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doi:10.2204/iodp.proc.339.109.2013

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, 34 headspace samples from Hole U1391A (sampling resolution of one per core) and 32 samples from Hole U1391C were analyzed (Fig. F24; Table T14), spanning the entire depth range of the site. In both Holes U1391A and U1391C, we detected methane (C1), ethane (C2), ethene (C2=), propane (C3), and propene (C3=). Methane ranges from 5.4 ppmv near the surface to a maximum of 41,703 ppmv at 69.9 mbsf (Section 339-U1391A-8H-7). Below this depth, methane displays a decreasing trend, reaching 1,276 ppmv at the base of Hole U1391C. Ethane, ethene, propane, and propene were all detected in both holes, but concentrations remained low. All C2 and C3 hydrocarbons do not exceed 8 ppmv for the entire depth profile. Ethane does not exceed 7.1 ppmv, ethene does not exceed 2.6 ppmv, propene does not exceed 0.6 ppmv, and propane does not exceed 4.8 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 U1391A (Table T15); however, because of time constraints at this final site of the expedition, we could not analyze samples below 351 mbsf. CaCO3 varies from 17.5 to 45.2 wt% (Fig. F25). The range is comparable, albeit greater, than the CaCO3 variability measured at nearby Site U1385. Organic carbon varies between 0.5 and 1.8 wt% (Fig. F26A), with no discernible trends at this resolution.

Nitrogen (Fig. F26B) was measured downhole to 351 mbsf in Hole U1391A and ranges from 0.06 to 0.1 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, varies between 7 and 22, indicating that the organic carbon is mainly of marine origin with a terrestrial component of varying contribution (Fig. F26C). Samples with a C/N ratio exceeding 10 indicate the presence of some terrestrial input, whereas those over 20 are predominantly terrestrial (Emerson and Hedges, 1988; Meyers, 1997). The terrestrial component calculated at Site U1391 is greater than that measured at Site U1385, which is expected given the closer proximity of Site U1391 to land. Total organic carbon and C/N ratios are positively correlated, which agrees with the relationships observed at Sites U1385–U1388 and Site U1390 but is in contrast with the relationship between total organic carbon and C/N at Site U1389.

Interstitial water chemistry

Major cations and anions

Sulfate concentrations are near seawater values at the top of the section and decrease to zero at ~20 mbsf (Fig. F27A; Table T16). At 407.5 and 482.55 mbsf, a small amount of sulfate (<1 mM) was detected, which is likely due to sample contamination from drilling fluid.

Ammonium concentrations increase from 467 µM at the surface to ~7,000 µM at 204 mbsf (Fig. F27B). Ammonium values remain roughly constant between 204 and 480 mbsf, after which they increase to a maximum of near 11,200 µM at 649 mbsf.

Alkalinity increases from 8.2 meq/L near the seafloor to peak values of 15.2 meq/L at 13 mbsf and decreases to 6 meq/L at 42 mbsf (Fig. F27C). Alkalinity remains low downhole to 100 mbsf, where it increases again to 10 meq/L at ~150 mbsf. Missing alkalinity data between 168 and 223 mbsf is the result of an electrode malfunction. Downhole from ~400 mbsf, alkalinity begins to rise markedly, reaching values as high as 39.5 meq/L at 592 mbsf.

Calcium, magnesium, and potassium display similar patterns in the upper part of Hole U1391A. All show a sharp decrease from seawater values near the seafloor to lower values between 22 and 32 mbsf. Calcium concentrations decrease from 8.6 mM near the seafloor to a minimum of 2.9 mM at 22 mbsf (Fig. F28A). Magnesium concentration is 52 mM at the seafloor and decreases to 32 mM at 33 mbsf (Fig. F28B). Potassium concentration is 12 mM at the seafloor and decreases to 8.6 mM by 33 mbsf (Fig. F28C). Between 33 and 460 mbsf, potassium and magnesium continue to decrease, reaching values of 6.22 and 23.93 mM, respectively, at 460 mbsf. From 460 mbsf to the base of Hole U1391C, potassium and magnesium increase to 10.44 and 33.5 mM, respectively. Calcium concentrations increase in a step-wise pattern from 22 to 241 mbsf. Between 241 and 396 mbsf, calcium varies between 6.7 and 7.8 mM. At 396 mbsf, calcium begins to decrease and reaches a minimum value of 2.66 mM at 552 mbsf. At 552 mbsf, calcium increases again to 7 mM at 620 mbsf and decreases slightly to reach ~6 mM at the base of Hole U1391C.

Chloride concentrations are 580 mM at the seafloor and decrease downhole, reaching minimum values of ~545 mM near the base of the site (Fig. F29A). Chloride decreases rapidly from the seafloor to ~70 mbsf, after which it continues to decline but at a slower rate.

Sodium at Site U1391 shows very high variability, ranging from 460 to 523 mM (Fig. F29B). This rapidly fluctuating signal is unlikely to be real, as it is not observed in any of the other profiles. With this in mind, the variation of Na/Cl ratios away from the seawater value of 0.86 (Fig. F29C) is probably largely a function of error in measurement of the sodium concentrations.

Minor elements

Barium (Fig. F30A) increases from the seafloor to the base of Site U1391. Sharp increases in barium concentration of about 30 µM are apparent between the seafloor and 22 mbsf and between 582 and 592 mbsf.

Boron decreases rapidly from ~500 µM at the seafloor to 257 µM at 146 mbsf (Fig. F30B). The concentration then varies between 208 and 283 µM downhole to 533 mbsf. From 533 to 552 mbsf, boron decreases sharply to 185 µM and increases to 284 µM at 563 mbsf, after which it decreases slightly to the base of Hole U1391C.

Iron concentration at Site U1391 decreases sharply from 37 to 1.6 µM between the seafloor and 42 mbsf (Fig. F30C). Several sharp increases in iron between 42 mbsf and the base of the hole that are as large as 19 µM are evident, but in general, below 42 mbsf the concentration remains close to our detection limit.

Lithium concentrations are <3 µM throughout Site U1391, which is very close to the detection limit (Fig. F31A).

Silicon varies but generally increases from a seafloor value of 320 µM to a maximum of 1020 µM at 620 mbsf (Fig. F31B). Silicon increases rapidly between 552 and 620 mbsf to ~700 µM.

Strontium increases smoothly from a seafloor value of 76 to 131 µM at 533 mbsf (Fig. F31C). The concentration decreases to 110 µM at 553 mbsf and rapidly increases to the base of the site, reaching a maximum of 184 µM.

Stable isotopes

Water isotopes were measured only in the upper 200 m of Site U1391 because of time constraints imposed by the end of the expedition (Table T17). At the seafloor, oxygen and hydrogen isotopes are ~0.9‰ and 3.8‰, respectively, reflecting the lower branch of MOW. δ18O oscillates around a mean value of 0.9‰ in the upper 50 mbsf and decreases to a minimum of 0.3‰ at 59.3 mbsf, followed by an increase between 60 and 89 mbsf to ~1.5‰. Below 89 mbsf, δ18O decreases, reaching 0.8‰ at 127 mbsf and remaining near this value to 187 mbsf. The deepest sample, measured at 194 mbsf, has an even lower value of 0.4‰.

δD increases from 3.8‰ at the sediment/water interface to 8.7‰ at ~42 mbsf. A distinct minimum in δD occurs at 59.3 mbsf, reaching values as low as 1.5‰, and followed by an increase in values averaging 5.7‰ between 70 and 200 mbsf. The δ18O and δD minimum at 59.3 mbsf was replicated by measuring the sample twice. Oxygen and hydrogen isotopes are positively correlated in the upper 115 mbsf (Figs. F32, F33).

Summary

It is useful to compare the interstitial water results from Site U1391 with those from Site U1385 because the former is influenced by the lower branch of MOW, whereas the latter reflects Northeast Atlantic Deep Water. The sulfate reduction zone is shallower at Site U1391 (20 mbsf) than at Site U1385 (50 mbsf), perhaps reflecting the higher sedimentation rate and accumulation rate of organic matter at Site U1391.

The rapid decrease in chloride in the uppermost 100 mbsf at Site U1391 is similar to the trend at Site U1386, which is located in the upper branch of MOW. Both sites are within MOW, but Site U1391 is in the lower branch.

The decrease of calcium and magnesium in the sulfate reduction zone reflects dolomite formation as a result of high alkalinity produced by sulfate reduction and anaerobic methane oxidation. The increase in calcium and decrease in magnesium below this level may reflect dolomitization of calcite that involves replacement of half the Ca2+ ions by Mg2+, thereby resulting in removal of magnesium and addition of calcium to interstitial water.

Alkalinity values as high as 40 meq/L are found near the base of Hole U1391C. If the alkalinity is due to dissolved carbonate, this high value may be related to in situ water-solid equilibrium with the dolomite layer found at the base of the hole (see “Lithostratigraphy”). Correspondingly, high carbonate concentrations would enable the in situ precipitation of dolomite from solution.

The organic geochemistry analysis shows similar trends and values at Sites U1385 and U1391. The CaCO3 values observed at this site is comparable to CaCO3 values at Site U1385, albeit with a greater range at Site U1391. Both sites exhibit a strong positive correlation between total organic carbon and C/N ratios; Site U1391 has a greater organic contribution from terrestrial matter compared to Site U1385, which can be explained by its closer proximity to land.