Concentrations of calcium carbonate and organic carbon were measured in samples obtained regularly from Holes 898 A and 898B. Organic matter atomic C/N ratios and Rock-Eval pyrolysis results were employed to determine the type of organic matter contained within the sediments. Routine monitoring of headspace gas contents, done for drilling safety, yielded interesting information about the formation and migration of biogenic methane in these passive margin sediments.
Concentrations of carbonate carbon vary between a high of 9.6% to essentially zero in sediments from Site 898 (Table 6). These carbonate carbon concentrations are equivalent to 80% 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 that initially had been deposited in shallower waters.
Concentrations of organic carbon are relatively elevated in the upper part of the Site 898 lithologic column (Table 6). Lithologic Unit I, a Pleistocene to lower Pliocene turbidite-containing sequence, averages nearly 0.5% organic carbon (Fig. 19). This average is significantly greater than the average of 0.2% calculated from DSDP Legs 1 through 31 by McIver (1975). The two principal sources of organic matter in oceanic sediments are from production of marine algal and land plant detritus supplied by rivers and winds. Algal organic matter typically is 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 transport to the ocean. Consequently, the elevated concentrations of organic carbon found in lithologic Unit I result from special depositional conditions. The unit is dominated by sediments that were displaced from shallower locations (see "Lithostratigraphy" section, this chapter), and downslope transport and rapid burial participated in delivering and preserving the organic matter.
The source of organic matter in Site 898 samples was determined by either organic C/N ratios or by Rock-Eval pyrolysis. Algal organic matter generally has C/N ratios of between 5 and 10, whereas organic matter derived from land plants has values of between 20 and 100 (e.g., Emerson and Hedges, 1988; Meyers, in press). C/N ratios for samples from lithologic Unit I average 8.2 (Table 6; Fig. 20), which suggests a predominantly marine source for the organic matter in these sediments. Some Unit I samples, however, have values above 15, that reflect the probable admixture of terrigenous organic matter. The C/N values of most samples from Unit II have low ratios (<5). These values probably are 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 (Muller, 1977). Thus, the C/N ratios in most of Unit II are not accurate indicators of organic matter source.
Rock-Eval pyrolysis of selected samples from Unit I provided further information about their organic carbon sources. 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 has high OI and low HI values (Table 7), which normally would indicate a land-derived origin. The relatively low C/N ratios, however, contradict this interpretation. It is likely that the organic matter has experienced considerable post-depositional oxidation, which would depress HI values while enhancing OI values. This evidence of alteration of 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. The generally low Tmax values (Table 7) suggest that thermal degradation of the organic matter can be excluded; the large range (301°-441°C) observed in this parameter probably reflects heterogeneous mixtures of relatively fresh marine organic matter and detrital organic matter in the turbidites.
Concentrations of headspace methane measured in Hole 898A are displayed in Figure 21. Concentrations of methane were high in Unit I, reaching values of nearly 50,000 ppm, before decreasing to near-background levels in Unit II (Table 8). This pattern is similar to the concentrations of headspace methane measured in sediments from Site 897 (see "Site 897" chapter, this volume). Two sources of the gas in Unit I are possible. First, gas from some deeper origin may have migrated into the unit, which consists of turbiditic sand, silt, and clay layers. The location of Site 898 on a basement high makes this an especially reasonable possibility. Evidence of diffusional migration of methane into porous sediments from deeper sources was found at Sites 762 and 763 on the Exmouth Plateau passive margin of northwestern Australia (Snowdon and Meyers, 1992). In the case of Sites 762 and 763, however, a known gas source existed in underlying Jurassic rocks; a suitable deeper source for the methane at Site 898 is presently unknown. Migration of methane into the Unit I turbidite sequence, consequently, is not a strong possibility.
A second, more likely possibility is in-situ formation by methanogenic bacteria. The high values occurred in the upper Pliocene sediments having elevated concentrations of marine organic matter (Table 6; Fig. 19), which is prone to microbial utilization. The absence of measurable amounts of C2 and heavier gases indicates that the gas is biogenic, as opposed to thermogenic. The source of the methane probably comes from in-situ microbial fermentation of the marine organic matter present in this turbiditic unit. Similar in-situ microbial production of methane from marine organic matter has been inferred from high biogenic gas concentrations in Pliocene-Pleistocene sediments from Site 532 on the Walvis Ridge (Meyers and Brassell, 1985) and in middle Miocene sediments from Site 767 in the Celebes Sea (Shipboard Scientific Party, 1990). The generally low amounts of organic matter and its inferred inert character in Unit II evidently preclude methanogenesis. Furthermore, Claypool and Kvenvolden (1983) observed that the presence of pore-water sulfate inhibits methanogenesis in marine sediments. High concentrations of headspace methane were not found in Site 898 sediments where pore-water sulfate concentrations were high within the upper part of Unit I and in Unit II (see "Inorganic Geochemistry" section, this chapter). This inverse relationship strengthens the possibility of the presence of active methanogenic bacterial populations in sediments as old as early Pliocene at this location.