The Centro Nacional de Investigación sobre la Evolución Humana (CENIEH) has participated in a study led by Virginia Martínez-Pillado, from the Universidad del País Vasco (UPV/EHU) and the Centro Mixto UCM-ISCIII, just published in the journal Quaternary International, which establishes the cause of the enigmatic and unusual red coloration of the stalagmites in Goikoetxe Cave, located in the Urdaibai Biosphere Reserve (Vizcaya, Spain)), and its possible use as an indicator of paleoclimatic changes in northern Spain.

As a general rule, the color red, both in geological and archaeological samples, is due to the presence of different oxides of iron, which stain minerals and rocks very intensely. Nevertheless, the analyses conducted in this work have shown that, in the case of the Goikoetxe stalagmites, the color is caused by the presence of organic substances derived from decomposition of the plant cover of the soils above the cavity.

“These organic substances, fundamentally humic and fulvic acids, are carried inside the cave by rainwater seeping through the rock, and for this to happen, there has to be a landscape with well-developed soils and thick plant cover, and relatively most climatic conditions”, explains Martínez-Pillado.

Spectroscopic techniques allow to detect compounds derived from the degradation of plant material inside the stalagmites

One of the collaborators in carrying out the analyses was the Archaeometry Laboratory at the CENIEH, where Ana Álvaro and her team used two spectroscopic techniques, Raman and FTIR, to identify different types of molecules and compounds. “These techniques have let us detect the presence of organic compounds derived from the degradation of plant material inside the stalagmites”, says Álvaro.

Moreover, using UV fluorescence measurements, it has been possible to identify different cycles related to the greater or lesser presence of organic compounds in the stalagmites and to changes in the environmental and climatic conditions outside the cavity. “These cycles detected, linked to the striking red color, have allowed us to embark on new investigations to reconstruct the climatic evolution of the Cantabrian coast between 5,000 and 7,000 years ago”, concludes Martínez-Pillado.

In addition to the UPV/EHU, the UCM-ISCII and the CENIEH, the Universidad de Burgos (UBU) also participated in this paper, entitled “The red coloration of Goikoetxe Cave’s speleothems (Busturia, Spain): an indicator of paleoclimatic change”.