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:
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). Liberated H2S was precipitated quantitatively in Zn acetate traps and measured spectrophotometrically (Cline, 1969). The residue was washed, dried, and weighed and analyzed for CNS contents by elemental analysis using a Fisons elemental analyzer. This fraction represents the sum of SORG and SRES. The Cr(II) residue was then tempered in a porcelain cruicible for several hours at 550°C to remove OM, weighed, and again analyzed for CNS (Böttcher and Schnetger, 2004). This fraction is considered to mainly represent residual barite sulfur. The SORG-I fraction was calculated from the difference of the two sulfur fractions. Additionally, the organic sulfur fraction was calculated from the difference of total sulfur and the sum of chromium-reducible sulfur and the sulfur content of the tempered Cr residue:
Organic sulfur results from both approaches agree well (Fig. F3). Still-reactive iron (FeD) was extracted from sediment samples using a buffered solution of Na dithionite (Canfield, 1989), which removes iron (oxyhydr)oxide phases (ferrihydrite, goethite, lepidocrocite, and hematite) but only small amounts of iron from silicates (Canfield, 1989; Haese, 2000). The iron concentration was determined spectrophotometrically (at 562 nm) with ferrozine in N-2-hydroxyethylpiperazine-N´-2-ethanesulfonic acid (HEPES) buffer at pH 7 (Stookey, 1970). The amount of pyrite iron (FeP) was calculated from the content of SP. The highly reactive iron fraction FeHR is calculated as the sum of extractable and pyrite iron:
The range of geochemical results in the Fe-S-C system for Unit IV sediments is summarized in Table T2. Finally, the iron fraction extractable by cold 0.5-M HCl (FeHCl) was determined in all samples (data not shown). Maximum dry weight contents of FeHCl are 140 mg/kg (Unit V, Site 1257), 70 mg/kg (Unit I, Site 1258), 199 mg/kg (Unit V, Site 1259), 83 mg/kg (Unit I, Site 1260), and 302 mg/kg (Unit I, Site 1261). FeHCl and FeD contents are positively correlated. Scanning electron microprobe analysis of gold-coated nonground sediment samples was carried out using a Hitachi S-3200N scanning electron microscope (SEM).