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doi:10.2204/iodp.proc.339.106.2013

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, 20 headspace samples were analyzed, including 1 from Hole U1388A, 18 from Hole U1388B, and 1 from Hole U1388C (Fig. F27; Table T13), spanning the entire depth range of the site. Methane (C₁), ethane (C₂), and ethene (C₂₌) were the only hydrocarbons detected. Methane ranged from 10.38 ppmv near the surface to a maximum of 65,682 ppmv at 160.1 mbsf (Section 339-U1388B-18X-2). Ethane was detected at and below 83.6 mbsf and remained less than 2.75 ppmv. Ethene was detected at and below 147 mbsf and did not exceed 0.82 ppmv.

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 Holes U1388A–U1388C (Table T14), spanning the maximum penetration at this site. CaCO₃ varies from 18.0 to 29.9 wt% (Fig. F8). Organic carbon is generally low and varies between 0.08 and 0.92 wt% (Fig. F28A).

Nitrogen was measured downhole to 224.74 mbsf, with values ranging from 0.02 to 0.11 wt% (Fig. F28B). The C/N ratio, used to distinguish the origin of organic matter (marine versus terrestrial) in sediment (Emerson and Hedges, 1988; Meyers, 1997), indicates that the organic carbon is mainly of marine origin with little terrestrial input (Fig. F28C). The highest C/N value of 16 is at 36.34 mbsf, corresponding to a portion of the core with fine-sand composition. The surrounding sediment is mainly clay and silt. A slight positive relationship appears to exist between organic matter content and C/N ratios, but it is weaker than at Sites U1385–U1387.

Interstitial water chemistry

Major cations and anions

Elemental analyses were made on 14 samples from whole-round samples taken at Site U1388 to a total depth of 211 mbsf, including 1 sample from Hole U1388A and 13 from Hole U1388B.

Sulfate concentrations are near seawater values at the top of the section and decrease to zero by ~50 mbsf (Fig. F29A; Table T15). Alkalinity generally increases downhole with the exception of a low interval between 125 and 150 mbsf (Fig. F29B). Ammonium gradually increases from near zero at the top of the hole to a maximum of 8000 µM at 200 mbsf (Fig. F29C). Methane begins to increase at 50 mbsf, marking the SMT (Fig. F29D).

Calcium, magnesium, and potassium profiles show similar patterns, with a sharp decrease between the top of the hole and 50 mbsf (Fig. F30). Potassium continues to gradually decrease downhole, whereas calcium and magnesium increase. Chloride and sodium increase downhole from seawater values at the top of the section to a maximum of 800 and 690 mM at 211 mbsf, respectively (Fig. F31). The Na/Cl ratio remains near the seawater value (0.86) throughout the section.

Minor elements

Strontium remains near seawater value in the upper 75 mbsf and increases thereafter, reaching its greatest values at the base of the hole (Fig. F32). Barium was below detection in the uppermost sample and increases downhole to a maximum of 57 µM between 107 and 128 mbsf. Thereafter, barium decreases downhole to 150 mbsf and averages ~25 µM below. Lithium decreases between the surface and 50 mbsf and then increases downhole, exhibiting a pattern similar to that of strontium. Boron is high in the upper 50 mbsf and undergoes a series of stepped decreases downhole at 70, 100, and 150 mbsf. Silicon concentration is 110 µM at the surface and generally increases downhole, with a distinct maximum of 647 µM between 160 and 177 mbsf.

Figure F33 shows the relationship of various elemental concentrations versus chloride. Sodium, calcium, magnesium, and strontium show strong positive relationships with chloride, whereas potassium, boron, and barium display a negative correlation with chloride.

Stable isotopes

Oxygen and hydrogen isotopes were measured on 14 samples from whole-round samples at Site U1388. At the top of the hole, δ¹⁸O and δD are at bottom water values of 0.8‰ and 6.5‰, respectively, and increase to 1.3‰ and 9.2‰ by 50 mbsf, respectively (Fig. F34; Table T16). δD gradually decreases below 50 mbsf toward the base of the hole, whereas average δ¹⁸O values remain between 1.3‰ and 1.4‰. δ¹⁸O and δD are generally positively correlated (Fig. F35).

Discussion

The interstitial water profiles at Site U1388 reflect a combination of processes, including microbial degradation of organic matter, dissolution, precipitation of authigenic minerals, and the likely influence of a brine. Sulfate reduction is complete in the uppermost 50 mbsf. The increase in alkalinity and decreases in calcium and magnesium reflect authigenic precipitation of calcite and dolomite. This interpretation is supported by a peak in dolomite observed in XRD data at 36.34 mbsf (see “Lithostratigraphy”). The linear downhole increase and high values of sodium and chloride at depth suggest a source for these elements in the deeper sediment. Some mud volcanoes in the Gulf of Cádiz have high salinities derived from dissolution of halite and late-stage evaporites (Hensen et al., 2007; Scholz et al., 2009). Nevertheless, no strong deviation from the seawater value for Na/Cl (0.86) at Site U1388 is apparent, as would be expected from the dissolution of halite (Na/Cl = 1). Vengosh et al. (1994) suggests that the high salinity values of interstitial waters in Mediterranean Deep Sea Drilling Project sites with Na/Cl ratios typical of seawater are the remnants of Miocene hypersaline lakes, and perhaps these brines also are present in the eastern Gulf of Cádiz.

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