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doi:10.2204/iodp.proc.308.103.2006

Geochemistry and microbiology

Inorganic geochemistry

Interstitial water chemistry

Interstitial water chemistry results are listed in Table T10 and shown in Figures F26, F27, F28, and F29.

Alkalinity was widely variable throughout Hole U1319A, ranging from a minimum value of 2.95 to a maximum of 19.45 mM (Table T10). Alkalinity increases to the peak concentration of 19.45 mM at 13.5 mbsf and then decreases ~4 mM at 40 mbsf. Below this depth alkalinity remains constant at ~4 mM (Fig. F26). Interstitial water pH varies from 7.18 to 7.84. The pH depth profile is similar in downhole variation trends to alkalinity but with different maxima depths. The pH maximum, at 24–36 mbsf, is below the alkalinity concentration maximum depth (15 mbsf). Below 50 mbsf, pH remains approximately constant, but with a slight downhole increase from 7.3 to 7.5 (Fig. F26).

Salinity varies from 3.2 to 3.7 parts per hundred (pph) (Table T10). Salinity decreases significantly downhole from the seafloor to ~20 mbsf. Below this depth, salinity shows more limited but irregular variation between 3.2 and 3.5 pph. A low salinity of 3.2 pph is again reached at ~110 mbsf, corresponding to a high methane content peak in this interval. Interstitial water chlorinity values are mostly ~560 mM, similar to the standard seawater value (559 mM), but several mimima are observed, the lowest value of 516 mM is coincident with the salinity minimum at 110 mbsf (Fig. F26).

The dissolved SO42– profile dramatically decreases from 26 mM at the seafloor to ~0.5 mM below 15 mbsf and then increases slightly downhole to 2.1–2.9 mM at the bottom of the core (Fig. F27). The sulfate/methane interface (SMI) is estimated to occur at 15 mbsf. Dissolved NH4+ concentration increases from 291 to 4411 µM downhole, although a mimimum occurs at 33 mbsf (Fig. F27). The depth profile for dissolved Si2+ is irregular, but slightly higher values occur at shallower depths (Fig. F27). Dissolved PO43– concentrations are highest at shallow depths, with a maximum value of 75.2 µM at 13.5 mbsf, which corresponds to the alkalinity maximum (Table T10). Below 40 mbsf, PO43– concentrations are generally low (several micromolar concentration) (Table T10).

The depth profile of Na+ is very irregular with several mimima at 13.5, 40.5, 53.5, and 133 mbsf (Fig. F28). Na+, K+, Mg2+, and Ca2+ profiles decrease similarly at shallow depths, reaching mimima at ~13.5 mbsf, corresponding to alkalinity and PO43– maxima at this depth. Below this depth, K+ and Mg2+ increase slightly and then decrease to TD. In contrast, Ca2+ increases to a maximum at 90 mbsf and then decreases slightly to the bottom of the hole. However, the sample from Section 308-U1319A-16X-3 at 133 mbsf shows distinct concentration minima for Na+, Mg2+, and Ca2+ relative to the respective downhole trends (Fig. F28).

Concentration-depth trends of B3+ and Ba2+ are similar to those of alkalinity and pH, with a peak at ~13.5 mbsf (Fig. F29). The dissolved Li+ depth profile is similar to those of K+ and Mg2+, and the Sr2+ profile mimics variations in Ca2+ (Fig. F29). The depth profile for dissolved Fe2+ is irregular, but slightly higher values are present at shallower depths. Mn2+ concentrations in interstitial water show a trend similar to that of SO42–; namely, Mn2+ concentrations decrease from 147 µM at the seafloor to 5 µM at 12 mbsf, followed by a small peak between 15 and 30 mbsf with values as high as 23.9 µM. Below this depth Mn2+ content remains extremely low (<4.0 µM) (Fig. F29).

In summary, interstitial water chemical compositions show great variability at shallow depths with maximum or minimum values centered at ~15 mbsf, which is coincident with lithostratigraphic Subunit IIB, homogeneous black clay. The sharp pore water chemistry changes at this very shallow subseafloor depth suggest very rapid anaerobic degradation of organic matter through sequential redox reactions within the uppermost 15 mbsf. Sulfate reduction of organic matter in this shallow interval is likely the cause for the decrease in SO42– concentrations and the increases in alkalinity, PO43–, and Ba2+ concentrations (e.g., Gieskes, 1983). The upper zone of increased alkalinity (centered at ~13.5 mbsf) coincides with decreases in Ca2+, Mg2+, and Sr2+ ions, which may suggest precipitation of diagenetic carbonates within this interval (e.g., Baker and Burns, 1985).

Solid-phase chemistry

Initial results for total inorganic carbon (TIC), total organic carbon (TOC), total nitrogen, and total hydrogen analyses on sediment squeeze cakes are listed in Table T11 and presented in Figure F30. TIC concentrations are highly variable throughout the hole, ranging from 0.87 to 4.08 wt%. The lowest concentration of TIC (0.16 wt%) occurs between 25 and 26.5 mbsf, directly below the highest peak in TIC content of 3.56 wt% at 23.5 mbsf. TIC concentrations initially decrease to ~1 wt% within the upper 39 mbsf, approaching an average concentration of 2.49 wt% below 39 mbsf to TD.

TOC content ranges between 0.16 and 1.9 wt% (average = 0.75 wt%). The result of 0 wt% for the sediment interval collected at 36 mbsf is an artifact of the method used to calculate TOC, where

TOC = TC – TIC,

and is not included in the data analysis. The TOC curve exhibits two concentration maxima at 6–13.5 and 34.5–39 mbsf, with maximum respective values of 1.06 and 1.9 wt%. TOC concentrations below 39 mbsf decrease to 0.67 wt% at TD.

Downhole variation is minimal in total nitrogen and hydrogen contents. Nitrogen contents range from 0.17 to 0.28 wt% (average = 0.23 wt%). Hydrogen concentrations range between 0.49 and 1.06 wt% (average = 0.77 wt%).

The molar ratio of organic carbon to total nitrogen (C/N) ranges from 0.88 to 8.5 (excluding results from 36 mbsf) and averages 3.77. C/N ratios tended to be higher within depth intervals 4.3 to 13.5 mbsf and 33.1 to TD, corresponding to respective lithostratigraphic Units II and VI.

Solid-phase initial interpretations

Comparison of organic and inorganic carbon concentrations with lithology suggests a relationship between carbon content and lithostratigraphic units. The TOC peak between 6 and 13.5 mbsf (Fig. F30) coincides with lithostratigraphic Subunit IIB, a homogeneous black clay. Elevated TOC concentrations are also observed between 34.5 and 39 mbsf within the upper portion of lithostratigraphic Unit IV. TIC maxima at 1.5 and 23.5 mbsf agree with sediment lightness in foraminifer-rich Units I and III (Figs. F9, F10). Coincidence between peaks in TIC contents and foraminifer-rich lithostratigraphic Units I and III suggests that carbon within these units is predominately inorganic.

The average C/N of 3.77 is indicative of organic matter derived primarily from algal material (Fig. F30). Marine organic matter has a C/N range of 4–10 compared to the typically high ratios (>20) associated with terrigenous organic matter (e.g., Bouloubassi et al., 1999). Conversely, C/N ratios <5 may suggest low-productivity conditions or poor preservation (e.g., Bouloubassi et al., 1999). The C/N maximum (5.92) observed at 13.5 mbsf is coincident with the black clay in lithostratigraphic Subunit IIB (Fig. F9). A second maximum C/N ratio of 8.53 at 34.5 mbsf suggests an organic-rich component in lithostratigraphic Unit VI (Fig. F9). High ratios (>8) may result from input of terrestrial material or preferential degradation of nitrogenous matter during early diagenesis (e.g., Bouloubassi et al., 1999). Bulk carbon and nitrogen isotopic analyses can provide an additional constraint with which to better access the significance of C/N ratios observed in the Gulf of Mexico.

Organic geochemistry

Hydrocarbon gas composition

Methane was the predominant hydrocarbon present in all cores of Hole U1319A. Concentrations of methane and ethane are shown in Table T12. Very minor amounts of ethane (~0.7 ppmv) were detected in a few sections, and no higher hydrocarbons were detected. The presence of only methane suggests that the hydrocarbon gas is a result of biogenic production rather than of thermogenic origin. The vertical distribution of headspace methane is shown in Figure F31. Methane concentrations fluctuate with depth; the highest concentration of methane (11,310 ppmv) occurs at 29 mbsf. Below this depth, methane concentrations decrease until 105 mbsf. Slight increases in methane concentrations are again observed between 105 and 153 mbsf. The SMI is estimated to occur at 15 mbsf, as shown in Figure F32.

Microbiology

Biomass enumeration

Samples for prokaryotic cell enumeration were taken from microbiology whole-round core samples (Table T13). Whole rounds were collected every ~4 m between 4.27 and 21.5 mbsf (5 samples), every ~9 m between 21.5 and 117.3 mbsf (10 samples), and every ~20 m between 117.3 and 152.4 mbsf (2 samples). The highest biomass was found near the seafloor (4.4 mbsf), which contained 1.2 × 106 cells/mL, and the number of cells decreased with depth in general (Fig. F33). The biomass depth profile is similar to results from previous Ocean Drilling Program (ODP) legs (Parks et al., 1994; D’Hondt, Jørgensen, Miller, et al., 2003; Newberry et al., 2004). However, there were several features observed in the vertical profile at this site. One was the significantly lower biomass compared to previous reports and the other was the drastic decrease in cell numbers near the seafloor (17 mbsf), <1.0 × 105 cells/mL. Considering the geological setting of this site and the low sedimentation rates, energy supply scarcity could be a possible reason for the low biomass and the drastic decrease in cell numbers in near-seafloor sediments. In addition, we observed an anomalously high biomass (1.2 × 105 cells/mL) at 37.5 mbsf, consistent with an increase in TOC concentrations (34.5 and 39 mbsf).