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doi:10.2204/iodp.proc.307.104.2006

Geochemistry and microbiology

Geochemistry

A total of 59 interstitial water samples were taken from Holes U1317A, U1317D, and U1317E (31, 20, and 8 samples, respectively). Sampling of Hole U1317A was completed at a depth of 137 mbsf and the sampling of Hole U1317D began at 130 mbsf in order to provide a complete depth profile with slight overlap. Sampling of Hole U1317E was restricted to the upper four cores. Two interstitial water samples per core were collected from Hole U1317A because interstitial water recovery was low, except from Cores 307-U1317A-17X and 18X, where one interstitial water sample was taken. Two interstitial water samples per core were collected from Holes U1317D and U1317E, except from Cores 307-U1317D-3R through 6R, where one interstitial water sample per core was collected. The APC was used in Holes U1317A and U1317E, and the RCB was used in Hole U1317D.

Syringe samples were collected for gas chemistry. One sample per core was collected for shipboard safety analyses of dissolved methane from Holes U1317A, U1317D, and U1317E, and one sample per core was taken from Hole U1317B from Cores 307-U1317B-1H, 2H, 4H, and 13H through 17X for a total of 62 samples (19, 8, 18, and 17 samples from Holes U1317A, U1317B, U1317D, and U1317E, respectively). One sample per core was collected for analysis of total methane (both dissolved and adsorbed) from Holes U1317A and U1317D for a total of 31 samples (15 and 16, respectively).

Interstitial water and gas

As at Site U1316, variations in interstitial water composition at Site U1317 are driven principally by microbially mediated reactions, diffusion of chemical species resulting from diagenesis at depths greater than those sampled, and varying rates of sediment burial (Table T5).

The concentration of Sr (Fig. F16A) increases from 212 µM at 1.4 mbsf, the shallowest depth sampled, to a maximum of 2320 µM at ~120 mbsf. From 1.4 to ~50–60 mbsf, the curvature of the profile is concave downward. The concave-downward profile is accentuated in Hole U1317E. Both profiles imply Sr input to the pore fluids, possibly the result of aragonite diagenesis. Below ~50–60 mbsf, Sr increases linearly, implying a diffusion-controlled profile, to a maximum concentration of 2320 µM at ~120 mbsf. This maximum is likely the result of aragonite diagenesis. From 110 to 140 mbsf, the Sr concentration decreases linearly to ~1150 µM at 170–180 mbsf and then slightly increases with increasing depth to >1200 µM at the bottom of the hole.

In contrast to Site U1316, Li and B (Fig. F16B, F16C) do not covary with Sr. They do covary linearly with each other, however, and both Li and B concentrations increase nearly linearly from the top to the bottom of each hole, implying diffusion. The higher concentrations of these elements at depth are most likely the result of silicate diagenesis. Li and B increase from values in Hole U1317A at 1.4 mbsf of 40 µM and 547 µM, respectively, to a maximum near the bottom of Hole U1317D at 250 mbsf of 363 µM and 2150 µM, respectively. An offset in the values between Holes U1317A and U1317D is prominent and is most likely the result of either lateral variation in the pore fluid chemistry between the two holes or a sampling/​analytical artifact. Site U1317 is on the slope of the mound and there are significant differences in depth to seafloor between the holes (815 meters below sea level [mbsl] for Hole U1317A and 794 mbsl for Hole U1317D). It is unknown whether the concentration gradients follow the same contours as the bathymetry; however, linking offsets in the geochemical profile to lateral stratigraphic variations requires that a similar depth offset be found in the profiles of each chemical species. This pattern is not observed (profiles described below); therefore, it is more likely that the offset is a result of error in the concentration measurement. These elements were diluted and analyzed on the inductively coupled plasma–atomic emission spectrometer (ICP-AES) from the same subsample. Variations in the calibration curve and the diluent between the ICP-AES subsamples from Holes U1317A and U1317D will lead to an offset in the concentration values.

Chlorinity behaves conservatively and the profile reveals a gradual increase in chlorinity with depth (Fig. F16D). It should be noted that Hole U1317E chlorinity measurements were determined by anion chromatography, rather than by titration. Variations in chlorinity may indicate errors introduced during sampling, loss of water associated with alteration of volcanic ash, or differences caused by the burial of different water masses during different levels of glaciation. The seawater in this region is dominated by high-salinity MOW, and the offset in chlorinity from 1.4 to ~30 mbsf may reflect variations in hydrography. A change in salinity of 1 psu would lead to a change in chlorinity of 15.6 mM.

pH is highest (7.69) at 1.4 mbsf, decreases to ~7.3 at 30–40 mbsf, and remains constant at ~7.3 to the bottom of the hole. This profile results from the diffusion of seawater into pore fluids that are influenced by microbial oxidation of organic carbon and buffered by carbonate phases (Fig. F16E). The profiles of dissolved inorganic carbon (DIC) and alkalinity are similar to one another (Fig. F17A, F17B). Both profiles increase with increasing depth from 1.4 mbsf and have coinciding maxima that extend from ~140 to 170 mbsf. Below ~170 mbsf, both DIC and alkalinity decrease with depth. Concentrations of both DIC and alkalinity are higher (~3 mM) in Hole U1317E, as compared to Hole U1317A.

Dissolved methane concentrations are low (<0.2 µM) to a depth of 132 mbsf in Hole U1317A and 148 mbsf in Hole 1317D (Fig. F17F; Table T6). Concentrations increase steeply below 132 mbsf, reaching 6.66 µM by 266 mbsf. This profile reflects methane production and diffusion from the deeper Miocene sediments and anaerobic methane oxidation occurring at ~140 mbsf. Adsorbed methane, the dissolved methane subtracted from the total methane in samples treated with 1M NaOH, is below the analytical detection limit throughout most of this hole (Fig. F17G). Dissolved ethane remains below the detection limit until 132 mbsf in Hole U1317A and 152 mbsf in Hole U1318D (Fig. F17H). Like methane, it generally increases with increasing depth, and the change in gradient at ~140 mbsf implies anaerobic consumption. The considerable scatter in the gas data appears to be an artifact of RCB drilling and the protocol involved with sampling fairly indurated sediments.

Ammonium (Fig. F17I) also increases with depth with a slight concave-upward shape from 30.5 µM to a maximum of >2.6 mM at 220 mbsf indicating microbial respiration of nitrogen-containing organic matter. Below 220 mbsf, it decreases in concentration to 2.46 mM at 266 mbsf and is most likely controlled by diffusion.

Dissolved sulfate (Fig. F17C) is 27.3 mM at 1.4 mbsf and decreases to below its analytical detection limit near ~170–180 mbsf. The double (because of an inflection point in the profile ~50–60 mbsf) concave-downward curvature of the profile is a result of sulfate removal by bacterially mediated sulfate reduction. The decrease in sulfate is particularly striking in Hole U1317E, where sulfate decreases from seawater values at 1.4 mbsf to <15 mM at 28 mbsf. A zone of apparent sulfate consumption, as indicated by the concave-downward shape of the sulfate profile between 150 and 190 mbsf, corresponds to the depths of a steep increase in methane concentration (Fig. F17C, F17F). The profiles imply that sulfate reduction coupled to anaerobic methane oxidation is occurring over this broad 50 m interval of sediments. The oxidation of methane and the reduction of sulfate increase the DIC and the alkalinity over this interval, potentially driving carbonate precipitation.

Dissolved Mn and dissolved Fe have maxima of 3.5 µM and 38 µM, respectively, at 1.4 mbsf and drop sharply at 4.9 mbsf to below analytical detection limit (Table T5).

Barium concentrations are below 0.5 µM until ~100 mbsf, at which point they increase to 11.2 µM at the bottom of Hole U1317D (266 mbsf) (Fig. F17J). Ba concentrations are controlled by sulfate concentrations. Decreasing sulfate promotes the dissolution of barite (BaSO₄), adding Ba to the pore fluid.

Calcium concentrations (Fig. F17D) range from 10.5 mM at 1.4 mbsf to a broad local maximum of ~11.5 mM at depths ranging from ~20 to 50 mbsf. The concomitant rise in Sr over this same interval suggests the broad increase in Ca is related to aragonite dissolution. In contrast, Ca exhibits a 2 mM drop in concentration over the same interval in Hole U1317E. This drop in Ca concentration, coupled with a rise in Sr, suggests mineral-controlled diagenesis of aragonite, such as the inversion of aragonite to calcite, rather than dissolution. Below this interval, calcium concentrations decrease to ~9–10 mM at ~100 mbsf, forming another broad local minimum in the profile from ~100 to 170 mbsf and then increase to 12–13 mM toward the bottom of the hole.

Magnesium concentrations (Fig. F17E) decrease from near-seawater values of >51 mM at 1.4 mbsf to ~35 mM at ~120 mbsf and then gently decrease to ~30 mM at the bottom of Hole U1317D (266 mbsf). The Mg profile shows a similar trend with depth as sulfate, including an inflection point in its curvature at 50–60 mbsf forming a double concave-downward profile. The Mg decrease is more accentuated in Hole U1317E. From ~140 to 170 mbsf, the Mg profile straightens and decreases linearly with depth, appearing to be diffusion controlled to the bottom of the hole. The concave-downward Mg profile from 1.4 to ~140–170 mbsf indicates Mg removal, possibly because of diagenetic carbonate formation.

Below 140–170 mbsf, alkalinity, DIC, and Mg decrease, Ca increases, and sulfate remains depleted. The alkalinity, DIC, Mg, and Ca profiles appear diffusion controlled and the net loss of Mg, DIC, and alkalinity, in contrast to the net gain in Ca, implies some degree of dolomitization at depth, although the addition of Ca from the breakdown of calcium-rich silicates at depth can also drive carbonate precipitation and contribute to a decrease in alkalinity, DIC, and Mg.

Sediments

Carbonate is present at significant levels throughout the site. In the full 260 mbsf profile, it scatters around an average of 45.4 wt% and principally ranges between 30 and 60 wt% (Table T7). The depth profile for Hole U1317A is shown in Figure F18.

Microbiology

Whole-round core and catwalk sampling

Whole-round cores (WRCs) were taken from Holes U1317A and U1317D for shore-based work. A 2 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 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°C 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 692 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 U1317A (16 samples between the near surface and 136.81 mbsf), U1317D (16 samples between 129.90 and 265.96 mbsf), and U1317E (5 samples between 1.35 and 38.15 mbsf). All the samples in Holes U1317D and U1317E were stored for later, shore-based processing.

Prokaryotes were present in all samples studied to a depth of 136.81 mbsf (Fig. F21). The largest population was 1.52 × 10⁷ cells/cm³ at 4.85 mbsf (Sample 307-U1317A-1H-3, 185–190 cm) and the smallest population was 1.86 × 10⁶ cells/cm³ at 68.35 mbsf (Sample 307-U1317A-8H-3, 185–190 cm). The detection limit was estimated at 2 × 10⁵ cells/cm³ based on calculations on 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 the lower prediction limits of the trend observed at other Ocean Drilling Program sites (Parkes et al., 2000), with a prokaryote population a factor of three times smaller than expected indicating that in the upper 30 m this is a low-activity site. Between ~30 and 70 mbsf (Samples 307-U1317A-4H-4, 185–190 cm, and 8H-3, 185–190 cm) there is an increase in cell numbers. Although this increase is small (a doubling of the population), it is persistent and accompanied by a significant rise in the dividing cell numbers indicating the population growth is real. It is not yet clear why this bulge in the prokaryote population occurs. From 68.35 mbsf (Sample 307-U1317A-8H-3, 185–190 cm) downhole, there is a gradual increase in the cell numbers toward the expected profile, after which they decrease gradually in line with it. At 125.25 mbsf (Sample 307-U1317A-14H-3, 185–190 cm), however, cell numbers show a steady increase to 7.86 × 10⁶ cells/cm³, approximately doubling the population size and giving the second highest count in this hole. This increase is concomitant with the appearance of significant methane, albeit at relatively low concentrations, and suggests methane oxidation and/or methanogenesis is occurring at these depths. Additionally, this increase in cell numbers is supported by a steady rise in dividing cell numbers, indicating active population growth. It is suspected that the initial increase in cell numbers and dividing cell numbers at 125.25 mbsf is associated with methane oxidation, hence the absence of methane at this particular depth. This hypothesis will be tested by measurements of methane oxidation activity in later shore-based work.

Contamination tests

Perfluorocarbon tracer

PFT was added continuously to the drilling fluid during coring of Holes U1317A and U1317D to evaluate the penetration depth of the drill mud into the core material. Subcores of 5 cm³ were taken between the center and the periphery from the bottom cut end of each microbiology (MBIO) section adjacent to the interstitial water WRC. In Hole U1317D, an additional subcore was taken from the outer core section (interface between liner and core). The delivery of PFT to the drill mud was again confirmed by analysis of several drill mud samples (Table T8). Air samples from the catwalk and the laboratory were collected to assess potential PFT background concentrations from the analytical procedures. All air samples tested negative for PFT (Table T8). The gas chromatograph system was tested by injecting 1 mL of PFT-free air. Even after a PFT standard, the subsequent blank run shows no PFT signal and indicates that no background signal is present from the analytical procedure.

The first 14 cores of Hole U1317A were cored using the APC system. All inner core subsamples were negative for PFT. Sections 307-U1317A-17X-1 and 18X-1 were cored with the XCB system, resulting in a greater disturbance of core integrity. Whereas the interior of Section 307-U1317A-17X-1 was negative, Section 307-U1317A-18X-1 revealed trace amounts of PFT, possibly caused by the XCB coring process.

Hole U1317D was drilled exclusively with the RCB drilling technique. Thus, the outer core surface is always in contact with drill fluid and so it is not surprising that in almost all cores taken, the outer core surface was contaminated with drill fluid (Table T8). However, with one exception (Section 307-U1317D-10R-1), the interior of the sample material did not have a PFC drill mud signal, indicating an intact and contamination-free core interior. Cores taken using RCB drilling will need to be inspected for fractures that allow the intrusion of drill mud prior to their use in MBIO. This may also be an explanation for the occurrence of PFT in Section 307-U1317D-10R-1.

Particulate tracer

Fluorescent microspheres were deployed in all cores from which MBIO samples were taken in Holes U1317A and U1317D. 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 six samples from Hole U1317A and in zero samples from Hole U1317D (Table T8). Contamination levels were generally higher in cores from Site U1317 than from Site U1316; however, contamination levels are still likely to be very low given that the original microsphere suspension contained 7,000,000 microspheres per microliter. The number of microspheres detected in Sample 307-1317A-3H-3, 20.85–20.9 cm (993/cm³), is still only equivalent to the estimated minimum penetration of 0.14 nannoliters of drill fluid per cubic centimeter, or 0.14 ppm. The relatively high numbers of microspheres observed in the upper part of Hole U1317A is probably due to the nature of the core material, which frequently contained pieces of coral. This would disrupt the integrity of the core allowing ingress of drill fluids. Interestingly, no microspheres were detected in Hole U1317D. This frequently is the case with RCB cores, as the sediments are much harder and more consolidated, therefore preventing drill fluid penetration.

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