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Biomineralisation and biogeochemical mobility of chemical elements: phytolith dissolution and its implication in geoarchaeology

Andriopoulou Nafsika-Chrysoula

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Summary

A growing amount of research has been done on biominerals produced in planttissues, microscopic SiΟ2-rich phytoliths included, because of the useful information they may provide on the natural and the anthropogenic environment. The relationship between nature and humanity is fundamental in archaeological interpretation. Environmental changes are associated with biogeochemical processes and with chemical element mobility in soil horizons, and the understanding of such processes is crucial for the comprehensive study of archaeological materials. Phytoliths are produced through biomineralisation processes that take place in most plant species, and they are eventuallyreleased in soils and sediments after the decomposition or/and burning of the plant material. Cereals, and especially wheat (Triticum spp.), produce great quantities of SiO2-rich phytoliths and and their impact on human economies over time has been well documented.Biogenic silicon (bSiO2) is the earliest known natural bioskeleton and its uniquephysicochemical properties make it a suitable material for a wide range of applications in the geosciences and archaeology. Even though the biogeochemical cycle of silicon has been studied extensively, most existing studies have restricted their focus on diatoms. Despite the study of phytoliths being an internationally emerging field, scientific interest has remained limited mainly to their systematic classification and morphometry. In particular, in the context of Greece and Cyprus, it is only the last decade that their significance has been acknowledged. Despite the significant contributions having been made in this area, recent studies on SiΟ2-rich phytoliths have not put enough emphasis on the investigation of phytolith stability in relation to the biogeochemical depositional environment and on the preservation of their morphotypes, relating it to important environmental parameters such as soil pH, temperature and water availability. Furthermore, the degree in which the presence of other chemical elements affect the dissolution of biogenic silicon, and therefore affecting phytolith preservation in soils andsediments, has been scarcely covered. Preservation status is one of the most important factors for the reliable interpretation of phytolith morphotypes. In most studied cases, the mineralogical or/and chemical composition of phytoliths is affected by a variety of morphological alterations that have been caused by in situ taphonomic or/and laboratory processes during their lifespan.The present doctoral thesis aims to make a contribution to the research lack outlined above and to expand this field of study. In particular, the dissolution mechanism of phytoliths is investigated in relation to variations of pH, temperature and time, by the means of long-term batch experiments. Moreover, the mobility of silicon (Si) and 7 other main and trace chemical elements (K, Mg, Al, Ca, Fe, Sr, Ba) is being studied with the use of inductively coupled plasma mass spectrometry (ICP-MS), and visualised with MATLAB software. In addition, a multi-analytical characterisation of phytoliths was performed in order to understand their microstructure and behaviour after their extraction from plantsby means of two conventional methods: the dry and the wet ashing method. For the high resolution characterisation of phytoliths extracted from either the entire plant (excluding the roots) or different parts of the plant, the inflorescence and the stems-leaves; a variety of analytical techniques were employed, including powder X-Ray Diffraction (XRD), Energy Dispersive X-Ray Fluorescence spectrometry (ED-XRF), Fourier-Transform Infrared spectroscopy (FTIR), Thermogravimetric and Differential Thermogravimetric Analysis (TGADTGA),elemental (Carbon-Hydrogen-Nitrogen-Sulphur) analysis (CHNS), ScanningElectron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS), optical microscopy and photomicrography. The quantification of morphotypes for phytoliths originating in the plants (counting at least 250 phytoliths per microscope slide and morphological classificating according to form, texture and anatomical origin on the basis of the International Code for Phytolith Nomenclature (ICPN Working Group: Madella et al. 2005) was carried out both before and after dissolution, and visualised using the python programming language. Phytoliths from soils and archaeological sediments were also extracted. Furthermore, the integrated use of microscopy and photogrammetry for thethree-dimensional (3D) representation of biominerals was proposed as a useful tool in geoarchaeology.The laboratory analyses took place at the Technical University of Crete (School of Mineral Resources Engineering and School of Environmental Engineering), the American School of Classical Studies at Athens (Malcolm H. Wiener Laboratory for Archaeological Science) and the British School at Athens (Fitch Laboratory). The studied phytoliths were parts of three sample groups: (1) ‘’modern’’ wheats (Triticum monococcum/Triticum durum) from five organic crops in Greece (Crete, Volos, Corfu and Pella) and one conventional crop in Cyprus (Pafos), (2) the corresponding soils for these samples, and (3) archaeological sediments from three nearby the cultivations archaeological sites in Greece (Knossos, Crete, Palea Castle Volos and Toumba Thessaloniki). The comprehensive mineralogical and biogeochemical study contributes to the understanding of the geochemical cycle of silicon and to geoarchaeological research, through the insights gained on the post-depositional alterations that SiΟ2-rich phytoliths are subjected to over time, and how this relates to the mobility of other soil elements. Furthermore, the mineralogical and chemical composition of the recovered phytoliths is controlled by the extraction method, strongly suggesting that comparison of phytoliths extracted from plants is meaningful only if the method of extraction remains the same. Physicochemical characteristics of fresh phytoliths extracted from plants provided useful information on their preservation state after laboratory processing, that may further contribute to the study of aged phytoliths. Dimensional characteristics and texture of the majority of the phytoliths changed with rising pH and temperature, suggesting that long cells dendritic morphotypes may alter to long cells echinates or/and to long cells verrucates morphotypes. Moreover, this study provided information about phytoliths characteristics after burning. Phytoliths obtained from plants using the dry method of extraction here are therefore suitable for fire incidents with implications to archaeology. The overall physicochemical characterisation of phytoliths may further contribute to the interpretation of phytolith assemblages from paleontological, archaeological and ethnographical contexts for the diachronic study of human-plant interactions.

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