Bacteria, guano and soot: Source assessment of organic matter preserved in black laminae in stalagmites from caves of the Sierra de Atapuerca (N Spain), Joeri Kaal, Virginia Martínez-Pillado, Antonio Martínez Cortizas, Jorge Sanjurjo Sánchez, Arantza Aranburu, Juan-Luis Arsuaga, Eneko Iriarte
International Journal of Speleology