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doi:10.2204/iodp.proc.344.107.2013

Geochemistry

Inorganic geochemistry

We collected 30 APC whole-round samples (12–24 cm long) and 9 XCB whole-round samples (25–36 cm) from Hole U1413A at a frequency of two samples per core through Core 344-U1413A-13H and one to two samples per core in the rest of the hole. Most XCB cores were of high enough quality for interstitial water processing. Hole U1413B consisted of only three APC cores, devoted to high-resolution biogeochemical processes studies at and in the vicinity of the sulfate–methane transition zone (SMTZ). A total of 26 whole-round samples (5–10 cm) were collected on the catwalk in the three cores from Hole U1413B: five samples from Core 344-U1413B-1H, twelve from Core 2H, and nine from Core 3H. All samples from Hole U1413B were processed in a glove bag under a nitrogen atmosphere. Real-time sulfate concentration analyses were used to accurately delineate the SMTZ depth at 16 mbsf. A 2 cm slice from each of the cleaned interstitial water sediment samples was collected for shore-based microbiological analyses in Hole U1413A to ~100 mbsf and in Hole U1413B. These microbiological samples were vacuum-sealed and stored at –80°C. In Hole U1413C, we collected 41 (30–32 cm long) whole-round samples at a frequency of one to two per core.

All samples were thoroughly cleaned for drill water contamination. The cleaned samples from all three holes were placed in Ti squeezers and squeezed at gauge forces of <30,000 lb. The inner diameter of the Ti squeezers is 9 cm; thus, the maximum squeezing pressure was 3043 psi (~21 MPa). The pore fluid was collected in syringes and passed through a 0.2 µm filter prior to analysis. The volume of pore fluid recovered varied with lithology and coring technique: 31–72 mL from the APC samples, 11–38 mL from the XCB samples, and 5.5–34 mL from the RCB samples. Specific aliquots of pore fluids were used for shipboard analyses, and the remaining fluid was sampled for shore-based analyses (see “Geochemistry” in the “Methods” chapter [Harris et al., 2013]). One sample from Section 344-U1413C-15R-1 was collected for shore-based He isotope ratio analysis.

Samples recovered from below the SMTZ should not contain sulfate. We used the sulfate concentrations reported in Table T6 as a tracer for drilling contamination with surface seawater and corrected the pore fluid concentrations in Hole U1413A and U1413C samples, as described in “Geochemistry” in the “Methods” chapter (Harris et al., 2013).

The uncorrected data for the major element concentrations are listed in Table T6, and in Table T7 we list the sulfate-corrected concentration data. Table T8 lists the uncorrected minor element concentrations, and Table T9 presents the sulfate-corrected data. Figures F18, F19, F20, and F21 illustrate only the sulfate-corrected data.

Salinity, chloride, and alkalis (sodium and potassium)

Downhole profiles of salinity, chloride, potassium, and sodium from Site U1413 are shown in Figure F18. Salinity values slightly decrease with depth from a seawater value at the seafloor to ~32.5 below the SMTZ at ~16 mbsf. Between the SMTZ and ~150 mbsf, which corresponds a probable MTD (see “Lithostratigraphy and petrology”), salinity values range from 32.5 to 33.0. Below 150 mbsf, salinity slightly decreases to ~30 at ~200 mbsf and remains constant at ~30 to the bottom of Hole U1413C. The drop of ~1.5 salinity units below the SMTZ is most likely related to authigenic carbonate precipitation in the vicinity of this transition zone. At this site, Cl concentrations are on average lower than modern seawater by 3–10 mM. Cl concentrations increase with depth to ~150 mbsf, where an MTD is observed (see “Lithostratigraphy and petrology”). Interestingly, the Cl concentration at this depth is that of modern seawater, 559 mM. From ~150 to ~500 mbsf, Cl concentrations decrease, with minimum concentrations of 522–526 mM observed at 475–510 mbsf, suggesting minor lateral fluid flow of fresher fluid in the more sandy section in lithostratigraphic Unit III (see “Lithostratigraphy and petrology”). Slightly elevated Li concentrations (Fig. F21) support the interpretation of slightly fresher and Li-rich fluid flow from a higher temperature source; the in situ temperature is only ~32°C. This observation is especially intriguing because a similar decrease in Cl concentrations and a corresponding increase in Li concentrations were observed at Site U1379, ~100 m deeper in the sediment section.

The cause for lower than modern seawater Cl values (98.5%–99% modern seawater Cl) in the uppermost ~150 m, where several MTDs were observed, is unclear.

Sodium concentrations decrease with depth from about the seawater value of 480 mM at ~3 mbsf to 435–440 mM at 45 mbsf, the depth of the lithostratigraphic Unit I/II boundary (see “Lithostratigraphy and petrology”). The decrease in sodium may be driven by volcanic tephra alteration to zeolites; in this depth interval, Cl concentrations do not increase. Below this depth, sodium concentrations remain approximately constant, varying between 445 and 460 mM to the bottom of Hole U1413C.

Similar to the other Expedition 344 drill sites, at and below the seawater/​sediment interface, K concentrations are slightly higher by 1–2 mM than the modern seawater value of 10.4 mM. This elevated concentration most likely relates to clay ion exchange reactions. K concentrations gradually decrease from the seafloor and reach ~8.5 mM (Fig. F18) at the base of Unit I (see “Lithostratigraphy and petrology”); this decrease may be caused by volcanic tephra alteration to clays. The decrease in K concentrations steepens in the lowermost 80 m of Hole U1413A and reaches a minimum of ~7 mM. Deeper in Hole U1413C, K concentrations are approximately constant to ~380 mbsf in lithostratigraphic Unit II (see “Lithostratigraphy and petrology”). Below 380 mbsf, K concentrations slightly decrease to a minimum of ~5 mM (~50% modern seawater value) at the bottom of this site. The uptake of K may be related to zeolite formation and/or to smectite/illite transformation at greater depth.

Alkalinity, sulfate, ammonium, calcium, and magnesium

Sulfate, alkalinity, and ammonium (Figs. F19, F20) have characteristic organic matter remineralization concentration-depth profiles in the uppermost ~150 m, which are impacted by MTDs at ~45 and ~150 mbsf (see “Lithostratigraphy and petrology”). Sulfate concentrations decrease from ~26 mM to zero between the seafloor and the SMTZ at 16 mbsf. Alkalinity increases from seawater value at the seafloor to a maximum of ~36 mM at ~42 mbsf. Below the MTD at ~45 mbsf, concentrations gradually decrease to ~27 mM at 68 mbsf, increase again to ~34 mM at 92 mbsf, and following this secondary maxima, alkalinity steadily decreases to a minimum of 2.7 mM at the bottom of Hole U1413C. Alkalinity is involved in carbonate diagenesis in the upper ~150 m, as suggested by the concomitant decrease in Ca and Mg concentrations. Similar to alkalinity, ammonium concentrations increase steeply from the seafloor, where concentrations are zero, to a maximum of ~8 mM just above the first slump at ~45 mbsf. Below this MTD, concentrations first increase and then decrease slightly to ~4.7 mM at the MTD at ~151 mbsf (see “Lithostratigraphy and petrology”). After increasing to 7 mM at ~200 mbsf, concentrations decrease to the bottom of Hole U1413C, as in typical ammonium concentration-depth profiles.

High-resolution sulfate, alkalinity, and methane data from the uppermost 30 m of Site U1413 are shown in Figure F19. The SMTZ is clearly observed at 16 mbsf; at this depth, alkalinity reaches a maximum of 26.4 mM. Below the SMTZ, alkalinity slightly decreases to 23 mM and then increases with depth. Sulfate has an atypical concentration-depth profile that suggests another small MTD in the uppermost 8–10 m; this is also supported by the alkalinity depth profile.

Ca and Mg concentrations decrease from seawater values at the seafloor to minima of 1.1 and 39.8 mM, respectively, at the lithostratigraphic Unit I/II boundary, where an MTD is observed (see “Lithostratigraphy and petrology”). This pattern reflects precipitation of authigenic carbonates. There are marked increases in Ca concentrations below this depth, in particular below the second MTD at ~150 mbsf to the bottom of Hole U1413C, where Ca concentration is ~15 mM (~50% higher than modern seawater value). Mg concentrations increase below the first and second MTDs, most likely driven by clay ion exchange with ammonium. However, below ~150 mbsf, the Mg concentration-depth profile is a mirror image of the Ca profile. Mg concentration at the bottom of Hole U1413C is ~19 mM (~53% seawater value). The inverse concentration-depth profiles of Ca and Mg in Hole U1413C suggest diffusional interaction with a higher temperature reaction zone at depth.

Strontium, lithium, manganese, boron, silica, and barium

Downhole distributions of Sr, Li, Mn, B, Si, and Ba are shown in Figure F21. Sr concentrations decrease steeply from seawater concentration at ~3 mbsf to ~60 ?M at the SMTZ (16 mbsf), indicating diagenetic carbonate precipitation. From the SMTZ to ~150 mbsf, Sr concentrations remain low, with values between 41 and 60 µM. This depth interval is associated with high alkalinity and carbonate diagenesis. Below ~200 mbsf, Sr concentrations slightly increase to a maximum of ~75 µM. Several diagenetic reactions may be responsible for this slight increase in Sr concentrations. However, it is difficult to uniquely identify the controlling reaction based on the data available.

Li concentrations rapidly decrease from the seafloor to ~10 µM just below the SMTZ and fluctuate between 10 and 16 µM to the large slump at ~150 mbsf. Li values increase irregularly with depth and peak at ~80 µM below ~400 mbsf in lithostratigraphic Unit III (see “Lithostratigraphy and petrology”) (Fig. F21). This unit is somewhat sandier, and the increase in Li and decrease in Cl concentrations within this zone may indicate transport of slightly fresher fluid originating at a higher temperature. However, besides Li and Cl concentrations, other solute concentration profiles do not indicate that the lower sandier lithostratigraphic Unit III acts as a fluid conduit. Below this depth to the bottom of the Hole U1413C, Li concentrations decrease to a minimum value of ~54 µM.

Mn concentrations are very low, mostly below 4 µM, throughout the section cored.

B concentrations are slightly higher than the bottom seawater value of ~450 nM in the shallowmost samples and steadily decrease in lithostratigraphic Unit I to ~140 µM at the 150 mbsf MTD. From ~180 mbsf, B concentrations vary and decrease slightly. In lithostratigraphic Unit III (below ~360 mbsf), concentrations fluctuate between 90 and 110 µM (Fig. F21). These lower B concentrations may be either lithologically or diagenetically controlled.

In the uppermost ~40 m, above the first slump (see “Lithostratigraphy and petrology”), Si concentrations are above bottom water concentrations and vary between 300 and 500 µM. Below this depth, Si concentrations decrease sharply in the upper ~150 m and then more gradually to a minimum of 65 µM at the bottom of Hole U1413C (Fig. F21).

Dissolved Ba concentrations are extremely low in the uppermost ~150 m. They increase in Hole U1413C and are variable, ranging between 1 and 5 µM. The concentration maxima at ~300 and 450 mbsf may reflect paleoredox boundaries.

Organic geochemistry

The organic geochemistry data for Site U1413 are listed in Tables T10 and T11 and plotted in Figures F22 and F23. In the headspace gases, methane concentrations range from 3 to 42,600 ppmv, ethane concentrations are 0.4 to 106 ppmv, and propane concentrations do not exceed 24.1 ppmv. C₄₊ was detected below 290 mbsf. N-butane concentrations range between 0.8 and 9.3 ppmv, and iso-pentane values range from 0.7 to 8.4 ppmv. In the void gases, methane concentrations range from 792,000 to 841,000 ppmv, ethane concentrations range between 4 and 11 ppmv, and propane concentrations are below 8 ppmv.

In general, methane concentrations increase rapidly below 15.5 mbsf, consistent with the depth of the SMTZ at this site (Fig. F19). From the top of the sediment section to 333 mbsf, headspace and void gas compositions indicate a biogenic hydrocarbon source. Below 333 mbsf, profiles of C₂ to C₅ and C₁/C₂₊ ratios of headspace gas as low as ~25 indicate the presence of thermogenic gases. The geothermal gradient at Site U1413 is 40°C/km (see “Physical properties”); thus, the temperature between 350 mbsf and the bottom of the cored section ranges from 25° to 33°C. This temperature range is too low for in situ production of thermogenic hydrocarbons, suggesting that the gases sampled here migrated from deeper, warmer sediments. This inference will be confirmed by isotopic analyses of the gases postexpedition.

Organic and inorganic carbon distributions are illustrated in Figure F24, and the data are listed in Table T12. Inorganic carbon fluctuates from ~0.6 to <1.6 wt%, and CaCO₃ ranges from 5.2 to 13.5 wt%. Total nitrogen concentrations do not exceed 0.2 wt%. The total carbon and total organic carbon concentrations in lithostratigraphic Unit II decrease slightly to ~1.5 and ~0.5 wt%, respectively. The calculated C/N ratio is ~10 throughout lithostratigraphic Unit I, and the C/N ratio decreases from 14.9 to 4.9 in lithostratigraphic Unit II, though with high data scatter. The organic carbon source and degree of degradation will be further characterized postcruise through carbon, hydrogen, and nitrogen isotopic ratio analyses.

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