Aliphatic and aromatic biomarkers from Gondwanan sediments of Late Ordovician to Early Devonian age: An early terrestrialization approach

摘要

Twenty one core samples of Late Ordovician to Early Devonian age from sections in southern Tunisia, North African Platform (Gondwana) and containing marine and terrestrial organic matter with microbial input were investigated to link the aliphatic and aromatic hydrocarbon distributions with the terrestrial and marine palynomorph content (e.g. acritarchs, prasinophytes, chitinozoans, cryptospores and trilete spores). In addition to several biomarkers of algal/bacterial origin, long chain n-alkanes that might be derived from land plants, as well as the terrestrial diterpane, norabietane, were found. Several land plant-derived biomarkers, such as retene, cadalene, simonellite, tetrahydroretene and C19 isohexylalkyl naphthalene were observed in the aromatic fractions. While these terrestrial biomarkers could be clearly recognized in the middle Silurian-lower Devonian samples, their presence in the upper Ordovician-lower Silurian sediments is more doubtful, because of much lower relative abundances. The land plant biomarkers show a fairly good correlation with the occurrence and abundance of cryptospores and trilete spores, derived from bryophytes and tracheophytes, which covered the emerged areas around the Ghadamis Basin during the Silurian and Devonian. The early tracheophytes (e.g. Cooksonia, lycophytes and zosterophylls) are therefore suggested as a new terrestrial source for most of the saturated and aromatic biomarkers found in sediments of Middle Silurian to Early Devonian age.