Thermal stability of freshwater fern Azolla biomarkers as assessed by hydrous pyrolysis

Abstract

The freshwater fern Azolla produces a unique series of ω20-hydroxy and ω9,ω10-dihydroxy lipids. These compounds were also detected in Eocene sediments (∼48.5 Ma) characterized by the occurrence of fossil remains of Azolla and are thought to be prime markers for the past occurrence of Azolla implying that they could serve as palaeo-environmental indicators of freshwater conditions. However, the suitability of these Azolla biomarkers for application to more mature sediments is unknown. In this study, Azolla caroliniana specimens were heated using hydrous pyrolysis at temperatures ranging from 220 to 365 °C for 72 h to examine the stability and degradation of the Azolla biomarkers during simulated diagenesis. Initially all ω20-hydroxy and ω9,ω10-dihydroxy lipids are converted to their ketone analogues. At 260 °C n-alkanes are formed. ω20-Hydroxy and ω9,ω10-dihydroxy lipids are degraded between 260 and 280 °C. At 300 °C, only ω20-keto-1-ols, ω20-keto fatty acids and ω20-ketones, along with their ω9 and ω10 analogues, remain to preserve their ω20, ω9 and ω10 functionalities, while at 320 °C and higher only the ω20-, ω9, and ω10-ketones were observed. At higher temperatures the proportions of n-alkanes became dominating. The high thermal stability of the ω20, ω9/ω10 functionality shows that it has a relatively high preservation potential. Hence, it may potentially be a robust source indicator of the depositional environment of Azolla-derived organic carbon reflecting freshwater or brackish surface waters.