Biomarker evidence for intermittent photic zone euxinia in the Aptian-Albian organic sedimentary record from the Ionian Zone (Epirus, Greece)

摘要

New biomarker evidence is presented that constrains the conditions of deposition and preservation of sediments rich in extractable organic matter from the Aptian-Albian sedimentary record of the Ionian Zone in NW Greece. Analyses were specifically carried out on total lipid extracts from 12 organic rich samples from the Vigla Shale Member, two of which represent the regional record for the early Albian Paquier Event of OAE1b. In the aliphatic fractions, compound groups such as n-alkanes, isoprenoid alkanes, steroids, hopanoids and terpenoids were recognized, pointing to a mixed origin from algae and prokaryotes with an additional measurable contribution from terrigenous plant matter. These compounds suggest deposition of the primary organic sediments under conditions of decreased bottom water oxygen concentrations. Mass spectral evidence from the aromatic fractions of most samples reveal the variable presence of diagenetic and catagenetic derivatives of carotenoid pigments (including some with bound sulfur) that are characteristic of photoautotrophic sulfur bacteria. The presence of specific biomarkers originating from photosynthetic anaerobic microorganisms, similar to those in ancient and contemporary euxinic basins such as the Black Sea, suggest that photic zone euxinic conditions were intermittently operative in the Aptian-Albian stages of the Ionian Basin, but were not exclusively characteristic of the black shale representing the Paquier Event itself.