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doi:10.2204/iodp.proc.341.103.2014

Geochemistry

Interstitial water chemistry

Interstitial water (IW) samples were taken in Holes U1417A and U1417C–U1417E at variable sampling resolution with depth in hole; sampling was designed to include overlaps in the sampled intervals between each successive hole. The following results are expressed on the CCSF-B scale for Site U1417 (see “Stratigraphic correlation”).

In Hole U1417A, 26 IW samples were taken with a resolution of three samples per core in Cores 341-U1417A-1H and 2H, two samples per core in Cores 3H through 6H, and one sample per core deeper than Core 6H. Whole rounds for IW analyses were 5 cm long in Cores 341-U1417A-1H through 16H and 10 cm long in Cores 17H through 22H. All samples from Hole U1417A were collected from APC cores (0–165.14 m CCSF-B). In Hole U1417C, 11 IW samples were taken from Cores 341-U1417C-17H through 28H (149.09–221.65 m CCSF-B), with a more irregular depth resolution (1–2 samples per core) than in Hole U1417A depending upon core recovery. In Hole U1417D, 13 IW samples were taken from Cores 341-U1417D-32H through 59X (207.15–421.22 m CCSF-B), with irregular spacing due to variable core recovery.

In Hole U1417E, 22 IW samples were taken from Cores 341-U1417E-7R through 39R (401.26–643.84 m CCSF-B). Hole U1417E 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 been reported (e.g., Shipboard Scientific Party, 2000; Fulthorpe et al., 2011). The intervals of higher (>40%) recovery in Hole U1417E allowed IW sampling on complete 10 or 15 cm long whole rounds, and we did not identify evidence for contamination with seawater in the chemical analyses (see below). The compositions of IW samples taken from overlapping depths between the different holes were in good agreement, allowing construction of composite IW records. All chemical parameters analyzed in the IW samples at Site U1417 are similar to respective low-resolution data from original Site 178 drilled in 1971 at the same location (Kulm et al., 1973; Waterman et al., 1973).

The applied squeezing pressures ranged from 8,000 to 28,000 psi, and the amounts of extracted IW ranged between 25 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). IW splits also 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

An alkalinity peak occurs at 28.9 m CCSF-B (16.1 mM), followed by a decrease to 3.6 mM by 218.0 m CCSF-B (Fig. F26A). A second peak occurs at 375.6 m CCSF-B (12.6 mM) before an overall downcore decrease, with some small oscillations, to 2.23 mM (702.8 m CCSF-B). The pH values fluctuate between 7.36 and 8.28. We observed an overall increase in both the range and absolute pH values downcore at Site U1417 (Fig. F26B).

Chloride (titrated) concentrations range between 546 and 565 mM (Fig. F26I), largely without a clear downcore trend apart from a chloride maximum at 37.6 m CCSF-B.

Salinity gradually and steadily decreases from 34 to 30 from 0 to 288.0 m CCSF-B (Fig. F26H). Salinity remains at 30 deeper than this depth, except for two slight increases to 31 at 375–430 m CCSF-B and deeper than 662.3 m CCSF-B.

Dissolved ammonium, phosphate, and silica

Dissolved ammonium at Site U1417 increases rapidly with depth in the uppermost 20 m, from 0.161 mM (1.3 m CCSF-B) to 1.104 mM (20.3 m CCSF-B) (Fig. F26D). Deeper than 20.3 m CCSF-B, ammonium concentrations remain between 1.0 and 1.2 mM and then decrease downcore deeper than 400 m CCSF-B to values <0.5 mM.

Phosphate concentrations are highest in the upper 30 m, with maxima at 14.6 m CCSF-B (38.2 µM) and 28.9 m CCSF-B (35.5 µM) (Fig. F27J). Deeper than 29 m CCSF-B, phosphate rapidly decreases to 12.9 µM at 37.6 m CCSF-B, followed by a more gradual decrease to 3.7 µM at 165.1 m CCSF-B. Phosphate concentrations remain low downcore (<4 µM), apart from a slight increase to ~6 µM between 300 and 375 m CCSF-B.

Silica concentrations display a number of significant and abrupt transitions (Fig. F26G). A steep increase from 600 to 800 µM within the uppermost 15 m CCSF-B is followed by a more gradual increase to a value of 1054 µM at 177.2 m CCSF-B. Silica concentrations of ~1000 µM are sustained downcore, apart from three intervals of low concentrations: 215–290 m CCSF-B (as low as 300 µM), 518–596 m CCSF-B (as low as 192 µM), and deeper than 643.8 m CCSF-B (as low as 160 µM).

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

Sulfate concentrations decrease continuously with depth in the upper 200 m CCSF-B, from 21.47 mM (1.3 m CCSF-B) to 0.93 mM (207.2 m CCSF-B) (Fig. F26C). Over this depth interval, the sulfate gradient is variable, with the steepest decrease occurring between 1.3 and 20.3 m CCSF-B (to 16.85 mM). Total sulfate depletion (to values below the detection limit) is not reached, but deeper than 200 m CCSF-B, sulfate concentrations remain below 2 mM. An increase in sulfate occurs deeper than 643 m CCSF-B, reaching ~7 mM at 681.8 and 702.8 m CCSF-B.

Calcium concentrations increase overall with depth at Site U1417, from 10.5 mM (1.3 m CCSF-B) to 30.3 mM (681.8 m CCSF-B) (Fig. F27A). Two intermediate maxima occur in the upper 90 m (12.6 mM at 14.6 m CCSF-B and 12.4 mM at 52.1 m CCSF-B). A steep increase in calcium concentrations deeper than 100.2 m CCSF-B leads to a third, broad maximum between 207.2 and 254.6 m CCSF-B (~16–17 mM). Calcium concentrations decrease slightly between 288.0 and 316.3 m CCSF-B and then increase again to reach values of ~30 mM at the bottom of the hole.

Magnesium concentrations decrease overall with depth, from 50.7 mM (1.3 m CCSF-B) to 15.3 mM (702.8 m CCSF-B) (Fig. F27C). This trend is interrupted by a magnesium increase from 24.0 to 30.5 mM between 254.6 and 375.6 m CCSF-B. Calcium and magnesium concentrations are negatively correlated (Pearson’s correlation coefficient of –0.93) at Site U1417 deeper than 90 m CCSF-B, indicating a common control over these downcore profiles (discussed further below).

The potassium profile at Site U1417 shows an overall downcore decrease in two main steps: from 11.2 mM at 1.3 m CCSF-B to 8.6 mM at 100.2 m CCSF-B and from 6.7 mM at 421.2 m CCSF-B to 3.8 mM at 574.9 m CCSF-B (Fig. F27B). Deeper than 600 m CCSF-B, potassium concentrations increase to reach 5.6 mM at the bottom of the hole (702.8 m CCSF-B).

Sodium concentrations at Site U1417 mostly range between 463 and 480 mM, with the highest values between 9.5 and 20.3 m CCSF-B (Fig. F26J). An increased range of sodium concentrations is recorded deeper than 100 m CCSF-B, with an overall downcore increase from 100 to 260 m CCSF-B. A sharp drop in sodium concentration occurs at 400 m CCSF-B, followed by an overall downcore increase to the bottom of the hole.

Bromide concentrations at Site U1417 increase steeply between 1.3 and 12.1 m CCSF-B and remain ~0.9 mM down to 700 m CCSF-B (Fig. F26E).

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

Both iron and manganese concentrations at Site U1417 are highest in the uppermost 100 m CCSF-B, decreasing downcore from maximum concentrations of 18 and 112 µM, respectively (Fig. F27H–F27I). Deeper than 100 m CCSF-B, iron concentrations are below the quantification limit. In contrast, manganese concentrations are more variable and show a number of maxima and minima (ranging between 5 and 43 µM).

Barium concentrations are <10 µM in the uppermost 100 m CCSF-B at Site U1417 (Fig. F27F). Between 100 and 215 m CCSF-B, barium concentrations double; they further increase to 440 µM in the interval between 200 and 620 m CCSF-B. Farther downcore, barium decreases to low values (~30 µM) by 643 m CCSF-B. Detectable barium concentrations are limited to the sediment interval with lowest sulfate concentrations.

Strontium concentrations at Site U1417 first increase slowly and then more rapidly from ~100 µM at the top of the record to as high as 180 µM by 215 m CCSF-B (Fig. F27G). Between ~215 and 600 m CCSF-B, strontium remains around 180 µM. Deeper than this interval, strontium concentrations slightly decrease to ~130 µM again to the bottom of the hole.

Boron concentrations display a pronounced maximum in the uppermost 45 m CCSF-B of Site U1417, reaching values of 650 µM (Fig. F27E). Deeper than this depth, boron decreases to values around 300 µM by 240 m CCSF-B and mostly ranges between 200 and 300 µM to the bottom of the hole.

Lithium concentrations at Site U1417 decrease within the uppermost 10 m CCSF-B from 32 to 12 µM and then slowly increase downcore and reach 24 µM at ~240 m CCSF-B (Fig. F27D). Deeper than 260 m CCSF-B, lithium concentrations increase more steeply, reaching values of 55 µM by ~410 m CCSF-B. Following a local minimum and another maximum, lithium concentrations decrease to 34 µM around 565 m CCSF-B, followed by a steep increase to maximum values of 103 µM at the bottom of the hole.

Volatile hydrocarbons

Headspace gas samples were collected at a resolution of one per core in Holes U1417A (Cores 1H through 22H), U1417B (Cores 21H through 47X), U1417D (Cores 32H through 65X), and U1417E (Cores 7R through 39R). Methane is by far the dominant hydrocarbon gas detected in Holes U1417A and U1417B and most of Hole U1417D and remains at low concentrations (generally <7 ppmv) to 424.7 m CCSF-B (Fig. F26F). Slightly elevated methane concentrations are found in the uppermost 130 m (5–13 ppmv). Deeper than 431.1 m CCSF-B, methane concentrations increase with depth by two to three orders of magnitude in Holes U1417D and U1417E, reaching a maximum of 5117 ppmv at 498.9 m CCSF-B. In Hole U1417E, methane concentrations decrease abruptly at 643.8 m CCSF-B. Ethane is present deeper than 439.2 m CCSF-B in low concentrations (<2 ppmv). C₁/C₂ ratios thus remain high, between 500 and 5000.

Bulk sediment geochemistry

IW squeeze cakes and discrete core samples were analyzed from Site U1417 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, 55 samples were analyzed from Hole U1417A (Cores 1H through 22H), 1 from Hole U1417B (Core 16H-CC), 11 from Hole U1417C (Cores 17H through 28H), 44 from Hole U1417D (Cores 7H through 64X), and 35 from Hole U1417E (Cores 4R through 39R). Discrete samples were selected in collaboration with the Lithostratigraphy group to ensure that the main lithologies were analyzed. All of these geochemical results are consistent with the low-resolution analyses of Site 178 (Kulm et al., 1973).

TOC contents mostly range between 0.1 and 0.6 wt% at Site U1417 (Fig. F28A–F28B). No consistent trend is discernible in the uppermost 200 m other than a very slight overall downcore decrease. Deeper than 300 m CCSF-B, TOC increases slightly downcore to 564.7 m CCSF-B (0.85 wt%) within lithostratigraphic Unit V, which contains visible plant debris and suspected fragments of coal or shale within a diamict facies (see “Lithostratigraphy”). Variable but slightly reduced TOC contents occur deeper than 600 m CCSF-B. Several discrete samples from specific lithologies show elevated TOC contents: two diatom oozes (0.85 wt%, 357.6 m CCSF-B; 1.39 wt%, 365.0 m CCSF-B), diamicts suspected to contain coal or shale clasts (9.08 wt%, 364.1 m CCSF-B; 2.38 wt%, 442.2 m CCSF-B; 1.14 wt%, 466.9 m CCSF-B; 1.49 wt%, 681.8 m CCSF-B), woody debris within a sandy bed (6.49 wt%; 535.2 m CCSF-B), and a dark gray mud (1.79 wt%; 565.0 m CCSF-B). A TOC content of 59.80 wt% was recorded for an angular clast suspected to be coal or shale within a diamict unit (441.4 m CCSF-B) (see “Lithostratigraphy”).

TN contents range between 0 and 0.2 wt% at Site U1417 (Fig. F28C). Nitrogen contents are higher overall in Hole U1417A (~0.1 wt%) than in the other holes at Site U1417, indicating a systematic bias related to a saturated nitrogen column in the gas chromatograph. There are small fluctuations in TN deeper than 200 m CCSF-B (0–0.1 wt%). Several discrete samples from specific lithologies record higher TN contents: in a diatom ooze (0.2 wt%; 364.1 m CCSF-B), in a suspected coal or shale clast (1.0 wt%; 441.4 m CCSF-B), and in sediment around a pyritized lonestone (1.1 wt%; 143.9 m CCSF-B).

Organic carbon to TN (C/N) ratios range between 0 and 30, increasing progressively downcore from 0–5 in the uppermost 100 m CCSF-B to values of 0–30 at the bottom of the hole. Considerably higher C/N ratios (50–60) are recorded in a suspected coal or shale sample (441.4 m CCSF-B) and in woody debris (535.2 m CCSF-B). C/N ratios in the uppermost 100 m CCSF-B (Hole U1417A) fall within the range of fresh organic matter and are consistent with contributions of terrigenous organics (both refractory eroded material and relatively fresh plant material) matter to modern sediments off Alaska (Walinsky et al. 2009), but these ratios may be underestimated from C/N ratios as a result of contributions from the inorganic N pool (e.g., clay mineral–bound ammonium) (Fig. F28F). This phenomenon has been observed in other low-TOC marine sediments such as in the Arctic Ocean (e.g., Schubert and Calvert, 2001; Jakobsson, 2004). The increasing C/N ratios deeper than 100 m CCSF-B are consistent with a mixed marine-terrigenous origin for the sediments at Site U1417 (see “Lithostratigraphy”). However, determination of the relative contributions of organic and inorganic N is required to assess this interpretation fully.

CaCO₃ contents range mostly between 0 and 1.5 wt% at Site U1417 (Fig. F28D–F28E). Between 200 and 265 m CCSF-B and deeper than 345 m CCSF-B, CaCO₃ contents are still low but range from 0 to 3 wt%. Contents exceeding 2 wt% include one data point associated with a diatom-rich interval (4 wt%, 61.1 m CCSF-B; interval 341-U1417A-9H-4W, 74–76 cm). XRD analysis identified the presence of dolomite (see XRD data in the Laboratory Information Management System [LIMS] database) in this sample, which likely accounts for the high inorganic C percentage and which is not corrected for in the calculations of CaCO₃ (see “Geochemistry” in the “Methods” chapter [Jaeger et al., 2014]). A suspected authigenic carbonate layer at 336.9 m CCSF-B (interval 341-U1417D-50X-2, 102–106 cm) has 21 wt% CaCO₃. The maximum CaCO₃ value (38 wt%) is recorded at 450.9 m CCSF-B (interval 341-U1417D-64X-1W, 0–3 cm) in a highly cemented sandstone (low-Mg calcite according to XRD analysis; see XRD data in LIMS).

Interpretation

Site U1417 geochemistry indicates relatively low rates of organic matter remineralization, reflecting limited input of organic matter to the sediments. The low TOC and TN contents at Site U1417 are typical for an oligotrophic deepwater setting, and sediments are not strongly influenced by diagenetic reactions. A contribution of inorganic N to the bulk sediment composition, most likely related to clay-bound ammonium (Schubert and Calvert, 2001), is supported by the continuous downcore depletion of ammonium deeper than ~30 m CCSF-B. The low CaCO₃ contents are equally typical for a subarctic deepwater setting with high terrigenous dilution, as well as with corrosive bottom waters and overall low biogenic carbonate preservation. The sparse occurrence and low preservation of calcareous micro- and nannofossils support this interpretation (see “Paleontology and biostratigraphy”).

A number of IW parameters point to a zone of increased organic matter remineralization between ~10 and 40 m CCSF-B at Site U1417 (Figs. F27, F29). Dissimilatory manganese and iron reduction release dissolved manganese, iron, and adsorbed phosphate (potentially also boron) into the pore waters (Froelich et al., 1979). Following upward diffusion, these metals reprecipitate as authigenic manganese and iron (oxyhydr)oxides (Froelich et al., 1979), as seen in the brown sediment layer directly beneath the mudline at Site U1417. Downward diffusion probably leads to precipitation of authigenic manganese and iron carbonates, iron sulfides, iron phosphates, and/or iron-rich clay minerals. Beneath the zone of dissimilatory manganese and iron reduction, the steep linear sulfate gradient from the sediment/water interface to ~20–30 m CCSF-B implies enhanced organoclastic sulfate reduction rates that lead to (1) degradation of organic matter, increasing alkalinity, ammonium, bromide, and phosphate concentrations and (2) production of hydrogen sulfide, reacting with iron oxides to form iron sulfides and releasing iron-bound phosphate (Gieskes, 1973, 1975). The variable pattern and overall low concentrations of methane in this part of the sediment succession (≤13 ppmv in all samples) do not support the establishment of a distinct sulfate–methane transition zone (SMTZ) but might indicate very low rates of methanogenesis occurring within the uppermost ~100 m of the sediment column. Deeper than ~30 m CCSF-B, the sulfate gradient becomes less steep, but sulfate is nevertheless decreasing, indicating continuous sulfate reduction at lower rates in the deeper sediments that leads to almost full sulfate depletion at ~200 m CCSF-B.

From the major diagenetic reaction zone between ~10 and 40 m CCSF-B, alkalinity, ammonium, bromide, and phosphate diffuse to respective sinks in the sediment or water column. Ammonium might be incorporated into clay minerals, whereas alkalinity and phosphate likely precipitate as authigenic carbonate and apatite, respectively (Hein et al., 1979a; Ruttenberg and Berner, 1993). A distinct sink for alkalinity is located between 215 and 290 m CCSF-B, and slightly higher inorganic carbon values in this zone (1–2 wt% CaCO₃) and lower IW magnesium concentrations suggest the precipitation of authigenic carbonate (Hein et al., 1979a; Raiswell and Fisher, 2004). This interpretation is supported by the presence of brownish, probably dolomite-coated foraminifers in this depth interval (see “Paleontology and biostratigraphy”), beneath which foraminifer abundance falls from few to barren as alkalinity increases. In addition, the very steep downward phosphate gradient might indicate the formation of Fe(II) phosphates (e.g., vivianite), as indicated by the possible detection of its oxidation products (e.g., koninckite) by XRD analysis (see XRD data in LIMS). Deeper in the sediment, a local alkalinity maximum around 375 m CCSF-B suggests another zone of enhanced organic matter remineralization, supported by slightly elevated ammonium and phosphate concentrations.

The finding of substantially increased methane concentrations only deeper than ~420 m CCSF-B but almost total sulfate depletion around 200 m CCSF-B suggests that sulfate is not currently consumed by anaerobic methane oxidation in an SMTZ. Nevertheless, a discrete sink for alkalinity and magnesium deeper than 375 m CCSF-B supports precipitation of authigenic carbonate. This process also could explain the occurrence of an indurated/cemented sandstone layer at the bottom of Hole U1417D (450.9 m CCSF-B) that was only partially recovered during coring. Because of problems with the disaggregation of core catcher samples from Hole U1417E (see “Paleontology and biostratigraphy”), foraminifer abundance (and preservation state) and any potential association with the alkalinity sink could not be investigated.

As sulfate concentrations reach lowest values at ~200 m CCSF-B, pore water barium concentrations begin to increase with depth, possibly because of the dissolution of biogenic barite (barium sulfate) at greater depths. This dissolution occurs because IW become undersaturated with respect to sulfate (Waterman et al., 1973). The top of the barite dissolution zone is clearly confined by the sulfate penetration depth (the depth of total IW sulfate depletion), and precipitation of authigenic barite is most likely occurring at this geochemical transition (von Breymann et al., 1992). Strontium concentrations seem to follow the barium pattern, which might be related to the relatively high strontium content in barite. The lower boundary of the zone of barium- and strontium-enriched IW is related to a slight but consistent increase in sulfate concentrations deeper than 500 m CCSF-B. Methane production is roughly confined to the sulfate-depleted zone as well, but a direct overlap of the methane and the sulfate profiles was not observed shallower than the methanogenic zone (possibly because of incomplete methane recovery in low-methane samples and/or a decrease of methane production over time). However, there is a second SMTZ located around 650 m CCSF-B. Sulfate may be diffusing upward, possibly from a source that exists at, or deeper than, the sediment/basalt contact (D’Hondt, Jørgensen, Miller, et al., 2003), ~80–100 m beneath the deepest pore water sample, based on a tentative shipboard correlation to DSDP Leg 18 results (Fig. F30; Kulm, Von Huene, et al., 1973). This hypothesis is supported by lithium concentrations higher than the seawater value of 26 µM toward the base of the IW record (>100 µM), which are due to leaching of the basalt by seawater (e.g., Mayer, Pisias, Janecek, et al., 1992; Zhang et al., 1998).

The patterns of dissolved calcium, magnesium, and potassium also indicate geochemical transformation processes occurring in the sediments. Calcium might precipitate as authigenic carbonate and/or apatite, possibly explaining its variable pattern in the uppermost ~100 m CCSF-B of sediment. However, sources of dissolved calcium located deeper than this may be related to the alteration of volcanic ash or the basalt underlying the sediments (Gieskes, 1975; Gieskes and Lawrence, 1981). Alteration of ash/basalt may be responsible for the downcore decrease of magnesium deeper than ~400 m CCSF-B, suggesting the neo-formation of magnesium-rich clay minerals (e.g., smectites), whereas the magnesium removal around 250 m CCSF-B is most likely caused by authigenic carbonate formation (see above). Potassium, magnesium, and iron might be incorporated into either clay minerals or zeolites formed by alteration of diatom frustules (Michalopoulos and Aller, 1995). Iron-rich clays may be forming (see XRD data in LIMS), but the terrestrial source area also contains iron-rich clay minerals (Hein et al., 1979b).

Dissolved silica concentrations exceed those of modern North Pacific Deep Water (~160 µM), indicating dissolution of silica below the seafloor, likely from volcanic ash and/or biogenic opal. High IW silica concentrations correspond to the presence of biosiliceous oozes, whereas IW silica concentrations are low when these oozes are absent between 200 and 300 m CCSF-B and between 500 and 600 m CCSF-B (see “Lithostratigraphy”). The potential precipitation of authigenic clay minerals and removal of dissolved silica from the IW might also account, in part, for the low silica concentrations observed between 200 and 300 m CCSF-B (lithostratigraphic Unit II). At these depths, volcanic ash is present as a silica source (see “Lithostratigraphy”). Diffusion upward from the underlying basalt may explain the trend in dissolved silica deeper than 600 m CCSF-B, but this depth interval is also marked by the presence of biosiliceous ooze, which may induce higher silica input.

The patterns in chloride, sodium, and (less indicative) salinity in the uppermost 60 m CCSF-B are most likely related to the burial of higher salinity Last Glacial Maximum seawater that is diffusing away from this subsurface maximum (Figs. F26, F29) (McDuff, 1985; Gieskes et al., 1998).

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