Understanding Fossil Phytolith Preservation: The Role of Partial Dissolution in Paleoecology and Archaeology

May Understanding Fossil Phytolith Preservation: The Role of Partial Dissolution in Paleoecology and Archaeology Dan Cabanes 0 1 2 Ruth Shahack-Gross 0 1 2 0 1 ERAAUB, Department of Prehistory , Ancient History and Archaeology , University of Barcelona , c/ de Montalegre 6-8, 08001, Barcelona , Spain , 2 Kimmel Center for Archaeological Science, Weizmann Institute of Science , Rehovot, 76100 , Israel 1 Funding: This research has been carried out with the support of European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n 229418 to S. Weiner and I. Finkelstein where R. S- G. acted as track leader. This contribution is part of the activities of the Equip de Recerca Arqueologica i Arqueometrica de la Universitat de Barcelona (ERAAUB), Consolidated Group (2014 SGR 845), thanks to the support of the Comissionat per a Universitats i Recerca del DIUE de la Generalitat de 2 Academic Editor: Karen Hardy, ICREA at the Universitat Autonoma de Barcelona , SPAIN Opaline phytoliths are important microfossils used for paleoecological and archaeological reconstructions that are primarily based on relative ratios of specific morphotypes. Recent studies have shown that phytolith assemblages are prone to post-depositional alteration involving partial dissolution, however, the manner in which partial dissolution affects morphotype composition is poorly understood. Here we show that morphotype assemblages from four different plant species subjected to controlled partial dissolution are significantly different from the original assemblages, indicating that the stability of various morphotypes differs, mainly depending on their surface area to bulk ratios. This underlying mechanism produces distorted morphotype compositions in partially dissolved phytolith assemblages, bearing vast implications for morphotype-based paleoecological and archaeological interpretation. Together with analyses of phytolith assemblages from a variety of archaeological sites, our results establish criteria by which well-preserved phytolith assemblages can be selected for accurate paleoecological and archaeological reconstructions. - Silica phytoliths are microscopic opaline (SiO2 nH2O) bodies found in many plant families and are especially prominent in the grass family [1]. Past research focused on the mechanism of their formation [24] and their use as proxies for past vegetation and climate [58]. Studies of the systematics of opaline phytolith morphologies have shown that many of these so-called morphotypes, especially in grasses, are in fact silicified replicas of plant cells or cell walls. Thus specific morphotypes reflect specific cells or tissues, and can be divided according to their origin within a plant (e.g., leaf vs. inflorescence phytoliths) and even serve as identifiers of plant species, genera and families [9]. This attribute in phytolith systematics is utilized to reconstruct past vegetation. In paleoecology, for example, phytolith morphotypes that are specific to C3 and C4 grasses are utilized to infer past climates [10]. In archaeology, phytoliths are used to Competing Interests: The authors have declared that no competing interests exist. detect concentrations of plant remains in archaeological sites [11], to determine the types of plants used by ancient humans [12], to detect (albeit with some controversy) the earliest appearance of domestic species such as maize and rice [13, 14], to recognize the types of plants utilized for fuel and bedding [1517], and to identify spatial activities in archaeological sites [18]. With this abundance of research and its far-reaching implications for the past, one must understand the effect of partial dissolution on the interpretations that are derived from fossil phytolith morphotype assemblages. Being an important source of silicon in the global terrestrial silicon cycle, a surge of interest in post-depositional alteration of phytoliths that involves partial dissolution is a hot research topic in recent years. Research in the last decade or so focuses on the role of phytoliths in the recycling of silicon by plants [1921] which is tightly related to the mechanism of opal dissolution [2226]. The latter studies highlight the fact that unlike previous assumptions, opaline phytoliths are relatively unstable in soils and sediments [2732]. Yet, despite this realization little attention has been given by the paleoecological and archaeological research communities to the effect of partial dissolution on interpretations based on the morphological composition of fossil phytolith assemblages. Recently we pinpointed this issue by studying the effect of partial dissolution on the stability of phytoliths from domestic wheat plants [27]. Yet, the mechanism that explains which morphotypes would be stable or unstable following partial dissolution is currently unknown. Several untested hypotheses have been raised to explain the differential stability of phytolith morphotypes. Bartoli and Wilding [33] identified Al in phytoliths and suggested that this impurity in the opal contributes to phytolith stability, a suggestion refuted by Fraysse et al. [23] and supported by Nguyen et al. [26]. The role of Al in stabilizing opal is thus not yet understood. Osterriech et al. [31] suggested that not only the presence of chemical impurities in phytolith opal, but also maturity of a phytolith morphotype, may relate to phytolith preservation. They did not, however, supply criteria for assessing phytolith maturity. In addition, sediment pH, the specific silicified plant taxon and localities protected from weathering, were also suggested as parameters that influence phytolith preservation and affect interpretation of phytolith assemblages [1]. A most interesting observation was made by Wilding and Drees [34] who showed that biogenic opal produced in the leaves of deciduous trees is 10 to 15 times more soluble than opal produced by grasses. This observation led them to suggest that the larger surface to bulk ratio of tree leaf phytoliths make them more soluble than grass phytoliths. Fraysse, Pokrovsky, Schott and Meunier [23] studied the specific surface area in phytolith assemblages from 4 different plant species, showing that despite differences in specific surface area among the 4 plant species, the solubility of these phytolith assemblages was similar (ranging from 2.5 to 3.0 mM Si at pH 8 and 50C). This indicates that specific surface area of a phytolith assemblage is not the main factor affecting phytolith solubility and relative stability. Additional research also highlighted the possibility that phytolith stability is related to the phytoliths surface to bulk ratio [31, 35], yet, no study to-date conducted direct measurements on individual phytoliths to test this hypothesis. To our knowledge, there is no currently available technique that enables direct measurement of specific surface area of single microscopic part (...truncated)