ORGANIC GEOCHEMISTRY

Headspace gas analysis was conducted as part of the standard protocols required for shipboard safety and pollution prevention monitoring. A total of 38 cores from Hole 1208A were evaluated (Table T6). The concentrations of CH₄ were low (10 µL/L [ppmv]), and no hydrocarbon gases higher than C₁ were detected. The downhole profile of CH₄ concentration shows a subsurface maximum in gas concentrations from ~50 to 200 mbsf in Subunit IA that is associated with a minimum in sulfate concentrations. At shallower depths (upper 52 mbsf), CH₄ generation increases exponentially as sulfate is depleted, a trend exemplified by the strong inverse correlation (R² = 0.959) between CH₄ and sulfate concentrations (Fig. F27). At greater depths (below 239 mbsf), CH₄ concentrations decrease toward background levels, whereas sulfate concentrations remain at 21-23 mM.

Significant production of CH₄ by methanogenesis generally does not occur until sulfate is depleted by bacterial sulfate reduction (Martens and Berner, 1974; Claypool and Kvenvolden, 1983). Exceptions to this principle have been noted recently in carbonate sediments (Mitterer et al., 2001), providing further convincing evidence that the two microbial processes are not necessarily exclusive (Oremland and Taylor, 1978; Oremland et al., 1982). At Site 1208 the coincidence of the peak in CH₄ concentrations with the minimum in sulfate concentrations is consistent with co-occurring sulfate reduction and methanogenesis, albeit at subsistence levels.

Examination of data from analyses of hydrocarbon gases and interstitial waters at other sites reveals other occurrences of coincident CH₄ generation and sulfate depletion in carbonate sequences. The depth trend relationships at Site 1208 show marked similarities to Sites 846 and 849 in the eastern equatorial Pacific (Shipboard Scientific Party, 1992a, 1992b) (Fig. F28): an initial rise in CH₄ concentrations with increasing depth coupled with a concomitant decline in sulfate concentrations, followed successively by a zone of elevated CH₄ concentrations and a subsequent decrease in CH₄ accompanying an increase in sulfate concentrations (Fig. F28). The trend is also similar to that observed at shallow depths at Site 1009 (Shipboard Scientific Party, 1997). All four sites show similar inverse exponential correlations between CH₄ and sulfate concentrations in the upper 50-80 m.

Carbonate determinations by coulometry were made for a total of 108 samples from Hole 1208A (Table T7). Samples were selected to provide a measure of the carbonate content within different units and to assess the influence of content on color reflectance. The values for carbonate range from 10 to 89 wt% (Table T7) in Unit I, reflecting variations in the relative proportions of clay, biogenic silica, and carbonate. The carbonate content profile shows alternating values downcore (Fig. F29). The variations observed are primarily an artifact of the dominant criterion for sample selection: namely the choice of intervals that are representative of extremes in color and lithology. Carbonate contents are high (~97 wt%) and show little variability in the calcareous oozes of the Campanian (Unit II), comparable to Site 1207.

Analysis of extractable organics in Sample 198-1208A-1H-1, 68-69 cm, revealed trace amounts of n-alkanes in the C₂₅₊ range, extending to C₃₃ with pronounced odd/even predominance. This profile is characteristic of vascular plant waxes and suggests contributions from eolian dust (Simoneit, 1978; Zafiriou et al., 1985; Gagosian and Peltzer, 1986; Gagosian et al., 1987). No other components could be detected from the small sample size (~1 g), and the low yield of extractable organics precluded effective investigations of lipid components in other samples.