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doi:10.2204/iodp.proc.339.107.2013

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, 39 headspace samples from Hole U1389A and 63 headspace samples from Hole U1389E (sampling resolution of one per core) were analyzed (Fig. F28; Table T21), spanning the entire depth range of the site. In Holes U1389A and U1389E, we detected methane (C₁), ethane (C₂), ethene (C₂₌), propane (C₃), and propene (C₃₌). Methane ranges from 106.3 ppmv near the seafloor to a maximum of 71,003 ppmv at 100.21 mbsf (Section 339-U1389A-12X-3). Ethene does not exceed 2.9 ppmv in the samples in which it was detected, and ethane remains below 50 ppmv in most samples, with a maximum of 244.4 ppmv at 905.1 mbsf. Generally, both ethene and ethane show higher concentrations with depth, whereas methane concentrations decrease. Propane and propene remain low, with propene not exceeding 1.5 ppmv. Propane is at concentrations below 1 ppmv above 750.72 mbsf. Below this depth, it displays an increasing trend, with a maximum of 92.0 ppmv at 905.1 mbsf.

As part of the safety protocol, we used ratios of hydrocarbon concentrations to determine their origin as biogenic versus thermogenic. Thermogenic gases can be identified by a C₁/(C₂ + C₃) ratio of <50 (Claypool and Kvenvolden, 1983). A few samples at this site drop below this level; however, the absolute concentrations of heavier gases in those samples are too low to be a cause for concern.

Sedimentary geochemistry

Sediment samples were collected for analysis of solid-phase geochemistry (inorganic and organic carbon and nitrogen) at a resolution of approximately one sample per core in Holes U1389A and U1389E (Table T22), thereby spanning the full depth of the site. CaCO₃ varies from 21 to 35 wt% (Fig. F29). At ~360 mbsf, CaCO₃ concentrations display a sharp drop and slow rebound with depth to values characteristic of the top of the site. This sharp transition corresponds to samples barren in nannofossils (see “Biostratigraphy”) and is close to but offset by ~60 m from the observed changes in interstitial water chemistry. Organic carbon varies between 0.4 and 1.2 wt% (Fig. F30A) from the seafloor to 355 mbsf in Hole U1389A and 0.3 and 1.8 wt% in Hole U1389E between 341.4 and 986.1 mbsf.

Nitrogen (Fig. F30B) was measured downhole to 355 mbsf in Hole U1389A and between 341.4 and 986.1 mbsf in Hole U1389E. Measured nitrogen ranges from 0.008 to 0.24 wt%, with Hole U1389E showing greater nitrogen concentrations than Hole U1389A, in particular an interval of much higher values between 400 and 600 mbsf. 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 a mix of marine and terrestrial inputs to a depth of 352 mbsf. Below this level, the ratios are distinctly marine, and the transition between the two regimes coincides with the observed transition in CaCO₃ content (Figs. F29, F30C; Table T22). Unlike at Sites U1385–U1388, at Site U1389 an inverse relationship exists between total organic carbon (TOC) and C/N ratios that is particularly evident in the uppermost 300 mbsf. The data suggest that at Site U1389, a greater input of organic matter was associated with greater marine productivity rather than a terrestrial source.

One hypothesis for the observed relationship between TOC and C/N ratio is that it is due to riverine input of iron from the Iberian pyrite belt (Almodóvar et al., 1997). We theorize that changing sea level (Miller et al., 2005) has altered the spatial extent of river outflow as well as the magnitude of iron input to the surface waters of the Gulf of Cádiz, resulting in changes in marine productivity. The magnitude of sedimentation (see “Biostratigraphy”) and iron-induced marine productivity at this site may have resulted in the marine values observed in the organic indexes measured at Site U1389 despite its proximity to land. We evoke this as a likely explanation for the differences between Sites U1389 and U1390. Further study at higher resolution, aided by carbon isotopic measurements, is needed to definitively determine the origin of the observed variability between sites.

Interstitial water chemistry

Sediment samples for postcruise high-resolution analysis were taken from the bottom of each section of the uppermost 200 mbsf in Hole U1389A using syringe plugs. Whole rounds for shipboard and shore-based interstitial water sampling were taken at a resolution of one per core in Hole U1389A and one every three cores in Hole U1389E. After encountering a sharp reversal in the downhole chlorinity trend in Hole U1389E around 565 mbsf, whole-round sampling resolution was temporarily increased to one per core until low recovery made this impractical.

Major cations and anions

Sulfate decreases sharply from 18.4 mM at seafloor to 0 mM at 13.3 mbsf at Site U1389 (Fig. F31A; Table T23) and remains undetectable downhole until the base of the site except for two samples between 730 and 750 mbsf in which 3–4 mM of sulfate was measured. These samples were taken in cores recovered by RCB in which drill-fluid contamination of the interstitial water sample is more likely than in cores recovered with the APC. The samples were also well compacted and crumbled easily because of their low porosity, which made proper removal of the contaminated outer rind more difficult. Drill fluid is composed of surface seawater, which has high (typically ~29 mM) concentrations of sulfate. Therefore, the sulfate signal is likely attributable to minor sample contamination.

Ammonium varies but generally increases from 1000 µM at the seafloor to a maximum of 6485 µM at 533 mbsf (Fig. F31B). Large increases in ammonium take place in the sulfate reduction zone, from 1000 to 2751 µM (~132 µM/m), and between 293 and 533 mbsf, where the concentration increases from 3864 to 6485 µM (~11 µM/m). The location of the ammonium concentration maximum is the same as the location of the maximums of many other elements. Below 533 mbsf, ammonium generally decreases and is 4320 µM at the base of the site.

Alkalinity is variable in the upper 292 m of the site, with a maximum in the uppermost 3 mbsf of 10.3 meq/L and a minimum of 5.01 meq/L at 60.6 mbsf (Fig. F31C). At 318 mbsf, alkalinity drops sharply and remains between 1.8 and 3.1 meq/L downhole to 565.3 mbsf, where its value begins to increase again. From 677.5 mbsf to the base of the site, alkalinity values vary between 3.5 and 8.4 meq/L.

Magnesium and calcium both decrease sharply, from 51 to 35 mM and from 7.7 to 5 mM respectively, in the sulfate reduction zone (Fig. F32A, F32B). The decrease in magnesium is approximately six times greater than that of calcium, which indicates that both in situ dolomitization of calcite and authigenic dolomite precipitation are occurring. The sharp decrease in alkalinity (Fig. F31C) and increase in ammonium (Fig. F31B) coincident with the sulfate decrease are also evidence that the sulfate reduction reaction is occurring:

The sharp sulfate–methane transition that occurs at the base of the sulfate reduction zone is 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).

From 13.3 to 263 mbsf, magnesium concentration varies around a constant value, but calcium concentration increases to 7.25 mM, indicating diagenetic dissolution of calcite (Fig. F32A, F32B). Between 263 and 533 mbsf, magnesium and calcium increase linearly in roughly a 5:2 ratio. Both have a maximum at 533 mbsf, magnesium at 63.6 mM and calcium at 20.0 mM. Concentrations then decrease to 11.1 and 2.68 mM, respectively, at 677 mbsf, below which they vary only slightly to the base of the site. Magnesium values at the base of the site are ~20 mM lower than those above the section in which concentration greatly increases, whereas calcium concentrations at the base of the site are similar to those in the top sections.

Potassium concentration in the interstitial water decreases sharply in the sulfate reduction zone, from 9.96 to 7.96 mM (Fig. F32C). Potassium then continues to decrease slowly over the next 200 m by 2 mM. In the depth interval between 263 and 533 mbsf, where many other large property changes were observed, the potassium concentration remains roughly constant, oscillating between 5.8 and 7 mM. From 533 mbsf to the base of the site, potassium concentration decreases another 3.5 mM to a minimum value of 2.63.

Sodium and chloride do not show much variation from their respective bottom water values of 490.8 and 584.8 mM in the uppermost ~250 m of Site U1389 (Fig. F33A, F33B). Starting at 263.4 mbsf, sodium and chloride both begin to increase and reach maxima at 533 mbsf of 691.9 and 942.2 mM, respectively. Below this point, sodium and chloride decrease sharply to 323.8 and 356.5 mM at 757.4 mbsf and do not vary significantly to the base of the site.

The sodium/chloride ratio in seawater of 0.86 remains constant under the influence of evaporative processes. Deviations from this value in interstitial water sodium/chloride ratios can be caused by diagenetic mineral or evaporite dissolution, clay mineral transformations, or interactions with ocean crust through tectonic activity (Kastner et al., 1991). Between 41 and 185 mbsf, the sodium/chloride ratio is slightly greater than that of seawater (Fig. F33C), which suggests mild leaching of buried salt evaporite, perhaps halite (NaCl). In the section of rapid change that precedes the maximum in many elements, an excursion in the sodium/chloride ratio to low values between 0.7 and 0.75 occurs. At the multi-elemental maximum at 533 mbsf, the sodium/chloride ratio reaches its minimum value of 0.7. Ratios of sodium/chloride below the seawater value with a simultaneous increase in both sodium and chloride concentrations suggest the waters are affected by hydrothermal reactions originating in the crust (Kelley et al. 2002).

Minor elements

Barium varies between 0 and 21.3 µM in the uppermost 230.6 m of Site U1389 (Fig. F34A; Table T23). The concentration then increases rapidly to a maximum of 136.1 µM at 533 mbsf that coincides with the maxima in strontium, chloride, sodium, magnesium, calcium, and ammonium. Barium then decreases rapidly to 7.1 µM at 706.5 mbsf, below which it varies between 0 and 12.4 µM to the base of the site.

Boron increases slightly in the uppermost ~13 mbsf from 479.7 to 510.7 µM (Fig. F34B). The concentration then decreases to ~220 µM at 318.1 mbsf. A slight increase from ~220 to 264 µM in the next 100 m occurs, after which boron decreases slowly to ~ 216 µM at 621.4 mbsf. From 621.4 mbsf to the base of the site, boron concentrations fluctuate between 164.3 and 267.5 µM.

Iron decreases from 77.5 to 5 µM in the uppermost 80 m of Site U1389 and then remains low to the base of the site (Fig. F34C). The majority of the decrease in iron, from 77.5 to ~30 µM, occurs in the sulfate reduction zone. The formation of iron sulfide minerals (FeS and FeS₂) is favored in sulfate reduction zones because it requires the presence of hydrogen sulfide, one of the products of the sulfate reduction reaction. The continuing decrease of iron concentration below the base of the sulfate reduction zone may indicate another source of hydrogen sulfide in the sediments.

Lithium concentration decreases from 43.4 to 24.8 µM in the uppermost 13.3 mbsf (Fig. F35A) but then increases to a maximum of 89.9 µM at 533 mbsf, where sodium, magnesium, chloride, calcium, ammonium, barium, and strontium also have concentration maxima. From 533 to 650.4 mbsf, lithium concentration decreases to 58.3 µM. Lithium then increases to 84.9 µM at 757.4 mbsf and proceeds to decrease yet again to ~65 µM near the base of the site.

Silicon varies between 110 and 397 µM between the seafloor and 420 mbsf but does not show a coherent trend with depth (Fig. F35B). Two high values of 394 and 397 µM occur at 193 and 231 mbsf, respectively. Below 420 mbsf, the range of silicon values decreases to a third of the upper site variability, and values vary between 90.7 and 179 µM.

Strontium increases gradually from 72 to 120 µM in the upper 292 m of Site U1389 (Fig. F35C). Around 292 mbsf, the rate of strontium change increases. Strontium concentration reaches a maximum of 539 µM at 533 mbsf. The concentration then decreases sharply to 134 µM at ~621 mbsf and continues dropping slowly to 84 µM at 707 mbsf. Strontium then varies slightly downhole to the base of the site.

Stable isotopes

Oxygen and hydrogen isotopes are ~0.8‰ and 6‰, respectively, at the top of the core, reflecting Mediterranean Overflow Water (Fig. F36A, F36B; Table T24). Oxygen isotope values generally increase downhole, whereas hydrogen isotopes decrease below 200 mbsf. δ¹⁸O increases by 0.8‰ between 320 and 400 mbsf, below which values average ~1.9‰ to the base of the hole except for two samples at 564 and 651 mbsf that gave low, surfacelike δ¹⁸O values. Below 200 mbsf, δD values steadily increase, reaching a minimum of –10.8‰ at 530 mbsf. Below 530 mbsf, δD increases to –3‰ at 565 mbsf and thereafter averages from –7‰ to –9‰ to the base of the hole. There is a strong negative correlation between δ¹⁸O and δD (Fig. F37), which indicates that their values are influenced by a diagenetic reaction.

Summary

Routine headspace analysis detected only methane, ethane, ethane, propane, and propene at this site. We determined the source of the hydrocarbons to be predominantly biogenic with some thermogenic input downhole, but absolute concentrations of thermogenically sourced gases remained low.

Sediment analysis shows total organic carbon content to be low, close to ~1 wt%. CaCO₃ content exhibits a sharp transition at ~360 mbsf, which is also observed in the nannofossil abundance and corresponds to a change in nitrogen content as well as a decreasing shift in the C/N ratios. Based on our data we ascertain that below 360 mbsf, organic matter is of marine origin, whereas above 360 mbsf it carries the signature of both marine and terrestrial end-members. Unlike Sites U1385–U1388, total organic carbon and C/N ratios at Site U1389 show a negative relationship that is stronger in the upper 300 m of the site but below statistical significance for the site as a whole.

The maxima observed in many of the interstitial water element concentration profiles around 533 mbsf suggest that the same process controls their concentrations. The sharp transition in concentrations both above and below this maximum is indicative of either a physical barrier to vertical diffusion in the sediment or the lateral intrusion of fluid across this depth interval. The increase in concentrations is likely due to dissolution of minerals, which may have occurred locally or at a remote source. Dissolution of carbonate minerals and/or shells would increase the alkalinity of the interstitial water because of an increase in dissolved CO₃^2– concentration. The observed drop in alkalinity at this depth interval thus requires that a lower alkalinity (higher acidity) and probably CO₂-rich fluid was the cause of the carbonate dissolution.

The strong negative correlation between δ¹⁸O and δD (Fig. F37) is characteristic of clay mineral dehydration reactions (Kastner et al., 1991; Dählmann and de Lange, 2003). The silicate framework of many clay minerals is light in δD and heavy in δ¹⁸O (Savin and Epstein, 1970). The breakdown of clays transmits this isotopic signature to the interstitial water. Furthermore, the release of sodium, which is found in the framework of many silicate clays, would explain the increase in sodium/chloride ratio in the interstitial water. The transition of smectite to illite, in particular, has been identified as an important process for fluid formation in deeply buried sediment of the Gulf of Cadiz (Hensen et al., 2007). However, clay diagenesis typically occurs at great depth (several km) and begins at temperatures exceeding ~60°C (Kastner et al., 1991). Downhole temperature profiles (see “Downhole measurements”) in the upper 100 m of the site fit a linear trend of 21°C/km. Extrapolation of this trend indicates that the temperature at the base of the hole is ~34°C. This supports the hypothesis that the low salinity signal is a result of fluid migration from a deeper, higher temperature source.

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