What are phytoliths?
Phytoliths, or plant stones, are formed when plants draw monosilicic acid from groundwater through their roots and deposit silica within and between plant cells. The silica can take on the form of the cells themselves, cell parts, or intercellular space. Although many plants silicify cells or cell parts, some actually form phytoliths that are diagnostic to particular plant families, genera, or even species. Being inorganic, the silica is deposited into soil when the plant dies and breaks down. Most phytoliths are between 10 and 70 microns in size, and are therefore found in the silt fraction of soils. Phytolith analysts process comparative plant and phytosoil samples to remove the organic and non-silica components, leaving phytoliths as proxy indicators for answering a broad range of research questions. Phytolith analysis is an excellent tool for complementing macroremain, pollen, and starch grain analyses. Because they are inorganic, phytoliths can survive in soils indefinitely.
What questions can be answered using phytoliths?
Archaeologists frequently want to know in what environment a particular culture was operating
Was the landscape covered with trees? Was it deforested by human agency? Did the climate change with the passage of time? Phytolith analysis can help to answer these sorts of questions by reconstructing the vegetation of the past, the paleoenvironment, and even the paleoclimate. Phytolith analysis has proven particularly valuable for work in the Neotropics (New World tropics), but also has applications in the temperate areas of the New World.
Other questions that archaeologists have involve the use of cultigens, or fully domesticated plants, by past peoples. When did a plant transition from a cultivar to a cultigen? When were domesticated crops brought into an area? What crops were grown, and where? What plants were eaten? What plants were used as tools? Phytolith analysis can use the diagnostic types produced by maize, beans, squash, arrowroot, palm, and some other economically important species of the New World to identify the presence of plants that were used by humans for food, shelter, or tools.
Since they are found in soils, phytoliths can also be recovered from archaeological contexts including agricultural field features, structure floors, various features, pot residues, grinding stones, and other contexts. It is very important, however, when testing features and archaeological sites using phytolith samples, to take control samples in order to establish a "normal" signature against which the feature sample can be compared. Phytoliths can even be recovered from dental calculus in humans and animals when the research question calls for it.
Because they have similar specific gravity and chemical makeup, non-phytolith biosilica such as sponge spicules and diatoms are often present in phytolith extracts. In phytolith analysis, we often track the patterns of diatoms, algae with silica skeletons, as indicators of wet habitats. Diatom taxa are often sensitive to different environmental conditions, such as still versus flowing water and brackish water, and can be used to complement the data presented by phytolith analysis.
While the following list is not exhaustive, please refer to some of these seminal works on paleoethnobotany and the archaeological uses of microremains such as phytoliths and starches.
Bohrer, Vorsila L. and Karen R. Adams
1977 Ethnobotanical Techniques and Approaches at Salmon Ruin, New Mexico. San Juan Valley Archaeological Project Technical Series No. 2. Eastern New Mexico University Contributions in Anthropology, Vol. 8, No. 1. Llano Estacado Center for Advanced Professional Studies and Research.
Although this volume predates phytolith analysis for archaeologists, it does give some excellent information regarding sampling practices for recovering microremains, especially in the Plant Microfossil section (pp. 13-31) in the Field Procedural Manual by Karen Adams.
Meunier, Jean D. and Fabrice Colin (editors)
2001 Phytoliths: Applications in Earth Sciences and Human History. Lisse, A.A. Balkema Publishers.
Neff, Hector, Deborah M. Pearsall, John G. Jones, Barbara Arroyo, Shawn K. Collins, and Dorothy E. Freidel
2006 Early Maya Adaptive Patterns: Mid-Late Holocene Paleoenvironmental Evidence from Pacific Guatemala. Latin American Antiquity 17(3):287-315.
A cross-disciplinary study that shows the complementarity of phytolith analysis with pollen analysis in reconstructing the paleoecology and paleoclimate of parts of Mesoamerica.
Nelson, Ben A., Christopher T. Fisher, Michelle Elliott, Roberto S. Molina Garza, Shawn K. Collins, Deborah M. Pearsall
n.d. Desertification in Relation to Episodes of Human Occupation in Arid Northern Mexico. Submitted to Ecology and Society, December 2007.
A cross-disciplinary study demonstrating the impacts of humans upon their environment.
Pearsall, Deborah M.
Pearsall, Deborah M.
2000 Paleoethnobotany: A Handbook of Procedures. 2nd Ed. San Diego, Academic Press.
Pearsall gives the history of paleothnobotanical analyses, their uses in archaeology, case studies, and sampling practices for plant macro- and microremains. The chapter on phytoliths is completely updated from the 1989 edition, reflecting the maturation of the field of phytolith analysis in archaeology.
Pearsall, Deborah M. and Dolores R. Piperno (editors)
1993 Current Research in Phytolith Analysis: Applications in Archaeology and Paleoecology. Philadelphia, The University of Pennsylvania Museum.
Pinilla, A., J. Juan-Tresserras and M.J. Machado (editors)
1997 Estado Actual de los Estudios de Fitolitos en Suelos y Plantas. (The State-of the Art of Phytoliths in Soils and Plants.) Monografias del Centro de Ciencias Medioambientales, No. 4. Madrid, Consejo Superior de Investigaciones Cientificas.
Piperno, Dolores R.
2006 Phytoliths: A Comprehensive Guide for Archaeologists and Paleoecologists. Lanham, Alta Mira Press.
This is a great update of Piperno's 1988 volume, reflecting the maturation of phytolith analysis as a discipline.
Piperno, Dolores R.
1988 Phytolith Analysis: An Archaeological and Geological Perspective. San Diego, Academic Press.
Rapp, George Jr. and Susan C. Mulholland (editors)
1992 Phytolith Systematics: Emerging Issues. New York, Plenum Press.