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Inorganic geochemistry

Interstitial water geochemistry by standard squeezing method

Interstitial water (IW) was analyzed according to the standard analytical procedures for cores taken from Holes C0002J–C0002L. Analytical results are given in Table T29. Samples were taken from 200 to 500 and 900 to 940 mbsf. In addition, selected samples were used for comparing results from the ground rock interstitial normative determination (GRIND) method and the standard squeezing method. IW samples taken from Holes C0002H (Section 338-C0002H-2R-2; 1111 mbsf) and C0002J (Section 338-C0002J-7R-1; 933 mbsf) were extracted using only the GRIND method because core recovery was too low to provide enough volume from the whole-round core (WRC) sample for the standard method. The results of the GRIND method are shown in Figures F71 and F72 and are described in “Interstitial water geochemistry by GRIND method.”

Concentrations of dissolved components with depth are shown in Figure F71, in which previously reported results from Expedition 315 are also included (Holes C0002B and C0002D; Expedition 315 Scientists, 2009b). Data obtained during Expedition 338 fill the gap in the previously obtained data, and continuous geochemical profiles with depth were documented to ~1000 mbsf at Site C0002.

Salinity decreases from the seafloor until it reaches a minimum value near 500 mbsf. Chlorinity and Na+ values have profiles similar to salinity from 300 to 500 mbsf. Between 400 and 500 mbsf, IW samples show low salinity, chlorinity, and Na+ concentrations. Because a bottom-simulating reflector (BSR) exists at ~400 mbsf at this site, this low dissolved salt concentration could be attributable to freshwater derived from dissociation of methane hydrate. Salinity, chlorinity, and Na+ concentrations gradually increase to 800 mbsf and then decrease again. Sulfate decreases rapidly beneath the seafloor surface and concentrations remain below 10 mM. However, IW samples from Holes C0002K and C0002L (200–500 mbsf) contain slightly higher SO42– than the shallower and deeper samples previously analyzed. Seawater and/or mud water contamination into the core is not obvious from the other elements analyzed, and the reason for this higher SO42– concentration cannot be explained at present. The reason for the higher SO42– in IW samples from Hole C0002J (at ~900 mbsf) can vary. The core was fragmented when it was recovered, and it was difficult to separate fresh sediment from disturbed samples. Oxidation of H2S is also a mechanism to increase SO42– concentrations in those sediments. Alkalinity, PO43–, and NH4+ all increase from the seafloor to 150 mbsf (roughly corresponding to the Unit I/II boundary) and then decrease with depth. Those components are produced via microbial decomposition of organic matter in the shallow sediment and then decomposed via further microbially anoxic decomposition.

Bromide increases in Unit I. Although Br is abundant in seawater, about the same as Cl, it was probably added to IW because of the decomposition of organic matter. After the decomposition of Br-bearing organic matter, Br follows the dilution of freshwater, similar to Cl. Potassium decreases with depth to 500 mbsf, becomes rather stable to 820 mbsf, and then drastically decreases below that depth. Magnesium varies in a similar manner to K+; however, its concentration does not change below 820 mbsf. Variation of Ca2+ with depth seems to mirror that of K+; it increases gradually to 820 mbsf then drastically increases with depth. Variations in concentrations of major cations (Na+, K+, and Mg2+) resemble that of chlorinity. Minor variations might be caused by the interaction between IW and detrital and authigenic minerals.

Among minor alkaline and alkaline earth elements, Rb and Sr variations with depth are similar to that of Na+; the minimum and maximum concentrations appear at the BSR (~400 mbsf) and the Unit II/III boundary, respectively. Lithium variation is also similar to Na+ variation. Boron and Ba variations are similar to each other: concentrations decrease in Unit I then slightly increase and decrease at 200–300 and 400–500 mbsf, respectively, and reach the maximum at ~815 mbsf. Cesium does not change with depth very much except in Unit I, where it increases with depth.

Silicon concentration generally increases with depth and the maximum concentration is found at the Unit III/IV boundary. Trace elements, although varying in large ranges, can be categorized into two groups based on the variations with depth. The first group is elements concentrated from 200 to 300 mbsf: V, Cu, and Pb. The second group is those that increase with depth: Fe, Mn, Zn, Mo, and U. In the latter group, Fe and Zn are enriched in the lower half of Unit II and the others are enriched in Unit IV. Although controlling factor(s) on the behaviors of those elements are not clear at present, lithology and the associated chemical composition would effect the vertical distribution of the elements.

Interstitial water geochemistry by GRIND method

The GRIND method was proposed as an alternative method for when core recovery was too low to provide enough volume from the WRC sample for the standard method and/or when the sample was too hard to squeeze for the standard squeezing method (Expedition 315 Scientists, 2009b). This method was applied to samples in Holes C0002H (Section 338-C0002H-2R-2; 1111 mbsf) and C0002J (Section 338-C0002J-7R-1; 933 mbsf). In addition, we selected 10 WRCs for IW obtained from Holes C0002J–C0002L to conduct a method comparison test; those samples were extracted by both standard and GRIND methods. Water content of the studied core samples was determined prior to the process (Table T30). Analytical results are shown in Table T31. Comparison between the values obtained from the standard squeezing method and the GRIND method is shown in Figure F72.

The GRIND method was evaluated in detail (see “Appendix A” in the “Methods” chapter [Strasser et al., 2014a]). Although the GRIND method is only applicable for limited components, it is useful to provide some geochemical profiles when a limited amount of sediment can be used to extract IW. As shown in Figure F72, concentrations of some elements determined by the GRIND method are consistent with those determined by the standard squeezing method. Section 338-C0002H-2R-2 (1111 mbsf) used for extracting IW was fine sand, which is not usually suitable for IW analyses because sand is permeable and its IW is easily contaminated with drilling mud. However, IW extracted from Section 338-C0002H-2R-2 is likely pure IW because its chemistry is different from that of the liquid in core liner (LCL), which is seawater mixed with soluble components derived from bentonite (see “Liquid in core liner chemistry”). Chlorinity is 461 mM (extracted with Milli-Q water), which is much lower than that of seawater and close to that of IW in core sediment taken from Hole C0002B at ~1000–1050 mbsf (Expedition 315 Scientists, 2009b). Na+, Mg2+, and Ca2+ concentrations are also close to those from Hole C0002B at ~1000–1050 mbsf. SO42– concentrations are slightly higher than those in nearby sediment; however, those are almost on the trend of SO42– concentration with depth. K+ concentration is much higher than in nearby sediment, as expected from the evaluation test of the GRIND method (see “Geochemistry” and “Appendix A” both in the “Methods” chapter [Strasser et al., 2014a]). Strontium concentration, which is expected to be almost the same as that obtained by the standard squeezing method, also lies on the previously obtained profile of Sr with depth. Boron concentration is also expected to be similar to that obtained by the standard squeezing method; however, it was much higher than those of IW at 1000–1050 mbsf in Hole C0002B. The reason for this large difference in B concentration is unknown at present.

Reasonable concentrations of dissolved components extracted using the GRIND method suggest that this method would be applicable to IW not only in clayey sediment but also in sandy sediment if the pore pressure of core sediment is high enough to prohibit the incorporation of mud invasion. Analysis of IW from sand intervals, however, should be carefully evaluated for contamination.

Interstitial water geochemistry of cuttings

Interstitial pore water was extracted from two underreamer samples (cuttings) at 1975 and 1982.5 mbsf using both squeezing and GRIND methods (see “Geochemistry” in the “Methods” chapter [Strasser et al., 2014a]). We also analyzed one drilling mud water sample from the mud tank. Aliquots were analyzed for anions and cations, as well as major, minor, and trace elemental concentrations following the detailed analytical methods (see “Geochemistry” in the “Methods” chapter [Strasser et al., 2014a]). The results are listed in Table T32. It is not possible to assess any trend from two data points; however, the absolute elemental values shed some light on whether or not the samples were contaminated by the drilling mud. Concentrations of dissolved species are higher in aliquots extracted using the traditional squeezing method compared to those of dissolved species extracted by the GRIND method. In general, concentrations of anions, cations, and major, minor, and trace elements are 2–100 times higher than those of pore water data from Site C0001 (Expedition 315 Scientists, 2009a) with the exception of Na+ and SO42–. Na+ concentration is closer to International Association for the Physical Sciences of the Oceans (IAPSO) standard seawater values and might originate either from contamination by seawater or from the formation. SO42– values are closer to the values of shallower depth (i.e., 3–4 mbsf) in Hole C0001E, although they were extracted from 1975.5 and 1982.5 mbsf in Hole C0002F. Taking into account the concentrations of elements, anions, and cations, these data suggest that IW of two samples are severely contaminated. In the following, we shed further light on the contamination using the chemical composition of the drilling mud water derived from one sample.

The major water-soluble constituents of drilling mud used during operations are generally 50 g KCl, 170 g NaCl, and 5 g soda ash (Na2CO3) dissolved in 1 L of seawater (assumed to contain 40 g/L salts) and adjusted to a pH of 9.0 to 10.5 with KOH solution. These salts were combined with a number of commercial products that act as colloidal lubricants and viscosifiers, including 5 g TelGel (bentonite), 8 g TelPolymerL (cellulose derivative), 10 g TelPolymerDX (starch derivative), 2.5 g Xanvis (xanthan gum derivative), 50 g CleanLubeL (lubricant and gas-hydrate inhibitor), 100 g RevDust (pseudocuttings), and 1 g TelniteGXL (antiseptic agent). K+ concentration could be used as a first-order proxy for contamination, as its concentration was ~30 times higher than that of K+ concentration in Hole C0001E, in which a water-based drilling mud was used. Furthermore, SO42– concentration is as high as seawater at 1975.5 or 1982.5 mbsf, which again suggests contamination by drilling mud. However, K+ and SO42– concentrations in drilling mud used during Expedition 338 were up to 6 and 10 times smaller, respectively, than in the cuttings. Following the analytical results shown in Table T32, the only proxy suitable for contamination of drilling mud would be Zn, Fe, and Si. Zn and Fe in particular are significantly enriched in drilling mud, whereas concentrations in cuttings samples are ~10 and ~100 times smaller, respectively. Consequently, contamination of the analyzed cuttings by drilling mud seems less likely.

Liquid in core liner chemistry

Analytical results of LCL samples are given in Table T33. LCL concentrations are almost the same as those of seawater: chlorinity varies within the range of 530 and 552 mM except for three samples from Hole C0002K. However, the differences in major element concentrations between LCL samples are larger than the analytical error (e.g., analytical error determined by repeating analyses of standard seawater was about ±0.6% for chlorinity). Thus, most LCL likely contains a small amount of IW, in which chlorinity is lower than that of seawater.

Organic geochemistry

Gas chemistry from cores

Headspace gas samples were taken from cores in Holes C0002H and C0002J–C0002L (Tables T34, T35). In addition, void gas samples were collected from gas pockets in core liners (Table T36). Methane, ethane, and propane concentrations in the headspace and void gases were measured by flame ionization detector, and carbon isotopic compositions of methane (δ13C-CH4) were measured by methane carbon isotope analyzer (MCIA) (see “Sampling and analysis of gas samples from cores” in the “Methods” chapter [Strasser et al., 2014a]). Concentrations of methane, ethane, and propane are shown in Tables T34, T35, and T36. Concentrations are shown in parts per million by volume (ppmv) and moles per kilogram, the gas molecules dissolved in 1 kg of interstitial water as calculated by the equation in “Sampling and analysis of gas samples from cores” in the “Methods” chapter (Strasser et al., 2014a). δ13C-CH4 and ratios of methane to ethane and propane (C1/[C2 + C3]) are also shown in Tables T34, T35, and T36.

Methane is the predominant hydrocarbon in all core samples. Ethane and propane are detected in most samples. These features are ubiquitous in deep-sea sediment as shown world-wide by Deep Sea Drilling Project, ODP, and IODP studies. Void gas is richer in methane than headspace gas (Tables T35, T36), which might be due to the lower solubility of methane compared to ethane. Because void gas originates from dissolved gas in interstitial water and headspace gas data show chemical features of gas in sediment, void gas should concentrate more volatile gas and headspace gas taken from sediment should have less volatile gas. Nevertheless, δ13C-CH4 values are not different between void and headspace gases, suggesting that degassing processes did not affect δ13C-CH4 values. The C1/(C2 + C3) ratios of headspace gas from core at 1101.9 and 1111.0 mbsf in Hole C0002H showed good agreement with those of mud gas from Hole C0002F (Table T35).

During this expedition, in addition to the conventional headspace gas extraction, headspace gases were extracted by alkaline solution (see “Sampling and analysis of gas samples from cores” in the “Methods” chapter [Strasser et al., 2014a]). We compared the gas data obtained by the two methods with respect to methane concentration, ethane concentration, C1/(C2 + C3), and δ13C-CH4 (Fig. F73). For ethane, the headspace gases obtained by the NaOH addition method showed lower concentrations than those obtained by the oven-heating conventional method. Such a difference was not observed in methane concentrations. This could be due to higher solubility of ethane compared to that of methane. Such a difference in ethane concentrations causes higher C1/(C2 + C3) ratios obtained by the additional headspace gas extraction than those obtained by the conventional headspace gas extraction. We did not find a significant difference in δ13C-CH4 data between the two methods. The depth profiles of methane, ethane, and propane concentrations are shown in Figure F74. Methane peaks at 30, 270, 920, and 1050 mbsf, whereas ethane peaks at 920 and 1050 mbsf. Propane was not detected at most depths. The methane peak at 30 mbsf could be produced by microbes utilizing relatively fresh organic matter. The methane peak at 270 mbsf could be caused by the presence of gas hydrates, indicated by IW data. The methane peak at 920 mbsf corresponds to the Unit III/IV boundary. Unit III is the transition unit from the Kumano Basin sediment to the old accretionary prism, which is characterized by multiple volcanic ash layers. Methane is concentrated just below Unit III, indicating that sediment in Unit III could play a role as a seal for methane. The methane peak at ~920 mbsf was also observed in mud monitoring data from Hole C0002F (Fig. F75). Additionally, the methane peak was accompanied by an ethane peak (Fig. F74), suggesting the hydrocarbons are derived from a common process.

The C1/(C2 + C3) and δ13C-CH4 data are shown in Figure F76. Data from this expedition are consistent with data from Expedition 315 (Expedition 315 Scientists, 2009b). The C1/(C2 + C3) ratio decreases with increasing depth to 1100 mbsf. The δ13C-CH4 values gradually increase from 100 to 600 mbsf, below which they decrease slightly. The C1/(C2 + C3) ratios and δ13C-CH4 values are generally used to consider the origin of methane (Bernard et al., 1978). Microbial origin methane has δ13C-CH4 values less than –55‰ Vienna Peedee belemnite (VPDB) (Rice and Claypool, 1981), and C1/(C2 + C3) ratios are as high as 1000 (Bernard et al., 1978). On the other hand, thermogenic methane has δ13C-CH4 values between –50‰ and –25‰ VPDB (Schoell, 1983) and C1/(C2 + C3) ratios lower than 100 (Bernard et al., 1978). The data from Site C0002 are plotted on the Bernard diagram (Fig. F77). The methane sampled during Expedition 338 primarily falls in the region of microbial origin. A few samples fall in the region of mixing with thermogenic methane or that of oxidized microbial methane.

Chemical components in IW, resistivity during LWD, and infrared camera data indicate the presence of gas hydrates between 200 and 400 mbsf. The methane taken from the methane hydrate zone corresponding to core samples from Holes C0002K and C0002L falls in the region of microbial origin. The data suggest the methane hydrates are composed of microbial methane. According to the temperature profile at Site C0002, obtained by downhole temperatures and thermal conductivity on cores measured on board the ship during Expedition 315 (Expedition 315 Scientists, 2009b), the temperature is ~43°C at ~1000 mbsf and estimated to be ~86°C at ~2000 mbsf. Because generation of thermogenic methane occurs at temperatures >80°C, any thermogenic methane should have originated from deeper than 2000 mbsf.

Gas chemistry of mud gas

Overview of mud-gas composition

Continuous drilling with mud-gas monitoring took place while drilling Hole C0002F from 875.5 to 2005.5 mbsf. In total, three autonomous data sets were generated during the operation (i.e., the SSX data set including data from the gas chromatograph [GC]-natural gas analyzer [NGA], Geoservices, and the MCIA; the process gas mass spectrometer (PGMS) data set; and the Rn data set) (see “On-line radon analysis” in the “Methods” chapter [Strasser et al., 2014a]). The SSX and PGMS data sets were generated by the newly installed mud-gas monitoring system on the Chikyu (see “Recording of on-line gas analysis and monitoring of drilling operations, time, and depth” in the “Methods” chapter [Strasser et al., 2014a]). Most likely due to air contamination in the onboard system (see “Background control, quality checks, and comparison of different sampling techniques” in the “Methods” chapter [Strasser et al., 2014a]), absolute gas concentrations determined by the MCIA, GC-NGA, and PGMS differ from those measured by Geoservices. For the PGMS, this is only of secondary importance because the concentrations of different gases were normalized to 100% (see “Geochemistry” in the “Methods” chapter [Strasser et al., 2014a]). At the same time, the individual data sets include data from different instruments, measurement techniques, and sampling intervals. Nonetheless, general trends in the hydrocarbon gas data and the nonhydrocarbon gas data can be correlated across the data sets (Figs. F75, F78, F79, F80, F81, F82).

Following the Geoservices data set (Fig. F78), the total gas content as well as the methane concentration ranged between 0% and 16.4%. Ethane concentrations reached 0.03%. Higher homologues (propane, n-butane, and i-butane) were below 0.01%, and consequently, did not add significantly to the total gas composition. Gas concentrations determined by the stationary onboard instruments are different than those from the Geoservices data set: up to 8.64%, 0.09%, and 0.23% for methane, ethane, and propane, respectively, whereas the remaining hydrocarbons remain below 0.01% (Figs. F75, F79).

For the nonhydrocarbons, the PGMS data set was dominated by nitrogen (76.6%) and oxygen (24.8%), whereas the concentrations of the remaining nonhydrocarbons ranged from 0.02% for Xe to 1.55% for H2 (Fig. F80). Although absolute concentrations are not reliable (see discussion in “Liquid in core liner chemistry”), general trends can be observed.

Mud-gas distribution with depth

From 875.5 to 2005.5 mbsf, the total gas concentration shows an overall decline from a maximum of 16.4% near 918 mbsf to values below 0.01% near 1996 mbsf (Fig. F78). Gases in this interval are mainly hydrocarbons. At 918 mbsf, a sharp increase in gas remains to 1000 mbsf and is composed of two different peaks, indicating concentrations of 16.4% at 936 mbsf and 13.8% at 944 mbsf. The total gas from 918 to 1000 mbsf is mainly methane with minor amounts of propane (Figs. F75, F78, F79). Below 1000 mbsf, gas concentrations generally decrease with increasing depth. The total gas concentration is still dominated by methane, and, in the Geoservices data, decreases, on average, by 75 ppm/m. Methane decreases by almost the same rate with 65 ppm/m, whereas no clear trend is visible in the higher homologues. Throughout the whole section, the relative changes of Rn, not in terms of trend but in terms of increase and decrease in concentrations, show a good correlation with relative changes in methane (Fig. F75). The same is true for ethane, except between 918 and 1000 mbsf (Fig. F79). Ethane determined by the GC peaks at 0.005% at 1122 mbsf, and although absent in the Geoservices data, this peak correlates well with the methane and propane data in all data sets (Figs. F78, F79). From 1100 to 1240 mbsf, generally high ethane and propane values are present, which correspond to elevated Rn concentrations (Fig. F79). Between 1240 and 1460 mbsf, generally higher ethane values are visible, which peak with 0.035% in the Geoservices data set and in the GC data (0.04%) at 1378 mbsf. This again corresponds well to relatively small increases in methane and propane to 2.7% and 0.003%, respectively. Between 1460 and 1600 mbsf, an overall decrease in ethane and propane occurs, and beginning near 1600 mbsf, propane shows overall higher values. Below 1827 mbsf, a drop in total gas concentration exists, which corresponds to decreases in methane and ethane concentrations in all data sets, whereas propane shows no overall difference in concentration.

The variations of the nonhydrocarbon components with depth are characterized by significant shifts in concentration of the individual gases at 918, 1000, 1060, 1240, 1540, 1600, 1855, and 1933 mbsf (Fig. F80). Almost all changes correspond to a downtime longer than 120 min (red arrows in Fig. F80; Table T3) (see “Operations”). Here, downtime is defined as the interval during operations when the bit is off bottom and the mud pumps might have been turned off. The shifts can be characterized as follows:

  • 918 mbsf: small increase in H2 and CO2 (Geoservices only) and decrease in N2, O2, and Ar; ~10 m deeper, Rn increases.

  • 1000 mbsf: sharp positive peak in the CO2 data, which correlates with a positive shift in air-derived gases like N2, Ar, O2, and Xe, as well as with a positive shift in the CO2 data provided by Geoservices. This anomaly correlates with a downtime period.

  • 1060 mbsf: decrease in H2, Ar, and Rn and a positive peak in CO2 (only PGMS data) and He. Here, a downtime period was reported, and calibration of the PGMS was carried out.

  • 1232 mbsf: increase in H2, Ar, CO2 (Geoservices), and He. Subtle positive peaks were identifiable in He, Xe, and N2, whereas CO2 (PGMS) shows a strong positive peak. Here, a downtime period of ~25 min was reported, and calibration of the PGMS was carried out.

  • 1540 mbsf: sharp decrease and peak in CO2 derived by Geoservices and PGMS analysis, respectively, as well as a decrease in Ar, an increase in N2, and positive peaks in H2 and O2. For O2, this peak is followed by an overall decrease in concentration. These changes in concentration correspond to a downtime period of almost 13 h and calibration of the gas monitoring instruments.

  • 1600 mbsf: increase in all gases except for N2 and Rn, which decrease. The data shifts correlate with a downtime period of ~16 h.

  • 1855 mbsf: sharp decrease in H2, Ar, O2, He, and Rn and increase in N2 and CO2 (Geoservices only; PGMS shows only a small peak). The shift at this depth corresponds to a downtime period of almost 7 h.

  • 1933 mbsf: Change in trends of Ar and O2 (increase) as well as N2 and Rn (decrease). This change is close to a downtime period of 39.5 min at 1921 mbsf.

Classification of hydrocarbons

A clear classification of the hydrocarbon (HC) gases proved to be difficult. Above 918 mbsf, the Bernard parameter [methane/(ethane + propane)] combined with the δ13C of methane (Bernard et al., 1978) indicates a mixed-gas regime (Fig. F81). A close examination of the gas signature of the major gas increases between 918 and ~1000 mbsf (Figs. F78, F79, F80), showed that the composition of HC gases is defined by the presence of methane and a small concentration of propane, as well as low δ13C values of around –70‰ (Figs. F75, F79, F81). Below 1000 mbsf, a steady decline of the Bernard parameter is evident, similar to the one found in the total gas and the methane data provided by Geoservices. The δ13C and the Bernard parameter point to a thermogenic source of HC gases below 1700 mbsf, which start to increase significantly at ~1830 mbsf.

Preliminary interpretation

The increase of gas at 918 mbsf correlates well with the LWD and lithology findings (see “Logging while drilling,” and “Lithology”). Based on the results of Expedition 315 Scientists (2009b) and Expedition 319 Scientists (2010), it is most likely that at 918 mbsf, an unconformity separates the Kumano Basin (Unit III) and the upper section of the accretionary prism (Unit IV). The dominance of bacterial methane between 918 and 1000 mbsf contradicts the findings on core samples from Hole C0002H and of previous investigations at Site C0002 (Expedition 315 Scientists, 2009b), where gas with a higher thermogenic signature was detected (i.e., Bernard parameter of 100–200). In contrast, the presence of microbial methane is also supported by the results of Expedition 319 Scientists (2010; see figures F48 and F49) and the relatively high amount of organic material found in cuttings from Hole C0002F (see “Lithology”) (Fig. F81). When considering the Rn data, which sharply increase 10 m below the inferred boundary, migration of the fluids from deeper sections might also be possible. Whether the source of the HC gases is the same as that for Rn remains speculative at this point. Based on the compartmentalization (i.e., high gas content below 918 mbsf compared to low gas content above) seen in the gas data, Unit III might act as a seal for upward migration of gases, allowing accumulation of gases below the unconformity. This corresponds with the sharp increase in gas concentration found in cores from Hole C0002J (Fig. F74). The interpretation that the sediment below the unconformity might act as reservoir is supported by an increase in porosity as determined based on LWD resistivity-derived porosity (see “Logging while drilling”).

With increasing depth, the presence of marine organic matter is supported by the Bernard parameter and the δ13C values (see Bernard diagram Fig. F18 in the “Methods” chapter [Strasser et al., 2014a]). During IODP Expedition 319, elevated methane concentrations of up to 12% were found at IODP Site C0009 between 1050 and 1300 mbsf (Expedition 319 Scientists, 2010). Based on the Bernard parameters and a very high percentage (up to 100%) of wood fragments found in this interval, the Expedition 319 Scientists (2010) suggested that the methane is of microbial origin. In Hole C0002F, fragments of wood and lignite were also found, relatively abundant in the upper part of the borehole <1685.5 mbsf and less common in depths >1685.5 mbsf (Figs. F75, F79, F81).

Based on the overall changes detected in hydrocarbons in the Geoservices and SSX data sets (see “Overview of mud-gas composition”) and the Rn data, six boundaries (seven gas packages) can be defined (Fig. F82). The first boundary, separating logging Unit III from Unit VI, is set at 918 mbsf, where the major gas increase exists (Fig. F78). A second boundary is proposed at ~1100 mbsf, followed by boundaries at 1240 and 1460 mbsf. The fifth boundary is set at 1600 mbsf, and the last boundary, where the overall gas concentration is declining and shifts to a more thermogenic regime, exists at 1827 mbsf (Figs. F81, F82). Considering other shipboard data allows correlation of some gas geochemical data to logging unit boundaries at 918 mbsf (logging Units III/IV), 1100 mbsf (logging Subunits IVB/IVC), and 1638 mbsf (logging Units IV/V). The logging unit boundaries at 1638 mbsf are 38 m deeper than the observed changes based on drilling mud–gas data (Fig. F82). There are many possible explanations for this observation, including upward migration pathways for gases, which cannot be adequately explained based only on the shipboard data. Postcruise research might elucidate these differences.

Defining additional boundaries and/or constraining unit boundaries by data shifts and peaks reported for the nonhydrocarbons is difficult because the sources of data shifts are not resolved. Most of the anomalies in the PGMS data and the CO2 data from Geoservices correlate well with downtimes >120 min (Fig. F80), although not all shifts appear after every downtime. Changes in mud composition, and in particular the pH of the mud, can have a significant effect on the CO2 data, but no clear correlation can be made based on the information provided by the daily mud report, even if the lag time is taken into account. Additionally, in the PGMS data after the sudden increase or decrease in concentration, the concentrations stay on this level until the next shift occurs. This might be due to instrument calibrations being carried out during longer downtimes. The downtime itself is likely not the sole reason for the anomalies because after one complete circulation (i.e., 1 lag time), the concentrations should return to previous values, which is not the case. Structural and/or lithologic features cannot be excluded and may have contributed to shifts. The changes at 1060, 1600, and 1933 mbsf are close to the boundaries observed in logging and structural data (see “Logging while drilling” and “Structural geology”), whereas the shifts at 1000, 1232, 1540, and 1855 mbsf correlate well with gas package boundaries found in the HC data (Fig. F82). Of particular interest is the change in trend at 1933 mbsf, which is different from the other signals. This boundary is well correlatable with the top of logging Subunit VB and, thus, the change in the gas data may be dominated by a change in lithology and/or structure. Because most of the anomalies are also visible in the CO2 data provided by Geoservices, a combination of real changes and calibration of the PGMS during downtimes, which superimposes the natural shift to higher/lower concentrations, might explain the observed shifts and peaks in the PGMS data set. Because of the several uncertainties, a clear distinction between artificially created data shifts and actual data cannot be resolved at this time.

The overall low CO2 concentrations (0.03% in the PGMS data, almost atmospheric values) were caused by the high pH of the drilling mud (9.9–10.6). Once CO2 enters the highly alkaline mud, hydrogen carbonate is generated:

CO2 + 2H2O ↔ H3O+ + HCO3. (1)

Occasionally, CO2 concentration increased to two times the atmospheric value (~0.07%), but this might be due to calibration issues associated with highly concentrated standard gases. The high He concentration (up to 0.03%) is also influenced by the standard gas used for the calibration having a He concentration of almost 1%, which is too high for defining low values such as atmospheric (~5.2 × 10–5%). Considering the overall constant He/Ar ratios and the overall low ethane values, He is most likely derived from air. The origin of He will be constrained by postcruise analysis of the 4He/3He ratio.

Similar to He and CO2, Xe concentration is unreasonably high (up to 500 times higher than the normal value in air, ~0.09 × 10–5%). Here again, calibration with a highly concentrated standard gas (Xe = 0.97%) is the most likely reason. Later shore-based analysis of noble gases will constrain the shipboard results.

For N2, although it can originate from various sources including clay-rich sedimentary rock (e.g., Krooss et al., 1995; Mingram et al., 2005), the N2/Ar ratio of 70–87 supports an atmospheric source (Jenden et al., 1988; Krooss et al., 1995). The dominance of O2 and N2 as well as the low values of the other nonhydrocarbon gases in the PGMS data set also indicate air contamination.

Inorganic carbon, total carbon, and total nitrogen

Calcium carbonate (CaCO3), total organic carbon (TOC), and total nitrogen (TN) concentrations were determined from total inorganic carbon, total carbon, and TN measurements of 237 cuttings samples from the >4 mm and 1–4 mm cuttings size fractions from 920.5 to 2004.5 mbsf in Hole C0002F as well as from core samples from Holes C0002K, C0002L, C0002J, and C0002H (see “Geochemistry” in the “Methods” chapter [Strasser et al., 2014a] for analytical procedures). CaCO3, TOC, and TN concentrations and TOC/TN (C/N) ratios are plotted in Figures F83 and F84. These data and total sulfur (TS) and the TOC/TS ratio are provided in Tables T13 and T37.

CaCO3 in core samples varies between 0.03 and 26.42 wt%, with a median of 5.33 wt%. Generally, concentrations fit well with the values found in Hole C0002B (Fig. F84). The highest values were found at 250 mbsf in Hole C0002K and 900 mbsf in Hole C0002J. The data obtained from cuttings correspond well with the results obtained from core samples with the highest CaCO3 concentrations close to 900 mbsf. Overall, CaCO3 in cuttings ranges from 2.63 to 15.76 wt% with a median of 4.20 wt% (Fig. F83). Between 920.5 and 1105.5 mbsf, CaCO3 decreases from 15.76 to 3.5 wt% (0.089 wt%/m) with a few scattered data points. High CaCO3 concentrations near 945.5 mbsf match those determined on cores from Expedition 315 (Expedition 315 Scientists, 2009b). In addition, the decreasing trends from 945.5 to 1049.5 mbsf match those of the decreasing CaCO3 values in Hole C0002B. The magnitudes, however, differ with CaCO3 concentrations ~7% lower based on cuttings. Between 1355.5 and 1895.5 mbsf, CaCO3 shows less variation (from 2.63 to 4.49 wt%), with a median value of 3.57 wt%. From 1895.5 to 1955.5 mbsf, CaCO3 concentration increases to 6.41 wt% then decreases to 3.35 wt% at 2004.5 mbsf. Two CaCO3 values of 1.25 and 1.20 wt% at 880.5 and 915.5 mbsf, respectively, were determined to be influenced by cement (Table T37). CaCO3 concentrations of eight mud water samples range from 2.48 to 2.84 wt% (see Table T13 in the “Methods” chapter [Strasser et al., 2014a]), and these data allow us to assess the background concentration and any potential contamination by mud water to those of the CaCO3 data of cuttings. We also report CaCO3 concentrations of 5.4 and 4.9 wt% at 1975.5 and 1985.5 mbsf, respectively, from the underreamer samples (Table T37). The trend of CaCO3 concentration and distribution with depth generally agrees with the downcore weight percent calcite data, which are derived from XRD measurements (see “Lithology”). However, there are differences in the absolute values (Fig. F23).

TOC found in core samples remains almost constant with depth, with values ranging from 0.21 to 0.97 wt% and a median of 0.58 wt%. Surprisingly, the TOC of cuttings is significantly higher and more variable. Concentrations between 0.80 and 3.89 wt% with a median of 1.61 wt% were detected. Between 920.5 and 1240 mbsf, TOC of cuttings is higher than those of core samples (920–1049.18 mbsf) (Fig. F83, F84). Between 1345.5 and 1655.5 mbsf, TOC ranges from 1.00 to 2.02 wt% and generally decreases downhole along a linear trend but also has a wide scatter. TOC changes to an increasing trend from 1655.5 to 1875.5 mbsf with values from 1.04 to 1.77 wt%. From 1875.5 to 2004.5 mbsf, TOC is almost the same (0.8–1.77 wt%). To some extent, the TOC data correlate with those of the methane gas data (Figs. F78, F83).

TN obtained from core samples varies between 0.01 and 0.12 wt% with a median of 0.08 wt%. TN of cuttings is in the same range, with concentrations from 0.03 to 0.07 wt% and a median of 0.06 wt% (Fig. F83). In the TN of cuttings, three trends can be identified: (1) a downward increase between 920.5 and 1050 mbsf, (2) a scatter of data with no apparent trend between 1050 and 1355.5 mbsf, and (3) an increasing trend between 1355.5 and 2004.5 mbsf with a more gentle slope in comparison to TN between 920.5 and 1050 mbsf. TOC and TN data show opposite trends downhole. It is noteworthy that the TN content of cuttings is generally lower compared to those of the TN data from the other holes at Site C0002 (Fig. F84) (Expedition 315 Scientists, 2009b) but consistent with TN at Site C0009 (Expedition 319 Scientists, 2010).

TS was only determined from core samples and ranges between 0.02 and 1.22 wt% with a median of 0.12 wt%. Although highly variable, TS usually stays below 1 wt% downhole. Between 930 and 940 mbsf, a general increase to values >1 wt% can be observed, but below 1000 mbsf, the concentrations are again <1 wt% (Fig. F84).

C/N ratios in core samples range between 4.5 and 13.1 with a median of 7.2. A clear trend is not visible; the data are scattered. C/N ratios in cuttings are generally higher and highly variable with concentrations between 14.7 and 69.7 and a median of 28.0 (Fig. F83). Between 920.5 and 1265.5 mbsf, an increasing trend in C/N from 22.6 to 69.7 with a median of 28.9 is observed. Relatively higher values from 20.9 to 40.1 between 1265.5 and 1635.5 mbsf also occur. Between 1635.5 and 1855.5 mbsf, C/N is constant with a median of 20.8. Again, C/N between 1875.5 and 2004.5 mbsf is almost constant with a median of 16.4, which is lower than that of the values between 1635.5 and 1855.5 mbsf.

It is commonly accepted that the C/N of marine organic matter typically ranges from ~4 to ~10 (Meyers, 1997), compared to higher values (>10) in terrestrial organic matter. This distinction arises from the absence of cellulose in algae, its abundance in terrestrial plants, and the protein richness of algae (Meyers, 1997). Therefore, data in Hole C0002F suggest that organic matter in the upper section (920.5–1355.5 mbsf) might be dominated by a terrestrial source, consistent with the more negative δ13C values of methane (Fig. F18 in the “Methods” chapter [Strasser et al., 2014a]). However, when the difference in TOC between cuttings and core is considered, the elevated TOC in cuttings is most likely spurious. To assess any contamination by drilling mud, we have determined TOC from eight drilling mud samples (see Table T13 in the “Methods” chapter [Strasser et al., 2014a]). Indeed, TOC is extraordinarily high with concentrations between 13.65 and 17.00 wt%. Consequently, whether terrestrial organic matter is more abundant in Hole C0002F than in the other holes or not can not be evaluated at this point because of the artificial increase in TOC.

The CaCO3, TOC, and TN data from Hole C0002F allow identification of two significant shifts: (1) between 1635.50 and 1645.50 mbsf and (2) between 1885.50 and 1895.5 mbsf (Fig. F83). These shifts are consistent with the boundaries defined by LWD data (Fig. F8; Table T4) that show significant changes in the rock physical properties. Furthermore, the overall trend of the CaCO3 data seem to match with those of the bulk mineralogical data derived from XRD (calcite, Fig. F28) and XRF (CaO, Fig. F30A) (see “Lithology”). However, the lithologic Unit III/IV boundary appears diffuse, although it is well identified in the LWD data at 918.5 mbsf as well as in earlier reports from Hole C0002B (Expedition 315 Scientists, 2009b) and bulk mineralogical data (Figs. F28, F30A).