Improved analysis of polycyclic aromatic hydrocarbons (PAHs) in speleothems

Speleothems are valuable paleoenvironmental archives that can grow continuously over thousands of years and are particularly favorable due to reliable dating using the ²³⁰Th/U-method. As the caves in which these mineral deposits are formed are largely closed systems and the speleothems resist chemical alteration, it is possible to analyse organic substances in addition to established proxies such as stable isotopes and trace elements. [1] The analysis of organic markers has only recently come into focus but shows promising results for the reconstruction of past climatic conditions, especially regarding vegetation changes and paleofires.  [2]

Polycyclic Aromatic Hydrocarbons (PAHs) are ubiquitous organic compounds consisting of one or more fused aromatic rings. They are formed, for example, during the incomplete combustion of biomass and enter caves with the infiltrating water, where they are incorporated into speleothems. [3] Alongside other proxies such as levoglucosan (combustion product of cellulose), they can provide information about past vegetation and fire activity.

As only a small fraction of PAHs reaches the speleothems, suitable sample preparation methods are required to maximize the amount of analyte while keeping the required amount of sample low. We present a rapid and easy extraction method using an ultrasonic bath and the addition of keepers to reduce the loss of low molecular weight PAHs like naphthalene during the evaporation process.

The developed method, as well as a new and fast GC-HRMS method, was applied to a particularly old flowstone from the Conturines Cave in northern Italy, dating back to the Pliocene. 

[1] A. Blyth et al. Quat. Sci. Rev. 149 (2016) 1-17. [2] J. Homann et al. Biogeosciences. 20 (2023) 3249–3260. [3] Y.Sun et al. Chemosphere 230 (2019) 616–627.