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doi:10.2204/iodp.proc.338.106.2014

Geochemistry

Inorganic geochemistry

Interstitial water geochemistry

Interstitial water (IW) was sampled (1 IW sample/core) from 3.7 to 186.4 mbsf in Hole C0021B, except for the interval between 5.9 and 80 mbsf, which was washed down without coring. Table T7 shows the analytical results of IW analyses, and Figures F22, F23, and F24 show the variations of IW chemistry with depth.

Although very close to the sea bottom, the uppermost sample at 3.7 mbsf shows IW chemistry different from that of seawater (e.g., pH 8.01 is slightly lower than that of seawater of ~8.2). The chemistry of samples below 80 mbsf is clearly different from the uppermost one: refractive index, pH, and chlorinity are lower than seawater, possibly indicating freshwater input and subsequent dilution. Chlorinity slightly increases with increasing depth between 80 and 170 mbsf and decreases again below 170 mbsf. The origin of freshwater could be dissociation of gas hydrates, clay mineral dehydration, and/or fluid flow along a fracture zone. The drilled interval lies within the hydrate stability zone; therefore, gas hydrate dissociation is a plausible scenario, although no hydrates were directly observed in the cores.

SO₄^2– concentration decreases from the uppermost sample to that of IW at 80 mbsf and does not change below that depth. Alkalinity and PO₄^3– are higher in IW at 80 mbsf than the uppermost one and then decrease with increasing depth to 170 mbsf and increase again to the bottom of the hole. These three components should behave concordantly with microbial activity (i.e., aerobic respiration consuming oxygen from oxoacid). In contrast, Br^– and NH₄⁺ increase with increasing depth, possibly because of decomposition and dissolution of organic matter. Alkaline metal elements (Li, Na⁺, K⁺, Rb, and Cs) and Mg²⁺ and Ba should behave according to those affinities of adsorption. Such an adsorption affinity depends on the radius of the hydrated ion and generally becomes stronger with increasing mass and valence among cations (Fuerstenau et al., 1981; Piasecki et al., 2010). Among those elements, K⁺, Rb, Cs, Mg²⁺, and Ba decrease and Na⁺ and Li increase with increasing depths to 170 mbsf, probably because of cation exchange reactions. The Ca²⁺ and Sr increase with depth may be caused by the higher dissolution rate of plagioclase and/or carbonate minerals than adsorption.

Among the trace elements not described above, Si, Fe, Mn, and Cu are higher in sediment deeper than 80 mbsf than in the sample at 3.7 mbsf. Si is released from minerals via hydration and dissolution. Mo behaves similar to Si. Fe and Mn should be partly released via dissolution of minerals and should also be partly released via desorption in association with a reducing condition of IW. Fe, Mn, and Cu decrease with increasing depth below 80 mbsf in a manner similar to the other elements (B, V, Pb, Zn, and U).

When investigating the different changes in element concentrations more closely, variations in trend and scatter seem to relate to MTDs (see “Lithology”). MTD A is located between ~94 and 116 mbsf, and MTD B occurs between ~133 and 176 mbsf. However, a systematic pattern that would describe every component is hardly recognizable (Figs. F22, F23, F24). For example, alkalinity, chlorinity, pH, Mg²⁺, B, Li, PO₄^3–, and Mn show a change in slope of the overall trend across MTDs tops and bases, whereas Ba, Rb, and NH₄⁺ remain unaffected. In general, the most prominent excursions occur close to the base of MTD B (i.e., close to the Subunit IA/IB boundary) at ~170 mbsf. At this depth, most data show an overall change or even a reversal in trend. The reasons for these observations remain unclear and will be the subject of future postcruise studies.

Geochemistry of liquid in core liner

Two water samples were taken from the core liner at 0 and 179.4 mbsf to check the chemical composition of mud water that may have contaminated the IW. Results of shipboard analyses are shown in Table T8. The liquid in core liner (LCL) from the top of the core (0 mbsf) is the same as seawater. The water from 179.4 mbsf has a slightly lower concentration of chlorinity (551 mM) than that of the LCL at 0 mbsf (555 mM) as well as SO₄^2– (25 mM at 0 mbsf and 20 mM at 179.4 mbsf).

Organic geochemistry

Gas chemistry

Headspace gas and void gas samples were taken from cores in Hole C0021B, and concentrations of methane, ethane, and propane, as well as carbon isotopic compositions of methane (δ¹³C-CH₄), were measured (Tables T9, T10, T11).

Gas chemistry data (methane concentration, ethane concentration, ratio of C₁/[C₂ + C₃], and δ¹³C-CH₄) obtained by conventional oven-heating gas extraction are compared with those obtained by NaOH-addition gas extraction (Fig. F25). Ethane concentration in headspace gases obtained by the oven-heating method was higher than that obtained by the NaOH addition method, whereas this difference was not observed for methane concentrations. This suggests that the difference likely results from higher solubility of ethane than that of methane. No significant difference between the two methods was found in the δ¹³C-CH₄ data.

The vertical distribution of methane, ethane, and propane concentrations are shown in Figure F26. Methane peaks appear at ~100 mbsf. Ethane increases with depth. Propane was not detected except at ~150 mbsf. Void gas samples were collected between 85 and 185 mbsf (Table T11), implying that degassing of the sediment took place at room temperature and atmospheric pressure. This suggests that gas generation took place at in situ conditions, as observed at Site C0022 (see “Geochemistry” in the “Site C0022” chapter [Strasser et al., 2014d]), and that sediments in the slope basins are rich in gas. Ethane increases with depth above 200 mbsf, as also observed at Site C0022.

Depth profiles of C₁/(C₂ + C₃) ratios and δ¹³C-CH₄ are shown in Figure F27. C₁/(C₂ + C₃) ratios decrease with increasing depth to 200 mbsf. δ¹³C-CH₄ values are almost constant throughout the whole cores at this site. C₁/(C₂ + C₃) ratios and δ¹³C-CH₄ values can be used to determine the origin of methane (Bernard et al., 1978). A plot of data from Site C0021 on the Bernard diagram (Fig. F28) indicates that methane distributed at Site C0021 is of microbial origin rather than of thermogenic origin.

Although the highest concentrations of methane and total gas occur within MTDs A and B, respectively, a direct correlation between gas concentration and MTDs is difficult to draw (Fig. F26). The negative trend with depth of C₁/(C₂ + C₃) ratios and constant δ¹³C-CH₄ values remain unaffected across MTDs or subunit boundaries, indicating no remarkable change in gas composition within MTDs or across the stratigraphic boundaries.

Inorganic carbon, total carbon, total nitrogen, and total sulfate

Calcium carbonate (CaCO₃) and total organic carbon (TOC) concentrations and TOC/total nitrogen (TN) (C/N) and TOC/total sulfur (TS) (C/S) ratios were determined from total inorganic carbon (IC), total carbon, TN, and TS measurements of sediment samples from 3.7 to 186.4 mbsf. CaCO₃, TOC, TN, and TS concentrations and C/N ratios are plotted in Figure F29 and listed in Table T12.

CaCO₃ in sediment at Site C0021 ranges from 2.9 to 23.6 wt%, with an average of 12.0 wt% (Fig. F29). Between 80 and 92 mbsf, CaCO₃ sharply decreases from 23.6 to 2.9 wt%. CaCO₃ then scatters around ~10 wt% between 100 and 175 mbsf before decreasing to lower values in Subunit IB, which is consistent with low calcite content values in Subunit IB determined from XRD analyses (see “Lithology”). TOC in sediment ranges from 0.12 to 0.76 wt% with a median of 0.46 wt%. TOC values in surface sediment above 5 mbsf are higher than those below 80 mbsf. TN concentration of sediment ranges from 0.020 to 0.090 wt% with an average of 0.070 wt%. TN values in surface sediment are higher than those below 80 mbsf. Below 80 mbsf, TN values are almost constant throughout the core. The lowest value was obtained at 92 mbsf, where CaCO₃ and TOC values also show the lowest values at Site C0021. At ~170 mbsf, CaCO₃, TN, and TS indicate a change in trend to lower values.

C/N ratio ranges from 5.0 to 8.4 with an average of 6.6 (Fig. F29) and is almost constant throughout the core. This suggests that organic matter at Site C0021 is dominantly of marine origin throughout the section (Meyers, 1997). TS concentration in sediment is generally low with an average of 0.31 wt%. A few higher values can be associated with the precipitation of sulfide, which will be evaluated by shore-based analyses of samples.

Like the gas data, correlations between CaCO₃, TOC, TN, and TS remain to be established. Moreover, the increase in data scattering and deviation from the overall trend observed across MTDs (Fig. F29) should be addressed by future postcruise studies.

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