Organic carbon–rich black shales are found extensively in widely different Cretaceous marine settings. Although globally distributed, the black shales are restricted to specific intervals during the Aptian–Santonian ages (see review in Arthur et al., 1990). The mid-Cretaceous is characterized by a coincidence of intensified volcanism, increased oceanic crust production, elevated sea level, and high paleotemperatures that peaked in the early Turonian (Larson, 1991a, 1991b). The concept of global oceanic anoxic events (OAEs) (Schlanger and Jenkyns, 1976) was proposed to explain times of enhanced organic carbon burial associated with the genesis of widespread marine black shales as at the Cenomanian/Turonian (C/T) boundary (OAE 2). The OAEs reflect periods of biotic crisis, likely linked to a limited oxygen supply in already oxygen-depleted warm oceans and epeiric seas. The oxygen restriction in these marine settings could have been reinforced either by an increase in primary organic matter production related to transgressions (Schlanger and Jenkyns, 1976; Jenkyns, 1980), by the onset of density stratification following a decrease in oceanic circulation (e.g., Ryan and Cita, 1977; Fischer and Arthur, 1977), or by a combination of both (Arthur and Schlanger, 1979; Stein, 1986).
During Leg 207, 56- to 93-m-thick intervals of lower Albian–Santonian organic matter–rich black shales were recovered from five different sites. All these sites are located on the northwestern slope of the Demerara Rise, a northwest-southeast–oriented prominent submarine plateau, ~380 km long, offshore Suriname and French Guyana (see the "Leg 207 Summary" chapter for more comprehensive information). The rock samples included in this study were taken from cores at Sites 1257 and 1258, representing, respectively, the second deepest (2951 meters below sea level [mbsl]) and the deepest (3192 mbsl) locations with regard to water depth.
Site 1257 was drilled to a total depth of 285 meters below seafloor (mbsf) on a terrace above the steep northern flank of the Demerara Rise at the same location as Deep Sea Drilling Project (DSDP) Site 144. A 57-m-thick interval of Cenomanian–Santonian black shales was recovered. These laminated sediments (lithostratigraphic Unit IV) have high total organic carbon (TOC) concentrations (up to 15.8 wt%; average = 6.6 wt%) and were deposited at an estimated sedimentation rate of 0.5 cm/k.y. (see "Lithostratigraphy," "Organic Geochemistry," and "Sedimentation Rates," all in the "Site 1257" chapter).
Site 1258 was drilled ~43 km west of Site 1257 on the gently dipping northwestern slope of the Demerara Rise to a total depth of 485 mbsf. The 56-m-thick black shale interval at Site 1258 (Unit IV) consists of an expanded Cenomanian section overlain by Turonian sediments that are truncated by an erosional unconformity. The sediments of Unit IV have average TOC = 7.9 wt%, but vary between 0.1 and 28.3 wt% (estimated sedimentation rate = 0.3 cm/k.y.) (see "Lithostratigraphy," "Organic Geochemistry," and "Sedimentation Rates," all in the "Site 1258" chapter). The lower to mid-Albian sediments (Unit V) at Site 1258 contain up to 5 wt% TOC and represent an atypical clay-rich facies different from all other sites, where Albian deposits are normally characterized by organic matter–lean siliciclastics (see "Leg 207 Summary" chapter).
The aim of our investigation focused on establishing biomarker-based evidence for the biological sources and the maturity of the organic matter and indicators of its depositional environment, especially preservation.