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doi:10.2204/iodp.proc.344.105.2013

Geochemistry

Inorganic geochemistry

We collected 29 APC whole-round samples (12–25 cm long), 9 XCB whole-round samples (20–25 cm), and 10 RCB whole-round samples (11–37 cm) on the catwalk at a frequency of 1–3 samples per core through Core 344-U1412A-15H and 1–2 samples per core in the XCB and RCB cored intervals of Holes U1412B–U1412D. Most XCB cores were too disturbed for adequate pore fluid sampling. All samples were thoroughly cleaned for drill water contamination. The cleaned samples were placed in Ti squeezers and squeezed at gauge forces of <30,000 lb. The inner diameter of the Ti squeezers is 9 cm; thus, the maximum squeezing pressure was 3043 psi (~21 MPa). The pore fluid was collected in syringes and passed through a 0.2 µm filter prior to analysis. The volume of pore fluid recovered varied with lithology and coring technique, from 23 to 56 mL from the APC cores, from 0.8 to 50 mL from the XCB cores, and from 3.5 to 35 mL from the RCB cores. The chemical composition of the samples below the sulfate–methane transition zone (SMTZ) at 14.7 mbsf was corrected for drill fluid contamination where the sulfate data indicated contamination <5%. Specific aliquots were used for shipboard analyses, and the remaining fluid was sampled for shore-based analyses (see “Geochemistry” in the “Methods” chapter [Harris et al., 2013]).

The uppermost three cores were processed under a nitrogen atmosphere and used for detailed studies of biogeochemical processes in the SMTZ. In addition to the 10 cm interstitial water samples, a 2 cm slice from the cleaned sediment was collected for shore-based microbiological analyses. These microbiological samples were collected through Core 344-U1412A-15H. The cleaned samples were vacuum-sealed and stored at –80°C.

To collect enough methane concentration data above the methane saturation depth, we sampled sediment immediately below the SMTZ at high resolution. We used real-time sulfate concentration analyses to accurately delineate the SMTZ at 14.7 mbsf. These real-time measurements allowed us to select the core sections immediately below the SMTZ (Sections 344-U1412A-2H-7 and 3H-1), which were sampled on the catwalk at 20 cm intervals. Three headspace samples were collected from each interval for shore-based concentration and isotope analyses of the dissolved gases. Additionally, we collected one whole round just above the SMTZ depth and one below for shipboard interstitial water analyses.

In Section 344-U1412C-7R-2 we collected one whole-round sample for He isotope analysis.

The APC cores recovered from Hole U1412A do not show significant drilling contamination. In contrast, samples from Holes U1412B and U1412C were highly disturbed by drilling. Because the samples from these cores were recovered below the SMTZ, no sulfate should be present in these samples. We used the sulfate concentrations reported in Table T6 as a tracer for drilling contamination with surface seawater and corrected the pore fluid concentrations in the samples from Holes U1412B and U1412C, as described in “Geochemistry” in the “Methods” chapter (Harris et al., 2013).

Raw data for major and minor element concentrations are listed in Tables T6 and T7. In Tables T8 and T9 we list the sulfate-corrected concentration data. Sulfate-corrected data for Holes U1412A–U1412C are illustrated in Figures F17, F18, F19, and F20.

Because of drilling problems and no core recovery suitable for pore fluid analyses, all figures show a gap in data points between ~200 and ~320 mbsf.

Salinity, chloride, and alkalis (sodium and potassium)

Downhole profiles of salinity, chloride, sodium, and potassium at Site U1412 are shown in Figure F17. Salinity values slightly decrease with depth from the seawater value at the seafloor, 35, to ~30 at the base of the hole. Chloride concentrations, however, do not vary significantly with depth. A few discrete excursions to lower values between 60 and 85 mbsf are consistent with observations of moussy sediments in the cores and of a BSR and may be indicative of gas hydrate dissociation during core recovery.

Sodium concentrations remain relatively constant in the APC cores recovered from Hole U1412A. They show a marked decrease toward the sediment of lithostratigraphic Unit II to a low value of 331 mM at 163 mbsf, which is also apparent in the low concentrations measured in the three samples collected at the bottom of this unit at ~187, 196, and 329 mbsf in Hole U1412C. The decrease may be driven by volcanic tephra alteration to zeolites. Sodium concentrations increase toward the bottom of the hole but still remain lower than seawater in the deepest sample, which has a concentration of 369 mM.

There is no significant change in the concentration of potassium in the sampled depth interval. In the uppermost ~100 m, potassium concentrations are slightly higher than the seawater value of 10.5 mM by 1–2 mM, most likely because of clay ion exchange reactions. The reason for scatter in the data points in the XCB core samples is unclear. Potassium concentrations in Hole U1412C (325–370 mbsf) are 1 mM lower than seawater value, which may be caused by volcanic tephra alteration to clays.

Alkalinity, sulfate, ammonium, phosphate, calcium, and magnesium

Pore fluid profiles of sulfate, alkalinity, ammonium, and phosphate in the uppermost 100 to 200 m at this site reflect characteristic organic matter remineralization (Figs. F18, F19). Sulfate concentrations decrease almost linearly from the seafloor to the SMTZ at 14.7 mbsf. Alkalinity increases from the seawater value at the seafloor to a maximum of 44 mM at 56 mbsf and gradually decreases with depth to ~4 mM in the deepest sediments cored at Site U1412. Alkalinity is involved in carbonate diagenesis in the uppermost ~150 m, as suggested by the Ca profile. Phosphate reaches its maximum concentration of 121 µM at 23 mbsf, at a shallower depth than alkalinity, and its concentrations decrease sharply with depth, possibly because of minor apatite precipitation. Ammonium peaks at a greater depth than alkalinity and reaches a maximum value of ~9.3 mM at 111 mbsf. Ammonium concentrations show an increasing trend with depth below ~300 mbsf. However, there are not enough data points to properly interpret this trend.

Ca and Mg concentrations decrease from seawater values at the seafloor to minima of ~3.9 and ~40 mM, respectively, at the SMTZ at 14.7 mbsf, reflecting precipitation of authigenic carbonates. There, however, the concentrations of both Ca and Mg, but in particular Ca, increase at the depth of the highest alkalinity values, and more marked increases are noted in the samples below 170 mbsf, indicating the presence of diagenetic sources for both cations. In lithostratigraphic Unit III, Ca concentrations reach a maximum value of ~13 mM, slightly higher than the seawater value, suggesting a source of Ca at greater depth. In contrast, Mg concentrations decrease with depth to a minimum of ~22 mM, accounting for only half of the seawater values. The inverse concentration-depth profiles of Ca and Mg also suggest interaction with a high-temperature reaction zone at depth.

Strontium, lithium, manganese, boron, silica, and barium

Downhole distributions of Sr, Li, Mn, B, Si, and Ba are shown in Figure F20. Sr concentrations remain near the seawater value of 87 µM throughout Hole U1412A, with only a small incursion to low concentrations and a minimum value of 68 µM at 8.8 mbsf. The overall lower concentrations in the uppermost 20 m are coincident with the observed decrease in calcium and suggest Sr uptake during carbonate precipitation. Sr in samples from Hole U1412B increases with depth, continuing toward the bottom of lithostratigraphic Unit III. The gap in data between ~200 and 320 mbsf precludes good interpretation of the diagenetic reactions responsible for the increase in Sr concentrations.

Li concentrations decrease in the uppermost 20 m from a seawater value of 25 to 5.9 µM at ~23 mbsf. Such a decrease in Li concentrations in the upper sediments offshore Nicoya was analyzed in the context of lithium isotope data by Chan and Kastner (2000), who interpret their results as a balance between ash alteration at low temperatures and ion exchange from clays; indeed, a rather thick ash layer is present at this depth (see “Lithostratigraphy and petrology”). In general, Li concentrations increase with depth from the minimum at 23 mbsf to ~100 µM at the bottom of the cored section. These elevated values suggest interaction with a high-temperature reaction zone at depth, consistent with the calcium data.

B values steadily decrease in lithostratigraphic Unit I from seawater value to ~100 µM at ~170 mbsf, perhaps driven by volcanic tephra alteration and/or siliceous ooze diagenesis in lithostratigraphic Unit II. However, because of a lack of data this interpretation cannot be verified.

Even the shallowest samples collected in Hole U1412A show an increase in dissolved Si to ~600 µM, a value that remains relatively constant throughout lithostratigraphic Unit I. Si concentrations are variable in Hole U1412C, ranging from 173 to 986 µM.

Dissolved Ba shows a marked increase in concentration below the SMTZ (Fig. F18), associated with dissolution of marine barite in environments depleted in sulfate (e.g., Torres et al., 1996). Ba concentrations remain relatively constant below ~40 mbsf and increase with depth in Holes U1412B and U1412C, suggesting a deeper source for this element.

Organic geochemistry

At Site U1412, we collected 64 samples for headspace gas, and 20 voids were also sampled for shore-based gas analyses. The data from these samples are listed in Tables T10 and T11 and plotted in Figures F21 and F22. In the headspace gas samples, methane concentrations range from 4 to 31,400 ppmv, ethane concentrations are <12 ppmv, propane concentrations do not exceed 3 ppmv, and C₄₊ was not detected at all. In the void gas samples, methane concentrations range from 770,000 to 850,000 ppmv, ethane ranges between 14 and 85 ppmv, propane concentrations are <11 ppmv, and C₄₊ was not detected at any depth.

In general, methane concentrations increase rapidly below 14.1 mbsf, consistent with the depth of the SMTZ at this site (Fig. F18). The gas composition of the headspace and void gas indicates that gas originates from a biogenic source. However, the decrease with depth in the C₁/C₂₊ ratios of headspace and void gas to ~400 indicates a mixture of biogenic gas and thermogenic hydrocarbons that likely migrated from depth.

The organic and inorganic carbon (IC) profiles are illustrated in Figure F23 and listed in Table T12. In lithostratigraphic Unit I, total carbon fluctuates between ~1.2 and 2.9 wt%; IC ranges from ~0.1 to <1.2 wt%, which corresponds to CaCO₃ concentrations ranging from 0.9 to 9.6 wt%; and total organic carbon (TOC) concentrations decrease from 1.8 to 0.8 wt%. There is a marked change to higher values in lithostratigraphic Unit II, with IC ranging from ~3 to 8.5 wt%, which corresponds to CaCO₃ concentrations ranging from 25 to 68 wt% and TOC concentrations decreasing from 1.8 to 3 wt%. In lithostratigraphic Unit III, IC ranges from ~0.4 to 0.9 wt%, which corresponds to CaCO₃ concentrations ranging from 3.4 to 7.7 wt%, and TOC concentrations are relatively constant at ~1 wt%.

Total nitrogen concentrations do not exceed 0.18 wt%. The calculated C/N ratio in Hole U1412A ranges between 5.4 and 15.6, suggesting that the sediment might be degraded marine organic matter. However, to fully characterize the organic carbon source we require information on the carbon isotopic ratios and other organic matter tracers.

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