Honey-like scented deposits in a lava tube from La Palma Island (Spain)

Speleothems, or secondary mineral deposits in caves, are usually formed due to the dissolution of primary minerals from the host rock, such as in limestone and dolostone caves. In volcanic caves, the vast majority of speleothems are siliceous, formed in the initial stage of lava tube formation or due to leaching and subsequent precipitation of secondary minerals [1]. Among them, organic ooze deposits that coats the walls of lava tubes have been reported [2,3]. In a lava tube (Llano de los Caños Cave) from La Palma Island, in Canary Islands (Spain), black ooze with honey-like scent was collected from the wall and ceiling of the cave where sub-horizontal fractures occur (Fig. 1A,B). Field emission scanning electron microscopy (FESEM-EDS), X-ray micro-computed tomography (micro-CT) and mineralogical analyses were conducted for characterizing the surface morphology, 3D microstructure and mineralogical composition of these black organic deposits. Abundant microbial structures were found in close association with the organic ooze, including reticulated filaments (Fig. 1B), bacterial cells, platelet microorganisms and pollen grains (Fig. 1B). The overall volume of the sample showed low opacity or high transparency to X-rays according to micro-CT (Fig. 1C), with the exception of a superficial deposition of calcium carbonate in the form of calcite (whitish layer), as also observed by stereomicroscopy (Fig. 1B, white particles). The X-ray pattern showed a broad hump typical of non-crystalline material and some calcite, suggesting that the black ooze samples are mainly composed of amorphous material and calcite. In an attempt to achieve an accurate characterization of the ooze deposits found in Llano de los Caños Cave in La Palma Island and unveil the nature of these unusual honey-like scented deposits, analytical pyrolysis (Py-GC/MS) and stable isotope analysis were performed. Py-GC/MS showed abundance of polysaccharides, plant lipids and specific terpenoids typical of the local vegetation (mainly Erica arborea). In addition, polycyclic aromatic hydrocarbons and N-containing heterocyclic compounds were detected, which probably derived from the leaching of charred vegetation resulting from a wildfire occurred in the area in 2012. Stable isotope analysis of δ 13C, performed on the cave black deposits, topsoil and Erica arborea collected from the area over the lava tube, confirmed that the organic fraction of the ooze deposits are a combination of partially charred vegetation and organic compounds from the andic soil located over the cave. Therefore, these cave deposits are the result of an input of plant organic matter and charred vegetation into the cave through rock fractures, which may constitute an important source of energy for cave organisms. The honey-like scent of the black ooze deposits is probably due to the thermal transformation of abundant plant polysaccharides during fire and their subsequent deposition on the cave wallAZM and JMR acknowledge the support from the Marie Curie Fellowships within the 7th European Community Framework Programme (Grants PIEF-GA-2012-328689-DECAVE and PCIG12-GA2012-333784-Biocharisma respectively). The authors acknowledge the Spanish Ministry of Economy and Competitiveness (project CGL2013-41674-P) and FEDER funds for financial support.Peer Reviewe