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doi:10.2204/iodp.proc.308.108.2006

Geochemistry and microbiology

Inorganic geochemistry

Interstitial water geochemistry

Interstitial water chemical data are listed in Table T11 and shown in Figures F26, F27, F28, and F29. Interstitial water alkalinity shows limited variation between 4.61 and 11.35 mM (Table T11). A downhole increase in alkalinity is observed with local maxima at 43, 131, 226, 411, 467, 507, and 584 mbsf (Fig. F26). The pH of the interstitial water varies between 6.59 and 7.60. The acidic nature of the interstitial water (pH < 7.0) above 200 mbsf in this hole is peculiar and rare for pore water from deep-sea marine sediments. Below 200 mbsf to the bottom of the sampled section, the pH values are higher and center around 7.3 ± 0.2 (Fig. F26).

Salinity varies between 3.3 and 3.8 parts per hundred (pph) (Table T11). At depths above 50 mbsf, the salinity is higher and centers at 3.7–3.8 pph. Between 50 and 94 mbsf, salinity decreases from 3.8 to 3.4 pph, and then it remains constant to the bottom of the hole. Interstitial water chlorinity varies from 554 to 570 mM, quite similar to the standard seawater value (International Association of Physical Sciences of the Ocean [IAPSO] = 559 mM), but a clear decrease in chlorinity is observed at 438 mbsf (Fig. F26).

Dissolved sulfate increases slightly from 30.6 mM at the seafloor to a maximum of 37.3 mM at ~35 mbsf (seismic Reflector S10). Below this depth, SO₄^2– decreases to 0 mM at 94 mbsf (Fig. F27), the sulfate/​methane interface (SMI) depth. Interstitial water ammonium concentrations are very high, from 1109 to 6820 µM, and a general downhole increase is observed within lithostratigraphic Unit I. Variations of ammonium contents occur in lithostratigraphic Unit II, but the values are generally high. Dissolved phosphate concentrations are very low (<4.0 µM) with one exception (9.2 µM) at 507 mbsf (Table T11). The interstitial water data also show elevated concentrations of dissolved silica (H₄SiO₄) from 185 to 601 nM. Fluctuations similar to those in ammonium are also observed in dissolved silica, and both elements show a slightly antithetical relationship (Fig. F27). Although ammonium and silica contents are higher at greater depths, at shallower depths ammonium, silica, and sulfate show local maxima at the same depth of ~25 mbsf (Fig. F27).

Na⁺ concentrations of the interstitial water did not show significant downhole variation. However, significant increases of Mg²⁺ and Ca²⁺ and decreases of K⁺ are observed from the seafloor to ~35 mbsf (i.e., seismic Reflector S10). Below 35 mbsf to 165 mbsf (seismic Reflector S30), concentrations of Mg²⁺, Ca²⁺, and K⁺ decrease. Between 240 and 280 mbsf, local maxima are obvious for Na⁺, K⁺, Ca²⁺, and Mg²⁺. Below 280 mbsf, the elemental concentrations stay relatively constant, except at 411 and 603 mbsf, where samples show significantly lower Na⁺ and Mg²⁺ values (Fig. F28).

Downhole variations of Li⁺, B³⁺, and Sr²⁺ mimic those of Ca²⁺ and Mg²⁺. Concentrations of Ba²⁺ are generally low (<1.7 µM) above 75 mbsf in Hole U1324B. Ba²⁺ peaks at 23.2 µM at 112 mbsf (just below seismic Reflector S20) and decreases to 9.9 µM at 150 mbsf. Between 150 mbsf and seismic Reflector S40, Ba²⁺ concentrations are constant at ~10–12 µM. Below seismic Reflector S40, Ba²⁺ concentrations fluctuate between 9 and 21 µM. The deepest sample (603 mbsf) shows significantly lower Sr²⁺ and Ba²⁺ and higher Li⁺ concentrations (Fig. F29). Dissolved Mn²⁺ concentrations decrease from 22.9 µM at the seafloor to a minimum of 5.7 µM by ~25 mbsf. Below 75 mbsf Mn²⁺ concentrations remain generally low, although the deepest sample shows an unusually high value of 25.2 µM. The interstitial water contains high concentrations of dissolved Fe²⁺ at shallow depths with local maxima of 479 and 354 µM at 18 and 94 mbsf, respectively (Fig. F29). High dissolved Fe²⁺ caused iron oxyhydroxide precipitation (lepidocrocite, FeO[OH], as deduced from XRD analyses of the precipitates) from the pore waters immediately after squeezing in the laboratory.

Eight pore water samples from Hole U1324C were also analyzed and show similar trends as the data from Hole U1324B.

In summary, the interstitial water chemistry at Site U1324 shows the largest variations at depths <100 mbsf. Below 100 mbsf, only limited changes are observed. Pronounced pore water chemical changes are particularly important from the seafloor to seismic Reflector S10 (~35 mbsf). Li⁺, B⁺, and Sr²⁺ maxima and a Mn²⁺ minimum occur at ~35–40 mbsf, whereas H₄SiO₄ and Fe²⁺ maxima occur between ~20 and 25 mbsf. Between seismic Reflectors S10 and S30, salinity, Li⁺, B³⁺, and Sr²⁺ decrease; Ba²⁺, Fe²⁺, and NH₄⁺ increase; and Cl^–, Mn²⁺, and H₄SiO₄ remain constant. The extremely high NH₄⁺ content (up to 6820 µM) is consistent with more reducing conditions at this site in comparison with the sites drilled in Brazos-Trinity Basin IV. The general downhole increase in NH₄⁺ likely reflects enhanced organic degradation at depth. The vertical profile, especially the surficial maximum and minimum in dissolved Fe²⁺ and Mn²⁺, are consistent with the hierarchy of redox reactions often observed in deep-marine sediments (Froelich et al., 1979). Elevated Fe²⁺ concentrations within shallow depths may reflect enhanced iron reduction and/or greater availability of detrital iron oxides/​oxyhydroxides or simply Fe-rich clays. The pore water chemistry at the seafloor is probably dominated by dissolution rather than organic matter degradation, which enhances alkalinity, Ca⁺, Mg²⁺, Sr²⁺, B³⁺, and Li⁺ concentrations at ~35 mbsf. The causes for the acidic nature of the pore water (pH < 7) at depths above 200 mbsf are unclear.

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 T12 and presented in Figure F30. With the exception of total hydrogen concentrations above 205 mbsf, Hole U1324C elemental data are coincident with concentration data from Hole U1324B.

TIC concentrations range from 0.60 to 2.92 wt% (average = 1.85 wt%). The organic-rich clays described in lithostratigraphic Subunit IA in the upper 15 mbsf contain <10 wt% CaCO₃. The color-banded laminations of green and red mud and clay within the remaining sections of lithostratigraphic Unit I are typically calcite rich, composed of ~15 wt% CaCO₃. The TIC and CaCO₃ curves exhibit a smooth transition between low and high concentrations from 49.3 to 332.1 mbsf. The excursion in inorganic carbon content observed in Section 308-U1324B-19H-3 at 166.5 mbsf is likely related to increased contribution of iron, indicated by the reddish color of the sediment (see Fig. F7). Inorganic carbon contents generally decrease with depth from 300 mbsf to 608 mbsf.

TOC contents are fairly uniform, ranging between 0.22 and 2.24 wt% (average = 0.75 wt%). Results ≤0 wt% for Sections 308-U1324B-3H-3, 5H-5, 13H-3, and 69X-2 and 308-U1324C-6H-2 are an artifact of the method used to calculate TOC, where

TOC = TC – TIC,

and are not included in the data analysis. TOC concentrations remain relatively constant downhole, with exception of a maximum in the upper 50 m that corresponds to organic-rich, black clays in lithostratigraphic Subunit IA. Mottled black sediments were observed throughout the color-banded laminations of lithostratigraphic Unit I and are likely related to increases in TOC concentrations at 198–227, 294–323, and 361 mbsf.

Nitrogen ranges from 0.10 to 0.53 wt% (average = 0.15 wt%). The total nitrogen curve is similar to the TOC curve, with the highest concentrations in organic-rich layers. Hydrogen contents of the sediments range between 0.17 and 1.00 wt% (average = 0.64 wt%).

Molar C/N ratio ranges between 2.46 and 9.24 (average = 5.85) (omitting results from Sections 308-U1324B-3H-3, 5H-5, 13H-3, and 69X-2 and 308-U1324C-6H-2). The broad range suggests fluctuations between hemipelagic and marine organic matter. The greatest C/N variability in lithostratigraphic Unit I occurs within the organic-rich layers of Subunits IA and ID. C/N is fairly uniform within the carbonate-rich sediments. The jagged ratio trace below 300 mbsf is associated with highly interbedded sand and silt layers in the lower portions of lithostratigraphic Unit I and throughout lithostratigraphic Unit II.

Solid-phase initial interpretations

TOC contents at Site U1324 are slightly higher than concentrations observed within sediments at Sites U1319 and U1320 (Brazos-Trinity Basin IV). An increase in available organic matter based on trends in the TOC, total nitrogen, and C/N curves at 0–50 and 320–608 mbsf suggests isolated periods of intensified organic matter sequestration possibly related to glacial–interglacial cycles and oxidation fronts associated with rapid sedimentation rates. The upper 60 m represents the Holocene/​Pleistocene boundary (see Fig. F22). Within the top 50 m, the maximum C/N value of 9.09 suggests increased input of terrigenous material during sea level rise. Provenance of sedimentary organic matter may be better assessed with carbon and nitrogen isotopic analysis (Jasper and Gagosian, 1990). Rapid sedimentation rates estimated at >25 m/k.y. (see Fig. F22) may be responsible for the preservation of organic matter below 320 mbsf.

Organic geochemistry

Hydrocarbon gas composition

Headspace methane is shown in Figure F31. Rapid increases in methane concentration begin in the middle of lithostratigraphic Unit I, whereas there are only minor amounts of methane from seafloor to 160 mbsf. Methane was the predominant hydrocarbon species determined in Hole U1324B. Concentrations of hydrocarbon gas components are shown in Table T13. The calculated SMI depth is 94 mbsf (Fig. F32). This inverse correlation may suggest that methane results from methanogenesis, which is inhibited in the presence of sulfate. Minor amounts of ethane (<3.5 ppmv) and ethylene (<1.5 ppmv) were detected below 100 mbsf, coincident with localized peak methane concentrations (100–320 mbsf and/or 600 mbsf) (Fig. F31). No higher hydrocarbons were detected in Hole U1324B. When interpreting C₁/C₂ ratios, we need to consider that minor amounts of C₂ (and 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). The C₁/C₂ ratios at Site U1324 are generally high, suggesting biogenic origin of the methane, which may be derived by in situ microbial activities or hydrogeologically driven migration. The almost exclusive presence of methane suggests that the hydrocarbon gases found at Site U1324 are of biogenic, not thermogenic, origin.

Methane concentrations fluctuate with depth. The two highest concentrations of methane were observed in the range of 40,339–41,620 ppmv at depths of 212 and 261 mbsf. According to the lithologic description, gas-bearing strata from 198 to 278 mbsf are a mixed association of muds and clays within lithostratigraphic Unit I. These strata are positively correlated with slight increases in diagenetic ethane. In situ methanogenesis is expected to be minor considering the low abundance of subsurface microbes. Hence, the anomalously high concentrations in the middle part of lithostratigraphic Unit I are inferred to reflect fluid flow and transport. Stratigraphic accumulations occur in relatively permeable sediments and form largely from bacterial methane generated in situ or slowly migrated from depth in the section.

Microbiology

Biomass enumeration

The prokaryotic cell distribution in Hole U1329B was determined using whole-round core samples. The sampling frequency was one sample every 4–7 m between 2.8 and 27.2 mbsf, one sample every 8–10 m between 27.2 and 111.7 mbsf, and one sample every 15–30 m between 111.7 and 603.1 mbsf. The maximum cell density, which was 2.0 × 10⁵ cells/mL, was observed at 2.8 mbsf. Cell density decreases with depth and was below detection limit (1.0 × 10⁴ cells/mL) within ~100 mbsf with the exception of one sample at 131.8 mbsf (2.8 × 10⁴ cells/mL) (Fig. F33). The biomass of microbial cells at Site U1324 was unexpectedly small considering the location and high sedimentation rate of this site. The predominance of clay-rich sediment at Site U1324 is interpreted to explain the low microbial abundance (Table T14).

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