycle of the silicon in the latosoil of Dimonika forest (Congo; modified from Alexandre et al., 1997).
understanding these pre-depositional and post-depositional effects related to human activities are a fundamental part for interpreting phytolith assemblages and the related social actions. 4. Sampling and recovery as taphonomy 4.1. Sampling natural and anthropic sediments If taphonomy is understood as the assortment of possible biases in phytolith assemblages, then sampling can be considered a taphonomical process because it introduces potentially strong biases for the interpretation of the record. Horizontal and vertical sampling should be carried out to be able to assess synchronic and diachronic variability of phytolith assemblages. In the field, it is extremely important to pay the utmost attention to the sediment structure, variability and stratigraphy. In fact, the sampled sediment must represent, at the highest possible confidence level, the deposit that it is going to be interpreted. A practical example is illustrated in Fig. 4, where the B horizon of a tropical soil is intersected by termite channels bringing down the
sediment from the overlying A horizon. Sampling must take care not to mix the two sediments; otherwise the phytolith assemblage extracted will originate from the mix of two very different depositional events. In case of archaeological sites, the finest possible episode of deposition (unless there is a specific different need) must be identified in the field and sampled. As an example, Fig. 9 shows the bottom part of a pit from the Mesolithic/Bronze Age site of Loteshwar in North Gujarat (India) where, during excavation, several depositional and post-depositional events have been identified and sampled separately to understand the construction strategy of this structure as well as the possible origin of the dumped fill. 4.2. Microsampling Microsampling is an approach that allows a strong control for the identification and study of single depositional episodes. It is carried out in the laboratory using as a guide the micromorphology thin section of the sediments under study. This type of sampling starts in the field with the collection of a micromorphology block from the deposit under study. In the laboratory the block is cut into two halves, one is sent for impregnation and the other is re-sealed and retained. When the thin section has been studied and the micro-layers have been identified, the un-impregnated half is sampled following the information gathered from the thin section. It is also possible to run parallel analysis such as infrared (FT-IR) to control pedogenesis and diagenesis processes related to the preservation of phytoliths. An example of this approach is given in Fig. 10, where a micro-sequence from the Toranagallu ash mound (Karnataka, India) is untangled to identify the finest, single depositional episodes (moments of cattle stabling) that were each sampled separately to identify seasonality of frequentation. 4.3. Recovery
Fig. 7. Comparison of dissolution rates of quartz (Dove and Eltson, 1992), vitreous silica (Wirth and Gieskes, 1979), and opal silica phytoliths from two plants: a grass (N. borbonicus Gmelin e bamboo) and a pteridophyta (E. arvense L). The pH was measured in a mixed-flow reactor at 25 C, I ¼ 0.01 M for 2 pH 12 and I ¼ 0.1 for pH < 2 (modified from Fraysse et al., 2009).
Phytolith recovery procedures follow a chain of operational steps defined by the type and condition of the soil or sediment under analysis. There are several different procedures that are commonly employed by researchers to extract phytoliths from soils (Rovner, 1972; Powers and Gilbertson, 1987; Madella et al., 1998;
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Fig. 8. Theoretical diagram of the depositional and post-depositional processes of phytoliths in anthropic deposits.
Zhao and Pearsall, 1998; Albert et al., 1999; Pearsall, 2000; Parr et al., 2001a; Parr, 2002; Coil et al., 2003; Horrocks, 2005; Piperno, 2006; Katz et al., 2010). They all rely on mechanical and chemical separation and have the purpose of concentrating and separating the silica fraction from other mineral and non-mineral components.
A few studies (Jones and Milne, 1963; Jenkins, 2009; Cabanes et al., 2011; Parr et al., 2001b) have explored the possible taphonomic effects of different types of extraction methods on phytoliths. Specifically, these studies have concentrated on wet- and dryashing techniques (i.e. elimination of the organic matter with and without chemicals) to extract phytoliths from plant samples. Parr et al. (2001b) conclude that the two techniques produce similar assemblages and that any difference in shape and/or size is due to the natural variability of phytoliths. The comparison between the effects of wet- and dry-ashing on the number of conjoined phytolith performed by Jenkins (2009) shows that both techniques introduce biases. Dry-ashing can produce the fusion of multicellular phytoliths (silica skeletons) because of the dehydration that occurs during heating at high temperatures. On the contrary, wet-ashing causes the breakdown of articulated phytoliths thus artificially generating much more single-cells phytoliths. More work is needed in order to fully understand the potential of taphonomic processes induced by extraction methods. However, biases caused by the recovery procedures can be reduced to a minimum when the extraction is carried out with a standardised method. Standard laboratory procedures and the careful control over human errors that can be attained in a laboratory environment should result in phytolith assemblages that are, if not unbiased, at least all affected by the same errors. 5. Checking phytolith assemblage representativeness
Fig. 9. Anarta period (Chalcolithic) pit from the site of Loteshwar (N Gujarat, India). The sequence of deposits was clearly identified during excavation and sampling was carried out to resolve the structure construction technique and possible use.
The representativeness of phytolith assemblages can be checked during analysis in several ways. The first to propose a method to assess the level of taphonomy action on phytolith assemblage were Fredlund and Tieszen (1997: p. 211) who evaluated phytolith preservation observing the degree of pitting in bulliforms and the degree of erosion in elongate (long) cells. This criterion offers a first and quick assessment but it is, as the authors themselves highlighted, quite subjective and dependent on the experience of the
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Fig. 10. Sampling strategy for ash mound sites of the Southern India Neolithic. The site of Toranagallu is a small ash mound formed by the cyclical deposition of dung from stabling episodes. The deposit is formed by a sequence of very fine layers (from 1 to 15 mm in thickness) that were sampled individually to unravel the history of occupation of the site. The micro-layers needed to be identified with the help of the micromorphology and sampled in the laboratory using a magnifying glass.
analyst in discriminating between fine differences in phytolith appearance. Also, some of these patterns identified as related to post-depositional processes can be present in fresh phytolith of living plants and they can be often observed on phytoliths from reference collections or in the plant tissues when observed under the microscope. Two mathematical measurements can be used to attain a more controlled evaluation of the impact of taphonomy on the entire assemblage. In order to rate the degree of preservation the ratio of short versus long cells can be applied. Long cells are more often less silicified and offer wider surface area to chemical and physical attack. Therefore they represent weaker typologies than short cells, and tend to disappear more easily. Assemblages with a high number of long cells versus short cells should emphasize a higher degree of preservation (Madella, 1997; Lancelotti, 2010). A correlation measurement between the number of morphotypes identified and the concentration of phytoliths per gram of Acid Insoluble Fraction (AIF) can also be used to assess whether taphonomy affects the richness of the phytolith assemblage (Lancelotti, 2010). This method implies that, when taphonomy influences the representativeness of a sample, the two values will display a high degree of correlation, i.e. the number of morphotypes
identified will increase in more concentrated samples (Fig. 11a) whereas less concentrated samples will produce less varied assemblages. On the contrary, where the two values are not correlated, the phytolith assemblage can be considered representative of the original phytolith input, i.e. more and less concentrated samples will show the same diversity of morphotypes (Fig. 11b).
Fig. 11a. The assemblage in this example shows a high degree of correlation between number of morphotypes identified (x-axis) and concentration of phytoliths per gram of AIF (y-axis). In this case, taphonomy highly affects the representativeness of the phytolith assemblage.
Fig. 11b. The assemblage in this example is not affected by taphonomy. There is no correlation between the number of morphotypes identified (x-axis) and the concentration of phytoliths per gram of AIF (x-axis) and the concentration of phytoliths per gram of AIF (y-axis).
6. Final remarks Understanding taphonomy is a fundamental step in any interpretation of fossil assemblages. Being able to assess the representativeness of the fossil assemblage in respect to the original input provides more robust interpretations of past processes. Phytoliths are today a fossil routinely analysed in geological and archaeological sediments to assess past vegetation and the use and processing of plant resources in the past. For this reason, it is essential to identify and comprehend the processes of formation, transport, sedimentation and biotic (including human) actions that intervene in the formation of the assemblage observed at the microscope. This paper highlighted the complex and multiform processes that can be at work to define a phytolith assemblage and that must be taken into
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consideration when interpreting the assemblage. With this in mind, it will be possible to untangle the useful evidence from the noise originated by taphonomy. Also, it will be possible to pick out the anthropic actions that are the focal interest for understanding past human activities as well as social and ecological processes. Acknowledgements The authors would like to thanks RM Albert, MA Cau and D Cabanes for the invitation to present the current work at an ICREA Conference in 2010. Both authors are part of the AGREST research group (Government of Catalunya) and SimulPast CONSOLIDERINGENIO2010 project funded by the Ministry of Science and Innovation (MICINN e Government of Spain). CL is part of the AGRIWESTMED Project. This work has been funded by the EU ERC230561 Project, the SimulPast CD2010-00034 Project and the NoGAP HAR2010-16052 Project. References Albert, R.M., Lavi, O., Estroff, L., Weiner, S., 1999. Mode of occupation of Tabun Cave, Mt Carmel, Israel during the Mousterian period: a study of the sediments and phytoliths. Journal of Archaeological Science 26, 1249e1260. Alexandre, A., Meunier, J.-D., Colin, F., Koud, J.-M., 1997. Plant impact on the biogeochemical cycle of silicon and related weathering processes. Geochimica et Cosmochimica Acta 61 (3), 677e682. Alexandre, A., Meunier, J.-D., Mariotti, A., Soubies, F., 1999. Late Holocene phytolith and carbon-isotope record from a latosol at Salitre, south-central Brazil. Quaternary Research 51 (2), 187e194. Bartoli, F., 1983. The biogeochemical cycle of silicon in two temperate forest ecosystems. Environmental Biogeochemistry Ecology Bulletin 35, 469e476. Blinnikov, M., Busacca, A., Whitlock, C., 2002. Reconstruction of the late Pleistocene grassland of the Columbia basin, Washington, USA, based on phytolith records in loess. Palaeogeography, Palaeoclimatology, Palaeoecology 177 (1e2), 77e101. Cabanes, D., Weiner, S., Shahack-Gross, R., 2011. Stability of phytoliths in the archaeological record: a dissolution study of modern and fossil phytoliths. Journal of Archaeological Science 38, 2480e2490. Calegari, M.R., Madella, M., Vidal-Torrado, P., Pessenda, L.C.R., Marques, F.A. Combining phytoliths and soil organic matterin Holocene palaeoenvironmental studies of tropical soils: The example of an oxisol in Brazil. Quaternary International, in press. Coil, J., Korstanje, A.M., Archer, S., Hastorf, C.A., 2003. Laboratory goals and considerations for multiple microfossil extraction in archaeology. Journal of Archaeological Science 30 (8), 991e1008. Delhon, C., Alexandre, A., Berger, J.-F., Thiébault, S., Brochier, J.-L., Meunier, J.-D., 2003. Phytolith assemblages as a promising tool for reconstructing Mediterranean Holocene vegetation. Quaternary Research 59 (1), 48e60. Dodd, J.R., Stanton, R.J., 1990. Paleoecology. Concepts and Applications. Wiley, New York. Dove, P.M., Eltson, S.F., 1992. The low-temperature dissolution kinetics of quartz in sodium chloride solutions: analysis of existing data and rate model for 25 C, pH 2e13. Geochimica et Cosmochimica Acta 56, 4147e4156. Efremov, I.A., 1940. Taphonomy: a new branch of paleontology. Pan-American Geology 74, 81e93. Farmer, V.C., 2005. Forest vegetation does recycle substantial amounts of silicon from and back to the soil solution with phytoliths as an intermediate phase, contrary to recent reports. European Journal of Soil Science 56 (2), 271e272. Fisher, R.F., Newell Bourn, C., Fisher, W.F., 1995. Opal phytoliths as an indicator of the floristics of prehistoric grasslands. Geoderma 68, 243e255. Fishkis, O., Ingwersen, J., Lamers, M., Denysenko, D., Streck, T., 2010. Phytolith transport in soil: a field study using fluorescent labelling. Geoderma 157, 27e36. Fraysse, F., Pokrovsky, O.S., Schotta, J., Meunier, J.-D., 2009. Surface chemistry and reactivity of plant phytoliths in aqueous solutions. Chemical Geology 258, 197e206. Fredlund, G.G., Tieszen, L.L., 1997. Phytolith and carbon isotope evidence for late Quaternary vegetation and climate change in the southern Black Hills, south Dakota. Quaternary Research 47, 206e217. Hart, D.M., 2003. The influence of soil fauna on phytolith distribution in a soil. In: Hart, D.M., Wallis, L.A. (Eds.), Phytoliths and Starch Research in the AustralianPacific-Asian Regions: The State of the Art. Terra Australis, vol. 19, pp. 83e91. Hart, D.M., Humphreys, G.S., 1997. The mobility of phytoliths in soils: pedological considerations. In: Pinilla, A., Juan-Tresserras, J., Machado, M.J. (Eds.), The Stateof-the-art of Phytoliths in Soils and Plants. Centro de Ciencias Medioambientales (CSIC), Madrid, pp. 93e100. Monograph.
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Plant and Soil 18 (2), 207e220. Katz, O., Cabanes, D., Weiner, S., Maeir, A.M., Boaretto, E., Shahack-Gross, R., 2010. Rapid phytolith extraction for analysis of phytolith concentrations and assemblages during an excavation: an application at Tell es-Safi/Gath, Israel. Journal of Archaeological Science 37 (7), 1557e1563. Kaufman, P.B., Dayanandan, P., Franklin, C.I., Takeoka, Y., 1985. Structure and function of silica bodies in the epidermis system of grass shoots. Annals of Botany 55, 487e507. Lancelotti, C., 2010. Fuelling Harappan Hearths: HumaneEnvironment Interactions as Revealed by Fuel Exploitation and Use. Unpublished PhD Dissertation, Department of Archaeology, University of Cambridge, Cambridge. Madella, M., 1997. Phytolith analysis from the Indus Valley site of Kot Diji, Sindh, Pakistan. In: Sinclair, A., Slater, E., Gowlett, J. (Eds.), Archaeological Sciences 1995. Oxbow, Oxford, pp. 294e302. Madella, M., Power-Jones, A.H., Jones, M.K., 1998. A simple method of extraction of opal phytoliths from sediments using a non-toxic heavy liquid. Journal of Archaeological Sciences 25, 801e803. Madella, M., Jones, M.K., Echlin, P., Powers-Jones, A.H., Moore, M., 2009. Plant water availability and analytical microscopy of phytoliths: implications for ancient irrigation in arid zones. Quaternary International 193, 32e40. Mann, S., Perry, C.C., Williams, R.J.P., Fyfe, C.A., Gobbi, G.C., Kennedy, G.J., 1983. The characterisation of the nature of silica in biological systems. Journal Chemical Society 1314, 168e170. Meunier, J.D., Colin, F., Alarcon, C., 1999. Biogenic silica storage in soils. Geology 27, 835e838. Meunier, J.D., Colin, F., 2001. Phytoliths: Applications in Earth Science and Human History. Swets & Zeitlinger, Lisse. Osterrieth, M., Madella, M., Zurro, D., Alvarez, M.F., 2009. Taphonomical aspects of silica phytoliths in the loess sediments of the Argentinean Pampas. Quaternary International 193, 70e79. Parr, J.F., 2002. A comparison of heavy liquid flotation and microwave digestion techniques for the extraction of fossil phytoliths from soil sediments. Review of Palaeobotany and Palynology 120, 315e336. Parr, J.F., Dolic, V., Lancaster, G., Boyd, W.E., 2001a. A microwave digestion method for the extraction of phytoliths from herbarium specimens. Review of Palaeobotany and Palynology 116, 203e212. Parr, J.F., Lentfer, C.J., Boyd, W.E., 2001b. A comparative analysis of wet and dry ashing techniques for the extraction of phytoliths from plant material. Journal of Archaeological Science 28, 875e886. Pearsall, D.M., 2000. Paleoethnobotany. A Handbook of Procedures. Academic Press, San Diego. Perry, C.C., 1989. Chemical studies of biogenic silica. In: Mann, S., Webb, J., Williams, R.J.P. (Eds.), Biomineralization: Chemical and Biochemical Perspectives. VCH Verlagsgellschaft, Weinheim, pp. 223e256. Piperno, D.R., 2006. Phytoliths. A Comprehensive Guide for Archaeologists and Paleoecologists. Altamira Press, Lanham. Powers, A.H., Gilbertson, D.D., 1987. A simple preparation technique for the study of opal phytoliths from archaeological and Quaternary sediments. Journal of Archaeological Science 14, 529e535. Raven, J.A., 2003. Cycling silicon e the role of accumulation in plants. New Phytologist 158 (3), 419e421. Rovner, I., 1972. Note on a safer procedure for opal phytolith extraction. Quaternary Research 2 (4), 591. Runge, F., 1999. The opal phytolith inventory of soils in central Africa d quantities, shapes, classification, and spectra. Review of Palaeobotany and Palynology 107, 23e53. Strömberg, C.A.E., 2004. Using phytolith assemblages to reconstruct the origin and spread of grass-dominated habitats in the great plains of North America during the late Eocene to early Miocene. Palaeogeography, Palaeoclimatology, Palaeoecology 207 (3e4), 239e275. Wirth, G.S., Gieskes, J.M., 1979. The initial kinetics of dissolution of vitreous silica in aqueous media. Journal of Colloid and Interface Science 68, 492e500. Zhao, Z., Pearsall, D.M., 1998. Experiments for improving phytolith extraction form soils. Journal of Archaeological Science 25, 587e598.
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Taphonomy and phytoliths: A user manual Marco Madellaa, *, Carla Lancelottib a b
ICREA e Department of Archaeology and Anthropology, Institució Milà i Fontanals, Spanish National Research Council (CSIC), C/Egipciaques 15, 08001 Barcelona, Spain Department of Archaeology and Anthropology, Institució Milà i Fontanals and CCHS, Spanish National Research Council (CSIC), C/Egipciaques 15, 08001 Barcelona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 16 September 2011
Taphonomy is commonly described as “the study of the environmental conditions affecting the preservation of animal or plant remains”. This broad definition is usually understood to refer to postdepositional processes. One of the main purposes of taphonomic studies is the understanding of possible biases in the fossil record. In this respect, different forms of pre- and post-depositional taphonomy can affect phytolith assemblages that scientists retrieve from sediments and soils. These can be both natural- and human-induced. This paper analyses the forms of possible biases during- and post-depositional, as well as during sampling, recovery and study and proposes some methodological adjustments as to reduce the possibility of errors as well as to check the representativeness of the assemblages retrieved. It is shown that control is possible when human action is involved in the form of attentiveness in sampling and recovery strategies. Pragmatic and statistic methods are proposed so to understand the impact of pre- and post-depositional taphonomic processes on the final phytolithic assemblage. With an increase of stratigraphic control and simple calculation of representativeness it is possible to achieve a confident level of standardisation so to reduce biases at minimum and obtain reliable results with phytolith analysis. Ó 2011 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction The main purpose of taphonomic studies is the understanding of possible biases in the fossil record, being this understood in a biological, mineralogical/geological or archaeological perspective. Taphonomy (from the Greek sάfo2 meaning burial and nόmo2 meaning law) is commonly described as the study of the environmental conditions affecting the preservation of animal or plant remains (e.g. Efremov, 1940), and of artefacts. This broad definition is usually understood to refer to post-depositional processes. However, to comprehend taphonomy in both an anthropic and a non-anthropic setting one needs to understand what happened to the subject of study before it was deposited and how its level of significance might have changed before entering the deposit. Different forms of pre- and post-depositional taphonomy can affect phytolith assemblages that are retrieved from sediments and soils. These taphonomical aspects can be natural- and/or humaninduced. The present paper highlights and discusses the forms of possible transformations (biases) that can enter during- and postdepositional, as well as in the course of sampling, recovery and
* Corresponding author. Fax: þ34 934 430 071. E-mail address: [email protected] (M. Madella). 1040-6182/$ e see front matter Ó 2011 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2011.09.008
study of the assemblage. Furthermore, some methodological adjustments are proposed as to reduce the possibility of inaccuracy as well as to check the representativeness of the retrieved assemblages.
2. Non-anthropic systems 2.1. Significance of phytolith assemblages Phytoliths, owing to their inherent characteristics (e.g. taxonomical significance, production related to physiological and environmental conditions, resistance to decay, ubiquity, etc.), are good indicators of past vegetation cover and environmental conditions (e.g. Blinnikov et al., 2002; Delhon et al., 2003; Strömberg, 2004; Iriarte, 2006). Phytoliths are incorporated in the sediments through a set of processes (Dodd and Stanton, 1990; Osterrieth et al., 2009) that comprise: (1) necrolysis, which refers to decomposition and disaggregation of the plant at the time of death; (2) biostratinomy, that relates to all the processes that take place after the plant death but before the phytoliths burial; (3) fossil diagenesis, which encompass the cumulative effects of the physical, chemical and biological processes that may alter or destroy the phytolith fossil (buried) record (Fig. 1).
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Fig. 1. Theoretical diagram of the depositional and post-depositional processes of phytoliths in natural deposits (from Osterrieth et al., 2009).
2.2. Living plant, necrolysis and biostratinomy (pre-depositional) Phytoliths are produced in and between the cells of living plants. Two primary mechanisms, genetically and environmentally determined, control the production of these bodies (Madella et al., 2009). Certain cells are primed (genetically determined) to deposit opal silica due to their role in the plant tissues (e.g. hairs, short cell of the grass epidermis, etc.) while others are filled by opal silica because of high evapotranspiration (e.g. bulliform cells or stomata cells in the grass leaf). The absorbed aqueous silicic acid [Si(OH)4] is moved through the plant along the transpiration stream (Raven, 2003) and its polymerisation can take place through several processes (Kaufman et al., 1985; Perry, 1989). Silica bodies are made of porous opal-A (SiO2$nH2O) (Mann et al., 1983) and their formation might not be complete, especially if the plant dies before the end of its life cycle. Observation under the microscope of fresh plant tissues from grasses (Gramineae) shows that phytoliths are present in the epidermal cells at different stages of formation (i.e. different degrees of silicification). Some of these silica bodies are therefore incomplete or display patterns that, in phytoliths from sediments or soils, could be interpreted as originating from taphonomy (Fig. 2). Cabanes et al. (2011) observed these pseudo-taphonomised phytoliths in dissolution experiments using fresh assemblages. Once the plant dies, and necrolysis sets in, phytoliths are released in the environment due to the decomposition of the organic matter. At this point the silica bodies are exposed to transport that could mechanically damage their bodies (this transport can also happen at the stage of dead plant but while phytoliths are protected within the tissues the damage should be negligible). Transport can happen by wind and/or by water (Fig. 3) and the result of it can be the breakage of phytolith finest appendices and weak points (e.g. the narrower part of a bilobate), the chipping of the main body and/or abrasion. The extreme
Fig. 2. Partial silicification of the epidermal cells from the grasses Setaria pumila and Urochloa ramosa. The silicification of the phytolith bodies is incomplete, both in the bilobates (short cells) of S. pumila and in the long cells of U. ramosa, showing patterns that might be interpreted as post-depositional dissolution. Photographs M. Madella.
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Fig. 3. Simplified line drawing of phytolith transport by wind. Depending on the strength of the wind and the weight and shape of the silica body, transport can happen according to surface creep, saltation and/or suspension with different intensity of damage to the phytolith body. Similar transport patterns will be in water.
consequence of transport could be the complete modification of the silica body characteristics (especially in the case of strong abrasion) that results in an impossibility of correct identification. In general, phytoliths are not transported over long distances because they are relatively “heavy” particles (as opposed to pollen, for instance) but there are situations in which transport can be an important factor in the making of an assemblage. This is the case of dry environments with sparse vegetation and strong winds (e.g. deserts, loess plateaus) or environment with heavy precipitations and important runoff. Finally, some dissolution may occur at this stage in environments with abundant water availability such as wet tropical areas (Alexandre et al., 1997). However, dissolution normally affects phytoliths as a post-depositional process and, as such, it will be discussed in the next paragraph.
The degree of phytolith translocation related to water seeping through the soils column has been open to debate. Whilst some authors considered this a significant process (Alexandre et al., 1997, 1999; Hart and Humphreys, 1997; Hart, 2003; Humphreys et al., 2003), others see phytoliths to be almost stationary (e.g. Fisher et al., 1995). Fishkis et al. (2010) demonstrated that downward movements of phytoliths (both for bioturbation and water seeping) could be an important process in soils (4 cm/year) and that interpretation on past vegetation based on fossil phytoliths must take into consideration these movements. Also, the transport rate of
2.3. Pedogenesis and fossil diagenesis (post-depositional) Once phytoliths are incorporated in to the soil/sediment system (post-deposition) they are subjected to pedogenesis (soil formation) and fossil diagenesis (rock formation). Soil formation results from the combined effect of physical, chemical, biological, and anthropogenic processes on the soil parent material. These processes involve additions, losses, transformations (physicochemical) and translocations of material that compose the soil. Diagenesis gradually transforms sediments in a rock by chemical and/or structural modifications of the original components. Another process to which phytoliths are subjected after deposition is bioturbation. The translocation of phytolith due to bioturbation has been broadly acknowledged (Hart and Humphreys, 1997; Runge, 1999; Humphreys et al., 2003; Farmer, 2005). Extreme bioturbation can give origin to two clearly different (in chronology and composition) phytolith pools existing side by side in the same soil horizon. It is the case, for instance, of tropical and sub-tropical soils with the presence of termites, such as the case illustrate in Fig. 4 where the oxisol developed under an Araucaria forest (Machado, southern Minas Gerais State, Brazil; Calegari et al., in press) shows channels filled with darker (more organic) A and A2 horizon constituents all the way down to a depth of 200 cm.
Fig. 4. This close up photograph shows the abundance of termite channels in a humic oxisol from Minas Gerais in Brazil. Abundant parts of the overlying A horizon have been vertically translocated for more than 100 cm in the soil profile.
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phytoliths can vary importantly depending on the soil’s physical characteristics (Fig. 5; Fishkis et al., 2010). Dissolution (chemical attack) during pedogenesis and diagenesis can lead to a substantial change in the phytolith pool, depending on pH, water availability and temperature. It has been shown that in forest environments an important part of the phytolith pool is quickly dissolved and absorbed by the plants and phytoliths contribution to the flux of dissolved silica is 3 times higher than that of the mineral component (Bartoli, 1983; Alexandre et al., 1997) (Fig. 6). The importance of the silicon reservoir and the recycling of the phytolith fraction by the vegetation are becoming clear (e.g. Meunier et al., 1999; Meunier and Colin, 2001). Dissolution rates for phytoliths seems to be similar and independent from the specific surface area of the morphotype (Fraysse et al., 2009) (Fig. 7) and experiments suggest that the major release of Si happens during litter degradation (Fraysse et al., 2009). Finally, phytoliths can be removed or added to a specific assemblage due to erosion and/or re-deposition. This scenario can be rather important in certain condition such as the ones of loess/ palaeosol formation (see Osterrieth et al., 2009) where phytoliths can be transported and re-deposited over long distances. 3. Anthropic systems 3.1. Significance of phytolith assemblages The same properties that make phytoliths an interesting microfossil in environmental studies are also fundamental in anthropic systems. In this case, the taxonomical and anatomical significance are paramount for their use in archaeology (see Piperno, 2006 for a general review). Phytoliths are incorporated in archaeological sediments through a set of processes similar to those that acts on natural environments but in this case the input that defines the phytolith assemblage is the one originating from the anthropic activities (Fig. 8). These multiform anthropic activities (e. g., exploitation of plant materials, crop processing, animal stabling, architectural structures, etc.) produce phytolith assemblages that are normally much larger than those produced from the natural vegetation or the phytolith soil bank. The processes of phytoliths incorporation, however, follow the same path: (1)
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necrolysis (decomposition and disaggregation of the plant); (2) biostratinomy (all the processes that take place after the plant death but before the phytoliths’ burial); (3) fossil diagenesis (the cumulative effects of the physical, chemical and biological processes that may alter or destroy the phytolith fossil e buriede record) (Fig. 8). 3.2. Plant material, necrolysis and biostratinomy (pre-depositional) Depending on the type of activities carried out by the human group or individual, the natural input of phytoliths can be more or less important in the constitution of the fossil assemblage. Indeed, a temporary hunteregatherer campsite would have (in general) a much weaker anthropic phytolith input than that of a tell from an agrarian society. Plant material will be arriving at the human frequentation spot because of the multiform use of plant material for food, utensils, construction, fuel as well as ornaments, just to list a few. All these human activities will leave behind plant organic matter that can undergo the same processes of transport highlighted in the previous section. In anthropic settings, however, there is also the action of the same people that might intervene in moving around the plant material. For example, cleaning can spread and dislocate the plant residues discarded on a floor, the majority of which could be retrieved from a completely different context (e.g. a midden or a fireplace) from where they originated. 3.3. Pedogenesis and fossil diagenesis (post-depositional) The effects of pedogenesis and diagenesis in deposits of anthropic origin are very similar to the ones already discussed for non-anthropic sediments. However, human actions can further intervene in the movement of the original deposits, sometime with drastic effects in which sediments are recycled or displaced (e.g. the use of older anthropic sediments for the construction of new structures). Human activities can also affect preservation (pre- or postdeposition) because of, for instance, increased trampling from people and/or animals or semi-industrial activities (e.g. use of high temperature furnaces). From all this, it is clear that anthropic systems are much more complex than the natural ones and
Fig. 5. Distribution of recovered phytoliths (%) with soil depth for two European soils. Concentrations are higher in the upper layers but the percentage of recovery shows that there is a rate of transport of ca. 4 cm/year in both soils (For more discussion see Fishkis et al., 2010). Error bars indicate standard deviations (n ¼ 3) (modified from Fishkis et al., 2010).
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Fig. 6. Biogeochemical cycle of the silicon in the latosoil of Dimonika forest (Congo; modified from Alexandre et al., 1997).
understanding these pre-depositional and post-depositional effects related to human activities are a fundamental part for interpreting phytolith assemblages and the related social actions. 4. Sampling and recovery as taphonomy 4.1. Sampling natural and anthropic sediments If taphonomy is understood as the assortment of possible biases in phytolith assemblages, then sampling can be considered a taphonomical process because it introduces potentially strong biases for the interpretation of the record. Horizontal and vertical sampling should be carried out to be able to assess synchronic and diachronic variability of phytolith assemblages. In the field, it is extremely important to pay the utmost attention to the sediment structure, variability and stratigraphy. In fact, the sampled sediment must represent, at the highest possible confidence level, the deposit that it is going to be interpreted. A practical example is illustrated in Fig. 4, where the B horizon of a tropical soil is intersected by termite channels bringing down the
sediment from the overlying A horizon. Sampling must take care not to mix the two sediments; otherwise the phytolith assemblage extracted will originate from the mix of two very different depositional events. In case of archaeological sites, the finest possible episode of deposition (unless there is a specific different need) must be identified in the field and sampled. As an example, Fig. 9 shows the bottom part of a pit from the Mesolithic/Bronze Age site of Loteshwar in North Gujarat (India) where, during excavation, several depositional and post-depositional events have been identified and sampled separately to understand the construction strategy of this structure as well as the possible origin of the dumped fill. 4.2. Microsampling Microsampling is an approach that allows a strong control for the identification and study of single depositional episodes. It is carried out in the laboratory using as a guide the micromorphology thin section of the sediments under study. This type of sampling starts in the field with the collection of a micromorphology block from the deposit under study. In the laboratory the block is cut into two halves, one is sent for impregnation and the other is re-sealed and retained. When the thin section has been studied and the micro-layers have been identified, the un-impregnated half is sampled following the information gathered from the thin section. It is also possible to run parallel analysis such as infrared (FT-IR) to control pedogenesis and diagenesis processes related to the preservation of phytoliths. An example of this approach is given in Fig. 10, where a micro-sequence from the Toranagallu ash mound (Karnataka, India) is untangled to identify the finest, single depositional episodes (moments of cattle stabling) that were each sampled separately to identify seasonality of frequentation. 4.3. Recovery
Fig. 7. Comparison of dissolution rates of quartz (Dove and Eltson, 1992), vitreous silica (Wirth and Gieskes, 1979), and opal silica phytoliths from two plants: a grass (N. borbonicus Gmelin e bamboo) and a pteridophyta (E. arvense L). The pH was measured in a mixed-flow reactor at 25 C, I ¼ 0.01 M for 2 pH 12 and I ¼ 0.1 for pH < 2 (modified from Fraysse et al., 2009).
Phytolith recovery procedures follow a chain of operational steps defined by the type and condition of the soil or sediment under analysis. There are several different procedures that are commonly employed by researchers to extract phytoliths from soils (Rovner, 1972; Powers and Gilbertson, 1987; Madella et al., 1998;
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Fig. 8. Theoretical diagram of the depositional and post-depositional processes of phytoliths in anthropic deposits.
Zhao and Pearsall, 1998; Albert et al., 1999; Pearsall, 2000; Parr et al., 2001a; Parr, 2002; Coil et al., 2003; Horrocks, 2005; Piperno, 2006; Katz et al., 2010). They all rely on mechanical and chemical separation and have the purpose of concentrating and separating the silica fraction from other mineral and non-mineral components.
A few studies (Jones and Milne, 1963; Jenkins, 2009; Cabanes et al., 2011; Parr et al., 2001b) have explored the possible taphonomic effects of different types of extraction methods on phytoliths. Specifically, these studies have concentrated on wet- and dryashing techniques (i.e. elimination of the organic matter with and without chemicals) to extract phytoliths from plant samples. Parr et al. (2001b) conclude that the two techniques produce similar assemblages and that any difference in shape and/or size is due to the natural variability of phytoliths. The comparison between the effects of wet- and dry-ashing on the number of conjoined phytolith performed by Jenkins (2009) shows that both techniques introduce biases. Dry-ashing can produce the fusion of multicellular phytoliths (silica skeletons) because of the dehydration that occurs during heating at high temperatures. On the contrary, wet-ashing causes the breakdown of articulated phytoliths thus artificially generating much more single-cells phytoliths. More work is needed in order to fully understand the potential of taphonomic processes induced by extraction methods. However, biases caused by the recovery procedures can be reduced to a minimum when the extraction is carried out with a standardised method. Standard laboratory procedures and the careful control over human errors that can be attained in a laboratory environment should result in phytolith assemblages that are, if not unbiased, at least all affected by the same errors. 5. Checking phytolith assemblage representativeness
Fig. 9. Anarta period (Chalcolithic) pit from the site of Loteshwar (N Gujarat, India). The sequence of deposits was clearly identified during excavation and sampling was carried out to resolve the structure construction technique and possible use.
The representativeness of phytolith assemblages can be checked during analysis in several ways. The first to propose a method to assess the level of taphonomy action on phytolith assemblage were Fredlund and Tieszen (1997: p. 211) who evaluated phytolith preservation observing the degree of pitting in bulliforms and the degree of erosion in elongate (long) cells. This criterion offers a first and quick assessment but it is, as the authors themselves highlighted, quite subjective and dependent on the experience of the
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Fig. 10. Sampling strategy for ash mound sites of the Southern India Neolithic. The site of Toranagallu is a small ash mound formed by the cyclical deposition of dung from stabling episodes. The deposit is formed by a sequence of very fine layers (from 1 to 15 mm in thickness) that were sampled individually to unravel the history of occupation of the site. The micro-layers needed to be identified with the help of the micromorphology and sampled in the laboratory using a magnifying glass.
analyst in discriminating between fine differences in phytolith appearance. Also, some of these patterns identified as related to post-depositional processes can be present in fresh phytolith of living plants and they can be often observed on phytoliths from reference collections or in the plant tissues when observed under the microscope. Two mathematical measurements can be used to attain a more controlled evaluation of the impact of taphonomy on the entire assemblage. In order to rate the degree of preservation the ratio of short versus long cells can be applied. Long cells are more often less silicified and offer wider surface area to chemical and physical attack. Therefore they represent weaker typologies than short cells, and tend to disappear more easily. Assemblages with a high number of long cells versus short cells should emphasize a higher degree of preservation (Madella, 1997; Lancelotti, 2010). A correlation measurement between the number of morphotypes identified and the concentration of phytoliths per gram of Acid Insoluble Fraction (AIF) can also be used to assess whether taphonomy affects the richness of the phytolith assemblage (Lancelotti, 2010). This method implies that, when taphonomy influences the representativeness of a sample, the two values will display a high degree of correlation, i.e. the number of morphotypes
identified will increase in more concentrated samples (Fig. 11a) whereas less concentrated samples will produce less varied assemblages. On the contrary, where the two values are not correlated, the phytolith assemblage can be considered representative of the original phytolith input, i.e. more and less concentrated samples will show the same diversity of morphotypes (Fig. 11b).
Fig. 11a. The assemblage in this example shows a high degree of correlation between number of morphotypes identified (x-axis) and concentration of phytoliths per gram of AIF (y-axis). In this case, taphonomy highly affects the representativeness of the phytolith assemblage.
Fig. 11b. The assemblage in this example is not affected by taphonomy. There is no correlation between the number of morphotypes identified (x-axis) and the concentration of phytoliths per gram of AIF (x-axis) and the concentration of phytoliths per gram of AIF (y-axis).
6. Final remarks Understanding taphonomy is a fundamental step in any interpretation of fossil assemblages. Being able to assess the representativeness of the fossil assemblage in respect to the original input provides more robust interpretations of past processes. Phytoliths are today a fossil routinely analysed in geological and archaeological sediments to assess past vegetation and the use and processing of plant resources in the past. For this reason, it is essential to identify and comprehend the processes of formation, transport, sedimentation and biotic (including human) actions that intervene in the formation of the assemblage observed at the microscope. This paper highlighted the complex and multiform processes that can be at work to define a phytolith assemblage and that must be taken into
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