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Organic geochemistry

Hydrocarbon gas

Concentrations of methane and ethane, as well as the ratios of methane to ethane (C1/C2) are shown in Table T13 and Figure F41. Methane is predominant in all Hole C0018A cores except in Core 333-C0018A-1H, where no hydrocarbon gas was detected. Methane concentration ranges between 0 and 19,338.7 parts per million by volume (ppmv), with an average of 6,949.5 ppmv. Ethane was sporadically detected in Hole C0018A and only present in low concentrations, with a maximum value of 1.6 ppmv. No heavier hydrocarbon gases were found. The C1/C2 ratios for all cores are >4000, suggesting methane is mostly biogenic and organic matter is immature in Hole C0018A.

Relatively high methane concentration occurs at two depth intervals. The first interval occurs approximately between 17 and 50 mbsf, corresponding to the SMT zone (see “Inorganic geochemistry”) and indicating increases in methanogenesis. The second interval is between 201.5 and 239.5 mbsf, where methane concentration rises to the maximum value.

Sediment carbon, nitrogen, and sulfur composition

Calcium carbonate (CaCO3) content varies in a wide range from 0.22 to 25.36 wt% and averages 8.52% throughout Hole C0018A (Table T14; Fig. F42). Most high values (i.e., >15 wt%) are found in the upper ~86 mbsf, where the values average 13.22 wt% and are highly variable between 0.22 and 25.36 wt%. CaCO3 concentration between ~86 and ~191 mbsf displays a lower average of 9.55 wt% and a smaller variation range of 1.77–17.85 wt%. Below 191 mbsf, the amounts of CaCO3 are less variable and generally low, averaging 3.13 ± 1.75 wt%. The change in CaCO3 concentration is correspondent with lithologic Subunits IA and IB (see “Lithology”).

Total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS) contents are low in the majority of cores (Table T14; Fig. F42). TOC and TN concentrations show a positive correlation (r2 = 0.74). Similar to the CaCO3 content, TOC and TN values show large variations in the upper 71 mbsf. TOC content ranges from 0.03 to 0.96 wt% with an average of 0.50 wt%, whereas TN values are from 0.01 to 0.12 wt% with an average of 0.07 wt%. Higher values are concentrated in the upper 20 mbsf, which is perhaps due to less diagenetic alterations in the shallow sediments. TOC and TN contents are less variable between ~87 and ~191 mbsf, averaging 0.49 ± 0.09 wt% and 0.07 ± 0.01wt%, respectively. Values become more sporadic below 191 mbsf, averaging 0.47 ± 0.11 wt% for TOC and 0.06 ± 0.02 wt% for TN. The TOC to TN atomic ratios (TOC/TNat) fall in the range of 3.5–15, suggesting the organic matter is predominately marine derived but also contains terrigenous material in some horizons. TS concentration varies from 0.04 to 1.19 wt% and averages 0.28 wt%. The samples show elevated TS content between ~90 and 130 mbsf, which can be associated with the occurrence of pyrite at ~120 mbsf (see “Lithology”).

Rock-Eval pyrolysis

Thirty samples were measured by Rock-Eval pyrolysis to characterize the type and maturity of organic matter (Table T15; Fig. F43). Rock-Eval Tmax values are <430°C in all but one sample, indicating that the organic matter in Hole C0018A is thermally immature. The amounts of free hydrocarbon (S1) range between 0 and 0.12 mg hydrocarbon/g sediment (mg HC/g sediment). Thermal cracking of nonvolatile organic matter released hydrocarbon (S2) is between 0.01 and 0.80 mg HC/g sediment. Production index (PI) ranges between 0 and 0.14, further indicating the immaturity of the organic matter.

Hydrogen index (HI) values <100 mg HC/g TOC are typical of terrigenous organic matter, whereas HI values of 300–800 are generally attributed to a marine source (Tissot and Welte, 1984). The HI values in Hole C0018A are rather low, varying between 9 and 93 mg HC/g TOC. The contradiction between low HI values and contemporarily low TOC/TNat ratios may be related to the low TOC content in our samples, which can artificially lower HI values due to the adsorption of hydrocarbons onto clay minerals (Espitalié et al., 1985). In addition, intensive degradation can preferentially remove hydrogen-rich organic matter and further lower HI values (Espitalié et al., 1977).