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doi:10.2204/iodp.proc.341.105.2014

Geochemistry

Interstitial water chemistry

Interstitial water (IW) samples were only taken in Hole U1419A. The following results are expressed on the CCSF-B depth scale for Site U1419 (see “Stratigraphic correlation”).

In Hole U1419A, 22 IW samples were taken with a resolution of two samples per core in Cores 341-U1419A-1H, 6H, and 9H; three samples per core in Cores 2H, 3H, and 8H; and one sample per core from Core 11H to the bottom of the hole. Whole rounds for IW analysis were 5 cm long in Cores 341-U1419A-1H through 9H and 10 cm long in Cores 11H through 26X. Samples from Hole U1419A were collected from APC cores (0–115.2 m CCSF-B) and XCB cores (140.9–168.5 m CCSF-B), 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 range between 20 and 40 mL. Splits of the IW samples were taken and processed following methods outlined in “Geochemistry” in the “Methods” chapter (Jaeger et al., 2014a) 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

Following a steep increase from the core top to 20.8 m CCSF-B, a broad alkalinity maximum occurs at Site U1419 from 23.4 to 70.4 m CCSF-B, with a maximum value of 37.1 mM recorded at 59.8 m CCSF-B (Fig. F19A). From 85.6 m CCSF-B, alkalinity decreases downcore to 13.7 mM by 140.9 m CCSF-B. The pH values range from 7.49 to 7.8 between 2.5 and 169.5 m CCSF-B without a systematic trend (Fig. F19B).

Chloride concentrations (titrated) decrease with depth from 550.2 to 446.5 mM in the upper 140 m CCSF-B of Site U1419 (Fig. F19I). Salinity gradually decreases from 33 at the top of the core to 32 by 59.8 m CCSF-B and then decreases more rapidly to a minimum of 25 at 140.9 m CCSF-B.

Dissolved ammonium, phosphate, and silica

Dissolved ammonium concentrations increase rapidly from 0.6 mM at 2.5 m CCSF-B to 3.0 mM at 20.8 m CCSF-B (Fig. F19D). They continue to increase more gradually downcore, reaching a maximum of 4.6 mM at 70.4 m CCSF-B, and then decrease to 1.9 mM deeper than 140.9 m CCSF-B.

Phosphate concentrations range from 7.4 to 78.6 µM at Site U1419 (Fig. F20J), with higher and more variable values in the uppermost ~100 m CCSF-B (25.4–74.0 µM) followed by a decrease to ≤16 µM to the bottom of the hole.

Silica concentrations at Site U1419 range from 602 to 843 µM and generally increase downcore (Fig. F19G).

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

Sulfate concentrations at Site U1419 decrease sharply with depth from 2.5 to 12.6 m CCSF-B (from 20.7 to 2.2 mM), with 12.6 m CCSF-B defined as the depth of total sulfate depletion (Fig. F19C). Deeper than this depth, sulfate concentrations range between 0.9 and 1.3 mM.

Calcium concentrations at Site U1419 decrease sharply with depth from 2.5 to 12.6 m CCSF-B (from 8.4 to 3.2 mM) (Fig. F20A), stabilize around 3.7–4.0 mM between 40 and 85 m CCSF-B, reach a local minimum at 90.5 m CCSF-B (3.1 mM), and increase to concentrations around 4.8 mM deeper than 106 m CCSF-B.

Magnesium concentrations vary between 34.6 and 49.6 mM at Site U1419 (Fig. F20C). They decrease sharply from 49.6 to 46.5 mM between 2.5 and 12.6 m CCSF-B and stabilize at ~47.5 mM to 85.6 m CCSF-B. Deeper, magnesium concentrations decrease, reaching a minimum value of 34.6 mM at 140.9 m CCSF-B, and remain at low concentrations to the bottom of the hole.

Potassium concentrations at Site U1419 decrease downcore from 10.6 to 8.4 mM (2.5–62.4 m CCSF-B) (Fig. F20B). Below a local maximum (8.5 mM) at 68.2 m CCSF-B, potassium concentrations decrease downcore to a minimum of 5.6 mM at 168.5 m CCSF-B.

Sodium concentrations show an overall downcore decrease that steepens deeper than 68.0 m CCSF-B (Fig. F19J). The maximum sodium concentration of 470 mM occurs at 2.5 m CCSF-B, and the minimum concentration of 377 mM is recorded at 140.9 m CCSF-B.

Bromide concentrations at Site U1419 increase downcore to reach a broad maximum of ~0.98 mM between 41.5 and 64.6 m CCSF-B (Fig. F19E). Deeper than 68.2 m CCSF-B, bromide concentrations decrease, reaching a minimum of 0.81 mM at 140.9 m CCSF-B.

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

Manganese concentrations at Site U1419 vary between 1.7 and 3.9 µM in the upper ~60 m CCSF-B (Fig. F20I). Manganese concentrations decrease to a minimum of 1.7 µM at 70.4 m CCSF-B. Farther downcore, a progressive increase is recorded to the bottom of the hole.

Dissolved iron concentrations at Site U1419 generally vary between 0 and 3.6 µM, excluding three isolated maxima at 23.4, 59.8, and 90.4 m CCSF-B (Fig. F20H). Local minima occur around 42 and 115 m CCSF-B.

Barium concentrations are below detection limit at Site U1419 from the top of the core to 18.2 m CCSF-B, where barium concentrations increase rapidly to 11.9 µM (Fig. F20F). Deeper than 68.2 m CCSF-B, a more gradual increase in barium occurs, including isolated barium maxima (e.g., 45.6 µM at 115.2 m CCSF-B). Detectable barium concentrations are limited to the sediment interval with lowest sulfate concentrations.

Strontium concentrations decrease from 85 to 76 µM between 2.5 and 12.6 m CCSF-B (Fig. F20G). Deeper than 12.6 m CCSF-B, strontium concentrations decrease gradually to ~100 µM at 90.4 m CCSF-B. Deeper than 97.6 m CCSF-B, strontium concentrations increase more rapidly, reaching values of ~128 µM deeper than 115 m CCSF-B.

Boron concentrations increase from ~500 µM at the top of the hole to a maximum of ~600 µM at 20.8 m CCSF-B. Deeper, boron concentrations decrease slightly to 85.6 m CCSF-B and then more rapidly to reach concentrations of <300 µM at the bottom of the hole.

Lithium concentrations at Site U1419 increase rapidly from 20.7 µM at the core top to 23.6 µM at 7.5 m CCSF-B (Fig. F20D). Deeper downcore, lithium decreases to 19.2 µM at 12.6 m CCSF-B and then decreases more slowly from 13 m CCSF-B to the bottom of the hole. A local minimum of 8.6 µM is recorded at 115.2 m CCSF-B.

Volatile hydrocarbons

Headspace gas samples were collected at a resolution of one per core in Hole U1419A only (Cores 341-U1419A-1H through 27X). Methane is the dominant hydrocarbon gas detected. Methane is in very low concentrations (<5 ppmv) in Cores 341-U1419A-1H and 2H but increases starting at Core 3H and in general ranges between 4,400 and 39,000 ppmv throughout Hole U1419A (Fig. F19F). Ethane is only intermittently present, and concentrations remain very low throughout Hole U1419A (<1 ppmv); the C₁/C₂ ratio is correspondingly high (>40,000), indicating no threat to drilling operations.

Bulk sediment geochemistry

Discrete core samples were analyzed from Site U1419 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., 2014a). In total, 20 samples were analyzed from Hole U1419A (Cores 341-U1419A-1H through 26X). Discrete samples were selected in collaboration with the Lithostratigraphy group to ensure that the primary lithologies were analyzed.

The TOC content at Site U1419 ranges between 0.4 and 1.0 wt% with no clear downcore trend (Fig. F21A). The highest TOC contents are recorded in lithologies with notable diatom content at 0.5 m CCSF-B (0.87 wt%) and 60.7 m CCSF-B (0.95 wt%) in Cores 341-U1419A-1H and 8H, respectively (see “Lithostratigraphy”).

The TN content at Site U1419 ranges between 0 and 0.1 wt% and shows no downcore trend (Fig. F21B). Slightly elevated TN contents are observed in the two samples with high TOC contents, but values remain very low. Organic carbon to TN (C/N) ratios range between 10 and 22 (Fig. F21C), consistent with a contribution from both marine and terrigenous organic matter (Hedges et al., 1986; Walinsky et al., 2009). The negative y-axis intercept in the correlation between TOC and TN (Fig. F21E) implies that organic N dominates the TN content. 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 U1419.

CaCO₃ contents range mostly between 1.5 and 4.0 wt% at Site U1419 (Fig. F21D). High CaCO₃ content is recorded in a TOC-rich diatom ooze sample (60.7 m CCSF-B; Core 341-U1419A-8H) that also contains shell fragments (see “Lithostratigraphy”) and in the sand lithology recorded in Core 341-U1419A-26X (167.5 m CCSF-B).

Interpretation

The IW and sediment composition at Site U1419 indicate relatively high rates of organic matter remineralization. Although TOC contents at Site U1419 are low (<1 wt%), the high sedimentation rates imply that respective TOC accumulation rates are also high (see “Stratigraphic correlation”). The potentially high organic matter accumulation rates are likely reflected in the pronounced diagenetic processes occurring at Site U1419, discussed below. CaCO₃ contents are also low but indicate a contribution of biogenic carbonate to the sediment composition, which is in agreement with high abundances and preservation of foraminifers at Site U1419 (see “Paleontology and biostratigraphy”).

A distinct interval with low MS at ~80–90 m CCSF-B (see “Lithostratigraphy”) is not paralleled by a TOC maximum, as would be expected if both the magnetic and geochemical records were influenced by enhanced deposition of organic matter and the resulting diagenetic dissolution of magnetite. However, it is possible that the measured TOC contents only represent the refractory fraction of the originally deposited organic matter, so a connection between diagenetic processes and the magnetic properties cannot be excluded. Dissolution of biogenic opal and/or volcanic glass is indicated by dissolved silica concentrations that are higher than those of the overlying North Pacific Deep Water (~160 µM). There is high variability in the dissolved silica profile at Site U1419, reflecting the potential influence of both sources (biosilica/ash dissolution) and sinks (formation of authigenic clay minerals).

At Site U1419, both ammonium and alkalinity show relatively high concentrations, indicating intense diagenesis likely driven by the input of reactive organic matter, possibly reflecting the location of Site U1419 adjacent to the coastal Alaskan high-productivity zone (Stabeno et al., 2004). Ammonium and alkalinity both exhibit broad maxima between ~20 and 90 m CCSF-B, indicating that the most intense organic matter degradation is occurring within the methanogenic zone. This process also seems to release organic matter–derived bromide, boron, and phosphate into solution.

The biogeochemical processes at Site U1419 follow the catabolic reaction sequence (Froelich et al., 1979), with the exception that manganese reduction is completed above the depth of the uppermost IW sample. Iron reduction appears to be quantitatively unimportant for organic matter remineralization, at least within the sampled sediment intervals, as indicated by low dissolved iron concentrations. However, rapid precipitation of iron monosulfides within sulfidic microenvironments might also contribute to the removal of dissolved iron from pore solution. Total sulfate depletion is reached at ~20 m CCSF-B, concomitant with the onset of methane production, indicating anaerobic oxidation of methane coupled to sulfate reduction in a sulfate–methane transition zone (SMTZ). Dissolved calcium, magnesium, and strontium exhibit sinks within the SMTZ, likely related to the formation of authigenic carbonates. Deeper than the SMTZ, the release of barium and strontium to the pore waters indicates dissolution of barite under sulfate-depleted conditions in methanogenic sediments. Authigenic barite reprecipitation at the SMTZ most likely acts as a sink for dissolved barium (von Breymann et al., 1992).

At Site U1419, chloride concentrations, 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. No bottom simulating reflector is observed at Site U1419 (see “Background and objectives”) Despite this lack of geophysical evidence for gas hydrate deposits at Site U1419, the water depth of ~750 m is within the gas hydrate stability window (Hesse, 2003). Finely disseminated gas hydrates would not necessarily be visible in seismic profiles as a bottom-simulating reflector (Shipley et al., 1979; Hesse, 2003), and significant gas expansion of Site U1419 cores proves that high concentrations of gas (possibly from dissociated hydrates) are present in these deposits. The moderate methane concentrations analyzed in headspace samples may be an underestimate of the real gas contents of the deposits in the coarser lithologies.

Alternatively, 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. It is likely that during glacial maxima an ice front extended from the southern Alaskan coastline adjacent to Site U1419 over much of the continental shelf, potentially reaching the shelf break (Carlson and Bruns, 1997; Davies et al., 2011). Subglacial meltwater or submarine groundwater discharge could have caused infiltration of freshwater into the underlying sediments, followed by its transport through permeable shelf sediments down to the upper slope. However, based on shipboard analyses alone, it is impossible to discern whether gas hydrates or glacial meltwater caused the observed IW freshening.

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