Concentrations of calcium carbonate were measured, on average, from three samples selected from each core in Hole 900A. Concentrations of organic carbon, C/N ratios, and Rock-Eval pyrolysis were employed to determine the type of organic matter contained within the upper sediment units. Routine monitoring of headspace gas con tents, done for drilling safety, yielded information that is interesting to compare with Sites 897, 898, and 899.
Concentrations of carbonate carbon vary between 9.5% and essentially 0% in sediments from Site 900 (Table 9). These concentrations of carbonate carbon are equivalent to 78% to 0% CaCO3 in the sediments, assuming that all of the carbonate is present as pure calcite. The variability in carbonate content reflects a history of generally low biological productivity and deposition of hemipelagic sediments below the CCD, combined with delivery of carbonate-rich turbiditic sediments initially deposited in shallower waters.
Concentrations of organic carbon were measured in a subset of sediment samples from the upper part of Hole 900A. The absence of significant amounts of headspace methane in Site 900 sediments suggested that organic carbon concentrations would be low, and therefore few organic carbon measurements were performed. Indeed, concentrations were low in nearly all of the Site 900 samples (Table 10). Unit I, a sequence dominated by Pleistocene to lower Pliocene turbidites, averages 0.3% organic carbon. This average is approximately the same as the average of 0.2% calculated from DSDP Legs 1 through 31 by McIver (1975). The equivalent lithological unit at Sites 897 and 898 contained 0.5% to 0.6% organic carbon (see "Site 897" and "Site 898" chapters, this volume). The two principal sources of organic matter in oceanic sediments are marine algal production and land plant detritus supplied by rivers and winds. Algal organic matter is typically oxidized and largely recycled during and shortly after settling to the seafloor (e.g., Suess, 1980; Emerson and Hedges, 1988). The land-derived organic matter that is delivered to deep-sea sediments is generally the less-reactive material that survives trans port to the ocean. The organic carbon found in Unit I at Site 900 evidently has been substantially oxidized prior to sedimentation and, consequently, is not very reactive.
Organic C/N ratios were measured for selected Site 900 samples to determine the source of the organic matter. Algal organic matter generally has C/N ratios of between 5 and 10, whereas organic matter derived from land plants has values between 20 and 100 (e.g., Emerson and Hedges, 1988; Meyers, in press). Variable C/N ratios of samples from Unit I (Table 10) indicate that some samples have a predominantly marine source for their organic matter, whereas other samples contain mostly terrigenous organic matter. The C/N ratios of some samples are low (<5). These values are probably an artifact of the low carbon contents, combined with the tendency of clay minerals to absorb ammonium ions generated during the degradation of organic matter (Müller, 1977). The C/N ratios in samples that are especially low in organic carbon consequently are not accurate indicators of the source of organic matter.
Concentrations of headspace methane were monotonously low throughout Site 900 (Table 11). These low concentrations contrast with the high levels of biogenic methane found in the upper, turbiditic units at Sites 897 and 898, but are similar to the low amounts present at Site 899 (see "Site 897," "Site 898," and "Site 899" chapters, this volume). The generally low amounts, and inferred inert character, of organic matter in sediments from Site 900 evidently preclude methanogenesis. In addition, Claypool and Kvenvolden (1983) observed that the presence of interstitial sulfate inhibits methanogenesis in marine sediments, and concentrations of sulfate are high throughout the recovered Site 900 sediments (see "Inorganic Geochemistry" section, this chapter).
Rock-Eval pyrolysis of organic matter from selected Unit I samples from Sites 897, 898, 899, and 900 provided insights into the differences in concentrations of headspace gas at these locations. Two Rock-Eval parameters are especially useful for characterizing sedimentary organic matter. The hydrogen index (HI) is the quantity of hydrocarbons generated from thermal decomposition of the organic matter, expressed as milligrams of hydrocarbons per gram of total organic carbon. Marine organic matter typically has high HI values (Espitalié et al., 1977). The oxygen index (OI) is the quantity of CO2 generated during pyrolysis and is given in the same units. Cellulose containing land plants produce organic matter having high OI and low HI values (Espitalié et al., 1977). Organic matter in Unit I samples from all four sites has high OI and low HI values (Table 12), which would normally indicate a land-derived origin. Relatively low C/N ratios, however, contradict this interpretation. It is likely that the organic matter in this turbiditic sedimentary unit has experienced considerable post-depositional oxidation, which would depress HI values while enhancing OI values. This evidence of alteration in the Rock-Eval source character implies that considerable microbial degradation of the marine organic matter in Unit I has occurred, which is consistent with an inferred history of downslope relocation of the Unit I sediments from a shallower site of initial accumulation. Sediments from Sites 899 and 900 have lower S1, S2, and HI values than those from Sites 897 and 898 and have been degraded to greater degrees. This difference in organic matter character in closely spaced locations on the Iberia Abyssal Plain implies different deliveries of turbidite components at the sites. Furthermore, the greater amount of degradation evident at Sites 899 and 900 appears to render the organic matter less supportive of methanogenic bacteria. The generally low Tmax values (Table 10) suggest that thermal degradation of the organic matter can be excluded; the large range of Tmax (309°-441°C) probably reflects heterogeneous mixtures of relatively fresh marine and detrital organic matter in the turbidites.