Ash bones and guano: A study of the minerals and phytoliths in the sediments of Grotte-XVI, Dordogne, France

Very few prehistoric cave deposits in Western Europe contain visible hearth remains, even though there is abundant evidence of fire use by cave inhabitants. Grotte XVI (Dordogne, France) is exceptional in this respect in that it contains a most conspicuous layer (Couche C) characterized by a series of brightly coloured bedded sediments. The formation of Couche C has previously been ascribed to the ash and charcoal produced by the cave's Mousterian occupants burning mainly lichens. The Mousterian sedimentary layers below Couche C, and the Upper Palaeolithic sediments unconformably overlying Couche C, are quite different in texture and colour, and show no visible evidence of ash deposits. Analyses of the minerals, microstructures and phytoliths in Couche C are consistent with the notion that these sediments were derived mainly from ash produced by burning wood and to a lesser extent grass. The original calcite component of the ash has been diagenetically altered to carbonated apatite around the cave periphery, and to even more insoluble phosphate minerals in the centre. One such mineral, newberyite, most likely formed by the reaction between guano degradation products and the ash. Under these circumstance bone preservation is also affected. The overlying Upper Palaeolithic sediments contain calcite. One of these layers (Abb) contains much more calcite than all the other layers, as well as abundant phytoliths derived from wood and grass. This layer also contained many artifacts and bones. It is thus concluded that ash is a major component of Layer Abb, even though it is not visible to the naked eye in the form of hearths.