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.