GEOCHEMISTRY OF ANOXIC MARINE SEDIMENTS OF LATE-MIOCENE AGE: VENA DEL GESSO BASIN, NORTHERN APENNINES, ITALY

The Vena del Gesso basin in the Northern Apennines, Italy contains an excellent sedimentary record of changing paleo-oceanographic conditions during the Mediterranean salinity crisis. A 10.3 m of section of organic rich pelites of Lower Messinian age intercalated with carbonates is exposed in the Monticino quarry, Brisighella, Northern Apennines. Current depositional models suggest that the organic-rich pelites (euxinic shales) were deposited under restricted marine conditions initiated as a response to the Mediterranean Salinity crisis whereas the carbonate intercalations were precipitated due to changing paleo-bathymetric conditions.

δ¹³C (down to -35 per mil PDB) analysis of authigenic carbonates indicate biogenic methane as a possible carbon source for the carbonates. Presence of framboidal pyrite, microbial biofilms and abundance of archaeal and bacterial biomarkers in the carbonates suggests biogenic origin for the carbonates deposited in an anoxic setting. Differences in δ¹⁸O (up to 4 per mil VPDB) and δ¹³C (up to 6 per mil PDB) values between benthic (Uvigerina, Bulimina) and planktonic (Orbulina) foraminifera indicates intense stratification of the Vena del Gesso basin. Relationship of “redox sensitive” trace elements (V, Zn) to total organic carbon (TOC) in the shales provides insights into paleo-oceanic conditions.

An array of factors like basin restriction during Mediterranean salinity crisis, decreased circulation, increased organic productivity and periodic dissociation of methane hydrates played an important role in basin anoxicity. Vena del Gesso basin provides a perfect paradigm for understanding similar anoxic settings and help comprehend oceanic anoxic events (OAE's) in the paleo-record.