ORGANIC GEOCHEMISTRY

The shipboard organic geochemistry program at Site 1169 included studies of volatile hydrocarbons, total nitrogen, and total organic and inorganic carbon. CNS analysis, gas chromatography, and carbon coulometry were performed (see "Organic Geochemistry" in the "Explanatory Notes" chapter). Sample spacing varied because of the core disturbance during drilling; therefore, a lower resolution geochemical record was produced here compared to Site 1168.

Sedimentary Geochemistry

Results

Carbonate (CaCO3) content values for the strata sampled at Site 1169 range from 60 to 95 wt% (Fig. F9; Table T11). In general, the profile exhibits an overall slightly decreasing upward trend. Sediments from ~250 to ~170 mbsf commonly contain >90 wt% CaCO3, except for one horizon with a value of ~85 wt%. An abrupt decrease to ~60 wt% is observable at ~135 mbsf. From this horizon, carbonate content values fluctuate between ~80 and 90 wt%.

The total organic carbon (TOC) content for most samples from Site 1169 is <0.7 wt%, except for one sample at ~200 mbsf that contains >1.3 wt% TOC (Fig. F9; Table T11). Organic matter type was assessed using C and N analyses (Fig. F9; Table T11); C/N ratios of ~5-8 are generally considered as indicative of marine organic matter, whereas ratios of ~25-35 suggest the presence of terrestrially derived organic matter (Bordovskiy, 1965; Emerson and Hedges, 1988). Total nitrogen content through the core is generally low, so C/N ratios must be considered with care.

Discussion

The high carbonate content of sediments at Site 1169 reflects a dominance of calcareous nannoplankton and foraminifers (see "Biostratigraphy"). The extremely high carbonate content may represent enhanced carbonate preservation, perhaps indicating relatively high biogenic productivity. This observation is similar to one made for middle Miocene through Quaternary sediments at Site 1168. Of note is the carbonate decline at ~135 mbsf. Several distinct carbonate declines are visible between ~100 and 200 mbsf at Site 1168 (see Fig. F27 in the "Site 1168" chapter), so these horizons with relatively low carbonate content may represent regionally correlatable horizons.

The generally low TOC contents in this sequence seem to record settling of organic matter through a well-mixed water column and/or to a well-oxygenated seafloor. The TOC content profile here is similar to the TOC content distribution at Site 1168, even considering the lower resolution sampling at Site 1169. Of particular interest are the TOC content variations between ~100 and 200 mbsf (Fig. F9), which are similar to variations observed between ~60 and 160 mbsf in Hole 1168A (see Fig. F27 in the "Site 1168" chapter). These peaks may represent similarities in organic carbon preservation recorded at both sites.

The C/N ratios show a wide range of values from 0 to >100 and covary with the TOC content. Some of these apparent variations may be described by the extremely low total nitrogen content of the samples. However, if these values indicate variations in organic matter type, then they may record variations in seafloor redox conditions. Low C/N intervals could record preservation of marine-derived organic matter perhaps associated with carbonate producers, or dinoflagellates (see "Biostratigraphy"), whereas relatively higher C/N intervals may record organic matter denitrification. Denitrification is usually limited in marine sediments (Killops and Killops, 1994) and does not generally occur until most of the sedimentary organic carbon has been metabolized (Mechalas, 1974). Alternatively, the high C/N units may represent total oxidation of labile-marine organic matter and subsequent preservation of minor quantities of refractory terrestrial residuum.

Volatile Hydrocarbons

Results

Concentrations of volatile hydrocarbon gases were measured from every core using the standard ODP headspace-sampling technique and gas chromatographic analysis. Methane content (Table T12) was extremely low (0 to <100 ppmv), although it generally increases downcore.

Discussion

The extremely low gas content at Site 1169 is likely a function of two characteristics of the sediment. First, the sediments contain little organic matter as a source of natural gas. Second, pore-water profiles show that appreciable SO42- exists to the bottom of Hole 1169A; thus, sulfate reduction processes may be limiting the onset of methanogenesis in this interval (see "Inorganic Geochemistry").

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