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doi:10.2204/iodp.proc.341.104.2014

Geochemistry

Interstitial water chemistry

Interstitial water (IW) samples were taken in Holes U1418A, U1418B, U1418D, and U1418F at increasing depth ranges; sampling was designed to include an overlap in the sampled interval between each successive hole. The following results are expressed on the CCSF-B scale for Site U1418 (see “Stratigraphic correlation”).

In Hole U1418A, 29 IW samples were taken with a resolution of two samples per core in Cores 341-U1418A-1H and 2H, three samples per core in Cores 3H and 4H, two samples per core in Cores 5H and 6H, and one sample per core from Core 7H to the bottom of the hole. Whole rounds for IW analysis were 5 cm long in Cores 341-U1418A-1H through 12H and 10 cm long in Cores 13H through 32H. All samples from Hole U1418A were collected from APC cores (0–204.15 m CSF-A; 0–205.0 m CCSF-B). In Hole U1418B, six IW samples were taken from APC cores, with a resolution of three samples per core in Cores 341-U1418B-1H and 2H (5 cm long whole rounds). In Hole U1418D, 12 IW samples were taken from Cores 341-U1418D-22H through 36X (148.9–223.1 m CSF-A; 289.3 m CCSF-B) (10 cm long whole rounds), with a more irregular sampling resolution than in Hole U1418A because of variable core recovery.

In Hole U1418F, 58 IW samples were taken from Cores 341-U1418F-4R through 65R (240.0–831.5 m CCSF-B). Hole U1418F was drilled using the RCB system, which has the potential for contamination of IW samples by surface seawater and drilling muds (e.g., Chambers and Cranston, 1991). However, successful recovery of uncontaminated RCB sequences for IW analysis has also been reported (e.g., Shipboard Scientific Party, 2000; Fulthorpe, Hoyanagi, Blum, and the Expedition 317 Scientists, 2011). The improved core recovery in Hole U1418F allowed IW sampling on complete whole rounds ranging in length from 9 to 19 cm, and we did not identify evidence for contamination with seawater in the chemical analyses (see below). In cases of strongly fractured and/or indurated core material toward the bottom of Hole U1418F (deeper than Core 341-U1418F-65R), no IW samples were taken. The compositions of IW samples taken from overlapping depths between the different holes were in good agreement, allowing for the construction of composite IW records.

The applied squeezing pressures ranged from 8,000 to 32,000 psi, and the volumes of IW recovered range between 12 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). IW samples were also 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 in the uppermost 30.1 m CCSF-B, an alkalinity maximum (31.8 mM) occurs at 76.1 m CCSF-B (Fig. F26A). Apart from a slight decrease around 93 m CCSF-B, alkalinity stays above 30 mM to 158.7 m CCSF-B and then decreases to 5.2 mM by 249.1 m CCSF-B. Deeper than this depth, alkalinity decreases more gradually and largely remains <4 mM. Deeper than 600 m CCSF-B, alkalinity remains <1 mM.

The pH values at Site U1418 decrease from 7.5 to 7.2 between 1.5 and 8.4 m CCSF-B (Fig. F26B). Deeper than this depth, pH values steadily increase to 7.8 mM (322.3–602.0 m CCSF-B), followed by a more rapid increase in pH deeper than 602 m CCSF-B to values ~8.6 deeper than 774 m CCSF-B.

Chloride (titrated) concentrations mostly range between 520 and 570 mM at Site U1418 (Fig. F26J). The uppermost chloride maximum occurs at 23.6 m CCSF-B (>571 mM), followed by another peak at 50.9 m CCSF-B (>570 mM). Deeper than this depth, chloride concentrations range between 520 and 570 mM, following an overall decreasing trend that is particularly pronounced from 500 m CCSF-B to the bottom of the hole.

A salinity maximum of 35 occurs at Site U1418 between 11.0 and 41.3 m CCSF-B (Fig. F26I), followed by a gradual downcore decrease to 30 at 390.1 m CCSF-B. Deeper than this depth, salinity remains near 30 to the bottom of the hole.

Dissolved ammonium, phosphate, and silica

In the uppermost 300 m CCSF-B of Site U1418, the dissolved ammonium profile broadly resembles that of alkalinity (Fig. F26D). Dissolved ammonium increases rapidly with depth in the upper 25 m, from 0.17 mM (1.3 m CCSF-B) to 1.83 mM (27.5 m CCSF-B). Ammonium concentrations stabilize at ~2.0 mM between 27.0 and 103.7 m CCSF-B and then increase to a maximum of 3.86 mM at 158.7 m CCSF-B. Deeper than 158.7 m CCSF-B, ammonium decreases to ~2.3 mM (340.2 m CCSF-B) and then decreases abruptly to ~1.8 mM between 400 and 448 m CCSF-B. Following a final increase in dissolved ammonium between 480 and 550 m CCSF-B (to ~2.2 mM), concentrations decrease downcore to ~700 m CCSF-B, where ammonium concentrations of ~0.9 mM are recorded.

Phosphate concentrations fluctuate between 3.7 and 45.4 µM in the upper 180 m CCSF-B at Site U1418 (Fig. F27J). Maxima in phosphate concentrations are recorded at 2.6, 30.1, 76.1, and 158.7 m CCSF-B. Phosphate concentrations decrease abruptly to 6.1 µM at 175.5 m CCSF-B and then remain low (0.5–5.0 µM) and show a gradual decrease to the bottom of the hole.

Silica concentrations have considerable variability, superimposed upon an overall downcore decrease, which occurs in several steps (Fig. F26H). In the upper 175 m CCSF-B, silica concentrations fluctuate between 540 and 885 µM. From 175 to 618 m CCSF-B, the variability increases (440–800 µM) and mean concentrations are lower. Deeper than 700 m CCSF-B, silica remains low (100–400 µM).

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

Sulfate concentrations decrease continuously over the uppermost 75 m at Site U1418, from 31.24 mM (1.3 m CCSF-B) to 1.34 mM (76.1 m CCSF-B) (Fig. F26C), which is defined as the depth of total sulfate depletion. Deeper sulfate concentrations range between the detection limit and 3 mM.

Calcium concentrations vary between 10 and 13 mM in the uppermost 65 m CCSF-B. A local calcium minimum is reached between 76.0 and 93.0 m CCSF-B (~7.4 mM), followed by a continuous downcore increase to ~20.3 mM by 424.5 m CCSF-B (Fig. F27A). Calcium concentrations show a local minimum at 474.5 m CCSF-B (17.7 mM) and then increase downcore, reaching 45–60 mM by ~800 m CCSF-B.

Magnesium concentrations vary between 50 and 65 mM in the uppermost 93.0 m CCSF-B (Fig. F27C). Deeper than 93.0 m CCSF-B, magnesium values decrease and reach a local minimum of 24.4 mM around 448.4 m CCSF-B. Magnesium concentrations show a broad local maximum between 500 and 600 m CCSF-B (up to 24.0 mM) and then decrease to a minimum of 5.6 mM at 802.5 m CCSF-B.

Potassium concentrations drop from 12.5 to 7.7 mM between 1.3 and 27.5 m CCSF-B (Fig. F27B). A more gradual decrease follows, reaching a minimum of 2.1 mM by 434.3 m CCSF-B, interrupted by a local maximum around 150 m CCSF-B. Deeper than this depth, potassium concentrations continue to decrease, reaching values around 1 mM at the bottom of the hole.

Sodium concentrations are 555 mM around 23.6 m CCSF-B and remain above 500 mM to 48.2 m CCSF-B (Fig. F26K). Deeper than this depth, sodium concentrations range from 432 to 485 mM to 200 m CCSF-B. A gradual increase in sodium concentrations occurs between 200 and 500 m CCSF-B, followed by a decrease to ~400 mM at the bottom of the hole.

Bromide concentrations are 1.2 mM around 23.6 m CCSF-B to 48.2 m CCSF-B (Fig. F26E). Deeper than this depth, bromide concentrations show a downcore increase to ~600 m CCSF-B, followed by a decrease to the bottom of the hole.

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

Manganese concentrations at Site U1418 are highest in the two uppermost samples (117.1 and 83.8 µM at 1.3 and 2.6 m CCSF-B, respectively), decrease rapidly to 4.8 µM by 21.3 CCSF-B, and stay <15 µM for most of the record to the bottom of the hole (Fig. F27I).

Iron concentrations are highest in the uppermost 30 m CCSF-B, reaching maximum values of 98.6 and 61.9 µM at 8.4 and 27.4 m CCSF-B, respectively (Fig. F27H). Deeper than 30 m CCSF-B, iron concentrations stay <6 µM, apart from elevated values (7.3–16.6 µM) between 122.0 and 149.8 m CCSF-B.

Barium concentrations are below detection limit in the uppermost 66.3 m CCSF-B at Site U1418 (Fig. F27F). At 76.1 m CCSF-B, barium concentrations increase rapidly downcore, with a number of distinct local maxima between 60 and 120 µM. Deeper than 424.4 m CCSF-B, barium increases continuously to 137.3 µM at 581.0 m CCSF-B and then decreases to 24.8 µM by 802.5 m CCSF-B. Detectable barium concentrations are limited to the sediment interval with lowest sulfate concentrations.

Strontium concentrations follow a distinct alternating pattern in the upper 200 m CCSF-B of Site U1418 (Fig. F27G). Following a steep increase from 89.8 µM at the core top to 107.7 µM at 18.5 m CCSF-B, strontium gradually decreases to a minimum of 92.7 µM at 57.4 m CCSF-B. A second local maximum of 129.3 µM is reached at 93.1 m CCSF-B, followed by a local minimum of 115.5 µM at 122.0 m CCSF-B. The third local maximum of 145.0 µM is reached at 175.5 m CCSF-B. Farther downcore, strontium gradually increases to values around 200 µM by ~650 m CCSF-B and then shows a minor decrease to the bottom of the hole.

The boron depth profile contains a number of distinct and abrupt changes (Fig. F27E). From 423 µM at the core top, boron increases to 537 µM at 32.6 m CCSF-B, with two further local maxima at 76.1 m CCSF-B (574 µM) and at 149.8 m CCSF-B (578 µM). Boron concentrations then decrease downcore to 230 µM at 367.8 m CCSF-B. Deeper than this depth, boron increases again to ~350 µM at 550 m CCSF-B, decreases to 250 µM at 670 m CCSF-B, and finally increases to 300 µM deeper than 670 m CCSF-B.

Lithium concentrations at Site U1418 increase rapidly from 24.0 µM at the core top to 29.5 µM at 11.0 m CCSF-B (Fig. F27D). Farther downcore, lithium decreases to 13.5 µM by 76.1 m CCSF-B and stays below 20 µM to 271 m CCSF-B. To 452 m CCSF-B, lithium fluctuates between 17 and 25 µM. Deeper than 452 m CCSF-B, lithium concentrations more than double to ~60 µM at 544 m CCSF-B and then decrease to 23.5 µM at 698.7 m CCSF-B before increasing to reach 46.5 µM at the bottom of the hole.

Volatile hydrocarbons

Headspace gas samples were collected at a resolution of one per core in Holes U1418A (Cores 341-U1418A-1H through 33H), U1418C (Cores 341-U1418C-30H through 32H), U1418D (Cores 341-U1418D-25H through 37X), and U1418F (Cores 341-U1418F-7R through 72R). Methane is the dominant hydrocarbon gas detected in all samples from Site U1418, and it remains at low concentrations (generally <5 ppmv) down to 80.3 m CCSF-B (Fig. F26F). Methane concentrations increase rapidly downcore, especially deeper than 93.2 m CCSF-B and deeper than 301.5 m CCSF-B, and largely remain above 5,000 ppmv to the bottom of the hole. High values (>30,000 ppmv) are recorded at 135.9, 327.6, and 494.0 m CCSF-B. Deeper than 150 m CCSF-B, methane concentrations at or below 10,000 ppmv tend to be associated with interbedded silt and mudstone lithologies in Holes U1418C and U1418D (see “Lithostratigraphy”), and in Hole U1418D sandy beds correspond with the low methane concentrations recorded in Cores 341-U1418D-35X (282.7 m CCSF-B). Inadvertent sampling from suck-in sediments (drilling disturbance) in Core 341-U1418C-32H is a possible explanation for these lower methane concentrations (see “Lithostratigraphy”). Deeper than 100 m CCSF-B, ethane is intermittently present in low concentrations (<4.15 ppmv). Ethane concentrations progressively increase downcore through Hole U1418F, from 0.5 ppmv (296.1 m CCSF-B) to 16.8 ppmv (768.6 m CCSF-B) (Fig. F26G). Between 820 and 871 m CCSF-B, ethane concentrations remain above 20 ppmv, reaching a maximum concentration of 119 ppmv at 858.1 m CCSF-B. However, the C₁/C₂ ratio remains high (generally 600–60,000), implying no hazard to drilling operations.

Bulk sediment geochemistry

Discrete core samples from Site U1418 were analyzed for total carbon, total nitrogen (TN), and total inorganic carbon (TIC). 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, 44 samples were analyzed from Hole U1418A (Cores 341-U14178A-1H through 33H), 14 from Hole U1418D (Cores 341-U1418D-22H through 36X), and 59 from Hole U1418F (Cores 341-U1418F-4R through 69R). Discrete samples were selected in collaboration with the Lithostratigraphy group to ensure that the main lithologies were analyzed.

The TOC content at Site U1418 ranges between 0.2 and 1.0 wt% (Fig. F28A). Variability in the TOC content is superimposed on an overall downcore increase from the surface to 320.8 m CCSF-B, which corresponds to lithostratigraphic Unit I and Subunit IIA (see “Lithostratigraphy”). Between 350 and 450 m CCSF-B, TOC is <0.6 wt% and then increases sharply to 0.89 wt% at 457.4 m CCSF-B. Deeper than 460 m CCSF-B, TOC contents decrease downcore and are largely <0.5 wt% deeper than 646 m CCSF-B.

The TN content at Site U1418 ranges between 0 and 0.01 wt% (Fig. F28B). Samples between 320 and 467 m CCSF-B are slightly lower than surrounding depths (<0.5 wt%), and although higher TN is observed in samples with higher TOC, TN contents remain extremely low throughout Site U1418.

Organic carbon to TN (C/N) ratios mostly range between 6 and 16, with an isolated maximum C/N ratio of 22 recorded at 457.4 m CCSF-B. These values are consistent with a contribution from both marine and terrigenous organic matter (Hedges et al., 1986; Walinsky et al., 2009). However, the ratio is likely underestimated as a result of contributions from the inorganic N pool (Fig. F28E) (e.g., clay mineral-bound ammonium as indicated by the positive intercept in a TOC/TN cross-plot) (e.g., Schubert and Calvert, 2001). C/N ratios increase downcore to 400 m CCSF-B, followed by a deeper downcore decrease to the bottom of the hole. Both trends are largely driven by variability in the TOC rather than the TN contents. 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 U1418.

CaCO₃ values range mostly between 0 and 5 wt% at Site U1418 (Fig. F28D). One sample, with a CaCO₃ content of 6 wt% (~630 m CCSF-B), was taken adjacent to a suspected calcite-cemented siltstone (see “Lithostratigraphy”), which likely explains its elevated values. An overall downcore decrease in CaCO₃ content occurs from 2.4 to 181.35 m CCSF-B. Deeper than ~200 m CCSF-B, CaCO₃ content is highly variable, and no clear downcore trends are observable. In general, CaCO₃ and TOC contents show inverse patterns at Site U1418.

Interpretation

Site U1418 is located close to the continental margin and the Alaska Current. The geochemistry at this site reflects the increased productivity and sediment burial associated with a proximal continental setting. The CaCO₃ contents are overall low but still support the preservation of foraminifers at Site U1418 (see “Paleontology and biostratigraphy”). Variability in TOC contents could be related to the amount of marine organic matter associated with biosiliceous productivity, which may be supported by the general association between peak TOC contents and intervals of high diatom abundance (e.g., between 80 and 100 m CCSF-B and at 320 m CCSF-B) (see “Paleontology and biostratigraphy”). Dissolution of biogenic opal and/or volcanic glass is indicated by dissolved silica concentrations that are almost an order of magnitude higher than those of the overlying North Pacific Deep Water (~160 µM). There is high variability in the silica profile at Site U1418, likely reflecting the combined influence of both silica dissolution and formation of authigenic clay minerals.

Both alkalinity and ammonium are produced near 30 m CCSF-B (likely related to organic matter degradation by dissimilatory iron reduction), at ~70 m CCSF-B (likely related to anaerobic oxidation of methane in a sulfate–methane transition zone [SMTZ]), and at ~150 m CCSF-B (likely related to an interval of intense methanogenesis, as indicated by a well-defined significant methane peak). The diagenetically most active interval between 0 and ~200 m CCSF-B coincides with elevated diatom abundance and preservation in lithostratigraphic Unit I (see “Lithostratigraphy” and “Paleontology and biostratigraphy”), and more intense diagenesis is driven by the availability of reactive organic matter. The initially steep then more gradual downcore decrease of alkalinity deeper than ~150 m CCSF-B is probably related to the precipitation of authigenic carbonate minerals, whereas the same pattern in the ammonium profile is probably related to its adsorption onto clay minerals (as discussed below). These processes are most intense directly beneath the lithostratigraphic Unit I/II boundary (i.e., at the top of the diamict facies) (see “Lithostratigraphy”).

Based on the composition of IW, Site U1418 is divided into two biogeochemical zones, one reaching from the seafloor to ~70 m CCSF-B and the other reaching from ~70 m CCSF-B to the bottom of the hole (Figs. F26, F27, F28, F29), but these zones do not directly correlate with lithostratigraphic unit boundaries. The shallower zone can be further divided, as the concentration profiles of manganese, iron, and sulfate follow the classical catabolic reaction sequence proposed by Froelich et al. (1979) in the uppermost 30 m CCSF-B at Site U1418 (Fig. F27). Dissimilatory reduction of manganese (0–2 m CCSF-B) and then iron (3–30 m CCSF-B) occurs, and diffusion of the dissolved manganese and iron upward into the oxic zone leads to precipitation of authigenic manganese and iron oxides and oxyhydroxides, as seen in the brown sediment layer directly beneath the mudline at Site U1418. Downward diffusion from relative near-surface maxima probably leads to precipitation of authigenic manganese and iron carbonates, iron sulfides, and/or iron phosphates. The precipitation of iron monosulfides (e.g., greigite) is documented by black micronodules and thin bands that occur throughout the upper part of the sediment record (see “Lithostratigraphy”). The steep linear sulfate gradient from the sediment/water interface to ~70 m CCSF-B terminates at an SMTZ, as clearly indicated by the increase in methane concentrations beneath this depth (Fig. F29). Consequently, the underlying sediments are methanogenic, although methane production rates appear to be variable within these deeper deposits. Slightly elevated sulfate concentrations (<3 mM) in some IW samples deeper than the SMTZ show tentative support for the notion of a “cryptic sulfur cycle” (Holmkvist et al., 2011), whereby iron sulfides are oxidized by manganese or iron oxides and oxyhydroxides (Bottrell et al., 2000). Contamination of subsurface waters by surface seawater pumped as part of the drilling process cannot explain the presence of deep sulfate; there are no similar anomalies in the dissolved barium profile and there is no evidence for systematic barite precipitation deeper than ~70 m CCSF-B.

The diagenetic zone deeper than 70 m CCSF-B is denoted by both less organic matter degradation and likely increased clay mineral adsorption-desorption processes that influence a number of IW constituents. Total sulfate depletion is reached at ~70 m CCSF-B, with the onset of methanogenesis; dissolved barium concentrations start to increase because of dissolution of barite (barium sulfate) as IW becomes undersaturated with respect to this mineral (Waterman et al., 1973). The top of the barite dissolution zone is clearly confined by the sulfate penetration depth, and precipitation of authigenic barite most likely occurs at this geochemical transition. Strontium concentrations seem to follow the barium pattern between ~100 and 200 m CCSF-B, which might be related to the relatively high strontium contents in barite. Deeper than ~200 m CCSF-B, however, dissolved barium and strontium are clearly decoupled; although strontium continues to increase downcore to ~700 m CCSF-B, the barium profile exhibits a number of sources and sinks that likely involve the interaction of barium (but not strontium) with clay minerals. At ~500–600 m CCSF-B, increased degradation of organic matter by methanogenesis may release both ammonium and alkalinity into the pore space (Fig. F26). This release might be related to the availability of more labile organic matter of marine algal origin (e.g., diatoms), as indicated by dissolved silica release due to biogenic opal dissolution in the same depth interval (Fig. F26) and the decrease in C/N ratios deeper than 500 m CCSF-B (Fig. F28). The similarity in the downcore profiles of dissolved lithium, barium, and boron deeper than 500 m CCSF-B is consistent with the preferential adsorption of ammonium to clay mineral surfaces, leading to its exchange for previously adsorbed lithium, boron, barium, or magnesium ions (Gieskes, 1975; Hanor and Chan, 1977; von Breymann et al., 1990; Zhang et al., 1998; James and Palmer, 2000) that are then released into solution. Re-adsorption of barium, lithium, and boron onto clay minerals in the sediments with lower dissolved ammonium concentrations might be assisted by the neoformation of authigenic clay minerals. The decrease in dissolved silica, magnesium, and potassium deeper than 600 m CCSF-B further supports this interpretation (Figs. F26, F27) (Gieskes, 1975; Michalopoulos and Aller, 1995; Gieskes et al., 1998). Alternatively, the removal of magnesium, potassium, barium, boron, and lithium from the IW might be related to authigenic feldspar formation (Gieskes, 1975; Kastner and Siever, 1979; Wallmann et al., 2008). It is unclear why strontium would not participate in this desorption–adsorption cycle. Overall, in this depositional setting strongly dominated by clay-rich terrigenous sedimentation, detrital and authigenic clay minerals may play a dominant role in the development of the IW chemistry in the methanogenic zone.

The patterns in IW chloride concentration, sodium, bromide, and (less indicative) salinity in the uppermost 100 m CCSF-B at Site U1418 are most likely related to the burial of glacial seawater in the interstitial space (Fig. F26) (McDuff, 1985; Gieskes et al., 1998). The decreases in chloride, bromide, and sodium toward the bottom of the hole are probably caused by IW dilution with freshwater released from clay mineral dehydration with increasing sediment burial depth, whereas the gradual loss of dissolved magnesium and potassium from the IW may also be due to authigenic clay mineral formation.

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