During Leg 207, sediments were recovered from five Sites (1257–1261) on Demerara Rise, located at ~9°N in the western tropical Atlantic (Fig. F1). 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. Whereas most of the plateau lies in shallow water (700 m), the northwestern margin forms a gentle ramp reaching water depths of 3000–4000 m. Nearly uniform, shallowly buried sections of Cretaceous and Paleogene age were drilled with good stratigraphic control. The five drill sites (Sites 1257–1261) constitute a depth transect with water depths ranging from 1900 to 3200 m. The sediments recovered can be divided into three broad styles of deposition: synrift clastics (lithologic Unit V), organic matter–rich, laminated black shales (Unit IV), and open-marine chalk and calcareous claystones (Units III–I) (Erbacher, Mosher, Malone, et al., 2004).
On board, interstitial waters from 152 samples from Sites 1257–1261 and covering a depth range from the sediment/seawater interface to 648 meters composite depth 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). On board the ship, splits of all squeeze cakes (pore water squeezing residues) were taken, freeze-dried, and stored in polyethylene bags. In the home laboratory, the squeeze cake samples were ground and homogenized in an agate ball mill. The powdered samples were used for all subsequent geochemical analyses. The classification of lithologic units follows the scheme given by Erbacher, Mosher, Malone, et al. (2004).
Total sulfur (TS) and total carbon (TC) were analyzed using a LECO SC-444 infrared analyzer. 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. For detailed information on precision and accuracy of the methods applied see Prakash Babu et al. (1999).
For X-ray fluorescence (XRF) analysis, 600 mg of sample was mixed with 3600 mg of a 1:1 mixture of dilithium tetraborate (Li2B2O7) and lithium metaborate (LiBO2), preoxidized at 500°C with NH4NO3, and fused into glass beads. The glass beads were analyzed by XRF (Philips PW 2400) calibrated with 29 carefully selected geostandards. Analytical precision was >2% for major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and >6% for minor elements (As, Ba, Co, Cr, Cu, Mo, Ni, Rb, Sr, V, Y, Zn, and Zr), except for Pb and U (6%–10%).
Samples from Site 1258 were also analyzed by inductively coupled–plasma mass spectrometry (ICP-MS). For acid digestion, 50 mg of sample was preoxidized with 1 mL of HNO3 (65%) in polytetrafluoroethylene (PTFE) vessels overnight and heated with 3 mL of HF (40%) and 3 mL of HClO4 (70%) in closed PTFE autoclaves (PDS-6) for 6 hr at 180°C. The acids were then evaporated on hot plates at 180°C to incipient dryness. Afterward, 3 mL of 6-N HCl aliquots were added and evaporated at 180°C. This step was repeated three times. The wet precipitate was dissolved in 1 mL of HNO3 (65%), diluted to ~10 mL, and simmered at 60°C for 1 hr. The acid digestions were brought up to 50 mL final volume with deionized water. Only acids purified by subboiling distillation (HNO3, HCl, and HClO4) or of suprapure (HF) quality were used. Rare earth element (REE) analyses were carried out using an Element (Finnigan MAT, Germany) high-resolution (HR)-ICP-MS. Analytical precision as checked by multiple analyses of international reference materials (see "Appendix;" Table AT1) is >7%.