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doi:10.2204/iodp.proc.341.107.2014

Geochemistry

Interstitial water chemistry

Interstitial water (IW) samples were taken in Hole U1421A. A total of 26 IW samples were taken, with a resolution of two samples per core in Cores 341-U1421A-1H and 6H, three samples per core in Cores 2H and 3H, and one sample per core from Core 7H to the bottom of the hole. Because of limited core recovery, the sampling resolution was low and irregular deeper than Core 341-U1421A-13H. Whole rounds for IW analysis were 5 cm long in Cores 341-U1421A-1H through 7H and 10 cm long in Core 8H and deeper cores. Samples from Hole U1421A were collected from APC cores (0–94.7 m CSF-A) and XCB cores (94.7–607.8 m CSF-A), and the results below refer to the combined APC and XCB sample record. The applied squeezing pressures ranged from 8,000 to 32,000 psi, and the volumes of IW extracted ranged between 10 and 40 mL. Splits of the IW samples were taken and processed following methods outlined in “Geochemistry” in the “Methods” chapter (Jaeger et al., 2014) using shipboard analyses or were preserved for shore-based analysis of dissolved trace metals, oxygen/sulfur/calcium/strontium isotopes, dissolved inorganic carbon, and silica.

Alkalinity, pH, chloride, and salinity

Alkalinity increases from 9.2 mM (3.0 m CSF-A) to a maximum of 18.5 mM at 47.8 m CSF-A (Fig. F17A). Following a decrease to 11.6 mM by 68.5 m CSF-A, alkalinity remains around 11.5 mM to 114.1 m CSF-A. An isolated alkalinity maximum of 33.7 mM occurs at 440.7 m CSF-A, followed by another decrease to 6.2 mM at 626.5 m CSF-A. The corresponding pH values at Site U1421 range between 7.4 and 7.8 in the uppermost 63 m CSF-A and between 7.6 and 7.8 at greater depths (Fig. F17B).

The chloride concentration profile at Site U1421 continuously decreases from 528.8 mM (3.0 m CSF-A) to 361.0 mM (49.3 m CSF-A), followed by a slight increase to 392.8 mM by 94.7 m CSF-A (Fig. F17J). Chloride concentrations remain between 380 and 415 mM to 494.3 m CSF-A. Starting at 577.7 m CSF-A, concentrations increase to 446 mM, reaching 467 mM by 626.5 m CSF-A.

The overall pattern of the chloride profile at Site U1421 is also reflected in salinity, with decreasing values from 31 mM (3.0 m CSF-A) to 20 mM (49.3 m CSF-A) followed by a slight increase to 22 by 94.7 m CSF-A. Salinity stays between 21 and 23 to 494.3 m CSF-A. At 577.7 m CSF-A, salinity values increase to 25 and reach 26 by 626.5 m CSF-A (Fig. F17I).

Dissolved ammonium, phosphate, and silica

Dissolved ammonium concentrations increase rapidly from 0.73 mM at 3.0 m CSF-A to 1.82 mM at 47.8 m CSF-A (Fig. F17D). Following a decrease to 1.48 mM by 88.0 m CSF-A, ammonium concentrations continue to rise more gradually downcore, reaching a maximum of 5.06 mM (485.7 m CSF-A). At greater depths, ammonium concentrations decrease to 4.05 mM (626.5 m CSF-A).

Phosphate concentrations at Site U1421 decrease rapidly from 29.1 µM at 3.0 m CSF-A to 1.9 µM at 94.6 m CSF-A (Fig. F18J). At greater depths, phosphate increases to 21.6 µM at 295.5 m CSF-A. An isolated phosphate maximum of 42.5 µM occurs at 440.6 m CSF-A, followed by another decrease to 1.1 µM by 607.8 m CSF-A.

Silica concentrations range from 394 to 861 µM (Fig. F17H). Values are highly variable in the uppermost 100 m CSF-A and deeper than 500 m CSF-A. Between 431.9 and 494.3 m CSF-A, silica values are consistently around 800 µM.

Dissolved sulfate, calcium, magnesium, potassium, sodium, and bromide

Sulfate concentrations at Site U1421 linearly decrease with depth between 3.0 and 19.3 m CSF-A (from 20.7 to 3.0 mM), with the latter depth defining the depth of total sulfate depletion (Fig. F17C). At greater depths, sulfate concentrations range between 2.9 and 3.7 mM.

Calcium concentrations decrease continuously with depth between 3.0 and 47.8 m CSF-A (from 8.5 to 3.9 mM) (Fig. F18A), followed by a minor increase to 5.5 mM at 81.7 m CSF-A. Starting at 431.8 m CSF-A, calcium concentrations increase gradually downcore (3.9 to 5.4 mM), with a steeper gradient between 577.6 and 626.4 m CSF-A (from 6.4 to 11.4 mM).

Magnesium concentrations decrease strongly from 47.7 to 24.7 mM between 3.0 and 47.8 m CSF-A (Fig. F18C). A minor increase in magnesium concentrations to 32.0 mM occurs at 81.7 m CSF-A. At greater depths, magnesium varies between 26.0 and 34.0 mM, returning to higher concentrations (36.6–42.3 mM) deeper than 577.7 m CSF-A.

Potassium concentrations strongly decrease downcore from 10.1 to 5.8 mM (3.0–22.3 m CSF-A) and then decrease more gradually to 4.6 mM at 114.1 m CSF-A (Fig. F18B). At greater depths, potassium concentrations remain low, between 4.2 and 5.7 mM.

Sodium concentrations continuously decrease from 445.7 mM (3.0 m CSF-A) to 292.2 mM (49.3 m CSF-A) (Fig. F17K). Sodium concentrations increase slightly to 411.0 mM at 94.7 m CSF-A and then fluctuate between 284.1 and 318.3 mM down to 494.3 m CSF-A. Deeper than 577.7 m CSF-A, sodium concentrations increase to 320.8 mM, reaching 337.8 mM by 626.5 m CSF-A.

Bromide concentrations continuously decrease from 0.81 mM (3.0 m CSF-A) to 0.60 mM (49.3 m CSF-A), followed by a slight increase to 0.67 mM at 94.7 m CSF-A (Fig. F17E). At greater depths, bromide concentrations range between 0.62 and 0.85 mM down to 494.3 m CSF-A. Between 577.7 and 626.5 m CSF-A, values range between 0.84 and 0.90 mM.

Dissolved manganese, iron, barium, strontium, boron, and lithium

Manganese concentrations at Site U1421 generally vary between 2.5 and 4.9 µM without any distinct downcore pattern apart from an increase to 7.5 µM at the bottom of the hole (Fig. F18I).

Dissolved iron concentrations at Site U1421 vary between quantification limit and 12.7 µM, with the highest values mostly limited to the uppermost ~120 m CSF-A (Fig. F18H). Also between 431.9 and 577.7 m CSF-A, iron concentrations reach 2.2 µM.

Barium concentrations are below detection limit at Site U1421 down to 15.7 m CSF-A and then gradually increase to values between 7.5 and 16.4 µM down to 114.1 m CSF-A (Fig. F18F). Deeper than 431.9 m CSF-A, barium shows an overall increasing trend to reach maximum concentrations of 50.4 µM at the bottom of the hole.

Strontium concentrations increase from 84 to 98 µM between 3.0 and 19.3 m CSF-A and then display a local minimum of 87 µM at 47.8 m CSF-A (Fig. F18G). At greater depths, an intermittent increase in strontium concentrations to 121 µM occurs at 88.1 m CSF-A. Deeper than 431.9 m CSF-A, strontium concentrations increase downcore from 70 to 189 µM by 626.5 m CSF-A.

Boron concentrations decrease from ~450 µM to <250 µM between the top of the hole and 114.1 m CSF-A. At greater depths, boron varies between 100 and 500 µM without any distinct depth trend.

Lithium concentrations at Site U1421 show a steep decrease in two steps, first from 20.8 to 15.4 µM by 22.3 m CSF-A and then from 16.2 to 9.6 µM by 68.5 m CSF-A (Fig. F17D). At greater depths, lithium concentrations are highly variable, ranging between 9.5 and 22.8 µM.

Volatile hydrocarbons

Headspace gas samples were collected at a resolution of one per core in Hole U1421A only, partly from core catchers (Cores 341-U1421A-2H through 76X). Methane is the dominant hydrocarbon gas detected. Methane is in very low concentrations (<5 ppmv) in Cores 341-U1421A-2H and 3H but increases starting in Core 4H and in general ranges between 1,000 and 34,000 ppmv throughout Hole U1421A (Fig. F17F). Three intervals of high methane concentrations occur (>10,000 ppmv): at 47.8–63.3, 256.1–304.6, and 501.3 m CSF-A. Ethane is only intermittently present, and concentrations remain very low throughout Hole U1421A (<6 ppmv; Fig. F17G). The C₁/C₂ ratio is correspondingly high (generally 1,000–36,000), indicating no threat to drilling operations.

Bulk sediment geochemistry

Discrete core samples were analyzed from Site U1421 for total carbon, total nitrogen (TN), and total inorganic carbon. From these analyses, total organic carbon (TOC) and calcium carbonate (CaCO₃) were calculated as described in “Geochemistry” in the “Methods” chapter (Jaeger et al., 2014). In total, 44 samples were analyzed from Hole U1421A (Cores 341-U1421-1H through 63X). Discrete samples were selected in collaboration with the Lithostratigraphy group to ensure that the primary lithologies were analyzed.

TOC contents mostly range between 0.4 and 1.0 wt% with no clear downcore trend (Fig. F19A). One sample with a TOC content of 1.26 wt% (171.2 m CSF-A) was sampled within a gray diamict (see “Lithostratigraphy”).

TN contents are <0.1 wt% (Fig. F19B). The highest TN contents primarily occur deeper than 431.5 m CSF-A.

Organic carbon to TN (C/N) ratios range between 9 and 42 (with one sample of 58) (Fig. F19C), consistent with a contribution from both marine and terrigenous organic matter (Hedges et al., 1986; Walinsky et al., 2009). The lowest C/N values occur deeper than 431.5 m CSF-A and correspond to the presence of dark greenish gray bioturbated and laminated mud lithologies (see “Lithostratigraphy”). Low C/N ratios at 113.8 and 295.2 m CSF-A also occur within gray diamict and dark greenish gray bioturbated mud lithologies. In contrast, high C/N ratios seem to dominate the upper and middle part of the record, where the lithology is dominated by mud with lonestones and diamicts. Determination of the contribution of inorganic N is required to fully assess the relative contributions of marine and terrigenous input to the organic matter at Site U1421.

CaCO₃ values range mostly between 1.5 and 3.0 wt% (Fig. F19D). The highest CaCO₃ contents of up to 4.8 wt% are recorded in dark greenish gray mud (295.2 m CSF-A; Core 341-U1421A-41X) and dark gray mud (500.7 m CSF-A; Core 63X), but not every mud-rich lithology appears to be equally enriched in carbonate (see “Lithostratigraphy”).

Interpretation

The IW and sediment composition at Site U1421 indicate moderate to high rates of organic matter remineralization. Although TOC contents at Site U1421 are low (mostly <1 wt%), the tentatively high sedimentation rates mean that respective accumulation rates are also high (see “Stratigraphic correlation”). The potentially higher organic matter accumulation rates seem to drive relatively intense diagenetic processes occurring at Site U1421, discussed below. The C/N ratios throughout the Site U1421 record show significant variability, and clear relationships exist between low C/N ratios and dark greenish gray mud-rich lithologies and between high C/N ratios and grayish lonestone-rich mud/diamict lithologies, implying changes in the dominant sources of organic material through time. The CaCO₃ contents are also generally low but indicate a significant contribution of biogenic carbonate to the sediment composition, which is in agreement with abundances and preservation of foraminifers at Site U1421, especially within mud-rich lithologies (see “Paleontology and biostratigraphy”).

At Site U1421, both ammonium and alkalinity show relatively moderate concentrations in the upper 100 m CSF-A, indicating that diagenesis is likely driven by the substantial input of refractory organic matter (supported by C/N ratios of 20 to 40), possibly reflecting the location of Site U1421 on a glacial trough mouth fan. In this upper part of the sediment record, organic matter degradation seems to proceed mostly via dissimilatory iron reduction and sulfate reduction, whereas manganese reduction is negligible in the studied depth interval. In the deeper part of the record (deeper than 400 m CSF-A), ammonium concentrations more than double, whereas alkalinity stays at moderate levels (apart from an alkalinity maximum at 440.7 m CSF-A within a mud-rich lithology; see “Lithostratigraphy”). The high ammonium concentrations deeper than 400 m CSF-A coincide with a clear decrease in C/N ratios, suggesting a sediment interval with more reactive marine organic matter that is being remineralized.

Methane concentrations within this high-ammonium interval are moderate, and sulfate is close to its detection limit, so neither sulfate reduction nor methanogenesis appear to be the dominant organic matter degradation pathways deeper than 400 m CSF-A. However, dissolved iron concentrations are slightly but consistently elevated at this depth in the sediment, suggesting that dissimilatory iron reduction might be occurring. This process requires the preservation of iron (oxyhydr)oxides in these deep, anoxic sediments, which is a distinct possibility given the tentatively very high sediment accumulation rates at Site U1421 (see “Stratigraphic correlation”). Both organic matter degradation and the dissolution of iron (oxyhydr)oxides are most likely responsible for the release of phosphate to the pore solution, not only in the deeper part of Hole U1421A, but also in the uppermost 60 m CSF-A.

At Site U1421, almost complete sulfate depletion is reached at ~20 m CSF-A, overlapping with the onset of methane production, which indicates the anaerobic oxidation of methane coupled to sulfate reduction in a sulfate–methane transition zone (SMTZ). Calcium, magnesium, strontium, and lithium exhibit sinks within the SMTZ, possibly related to the formation of authigenic carbonates. Deeper than the SMTZ, the release of barium to the IW indicates dissolution of barite under sulfate-depleted conditions in the methanogenic zone. Contamination by seawater sulfate during drilling is unlikely, as the elevated sulfate values occur in both APC and XCB cores.

At Site U1421, chloride concentration, salinity, and sodium decrease substantially with depth. Dehydration of clay minerals should not occur at such shallow burial depths (Saffer and McKiernan, 2009). Reduced IW salinities have been frequently observed in sediments overlying methane hydrate reservoirs (e.g., Hesse, 2003; Torres et al., 2004) because of the low-salinity water released upon hydrate dissociation. However, no bottom-simulating reflector was observed at Site U1421 (see “Background and objectives”), and we observed only moderate (mostly <20,000 ppmv) methane headspace concentrations, so gas hydrate dissociation is an unlikely explanation for the IW freshening. Studies on marine IW salinities adjacent to modern ice sheets of Greenland (Ocean Drilling Program Leg 152; Gieskes et al., 1998; DeFoor et al., 2011) and Antarctica (Lu et al., 2010) have revealed that substantial IW freshening might be related to glacial meltwater discharge events. To estimate the impact of freshwater dilution on the IW profiles, concentrations of all chemical parameters are normalized to downcore chloride concentrations. Selected normalized profiles are shown in Figure F20. Although the shape of the bromide profile is significantly affected by dilution from fresher waters, all other IW profiles maintain their original downcore trends after normalization.

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