Perylene preservation in an oxidizing paleoenvironment and its limitation as a redox proxy

Abstract Sedimentary rocks from the Eocene Shahejie Formation including the third (Es3) and fourth (Es4) members, in the Dongying Depression, Bohai Bay Basin, NE China, have been analyzed to determine bulk and molecular geochemical characteristics. Sedimentary rocks from the Es4 Member contained no or trace amounts of perylene, but C32–C35 17α(H),21β(H) homohopanes were well preserved along with concomitant high concentrations of β-carotane and phytane, suggesting reducing depositional conditions. However, some samples, mostly from the Es3 Member contained abnormally high concentrations of perylene with high dibenzofuran and 9H-fluoren-9-one consistent with oxidizing environments. On the other hand, indications from the relatively high pristane/phytane (Pr/Ph) values, the absence of C32–C35 homohopanes and β-carotane are not consistent with reducing conditions in these sedimentary rocks, indicating the perylene of the investigated samples was deposited in oxidizing conditions. In combination with the sample locations in the depression, bulk geochemical characterization including low total organic carbon (TOC) contents and poor kerogen quality, and a series of molecular geochemical records, perylene has a high probability of being preserved by rapid deposition under oxidizing environments. As the conversion of quinones to perylene must ultimately involve a reduction stage, perylene preservation under oxidizing conditions seems to be formed by alternative precursors during oxidation, such as by cyclodehydrogenation of 1,1′-binaphthyl. Hence, caution should be taken when perylene is used as an indicator of strongly reducing environments.