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doi:10.2204/iodp.proc.339.108.2013

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, 37 headspace samples from Hole U1390A (sampling resolution of one per core) were analyzed (Fig. F26; Table T16), spanning the entire depth range of the site. In Hole U1390A, methane (C₁), ethane (C₂), ethene (C₂₌), propane (C₃), and propene (C₃₌) were detected. Methane ranged from 2.9 ppmv near the seafloor to a maximum of 63,991 ppmv at 49.2 mbsf (Section 339-U1390A-6H-6). At the base of Hole U1390A, methane is 9,830.5 ppmv. Both ethane and ethene were detected starting at 31.60 mbsf, but concentrations remain low. Ethane reached a maximum value of 46.3 ppmv, and ethene reaches a maximum of 2.5 ppmv. Propene and propane concentrations at Site U1390 are also low; their concentrations are below 1 ppmv except for the last sample measured (Section 339-U1390A-38X-4), in which propane reaches 11.52 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 Hole U1390A (Table T4), thereby spanning the full depth of the site. CaCO₃ varies from 21.1 to 34.5 wt% (Fig. F27), and no discernible downhole trends appear. Organic carbon is low, generally <1 wt%, and varies between 0.4 and 1.12 wt% (Fig. F28A).

Nitrogen (Fig. F28B) was measured downhole to 351 mbsf in Hole U1390A. Measured nitrogen ranges from 0.05 to 0.11 wt%. We did not observe any notable trends in nitrogen content with depth. The C/N ratio, used to distinguish the origin of organic matter (marine versus terrestrial) in sediment (Emerson and Hedges, 1988; Meyers, 1997), varies between 6 and 13 and indicates that the organic carbon is mainly of marine origin (Fig. F28C). Samples with C/N ratios exceeding 10 indicate some terrestrial input. Total organic carbon and C/N ratios are generally positively correlated, which is in agreement with the relationships observed at Sites U1385–U1388 but is in contrast with the relationship between total organic carbon and C/N at Site U1389.

Interstitial water chemistry

Major cations and anions

Whole-round samples were taken for interstitial water analysis at a resolution of one per core downhole to 346 mbsf in Hole U1390A. Sulfate concentrations drop to zero in the uppermost 20 mbsf (Fig. F29A; Table T17). Ammonium concentrations steadily increase downhole from 1,200 µM at the seafloor to 15,500 µM at 170 mbsf (Fig. F29B). From 170 to 295 mbsf, ammonium concentrations oscillate around 14,000 µM, reaching a minimum of 11,842 µM and maximum of 15,321 µM followed by a small increase at ~300 mbsf to values between 16,000 and 17,000 µM at the base of the hole. Alkalinity varies between 10.9 and 15.4 meq/L in the upper 70 mbsf and decreases to 5.67 meq/L at 107 mbsf (Fig. F29C). After a slight increase to 7.97 meq/L at 137 mbsf, alkalinity decreases again downhole to 290 mbsf and averages ~2 meq/L downhole to the base of the hole.

Calcium and magnesium display similar patterns of variation (Fig. F30A, F30B). Both decline in the upper 20.6 mbsf in the sulfate reduction zone, from 7.7 to 3 mM and 51.6 to 35.9 mM, respectively. The ratio of magnesium to calcium decreases from ~6.7 to 3.3, which indicates that both dolomite and calcite are precipitating. Calcium and magnesium remain roughly constant from 20 to 58.4 mbsf, after which they increase to17.3 and 85.1 mM, respectively at 194.7 mbsf. Between 194.7 and 290.6 mbsf, calcium continues to increase and reaches a maximum value of 31.6 mM, whereas magnesium oscillates between 83 and 87 mM before decreasing slightly to 81 mM at 244 mbsf and increasing to 86.1 mM at 290.6 mbsf. Between 290.6 mbsf and the base of the hole, calcium and magnesium drop slightly to 14.2 and 80.5 mM, respectively.

Potassium decreases from 10.7 to 8.9 mM in the sulfate reduction zone between the seafloor and 20.6 mbsf (Fig. F30C). Potassium then remains constant until 39.3 mbsf, after which it increases gradually to 14.2 mM at the base of the hole.

Sodium and chloride concentrations have almost identical trends; however, the magnitude of changes in chloride is greater than that in sodium, which can be identified in the Na/Cl ratio. Sodium remains close to its core-top value of 479 mM downhole until 47.7 mbsf and increases to 1249 mM at 290.6 mbsf (Fig. F31B). After remaining constant for ~20 m, sodium increases to 1309 mM at 328.9 mbsf and decreases to 1169 mM at the base of the hole. The maximum sodium value is 2.7 times greater than its concentration at the top of the hole. Chloride is approximately constant and close to its core-top value of 586 mM in the upper 47.7 mbsf, after which it increases to 1745 mM at 290.6 mbsf (Fig. F31A). From 290.6 to 317.3 mbsf, chloride is roughly constant. The concentration increases to 1788 mM at 328.9 mbsf and decreases to 1604 mM at the base of the hole. The maximum chloride concentration is three times greater than its value in the top of the hole. Na/Cl ratios are all below the modern seawater value of 0.86, except at 81.1 mbsf (Fig. F31C). In the upper ~80 mbsf, Na/Cl varies around 0.8 but shows no significant trend. From 81.1 to 253.7 mbsf, Na/Cl decreases to a minimum value of 0.694. The ratio increases to ~0.73 from 253.7 mbsf to the base of the hole.

Minor elements

Barium concentration in the upper 11 mbsf is ~2 µM and increases to 27.3 µM at 20.6 mbsf (Fig. F32A). From 20.6 to 107 mbsf, barium varies between 13.6 and 24 µM. Between 107 and 118 mbsf, barium concentration increases to 40.9 µM. Barium increases between 118 and 290.6 mbsf to a maximum of 1131.2 µM. The concentration then decreases to 470.4 µM at the base of the hole, except for an excursion at 329 mbsf to 894.6 µM.

Boron increases from 493.6 to 558.8 µM in the sulfate reduction zone (upper 20.6 mbsf; Fig. F32B). The concentration is constant between 20.6 and 39.3 mbsf, after which it decreases to 346 µM at 118 mbsf. The concentration oscillates slightly until 156 mbsf and then decreases to 256.5 µM at 214 mbsf. After a slight positive excursion to 325.7 µM at 223.4 mbsf, boron concentration continues to decrease to the base of the hole, reaching a minimum of 242.3 µM.

Iron increases from 27.4 to 53.9 µM between 2.85 and 11.1 mbsf (Fig. F32C). The concentration of iron then decreases to ~2.3 µM at 67.8 mbsf and remains low until an increase to 60.2 µM at 162.8 mbsf. Iron concentration oscillates over a large range between 162.8 mbsf and the base of the hole, with the most significant jump being between 3.64 and 122.3 µM at 223.4 mbsf. At the base of the hole, the concentration of iron is ~32 µM.

Lithium decreases from 25.7 to 16 µM in the upper 20.6 mbsf (Fig. F33A). It then remains approximately constant until 107.1 mbsf, after which it increases to 69.3 µM at 253.7 mbsf. Between 253.7 and 280.9 mbsf, lithium increases more rapidly with depth to 156.9 µM. The lithium concentration decreases very slightly from 280.9 mbsf to a value of 134.3 µM at 328.9 mbsf. Lithium then increases to a value of 170 µM at the base of the hole.

Silicon concentration is highly variable and has no significant trend in the upper 156 mbsf, varying between a minimum of 270 µM and a maximum of 413.4 µM (Fig. F33B). The concentration increases to 805.1 µM at 162.8 mbsf. Silicon then decreases to 307.5 µM at 213.8 mbsf and increases to 544.9 µM at 223.4 mbsf. From 223.4 to 271.7 mbsf, silicon decreases to 292.5 µM. The concentration increases to 414.3 µM at 309.8 mbsf. After decreasing again to ~295 µM, silicon concentration increases to 561.8 at the base of the hole.

Strontium is variable in the top part of the hole, with values ranging between 57.5 and 97.9 µM (Fig. F33C). At 107.1 mbsf, strontium starts increasing monotonically and reaches a maximum of 2793 µM at 262.3 mbsf. The concentration of strontium decreases from 271.7 mbsf to 1925 µM at the base of the hole.

Stable isotopes

Oxygen and hydrogen isotopes are ~0.9‰ and 5.8‰, respectively, at the seafloor, reflecting MOW (Fig. F34A, F34B; Table T18). Oxygen isotopes are variable, with generally lower values between 100 and 200 mbsf and higher values above and below this interval. Maximum δ¹⁸O values of ~1.4‰ are found at the base of the hole.

Hydrogen isotope values average ~7‰ in the upper 107 mbsf. Below this level, δD decreases markedly, reaching minimum values of –6.5‰ at 300 mbsf and increasing slightly toward the base of the hole. The very high concentrations of chloride and sodium are not associated with increases in δ¹⁸O and δD. In fact, δD and chloride are highly negatively correlated (Fig. F35), with the lowest δD values recorded in the interval of highest chloride concentrations (Fig. F34B, F34C). The low δD signature is characteristic of clay mineral dehydration reactions (Kastner et al., 1991; Dählmann and de Lange, 2003). As clay dehydration requires higher temperatures than those inferred from the downhole temperature trend (see “Downhole measurements”), we speculate that the isotopic and elemental composition of interstitial water at Site U1390 are a product of the upward movement of interstitial water from the deep subsurface, which has been affected by clay dehydration reactions and dissolved salt layers at greater depth.

Summary

At Site U1390, organic sediment analysis shows that organic carbon is generally low. CaCO₃ content varies between 21 and 34 wt%. C/N ratios indicate that the source of organic carbon to this site is primarily of marine origin. We did not observe notable trends with depth in any of the organic or inorganic carbon indexes.

In Hole U1390A, the most distinctive feature of the interstitial water concentration profiles is the relatively high values as compared to those measured at other sites during Expedition 339. The maximum interstitial water concentrations of ammonium and strontium are an order of magnitude greater than those at any other previous site (U1385–U1388), and the maximum barium concentration is two orders of magnitude higher than concentrations at other sites in the Gulf of Cádiz and an order of magnitude higher than concentrations measured at Site U1385. The maximum values of chloride and sodium concentrations at Site U1390 are approximately double those at Site U1389. Maximum strontium concentrations are higher by an order of magnitude than values measured at other sites. Calcium and magnesium concentrations are elevated as well with respect to maximum values at other sites, although only by ~1.5–2 times.

All the alkali earth elements (Ca, Mg, Sr, and Ba) have very similar downhole patterns, suggesting that the same process controls their concentrations.

The downhole increases of sodium and chloride are quite notable. Chloride is 3 times the bottom water value and sodium is 2.75 times the bottom water value. Although the sodium and chloride concentrations reach high values, the Na/Cl ratios decrease to significantly less than the modern seawater value of 0.86. It is possible that part of the trend in Na/Cl ratios is false, as the reliability of the sodium measurements at high concentration has not been verified. However, the high chloride and sodium concentrations are associated with low δD values, and no evidence exists for evaporite deposits in the sediment (see “Lithostratigraphy”). This indicates that high salinity may not be the result of in situ salt dissolution. Instead, we suggest that salts were dissolved at depth by interstitial water that was altered by clay mineral dehydration reactions, which can also the affect water sodium concentration.

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