Assessing paleoclimatic changes archived in speleothems from volcanic Caves by pyrolysis gas chromatography‐based analyses

Abstract

Speleothems, or secondary mineral deposits found in caves, are formed due to dissolution of primary minerals from the host rock and subsequent precipitation. Their formation is greatly prompted by water‐rock interactions and climate conditions, which dictate how much water drip into the cave system [1]. In humid climates or during heavy precipitation a relatively rapid speleothem growth may occur, whereas in arid climates or during drought the growth is moderated or ceased. Hence, the composition, abundance and growth pattern of speleothems may be indicative of climate changes, as reported by several authors [1‐2]. This study comprises a multidisciplinary approach based in the combination of chromatography (analytical pyrolysis and pyrolysis compound‐specific isotope analysis) and stable isotope analysis for characterizing organic compounds entrapped in speleothems from volcanic caves. Samples collected in lava tubes were selected for this study: (i) coralloid speleothems rrom Easter Island (Chile), and (ii) moonmilk deposits from La Palma (Canary Islands, Spain). The aim was to recognize environmental changes during speleothem formation. The coralloid speleothems from Easter Island consisted of three major layers with different mineralogical composition and a significant contribution of organic carbon. Analytical pyrolysis (Py‐GC/MS) revealed contributions from higher plants and microorganisms to the organic matter entrapped in the coralloid speleothems. Biogeochemical analyses based on isotopic signatures and pyrolysis compound‐specific isotope analysis (Py‐CSIA) revealed that the genesis of the three colored layers was related to two different stages of speleothem formation caused by environmental changes on Easter Island. Variations in δ13C values pointed to wetter conditions during the formation of the innermost layer and a water shortage during the latest stage of speleothem formation. The trend observed for δ15N values suggested an increase in theaverage temperature over time, which is consistent with the so‐called climate warming during the Holocene [3]. The chromatograms of the moonmilk deposits from Canary Islands evidenced the presence of organic compounds in their composition, in particular fatty acids, polysaccharides, phytosterols and oleanane‐type triterpenes. Most of them have been previously found in sedimentary records. Oleananes and specific steranes are believed to derive from the early diagenesis of gymnosperms [4]. Our data suggest that the organic compounds associated with moonmilk deposits are partially driven by the topsoil and vegetation overlying the cave system. Hence, Py‐GC/MS and Py‐CSIA could be used as a valid proxy for paleoenvironmental research.

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