Depositional Settings

The sedimentary sequences at the six drill sites provide a composite late Miocene to present history of the upwelling system that currently extends from the mid-Namib margin to north of Walvis Ridge. The drill sites (Fig. F1) are seaward of the main upwelling core, but they contain second-order signals of the enhanced marine productivity associated with the upwelling that are transported to their respective locations by the Benguela Current and its meanders and eddies (cf. Lutjeharms and Stockton, 1987). The cored locations (Table T1) and their sedimentary sequences are briefly described here in the south-north direction of the current flow.

Site 1087 (3128S, 1519E) is in 1372 m of water on South Africa slope. Its sediments are greenish gray nannofossil oozes with varying amounts of foraminifers and clay. Site 1085 (2922S, 1359E) is at a water depth of 1713 m on Namibia slope. Sediments at this site are greenish gray foraminifer-bearing nannofossil oozes. Site 1084 (2531S, 1302E) is in 1992 m of water in the northern landward part of Cape Basin. The sediments contain alternations of bioturbated olive-gray to black clays rich in nannofossils and diatoms that overlie an olive-gray clayey nannofossil ooze. Site 1082 (2106S, 1149E; water depth 1279 m) is in Walvis Basin on the southern flank of Walvis Ridge. Its sediments also exhibit alternations of olive-gray to black bioturbated clays rich in nannofossils and diatoms. Sites 532 and 1081 are on the continental shoulder of Walvis Ridge. Site 532 (1945S, 1031E) is on a ridge terrace in 1331 m of water. Sediments here consist of alternations of olive-to-brown nannofossil ooze with varying amounts of foraminifers, diatoms, and clay. Site 1081 (1937S, 1119E) is more landward of Site 532 in water 794 m deep. Site 1081 sediments have alternations of olive-gray to black clays with varying amounts of nannofossils and diatoms that overlie an olive-gray clayey nannofossil ooze.

Inorganic and Organic Carbon
Measurements and C/N Ratios

The total organic carbon (TOC) contents of sediment samples from Sites 1081, 1082, 1084, and 1085 were determined onboard the JOIDES Resolution by the difference between total carbon concentrations as measured by a Carlo Erba NA 1500 NCS analyzer (Verardo et al., 1990) and the carbonate-carbon concentrations as measured by a Coulometrics 5011 inorganic carbon analyzer (Engleman et al., 1985). Freeze-dried samples were combusted at 1000C in an oxygen atmosphere in the NCS analyzer, and the resulting combustion products were chromatographically separated and quantified to yield the concentrations of total carbon and nitrogen. The TOC and total nitrogen values were used to calculate atomic C/N ratios of the samples. These C/N ratios are assumed to represent those of sediment organic matter in this report.

Carbon analyses of sediments from Site 1087 were done onshore (Meyers and Robinson, Chap. 2, this volume) and those from Site 532 were done onboard the Glomar Challenger by procedures different from JOIDES Resolution shipboard procedures. For samples from both sites, calcium carbonate concentrations were determined using the carbonate bomb technique of Mller and Gastner (1971), which yields results comparable to the coulometrics procedure (P. Meyers, unpubl. data). Weighed samples were reacted with 3-N HCl, and the volume of CO2 released from each sample was measured and compared to the volumes released from known amounts of pure CaCO3 to determine the percentage in the sample. The carbonate-free residue remaining after acid treatment was collected, rinsed, and dried. Concentrations of TOC and total residual nitrogen (TN) in the carbonate-free residues were directly measured with a CHNS analyzer. This procedure involves heating the carbonate-bomb residue at 1020C and measuring the combustion products by gas chromatography (e.g., Verardo et al., 1990). TOC concentrations are reported on a whole-sediment basis, adjusting for the carbonate concentrations determined from the bomb technique. Atomic C/N ratios have been calculated from the TOC and TN concentrations.

Meyers and Silliman (1996) compared the results of the direct TOC procedure with those calculated by difference. The direct procedure is more reliable at TOC concentrations <0.1 wt%, but agreement between the two procedures is good above this value. Because most of the sediments from beneath the Benguela Current upwelling system are rich in organic matter, the results of the two procedures are considered equivalent.

Rock-Eval Pyrolysis

Rock-Eval pyrolysis of organic matter consists of heating samples at a rate of 25C/min between 300 and 600C to yield the amount of volatile hydrocarbons present in the sediment (S1), the amount of hydrocarbons released during pyrolysis (S2), and the amount of CO2 released during heating to 390C (S3). These values are combined with TOC values to provide the information necessary to calculate the hydrogen index (HI = 100 x S2/TOC in milligrams hydrocarbon per gram of TOC) and the oxygen index (OI = 100 x S3/TOC in milligrams CO2 per gram of TOC). The temperature of maximum hydrocarbon release during pyrolysis (Tmax) is also obtained and provides a measure of organic matter thermal maturity (Espitali et al., 1977).