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doi:10.2204/iodp.proc.334.106.2012

Geochemistry and microbiology

Geochemistry

We collected 12 whole-round samples from Hole U1381A for pore fluid analysis at a frequency of one or two samples per core, depending on recovery. All the samples were exposed to the atmosphere prior to squeezing. Hole U1381A was cored by RCB, with no recovery in the uppermost 13 m of the sediment column. The first whole round was collected from Core 334-U1381A-3R. To acquire pore fluids from the uppermost 13 m at this site, two cores were collected in Hole U1381B by “punching” the sediment with the RCB system, much like a piston core. To connect with the previous profile, a third core was collected by RCB. We sampled three whole rounds from the first core in Hole U1381B and one whole round each in the second and third cores. Because of time constraints, we focused our efforts on collecting samples for postcruise studies in Hole U1381B and only a limited number of analyses were carried out on board.

No gas samples were collected in Hole U1381B. Nine headspace (HS) samples were collected for safety monitoring in Hole U1381A and analyzed on the gas chromatograph–flame ionization detector on the natural gas analyzer (NGA). Methane concentrations were at background concentrations at Site U1381, and ethane and higher hydrocarbons were not detected. Thus, the organic geochemistry at this site is not reported in the data tables. The inorganic geochemistry data are listed in Table T4 and plotted in Figure F13.

From 13 to 24 mbsf, salinity is lower than the seawater value (35) and increases to 34 at the base of the hole. Chloride concentrations are slightly below the modern seawater value (559 mM) through the cored section, averaging 554 mM with a minimum concentration of 550 mM (~1.5% lower than seawater). A similar dilution of Cl concentrations of ~2.5% lower than modern seawater was observed in the uppermost 100 m of the sediment column cored at the reference site (ODP Site 1039) offshore the Nicoya Peninsula (Kimura, Silver, Blum, et al., 1997). Sodium concentrations are also below the seawater value throughout the cored section and reach a minimum of 437 mM at 35 mbsf. Sodium concentrations are relatively constant below this depth, averaging ~465 mM.

Sulfate concentrations decrease from 15 mM at 13 mbsf to a minimum of 11 mM at 23 mbsf. Sulfate concentrations then increase nearly linearly with depth to 24 mM at the sediment/basement interface. The return to seawater-like values below 23 mbsf in the sulfate profile indicates lateral flow of altered seawater in the basement at this site. The diffusion of sulfate from this basement fluid to the sediment column and from the overlying water column must be faster than the rates of microbial sulfate reduction, keeping sulfate from reaching depletion in the reference section. This trend was also observed in the reference sediment column offshore Nicoya Peninsula (Kimura, Silver, Blum, et al., 1997). The alkalinity concentration-depth profile is a mirror image of the sulfate profile, reaching a maximum of 17.7 mM at 23 mbsf and decreasing to 4 mM at the base of the hole. Organic matter diagenesis in the uppermost part of the sediment section is also observed in the ammonium profile, which reaches a maximum value of 1.45 mM at 23 mbsf. Ammonium concentrations remain nearly constant to 35 mbsf and decrease nearly linearly to 0.42 mM at the base of the hole. Calcium concentrations reach a minimum value of 4.8 mM just below the sulfate minimum at 23 mbsf, suggesting precipitation of authigenic carbonates in the zone of active sulfate reduction and alkalinity production. Below this depth, Ca concentrations increase to 13.5 mM at the base of the hole. The increase in Ca with depth likely reflects both ash alteration in the sediment column and diffusional communication with the basement fluid.

Potassium concentrations decrease gradually with depth, reaching minimum values at the base of the hole. Magnesium concentrations decrease to 35 mM at 35 mbsf, suggesting volcanic ash alteration within a depth interval consisting of abundant tephra layers (34–48 mbsf) (see “Lithostratigraphy and petrology”). Magnesium concentrations remain relatively constant between 45 and 81 mbsf and decrease to 46 mM at the sediment/basement interface. In summary, the pore fluid profiles in the uppermost ~40 m of the sediment section at this site reflect reactions related to organic carbon cycling as well as volcanic ash alteration, and, to a lesser extent, clay-ion exchange reactions. The pore fluid profiles of SO₄, alkalinity, Ca, and Mg below this depth reflect ongoing ash alteration and ion exchange reactions with the sediment column and diffusional communication with a basement fluid. The estimated composition of this fluid (Ca = ~13 mM, Mg = ~46 mM, and SO₄ = ~24 mM) suggests that it is likely seawater that is only moderately modified by reaction with basalt along the fluid flow path.

Microbiology

Microbiological sampling consisted of 5 cm whole-round samples cut on the catwalk and subsampled in the laboratory using sterile techniques. In Hole U1381A, which was all RCB coring in soft sediment, one whole-round sample was taken in each of Cores 334-U1381A-5R through 9R and 11R. The choice of cores was guided by core recovery. Whole-round samples were not taken once basement was reached. In Hole U1381B, three whole-round samples were taken in Core 334-U1381B-1R, and one sample was collected in Cores 2R and 3R. Subsampling of the whole-round samples was performed for three different categories of research endeavors: (1) frozen samples for molecular analyses, (2) refrigerated samples for cultivation studies, and (3) paraformaldehyde-fixed samples for cell counting and contamination testing.

Fixed samples for cell counting were further prepared on board with a SYBR Green I staining procedure; however, because of the short time remaining, enumeration estimates will be performed postcruise along with the other analyses. No microspheres were deployed during RCB coring because of potential damage to the soft sediments from the microsphere bags.

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