As a general rule, the colour red in geological and archaeological samples tends to be due to the presence of various iron oxides that dye minerals and rocks a very deep colour. However, “the studies conducted in this piece of work and published in the Quaternary International journal have shown that in the case of the stalagmites in the Goikoetxe Cave located in the Basque Country’s Urdaibai Biosphere Reserve, the red colouration is due to the presence of organic substances produced by the decomposition of vegetation cover in the soils located on top of the cave”, said Virginia Martínez-Pillado, researcher in the UPV/EHU’s Department of Mineralogy and Petrology and lead author of this work. “All along the Cantabrian seaboard there is a lot of precipitation and vegetation, and that is why a large vegetation cover forms on top of the cave; when this vegetation degrades, these organic substances, mainly humic and fulvic acids, are dragged into the cave by rainwater, and dye the speleothems red when the red from the calcite that forms them is incorporated,” explained Virginia Martínez-Pillado.
The authors of this work have combined various analysis techniques for these stalagmites of different colours, such as uranium-series radiometric dating, petrography, X-ray fluorescence, spectroscopy and ultraviolet light luminescence. “To conduct the analyses, two spectroscopic techniques known as Raman and FTIR were used; their purpose is to identify various types of molecules and compounds. These techniques were the ones that established the presence of organic compounds produced by the degradation of plant matter inside the stalagmites,” said the lead author of the work.
Read more at University Of The Basque Country
Photo: Virginia Martínez Pillado. CREDIT: UPV/EHU