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doi:10.2204/iodp.proc.339.103.2013

Geochemistry

Volatile hydrocarbons

Headspace gas analysis was performed as a part of the standard protocol required for shipboard safety and pollution prevention monitoring. In total, 17 headspace samples from Hole U1385A (sampling resolution of one per core) were analyzed (Fig. F25; Table T21). Methane (C₁), ethane (C₂), and ethene (C₂₌) were detected at this site. Methane is the only hydrocarbon present from the seafloor to 58.3 mbsf (Section 339-U1385A-7H-7) (Fig. F25). Ethane appears at 67.5 mbsf (Section 8H-7), and ethene begins to increase at 105.5 mbsf (Section 12H-7).

Methane varies between 3.7 and 6.7 ppmv in the uppermost five cores of Hole U1385A, increases to a maximum of 87,447 ppmv (Section 339-U1385A-11H-6), and then decreases to 56,190 ppmv at the base of the hole (150.11 mbsf).

Ethane ranges from 0.7 ppmv at 67.5 mbsf (Section 339-U1385A-8H-7) to 1.87 ppmv at 150.1 mbsf (Section 17H-5). The maximum ethane concentration is 2.4 ppmv at 94.5 mbsf (Section 11H-6).

Ethene ranges from 1 ppmv at 105.5 mbsf (Section 339-U1385A-12H-7) to 0.7 ppmv at 150.1 mbsf (Section 17H-5). The maximum concentration is 1.23 ppmv at 142 mbsf (Section 16H-6).

Sedimentary geochemistry

Sediment samples were collected for analysis of solid-phase geochemistry (inorganic and organic carbon) at a resolution of approximately one sample per core in Hole U1385B (Table T2). CaCO₃ content varies between 23 and 39 wt% (Fig. F4). Organic carbon is generally low (<1 wt%) (Fig. F26), except for one sample at 81.13 mbsf, which is 2.7 wt%.

Nitrogen (Fig. F26) varies between 0.04 and 0.09 wt%. Total nitrogen shows greatest variability in the upper 50 m of Hole U1385B.

The C/N ratio, used to distinguish the origin of organic matter (marine versus terrestrial) in the sediments (Emerson and Hedges, 1988; Meyers, 1997), indicates that the organic carbon is mainly of marine origin (C/N = 9–14.5) (Table T2; Fig. F26). One sample, at 81.13 mbsf, has a C/N ratio of 42.6, which indicates either greater terrestrial input or degradation of organic matter for this sample. Further analysis is required to determine the cause for this high ratio.

Interstitial water chemistry

Major cations and anions

Sulfate concentrations are near seawater values at the top of the section and decrease to zero at ~50 mbsf (Fig. F27A; Table T22). Alkalinity averages 9 meq/L from the top of the hole to 35 mbsf and progressively increases downhole (Fig. F27B; Table T22). Ammonium is 1000 µM at the surface and increases downhole, reaching 4800 µM near the base of the hole (Fig. F27C; Table T22). Diagenesis of organic matter at Site U1385 has led to the depletion of dissolved sulfate in interstitial water by the reaction of sulfate reduction:

In this process, sulfate is consumed and alkalinity (i.e., HCO₃^–, HS^–, and HPO₄^2–), ammonium, and phosphate are byproducts. The ratio of alkalinity increase to moles of sulfate reduced is ~2, but the increase in alkalinity in the sulfate reduction zone is only ~5 meq/L rather than the ~40 meq/L implied by the extent of sulfate reduction. This supports the hypothesis that authigenic precipitation of dolomite in the sulfate reduction zone is driving the 1:1 decrease in magnesium and calcium. In the presence of dolomite formation, each unit of decrease in both magnesium and calcium would be accompanied by a 2-unit decrease of interstitial water alkalinity. The observed loss of ~10 mM of magnesium and calcium in the sulfate reduction zone, if a result of dolomite formation, in part explains the smaller than expected but still significant alkalinity increase from sulfate reduction. The hydrogen sulfide ion produced by sulfate reduction can react with iron to form iron sulfide minerals (e.g., FeS and FeS₂), which are paramagnetic and have relatively low magnetic susceptibility. Sulfate reduction can also lead to the dissolution of magnetite and formation of minerals with lower susceptibility. This process may explain the decrease in magnetic susceptibility below ~40 mbsf at Site U1385 (see “Physical properties”).

Once sulfate is depleted, methanogenesis becomes an important process in the sediment and methane begins to increase starting at ~50 mbsf, reaching a maximum concentration of >80,000 ppmv at ~95 mbsf (Fig. F27D; Table T22). In this process, organic matter is degraded and carbon dioxide, methane, ammonium, and phosphate are produced according to Redfield stoichiometry:

The SMT is very sharp at Site U1385 at ~50 mbsf, presumably because anaerobic methane oxidation in the presence of sulfate (i.e., CH₄ + SO₄^2– ‡ HCO₃^– + HS^– + H₂O) erases the signal of diffusive penetration of methane into the sulfate reduction zone above (Gieskes, 1983; Boetius et al., 2000).

Magnesium concentrations average ~45 mM in the uppermost 35 m of the core and then decrease to ~35 mM between 35 and 62 mbsf; thereafter, values average 32 mM to the base of the hole (Fig. F28B; Table T22). Calcium concentrations are 7.5 mM at the top of Hole U1385B, decrease to a minimum of 2.7 mM at 50 mbsf, and increase to 5.3 mM at the base of the hole (Fig. F28A; Table T22). The 1:1 decrease in calcium and magnesium in the zone of sulfate reduction suggests dolomite authigenesis, which was noted in smear slides and XRD analysis (see “Lithostratigraphy”). Potassium concentrations average 11.5 and 12 mM in the uppermost 25 mbsf and decrease downhole (Fig. F28C; Table T22). Below the sulfate reduction zone, the increase in calcium with a simultaneous decrease in magnesium and potassium may reflect the in situ alteration of volcanic detritus to form smectite (Perry et al., 1976). The possibility of this reaction is supported by the presence of dispersed volcanic glass noted in some sections (see “Lithostratigraphy”).

Minor elements

Barium, strontium, and lithium have similar patterns with near-constant values for the upper 50 mbsf, increasing rapidly between 50 and 82 mbsf, and gradually increasing below ~90 mbsf to the base of the hole (Fig. F29A–F29C; Table T22). Increases in concentrations of strontium may reflect dissolution of biogenic calcite (high in Sr) and secondary precipitation of inorganic calcite (low in Sr), although small contributions of strontium can also be derived from alteration of volcanic material (Gieskes, 1981). The similar patterns of barium and lithium suggest similar processes are controlling these elements. Boron shows an inverse pattern to barium, strontium, and lithium (Fig. F29D; Table T22). The most prominent feature of the silicon profile is the peak value at 100 mbsf (Fig. F29E; Table T22). Manganese concentrations decrease sharply from values of 18 µM at the surface to 12 µM at 43 mbsf. Iron shows a similar pattern, with values of 120 µM decreasing to 30 µM at 43.6 mbsf (Fig. F30; Table T22). Manganese and iron are near unchanging below 50 mbsf to the base of the hole.

Stable isotopes

Oxygen isotope values in interstitial water show considerable variability in the upper 30 mbsf, which is unexpected from a profile that should be dominated by diffusion (Fig. F31A; Table T23). An interval of generally higher δ¹⁸O exists between 7.5 and 27 mbsf, reaching values as high as 0.41‰, which may reflect the increase in δ¹⁸O of seawater during the last glacial period that has been attenuated by diffusion (McDuff, 1984; Schrag and DePaolo, 1993). The absence of a smooth δ¹⁸O profile at Site U1385 suggests that either reactions are occurring in the sediment to maintain the sharp δ¹⁸O gradients or that interstitial water samples have been contaminated by seawater circulated in the borehole during drilling operations. The contamination hypothesis is not supported by sulfate measurements (see “Sampling contamination”); however, the extent of contamination could be below our level of detection of sulfate. Another possibility is that the oscillations are higher-than-normal measurement noise of the Picarro water isotope analyzer, which was observed during the expedition. These possibilities are being investigated during postcruise analysis.

The δD value of 1‰ at the top of the hole reflects bottom water. Values of δD increase between 20 and 25 mbsf to 3.1‰ and fall between 25 and 44 mbsf before increasing to a maximum of 3.62‰ at 62.6 mbsf (Fig. F31B; Table T23). Following the δD maximum, values decrease downhole to 1.9‰ at 81.6 mbsf and remain more or less the same to the base of the hole. Lack of a strong correlation between δ¹⁸O and δD suggests the two isotopologues of water are controlled by different processes (Fig. F32). The interval of high δD corresponds to a low in magnesium, which may partly indicate alteration of volcanic detritus to smectite. This process can increase the δD of interstitial water because the hydroxyl groups in clay minerals preferentially incorporate the light isotope of hydrogen relative to water. However, the interlayer waters of smectites are isotopically heavy with respect to the water in which they form, which would decrease pore water δD. The relative isotopic fractionation of the structural hydrogen with respect to its formation water is greater than that of the interlayer water, but more information about the smectite structure is necessary to know which process is dominant in setting the pore water value in this case (Savin and Epstein, 1970; Suzuki and Epstein, 1976; Yeh, 1980; Lawrence and Gieskes, 1981; Liu and Epstein, 1984; Cerling et al., 1985).

Chloride is nonreactive in ocean sediment, and its concentration is dominantly controlled by diffusion and compaction-driven advection. However, uptake/release of water caused by formation/degradation of hydrated solids will increase/decrease the chloride concentration. The sharp gradients in the chloride profile at Site U1385 occurring simultaneously with changes in δD (Fig. F31C) suggest that the same process is controlling chloride and δD and that this process may be indeed the formation of smectite.

Sampling contamination

In order to evaluate whether the Rhizon samples were contaminated with drilling fluid, the sulfate concentration profile in the Rhizon samples from Hole U1385B was compared with that of the whole-round samples taken in Hole U1385A. Rapid sulfate reduction in the sediment makes sulfate concentration a good marker of drilling-fluid contamination; any contaminated Rhizon samples would have higher sulfate concentrations than the whole-round samples taken at the same depths. This would be particularly noticeable below the base of the sulfate reduction zone (~50 mbsf at Site U1389), after which sulfate concentration is 0 until the base of the hole. Also, in order to more precisely constrain the sulfate reduction zone, the sulfate concentration in the syringe samples taken from every section of Cores 339-U1385B-5H and 6H was measured. This enabled higher resolution comparison between Rhizon and squeeze samples in those cores.

All Rhizon samples analyzed for sulfate were taken from water following measurements with the Picarro water isotope analyzer. All samples were refrigerated at 4°C after isotope analysis, but the septa caps of the sample vials had been punctured by the Picarro sampling needle. To quantify the potential amount of evaporation that may affect our analysis, sulfate concentration was re-measured in some of the whole-round samples that had been analyzed for oxygen isotopes and stored in the same way. In Figure F33, the replicate whole-round measurements are identified as “Whole Rounds, post-isotope.” If evaporation of the samples occurred, the replicates would be expected to all have higher sulfate concentrations than the original samples, but this is not the case. Instead, the whole-round sample sulfate concentrations that were re-measured after water isotope analysis are lower than the initial values. This indicates that the change was not due to sample evaporation; instead, the difference in the two measurements is likely due to different dilution methods used for sample preparation before analysis. The original whole-round measurements were automatically diluted by the ion chromatograph, whereas all of the later measurements were diluted using a Hamilton Microlab 500 autodiluter/dispenser prior to sample introduction to the ion chromatograph.

Where sulfate is present, samples diluted in the same way have indistinguishable sulfate concentration profiles within the error of the measurements and detection limits of the instrument. The lowest measured sulfate concentration in one of the Rhizon samples was 1.43 mM, which is half the concentration of our lowest measured calibration standard of ~2.7 mM. Below the end of the sulfate reduction zone, as measured in the squeezed samples, no detectable sulfate was present in any of the Rhizon samples.

The contamination from drilling fluid in the Rhizon samples is minimal, if present at all. Postcruise analysis will determine whether the downhole δ¹⁸O, δD, and chloride profiles are more sensitive to low levels (micromolar) of contamination below the level of detection of the ion chromatograph. This remains possible because the expected downhole changes in δ¹⁸O, δD, and chloride are on the order of 1%–3% of the present bottom water values, and the lowest sulfate measurement registered was ~6% of typical modern seawater sulfate concentration.

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