Exploring organic biomarkers in lava tube gypsum deposits using analytical pyrolysis

Speleothems are recognized as valuable archives of past environmental conditions. However, speleothems and sediments within lava tubes have remained largely unexplored until recently. In this study, we used a multiproxy approach involving mineralogical, elemental, isotopic, and analytical pyrolysis techniques to investigate the origin, source, and dynamics of organic matter in a layered gypsum deposit from Paso Esqueleto Lava Tube at Timanfaya National Park in Lanzarote, Canary Islands, Spain. Significant differences in the composition and distribution of organic matter were observed between gypsum samples and the surface soil overlying the lava tube. The surface soil was characterized by high levels of total organic carbon (TOC) and total nitrogen (TN), consistent with an andosol enriched by surface vegetation and microbial biomass inputs. Analytical pyrolysis revealed a diverse array of lipid compounds, predominantly n-alkane/alkene pairs, confirming contributions from both plant and microbial sources. Variations in elemental, isotopic, and molecular composition across gypsum layers indicated contrasting stages of organic matter transformation. The upper layers contained recent organic matter inputs likely transported from the surface, while the lower layers were dominated by older, more degraded surface-derived organic matter, indicative of advanced microbial reworking or diagenetic alterations over time. These findings provide a comprehensive understanding of the biogeochemical dynamics within lava tubes, illustrating the interplay between surface-derived organic inputs and in-situ microbial activities in subterranean gypsum deposits.