Relationships between colour and diagenesis in the aragonite-calcite speleothems in Basajaun Etxea cave, Spain

摘要

Basajaun Etxea Cave, North Spain, contains a wide morphological and colour variety of speleothems. Most of them are composed of aragonite, but calcite speleothems are also found. Their most common colour is white, but there are also pink, green and turquoise speleothems, in different areas of the cave. Mg-rich dissolution waters from the cave's crystalline dolostone and magnesite host rock favour aragonite precipitation and drive important diagenetic changes. In this paper we will discuss how diagenesis modifies speleothem texture, mineralogy and geochemistry, causing significant changes in the colour. We also discuss how speleothems that have undergone diagenesis may also be useful indictors of paleoclimatic conditions. Our work shows that diagenesis changes the colour of the speleothems because of the mobility of the chromophore elements during this process. Along with this elemental alteration comes the loss of the primary isotopic signals of speleothems. Detailed petrological studies of speleothems should precede their analysis for palaeoclimatic reconstruction so that influence of areas affected by diagenesis can be eliminated. In addition, diagenetic signals also contain valuable information on the changes of the waters within the cave, and the overall evolution of speleothems and the cave itself.