Isorenieratene derivatives in sediments: possible controls on their distribution

Abstract

Diagenetic products of the green sulfur bacterial pigment isorenieratene have been studied in sediments from the Jurassic Kimmeridge Clay Formation and in a Pliocene Mediterranean sapropel to better understand their formation. A large number of previously unknown derivatives have been identified, including one possessing a naphthyl moiety, whose structure was determined by isolation and nuclear magnetic resonance studies. Fully cyclized isorenieratene derivatives are abundant in sediments from both environments. They are formed by four-ring closure reactions of the polyene system of double bonds in isorenieratene. Molecular mechanics calculations indicate that these reactions are thermodynamically favorable. Fully cyclized isorenieratene derivatives are subsequently transformed by double-bond isomerization, reduction and dehydrogenation reactions into species that are thermodynamically more stable. These reactions can take place during the first 50 m of sediment burial. Our findings indicate that a complex pathway of diagenetic reactions determines the fate of isorenieratene, leading to a complex distribution of derivatives controlled by numerous factors, which are presently only partly understood. Isorenieratane, the most widely used isorenieratene derivative in paleoenvironmental studies, typically only represents 25% of the major derivatives. Aryl isoprenoids are only trace components in sediments with thermal maturities below the oil window, limiting their use as markers for photic zone euxinia in the immature zone.