The paleoenvironmental conditions during organic matter (OM)-rich black shale formation have been an important scientific issue in the field of global and regional biogeochemical element cycling for a considerable period of time (e.g., Gauthier, 1987; Arthur et al., 1988; Arthur and Sageman, 1994; Brumsack, 2006, and references therein) but are still far from being completely understood. A number of different geochemical approaches have been applied to approach these questions, including trace element, biomarker, and stable isotope studies. As examples of more recent analogs for OM-rich sediment deposition, the formation of sapropels in the Black Sea and the eastern Mediterranean has been investigated in detail (e.g., Brumsack, 1986; Calvert et al., 1996; Lyons, 1997; Arthur and Dean, 1998; Emeis et al., 2000; Rinna et al., 2002; Lourens et al., 2001; Brumsack and Wehausen, 1999; Böttcher et al., 2003).
Accumulation of OM in sediments is often associated with the enrichment of sulfur and iron. The systematics behind the combined (bio)geochemistry of sulfur, iron, and organic carbon have been evaluated for the modern Black Sea (e.g., Leventhal, 1983; Arthur and Dean, 1998; Canfield et al., 1996; Raiswell and Canfield, 1998; Anderson and Raiswell, 2004) and successfully applied by analogy to the ancient depositional environments of OM-rich sediments (e.g., Dean and Arthur, 1989; Raiswell et al., 2001; Shen et al., 2003; Grice et al., 2005). Interpretation of ancient black shales is often complicated because of the modification by deeper burial and associated geochemical overprints. Close to the Earth's surface, modification of the geochemical composition of black shale can take place by weathering that may be induced by flow of rain and ground water (Petsch et al., 2000, 2001, 2005). Black shale sequences as well as corresponding pore water gradients obtained by deep sea drilling, on the other hand, have seldom been analyzed at a resolution sufficient for a detailed interpretation of past environmental change and possible diagenetic overprints. First analyses of pore waters associated with frequent sapropel layers from the Mediterranean gave no indication for a contribution of OM-rich zones to the shapes of present pore water profiles (Böttcher et al., 1998, 2003).
Widespread black shale formation took place during the global ocean anoxic events of the Cretaceous period (e.g., Schlanger and Jenkyns, 1976; Jenkyns, 1980), the causes still being a matter of intense debate (e.g., Arthur and Sageman, 1994; Arthur et al., 1988; Brumsack, 1986; Sinninghe Damstae and Koester, 1998). In the present study, we carried out a detailed geochemical investigation on Cretaceous black shale samples from the southern North Atlantic not previously affected by surface weathering. Expanded, shallowly buried Cretaceous sediments were recovered during Ocean Drilling Program (ODP) Leg 207 from the Demerara Rise off Suriname, South America, including multiple sequences of Cretaceous black shales. By means of a solid phase geochemical approach we aimed to characterize the sulfur-iron-carbon (S-Fe-C) systematics of these sediments and their use as indicators for the depositional paleoenvironment. Results are compared to the composition of the overlying younger organic-poor sediments. This communication is accompanied by reports on the bulk inorganic geochemistry including trace element contents (Hetzel et al., this volume), a high-resolution geochemistry study of Cretaceous black shales (Hetzel et al., unpubl. data), and the biogeochemistry of stable sulfur and oxygen isotope fractionation in pore waters and authigenic sulfur phases (Böttcher et al., unpubl. data).