A Comparison Study on the δD Value of Polycyclic Aromatic Hydrocarbons in Hydrous Pyrolysate of Herbaceous Peat with Different Water Medium

Abstract

To investigate the influence of diagenetic water media on the hydrogen isotopes of individualsedimentary aromatic compounds, a series of hydrous pyrolyses were conducted on herbaceous peat. Polycyclic aromatic hydrocarbons (PAHs) in hydrous pyrolysed samples and their hydrogen isotopic composition characteristics were studied. The aqueous medium demonstrated a significant influence on the hydrogen isotopic composition of the individual PAHs generated during pyrolysis. The results showed that the PAHs formed after pyrolysis in the presence of a saltwater medium with high δD value from a salt lake had a heavy hydrogen isotopic composition. The PAHs formed after pyrolysis in the presence of a freshwater medium with low δD value from a swamp had a light hydrogen isotopic composition. The difference in the average PAH δD value between the two hydrous experiments varied from ‐109 to ‐174‰, suggesting that the hydrogen isotopic composition of individual sedimentary PAHs can reflect the source of the diagenetic water medium. In addition, acomparative study found that the hydrogen isotopes of PAHs were superior to those of n‐alkanes in the same sample for diagenetic water indications. The results indicated that the exchange of water‐derived inorganic hydrogen and organic hydrogen was more intensive in freshwater experiments than in saltwater experiments. With an increase in the simulation temperature, the average δD value of PAHs generated in the hydrous simulation experiments showed an increasing trend, reflecting that the δD value of sedimentary PAHs formed with the participation of diagenetic water media was still closely related to the thermal maturity of organic matter. Comparative studies showed that the δD values of different types of organic compounds produced by hydrous pyrolysis of peat were in the order, PAHs > n‐alkanes > methane.