Samples of Pliocene-Pleistocene sediments were collected for organic-matter analysis at frequencies of two to five per 9-m core at each of the six sites investigated during Leg 161 (Fig. 1). The amount and type of organic matter were determined both in the organic-carbon-lean sediments that constituted most of the sequences found at these sites and in the dark-colored, organic-carbon-rich sapropels and sapropel-like layers that were minor parts of the sequences. The six sites encompass a variety of water depths and sedimentation rates, and they represent the major depositional basins of the western Mediterranean Sea.
Site 974 is located at a water depth of 3454 m in the central Tyrrhenian Basin (Fig. 1). Pliocene-Pleistocene sediments consist of bioturbated pelagic to hemipelagic nannofossil-rich clays and nannofossil oozes that were deposited above the carbonate compensation depth (CCD). Twenty-eight sapropel layers occur in this sequence. The average sedimentation rate of Pliocene-Pleistocene sediments at this site is 5 cm/k.y.
Site 975 is situated on the Menorca Rise between the Balearic Basin to the east and the Algerian Basin to the south (Fig. 1). The water depth at this site is 2116 m, which is above the CCD. Pliocene-Pleistocene sediments consist of bioturbated pelagic to hemipelagic nannofossil clays and oozes. Thirty-eight sapropel layers occur in the upper part of this sequence. The average sedimentation rate at this site is 7 cm/k.y.
Site 976 is located on the southern Iberian continental slope at a water depth of 1108 m in the Western Alboran Basin. Pliocene-Pleistocene sediments consist of hemipelagic nannofossil-rich clay, nannofossil clay, and nannofossil silty clay and contain 28 sapropel-like layers. The average sedimentation rate at this site is 23 cm/k.y.
Sites 977 and 978 are in the Eastern Alboran Basin. Site 977 is located south of the Al-Mansour Seamount at a water depth of 1984 m, and Site 978 is north of this seamount at a depth of 1929 m (Fig. 1). Sediments at Site 978 were continuously cored below 213 mbsf (meters below seafloor), but only spot-cored at shallower depths. Pliocene-Pleistocene sediments recovered at both sites consist of bioturbated hemipelagic nannofossil clay and nannofossil-rich silty clay. Thirty-nine sapropel layers occur in the Site 977 Pliocene-Pleistocene sediments, and the sedimentation rate for this sequence is 16 cm/k.y.
Site 979 is situated on the southern margin of the Alboran Ridge in the South Alboran Basin (Fig. 1). The water depth at this site is 1062 m. Pliocene-Pleistocene sediments consist of bioturbated hemipelagic nannofossil clay, which accumulated at an average rate of 21 cm/k.y. Twenty-nine sapropel-like layers occur in the Pliocene-Pleistocene sequence at this site.
The total organic-carbon contents (TOC) of Leg 161 samples were determined 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 1000°C 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 values of the samples. These C/N values are assumed to represent those of sediment organic matter in this report.
This procedure, in which TOC concentrations are determined from the difference between total carbon and carbonate-carbon concentrations, has been tested against the direct-determination analysis of carbonate-free sediment samples by Meyers and Silliman (1996). Agreement is good for TOC concentrations ranging between 0.2% and 2% and for C/N values between 5 and 15. Both procedures, however, cease to measure accurately TOC concentrations that are below 0.1%, and this level is considered the limit of detection in this report.
Rock-Eval pyrolysis of organic matter consists of heating samples at a rate of 25°C/min between 300°C to 600°C 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 390°C (S3). These values are combined with TOC values to provide the information necessary to calculate the hydrogen index (HI = 100 × S2/TOC, or milligrams hydrocarbons/gram organic carbon) and the oxygen index (OI = 100 × S3/TOC, or milligrams CO2/gram organic carbon). The temperature of maximum hydrocarbon release during pyrolysis (Tmax) is also obtained and provides a measure of organic matter thermal maturity (Espitalié et al., 1977).