Coniacian-Santonian deep ocean anoxia/euxinia inferred from molecular and inorganic markers: Results from the Demerara Rise (ODP Leg 207)

We investigated Coniacian to early Santonian sediments from ODP site 1261, Demerara Rise, using organic, inorganic and molecular methods to reconstruct the development of ocean water anoxia/euxinia. High lycopane/n-C31 values of up to 4.5 suggest oxygen-depleted conditions in the water column for most of the time. Lower values (∼ 1) across the early to mid-Coniacian transition probably mark a period of stronger oxygenation. In two higher resolution intervals we observe strong enrichment in Zn, concurrent with an increase in the lycopane/n-C31 ratio, marking the onset of euxinic bottom waters, whereas decreases in lycopane/n-C31 values and decreases in Ni enrichment characterize the termination of sulfidic conditions. Photic zone euxinia appears to be restricted to the early Coniacian interval, as isorenieratene derivatives were exclusively detected in this part of the section. The combination of biomarker and inorganic geochemical proxies suggests a general decoupling of surface and mid-water oxygenation from deep water oxygenation from the mid-Coniacian onwards. This process might be related to the successive opening of the Equatorial Atlantic Gateway, allowing a migration of surface and intermediate water masses before the establishment of a deep water connection.