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doi:10.2204/iodp.proc.346.106.2015

Geochemistry

Site U1425 is located on the Yamato Rise, ~54 km northeast of Site 799 (see “Background and objectives”). With the fairly comprehensive geochemistry data sets of Site 799 (Shipboard Scientific Party, 1990) as guidance, we returned to our basic Expedition 346 sample program (“Geochemistry” in the “Methods” chapter [Tada et al., 2015b]), but with three modifications.

We initially planned to collect two squeezed interstitial water (IW-Sq) samples per core on the deepest hole until the base of APC coring. However, we scaled this back, given our newly acquired appreciation for sample flow through the Chemistry Laboratory. We instead took two IW-Sq samples per core through the upper 100 m and one IW-Sq sample per core deeper than this depth.

By arrival at Site U1425, some geochemical results from Site U1424 had emerged. Among these exciting results, the analyses of Rhizon interstitial water (IW-Rh) samples from across Core 346-U1424A-1H (a “mudline” core) suggested prominent inflections in dissolved NH₄⁺, Mn, and Fe profiles within the uppermost 1 m below the seafloor (see “Geochemistry” in the “Site U1424” chapter [Tada et al., 2015e]). Presumably, the inflections represent thin depth horizons where microbes consume the three species. Encouraged by our ability to connect detailed interstitial water profiles across key intervals to broader interstitial water profiles generated through conventional squeezing, we offered a plan to collect numerous Rhizons across mudline cores at Site U1425 (Fig. F37).

A series of light and dark intervals characterize the uppermost lithologic unit at Sites U1422–U1424 as well as at other locations throughout the marginal sea (Föllmi et al., 1992; Tada et al., 1992). To aid shore-based sampling and research, we thought it desirable to quantify differences in organic carbon between some of these intervals.

Sample summary

During operations at Site U1425, the geochemistry group collected and analyzed a range of samples. These included the following (Tables T10, T11, T12, T13, T14):

• 2 mudline (ML) samples from bottom water poured from the uppermost core liner at Holes U1425B and U1425D.

• 55 interstitial water samples from whole-round squeezing (IW-Sq) from Hole U1425B. Most whole rounds were nominally 5–6 cm long.

• 84 interstitial water samples from Rhizons (IW-Rh) from Holes U1425A (7), U1425B (27), U1425D (23), and U1425E (27). The samples from Hole U1425A were used to evaluate whether water flow increased and sediment cracking decreased if Rhizons were dipped in deionized water prior to insertion. The Rhizons from Hole U1425D were taken because during operations the presence of a mudline was not certain for Hole U1425B. The Rhizons from Hole U1425E were taken on our second visit to the site to complement onshore studies.

• 56 sediment samples from the interstitial water squeeze cakes.

• 12 additional sediment samples from Core 346-U1425B-1H to examine changes in solid-phase chemistry associated with light and dark cycles (Table T11).

• 71 headspace (HS) gas samples.

No Vacutainer (VAC) samples were taken at this site.

Carbonate and organic carbon

The solid-phase geochemistry profiles at Site U1425 (Fig. F38) are fairly similar to those at Site 799 (Shipboard Scientific Party, 1990). In general, sediment samples have low CaCO₃ contents. Higher quantities (0.4–11.2 wt%; mean = 4.6 wt%) are found at the top within Unit I, whereas very small amounts (mean = 0.9 wt%) are found in basal units. Nonetheless, compared to results from Sites U1422–U1424, Unit I at Site U1425 has the highest overall CaCO₃ contents observed so far during Expedition 346. One sample at 100.25 m CSF-A in Subunit IIA has a very high CaCO₃ content of 17.7 wt%.

Site U1425 appears to have lower CaCO₃ values than sediment at Site 799 (Shipboard Scientific Party, 1990). Carbonate contents over the uppermost 400 m at Site 799 average 6.9 wt%, with a range between 0.2 and 51.7 wt%. As the water depths of Sites U1425 (1971 m) and 799 (2073 m) are similar, the difference in CaCO₃ content may be a consequence of our sampling strategy, where most sediment samples are the remains of IW-Sq samples. Certainly, numerous carbonate layers exist at both locations (see “Lithostratigraphy”).

In contrast to CaCO₃, organic carbon contents are moderately high for marine sediment (Müller and Suess, 1979; Emerson and Hedges, 1988). This is particularly true for the upper portion of Subunit IA and for Unit III (Fig. F38). The total organic carbon (TOC) contents of Subunit IA average 1.23 wt% with a range of 0.29–2.46 wt%. Subunits IB and IIA have lower TOC contents, on average 0.8 and 1.0 wt%, respectively. Subunit IIB has low TOC contents until ~200 m CSF-A, where values increase with depth, eventually reaching 2.4 wt%. Subunits IIIA and IIIB are the richest regarding TOC, with an average of 3.1 wt% and a range of 1.5–4.7 wt% across both units. Interestingly, the transition between these two units has lower TOC, although values are still ~2 wt%, which is high for marine sediment (Müller and Suess, 1979; Emerson and Hedges, 1988).

When examining the overall TOC profile (Fig. F38), it is important to consider that squeeze cakes are compressed intervals 5–10 cm in length and arbitrarily selected with regard to lithologic horizons. As such, a different result is obtained when centimeter-scale samples are taken from light and dark sediment layers of Subunit IA (Table T11). Between 0 and 7.4 m CSF-A, discrete sediment samples representing different color intervals (see “Physical properties”) show high variability, from 0.4 to 5.1 wt% (average = 2.3 wt%).

In summary, the total carbon (TC) contents of sediment at Site U1425 derive from organic and inorganic carbon at shallow depths (<200 m CSF-A), especially in Unit I. At deeper depths, organic carbon content mainly controls the abundance of TC.

The mean value of total nitrogen (TN) is 0.24 wt%. With the exception of some samples from Subunit IA where TN can exceed 0.5 wt%, the maximum TN content is 0.41 wt% at 328.6 m CSF-A. Minima in TN contents of 0.16 wt% are found at 100.25 and 176.05 m CSF-A. Deeper than 200 m CSF-A, TN contents clearly track TOC contents. In fact, the C:N ratio for samples from Unit III averages 8.2.

Manganese and iron

Dissolved Mn and Fe exhibit intriguing profiles with respect to depth when analyses from mudline, IW-Sq, and IW-Rh samples are merged (Figs. F39, F40). Notably, downhole changes in these species, particularly Fe, occur over stratigraphic distances too short to examine with the size (10 cm long) and spacing (>1 m) of samples typically taken for squeezing (Fig. F39).

Manganese is ~1 µM in the mudline sample. Within 5 cm of the apparent seafloor, concentrations rise to 76 µM (Fig. F40). From this depth, dissolved Mn decreases to 35 µM at 0.9 m CSF-A and then more gradually decreases to 2 µM by 17 m CSF-A (Fig. F39). Manganese concentrations remain low until 52 m CSF-A then increase, reaching almost 77 µM by 245 m CSF-A. Dissolved Mn decreases to <35 µM between 320 and 360 m CSF-A, which is the opal-A–opal-CT transition (see “Silica”). Mn concentrations of IW-Rh samples are similar to those of IW-Sq from similar depths (Fig. F40).

The Mn profile at Site U1425 is similar to that at Site 799 (Shipboard Scientific Party, 1990), except that concentrations are significantly higher at the new drill location. The cause of the basic Mn profile at both sites has been discussed, in both previous chapters of this volume and in chapters of ODP Leg 128 (Ingle, Suyehiro, von Breymann, et al., 1990). Assuming that dissolved Mn concentrations at depth are related to equilibrium with Mn carbonate phases (Middelburg et al., 1987), a basic explanation for the differences in dissolved Mn concentrations between the sites is that alkalinity is lower at Site U1425 compared to Site 799. This is the case, as discussed below.

The mudline sample has an Fe concentration of 1.1 µM, which is near the detection limit (1 µM at this site). Below the seafloor and as observed at previous sites drilled during Expedition 346, the dissolved Fe profile is complicated (Fig. F39). Considering only the IW-Sq samples, Fe concentrations fluctuate between 2 and 6 µM until ~180 m CSF-A. From here to the base of the hole, Fe concentrations are near detection limit, except for a peak centered at ~320 m CSF-A. Importantly, the Fe profile is not “smooth,” suggesting either problems obtaining precise dissolved Fe concentrations using samples from the squeezers, numerous depth zones where Fe-bearing minerals dissolve or precipitate, or some combination of both. Analyses of the IW-Rh samples from the upper 9 m (Fig. F40) are interesting in this regard. The Fe profile constructed using Rhizons shows peaks that suggest dissolution of a solid Fe phase in the upper 1 m and perhaps at ~2.4 m CSF-A, but almost all Fe existing in solid Fe phases across other depths. The difference between IW-Sq and IW-Rh Fe concentrations at ~6 m CSF-A could represent Fe precipitation during Rhizon collection, Fe contamination during squeezing, or both. Iron was not measured at Site 799, so there is no comparison to previous studies.

Sulfate and barium

Dissolved SO₄^2– concentration at the mudline was 28 mM, which is close to the expected 28.1 mM for Japan Sea Proper Water (JSPW) (Table T10 in the “Methods” chapter [Tada et al., 2015b]). Below the seafloor, the dissolved SO₄^2– profile systematically decreases with a concave downward shape to between 60 and 70 m CSF-A, where SO₄^2– concentrations decrease below 0.5 mM (Fig. F41). SO₄^2– concentrations of IW-Sq and IW-Rh samples are similar.

The dissolved Ba profile displays a shape opposite that of SO₄^2– (Fig. F42), similar to other sites drilled in the marginal sea (von Breymann et al., 1992) and during Expedition 346. Barium concentrations are <25 µM from the seafloor to 48 m CSF-A. At this depth, values rise considerably. As described in the “Methods” chapter [Tada et al., 2015b]), high Ba concentrations are unconstrained and require postcruise analyses to check and rescale high Ba concentrations. Similar to previous sites, Ba concentrations of IW-Sq samples are consistently higher than those of IW-Rh samples (Fig. F42).

Given Fick’s law (Fick, 1855) and assuming constant porosity and temperature with depth, the shape of the SO₄^2– profile suggests continual loss of SO₄^2– over the uppermost ~70 m below the seafloor. Such removal contrasts to that inferred from a linear SO₄^2– gradient, which suggests focused SO₄^2– consumption over a short depth horizon. The SO₄^2– profile, therefore, may indicate that organoclastic sulfate reduction, rather than anaerobic oxidation of methane (AOM), removes much of the SO₄^2– from interstitial water at Site U1425. The general lack of CH₄ until 60 m CSF-A (Fig. F41) may provide additional support for this hypothesis. Note, however, that a concave downward SO₄^2– profile will appear when AOM dominates net SO₄^2– loss across a short depth horizon if temperature increases significantly between the seafloor and the horizon of interest (Dickens, 2001). With the steep geothermal gradient of 104°C/km at Site U1425 (see “Downhole measurements”), numerical modeling is needed to elucidate the cause of SO₄^2– depletion and check the sensitivity of the system to variations in other parameters (e.g., porosity).

As discussed at previous sites, the removal of interstitial water SO₄^2– eventually leads to barite dissolution. In turn, such dissolution releases large quantities of dissolved Ba to interstitial water (von Breymann et al., 1992). Passage of very small barite crystals through 0.45 µm filters during squeezing or barite precipitation during Rhizon sampling may explain the differences in dissolved Ba concentrations across IW-Sq and IW-Rh samples (see “Geochemistry” in the “Site U1424” chapter [Tada et al., 2015e]).

Volatile hydrocarbons

The downhole CH₄ profile at Site U1425 (Figs. F41, F43) reflects several processes, including CH₄ production and consumption in the sediment column and degassing during coring and core retrieval.

Methane concentrations in HS samples are <20 ppmv from the seafloor to ~60 m CSF-A (Table T14). At this depth, which corresponds to the horizon where interstitial water SO₄^2– concentrations approach detection limit (Fig. F41), CH₄ values increase relatively slowly, reaching 17,300 ppmv at 185 m CSF-A. Deeper than this depth, CH₄ concentrations fluctuate between 19,720 and 8,800 ppmv until 320 m CSF-A. Relatively low CH₄ concentrations appear at depths of XCB drilling (i.e., 100, 283, and 330–403 m CSF-A).

Ethane concentrations range between 0 and 42 ppmv across all samples. Consequently, the C1:C2 ratio generally exceeds 500 at Site U1425.

At Site U1425, microbes use dissolved SO₄^2– to consume significant amounts of organic matter. However, a small fraction of organic matter passes through the sulfate reduction zone, where microbes use it to form biogenic gas, composed of almost pure CH₄. The methane gradient between 60 and 120 m CSF-A, although difficult to quantify with HS samples from indurated sediment, is very low. The upward diffusive flux of CH₄ implied by this gradient may not be sufficient to counteract the downward burial flux of CH₄. Consequently, a prominent sulfate–methane transition, such as that found at Site U1422, does not occur at Site U1425 (Fig. F41).

Alkalinity, ammonium, and phosphate

The alkalinity, NH₄⁺, and PO₄^3– profiles at Site U1425 (Fig. F44) are broadly similar to those at sites previously drilled during Expedition 346. The profiles using Rhizons through the upper 2 m CSF-A appear similar to those generated using squeezers (Fig. F45), although it is difficult to make rigorous comparisons given the depth frequency of samples, depth offsets between holes, and times between sample recovery and analyses. In particular, because of precruise concerns that alkalinity could not be measured on IW-Rh samples accurately, alkalinity was measured on squeezed samples as soon as water was collected but was measured on IW-Rh samples typically after waiting for >12 h. During this waiting period, IW-Rh samples were kept in original sampling syringes without any headspace and refrigerated.

Alkalinity steadily increases from 2.5 mM at the seafloor to 32 mM at 106 m CSF-A and remains at 32 mM until 133 m CSF-A (Table T12). This increase exhibits a clear concave downward profile with no prominent concave downward inflections, such as at Site U1422. The alkalinity maximum coincides with a dolomite horizon (see “Lithostratigraphy”). Below the maximum, alkalinity decreases steadily to 9 mM at ~400 m CSF-A. The lack of a kink in alkalinity within the upper sediment column is informative: at Site U1425, there is no evidence for significant addition of alkalinity across a short depth horizon, as would be expected if AOM was an important process.

Ammonium increases from 21 µM in the mudline sample to 4100 µM at ~329 m CSF-A. Deeper than this depth, NH₄⁺ decreases to ~2800 µM at the base of the Hole U1425B (~400 m CSF-A). The profile is generally concave downward until 325 m CSF-A. Analyses of closely spaced Rhizon samples across the upper 2 m in both Holes U1425B and U1425D indicate that the steep NH₄⁺ profile is nearly linear and intersects the seafloor (Fig. F45).

Phosphate increases from 9 µM at the seafloor to 83 µM at ~15 m CSF-A. Deeper than this depth, PO₄^3– decreases steadily to <20 µM at ~300 m CSF-A. This decrease in PO₄^3– has a concave downward profile. A few apparently anomalous PO₄^3– values occur at depth.

The general explanations for downcore variations in alkalinity, NH₄⁺, and PO₄^3– profiles are the same as those provided in previous chapters of this volume and elsewhere (Murray et al., 1992). The only significant additional commentary on this matter is that Site U1425, along with the other sites, provides a spectrum of sites to investigate organic carbon decomposition.

Assuming the seafloor (mudline) was not missed twice during coring, NH₄⁺ seems to be escaping to deep water at this location (Fig. F45). The finding is especially intriguing when one considers two comparisons: (1) the 400 m of sediment at Site U1425 is approximately one-fifth the thickness of the overlying water mass and (2) the average NH₄⁺ concentration in interstitial water (~3000 µM) is ~120 times greater than the average NO₃^– concentration of water in the marginal sea (~25 µM) (Kim et al., 1992). Even considering sediment porosity, the apparently leaky subseafloor nitrogen pool, if extensive, must be extremely large relative to the overlying water mass nitrogen pool.

Bromide

The two mudline samples contain 0.84 mM Br^– (Table T12; Fig. F46). This value is identical to that expected for JSPW (Table T10 in the “Methods” chapter [Tada et al., 2015b]). Below the seafloor, Br^– concentrations slowly increase with depth, such that interstitial water samples at the base of the cored sequence have values of 0.93–0.94 mM.

The Br^– profile has a fair amount of scatter because of analytical precision. Nonetheless, the overall consistency of the profile at Site U1425 suggests that meaningful Br^– profiles might be generated using the new ion chromatograph, especially if sites with higher Br^– concentrations are encountered in the future. In terms of interpretation, the increase in dissolved Br^– with depth suggests that modest amounts of organic carbon have decomposed over time (Martin et al., 1993).

Yellowness/Absorbance

A total of 79 interstitial water samples were measured for absorbance at 325 and 227 nm (Table T13; Fig. F47). The totality examined included both IW-Sq (51) and IW-Rh (28) samples taken from three holes (Holes U1425A, U1425B, and U1425D). Because IW-Sq samples were collected from three holes with depths drilled at different times over 4 days, samples were analyzed in several batches. To assess absorbance precision, several samples were examined repeatedly.

As at previous sites during Expedition 346, interstitial water at Site U1425 did not visually appear yellow. Nevertheless, for both wavelengths examined, the absorbance of interstitial water changes systematically downhole (Fig. F47). At 325 nm, absorbance increases from 0.009 in the mudline sample to 0.257 at ~34 m CSF-A. Values then decrease to 0.034 at ~310 m CSF-A. At 227 nm, absorbance increases from 0.092 in the mudline sample to 1.638 at ~34 m CSF-A. Values then slowly decrease to 0.892 at ~310 m CSF-A. Thus, for both wavelengths, there is a steep increase in absorbance to a shallow maximum, as well as a gradual decline in absorbance with further depth, but the decrease is more pronounced at the 325 nm wavelength. However, it is not entirely certain how interstitial water absorbance scales with concentration. Samples should be diluted serially and reexamined, especially at 325 nm, where absorbance exceeds 1.0.

The downhole changes in absorbance broadly correspond to those of alkalinity and PO₄^3– (Fig. F44). This supports the idea that the color of interstitial water derives from some component released during the decomposition of solid organic matter, such as a fraction of dissolved organic carbon (Briucaud et al., 1981; You et al., 1993).

The above reporting on interstitial water color at Site U1425 raises two additional questions. First, for several samples from Core 346-U1425A-4H, the absorbance at 325 nm decreased after several days. Second, when dissolved HgCl₂ was added to several interstitial water samples from Core 4H (during poisoning for shore-based carbon isotope analyses), a yellow-brown color immediately appeared and a black solid precipitated within 2 days. The second change most likely resulted from the following reaction:

If this is the case and although fine-grained HgS precipitates do not exist in the interstitial water naturally, the above observations raise the possibility that, at least for some samples, some of the absorbance at low wavelengths may result from hydrogen sulfide or a fine-grained precipitate thereof.

Splits of several interstitial water samples from Site U1425 were removed for testing of absorbance after time and after experimentation, such as purging with N₂. The results are not completed at this juncture.

Calcium, magnesium, and strontium

The dissolved Ca, Mg, and Sr profiles (Table T12; Fig. F48) have similar shapes to those at several previously drilled sites in the marginal sea (Murray et al., 1992) and other locations (Gieskes and Lawrence, 1981). Mg and Ca concentrations of IW-Sq and IW-Rh samples are similar (Fig. F49). As at previous sites, Sr concentrations of interstitial water from Rhizons are significantly higher than those from squeezers.

Dissolved Ca is 10.0 mM in the mudline sample, which is slightly less than the 10.2 mM expected for JSPW (Table T10 in the “Methods” chapter [Tada et al., 2015b]). From the seafloor, Ca concentrations decrease to ~4.2 mM at ~53 m CSF-A. Deeper than this depth, Ca concentrations steadily increase to ~23 mM at ~400 m CSF-A, the base of the cored hole.

The mudline sample has a dissolved Mg concentration of 51.3 mM, which compares to 52.7 mM inferred for JSPW (Table T10 in the “Methods” chapter [Tada et al., 2015b]). Interstitial water Mg decreases below the seafloor to ~40 mM at ~53 m CSF-A. As observed at previously drilled sites during Expedition 346, the decrease in dissolved Mg concentrations within shallowly buried sediment is about twice the decrease in dissolved Ca concentrations. As also observed at the other drilled sites, Mg concentrations decrease deeper than 53 m CSF-A, reaching ~16 mM at ~400 m CSF-A.

Dissolved Sr is 92.8 µM in the uppermost IW-Sq sample below the seafloor (1.45 m CSF-A), which is slightly higher than the 90.5 µM expected for JSPW (Table T10 in the “Methods” chapter [Tada et al., 2015b]). Deeper than this depth, Sr concentrations slowly rise, reaching 234 µM at ~100 m CSF-A. Values are erratic (but high, >200 µM) from ~100 m CSF-A to the base of the cored hole.

Dolomite formation likely drives the decreases in dissolved Ca and Mg (Sayles and Manheim, 1975; Moore et al., 2004), even though AOM probably does not dominate SO₄^2– consumption at Site U1425. As noted previously (see “Geochemistry” in the “Site U1422” chapter [Tada et al., 2015c]), the key to suggesting dolomite precipitation lies in Ca and Mg flux calculations throughout the upper sediment column (Moore et al., 2004) rather than changes in Ca and Mg concentrations between the seafloor and the minimum in dissolved Ca and Mg. In any case, although AOM may enhance dolomite precipitation (Moore et al., 2004), it is not the proximal cause because all sites drilled so far have compelling evidence for dolomite precipitation, independent of whether AOM appears to be dominant or subordinate. The loss of SO₄^2–, irrespective of cause, may be the primary driver for dolomite formation in the sediment (Baker and Kastner, 1981).

Chlorinity and sodium

Cl^– concentration of the mudline sample is 544 mM, which compares to 544 mM for inferred JSPW (Table T10 in the “Methods” chapter [Tada et al., 2015b]). Below the seafloor, the Cl^– profile (Table T12; Fig. F50) shows anomalously low concentrations (<540 mM) until 77 m CSF-A. This pattern is consistent with those found at Sites U1422–U1424 and supports the idea that deep water of the marginal sea was less saline sometime in the recent past, perhaps during the Last Glacial Maximum. Interestingly, deeper than 50 m CSF-A, most Cl^– values are less than that of present-day bottom water. We speculate that, on average and over the long term, deep water of the sea was less saline than modern JSPW.

The Na profile at Site U1425 (Table T12; Figs. F50, F51) shows considerable scatter, and the mudline sample has a value of 454 mM, which is significantly less than the 468 mM expected for JSPW (Table T10 in the “Methods” chapter [Tada et al., 2015b]). Nonetheless, the Na profile generally shows lower values than present-day bottom water over much of the cored sedimentary sequence, including the upper 50 m below the seafloor.

Potassium

The K profile at Site U1425 (Table T12; Figs. F50, F51) resembles that at other sites drilled to date during Expedition 346. K concentration of the mudline sample is 10.8 mM, which is close to the 10.2 mM expected for JSPW (Table T10 in the “Methods” chapter [Tada et al., 2015b]). Immediately beneath the seafloor, K concentrations increase significantly, a phenomenon observed at all sites. The increase in K is greater in IW-Sq samples (to ~12.9 mM) than in IW-Rh samples (to ~11.5 mM) (Fig. F51). K then systematically decreases with depth, reaching 4 mM at the base of the cored sequence.

The rapid increase in dissolved K beneath the seafloor, consistent at all sites drilled, may represent a temperature effect. When clay-rich cores recovered from cold temperatures at in situ conditions are squeezed at room temperature, several studies have shown that certain ions, especially K⁺, are released to interstitial water (Sayles and Manheim, 1975). In fact, the typical increase in K⁺ concentration for a sample just below the seafloor is ~1.5 mM (Sayles and Manheim, 1975), which is approximately what we observe. If this explanation is correct, the Rhizons may be collecting water at colder temperature, at least for shallowly buried samples, presumably because samples are disaggregated before squeezing. The steady decrease in K toward basement results from alteration with basalt (Murray et al., 1992).

Boron and lithium

The dissolved B and Li profiles (Table T12; Fig. F52) resemble those at previous sites drilled during Expedition 346. Li concentration is 29 µm at the seafloor and increases to 45 µM at ~72 m CSF-A. Concentrations then rise more rapidly, such that Li reaches 201 µM at ~400 m CSF-A.

The B profile shows much scatter, but overall trends can still be recognized. Close to the seafloor, B concentrations range from 600 ± 50 µM. They decrease to range from 500 to 550 µM at ~10 m CSF-A. At deeper depths, B resembles the dissolved Si curve (see next section) as it increases to a maximum concentration of 826 µM and decreases rapidly at the opal-A/opal-CT horizon.

Silica

Dissolved silica (H₄SiO₄) concentrations exhibit a classic profile at Site U1425 (Table T12; Fig. F52). Concentration is 447 µm in the mudline sample, which is at 0.4°C (see “Downhole measurements”), and rapidly increases to >600 µm by 10 m CSF-A. Deeper than this depth, H₄SiO₄ steadily increases to ~1430 µm at 320 m CSF-A, which is at ~34°C, given the geothermal gradient of 104°C/km (see “Downhole measurements”). From ~320 to 360 m CSF-A, concentrations decrease to ~600 µm.

The H₄SiO₄ profile at Site U1425 is similar to those determined at other drill sites in the marginal sea (Murray et al., 1992). Consistent with this work, there appears a relatively straightforward explanation (Kastner et al., 1977; and many others). Biogenic silica begins dissolving on the seafloor and within the uppermost few meters below the seafloor. However, the amount of dissolution is limited by the solubility of opal-A, which is modest but increases with temperature. Hence, dissolved H₄SiO₄ concentrations quickly reach equilibrium with opal-A and slowly increase with depth along the geothermal gradient. At 320 m CSF-A, opal-CT begins to form, and by 360 m CSF-A, H₄SiO₄ concentration is in equilibrium with opal-CT.

Rhizon commentary

Our experimentation with Rhizon samples during Expedition 346 continued at Site U1425. As at previous sites (U1422 and U1424), the use of Rhizons during scientific drilling expeditions shows great potential but also raises some issues. Clearly, over wide concentration ranges, samples taken through conventional squeezing and Rhizons give very similar values for multiple dissolved species (Figs. F40, F41, F45, F49). In detail, however, there remain some issues.

Following the Rhizon campaign at Site U1424, the geochemistry group constructed a detailed plan for Rhizon sampling at Site U1425 before drilling commenced (Fig. F37). It was not clear if the mudline was captured in Hole U1424B, so additional Rhizons were placed at the top of Hole U1424D.

We took several Rhizons from Cores 346-U1424D-4H and 5H to see if the difficult penetration and low water flow encountered at Site U1423 recurred. They did, as these Rhizons also had water flow problems. In addition to lithology, syringe suction capabilities had an effect on extraction time. Newer syringes performed better than preused and acid-washed syringes.

After completing the interstitial water program at Site U1424 (and after collecting samples at Site U1425), we noted several issues when examining concentration data from IW-Sq and IW-Rh samples together. First, for some elements, notably Ba and Fe, samples collected from Rhizons consistently and systematically gave different values than samples obtained from squeezing. The same is true at Site U1425, as evidenced by comparing dissolved concentrations in IW-Sq and IW-Rh samples taken from short depth intervals in the same hole. For example, Ba concentrations in IW-Sq samples are consistently higher than in IW-Rh samples (Fig. F42), and Fe concentrations in IW-Sq samples are generally higher and show more scatter than in the IW-Rh samples (Fig. F40). We have not yet identified the cause of such variance, but we speculate that it may involve tiny particles of perhaps barite or microbially synthesized magnetite passing through the squeezer filter (0.45 µm) and not through the Rhizon porous membrane (0.2 µm). To examine the possibility of a Rhizon blank, we soaked two sets of Rhizons for 1 h, one in deionized water and one in synthetic seawater. We then pulled the water through the Rhizons and analyzed the water by inductively coupled plasma–atomic emission spectroscopy. Concentrations of all elements examined were below detection.

Preliminary conclusions

Similar to previous sites drilled during Expedition 346, the accumulation of organic matter and subsequent microbial diagenesis of this material strongly affect the geochemistry at Site U1425. Sediment in the upper ~200 m below seafloor averages ~1 wt% organic carbon. In the upper 1 m, reactions between this organic matter and metal oxides lead to maxima in dissolved Mn and dissolved Fe, along with increases in HCO₃^–, NH₄⁺, and PO₄^3–. With continued burial, organic matter reacts with dissolved SO₄^2–, which releases additional HCO₃^–, NH₄⁺, and PO₄^3– to interstitial water. Much of the organic matter decomposition at Site U1425 occurs through sulfate reduction, as evident by the high-resolution concave upward SO₄^2– profile that extends to zero concentration at 60 m CSF-A. This consumption of SO₄^2– and production of alkalinity also drives precipitation of authigenic carbonate, probably dolomite, within the uppermost 60 m of the sequence. Deeper than this depth, CH₄ concentrations slowly increase, eventually surpassing shipboard saturation conditions (1 atm) at ~150 m CSF-A. Dissolved H₄SiO₄ concentrations steadily increase below the seafloor until ~320 m CSF-A, where upon they decrease rapidly, presumably because opal-A converts to opal-CT.

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