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Depth profile of physical properties

The Aw for the core samples from Expedition 902 ranged from 0.975 to 0.981 and did not show a clear correlation with depth (Fig. F3A; Table T1). The Aw values from Expedition 337 were highly variable below 1276 mbsf, ranging from 0.954 to 0.983, whereas the Aw values for samples collected from deeper horizons appeared to be lower than those from shallower depths. Compared to the sequences above and between the coalbed layers, Aw values were relatively lower in coalbed layers. Lower Aw values were also observed at around 1300 mbsf (upper portion of Unit II, near the starting depth of coring in Expedition 337). The Aw values in coal samples, which ranged from 0.977 to 0.979, were higher than those in the sandstone and siltstone within coalbed layers; however, on the whole, no obvious relationship was observed between Aw and lithology (Table T2).

The porosity of the core samples used in Aw measurement decreased with depth, from 0.81 on the seafloor to 0.2 at the bottom of the core hole (Fig. F3B). The porosity reduction curve is well described by an exponential curve. A very low porosity of <0.05 was observed in several depth intervals, and these were considered to represent carbonate-cemented sandstone and siltstone. The logging porosity values calculated using gamma ray bulk density data from the Three-Detector Lithology Density logging tool (see the “Methods” chapter [Expedition 337 Scientists, 2013a]) were relatively lower than core sample porosity values, though the overall agreement between both porosity series was quite similar. Higher porosity zones were observed at around 1950 and 2450 mbsf, where partial coalbed layers were formed, corroborating previously reported porosity-depth trends (Tanikawa et al., 2016).

Effect of physical and chemical properties on Aw

Figure F4A (Tables T3, T4) shows Aw as a function of the weight fraction of NaCl solution saturated in the core samples. The result shows that the Aw for all samples decreased with increasing NaCl concentration and that the difference in Aw among the samples was relatively small between replicates at the same concentration. Moreover, the water activities of the sediments were similar, although somewhat lower, than those calculated using Raoult’s law for aqueous solutions of NaCl and the empirical equations introduced by Chirife and Resnik (1984). At higher salinities (>15%), the empirical curve of Chirife and Resnik (1984) fits the Aw values obtained for the core samples considerably better than Raoult’s law. Raoult’s law is generally applied to dilute aqueous solutions and is very accurate for Aw values >0.95, although marked deviations from the law were observed in more concentrated solutions. The discrepancy between the modeled and measured Aw values can be explained by intermolecular forces and solvation effects in a solution (Fontán and Chirife, 1981; Lilley and Sutton, 1991). The discrepancy at lower salinities (<5%) can be explained by incomplete desalination in our procedures.

Volumetric water contents normalized by saturated volumetric water content (degree of water saturation) were plotted against Aw to plot the SWCC (Fig. F4B; Table T5). The water activities for six samples did not decrease at higher water contents, and Aw remained above 0.9, even after a 10% reduction in water content (Table T3). Aw in coal samples gradually decreased with decreasing water content at the onset of drying the samples, whereas the water activities for the other four samples were not affected by drying, even when their water contents were reduced by 50%. The sensitivity of Aw to water content can be classified into one of three groups based on the shape of the SWCC, and Aw values for clay at 178 mbsf (Expedition 902 Section 20H-4) and sandstone at 1971 mbsf (Section 337-C0020A-21R-4) showed the lowest sensitivity to water content at higher normalized water contents. This result suggests that the loss of small amounts of pore fluid by drainage or evaporation during shipping and sample treatment did not affect the Aw of the samples if one assumes that pore water chemistry did not change.

No significant relationship was observed between Aw and porosity (Fig. F4C). Aw values were highly variable at porosity values below 0.4 and relatively high (0.972–0.974) in samples with very low porosity values (porosity < 0.06). However, as shown in Figure F4C, Aw appears to decrease with porosity when the lower Aw values are selected from a similar porosity range.

Predicted Aw from water chemistry

Figure F5A shows a comparison of the predicted and measured Aw values for core samples and drilling mud water at 25°C. The predicted Aw values calculated from the pore water chemistry data were plotted against the Aw values obtained at the same depth as the samples used for the analysis of interstitial water chemistry (<1.5 m apart). The values obtained from the analysis with GWB were larger than those obtained with PHREEQC, with deviations from the measured values being slightly smaller for PHREEQC than GWB. The lower predicted Aw values (<0.96) were higher than the measured values, although the deviation between the two values was small at higher Aw values. On the whole, the predicted values were moderately consistent with the measured values, implying that Aw for subseafloor sediments is influenced by the interstitial water chemistry and that water chemistry data can be used to predict Aw for sediments.

The Aw depth profile estimated based on the interstitial water chemistry inferred using PHREEQC is shown in Figure F5B. As core samples from Expedition 902 had been stored for 7 y before water chemistry analyses, oxidation and biochemical reactions might occur to change pore water chemistry. Chemical diffusion between the contaminated outer part and the inner part of the core sample is also expected to occur during storage, and it may result in a mixture of pore water and drilling fluid. Therefore, we did not use pore water chemistry data from Expedition 902 newly measured in this study to estimate the Aw depth profile (Fig. F5B). The predicted Aw for shallow sediments above 350 mbsf from Expedition 902 decreased slightly with depth (0.9826 to 0.982), and the predicted Aw was ~0.04 higher than the values obtained from the core samples. The predicted Aw values for core samples collected from Expedition 337 ranged from 0.959 to 0.984, which is consistent with the measured Aw values. The predicted Aw values at the top of Unit II (1277–1378 mbsf; Table T1) were lower than those at other depths but were similar to the measured Aw in core samples, cuttings, and drilling mud at the same depths. The Aw values for cuttings were similar to those for mud water, and both measures were lower than the Aw values measured from core samples. No low anomalies were observed in the predicted Aw values from the coal-bearing layers, but drilling mud fluid had relatively low Aw values in the coalbed. Similarities in the Aw values for mud water samples and cuttings could be explained by high levels of contamination with drilling mud fluid in cuttings (Inagaki et al., 2015), as higher Cl concentrations in cuttings and drilling mud due to contamination by drilling fluid was reported.