Integrated study of archaeomagnetism, magnetic properties and Raman spectroscopy of experimentally heated limestones from the Paleolithic site of Pinilla del Valle (Madrid)

Abstract

The palaeolithic archaeological sites of El Calvero de la Higuera are located in the Eastern part of the Central Iberian Range, in the karstic system of Pinilla del Valle (Madrid). The archaeo-palaeontological record yielded by the sites is allowing a detailed study of the occupation and activity of Neanderthal groups at the Iberian Peninsula during the Middle-Upper Pleistocene. A particularly interesting aspect is the presence of fire use by these human groups. We report here the archaeomagnetic and Raman spectroscopy results obtained from the analysis of samples from two types of non-archaeological limestones experimentally heated. These lithologies have been commonly identified at the sites of Descubierta Cave and Navalmaillo rock-shelter. By means of the thermocouple´s system, maximum heating temperatures of 400- 450 ºC were recorded during one hour. The main goal is to characterize the variations in the magnetic mineralogy and (in)organic compounds as well as study the record of the Earth´s magnetic field direction to obtain an pattern applicable to the archaeological limestones, in order to identify and characterize combustion structures at the site. All experiments of both techniques were carried out on representative samples of both lithologies before and after the experimental heating. Raman results show that the limestones are mainly composed by calcite and dolomite. The dominant iron oxide in the fresh (unburned) area is goethite/limonite, whereas in the burned area most affected by the highest temperatures is haematite. Mineral magnetic analyses have identified magnetite as the main magnetic carrier both in pre- and post-burning samples. In addition, the reversibility of thermomagnetic curves agrees well with the temperatures recorded in the field, indicating maximum heating temperatures of 400 – 450 ºC. The mineralogical results are discussed as well as their implications and applications in the identification of fire at the site.