Concentrations of calcium carbonate and organic carbon were measured in samples obtained regularly from Holes 899A and 899B.
Atomic C/N ratios of organic matter were employed to determine the type of organic matter contained within the sediments. Routine monitoring of headspace gas contents, done for drilling safety, yielded information that is interesting to contrast with results from Sites 897 and 898.
In sediments from Site 899, concentrations of carbonate carbon vary between a high of 9.3% to essentially 0% (Table 14), equivalent to 77% to 0% CaCO3, respectively, 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 on a subset of sediment samples from the upper part of Hole 899A, plus one sample from near the bottom of Hole 899B. The absence of significant amounts of headspace methane in Site 899 sediments suggested that organic carbon concentrations would be low and, therefore, few organic carbon measurements were performed. Indeed, concentrations are low in nearly all of the Site 899 samples (Table 14). Lithologic Unit I, a sequence dominated by late to early 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 lithologic 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 the transport to the ocean. The organic carbon found in Unit I at Site 899 evidently had been substantially oxidized prior to sedimentation and, consequently, is not very reactive.
Organic C/N ratios were measured in selected Site 899 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 and the uppermost part of Unit II (Table 15) indicate that some samples have a predominantly marine source for their organic matter, whereas other samples contain mostly terrigenous organic matter. The C/N values of some samples are low (<5). These values 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 (Müller, 1977). Consequently, the C/N ratios in samples especially low in organic carbon are not accurate indicators of organic matter source.
The sample selected from near the bottom of Unit III (149-899B-15R-2, 70-71 cm) was a red brown Eocene claystone that contained burrows filled with black claystone. The organic carbon content of this sample is 0.31%, and the organic C/N ratio is 6.2 (Table 15). The black infilling material evidently is rich in marine organic matter that has been diluted by the red brown claystone matrix in this sample.
Concentrations of headspace methane were low throughout Site 899 (Table 16). These low concentrations contrast with the high levels of biogenic methane found in the upper, turbiditic units at Sites 897 and 898 (see "Site 897" and "Site 898" chapters, this volume). It is possible that the uncored upper 80 m of sediment at Site 899 contained elevated headspace gas concentrations, but this remains unknown. The generally low amounts of organic matter, and its inferred inert character in sediments recovered from Site 899, evidently preclude methanogenesis. Shallower sediments may have similarly been low in organic matter and, therefore, low in methane concentrations. In addition, Claypool and Kvenvolden (1983) observed that the presence of interstitial sulfate inhibits methanogenesis in marine sediments, and sulfate concentrations were high throughout the recovered Site 899 sediments (see "Inorganic Geochemistry" section, this chapter).