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doi:10.2204/iodp.proc.339.105.2013

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 U1387A and 49 headspace samples from Hole U1387C (sampling resolution of one per core) were analyzed (Fig. F32; Table T17), spanning the entire depth range of the site. In Hole U1387A, methane (C₁), ethane (C₂), and ethene (C₂₌) were detected. Methane ranges from 6.3 ppmv near the surface to a maximum of 41,831 ppmv at 50.7 mbsf (Section 339-U1387A-7X-3). At the base of Hole U1387A, methane is 6,432 ppmv. Both ethane and ethene were detected from the uppermost core of this hole, but concentrations remain low. Ethane remains <2.5 ppmv and ethene reaches a maximum of 1.3 ppmv for the entire 351.8 m of Hole U1387A.

In Hole U1387C, methane is 5,206.3 ppmv at 353.6 mbsf and 10,042.8 ppmv at the base of the hole at 865 mbsf, with a maximum of 17,762.2 ppmv at 814 mbsf (Section 339-U1387C-56R-5). In Hole U1387C, ethene does not exceed 4.5 ppmv and generally stays at concentrations less than ~1 ppmv, whereas ethane does not exceed 20 ppmv. Propane was detected in Cores 339-U1387C-33R, 45R, and 55R at concentrations of 8.73, 6.52, and 18.4 ppmv, respectively.

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 U1387A and U1387C (Table T18), thereby spanning the full depth of the site. CaCO₃ varies from 12 to 38 wt%, with two samples in Hole U1387C with much higher values (Fig. F33). At 457.8 mbsf, CaCO₃ reaches 78.6 wt%, which corresponds to a dolomite layer in the core (see “Lithostratigraphy”). The other sample with high CaCO₃ (71.7 wt%) occurs at 693.2 mbsf (Section 339-U1387C-44R-1) and corresponds to an interval described as sandstone (see “Lithostratigraphy”). Organic carbon varies between 0.5 and 1.15 wt% (Fig. F34A) downhole to 351 mbsf in Hole U1387A and between 0.35 and 2.38 wt% in Hole U1387C between 356.4 and 863.3 mbsf, with one peak of 3.75 wt % at 703.3 mbsf.

Nitrogen (Fig. F34B) was measured downhole to 351 mbsf in Hole U1387A and between 356.4 and 863.3 mbsf in Hole U1387C. Measured nitrogen ranges from 0.04 to 0.19 wt%, with one sample in Section 339-U1387C-45R-1 having very low values (~0 wt%). This low nitrogen sample also corresponds to a sharp peak in total organic carbon, causing a peak in the C/N ratio. 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 of marine origin with a terrestrial component for the uppermost ~350 mbsf and of increasingly terrestrial origin below (i.e., older than 1.6 Ma; see “Biostratigraphy”). Total organic carbon and C/N ratios track each other well. Between 322 and 356 mbsf, a change occurs in the nature of organic input. The terrestrial component gains importance and dominates the signal, and additionally there is an increase in the C/N variability (Fig. F34C; Table T18). A similar trend is also observed in the pollen data from this site (see “Biostratigraphy”). Furthermore, abrupt changes are apparent in the calculated C/N signal at depths coinciding with the position of several hiatuses (~460, ~630, and ~730 mbsf) and lithostratigraphic changes (see “Lithostratigraphy”). The inferred terrestrial input could have potentially been even larger, but degradation of organic matter with time would have decreased the C/N ratio because nitrogen compounds are typically more labile (Ruttenberg and Goñi, 1997).

Interstitial water chemistry

Major cations and anions

Whole-round samples were taken for interstitial water analysis at a resolution of one per core from Hole U1387A downhole to 350 mbsf. We did not take interstitial water samples below 350 mbsf because of severe biscuiting of the XCB cores and drill water contamination of the interstitial whole rounds during RCB drilling.

Sulfate concentrations drop markedly in the uppermost part of Hole U1387A from 19.3 mM at 4 mbsf to 0 mM at 12.5 mbsf (Fig. F35A; Table T19), 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, and decreases to ~4.1 meq/L at 72 mbsf (Fig. F35B). Alkalinity then increases slightly again to 5.9 meq/L at 140 mbsf, after which it decreases and averages ~4 meq/L downhole to the base of the hole. Ammonium concentrations increase three-fold in the upper 32 mbsf and continue to rise to the base of Hole U1387A (Fig. F35C).

The sulfate–methane transition (SMT) is marked by sulfate depletion at 12.5 mbsf and a sharp increase in methane between 14 and 24 mbsf (Fig. F35D). Methane reaches peak concentrations of 41,800 ppmv at 51 mbsf, decreases to 5,400 ppmv at 130 mbsf, and fluctuates but remains below 10,000 ppmv to the base of the hole.

Calcium concentrations are 8 mM at the surface and decrease rapidly to a minimum of 4 mM at 12.5 mbsf (Fig. F36A; Table T19). From 12.5 mbsf to the base of the hole, calcium concentration increases. Magnesium concentration decreases from a surface value of 52 to ~30 mM at 32 mbsf and remains relatively unchanged to the base of Hole U1387A (Fig. F36B). Potassium decreases gradually downhole from 11 mM near the surface to ~6 mM toward the base of the hole (Fig. F36C). Sodium concentrations vary between 430 and 515 mM, with a single peak at 168 mbsf (Fig. F36D). The peak in sodium shows up as an aberration of smaller magnitude in the potassium, magnesium, and calcium profiles (Fig. F36A–F36C) and may be simply due to dilution error during sample preparation for measurement.

Chloride concentrations decrease downhole from 582 mM near the surface to 570 mM at 23 mbsf. Below 23 mbsf, chloride concentrations are scattered around 570 mM (Fig. F37; Table T19). Sodium to chloride ratios are all near the seawater value of 0.86.

The interpretation of the major element interstitial water profiles at Site U1387 is similar to that of Site U1386. High rates of organic matter accumulation have resulted in a very shallow sulfate reduction zone and SMT. The decline in calcium and magnesium in the upper ~20 mbsf is explained by precipitation of dolomite, which occurs near the SMT as a result of increased alkalinity related to sulfate reduction and anaerobic methane oxidation. Elimination of sulfate also reduces ion pairing between sulfate and magnesium, thereby removing a kinetic barrier to dolomite formation.

Minor elements

Strontium and barium have similar patterns of increasing concentrations downhole (Fig. F38A, F38B; Table T19), whereas boron shows an inverse pattern and generally decreases downhole (Fig. F38C). Silicon concentrations range between 100 and 300 µM in the upper part of Hole U1387A (Fig. F38D). Iron and manganese are below detection limits in the majority of the core. Calcium, strontium, and barium are positively correlated, suggesting they are controlled by similar processes, namely carbonate diagenesis (Fig. F39).

Speculation on the origin of the dolomite layers

The dolostone layers found at ~460 mbsf are a geochemical curiosity. Although we did not take interstitial water samples from below 350 mbsf because of severe XCB biscuiting, the upper interstitial water profile permits us to speculate upon its origin. A key feature is the association of the dolostone with a hiatus from 1.8 to at least 3.19 Ma (see “Biostratigraphy”). We suggest the unconformity represents, at least in part, a period of halted deposition, which resulted in the interface between sulfate reduction and methanogenesis remaining relatively stationary in the sediment column for an extended period of time. Dolomite forms at the SMT and complete layers can develop if the sulfate–methane interface stays at a fixed depth for a prolonged amount of time (Moore et al., 2004; Meister et al., 2008).

Summary

The observed interstitial water profiles at Site U1387 are very similar to those at Site U1386, which is only ~100 km away. This suggests that bottom water conditions and sedimentation histories have been similar for the time period represented by the uppermost several hundred meters of both sites.

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