Concentrations of calcium carbonate and organic carbon were measured in samples obtained regularly at selected intervals from Holes 897A, 897C, and 897D. Results of organic-matter atomic C/N ratios and pyrolysis were employed to determine the type of organic matter contained within the sediments. Routine monitoring of head-space gas contents (done for drilling safety) yielded interesting information about the evolution and migration of biogenic gas in these passive margin sediments.
Concentrations of carbonate carbon vary between a high of 9.4% to essentially 0% in sediments and rocks from Site 897 (Table 12). These concentrations of carbonate carbon are equivalent to 78% to 0% CaCO3 in the sediment, assuming that all of the carbonate is present as pure calcite. Lithologic Unit III is uniformly low in carbonate carbon; the other three units are highly variable. This variability 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 are relatively high in several parts of the Site 897 lithologic column (Table 12). Unit I, a Pleistocene to upper Pliocene turbidite-containing sequence, averages 0.6% organic carbon, and Unit IV, a Cretaceous sandstone-claystone-clast sequence, averages 0.9% (Fig. 55). Both units contain substantially more organic carbon than the average of 0.2% that was calculated from DSDP Legs 1 through 31 by McIver (1975). 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 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 generally is the less-reactive material that remains after transport to the ocean. Consequently, the elevated organic carbon concentrations found in Units I and IV result from special depositional conditions. Both units are dominated by sediments 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 897 samples was determined two ways: by organic C/N ratios and GHM pyrolysis. 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). C/N ratios for samples from Unit I averaged 6.7 (Table 12; Fig. 56), suggesting a predominantly marine source for the organic matter in these sediments. The C/N values of samples from Units II and III are highly variable, largely because the low concentrations of organic carbon and nitrogen in these units introduced large uncertainties in the measurement. Many samples that contain little organic carbon have relatively high C/N ratios; others have very low ratios (<5). The latter probably are an artifact of the low carbon contents, combined with the tendency of clay minerals to absorb ammonium ions that were generated during the degradation of organic matter (Mller, 1977). Consequently, the extreme C/N ratios in Units II and III are not accurate indicators of the source of organic matter. Samples from Unit IV are relatively rich in organic carbon and have elevated C/N ratios that average 23.5, indicating that the organic matter in this unit is mostly derived from land plant detritus.
Pyrolysis of samples rich in organic carbon from Units I and IV provided verification of the characterizations of the source of organic carbon inferred from the C/N ratios. Unit I samples yielded small S1 peaks, but substantial S2 peaks, as expected from thermally immature marine organic matter. The small S1 peaks imply that the organic matter has experienced considerable microbial degradation, which is consistent with an inferred history of downslope transport from a shallower site of initial sedimentation. Samples from Unit IV differed significantly from those in Unit I: these completely lack S1 peaks and have small-to-absent S2 peaks. The organic matter in this unit is virtually geochemically inert and, evidently, has undergone extensive degradation prior to sedimentation. This characteristic is consistent with a continental source.
Concentrations of headspace methane measured in Hole 897C are displayed in Figure 57. These were high in Unit I, reaching values of more than 100,000 ppm before decreasing to near-background levels in Units II, III, and IV. C1/C2 ratios were high in Unit I (Table 13). 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 897 on a basement high makes this an especially reasonable possibility. Evidence of migration of methane into porous sediments from deeper sources was found at Sites 762 and 763 on the Exmouth Plateau (Snowdon and Meyers, 1992). In the latter case, however, a known source existed in underlying Jurassic rocks; a suitable deeper source for the methane at Site 897 is unknown. A second 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 12; Fig. 55), which is prone to microbial utilization. High C1/C2 ratios indicate that the gas is biogenic, as opposed to thermogenic (Table 13). The source of the methane may be 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), although at a shallower depth (100 mbsf) than at Site 897 (292 mbsf). The generally low amounts of organic matter in Units II and III and its inert character in Unit IV evidently preclude methanogenesis. Furthermore, Claypool and Kvenvolden (1983) observed that the presence of pore-water sulfate inhibits methanogenesis in marine sediments, and concentrations of sulfate are anomalously high in Units II and III (see "Inorganic Geochemistry" section, this chapter). Consequently, head-space methane concentrations are low in sediments below Unit I.