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doi:10.2204/iodp.proc.307.105.2006

Geochemistry and microbiology

Geochemistry

Interstitial water and gas

A total of 31 interstitial water samples were obtained from Site U1318. One interstitial water sample per core was collected from Hole U1318A, except Cores 307-U1318A-1H, 2H, and 3H, where resolution was increased to two samples per core. One interstitial water sample per core was collected from Hole U1318B, except Cores 307-U1318B-19X and 20X, from which no samples were recovered. Filtered (0.45 µm) samples were analyzed for pH, salinity, chlorinity, alkalinity, sulfate (SO₄^2–), chloride (Cl^–), ammonium (NH₄⁺), silica (Si[OH]₄), boron (H₃BO₃), dissolved inorganic carbon (DIC), and major (Mg²⁺ and Ca²⁺) and minor (Fe²⁺, Li⁺, Sr²⁺, and Ba²⁺) elements. The interstitial water data are provided in Table T5. Details of analytical methods are provided in “Geochemistry and microbiology” in the “Methods” chapter.

Chlorinity (Table T5) shows an increase from ~575 mM in the shallowest sample to 589 mM at ~150 mbsf in Hole U1318A (Fig. F14). In the interval from 150 to 200 mbsf in Hole U1318B, chlorinity decreases downcore to ~580 mM. Below this interval and to the base of the hole, chlorinity increases to ~585 mM. The downcore trend in chlorinity may reflect a combination of (1) ongoing adjustment of pore water chlorinity to variations in mean ocean salinity that has generally increased over the past few million years in response to increasing continental ice volume (McDuff, 1985), (2) diffusion of chloride from high-chlorinity glacial seawater out of the sediment column (McDuff, 1985), and (3) hydration of silicates.

At Site U1318, 27 samples were collected for safety measurements of methane levels and 25 samples were obtained for analysis of adsorbed methane concentrations (Table T6). Dissolved methane (based on analyses of the safety gas samples) (see “Geochemistry and microbiology” in the “Methods” chapter) remains at or near the detection limit of 0.03 µM throughout the extent of the profile at Site U1318, suggesting little to no methane production. Concentrations of adsorbed methane, based on the difference between samples treated with NaOH and the safety gas samples, were significantly higher throughout the hole (Table T6). Adsorbed methane concentrations were in the range of 0.01 to 0.05 µmol/g throughout most of the site, although there were some distinct excursions to tenfold or higher concentrations in sediments deeper than 162 mbsf.

In Hole U1318A, the highest measured sulfate concentration is 26 mM, close to the seawater value of 28 mM (Fig. F15A). Throughout the extent of the profile, sulfate concentrations never decrease to values <11 mM. The downcore variations with multiple extremes (Fig. F15A) are somewhat enigmatic. A correspondence to lithologic variations, however, suggests a link between lithology, intervals of erosion or nondeposition, and/or sediment accumulation rate. These relationships are discussed in further detail below.

Alkalinity (Fig. F15B) at Site U1318 varies between 5 and 13 mM. In the uppermost part of the site, alkalinity increases from 5 mM near the top to ~12 mM at 10 mbsf. Values then decrease over the underlying 10 m to 5 mM. Below this interval, alkalinity shows a gradual increase to a value of 13 mM near the base of Hole U1318B. The general increasing trend that characterizes the pore water alkalinity profile below 20 mbsf is overprinted by an excursion toward slightly higher values between 40 and 90 mbsf. The downcore trend suggests that the highest rates of alkalinity production, generated mostly as bicarbonate ion (HCO₃^–), are within the upper 10 m of the core, between 40 and 90 mbsf, and at some depth below the base of Hole U1318B. The variations in the alkalinity profile are matched by similar variations in concentrations of DIC (Fig. F15C).

Ammonium in marine pore waters is produced largely through anaerobic reduction of detrital organic matter (Gieskes, 1983). Ammonium concentrations at Site U1318 range between ~400 and 1700 µM, with the highest values found in the upper half (<120 mbsf) of the site. Variations in the ammonium, alkalinity, and DIC profiles (Fig. F15D) mirror the sulfate curve, suggesting that the minor amounts of sulfate reduction observed at Site U1318 led to alkalinity production by anaerobic degradation of organic matter.

Iron concentrations at Site U1318 range from 0 to 68 µM (Fig. F15E), with the higher concentrations occurring in the upper 100 and lower 50 m of the profile (<100 and >200 mbsf, respectively). Variations in the Fe profile are attributed to sulfate reduction and Fe availability for sulfide precipitation. At Site U1318, higher Fe concentrations correspond roughly to depths where dissolved sulfate values decrease slightly, suggesting that reduction of Fe(III)-bearing minerals by sulfide is occurring.

Pore water Mn²⁺ concentrations (Fig. F15F) remain below 5 µM throughout the extent of the profile. Values decrease from 4.6 µM in the shallowest sample to a minimum of 0 µM at 90–120 mbsf. Slightly higher Mn²⁺ concentrations in the shallowest samples imply that the oxidation of organic matter is sufficient to deplete oxygen above the depth of sulfate reduction in the upper 10 m of the sediment column. The interval from 120 to 200 mbsf is marked by a transient excursion to 2 µM, which peaks at ~165 mbsf. Below 200 mbsf, Mn²⁺ is absent from the pore fluid.

The Ba²⁺ profile at Site U1318 is somewhat enigmatic. Although much of the profile is characterized by concentrations <1 µM, a few samples above 85 mbsf in Hole U1318A have unusually high concentrations (6.1 µM in Sample 307-U1318A-2H-1, 140–150 cm; 5.7 µM in Sample 4H-3, 140–150 cm; and 4.0 µM in Sample 6H-3, 140–150 cm) (Fig. F15G). These high concentrations were confirmed by running duplicate samples on the inductively coupled plasma–atomic emission spectrometer. Given that Ba²⁺ in pore fluids is controlled mainly by the solubility of barite, which becomes soluble only when sulfate concentrations are <5 µM, the higher values observed in the upper half of the profile are not clearly understood.

Excursions in the profiles of alkalinity, DIC, sulfate, ammonium, Fe, and Mn²⁺ show a striking correspondence with lithologic variations (Fig. F15), in particular, unconformable surfaces and the contacts between lithostratigraphic units (see “Lithostratigraphy”). In each case, the lithologic boundaries are represented by terminal electron acceptor process successions in the pore water profile, namely Mn²⁺, followed by Fe²⁺ and SO₄^2–. This association suggests that downcore variations in the profiles of sulfate, alkalinity, DIC, ammonium, Fe²⁺, and Mn²⁺ record the positions of past sediment/​water interfaces, below which an electron acceptor profile developed. The increasingly broad nature of the geochemical excursions with depth is interpreted as reflecting increasing age of the pore water and time available for diffusion.

The base of lithostratigraphic Subunit 1A (33.3 mbsf in Hole U1318A and 35.03 mbsf in Hole U1318B), for example, corresponds to a peak (excursion to higher values) in the sulfate profile and a valley (lower values) in the alkalinity, DIC, and ammonium profiles. Given the sharp decrease in the sulfate profile and increases in alkalinity, DIC, and ammonium profiles within the upper ~10 m of Hole 1318A, the underlying transition to higher SO₄^2– and lower alkalinity, DIC, and NH₄⁺ values implies that the contact between lithostratigraphic Subunits 1A and 1B records an interval of relatively oxidizing conditions and low organic carbon flux at the seafloor and within in the shallow subsurface. These conditions may have caused a decrease in the rate of sulfate reduction.

The excursion to lower dissolved sulfate values centered between 70 and 80 mbsf in Hole U1318A is matched by shifts to higher values in the alkalinity, DIC, and ammonium profiles (Fig. F15). These excursions correspond roughly to the contact between lithostratigraphic Units 1 and 2 (see “Lithostratigraphy”). This contact is defined by a shift from sandy clays (Unit 1) to dropstone-bearing fine sand and clay underlain by a conglomerate containing probable phosphorite clasts (Unit 2) and marks an unconformity defined on the basis of biostratigraphy (see “Biostratigraphy”). Phosphorite deposition commonly records periods of slow deposition and upwelling that lead to the development of reducing conditions at the seafloor.

In the lower part of Hole U1318B, the contact between lithostratigraphic Subunits 3B and 3C corresponds to shifts in the profiles of sulfate (beginning of a downcore trend toward lower values), ammonium (top of transient excursion to higher values), Fe²⁺ (shift to higher concentrations), and Mn²⁺ (shift to lower values) (Fig. F15). This correspondence suggests that these geochemical parameters were influenced by the lithologic change from silty clays (Subunit 3B) to dolomite-bearing silty clays and fine sands (Subunit 3C).

Concentrations of Ca²⁺ are similar to that of seawater at the top of the pore water profile (9.8 mM) and show an overall increase downcore to 22.7 mM in the deepest sample (Fig. F16A). This general trend is overprinted by two concave-downward excursions. At Hole U1318A, Ca²⁺ values decrease to 5.6 mM within the uppermost 15 m then increase to 8.1 mM at 31.45 mbsf. Concentrations remain relatively uniform in the interval from 31.45 to 70.57 mbsf and then gradually increase to the base of the profile in Hole U1318B.

Relative to calcium, Mg²⁺ concentrations (Fig. F16B) remain relatively uniform throughout the extent of the pore water profile (42.7 ± 3.3 mM). The greatest changes in magnesium occur within the upper 15 m of Hole U1318A and below 190 mbsf in Hole U1318B.

Dolomitization in lithostratigraphic Subunit 3C (Hole U1318B, 190.3–241.0 mbsf) (see “Lithostratigraphy”) may account for shifts to higher Ca²⁺ and lower Mg²⁺ pore water concentrations in the lower part of the profile (Fig. F16). The process of dolomitization, which generally involves the replacement of half of the Ca²⁺ ions in calcite or aragonite with Mg²⁺, results in removal of Mg and addition of Ca to the pore water system.

The silica (Si[OH]₄) profile at Site U1318 appears to be influenced by lithologic changes associated with the unconformity surface that marks the base of lithostratigraphic Unit 2 (see “Lithostratigraphy”). Above this surface, silica concentrations are relatively uniform, averaging 251 ± 45 µM (Fig. F17A). Below the unconformity, silica concentrations are relatively high (928 ± 76 µM) and show a slight increase with depth. The observed trend suggests equilibrium control by different siliceous phases above and below the unconformity.

As observed at Site U1316 (see “Geochemistry and microbiology” in the "Site U1316" chapter), B, Sr²⁺, and Li⁺ have similar profiles (Fig. F17B–F17D). The concentrations of these elements remain uniform in the upper 70 m of the sediment column before smoothly increasing in a concave-upward fashion to maxima at 236.35 mbsf in Hole U1318B, the depth of the deepest sample collected. The decrease in the slope of the profiles and higher concentrations of B, Sr²⁺, and Li⁺ with depth may reflect a combination of high rates of sediment accumulation, decreasing diffusivity downcore, and accelerated rates of silicate (Li and B) and carbonate (Sr) diagenesis below the deepest sample obtained.

Sediments

Carbonate determinations by coulometry were made on 53 samples from Site U1318 (Table T7). Samples were chosen to coincide with those used to determine dry bulk density (see “Physical properties”). Results provide a measure of the carbonate content within different units and allow assessment of the influence of carbonate content on parameters such as color reflectance. Carbonate concentrations, measured as dry weight percent CaCO₃, show less variation (~10–50 wt%) than at Sites U1316 and U1317 (Fig. F18). There appears to be a relationship between carbonate content and changes in lithology. Carbonate concentrations are relatively low and uniform (16 ± 3 wt%) throughout the upper part of Hole U1318A (0 to ~86 mbsf) over the depths that comprise lithostratigraphic Units 1 and 2 (see “Lithostratigraphy”). Below the base of lithostratigraphic Unit 2 and to a depth of ~160 mbsf in Hole U1318B, the carbonate profile shows a transient excursion to concentrations as high as 50 wt%. In the lower part of the profile (>160 mbsf), CaCO₃ concentrations tend to increase downcore, reaching a value of ~35 wt% in the deepest sample obtained. Within this interval, two outlying data points (Samples 307-U1318B-24X-1, 15–16 cm, and 25X-1, 7–8 cm) reach higher values of 70 and 72 wt%, respectively. These higher concentrations likely correspond to lithified dolomite-bearing layers discussed in “Lithostratigraphy.”

Microbiology

Whole-round core and catwalk sampling

Whole-round cores (WRCs) were taken from Holes U1318A and U1318B for shore-based work. A 2 or 1.5 m section from each core was selected on the catwalk. After removal of the interstitial water sample together with a variety of small syringe samples, the remainder of the section was taken directly from the catwalk to cold storage at 10°C, where WRC sections were cut and packed. Appropriately packed samples were stored at either +4° or –80°C. The distribution and packing/storing requirements of all samples and their sample codes are given in Figures F19 and F20. A total of 410 WRC/syringe samples were obtained.

Total prokaryote enumeration

Samples of 1 cm³ plugs for total prokaryote enumeration were taken during core processing on the catwalk from Holes U1318A (14 samples between the near surface and 135.55 mbsf) and U1318B (11 samples between 151.95 and 236.35 mbsf). All the samples in Hole U1318B were stored for later shore-based processing.

Prokaryotes were present in all samples studied, to a depth of 135.5 mbsf (Fig. F21). Maximum numbers observed were 2.7 × 10⁷ cells/cm³ in the shallowest sample at 4.85 mbsf (Sample 307-U1318A-1H-3, 185–190 cm) and the smallest population was 1.42 × 10⁶ cells/cm³ in the deepest sample at 135.5 mbsf (Sample 15H-2, 130–135 cm), a 19-fold decrease over 130 m. The detection limit was estimated at 2 × 10⁵ cells/cm³, based on calculations of a single membrane filter, and for each sample duplicate filters were used to provide a measure of variability. Where a zero count occurred, the prokaryote population was estimated by combining the data from both membranes and treating it as one subsample. This provides the only possible estimate of the population size in such samples, but does not allow any measure of variability.

The overall depth profile of cell numbers per cubic centimeter initially follows a trend observed at other Ocean Drilling Program sites (Parkes et al., 2000). This trend was accompanied by a significant decrease in sulfate concentrations in the upper 15 m (26.57–10.36 mM) indicating active sulfate reduction. From approximately the base of lithostratigraphic Subunit 1A (33.3 mbsf) (see “Lithostratigraphy”), cell numbers remain almost constant to 70.47 mbsf (Sample 307-U1318A-8H-3, 130–135 cm). Given that the prediction line is constantly decreasing, this suggests active population maintenance. This is supported by high proportions of dividing cells and a concavity in the sulfate profile over this depth range indicating continuing, although less active, sulfate reduction. At 81.7 mbsf, there is a significant erosion boundary separating sediments of substantially different ages (see “Biostratigraphy”). Across this boundary there is a factor of 3 decrease in prokaryote numbers from 1.24 × 10⁷ cells/cm³ at 70.47 mbsf (Sample 307-U1318A-8H-3, 130–135 cm) to 3.96 × 10⁶ cells/cm³ at 89.5 mbsf (Sample 10H-3, 130–135 cm). Below 89.5 mbsf, prokaryote populations decrease logarithmically and at a much greater rate than the prediction line. Within this depth range, few or zero dividing cells were observed, indicating that populations were in significant decline. The fact that this was occurring despite increasing sulfate concentrations between 80.5 and 135.5 mbsf (11.36–16.09 mM) suggests that lack of prokaryote population growth was because of low bioavailable carbon and that these ancient sediments, between 80.5 and 135.5 mbsf, lack an organic carbon substrate for metabolism. Total organic carbon will be determined as part of the shore-based work. Methane is often a significant carbon source for deep prokaryotes, but dissolved methane concentrations never rose above ~0.16 µM at Site U1318. Both the geochemical data and prokaryote profile suggest that neither methanogenesis nor methane oxidation are occurring at this site.

Contamination tests

Perfluorocarbon tracer

PFT was added continuously to the drilling fluid during coring of Hole U1318A to evaluate the penetration depth of the drill mud into the core. Drill mud samples were investigated to confirm the presence of PFT in the drill mud (Table T8). Air samples from the catwalk and the laboratory and several blanks were processed on the gas chromatograph system to exclude background contamination of PFT during analysis (Table T8).

Hole U1318A was cored using the APC. All inner core subsamples were negative for PFT. Although the hydraulic piston corer technique should theoretically reduce the contact of the drill fluid with the core material, PFT was detected on the outer surface of four cores (Cores 307-U1318A-9H, 11H, 12H, and 13H).

The first 14 cores in Hole U1318B were drilled with the APC followed by XCB coring down to the base. In Hole U1318B, PFT was not delivered to the drill mud before coring of Core 307-U1318B-19X because initially there was no intention to take samples from Hole U1318B. Thus, there is no contamination control for the first samples of this core taken from Sections 307-U1318B-17X-3 and 18X-3. All other deeper core sections selected for microbiology (MBIO) and geochemistry subsampling tested negative for PFT (Table T8).

Particulate tracer

Fluorescent microspheres were deployed in all cores from which MBIO samples were taken in Holes U1318A and U1318B. Subcores of 5 cm³ were taken from the bottom cut end of each MBIO section adjacent to the interstitial water WRC. Microspheres were detected in three samples from Hole U1318A and two samples from Hole U1318B. These data included the highest number of microspheres detected in contamination tests at all three sites of this expedition (2425 microspheres/cm³ in Section 307-U1318A-3H-3); however, this is still small compared to the original microsphere suspension at 7,000,000 microspheres/mL. The number of microspheres detected in Sample 307-U1318A-3H-3, 23.55–23.6 cm (2425 microspheres/cm³) is still only equivalent to the estimated minimum penetration of 0.35 nL of drill fluid/cm³, or 0.35 ppm. Given this is the only core in the upper 100 m that exhibited contamination, it is likely to be a single problem core rather than a general coring problem. Contamination toward the base of Hole U1318A is not surprising as core material is usually more prone to cracking at the extremes of APC coring capability, as in this case. XCB coring is generally more prone to contamination than APC, although that did not appear to be so here with only two instances of contamination detected in Hole U1318B.

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