Combined study of archaeomagnetism and Raman spectroscopy of experimentally burnt limestones from the middle-palaeolithic site of Pinilla del Valle (Madrid, Spain)

Abstract

Here we report the archaeomagnetic, rock-magnetic and Raman spectroscopy results of sample analysis from two types of limestones that were experimentally heated under controlled conditions. These lithologies have been commonly identified at the middle-palaeolithic sites of Descubierta Cave and Navalmaillo rock-shelter (Pinilla del Valle, Madrid, Spain). By means of a thermocouple system, maximum heating temperatures of 400 - 450 °C were recorded during one hour. The main goal was to characterize the variations in the magnetic mineralogy and (in)organic compounds, and to study the record of the Earth's magnetic field direction at the time of cooling. This allowed us to obtain a pattern that is potentially applicable to archaeological limestones in order to identify combustion structures at the site. All experiments were carried out on representative samples both before and after experimental heating. Before heating, both types of limestones were dominated by dolomite and calcite and as main iron oxides, goethite and magnetite. After heating, the original goethite persisted in samples heated to low temperatures (<200 °C) whereas it transformed to haematite and/or magnetite in those heated to 450 °C. Mineralogical alterations mostly focused on the outermost 1st cm of depth. The neoformation of magnetite as intermediate step in the dehydration process of goethite to haematite has been barely documented and has palaeomagnetic implications. The mineralogical results obtained, their implications for the identification of fire at the site as well as potential applications to similar cases studies are discussed.