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doi:10.2204/iodp.proc.338.107.2014

Geochemistry

Inorganic geochemistry

Interstitial water geochemistry

Slope sediment was sampled from the seafloor to ~415.9 mbsf in Hole C0022B. Interstitial water (IW) samples from those cores were extracted using the standard squeezing method (see “Geochemistry” in the “Methods” chapter [Strasser et al., 2014a]). Only time-sensitive parameters and elements (pH, alkalinity, chlorinity, PO₄^3–, and NH₄⁺) and chlorinity were analyzed during the expedition because of the limited time available; the rest of the parameters were measured on board the ship within 2 months after the expedition. The analytical results are listed in Table T10, and the variations with depth are in Figure F28.

The shallowest samples yield values almost similar to those of shallowest liquid in core liner (LCL) samples (Table T11), which consist of ambient seawater mixed with bentonite. Chlorinity increases to 140 mbsf and is stable with values between 630 and 650 mM below that depth. The variation with depth is similar to that observed at Site C0004, which is close to and located landward from this site (Expedition 316 Scientists, 2009a). Such an increase in chlorinity can be attributed to the hydration of sediment following diagenetic alteration of detrital minerals and volcanic glass. Chlorinity of IW at 90–130 mbsf is occasionally lower than the average values nearby. Chlorinity in this interval could be lowered by the following three mechanisms: infiltration of seawater (~550 mM chlorinity), clay mineral reactions, or upwelling of deep dehydrated water along a fault. The depth interval is almost concordant with the depth of the fracture zone of the megasplay fault tip (80–100 mbsf; see “Structural geology”). However, updip migration of deep-sourced fluid along the fault is not clear at present because other elements are not indicative of a deep-sourced fluid.

The IW geochemistry drastically changes just beneath the seafloor as a result of biochemical reactions; those changes are especially prominent in salinity, alkalinity, and PO₄^3–. Alkalinity and PO₄^3– vary in similar ways: they increase just beneath the seafloor before gradually decreasing with increasing depth. Such concentration profiles were also observed in the IW samples at Site C0002, but the peak maximum of those components appeared between 150 and 200 mbsf (Fig. F28). NH₄⁺ gradually increases to 150 mbsf and then decreases with depth, which is similar to the concentration profile at Site C0002 (although the concentration is lower at this site). Compared to NH₄⁺, pH shows an opposite trend with depth. The peak depth of alkalinity and PO₄^3– concentration of IW at Site C0022 is similar to that at Site C0004, whereas NH₄⁺ concentration at Site C0004 gradually increases with depth to 250 mbsf without showing any peak unlike at Site C0022 (Expedition 316 Scientists, 2009a). Concentrations of PO₄^3– and NH₄⁺ components are primarily controlled by in situ microbial activity. Compared to Site C0002, the lower concentrations in IW at Sites C0022 and C0004 are attributable to the lower organic matter content. Less organic matter in the sediment can be related to the increased distance from the land.

Liquid in core liner geochemistry

Mud water remaining in the core liner was sampled and measured to assess potential contamination of the interstitial water. Results of onboard analyses are listed in Table T11. Most LCL samples give chlorinity similar to that of seawater (~550 mM), whereas occasionally it is >580 mM. The LCL samples with higher chlorinity than seawater contain higher alkalinity. This suggests some contribution from IW, which is more saline than seawater.

Organic geochemistry

Gas chemistry from cores

The gas data are shown in Tables T12, T13, and T14. In these tables, gas content, gas concentration assuming the detected gases are dissolved in IW, molecular ratios (C₁/[C₂ + C₃]), and carbon isotopes of methane (δ¹³C-CH₄) are listed. The C₁/(C₂ + C₃) ratios range from 600 to 16,000, indicating that methane is predominant. These compositions are usually observed in the surface of deep-sea sediment. Especially, the ratios of void gas showed CH₄ enrichment, which would reflect the high volatility of methane. The δ¹³C-CH₄ ratio of void gas, however, is not so different from that of headspace gas, suggesting δ¹³C-CH₄ values are not influenced by degassing processes.

Vertical profiles of hydrocarbons are shown in Figure F29. CH₄ peaks are found at ~30, 100, and 400 mbsf. The CH₄ peak at ~30 mbsf is interpreted to be due to early diagenesis, which was also observed in shallow slope sediment in the Nankai accretionary prism at Sites C0004 and C0008 (Expedition 316 Scientists, 2009a, 2009b). Below the sulfate reduction zone, CH₄ is generally produced by microbes (e.g., Reeburgh, 2007). In Hole C0022B, sulfate was completely depleted below 23.4 mbsf (Table T10), suggesting a strongly reductive near-surface sediment. In such an environment, coupled CO₂ reduction and H₂ oxidation by methanogens generate CH₄ (e.g., Reeburgh, 2007). On the other hand, the CH₄ peaks at ~150 and 350 mbsf are accompanied with C₂H₆ peaks (Fig. F29), implying that the hydrocarbons are derived from a different process than microbial production. Between 87 and 105 mbsf, a fracture zone occurs (see “Logging while drilling”), which could account for the prominent peak in CH₄ close to 100 mbsf.

The vertical profiles of C₁/(C₂ + C₃) ratios and δ¹³C-CH₄ are shown in Figure F30. CH₄ with δ¹³C-CH₄ less than –80‰ Vienna Peedee belemnite (VPDB) is observed at ~30 mbsf, suggesting microbial CH₄ generation is active at ~30 mbsf. δ¹³C-CH₄ values increase with depth, reach –70‰ VPDB at ~100 mbsf, and remain constant at about –70‰ VPDB below 100 mbsf. This δ¹³C-CH₄ profile could be attributed to isotopic fractionation during methanogenesis, where microbes preferentially utilize ¹²CO₂ to leave ¹³CO₂ in the CO₂ reservoir, causing ¹³C-enrichment in CH₄ when methanogens produce CH₄ from ¹³C-enriched CO₂. Below 100 mbsf, the δ¹³C-CH₄ values become constant, suggesting methanogenesis is less active. In addition, the C₁/(C₂ + C₃) ratios decrease from 10⁴ to 10³, which also suggests that the generation process of hydrocarbon becomes different from the microbial process in the surface sediment.

The C₁/(C₂ + C₃) ratios and δ¹³C-CH₄ data are plotted on a Bernard plot (Bernard et al., 1976) (Fig. F31). The diagram shows that most data fall into the region of microbial origin, so CH₄ is of microbial origin. Thermogenic methane, however, mixes to some extent with the microbial CH₄.

Hole C0022B is located near the surface of the Nankai accretionary prism, where slope collapse deposits are observed (Strasser et al., 2009). Expedition 316 drill Sites C0004 and C0008 are located landward and seaward of Hole C0022B, respectively (Screaton et al., 2009). Gas hydrates were recovered at Site C0008 (Expedition 316 Scientists, 2009b). During this expedition, gas hydrates were not found in core sediment, but gas-rich sediment can be inferred from observed void spaces in core liners. As mentioned by Toki et al. (2012), the input of organic matter from land into the Kumano Basin is high because of the short distance from the source region (Japanese Islands) and the dam effect of the outer Kumano Ridge. This high input leads to a high production rate of methane, resulting in accumulation of methane and formation of gas hydrates. At Sites C0004, C0008, and C0022 in the slope of the accretionary prism, NH₄⁺ concentrations were lower than those at Site C0002. This suggests that early diagenesis at sites in the slope is less active than at Site C0002 in the Kumano Basin.

Anomalies related to the fracture zone near 100 mbsf (see “Logging while drilling” and “Structural geology”) are not detected in any components in interstitial water (Fig. F28). Close to the fracture zone, a radical reaction generates H₂, which could stimulate methanogenesis via H₂ oxidation (e.g., Stevens, 1997). In addition, if deep-sourced fluid flowed from a zone deeper than 2000 mbsf, expected fluid temperatures would be high enough to lead to the formation of thermogenic methane. The hydrocarbons show anomalies around the ~100 mbsf fracture zone (Fig. F29), implying active gas migration along the fracture zone. However, the lack of hydrocarbon isotopic anomalies (Fig. F30) suggests that the hydrocarbons do not come from a deep zone but rather from a near-surface zone.

Inorganic carbon, total carbon, total nitrogen, and total sulphate

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, total carbon, TN, and TS measurements of sediment samples from 1.2 to 415.3 mbsf (Fig. F32; Table T15).

CaCO₃ varies from 0.3 to 21.7 wt%, with a median of 3.9 wt%, and this variance occurs within the uppermost 50 m. Below 50 mbsf, the data are still scattered but show an overall decline downhole.

TN concentration of sediment ranges from 0.03 to 0.09 wt%, with a median of 0.06 wt%. With up to 0.09 wt%, TN is slightly higher in the surface sediment and drops to 0.05 wt% within the upper 23 m. Below this depth, TN values remain almost constant downhole but show a slightly positive trend until ~270 mbsf. Deeper than 270 mbsf, concentrations decline similar to CaCO₃.

TOC varies between 0.2 and 0.7 wt%, with a median of 0.4 wt%, and shows a similar development with depth as CaCO₃ and TN. TOC starts at 0.7 wt% and drops to 0.4 wt% within the top 23 m. Between 23 and 270 mbsf, TOC scatters between 0.3 and 0.6 wt%. Below 270 mbsf, again similar to TN and CaCO₃, concentrations decline constantly.

TS concentration is between 0.0 and 0.9 wt%, but the median is 0.1 wt%., indicating that overall TS values remain low (i.e., <0.2 wt%). In the top 23 m, TS increases slightly in contrast to TOC, CaCO₃, and TN but remains trendless and uncorrelatable afterward. Clusters of elevated and scattered TS were only detected between 23 and 85 mbsf and 359 and 415 mbsf.

The C/N ratio ranges from 4.5 to 9.3, with a median of 6.6. Following TOC, TN, TS, and CaCO₃, a trend is visible between 0 and 23 mbsf, where C/N drops from 8.4 to 7.1. Below 23 mbsf, the C/N ratio scatters without trend between the aforementioned values. It can be assumed that the organic matter in Hole C0022B is mainly of marine origin because the C/N ratio remains below 10 (Meyers, 1997).

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