Sulfur-iron-carbon geochemistry in sediments of the Demerara Rise

Abstract

The geochemical composition of sediments (squeeze cake samples) from five drill sites (Ocean Drilling Program Sites 1257–1261) on the Demerara Rise in the tropical Atlantic was determined, with special regard to a sequence of Cretaceous black shales. Sediments were analyzed for different iron (total, pyrite, Na dithionite, and HCl leachable) and sulfur (total, pyrite, acid volatile, and organic bound) fractions, in addition to total organic carbon (TOC) and total inorganic carbon. The relative abundance of highly reactive iron (FeHR/FeT) in the investigated black shale samples indicates that pyrite was formed both in the water column and the sediment. This corresponds to euxinic paleoenvironmental conditions, a situation similar to the modern deep Black Sea. This geochemical approach is independent of a possible minor contribution from ongoing sulfate reduction which is triggered by anaerobic methane oxidation above the black shale sequence. Pyrite sulfur in black shales makes up between 30% and 100% of total sulfur. In addition to fixation of sulfide with iron, organic matter (OM) acted as an important sulfur trap during early diagenesis, with organic sulfur composing between 5 and 10 atom% of TOC. The relative importance of OM sulfurization is increasing with its content. INTRODUCTION 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 con1Bottcher, M.E., Hetzel, A., Brumsack, H.-J., and Schipper, A., 2006. Sulfuriron-carbon geochemistry in sediments of the Demerara Rise. In Mosher, D.C., Erbacher, J., and Malone, M.J. (Eds.), Proc. ODP, Sci. Results, 207: College Station, TX (Ocean Drilling Program), 1–23. doi:10.2973/odp.proc.sr.207.108.2006 2Department of Biogeochemistry, Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, D-28359 Bremen, Germany. 3Present address: Marine Geochemistry-Geology, Leibniz Institute for Baltic Sea Research, Seestrasse 15, D-18119 Rostock, Germany. [e-mail] 4Institute of Chemistry and Biology of the Marine Environment, Carl-vonOssietzky University of Oldenburg, PO Box 2503, D-26111 Oldenburg, Germany. Initial receipt: 20 July 2005 Acceptance: 30 December 2005 Web publication: 17 November 2006 Ms 207SR-108 M.E. BOTTCHER ET AL. SULFUR-IRON-CARBON GEOCHEMISTRY IN SEDIMENTS 2 siderable 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; Bottcher 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 (Bottcher 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 (Bottcher et al., unpubl. data). M.E. BOTTCHER ET AL. SULFUR-IRON-CARBON GEOCHEMISTRY IN SEDIMENTS 3 MATERIALS AND METHODS During Leg 207, sediments on the Demerara Rise were cored at ~9°N in the tropical Atlantic (Fig. F1; Table T1). The rise stretches ~380 km along the coast of Suriname and reaches a width of ~220 km from the shelf break to the northeastern escarpment, where water depths increase sharply from 1000 to >4500 m. Although most of the plateau lies in shallow water (700 m), the northwest margin is a gentle ramp that reaches water depths of 3000–4000 m. Nearly uniform, shallowly buried stratigraphically expanded sections of Cretaceous and Paleogene age exist with good stratigraphic control. Five drill sites (Sites 1257–1261) constitute a depth transect ranging in water depths from 1900 m to 3200 m (Fig. F1). The recovered sediments include multiple sequences of Cretaceous black shales (Erbacher, Mosher, Malone, et al., 2004; Erbacher et al., 2005) pointing to varying levels of bottom water dysoxia and/or enhanced surface water productivity. Five units were identified: Unit I: consisting of modern, Pleistocene, and Pliocene sediments. Unit II: consisting of Oligocene and Eocene sediments. Unit III: consisting of late Paleocene–Campanian sediments. Unit IV: consisting of Santonian–Cenomanian black shales. Unit V: consisting of Albian sediments. Interstitial waters from 152 samples from Sites 1257–1261, covering a depth range from the sediment/seawater interface to 648 meters composite depth (mcd), were collected and processed using standard ODP methods. Interstitial water samples were squeezed from sediment samples immediately after retrieval of the cores using titanium squeezers, modified after the standard ODP stainless steel squeezer (Manheim and Sayles, 1974). Results for dissolved species relevant to the present study are summarized in Figure F2. On board the ship, splits of all squeeze cakes were taken, freeze-dried, and stored in polyethylene bags. In the shore-based laboratory, the samples were ground and homogenized in an agate mill. X-ray fluorescence analysis for main elements (Philips PW 2400 X-ray spectrometer) using fused glass beads were conducted as described by Schnetger et al. (2000). Detailed results are presented in Hetzel et al. (this volume); the present communication only refers to the total iron (FeT) measurements. Total sulfur (ST) and total carbon (TC) were analyzed using a LECO SC-444 infrared analyzer for squeeze cake samples. Total inorganic carbon (TIC) was determined coulometrically using a UIC CM 5012 CO2 coulometer coupled to a CM 5130 acidification module. Total organic carbon (TOC) was calculated as the difference between TC and TIC (e.g., Babu et al., 1999). Different sedimentary sulfur fractions, acid volatile sulfur (SAVS), chromium-reducible sulfur (SP, essentially pyrite), OM (essentially kerogen)-bound organic sulfur (SORG), and residual sulfur (SRES) were separated quantitatively on freeze-dried powdered samples. SAVS was obtained using anaerobic distillation with 6-M HCl (1 hr). Because FeS is not expected to survive the diagenetic pyritization and laboratory-based freeze-drying process in the black shale samples, the SAVS fraction is assumed to dominantly represent water column– derived ZnS and/or CuS (Brumsack, 1980). SAVS contents (data not shown) in the investigated black shale samples are <270 mg/kg. These results will be discussed in the light of trace element enrichments in more detail in a later contribution. Pyrite sulfur, SP, was extracted using hot acidic Cr(II)Cl2 (2 hr) (Zhabina and Volkov, 1978; Canfield et al., 1986). Site 1258