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doi:10.2204/iodp.proc.308.104.2006

Geochemistry and microbiology

Inorganic geochemistry

Interstitial water geochemistry

Interstitial water chemistry results are listed in Table T9 and shown in Figures F18, F19, F20, and F21.

Alkalinity varies from 3.05 to 15.99 mM (Table T9). Alkalinity rapidly increases from 4.77 mM near the seafloor to a maximum of 15.99 mM at 20 mbsf and then decreases to 5.28 mM at 121 mbsf. The depth of 150 mbsf is marked by an alkalinity maximum of 10.27 mM. Below this depth, alkalinity remains constantly low at ~4 mM (Fig. F18). The pH of the interstitial water varies between 7.09 and 7.84. In general, pH values are highest (>7.5) above 150 mbsf. The 150–250 mbsf interval is characterized by lower pH values, as low as 7.09. Below this interval, slightly higher pH values (7.37–7.54) are recorded (Fig. F18).

Salinity varies between 3.3 and 3.6 parts per hundred (pph) (Table T9). A significant downhole decrease in salinity is observed from the seafloor to ~30 mbsf. Below this depth, salinity increases from the minimum value of 3.3 to 3.5 pph at ~70 mbsf. Below 70 mbsf, salinity shows limited variation between 3.4 and 3.5 pph (Fig. F18). Interstitial water chlorinity varies from 547 to 593 mM with a slight increase downhole (Fig. F18).

The dissolved SO₄^2– profile dramatically decreases from 24.4 mM at the seafloor to ~0.6 mM at 21 mbsf. In the interval 28–63 mbsf, SO₄^2– concentrations remain low at 0.6–2.7 mM and then increase to slightly higher values of 2.1–6.8 mM downhole to the bottom of the core (Fig. F19). The calculated sulfate/methane interface (SMI) depth is 22 mbsf. Dissolved NH₄⁺ concentrations generally increase from 132 µM at the seafloor to a maximum of 3816 µM at 92 mbsf. Concentrations then remain constantly high at ~2000–3000 µM downhole to the bottom of the hole (Fig. F19). Dissolved PO₄^3– concentrations vary from 0.7 to 68.1 µM, with maximum values occurring at shallow depths between 14 and 21 mbsf. Below 100 mbsf, PO₄^3– concentrations are only several micromolar with one exception (32.4 µM) at 150 mbsf. Interstitial water data also show elevated concentrations of dissolved silica (H₄SiO₄) from 95 to 609 nM. Higher silica concentrations occur at shallow depths, with a maximum at 21.5 mbsf. Silica decreases slightly downhole (Fig. F19).

Concentrations of Na⁺ and Mg²⁺ are slightly lower than seawater values (Na⁺_sw = 480 mM, Mg²⁺_sw = 54 mM) and decrease slightly in the uppermost 40 mbsf. Values then remain nearly constant downhole (Fig. F20). Ca²⁺ concentrations generally decrease to ~40 mM at shallow depths (uppermost 40 mbsf), which is lower than seawater value (10.55 mM). In the interval between 40 and 140 mbsf, Ca²⁺ contents are constantly low. Below this depth, concentrations slightly increase to 10–12 mM to the bottom of the cored section (Fig. F20). Concentrations of K⁺ generally decrease downhole from nearly seawater value (10.44 mM) at the seafloor to a minimum of 3–4 mM at the bottom of the cored section (Fig. F20).

Concentration-depth profiles for B³⁺, Ba²⁺, and Fe²⁺ define two distinct higher-concentration zones, one shallow at 20–40 mbsf, corresponding to the upper part of lithostratigraphic Unit II, and another at 160–180 mbsf, corresponding to the lower part of lithostratigraphic Unit IV (Fig. F21). Li⁺ concentrations decrease similarly to sulfate concentrations at shallow depths and reach a minimum at ~40 mbsf. From 60 to 130 mbsf, Li⁺ contents increase from 11.78 to 28.04 µM. Below this depth, a general decreasing trend for Li⁺ is apparent (Fig. F21). Sr²⁺ contents show a general decrease to ~70 µM at shallow depths (uppermost 40 mbsf), slightly lower than seawater value (Sr²⁺_sw = 87 µM). Sr²⁺ concentrations remain low between 40 and 140 mbsf. Below this depth, Sr²⁺ values slightly increase to 100–120 µM to the bottom of the cored section (Fig. F21). Mn²⁺ concentrations in interstitial water show a similar trend to sulfate values, with a dramatic decrease from 145.4 µM at the seafloor to 1.94 µM at 30 mbsf. Below this depth, Mn²⁺ contents remain extremely low (<4 µM), but higher values are observed at 131 mbsf (18.03 µM) and in the interval from 168 to 176 mbsf (5.78–9.82 µM) (Fig. F21).

In summary, rapid changes in interstitial water chemistry profiles occur at shallow depths down to 40 mbsf, corresponding to the upper part of lithostratigraphic Unit II. The decrease in SO₄^2– concentrations coincides with a concomitant increase in alkalinity and decreases in Mn²⁺, Ca²⁺, Mg²⁺, Sr²⁺, and Li⁺ concentrations. These downhole variations may reflect a combination of processes, including rapid anaerobic degradation of organic matter, redox pathways, and diagenetic carbonate precipitation at shallow depths (e.g., Gieskes, 1983; Baker and Burns, 1985).

Solid-phase chemistry

Initial results for total inorganic carbon (TIC), calcium carbonate (CaCO₃), total organic carbon (TOC), total nitrogen, molar ratio of organic carbon to total nitrogen (C/N), and total hydrogen analyses on sediment squeeze cakes are listed in Table T10 and presented in Figure F22. TIC contents range from 0.07 to 3.14 wt% (average = 1.46 wt%). Inorganic carbon concentrations initially decrease to <0.5 wt% within the upper 13.5 mbsf and then steadily increase to 2.5 wt% at TD.

TOC contents range 0.03–1.99 wt% (average = 0.53 wt%). The results of 0 wt% for the sediment intervals collected at 13.5, 232.9, and 68.8 mbsf are artifacts of the method used to calculate TOC (where TOC = TC – TIC) and are not included in the data analysis. TOC contents remain relatively constant throughout the hole with exception of maxima at 14.04, 167.8, and 201.3 mbsf. The maximum concentration (1.99 wt%) occurs below the sulfate reduction zone.

Nitrogen contents of the sediments range between 0.09 and 0.26 wt% (average = 0.16 wt%). Hydrogen concentrations of the sediments range between 0.06 and 1.14 wt% (average = 0.71 wt%). The total nitrogen and hydrogen curves exhibit greater downhole variability than observed in Hole U1319A. The sawtooth shape of both profiles terminates at ~100 mbsf, where concentrations approach near-uniform concentrations.

Molar C/N ratios range from 0.3 to 12.35 and average 4.21 (omitting results from 13.5, 232.85, and 68.8 mbsf). The considerable variability in the ratio trend can be described in three subsections of the hole. The ratio within the upper 80 mbsf is variable with four minima at or near zero (calculations explained above). C/N ratios are near constant values of 3.2 from 79.3 to 159.7 mbsf. The ratio rises to 7.5 below 167.8 mbsf and then declines in a sawtooth fashion to a value of 3.2 at TD.

Solid-phase initial interpretations

Trends in data correspond to locations of seismic Reflectors R10, R20, R30, R40, and R50 (Fig. F22). Peaks occur for TIC, CaCO₃, TOC, nitrogen, C/N, and hydrogen at seismic Reflector R10, and concentration minima occur directly above seismic Reflector R20. Elevated inorganic carbon concentrations are also found at seismic Reflector R30, coincident with foraminifer-bearing clay of lithostratigraphic Unit III. TOC peaks occur across seismic Reflectors R40 and R50.

The average C/N ratio of 4.21 is indicative of organic matter derived primarily from algal material (Fig. F22) or the presence of inorganic nitrogen. Ratios are most variable within the sand and mud of lithostratigraphic Unit II and the bioturbated clays of Unit V.

Organic geochemistry

Hydrocarbon gas composition

The vertical distribution of headspace methane concentrations is shown in Figure F23. The rapid increases in methane concentrations begin with the disappearance of sulfate (Fig. F24). The calculated SMI depth is 22 mbsf. This inverse correlation suggests that methane results from methanogenesis, which is inhibited in the presence of sulfate. The predominant hydrocarbon found at Site U1320 was methane. Concentrations of hydrocarbon gas components are shown in Table T11. Only very minor amounts of ethane (<1.9 ppmv) and ethylene (<0.6 ppmv) were detected in a few samples. No higher hydrocarbons are detectable at Site U1320. When interpreting C₁/C₂ ratios, one should consider that minor amounts of C₂, C₃, C₄, and C₅ compounds can also be generated in situ during early (low temperature) diagenesis of organic matter (e.g., Kvenvolden and Barnard, 1983). C₁/C₂ ratios at Site U1320 are generally high, suggesting a biogenic origin of the methane from in situ microbial activities or hydrogeological migration. Hence, the near exclusive presence of methane suggests that the hydrocarbon gases found at Site U1320 are of biogenic, not thermogenic, origin.

Concentrations of methane fluctuate with depth. The highest concentration of methane (57,714 ppmv) is observed at 122 mbsf. There is a mud layer above ~79 mbsf (lithostratigraphic Unit II) that may play a caprock role, sealing the underlying strata. According to the lithologic description, gas-bearing strata from 79 to 159 mbsf (lithostratigraphic Units II, III, and IV) are mixed associations of sand and mud, which might serve as a natural reservoir for biogenic methane. Below 159 mbsf, methane concentrations decrease to 219 mbsf. The slight increase in methane concentrations observed between 228 and 294 mbsf is similar to the trend observed at Site U1319.

Microbiology

Biomass enumeration

Prokaryotic cell enumeration at Site U1320 was conducted using whole-round core samples for microbiology. The sampling frequency for microbiology was every ~5–10 m between 2.92 and 92.4 mbsf (nine samples) and every ~20–30 m between 92.4 and 284.8 mbsf (seven samples). The highest biomass was found in the sample nearest to the seafloor (7.5 mbsf), which contained 1.2 × 10⁶ cells/mL. Cell numbers sharply decrease with depth without anomalies (Fig. F25; Table T12). The trend of the vertical profile of cell density is normal, but the amount of biomass was low considering the high sedimentation rates at Site U1320.

Methane bubbles were observed in several core samples, perhaps due to biogenic methanogenesis. However, the low biomass suggests low in situ microbial activity at Site U1320, and large anomalous cell numbers in the sediments that could be related to methane production were not observed. This indicates that the biogenic methane found in high concentrations 79 to 159 mbsf (see above) were either generated in the geological past or migrated into their present location.

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