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doi:10.2204/iodp.proc.344.106.2013

Geochemistry

Inorganic geochemistry

We collected 25 whole-round samples (15–40 cm long) on the catwalk at a frequency of 1–4 samples per core through Core 344-U1380C-17R. After washing and installing casing, the first interstitial water sample was collected. These whole-round samples were thoroughly cleaned to remove drilling contamination prior to squeezing. The cleaned samples were placed in Ti squeezers and squeezed at gauge forces up to 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 burial depth from 2 mL to a maximum of 25 mL. 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., 2013b).

Sediment samples from Core 344-U1380C-18R to the bottom of the hole were compacted or cemented, and squeezing at gauge forces <30,000 lb did not yield pore fluid. Therefore, 12 whole-round sediment samples with lengths ranging from 20 to 30 cm were collected between 613 and 788 mbsf for shore-based handling and analyses. These samples were cleaned in the same manner as the shipboard interstitial water samples and stored in double vacuum-sealed bags.

Two additional 25 to 30 cm long whole rounds were acquired from Sections 344-U1380C-3R-5 and 9R-3 for He isotope analyses. These samples were handled in the lower tween deck to avoid contamination with gas tank He present in the Chemistry Laboratory. They were stored and cleaned in a N₂ glove bag at 4°C, squeezed as described above, and stored in Cu tubes that were crimped tightly.

Below the sulfate–methane transition zone, sulfate is depleted in the pore fluids and any sulfate present in a sample is a result of contamination with surface seawater that was pumped down the hole while drilling. Based on the sulfate concentration of each interstitial water sample, we used the chemical composition of the surface seawater to correct each analysis for contamination. The chemical composition of the surface seawater used during drilling at this site is listed in Tables T5 and T6. Raw data for the major element concentrations in pore water are listed in Table T5, and in Table T7 we list the sulfate-corrected concentration data. Similarly, Tables T6 and T8 list the uncorrected and corrected minor element concentrations, respectively. Only the sulfate-corrected data are illustrated in Figures F18, F19, and F20.

For gas analyses, 61 headspace samples were collected using a 5 mL cut-off syringe directly adjacent to the pore water. Samples were analyzed following protocols described in “Geochemistry” in the “Methods” chapter (Harris et al., 2013b), using a gas chromatograph equipped with a flame ionization detector and on the natural gas analyzer.

In addition to the fluid samples, 65 sediment samples were collected from the working half of the core, adjacent to the pore water samples. These samples were freeze-dried, ground, and analyzed for inorganic carbon (IC) and total carbon (TC) concentrations. The data are listed in Table T9.

Salinity, chloride, and alkalis (sodium and potassium)

Downhole profiles of salinity, chloride, sodium, and potassium in Hole U1380C are shown in Figure F18, which also includes data collected in Holes U1378B and U1380A during Expedition 334 for comparison (Expedition 334 Scientists, 2012a, 2012c). Salinity values at Site U1378 decrease steadily from seawater value (35) at the seafloor to ~18 at the base of the hole. Salinity values measured in Hole U1380C are consistent with those measured at Site U1378 and vary between 20.5 and 23.5. These low values suggest the presence of a fresher fluid resulting from mineral dehydration reactions at depth. Chloride concentrations at Site U1378 also decrease steadily with depth, reaching a minimum value of ~358 mM at the base of the hole, and are also well below the modern seawater value (559 mM). Concentrations measured in Hole U1380C are consistent with those measured at the base of Site U1378 and range between 371 and 392 mM. These concentrations are 36%–30% fresher (depleted in chloride) than modern seawater and change only slowly with depth below ~500 mbsf. Overall, Cl values are consistent with those observed at equivalent depths at Site U1378 and in Hole U1380A. The in situ temperature within this depth range is too cold to support local clay dehydration (Expedition 334 Scientists, 2012a, 2012c; see “Physical properties”); thus, the Cl data suggest interaction with a fluid that has migrated from a region with temperatures >60°C, high enough to support clay dehydration (Perry and Hower, 1970; Bekins et al., 1994, and references therein).

Sodium concentrations range between 294 and 330 mM throughout the cored section (Fig. F18), which corresponds to 39%–30% freshening of seawater and is consistent with the dilution values obtained for Cl. The near-constant Na/Cl molar ratio with depth suggests that the depletion in pore fluid Na is almost entirely related to dilution rather than mineral-fluid reactions. In addition to being influenced by the same freshening signal observed in the Cl and salinity data, potassium concentrations decrease with depth from 5.95 mM at 452 mbsf to 1.94 mM at 584 mbsf in Hole U1380C, indicating that K is involved in fluid-mineral reactions, likely the smectite to illite transformation. This reaction requires a minimum temperature of ~50°–60°C. The observed temperature gradient measured at Site U1378 is 51.4°C/km (Expedition 334 Scientists, 2012a; see “Physical properties”), yielding an in situ temperature of ~25°–32°C in the depth range of the Hole U1380C chemical profiles. This temperature range is below the minimum required for the transformation of smectite to illite, supporting the interpretation that the recovered fluids have interacted with a deeper fluid source.

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

The depth interval cored at this site is well below the sulfate–methane transition zone, so no sulfate should be present in the pore fluids, making sulfate an excellent tracer for contamination with surface seawater that was used as drilling fluid. Sulfate concentrations measured after thoroughly cleaning the samples prior to squeezing range from 1.2 to 2.4 mM (Table T5). These data were used to correct the pore fluid concentrations reported in Tables T7 and T8.

Because of the low volumes of pore fluid recovered through the depth interval cored and the rather large pore fluid volume (3 mL) required for shipboard alkalinity analysis, only select samples were analyzed for alkalinity. Alkalinity values are rather low in the depth range cored in Hole U1380C, having a maximum value of 7.20 mM at 473 mbsf and decreasing with depth to 1.53 mM at 522 mbsf (Table T7; Fig. F19).

Organic matter diagenesis in the upper sediment section is observed in the ammonium concentration-depth profile of Site U1378 (Fig. F19) (Expedition 334 Scientists, 2012a), where ammonium values reach a maximum of ~8 mM at ~320 mbsf and then decrease to 4.74 mM at the top of the equivalent section drilled in Hole U1380C. Similar to alkalinity, the ammonium concentrations continue to decrease nearly linearly with depth to 1.98 mM in the deepest sample analyzed at 584 mbsf.

In the uppermost part of the drilled section at this site (~450 mbsf) both Ca and Mg concentrations are depleted relative to the corresponding seawater values of 10.55 and 54.0 mM, respectively. The depletions are caused by carbonate diagenesis and some volcanic tephra alteration reactions in the upper sediment section as seen in the adjacent Site U1378 data (Fig. F19) (Expedition 334 Scientists, 2012a). Proceeding from ~450 mbsf to the deepest sample analyzed, Ca concentrations steeply increase by 23 mM, from 8.14 to 31.2 mM, a value that represents almost three times the seawater concentration. In contrast, Mg concentrations decrease with depth by 18 mM, from 18.5 to 0.40 mM, at a Ca:Mg ratio close to 1:1. The increase in Ca and decrease in Mg with depth likely reflect diffusional interaction with a deeper fluid that reacted with volcanic tephra and/or oceanic basement at elevated temperatures. This observation, coupled with the low K values discussed above, suggests that the reaction temperature is <150°C.

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

Downhole distributions of Sr, Li, Mn, Si, B, and Ba are shown in Figure F20, which includes data collected from Hole U1378B during Expedition 334 (M.E. Torres et al., unpubl. data) for comparison.

Sr concentrations increase downhole from 44 µM at ~450 mbsf (~50% of modern seawater value) to a maximum of 75 µM (~86% seawater value) at ~550 mbsf. This zone of increasing Sr concentrations correlates with the base of a shear zone that extends from ~480 to 550 mbsf (see “Structural geology”). Sr concentrations decrease sharply to ~63 µM in the lowermost ~30 m sampled and squeezed onboard for interstitial water (552–584 mbsf). Shore-based Sr isotope ratio determinations may help to identify the various reactions that are responsible for the observed Sr concentration-depth profile.

Li concentrations measured in samples from Site U1378 are lower than seawater values (26 µM) and are relatively constant from ~20 to 450 mbsf (Fig. F20) (M.E. Torres et al., unpubl. data). At Site U1378, Li concentrations increase below 450 mbsf to a maximum value of 59 µM at 489 mbsf and then decrease slightly to the base of the hole. In Hole U1380C, the Li concentration is already 80 µM at 452 mbsf and remains elevated between this depth and 540 mbsf, ranging from 58 to 69 µM. Below this depth, Li concentrations decrease sharply to ~10 µM at 584 mbsf. The zone of elevated Li concentrations correlates with the broad shear zone extending from ~480 to 550 mbsf (see “Structural geology”), as well as the horizon with depleted Cl and Ca concentrations, elevated Sr concentrations, and the only interval where thermogenic hydrocarbons (propane, butane, and pentane) were observed at this site (Fig. F21). Based on smectite-seawater hydrothermal experiments (You and Gieskes, 2001; Wei et al., 2010), the Li concentrations indicate fluid-rock reaction at source temperatures greater than ~80°C. Furthermore, temperatures within the shear zone are too low to support in situ formation of thermogenic hydrocarbons. Collectively, the Cl, Li, and hydrocarbon data clearly indicate that fluid migration along this horizon originated from a source depth where temperatures are >90°C. The decrease in Li concentrations below the shear zone reflects the presence of a fluid at depth that has interacted with volcanic material and/or oceanic basement at a lower temperature. This is also indicated by the decrease in Mg concentrations and increase in Ca concentrations below the uncomformity at 550 mbsf. This fluid is likely sourced at temperatures <80°C but >60°C, as Cl is depleted (indicating clay dehydration reactions) and there are no thermogenic hydrocarbons present (Figs. F20, F21, F22).

Mn concentrations in pore fluids sampled from Hole U1380C are variable and range from 0.11 to 1.4 µM. B concentrations are below the modern seawater value and vary between ~58 and 187 µM, with the band of lowest concentrations in the broad shear zone that extends from ~480 to 550 mbsf (see “Structural geology”), where Cl concentrations are depleted and Ca and Sr concentrations are elevated.

Si concentrations in Hole U1380C are rather low and relatively constant with depth, ranging from 96 to 163 µM, indicating that the solubility of quartz in the sand, instead of feldspars or clay minerals, controls Si concentrations in the pore fluids.

Ba concentrations are relatively low and constant with depth, ranging from 1.22 to 2.82 µM. No obvious trends are observed in the Ba concentration-depth profile in Hole U1380C, and the data are consistent with values obtained for the equivalent depth range in Hole U1378B (M.E. Torres et al., unpubl. data).

Organic geochemistry

Organic geochemistry data for Hole U1380C are listed in Tables T9 and T10 and plotted in Figures F21 and F22. In the headspace gases, methane concentrations range from 900 to 43,000 ppmv. Ethane concentrations range between 2 and 32 ppmv. Heavy hydrocarbons (C₃₊) were only detected between 452 and 552 mbsf (Section 344-U1380C-13R-7). No heavy hydrocarbons were detected below 552 mbsf. The C₁/C₂₊ ratio was <400 above 551.9 mbsf, and below that depth values are consistently >400. This finding likely indicates that the hydrocarbons above 552 mbsf have a thermogenic component consistent with the Cl and Li concentration profiles, whereas the sediments below 552 mbsf are predominantly from biogenic sources.

Organic and IC distributions are illustrated in Figure F23. TC increases from <0.9 to ~2.5 wt% at 553 mbsf; below that depth, TC ranges between 0.4 and 1.4 wt%. This remarkable concentration change is consistent with the observed changes from lithostratigraphic Unit I to Unit II (see “Lithostratigraphy and petrology”). IC also increases from ~0.1 to <1.5 wt% down to the lithostratigraphic Unit I/II boundary and then decreases to 0.2 wt% below that depth. The total organic carbon concentration also has a marked change in concentration at this boundary, from 0.9–1.4 to 0.1–0.8 wt%.

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