Occurrence of photosynthetic microbial mats in a LowerCretaceous black shale (central Italy): a shallow-water deposit

摘要

Cretaceous oceanic anoxic events (OAEs) wereperiods of high organic carbon burial corresponding tointervals with excellent organic matter (OM) preservation.This work focuses on the Urbino level, i.e., OAElb, whichis thought to be of regional extent. A detailed microscopicalstudy of OM shows a dominance of microbial activity,characterized by a typical arrangement of exopolymericsubstances (EPS) related to microbial mats, bacterial bod-ies, and some photosynthetic microorganisms, as shown bythylakoids. The latter lived where they have been found,i.e., at the sea bottom, which indicates that OM results fromthe diagenesis of benthic photosynthetic microbial mats, aninterpretation supported through the comparison with arecent analogue. The exceptional preservation of suchorganic structures in OM points to the joint role of theselective and sorptive preservation pathways. These dataand interpretation strongly differ from previous observa-tions in OAE 1 b equivalents. They suggest that the Urbinolevel might be an atypical OAE of regional/local extentwhich was formed within the photic zone.