Analytical pyrolysis and stable isotope analyses reveal past environmental changes in coralloid speleothems from Easter Island (Chile)

9 páginas.-- 3 figuras.-- 2 tablas.-- 83 referencias.-- Selected paper from the XVI Latin-American Congress on Chromatography (XVICOLACRO) and the 9th National Meeting on Chromatography (9ENC), 5–9 January 2016, Lisbon, Portugal.
This study comprises an innovative approach based on the combination of chromatography (analytical pyrolysis and pyrolysis compound-specific isotope analysis (Py-CSIA)), light stable isotopes, microscopy and mineralogy analyses to characterize the internal layering of coralloid speleothems from the Ana Heva lava tube in Easter Island (Chile). This multidisciplinary proxy showed that the speleothems consist of banded siliceous materials of low crystallinity with different mineralogical compositions and a significant contribution of organic carbon. Opal-A constitutes the outermost grey layer of the coralloids, whereas calcite and amorphous Mg hydrate silicate are the major components of the inner whitish and honey-brown layers, respectively. The differences found in the mineralogical, elemental, molecular and isotopic composition of these distinct coloured layers are related to environmental changes during speleothem development. Stable isotopes and analytical pyrolysis suggested alterations in the water regime, pointing to wetter conditions during the formation of the Ca-rich layer and a possible increase in the amount of water dripping into the cave. The trend observed for δN values suggested an increase in the average temperature over time, which is consistent with the so-called climate warming during the Holocene. The pyrolysis compound-specific isotope analysis of each speleothem layer showed a similar trend with the bulk δC values pointing to the appropriateness of direct Py-CSIA in paleoenvironmental studies. The δC values for n-alkanes reinforced the occurrence of a drastic environmental change, indicating that the outermost Opal layer was developed under drier and more arid environmental conditions.
A.Z. Miller acknowledges the support from the Marie Curie Intra-European Fellowship of the European Commission’s 7th Framework Programme (PIEF-GA-2012-328689). The Spanish Ministry of Economy and Competitiveness (MINECO, project CGL2013-41674-P) and the FEDER Funds are acknowledged for the financial support. N.T. Jiménez-Morillo thanks the MINECO for funding his pre-doctoral FPI fellowship (BES-2013-062573).