BayCEER Workshop 2020

Tracing Sources of Organic Matter in the Atacama Desert using Polycyclic Aromatic Hydrocarbons

The Atacama Desert is coated in dust, harbouring a broad inventory of traces of life. Until now, it could not be clarified whether these organic molecules are derived by past pluvial phases or organic carbon in the desert stems from external sources. To evaluate the impact of allochthonous combustion processes, we analysed polycyclic aromatic hydrocarbons (PAHs) in topsoils and dust particles of two transects in the eastern (Aroma) and western (Paposo) parts of the Atacama Desert. In detail, we aimed at i) identifying PAHs in topsoils and dust particles of the desert, ii) quantifying how much of the OC fraction is composed of PAHs, and iii) find out what kind of combustion processes causes the accrual of PAHs in desert soil. PAHs were indeed abundant in all samples. Thereby, total PAH concentration ranged from 0.001 to 0.31 ng g^-1 in Paposo and from 0.31 to 0.62 ng g^-1 in Aroma with a moderate correlation between OC and particle-phase PAH, pointing to the external input of soil organic matter. The amount of PAHs in dust particles was even higher and varied between 0.45 and 5.31 ng g^-1. The ratio of Fl/(Fl+Py) reveals that while vehicle emissions play a crucial role in the western Atacama, biomass burning is the key factor for PAH accrual in the eastern Atacama. Contrary to PAHs in soil samples, dust samples exhibited a different trend with larger pyrene and fluoranthene proportions, followed by phenanthrene, methyl-phenanthrene, and minor contributions of high molecular weight PAHs. We conclude that PAHs are detectable in dust and topsoils of the desert, with sources that are variable, either stemming from fossil fuel combustion (western Atacama) or biomass fires (eastern Atacama). Our findings confirm that PAHs are deposited in soil from atmospheric deposition, which shows that the inventory of organic matter is variable over time. Hence, traces of life are not always representative of the habitat they are found today, making a complete source assignment more complex and possibly leads to misinterpretations.