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doi:10.2204/iodp.proc.334.105.2012

Geochemistry and microbiology

Geochemistry

We collected 10 whole-round samples in Hole U1380A for pore fluid analysis at a frequency of one sample per core, except for Core 334-U1380-3R, from which we took an additional sample for helium analyses. All the samples were exposed to the atmosphere prior to squeezing, but the sample for helium analyses was processed in a helium-free environment (see “Geochemistry and microbiology” in the “Methods” chapter [Expedition 334 Scientists, 2012a]). Because of time constraints, we focused our efforts on collecting samples for postcruise studies and only a limited number of analyses were carried out onboard. A total of nine headspace (HS) samples were collected for safety monitoring and were analyzed on the gas chromatograph–flame ionization detector on the natural gas analyzer (NGA). In addition, eight void gas samples (VAC) were collected. Four gas samples preserved with 10% KCl solution (NZ) were also sampled for shipboard analysis. All of these samples were measured using the NGA. The data reported are from the HS and NZ samples. The inorganic and organic geochemistry data are listed in Tables T5 and T6 and are plotted in Figures F7, F8, and F9.

Salinity, Cl, and Na concentrations follow the same trends with depth observed at Site U1378, indicating diffusional communication with fluids at depth (Figs. F7). The low chloride concentration and salinity reflect significant freshening, with values as low as 380 mM and 20 (~60%–67% of seawater value), respectively (Fig. F7).

Methane concentration ranges between 3,347 and 11,926 ppmv in pore fluids from sediment sampled between 398.1 and 475.1 mbsf in Hole U1380A (Fig. F8). The gas at these depths is thermogenic in origin, as indicated by the low ratios of methane to heavier homologs (ethane and propane), with CH₄/(C₂H₆ + C₃H₈) ratios ranging from 458.78 to 551.80 (Fig. F9). The C₂H₆ concentrations range between 3.89 and 15.69 ppmv, with the highest concentration at 475.1 mbsf. Propane is also present in the cores with concentrations ranging from 1.39 to 7.33 ppmv at depth. Iso- and n-butane are detected from 398.1 to 475.1 mbsf, with concentrations between 0.55 and 3.45 ppmv. Iso-pentane concentrations range from 0.64 to 2.34 ppmv. Propane, butane, and pentane concentrations were all highest at 475.1 mbsf.

At Site U1378, pore fluid chemical profiles suggest the presence of a unique fluid between 420 and 500 mbsf characterized by relatively low salinity, Cl, Mg, K, and alkalinity and elevated Ca concentrations (Figs. F7; data plotted in blue). This depth interval also corresponds to a marked increase in thermogenic hydrocarbons (propane, n-butane, and iso-butane; Figs. F19, F20 in the “Site U1378” chapter [Expedition 334 Scientists, 2012b]). These anomalies are not present within the interval cored at Site U1380 (Figs. F7; data plotted in red). A clear offset exists in the Ca, Mg, K, NH₄, and alkalinity profiles at ~400–500 mbsf between Sites U1380 and U1378. Ca and NH₄ concentrations at Site U1380 are lower than at Site U1378, whereas K, Mg, and alkalinity concentrations are higher at Site U1380. Ca, Mg, and K concentrations were determined twice by inductively coupled plasma–atomic emission spectroscopy at both Sites U1380 and U1378, and NH₄ and alkalinity concentrations were determined by different methods; therefore, the offset is not an analytical artefact. Site U1380 is ~1 km northeast of Site U1378, and the lithostratigraphy at Site U1378 is only partially represented at Site U1380 (see Fig. F4 in “Site U1378” chapter [Expedition 334 Scientists, 2012b]; Fig. F2). The interval between ~400 and 500 mbsf at Site U1378 contains more and thicker sections of coarser grained sediments than at Site U1380, indicating that these horizons are either not present at Site U1380 or were not recovered and sampled. It is likely that these coarser grained horizons are the conduits for the laterally migrating fluids observed at Site U1378 (see “Geochemistry and microbiology” in the “Site U1378” chapter [Expedition 334 Scientists, 2012b]), which may explain why the geochemical profiles at Site U1380 show a steady increase (Ca and NH₄) or decrease (Mg and K) through the depth cored and do not display the marked anomalies observed at Site U1378.

Microbiology

No whole rounds were collected for microbiology at Site U1380.

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