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doi:10.2204/iodp.proc.344.103.2013

Geochemistry

For pore fluid geochemistry measurements, we collected 20 whole-round samples that were 10 cm long and 8 that were 15 cm long, at a frequency of 2–4 samples per core. These samples were stored and cleaned in a N₂ glove bag to remove drilling contamination. 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 filtered prior to analysis. The volume of pore fluid recovered varied with lithology and burial depth from 22 mL to a maximum of 72 mL in lithostratigraphic Unit II (see “Lithostratigraphy and petrology”). Specific aliquots were used for shipboard analyses, and the remaining fluid was sampled for shore-based analyses, following protocols specified by the scientists that will be involved in these analyses (see the “Methods” chapter [Harris et al., 2013]). Three additional 10 cm long whole rounds were acquired from Sections 344-U1381C-7H-6, 9H-6, and 11H-1 for He isotope analyses. These samples were handled on 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.

For gas analyses, 29 headspace samples were collected using a 5 mL cut-off syringe directly adjacent to the pore water sample. Samples were analyzed following the protocol described in “Geochemistry” in the “Methods” chapter (Harris et al., 2013).

In addition, 28 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), total carbon (TC), and total nitrogen (TN) concentrations.

Inorganic geochemistry

Salinity, chloride, and alkalis (sodium and potassium)

Downhole profiles of salinity, chloride, sodium, and potassium in Hole U1381C are shown in Figure F22, which includes data collected from Hole U1381B during Expedition 334 for comparison (Expedition 334 Scientists, 2012) (Table T6). From the seafloor to ~22 mbsf, salinity has a seawater value of 35.0 and shows slightly lower values (34.0–34.5) to ~100 mbsf. The lowest salinity value of 33.5 was obtained in the lowermost ~15 m of this site, above the sediment/​basement contact where the sediment is more indurated. Chloride concentrations are slightly below the modern seawater value (559 mM) in the uppermost ~7 m (554–557 mM), are equal to modern seawater at ~14 mbsf, and are 1%–2% (561–570 mM) higher than modern seawater between ~15 and 95 mbsf. This slight increase in chloride concentration is most likely caused by minor hydration reactions of volcanic tephra to clay minerals. Similar to salinity, Cl concentrations slightly decrease above the sediment/​basement contact.

Sodium concentrations range between 462 and 479 mM throughout the cored section (Fig. F22). Only three samples have near-seawater Na concentrations (480 mM). It is interesting to note that at this site, Na behaves differently from Cl, possibly because of volcanic tephra alteration reactions. Potassium concentrations decrease from a seafloor value of 12.2 mM (17% higher than the modern seawater value of 10.4 mM) to the sediment/​basement interface, where they reach modern seawater concentrations (Fig. F22). Higher concentrations of K than in modern seawater have been observed at numerous sites near the seafloor and have been attributed to ion exchange with clay minerals where ammonium values are elevated and to volcanic ash alteration. The latter reaction, however, could also be responsible for decreasing K concentrations at greater depths (see “Lithostratigraphy and petrology”).

Alkalinity, sulfate, ammonium, calcium, and magnesium

The alkalinity and sulfate depth profiles are approximate mirror images of each other (Fig. F23) and are caused by oxidation of organic matter according to the simplified reaction

Alkalinity reaches a maximum value of 17.4 mM at ~32 mbsf, and the sulfate minimum value of 12.1 mM was observed at 36–41 mbsf. Elevated alkalinities cause carbonate precipitation, as indicated by a decrease in Ca (and Mg) concentrations at the same depth interval, as discussed below. Organic matter diagenesis in the uppermost part of the sediment section is also observed in the ammonium concentration-depth profile, which mimics the alkalinity profile. Ammonium reaches a maximum value of 1.6 mM at the depth of the alkalinity maximum (32 mbsf) and decreases nearly linearly to 0.29 mM at the base of the hole (Fig. F22). Below this maximum value at ~32 mbsf, alkalinity concentrations also decrease to a minimum of ~2.7 mM near the sediment/​basement interface. In contrast, sulfate concentrations increase to 25 mM in the deepest sample, which is close to the modern seawater value of 28.9 mM. The observation of an increase in dissolved sulfate at depth indicates that the diffusion of sulfate from basement into the sediment section is faster than the rate of microbial sulfate reduction. A similar sulfate concentration-depth profile was observed at the reference sediment section offshore Nicoya Peninsula, at ODP Leg 170 Site 1039 (Kimura, Silver, Blum, et al., 1997) and in Expedition 334 Hole U1381B (Expedition 334 Scientists, 2012). The increase in sulfate concentrations in the deeper section is evidence for diffusional communication of the sediment column with fluid in the oceanic basement.

Both Ca and Mg concentrations are slightly depleted at ~1.5 mbsf, the shallowest pore fluid sample obtained, and continue to decrease parallel to the increasing alkalinity pattern (Fig. F23), suggesting carbonate diagenesis. In the shallowest pore fluid sample, the Ca concentration is 9.6 mM instead of the modern seawater value of 10.55 mM and the Mg concentration is 49.5 mM instead of the modern seawater value of 54 mM. The depletion in Mg is more than four times that of Ca, suggesting that more than one diagenetic reaction controls Mg (i.e., carbonate diagenesis, clay ion exchange, and minor ash alteration). Ca continues to decrease downhole to the depth of maximum alkalinity and minimum sulfate concentrations, suggesting further precipitation of authigenic carbonates in the zone of active sulfate reduction and alkalinity production. The Mg profile, however, remains almost constant in this depth range (Fig. F23). The consumption of Mg by carbonate diagenesis may be countered by an Mg increase due to ion exchange in clay minerals driven by the produced ammonium. Below this calcium consumption depth, Ca concentrations increase downhole to a maximum of 16.2 mM at the sediment/​basement interface (~150% modern seawater value). The increase in Ca with depth likely reflects both ash alteration in the sediment column and diffusional interaction with an altered basement fluid. Because basement fluids are depleted in Mg, diffusional interaction with the basement fluid should decrease the Mg concentration with depth; however, Mg concentrations remain almost constant to the bottom of the hole. A combination of reactions involving volcanic tephra alteration and clay ion exchange may explain the observed unusual Mg concentration profile. Reaction modeling will be required to explain the controlling reactions of Mg in this hole.

Concentrations of phosphate, an important nutrient (the Redfield ratio of marine organic matter is ~106C:16N:1P), increase rapidly from a seafloor value of ~2 µM to a maximum of ~38 µM, which corresponds in depth to the maximum in alkalinity concentration. Below this depth, phosphate decreases sharply, and in sediments corresponding to lithostratigraphic Unit II (see “Lithostratigraphy and petrology”), pore fluid phosphate concentrations are very low, just 1–2 µM.

Strontium, lithium, magnesium, silica, boron, and barium

Downhole distributions of Sr, Li, Mn, Si, B, and Ba are shown in Figure F24, which includes data collected from Hole U1381B during Expedition 334 (M.E. Torres et al., unpubl. data) for comparison (Table T7). Sr concentrations are close to a modern seawater value of 87 µM from the seafloor to ~45 mbsf and increase slightly downhole from ~45 to ~95 mbsf to a maximum concentration of 108 µM. In the deepest ~10 m of the hole, however, Sr concentrations decrease slightly, reaching ~104 µM at the sediment/basalt interface. The minor decrease in Sr concentrations observed at the base of the hole must be related to reactions in the oceanic basement. Shore-based Sr isotope data will identify the source(s) of Sr below ~45 mbsf.

Li is highly sensitive to volcanic tephra alteration and to the temperature of the alteration reaction. In the uppermost ~25 m, Li concentrations decrease from 25 µM (approximately the seawater value of 26 µM) to a minimum of 21 µM at 17 mbsf (Fig. F24), probably because of low-temperature minor tephra alteration. Deeper in the sediment, Li concentrations increase to a maximum of ~40 µM at the bottom of the hole. This concentration-depth profile suggests diffusional interaction with fluid at a greater depth, where higher temperatures prevail. Concentrations of Li, one of the most “fluid-loving” elements, increase with increasing temperatures during silicate-fluid diagenetic reactions.

Concentrations of Mn, a redox-sensitive element, decrease in the uppermost ~25 m, the zone of active sulfate reduction (Fig. F24). Usually in this zone, Mn concentrations increase with increasing sulfate reduction; oxidized Mn is being reduced and released into the pore fluid as Mn²⁺, a more soluble species than oxidized Mn. The decrease in Mn concentrations thus suggests diagenetic uptake of Mn; it most likely co-precipitates with the diagenetic carbonates. From ~25 mbsf to the bottom of the hole, Mn concentrations increase with depth, similar to the Li profile. This Mn increase has a diffusional shape, suggesting communication with deeper fluid in the oceanic basement that has a higher Mn concentration.

In the shallowest sample analyzed at ~1.5 mbsf, Si concentrations (523 µM) are higher than bottom water concentration, which in this region is ~225 µM Si. Silica concentrations increase with depth to ~1000 µM (Fig. F24). Dissolution of siliceous phyto- and zooplankton diatoms, radiolarians, and sponge spicules observed in larger abundances in lithostratigraphic Unit II (see “Lithostratigraphy and petrology”) is responsible for the high Si concentrations. The solubility of nonbiogenic silicates, such as feldspars, is only ~400–500 µM at ~25°C, the estimated temperature at the bottom of the hole. Si solubility increases with increasing temperature.

Ba concentrations throughout the sediment section are very low, between ~200 and 530 µM, slightly above the bottom seawater concentration of ~200–300 µM. These concentrations are typical of pore fluid Ba values in the pelagic environment, where sulfate concentrations are too high for barite remobilization. No obvious trends are observed in the Ba concentration-depth profile, as expected for this environment.

In summary, the concentration-depth profiles of Ca and SO₄, as well as those of Li and Mn and possibly Si, indicate diffusional interaction with altered seawater fluid in the oceanic basement. The Sr profile, however, shows a reversal close to the sediment/​basement contact, suggesting that fluid-basement reactions are influencing the reversal in the Sr profile.

Organic geochemistry

Organic geochemistry data for Hole U1381C are listed in Table T8 and plotted in Figures F25 and F26. In the headspace gases, methane concentrations range from 2 to 262 ppmv. Ethane concentrations range between 2 and 256 ppmv. We observed the highest methane concentrations in the shallowest sample at 1.5 mbsf, together with high levels of ethane and propane. This observation likely indicates potential contamination of the sediment sample by drilling fluid or during sampling on the catwalk. In all other samples, methane concentration was <7 ppmv, and no heavy hydrocarbons were detected.

The IC distribution (Table T9), shown in Figure F26, shows increasing concentrations from <2 to ~6 wt% at 55 mbsf, consistent with observed changes from the silty clay sediment of lithostratigraphic Unit I to the foraminiferal carbonate ooze of lithostratigraphic Unit II (see “Lithostratigraphy and petrology”). TC concentrations also increase from ~3 to <7 wt% at that depth. Total organic carbon concentrations range from 0.97 to 2.1 wt%. TN concentrations range between 0.049 and 1.7 wt%. TN concentrations are highest between 30 and 44 mbsf.

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