Analytical pyrolysis (Py-GC/MS) and Py compounds specific isotopic analysis (Py-CSIA) as proxy for the characterization of coralloid speleothems from lava tubes

Galardonado por el comité científico de dicho congreso con el premio a la mejor contribución científica.Lava tubes are a type of volcanic caves formed when lava flows down along the slopes after the eruption of the magma. They develop as a hard crust on the surface of the basaltic lava flow due to rapid cooling, while inside the lava remains liquid. Secondary mineral deposits, or speleothems, mainly of siliceous composition have been reported on the walls and ceiling of many lava tubes [1, 2]. However, lava tubes have traditionally been considered of little interest from a mineralogical point of view and the studies devoted to those secondary mineral deposits are scarce. Cave corals or coralloid-like speleothems are found either on the basalt surface or on the surface of other speleothems. Hill and Forti [3] suggested that subaerial cave corals are generated by capillary-film water, whereas other authors attribute a microbially mediated origin to their formation [2]. Siliceous minerals are sometimes intercalated with carbonate minerals [4] due to the change in the chemical composition of supplying water, environmental changes of the surface area overlying caves and probable diagenesis. However, most of the literature devoted to cave corals merely describes its geology, morphology and mineralogy. The application of chromatographic techniques to the study of speleothems is scarce due to their low content of organic compounds and low volatility. This study comprises an innovative approach based in the combination of chromatographic and isotopic analyses, which were applied to characterize coralloid speleothems from the Ana Heva lava tube in Easter Island (Chile). Analytical pyrolysis combines GC/MS with pyrolysis and has become an important tool for the characterization of sediments [5]. In addition, this study takes advantage of Py-CSIA, a relatively novel technique in which the pyrolysate is directed through a combustion into an isotope ratio mass spectrometer (Py-GC-C/TC-IRMS) to measure stable isotope proportions i.e., ¿13C, ¿15N respectively of specific pyrolysis compounds. Figure 1. A) Coralloids, B) corresponding cross-section and C) Py-GC/MS at 500ºC of the three coralloid layers from Ana Heva lava tube. Pyrochromatograms of the three different layers of this coralloid speleothem were composed by alkyl benzenes, naphthalenes, n-alkanes and a few N-containing compounds, which are typical compounds from plant remains and microorganisms. Variations in ¿13C values suggested changes in water surplus. They suggest wetter conditions during the formation of the Ca-layer and a possible increase in the amount of incoming fluids from the covering andosol. The trend observed for ¿ 15N values could indicate an increase on the average temperature along the time, which is consistent with the so-called climate warming during the Holocene.Dr. J.M. de la Rosa and A.Z. Miller thank the ¿Biocharisma¿ (PCIG12-GA-2012-333784) and ¿DECAVE¿ (PIEF-GA-2012-328689) projects, respectively, funded by the Marie Curie Actions of the EC 7th Framework Programme. The Spanish Ministry of Economy and Competitiveness (project CGL2013-41674-P) and the European Social Funds are acknowledged for the financial support.Peer reviewe