At Site 1127, in addition to routine monitoring of hydrocarbon and hydrogen sulfide (H2S) gases for safety, analyses were conducted for inorganic carbon, total carbon, nitrogen, and sulfur. The analytical procedures are described in "Organic Geochemistry" in the "Explanatory Notes" chapter.
Concentrations of volatile hydrocarbons and H2S were routinely determined from every core in Hole 1127B using standard ODP headspace or vacutainer sampling procedures. The striking results from Hole 1127B are the very high concentrations of methane (C1) and H2S throughout most of the section (Tables T3, T4). Gas pockets occurred commonly from 47 to 312 mbsf. These pockets were sampled directly through the core liner using a gas-tight syringe (vacutainer). Methane concentrations in these gas pockets range from 329,045 to 585,009 ppm (Fig. F11). Although concentrations of C1 are high compared to those from most previous ODP sites, methane concentrations exceeding 880,000 ppm have been reported for Sites 892, 994, and 996 (Westbrook, Carson, Musgrave, et al., 1994; Paull, Matsumoto, Wallace, et al., 1996).
Methane occurs at lower and variable concentrations in the headspace samples, with the lowest values at the tops and bottoms of the sedimentary sections (Fig. F12). From 20 to 80 mbsf, methane increases to 46,234 ppm, the highest value observed, and then decreases sharply to values between 885 and 3254 ppm from 96 to 169 mbsf. Between 169 to 304 mbsf, methane increases to 18,328 ppm. Below 304 mbsf, methane shows a general but variable decrease to <400 ppm. The bimodal C1 headspace profile vs. depth apparently tracks organic carbon (Corg) values. Low C1 concentrations between 100 and 200 mbsf and higher values from 200 to 350 mbsf correlate to comparable patterns of Corg concentration in the same depth intervals (see "Inorganic and Organic Carbon, Sulfur, and Nitrogen").
Ethane (C2) appears at depths below 227 mbsf at concentrations as high as 605 ppm in vacutainer samples and as high as 17.6 ppm in headspace samples (Figs. F11, F12). The C1/C2 ratio in vacutainer samples decrease from 1116 to 644; in headspace samples, the ratio decreases to a value of 82. Although the latter ratio is in the range indicative of thermogenic hydrocarbon sources, the concentrations of both C1 and C2 in the deeper section are too low to be of safety concern.
Heavier hydrocarbon gases (C3+) were detected in the headspace samples from 124 mbsf down to the deepest gas pocket occurrence at 311.6 mbsf and from 196.4 mbsf down to the base of the hole (except for one shallow sample at 67.4 mbsf). In addition, three unknown peaks appeared at approximate concentrations of 50-100 ppmv on the flame ionization detector gas chromatogram at the following retention times: (1) peak 1-4.67 min, following n-C4; (2) peak 2-6.07 min, between i-C5 and n-C5; and (3) peak 3-7.17 min, between n-C5 and i-C6. Reference standards were not available to ascertain the identity of these compounds.
Concentrations of H2S in the gas pockets range from 60,000 to 138,000 ppm, possibly the highest level found at any ODP site (Fig. F11). The maximum H2S concentration found at any previous site is ~100,000 ppm at Site 1005 (Eberli, Swart, Malone, et al., 1997). The concentrations of H2S in headspace samples are much lower and highly variable, ranging from below detection in the deepest intervals to as high as 28,218 ppm at ~350 mbsf (Fig. F12).
Inorganic carbon concentrations were converted to calcium carbonate percentages (see "Organic Geochemistry" in the "Explanatory Notes" chapter). Throughout Hole 1127B, carbonate values range primarily from 85 to 92 wt% (Table T5; Fig. F13). From the sediment surface to ~340 mbsf, except for sporadic lower values, the carbonate content is toward the higher end of the range (88-92 wt%). Below 340 mbsf, carbonate values are toward the lower end of the range (85-89 wt%).
Organic carbon reaches values as high as 1.15 wt% (Fig. F13). From the surface to a depth of ~240 mbsf, Corg values are primarily <0.5 wt%. In the interval from 240 to 360 mbsf, Corg ranges between 0.5 and 1.1 wt%. Concentrations of methane in this interval are also relatively high, apparently related to the higher Corg content. Below 360 mbsf, Corg values again are generally <0.5 wt%. Because of the generally low organic carbon content in Hole 1127B, no samples were analyzed for Rock-Eval parameters.
Nitrogen content reaches a maximum concentration of 0.14 wt%, although most samples have <0.1 wt%. Sulfur concentrations peak at 0.43 wt%, and most samples have no measurable sulfur. Sulfur concentrations are highest (0.1-0.43 wt%) between 250 and 320 mbsf, the same interval having the highest Corg content (Table T5).