The concentration of methane (headspace analysis) in sediments from Hole 1146A increased downhole from <10 ppmv at the top to a maximum of 85,000 ppmv at 599 mcd. Ethane (C2H6) and propane (C3H8) initially appeared at 536 mcd and peaked at 608 mcd with concentrations of 155 and 7 ppmv, respectively. The C1/C2 ratio reached a minimum of 345 at the bottom of the hole. A similar distribution was detected in the lower 100 m of Hole 1146C. Carbonate concentrations varied from <10 to >60 wt%, with the highest values in a distinct lithologic unit below 225 mcd. Total organic carbon (TOC) obtained by difference (total carbon [TC] - inorganic carbon [IC]) decreases systematically from 1 wt% at the top of Hole 1146A to trace abundance (<0.2 wt%) below 225 mcd, with occasional exceptions coincident with low carbonate. All sediments exhibit low C/N values, suggesting a marine organic source. However, organic C and total N are very low, and the C/N ratio is only reliable for a limited number of samples that yield higher C concentrations in the top 100 m of the hole.
Headspace gas analysis was performed on every core taken from Hole 1146A and on the bottom 100 m of Hole 1146C. Sampling and analysis were conducted as described in "Organic Geochemistry" in the "Explanatory Notes" chapter. Methane concentrations are low (<10 ppmv) above 110 mcd and relatively constant (<30 ppmv) between 110 and 231 mcd in Hole 1146A. Below this depth, concentrations rise sharply, reaching 1000 ppmv by 303 mcd, 10,000 ppmv by 415 mcd, and a maximum of 85,000 ppmv at 599 mcd (Table T12; Fig. F16A). Ethane (C2H6) was first detected at 536 mcd and peaked (155 ppmv) at 608 mcd. Propane (C3H8) was first detected at 568 mcd and peaked (7 ppmv) between 608 and 619 mcd. The C1/C2 ratio rapidly decreased from 2460 at 536 mcd to 345 at the bottom of the hole (Fig. F16B). No alkenes or higher (than C3) hydrocarbons were detected. A very similar pattern was observed in Hole 1146C.
Dissolved sulfate in interstitial waters was essentially zero (<0.5 mM) by 65 mcd (see "Inorganic Geochemistry"). A constant but low level of methane (10-30 ppmv) along with relatively low TOC (0.2-1.0 wt%) (Fig. F17B) was detected between this depth and the lithologic change at 225 mcd. This suggests predominantly microbial generation of methane in this upper unit. Dissolved NH4+, a product of both sulfate reduction and methanogenesis, is present below the zone of sulfate reduction (0-68 mcd) to ~225 mcd (see "Inorganic Geochemistry").
Sediment methane concentrations begin to increase substantially below 231 mcd. Because TOC decreases downhole and is <0.2 wt% (with a few exceptions) below ~210 mcd, this methane cannot be produced from an increased amount of organic matter (OM) for methanogens (see "Organic Matter Characterization"). Further, the presence of ethane (below 500 mcd) and propane (568 mcd), as well as the decreasing C1/C2 ratio, suggest the presence of thermogenic hydrocarbons. These increased HC concentrations also coincide with a significant decrease in chloride ions in interstitial water at 568 mcd (see "Inorganic Geochemistry"). However, from the lack of evidence of thermal maturity from thermal gradient measurements and Rock-Eval pyrolysis (see "Organic Matter Characterization"), it appears that these HC gases are migrated from either a deeper or a lateral source. Seismic survey profiles indicate a possible basement high ~2 km north-northwest and two prominent faults at a similar distance. A marked seismic reflector, T4 (see "Background and Objectives"), exists near the depth of maximum HC abundance. Physical properties measurements (see "Physical Properties") also indicate a lithologic change, which may be associated with a conduit for HC migration. If lateral instead of vertical migration is most significant, then the nearby fault scarp is a good source candidate. However, methane concentrations are persistently >25,000 ppmv to the bottom of Holes 1146A and 1146C, and the C1/C2 ratios appear to decrease with depth (Fig. F16; Table T12), indicating possible vertical HC migration.
Sampling for carbonate analysis was conducted in three sections per core from Hole 1146A (Table T13). Carbonate varies from 7.9 to 68.1 wt%, with low but gradually increasing concentrations from 0 to 225 mcd (average [AV] = 22.2 wt%; standard deviation [SD] = 5.9). Carbonate undergoes a marked increase from 225 to 425 mcd from ~20 to generally >50 wt% (AV = 55.4 wt%; SD = 6.0). Carbonate abundance again decreases to ~20-40 wt% (AV = 34.4 wt%; SD = 9.1) below 425 mcd to the bottom (641 mcd [Fig. F17A]). The upper two zones of significantly different carbonate abundance approximate lithologic Units I and II (see "Lithostratigraphy").
Total organic carbon concentration by difference (TC - IC) was determined for one sample per core (Table T13). The TOC decreases systematically from ~1.0 wt% at the top of Hole 1146A to 0.2 wt% at 220 mcd and remains below 0.2 wt% to the bottom of the hole, with a few notable exceptions (Fig. F17B). Samples 184-1146A-35X-3, 107-108 cm; 50X-3, 107-108 cm, and 56X-3, 107-108 cm, all correspond to intervals of higher TOC levels and may represent localized OM enrichment. Likely explanations include bioturbation, burrows, and the numerous dark bands observed in the lower cores from Hole 1146A (see "Lithostratigraphy"). On close inspection, the presence of green layers does not correlate with the high or anomalous TOC values. Rock-Eval measurement of eight samples from below 280 mcd that gave TOC (by difference) results >0.3 wt% produced no values above 0.1 wt% TOC (Table T14). This is consistent with lower values obtained by Rock-Eval analysis at previous sites.
Our analytical methods only allow organic matter to be characterized where TOC exceeds 0.5 wt%. Most of the TOC values (by difference) for Hole 1146A are therefore too low to characterize sediments precisely. Ratios of TOC (by difference) to total nitrogen (C/N) (Table T13; Fig. F17C) range from ~5 to 9 in the upper 225 m of the hole, suggesting a dominance of marine organic material in these immature sediments. The C/N values below 225 mcd are not necessarily diagnostic of marine OM because the TOC concentration is low, and the N is probably associated with clay as NH4+ (see "Organic Geochemistry" in the "Site 1145" chapter).
Rock-Eval results (Table T14) for samples from the upper 300 m of Hole 1146A show low Tmax indicating immature OM, but a poorly defined S2 peak may render this unreliable. Samples from the zone of high methane and higher HC give similarly low Tmax values, suggesting thermally immature sediments. This supports the earlier conclusion that the abundant HCs are migrated. Known gas and oil reservoirs are hosted in lower Miocene formations of the nearby continental shelf in a stratigraphic unit that may exist only a few hundred meters below the bottom of Hole 1146A (Wang and Wong, 1998). Drilling at Site 1148 through the Miocene (see "Organic Geochemistry" in the "Site 1148" chapter), however, has not revealed a substantial reservoir or source rocks. The source of the high values for methane and accompanying ethane and propane, therefore, awaits explanation from shore-based laboratory analysis.
Total sulfur was measured with the carbon-nitrogen-sulfur analyzer (see "Organic Geochemistry" in the "Explanatory Notes" chapter). Sulfur is higher in the top 170 mcd of the core, varying between 0.1 and 0.5 wt% (except for three lower values [Table T13]). These are also the sediments with the highest TOC values, and this occurrence of higher S and TOC together is expected (Berner, 1984). Because of the depositional environment and the presence of pyrite in the sediments (see "Lithostratigraphy"), we assume that the sulfur measured is pyrite. Below 170 mcd, sulfur is <0.1%.
Examination of the S data (Table T13) reveals discrepancies with the abundance of observed sulfides (see "Lithostratigraphy"). Observed "iron sulfide" that is disseminated in millimeter- to centimeter-sized areas (but is amorphous or microcrystalline) is greatest from 110 to 210 mcd, from 275 to 360 mcd, and in the lowermost 30 m. Pyrite as centimeter-sized crystal masses filling burrows is reported mainly between 425 and 530 mcd and also in the bottom 30 m. Although not quantitative, these lithology descriptions seem at odds with the measured S contents. This can be explained by two factors. First, pyrite concretions are rare and almost never found in the discrete 2-cm carbonate (CARB) sample that is taken for S analysis. Second, the finely disseminated S is too small to be recognized by the unaided eye or hand lens in the dark brown-gray matrix encountered in the top 170 mcd of the core, where our measured S abundance is greatest.