Isorenieratene derivatives in sediments: Possible controls on their distribution

Diagenetic products of the green sulfur bacterial pigment isorenieratene ha ve been studied in sediments from the Jurassic Kimmeridge Clay Formation an d in a Pliocene Mediterranean sapropel to better understand their formation . A large number of previously unknown derivatives have been identified, in cluding one possessing a naphthyl moiety, whose structure was determined by isolation and nuclear magnetic resonance studies. Fully cyclized isorenier atene derivatives are abundant in sediments from both environments. They ar e formed by four-ring closure reactions of the polyene system of double bon ds in isorenieratene. Molecular mechanics calculations indicate that these reactions are thermodynamically favorable. Fully cyclized isorenieratene de rivatives are subsequently transformed by double-bond isomerization. reduct ion and dehydrogenation reactions into species that are thermodynamically m ore stable. These reactions can take place during the first 50 m of sedimen t burial. Our findings indicate that a complex pathway of diagenetic reacti ons determines the fate of isorenieratene, leading to a complex distributio n of derivatives controlled by numerous factors, which are presently only p artly understood. Isorenieratane, the most widely used isorenieratene deriv ative in paleoenvironmental studies, typically only represents 15% of the m ajor derivatives. Aryl isoprenoids are only trace components in sediments w ith thermal maturities below the oil window, limiting their use as markers for photic zone euxinia in the immature zone. Copyright (C) 2001 Elsevier S cience Ltd.