SAMPLING AND ANALYSIS

Samples and Setting

Seismic surveys performed in preparation for Leg 149 drilling identified a series of seaward-dipping sedimentary reflectors on the landward edge of the Iberia Abyssal Plain (Fig. 1). A transect of four sites drilled during Leg 149 revealed that these seismic reflectors represent a succession of Neogene-to-Quaternary turbidite layers (Table 1).

Sediment samples from the Unit I turbiditic sequences at Sites 897, 898, 899, and 900 were employed to compare the results of the shipboard and shorebased analytical schemes. Samples were selected to represent the variety of lithologies present in the turbidites. They were freeze-dried and ground prior to analysis. The two procedures for organic carbon concentrations and organic C/N ratios were performed on subsamples of the same samples. Organic carbon isotope and Rock-Eval pyrolysis analyses were done on subsets of these samples.

Organic Carbon Concentrations

Two procedures were used to determine the total organic carbon contents (TOC) of Leg 149 samples. Shipboard TOC analyses employed the difference between total carbon concentrations as measured by a Carlo Erba NA 1500 NCS analyzer and the carbonate carbon concentrations as measured by a Coulometrics 5011 inorganic carbon analyzer (Engleman et al., 1985). Shorebased TOC analyses involved first removing carbonate carbon from samples by treatment with 3N HCl, washing and drying the Carbonate-free residue, and then measuring the carbon content of the residue with a Carlo Erba 1108 CHNS analyzer. In both procedures, freeze-dried samples were combusted at 1000°C in an oxygen atmosphere in the Carlo Erba elemental analyzer, and the resulting combustion products were chromatographically separated and quantified to yield the concentrations of carbon and nitrogen (Verardo et al., 1990). These values were also used to calculate atomic C/N ratios of sediment organic matter.

C/N ratios help to distinguish between algal and land-plant origins of sedimentary organic matter. Algae typically have atomic C/N ratios between 4 and 10, whereas vascular land plants have C/N ratios of 20 and greater (Premuzic et al., 1982; Jasper and Gagosian, 1990; Meyers, 1994; Prahl et al., 1994). This distinction arises from the absence of cellulose in algae and its abundance in vascular plants and is largely preserved in sedimentary organic matter.

TOC concentrations are reported on a whole-sediment basis. As observed by Arthur et al. (1987), reliable and consistent CaCO3 measurements are critical to using the whole-sediment basis. The coulometric carbonate carbon analysis procedure used by ODP is well-tested (Engleman et al., 1985). In addition, we have tested the coulometrics procedure against the DSDP-standard carbonate bomb procedure of Müller and Gastner (1971) and found the results to agree very well (Meyers, unpubl. data). Finally, the TOC concentrations determined by the two procedures are converted to whole-sediment percentages using the same CaCO3 values for each sample, thereby eliminating this source of possible variability.

Organic Carbon Isotope Ratios

Organic carbon isotope measurements of Leg 149 samples were done in the Stable Isotope Laboratory at The University of Michigan. Organic carbon 13C/12C ratios of the Carbonate-free residues used for shorebased TOC analyses were measured after reacting the samples with CuO in evacuated and sealed quartz tubes for 3 hours at 800°C. The CO2 produced by oxidation of the organic matter was analyzed with a Finnigan Delta S mass spectrometer. National Bureau of Standards carbon isotope standards were routinely used to calibrate the instruments. Results are reported relative to the PDB standard.

Organic carbon isotopic ratios are useful to distinguish between marine and continental plant sources of sedimentary organic matter. Most photosynthetic plants incorporate carbon into organic matter using the C3 Calvin pathway which biochemically discriminates against 13C to produce a 13C shift of about -20 from the isotope ratio of the inorganic carbon source. Organic matter produced from atmospheric CO2 (13C -7‰) by land plants using the C3 pathway consequently has an average 13C (PDB) value of ~-27 (cf. O'Leary, 1988). The source of inorganic carbon for marine algae is dissolved bicarbonate, which has a 13C value of ~0. Marine organic matter consequently typically has 13C values between -20 and -22. The 7‰ difference between organic matter produced by C3 land plants and marine algae has been used to trace the delivery and distribution of organic matter to sediments of ocean margins (Newman et al., 1973; Prahl et al., 1994). Carbon isotope ratios can be affected, however, by photosynthetic dynamics and by post-depositional diagenesis (Dean et al., 1986; McArthur et al., 1992) and consequently must be interpreted cautiously. Prominent among these are the availability of CO2 during photosynthesis and the possibility of selective diagenesis of organic matter fractions that are isotopically heavy or light. Any diagenetic isotope shift appears to be small, less than 2 (Hayes et al., 1989; Fontugne and Calvert, 1992; McArthur et al., 1992; Meyers, 1994). Increased availability of dissolved CO2 to algae, however, would enhance their isotopic discrimination and produce marine organic matter that is isotopically light (Hayes et al., 1989), as would increased delivery of isotopically light fluvial dissolved inorganic carbon (Fontugne and Calvert, 1992).

Rock-Eval Pyrolysis

Rock-Eval analyses were done onboard the JOIDES Resolution using a Girdel II instrument. 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 (S1), the amount of thermogenic hydrocarbons (S2), and the amount of CO2 released during pyrolysis 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 mg hydrocarbons/g organic carbon) and the oxygen index (OI = 100 × S3/TOC, or mg CO2/g organic carbon). The temperature of peak hydrocarbon release during pyrolysis (Tmax) is also obtained and provides a measure of organic matter thermal maturity (Espitalié et al., 1977).

Rock-Eval pyrolysis was originally developed to characterize the organic matter present in oil source rocks, which typically contain more thermally mature organic matter that is at higher concentrations than commonly found during scientific ocean drilling. Rock-Eval analyses have nonetheless proved valuable in helping to determine organic matter sources in DSDP and ODP samples. Land-plant organic matter is usually rich in woody components and consequently has lower hydrogen indices and higher oxygen indices than found in lipid-rich and cellulose-poor algal organic matter. This distinction between organic matter from continental and marine sources becomes blurred by diagenesis as marine matter oxidizes and gradually takes on HI and OI values similar to those of land-plant material.

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