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Organic geochemistry

Hydrocarbon gas

At Site C0011, no hydrocarbon gases other than methane were detected in the cored sequence (Table T14). Methane is either below detection or present at low concentrations in the uppermost 250 mbsf with an average value of 1.7 parts per million by volume (ppmv). Methane concentration increases below ~250 mbsf and reaches the maximum value of 873.6 ppmv at 376.4 mbsf (Fig. F46). Results of the present study are thus in agreement with the data from Expedition 322, which reported the presence of heavier hydrocarbons gases (i.e., ethane, propane, and butane) only below 425 mbsf at Site C0011 (Expedition 322 Scientists, 2010).

Sediment carbon, nitrogen, and sulfur composition

A total of 152 sediment samples were collected at Site C0011 and analyzed for concentrations of calcium carbonate (CaCO3), TOC, total nitrogen (TN), and total sulfur (TS) (Table T15; Fig. F47). The atomic ratios of TOC and TN (TOC/TNat) were calculated.

CaCO3 content is highly variable at Site C0011, ranging from 0.25 to 24.51 wt%. From the top of the core to 146.0 mbsf, the sediments are enriched in CaCO3, averaging 5.33 ± 4.60 wt% with a maximum of 17.66 wt% (Fig. F47). Thereafter, CaCO3 content drops to low values (average = 2.80 ± 2.08 wt%) and shows little variation until 238.9 mbsf. From 238.9 to 318.6 mbsf, CaCO3 content increases again and averages 6.70 ± 5.77 wt%. Sediments below 313.8 mbsf are characterized by low CaCO3 values averaging 2.14 ± 1.13 wt%.

TOC content is low throughout the cored sequence and ranges from 0.09 to 0.59 wt% (average = 0.31 ± 0.09 wt%). TOC values are relatively high in the upper ~215 mbsf, where the TOC average is 0.34 ± 0.08 wt%. Beneath an abrupt decrease to 0.21 wt% at 216.0 mbsf, TOC values remain low and uniform until 340.8 mbsf. The values in the lowermost cored sequence are highly variable, ranging between 0.06 and 0.56 wt%. TN content fluctuates between 0.02 and 0.08 wt% with an average of 0.06 ± 0.01 wt% and generally follows the TOC variation pattern. TOC/TNat ratios vary from 3.2 to 10.7 with an average of 5.6 ± 1.3, suggesting a predominant input of marine-derived organic matter. However, the TOC and TN values at Site C0011 show a relative weak correlation (r2 = 0.55), possibly suggesting an important contribution of inorganic nitrogen sources. Clay-bound ammonium can make up a significant TN fraction in organic-poor marine sediments (Müller, 1977). The admixtures of this inorganic nitrogen species and organic nitrogen may lead to an overestimate of marine-derived organic matter using the TOC/TNat ratios at Site C0011.

TS content is generally low, varying between 0.02 and 0.34 wt% (average = 0.23 ± 0.96 wt%) throughout most of the cores. Higher average TS values (0.38 ± 1.42 wt%), however, are observed between 84.5 and ~251.1 mbsf. Notably, the sediment at 238.4 mbsf has the maximum TS content of this site (11.86 wt%). This remarkably elevated TS value coincides with the maximum Fe content (see “Lithology”), suggesting the enrichment of pyrite at this depth.

Rock-Eval pyrolysis

Twelve selected samples were subjected to shipboard Rock-Eval pyrolysis in order to determine the type and maturity of the sedimentary organic matter preserved in Holes C0011C and C0011D. Thermal distillation of sedimentary organic matter (S1) released <0.1 mg hydrocarbon/g sediment (mg HC/g sediment), while the pyrolytic degradation of nonvolatile organic matter (S2) yielded <0.3 mg HC/g sediment (Table T16). The hydrogen index (HI) is rather low and ranges from 26 to 41 mg HC/g TOC (Fig. F48). These low HI values are common for a Type III kerogen (Tissot and Welte, 1984) and suggest a terrestrial origin of the sedimentary organic matter. This finding, however, contradicts with the interpretation of the TOC/TNat ratios, which indicate a predominantly marine derived nature of the organic matter. This contradiction can be due to intense degradation, which has been shown to reduce the hydrogen richness of sedimentary organic matter and ultimately lower HI values (Espitalié et al., 1977). An alternative explanation is that a large fraction of hydrocarbons may be adsorbed by clay minerals in TOC-poor sediments, which also can result in artificially low HI values (Espitalié et al., 1984).

Tmax values range from 336° to 414°C and slightly increase with depth, suggesting a higher degree of thermal alteration of the organic matter at the base of the cored sequence. However, all Tmax values fall below 430°C, which indicates that the organic matter is at a thermally immature stage.