Speleothems from volcanic caves as records of environmental changes

Abstract

Speleothems are secondary mineral deposits formed in caves, such as stalactites and stalagmites, due to water-rock interactions. They are typically used as climate archives in karstic caves, as their formation depends on the amount and geochemistry of water dripping into the cave. Likewise, soil minerals and organic matter from the surface are transported along bedrock discontinuities and deposited on speleothem surfaces during rain events. Hence, speleothems may provide information on local climate and other variables, such as changes in vegetation, precipitation and the occurrence of floods, droughts or fires. In volcanic islands, the most common caves are lava tubes, but they were considered of little interest and scarcely studied. Thus, siliceous speleothems are rarely used in paleoenvironmental studies due to their low carbon content. However, they can give us important information on past environmental disturbances. Forests in volcanic islands have been frequently affected by wildfires, which alter the physical properties of volcanic-ash soils (andosols) and drastically decrease their water retention capability, leading to extensive soil erosion after torrential rains. These changes may promote the genesis of speleothems in the underground environment which may grow extremely fast once the overlying andosol is burnt. For instance, in lava tubed from La Palma Island (Canary Islands, Spain), we found unusual black speleothems with gelatinous texture in caves located below a laurel forest burnt in 2012. For unveiling their origin and composition, we conducted FESEM-EDS, XRD, isotope analysis, 13C NMR and analytical pyrolysis (Py-GC/MS). We demonstrated that these gelatinous speleothems are entirely composed of highly hydrated non-crystalline materials. We described them as aluminum silicate gel coated with charred vegetation and thermally degraded resins or triterpenoids from the overlying burnt forest. This demonstrates that environmental changes in volcanic regions may change the underground environment. In this presentation we will show examples of how the combined use of contrasting and complementary analytical tools allow us to successfully discern environmental changes using speleothems found in volcanic caves from the Canary Islands (Spain) and Easter Island (Chile).