The oldest sediments recovered during Leg 171B were lower Albian and upper Aptian calcareous claystone and chalk containing a prominent ~40-cm-thick band of organic shale (Barker et al., 2001). Planktonic foraminifer biostratigraphic correlation suggests that the black shale horizon correlates with a series of marine sapropels deposited during Oceanic Anoxic Event (OAE) 1b in Tethys (Bellier et al., Chap. 3, this volume). Investigations of benthic foraminifer assemblages (Erbacher et al., 1999; Holburn and Kuhnt, 2001; Holbourn et al., in press) and stable isotope stratigraphy (Erbacher et al., 2001) suggest that the black shale recovered on Blake Nose represents a sapropel analogous to those formed during the Pliocene–Pleistocene in the Mediterranean Sea. Stable isotope data suggest that the black shale formed when the overlying water column became well stratified, perhaps by reduced salinities in the surface waters, and restricted ventilation of the ocean floor (Erbacher et al., 2001). Biomarker studies of organic carbon in the black shale have identified the molecular fingerprints of green sulfur bacteria that thrive where anoxic waters reach into the photic zone (J.S. Sinninghe Damsté, pers. comm., 2000). Cyclostratigraphic studies of OAE 1b on Blake Nose suggest that the black shale deposition occurred over ~46 k.y. (Erbacher et al., 2001).
During the Pliocene–Pleistocene, most sapropels in the Mediterranean basins formed when low-salinity surface waters prevented overturning and deep-water ventilation for periods of a few thousand years. The OAE 1b sapropel on Blake Nose apparently persisted an order of magnitude longer and did so within a much larger and less restricted basin. The biomarker evidence and organic geochemical studies of the type and maturity of organic carbon suggest that the North Atlantic was anoxic below the surface mixed layer (Barker et al., 2001). Erbacher et al. (2001) have suggested that the OAE 1b black shale represents a "super-sapropel" formed when sills to the North Atlantic shoaled and patterns of continental runoff contributed to salinity stratification throughout the basin. Extreme stratification was maintained in the western North Atlantic for only a fraction of the time that the eastern basins of the North Atlantic and Tethys remained stratified because equivalent sections in the Vacontian Basin of southern France and northern Italy are represented by a succession of black shales.