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doi:10.2204/iodp.proc.339.104.2013

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, 36 headspace samples from Hole U1386A, 9 headspace samples from Hole U1386B, and 10 headspace samples from Hole U1386C (sampling resolution of one per core) were analyzed (Fig. F37; Table T19). In Hole U1386A, methane (C₁), ethane (C₂), and ethene (C₂₌) were detected. Methane ranges from 6 ppmv near the surface to a maximum of 47,727 ppmv at 41.3 mbsf (Section 339-U1386A-5H-7). At the base of Hole U1386A, methane is 5,963 ppmv. Ethane was first detected also at 41.3 mbsf (Section 5H-7) and ethene at 176.9 mbsf (Section 20H-8). In Hole U1386A, ethane reaches a maximum of 3.5 ppmv at 337.6 mbsf (Section 38X-6). Ethene reaches a maximum of 1.1 ppmv at 337.6 mbsf (Section 38X-6).

In Holes U1386B and U1386C, methane, ethane, and ethene were detected. In Hole U1386B, methane is 11,109 ppmv at 358.1 mbsf, reaches a secondary peak of 8,367.9 ppmv at 428 mbsf, and decreases to 4,691.9 ppmv at the base of Hole U1386B (448.9 mbsf). Ethene ranges between 1.7 ppmv at 358.1 mbsf and 0.5 ppmv at 387.27 mbsf. Ethane was detected in all measured headspace samples from Hole U1386B and ranges between 0.7 and 6 ppmv.

In Hole U1386C, methane is 1191.4 ppmv at 420.6 mbsf and 3968.6 ppmv at the base of the hole at 524.2 mbsf, with a maximum of 5304.3 ppmv at 450.6 mbsf (Section 339-U1386C-10R-6). Ethene varies between 0.4 ppmv and a maximum of 1.3 ppmv at the base of Hole U1386C (524.2 mbsf), whereas ethane varies between 1.3 and 11.5 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 U1386A–U1386C (Table T2). Weight percent CaCO₃ varies from 10 to 40 wt% (Fig. F38; Table T2). Organic carbon is calculated to be between 0.4 and 1.5 wt% (Fig. F39A; Table T2).

Weight percent nitrogen (Fig. F39B; Table T2) was measured downhole to 270.8 mbsf and ranges from 0.04 to 0.1 wt%. Total nitrogen variability decreases below ~160 mbsf. The C/N ratio, used to distinguish the origin of organic matter (marine versus terrestrial) in sediment (Emerson and Hedges, 1988; Meyers, 1997), ranges from 7.57 to 25.83 and indicates that the organic carbon is mainly of marine origin with a contribution from terrestrial matter (Fig. F39C; Table T2). The higher C/N ratios, especially in the uppermost 31 mbsf, are not unexpected given the proximity of Site U1386 to land. Below 31 mbsf, the C/N ratios decrease sharply, ranging from 7.57 to 12.83 and indicating degradation of organic matter at this site (Ruttenberg and Goñi, 1997).

Interstitial water chemistry

Major cations and anions

Whole-round samples were taken downhole for interstitial water analysis from Hole U1386A to 337.5 mbsf and from Hole U1386B to 448.5 mbsf. Biscuiting of XCB cores made proper removal of contamination (i.e., drilling slurry) from whole-round samples progressively more difficult farther downhole. In particular, the presence of sulfate was noted in at least one Hole U1386B sample far below the base of the sulfate reduction zone, which is indicative of contamination by drilling fluid. For this reason, the measurements from Hole U1386B are excluded from the following analysis.

Sulfate concentrations drop markedly in the uppermost part of Hole U1386A, from 28.9 mM for seawater at the seafloor to 20 mM at 3 mbsf to 0 mM at 12.5 mbsf (Fig. F40A; Table T20), indicating very high rates of sulfate reduction at this site with high sediment accumulation rates. Alkalinity is elevated in the upper 12.5 mbsf, averaging 8–10 meq/L, decreases to ~5 meq/L at 32 mbsf, and remains low downhole to 175 mbsf (Fig. F40B; Table T20). An alkalinity peak of 8 mM occurs at 187 mbsf followed by a decline to 250 mbsf and an increase to a maximum near the base of Hole U1386A (Fig. F40B; Table T20). Ammonium concentrations increase rapidly in the upper 32 mbsf and slowly continue to climb to the base of Hole U1386A (Fig. F40C; Table T20).

The sulfate–methane transition (see discussion in “Geochemistry” in the “Site U1385” chapter [Expedition 339 Scientists, 2013c]) is marked by sulfate depletion at 12.5 mbsf and a rise in methane between 12.5 and 22 mbsf (Fig. F40D). Methane reaches peak concentrations of 48,000 ppmv at 42 mbsf, decreases downhole to 100 mbsf, and remains relatively low to the bottom of the hole.

Calcium concentrations are 8.5 mM at the surface and decrease rapidly to a minimum of 3.5 mM at 12.5 mbsf (Fig. F41A; Table T20). Magnesium concentrations decrease from surface values of 50–55 to ~30 mM at 100 mbsf and remain relatively unchanged to the base of Hole U1386A (Fig. F41B; Table T20). Potassium shows a similar pattern of values decreasing downhole from 11 mM near the surface to ~6 mM near the base of the hole (Fig. F41C; Table T20).

Chloride concentration decreases downhole from 586 mM near the surface to 569 mM at 60 mbsf. Below 60 mbsf, chloride values are scattered around 570 mM (Fig. U1386-H-F6). Sodium to chloride ratios are all near the seawater value of 0.86, indicating that downhole changes in chloride are not affected by underlying brines (Fig. F42).

The observed interstitial water profiles are largely the result of microbially mediated reactions involving organic matter, carbonate diagenesis, and diffusion. Sulfate reduction and methanogenesis are the dominant processes in the upper 100 mbsf. Methane diffusing upward into the sulfate reduction zone is rapidly consumed by anaerobic methane oxidation. Both sulfate reduction and anaerobic methane oxidation produce alkalinity (Gieskes, 1983; Boetius et al, 2000):

High alkalinity promotes authigenic carbonate precipitation, which explains the minimum in calcium at 12.5 mbsf (Fig. F41A; Table T20). Calcium and magnesium are negatively correlated with a slope of ~2 (Fig. F43A; Table T20). Dolomitization of calcite may account for this relationship because one mechanism of dolomite formation involves replacement of half of the Ca²⁺ ions by Mg²⁺, resulting in removal of magnesium and addition of calcium to interstitial water. Bacterial sulfate reduction can eliminate ion pairing between magnesium and sulfate in sediment pore waters (Katz and Ben-Yaakov, 1980) and create a high-magnesium environment conducive to dolomite formation.

Minor elements

Strontium, barium, and lithium have similar patterns showing increasing concentrations downhole (Fig. F44A–F44C; Table T20), whereas boron shows an inverse pattern and generally decreases downhole (Fig. F44D; Table T20). Silicon concentrations range between 100 and 200 µM in the upper 150 m of Hole U1386A and increase to an average of 250 µM below (Fig. F44E; Table T20). Iron and manganese are below detection limits in all but the uppermost core. Calcium, strontium, barium, and lithium are positively correlated, suggesting they are controlled by similar processes, namely carbonate diagenesis (Figs. F43B, F44A–F44C).

Stable isotopes

Samples for stable isotope analysis were collected from the base of each section of every core taken in Hole U1386A for the upper 350 mbsf using a syringe, with the exception of Section 6 in each core where a 5 cm whole-round sample was taken. Rhizon samples were taken from the middle (~75 cm) of Section 6 for comparison. Only the upper 83 mbsf were analyzed shipboard for δ¹⁸O and δD.

Interstitial water δ¹⁸O and δD values increase downhole from bottom water values at the top of Hole U1386A to a broad maximum between ~27 and 55 mbsf (Fig. F45; Table T21). δ¹⁸O and δD are positively correlated, with a slope that is indistinguishable from the modern δ¹⁸O-δD relationship from a nearby hydrographic profile (Fig. F46). The downhole peak in δ¹⁸O and δD is suggestive of an increase in the isotopic composition of seawater during the last glacial period, which has been attenuated by diffusion (McDuff, 1984; Schrag and DePaolo, 1993). However, this interpretation is not supported by the downhole chloride profile. Contrary to the expected trend caused by glaciation (Adkins and Schrag, 2001, 2003), chloride concentrations decrease in the upper 50 mbsf where δ¹⁸O and δD increase (Fig. F45). The decreasing downhole trend in chloride concentration paired with increases in both δ¹⁸O and δD likely indicates the in situ presence of gas hydrates that dissociated upon sediment recovery (Kastner et al., 1991; Dählmann and de Lange, 2003).

The opposite trends in chloride and water isotopic ratios will need to be investigated further postcruise after chloride measurements are measured at higher resolution.

Summary

The interstitial water profile at Site U1386 is dominated by organic matter diagenesis with a sharp decrease in C/N ratios at ~31 mbsf, a shallow sulfate reduction zone in the uppermost 12.5 mbsf, and methanogenesis below. High alkalinity associated with sulfate reduction and anaerobic methane oxidation has resulted in authigenic calcite and dolomite formation. Iron sulfide minerals formed as a consequence of sulfate reduction. A simultaneous downhole increase in interstitial water δ¹⁸O and δD and decrease in chloride concentration suggests that gas hydrates are present in the sediments at Site U1386 and have dissociated upon core recovery.

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