Die Rolle von Oxalat in der isotopischen Zusammensetzung von Aschen

Haydar Martinez-Dyrzo ^a, Susan Mentzer ^a,b, Christopher Miller ^a,c

^aInstitute for Archaeological Sciences, University of Tübingen
^bDepartment of Anthropology, University of Arizona
^cSenckenberg Centre for Human Evolution and Paleoenvironment, Universität Tübingen

The study of prehistory involves necessary the paleoenvironmental reconstruction. An important technique is the use of soil and sedimentary micromorphology to recognize anthropogenic patterns. One of this patternsare the combustion features, in particular wood-derived ashes. Their presence is a proxy to testify the production of fire during the Pleistocene and Holocene by natural variables or by human factors. Their identification is largely based on the observation of rhombic, calcium oxalate pseudomorphs of calcite .Sometimes is difficult to differentiate wood-derived ash from other materials such as plasters, geogenic and biogenic calcite, since the principal component of wood ash is calcium carbonate. For instance, some complementary techniques to micromorphology has been developed.

The present study shows a collection of 200 grain mounts of six different species of trees from the Swabian Alps, Germany (fir tree, oak, beech, ashtree, pine and birch), four species from the Trans-Volcanic Belt, Mexico (yucca, cuammochitl, violett copaland white copal), one representative from Australia (eucalyptus) and a grass representative from tropical Africa (gigant timber). All the species are C3 plants with the exception of yucca, an actually CAM plant. For each specie, a sample from the trunk bark, leaves and branch were taken. The samples were burnt at different conditions. In addition, a Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy analysis was performed for each sample, using a Cary 660 (Agilent Technologies) spectrometer. The isotopic analysis of the 200 samples were made using a Gas bench II connected to a Mat252 mass-spectrometer with a precision of ± 0,1 ‰ for δ¹³C and δ¹⁸O. 30 more samples of leaves were analyzed by this method in order to observe their isotopic signature depending of their position in a tree. Results shows, the existence of charcoal and organic material (a polymer organic component) at 400 °C. The samples at 900 °C shows in some cases an incomplete combustion. For some species (Pine, eucalyptus and yucca) the pyrogenetic calcite do not form the calcium oxalates pseudomorphs, but nails or styloids of micrite aggregates. The isotopic analysis shows important differences in the signature of both nuclides, not only between different taxa but between different parts from the same specimen. The mineralogical analysis shows that these differences are explained by the production of oxides since 400 C°. These chemical compounds tend to recarbonate using the atmospheric CO­₂. Despise this effect, the isotopic composition of ashes is still different from the bio- and geogenic carbonates. This study has important repercussions in the identification and dating of wood-derived ashes in archaeological context.