Previous   |    Next

doi:10.2204/iodp.proc.346.110.2015

Geochemistry

Site U1430 is located near two previously gathered piston cores (05PC-21 and KH-77-3 M3). Core 05PC-21 was collected from the Ulleung Basin during a 2005 cruise of the R/V Tamhae II. Radiocarbon data and color coordination indicated fairly rapid sedimentation rates (~4 cm/k.y.) for this core (Lee et al., 2008). Core KH-77-3 M3 was collected from the Tsushima Basin during a 1977 cruise of the R/V Hakuho Maru. Interstitial water data showed a rapid increase in alkalinity (to 30 mM) and fast decrease in SO₄^2– (to 9 mM) over this 10 m core (Masuzawa and Kitano, 1983). The data from these other piston cores were reminiscent of some of the previous Expedition 346 drill sites. For example, over the upper ~20 m at Site U1422, sedimentation rates are ~7 cm/k.y., alkalinity increases to 20 mM, and SO₄^2– decreases to 9 mM (see “Stratigraphic correlation and sedimentation rates” and “Geochemistry” both in the “Site U1422” chapter [Tada et al., 2015c]).

Sample summary

During operations at Site U1430, the geochemistry group collected and analyzed a range of samples. These included (Tables T10, T11, T12) the following:

• 2 mudline (ML) samples from bottom water poured from the uppermost core liner in Holes U1430A and U1430B.

• 24 interstitial water samples from whole-round squeezing (IW-Sq) from Hole U1430A; all were nominally 5 cm long. As at Sites U1428 and U1429, IW-Sq samples were passed through 0.45 and 0.2 µm filters.

• 28 interstitial water samples from Rhizons (IW-Rh) from the top of Hole U1430B.

• 37 sediment samples from the CARB samples.

• 30 headspace (HS) gas samples.

No Vacutainer samples were taken at this site.

Alkalinity, ammonium, and phosphate

Alkalinity, NH₄⁺, and PO₄^3– exhibit similar profiles as at other numerous locations with significant amounts of organic matter degradation (Fig. F33). Alkalinity increases from 2.4 mM in the mudline sample to 26 mM at 24 m CSF-A. The value for the seafloor is close to that measured for bottom water in the region (2.4 mM) (Sudo, 1986), and the increase is slightly steeper than at Site U1422. Below this rapid rise, alkalinity greatly increases to 40 mM at 62 m CSF-A and then remains close to this level until the base of the drilled sequence.

Ammonium concentrations increase gradually from a very low value in the mudline sample (38 µM) to >1440 µM at 34 m CSF-A (Fig. F33). The profile is slightly concave downward, at least over this depth range. At Site U1430, there is a prominent inflection in the NH₄⁺ profile at ~30 m CSF-A, and the molar ratio of alkalinity to NH₄⁺ is ~50:1 at 200 m CSF-A. Alternatively, this inflection may be an analytical artifact of the higher NH₄⁺ values, as the increase from the seafloor may instead increase to an inferred maximum at ~60–70 m CSF-A before decreasing to the bottom of the hole. However, the analyses were all well within the calibration standards, which had a maximum value of 5000 µM.

Phosphate concentrations increase rapidly from below the detection limit in the mudline sample to 79 µM at ~15 m CSF-A (Fig. F33). The rise is greatest over the uppermost 5 m below the seafloor. Deeper than a prominent maximum at 15 m CSF-A, the PO₄^3– profile decreases gradually, with values below 20 µM at 157 m CSF-A.

The profiles reflect a combination of several processes. As occurs at many locations, solid organic carbon, with a generic Redfield composition of [(CH₂O)₁₀₆(NH₃)₁₆(H₃PO₄)], decomposes to release HCO₃^–, NH₄⁺, and PO₄^3– (Froelich et al., 1979, Masuzawa and Kitano, 1983). Alkalinity often approximates HCO₃^– concentrations in the marine environment. Therefore, there are downhole increases in the three profiles. Both HCO₃^– and PO₄^3– can precipitate into (or onto) authigenic minerals, so their concentrations can decrease with depth.

The two general reactions that may contribute significant alkalinity in clay-rich sediment are (above references)

and

The alkalinity of 40 mM deeper than 80 m CSF-A would predict that SO₄^2– is <8 mM in the interstitial water. This prediction is based upon three main points: (1) the 1:2 stoichiometry in Reaction 1, (2) the absence of H₂S odor deeper than 80 m CSF-A, and (3) Reaction 2 only proceeds at minimal SO₄^2– concentrations. Thus, for each 2 moles of HCO₃^– (alkalinity) produced, at least 1 mole of SO₄^2– has been consumed. However, SO₄^2– concentrations greatly exceed the 8 mM prediction, and thus other processes must be acting. This difference between the prediction and measured values will be discussed below.

Calcium, magnesium, and strontium

At Site U1422 and most other sites drilled in the marginal sea (Ingle, Suyehiro, von Breymann, et al., 1990; Tamaki, Pisciotto, Allan, et al., 1990), profiles of dissolved Ca, Mg, and Sr are broadly similar between sites. The profiles of these elements in interstitial water at Site U1430 (Table T10; Fig. F34) continue this trend.

The dissolved Ca concentration in the mudline sample is 9.7 mM, which is slightly lower than the value expected for theoretical Japan Sea Proper Water (JSPW) (10.4 mM) (see “Geochemistry” in the “Methods” chapter [Tada et al., 2015b]). As such, either this sample is influenced by water from beneath the seafloor during sampling or the actual seawater value may be different. Nonetheless, from the seafloor values decrease to ~6 mM between 40 and 55 m CSF-A. Ca concentrations then increase fairly steadily with depth, reaching 13.2 mM at ~242 m CSF-A. This is very similar to the Ca profile at Site U1422, although the concentration at 200 m CSF-A is higher at Site U1430 (11.8 mM) than at Site U1422 (9.2 mM).

The dissolved Mg concentration in the mudline sample is 52.0 mM, which is slightly lower than the value expected for inferred JSPW (53.3 mM) (see “Geochemistry” in the “Methods” chapter [Tada et al., 2015b]). Between 1.45 and ~60 m CSF-A, Mg concentrations decrease to ~48 mM at 19 m CSF-A and then increase to ~54 mM at 62 m CSF-A. At Site U1422, Mg concentrations decreased to 40 mM in the uppermost 30 m, most likely because of dolomite precipitation or perhaps SO₄^2– driven by removal from interstitial water (Baker and Kastner, 1980). Mg concentrations between ~70 and ~242 m CSF-A show a slight decrease to ~49 mM at the base of the cored hole.

The dissolved Sr concentration is 88 µM in the mudline sample, which is lower than the 92 µM that is expected for JSPW (see “Geochemistry” in the “Methods” chapter [Tada et al., 2015b]). Below the seafloor, Sr generally increases, reaching 139 µM at ~242 m CSF-A. An inflection in dissolved Sr seems to occur at 19.05 m CSF-A.

At previous sites drilled in the marginal sea, both during Expedition 346 and during Ocean Drilling Program Legs 127/128, two processes were invoked to explain the primary features in dissolved Ca, Mg, and Sr profiles. Authigenic carbonate formation removes Ca and Mg and releases Sr in shallowly buried sediment. Reactions with ash and basalt remove Mg and release Ca and Sr at depth.

Sulfate

Sulfate concentrations in the mudline are 28.4 mM in Hole U1430A and 28.1 mM in Hole U1430B (Table T10; Fig. F35). These values compare to 28.2 mM expected for JSPW (Table T10 in the “Methods” chapter [Tada et al., 2015b]). From the seafloor, SO₄^2– concentrations decline, expressing a smooth concave downward profile that dips to 13.4 mM at 62 m CSF-A. Deeper than this depth, SO₄^2– concentrations rise to ~17.7 mM at 242 m CSF-A. Earlier in the discussion, we had predicted (based on stoichiometry) that SO₄^2– should decrease with depth to ~8 mM, but values remain at 15 mM, which will be discussed later below.

Volatile hydrocarbons

Methane is the only hydrocarbon gas in the headspace samples at Site U1430. No ethane or heavier hydrocarbons were detected. Compared to CH₄ concentrations at other marginal sea sites drilled during Expedition 346, values at Site U1430 are extremely low, consistently less than ~10 ppmv to the bottom of the hole (Table T11; Fig. F36). The CH₄ concentration is zero in the shallowest sample at 1.5 m CSF-A. Deeper than 1.5 m CSF-A, values increase to a maximum of ~9 ppmv at 81 m CSF-A and then decrease to 3 ppmv at 246 m CSF-A. A sulfate–methane transition does not exist at Site U1430 because both species are clearly present throughout.

Carbonate and organic carbon

A simple explanation for the high interstitial water SO₄^2– values and very low CH₄ concentrations at Site U1430 concerns organic carbon content. Although seemingly conflicting with the modest alkalinity concentrations, low amounts of organic matter would maintain high SO₄^2– concentrations, which would preclude significant methanogenesis. The solid-phase profiles at Site U1430 (Table T12; Fig. F37) make this interpretation very problematic.

Organic carbon contents across the upper ~70 m below seafloor average 1.1 wt% and range from 0.5 to 3 wt%. Deeper than this depth, values increase, averaging 3.3 wt% and fluctuating between 1.7 and 4.5 wt%. For the entire sediment column drilled, organic carbon contents are generally higher at Site U1430 than at other sites drilled during Expedition 346.

The C:N ratio in the upper ~70 m of sediment is relatively low with an average value of ~6. Between this depth and ~242 m CSF-A, the C:N ratio averages 11. The organic matter in the upper sediment is of marine origin, whereas that in the lower sediment appears to be derived from marine and terrestrial sources.

The carbonate contents of sediment at Site U1430 vary significantly. They are relatively high in the upper ~70 m CSF-A, although values fluctuate between 2 and 33 wt%. Deeper than ~70 m CSF-A, carbonate contents of sediment show little variation, except at the very base of the cored sequence. Values are <1 wt% until ~230 m CSF-A. Near the bottom of the site (232–245 m CSF-A), carbonate contents increase to 5 wt%.

The problem and a solution

Despite some similarities to other drill sites of Expedition 346, especially Site U1422, and other sites from many other legs and expeditions, Site U1430 is different. The basic geochemical puzzle at Site U1430 is the combination of (1) interstitial water with fairly high alkalinity, moderate SO₄^2–, and effectively no CH₄ and (2) clay-rich sediment with high organic carbon contents. We think the answer takes into consideration the site’s location and two reactions potentially occurring near the base of the sediment sequence:

and

The first of these reactions represents the dissolution of pyrite, whereas the second represents the dissolution of carbonate. The net result is the generation of SO₄^2–, H⁺, Ca²⁺, and HCO₃^–. The reactions are well known from studies of different environments. Indeed, considerable effort was expended during Expedition 346 to prevent these reactions from occurring after cores are recovered (see “Lithostratigraphy”), as these reactions cause the dissolution of carbonate microfossils during core storage.

In situ Fe sulfide oxidation explains the geochemical puzzle at Site U1430. High SO₄^2– concentrations are maintained at depth, indeed generated at depth, through Reaction 3.

Alkalinity is maintained because of carbonate dissolution, especially near the base of the cored sequence. In fact, carbonate contents at this location, including authigenic phases (see “Lithostratigraphy”), may be very low because of Reaction 4 (and derivatives thereof for other carbonate phases, such as dolomite). Generation of high alkalinity without degrading organic matter also yields the extremely high alkalinity:NH₄⁺ ratio, a ratio much greater than that of the Redfield ratio, the surrounding sedimentary organic carbon, or that observed at the other drill sites. We suggest the evidence for these paired reactions is compelling, both from the geochemistry and the lithology, as elaborated below.

pH

Throughout Expedition 346, we generally have omitted discussion of interstitial water pH. This is because it invariably lies between 7.1 and 8.1 and reflects the combination of many factors, not the least of which is the difficulty of the measurement itself. For example, the pH at Site U1422 ranges between 7.6 and 8.1. At Site U1430, the pH at the bottom of the hole is <6.3. This pH is exceptionally low for interstitial water and is consistent with input of acid.

Manganese and iron

The Mn and Fe profiles (Table T10; Fig. F38) are similar to those at other locations, with the notable exception of peaks near the bottom of the cored sequence. Mn concentrations of the mudline samples are below the detection limit (0.3 µM) in Hole U1430A and 6.76 µM in Hole U1430B. Two local maximum concentrations of Mn are 55 µM in the first Rhizon sample at 0.05 m CSF-A and 18 µM at 0.8 m CSF-A (Table T10). A local minimum of 5 µM at 0.15 m CSF-A separates the two maxima. Below the second maximum, concentrations decrease to <4 µM until 138 m CSF-A. However, toward the base of the sequence, several IW-Sq samples have elevated dissolved Mn concentrations of 6–9 µM.

Dissolved Fe concentrations are below the detection limit (0.9 µM) for the mudline samples. The maximum Fe concentration (202 µM) occurs just below the seafloor in the uppermost Rhizon sample at 0.05 m CSF-A (Table T10). Concentrations decrease to 11 µM at 0.15 m CSF-A before increasing again to 19 µM at 0.3 m CSF-A. Fe concentrations in IW-Rh samples then decrease to below the detection limit at 0.8 m CSF-A, except for a few values just above the detection limit. Fe concentrations in the IW-Sq samples remain above the detection limit until 34 m CSF-A, showing a local maximum of 13 µM at 19.05 m CSF-A. Although Fe concentrations of IW-Sq and IW-Rh samples do not match perfectly, “double filtering” with a 0.45 µm and 0.2 µm filter of the IW-Sq samples may have resulted in Fe concentrations remaining below the detection limit for most deep samples at Site U1430. This contrasts with the scattered “dissolved” Fe records at Sites U1422–U1427, where only a 0.45 µm filter was used. Most crucially, and despite the double filtering, several samples near the base of the cored sequence have high dissolved Fe concentrations. This is consistent with interstitial water coming from sediment where Fe sulfides are being oxidized.

Sediment color

Upon realizing the possibility of deep oxidation of sediment, we examined cores displayed in the Sedimentology Laboratory. Unit III, generally composed of green-gray sediment, has abundant intervals rich in diatoms with relatively high amounts of pyrite (see “Lithostratigraphy”). However, there are also several diatom-rich horizons deeper than 190 m CSF-A where sediments have a burnt red color. We suggest this red location is where pyrite has been oxidized.

Once one realizes that the co-occurrence of high alkalinity and high SO₄^2– at depth at Site U1430 may result from oxidation of Fe sulfides, the remaining constituents measured on the ship are relatively straightforward to understand because they are not affected too much by Fe sulfide oxidation.

Barium

Dissolved Ba concentration is below the detection limit (5 µM) in the mudline samples, as expected for seawater (Table T10). Below the seafloor, Ba is below the detection limit in all Rhizon samples and low but detectable (6–21 µM) in all squeezed samples. The discrepancy in Ba values between sample techniques, identified at previous sites, remains at Site U1430, despite additional filtering when collecting IW-Sq samples. The main point of interest is that the relatively high SO₄^2– concentrations at depth preclude significant dissolution of barite in the sediment column.

Potassium

K concentration is ~10.5 mM in the mudline sample. Just below the seafloor, concentrations increase to 12.0 mM at 1.45 m CSF-A (Table T10). K concentrations generally increase deeper than this depth, such that they reach 12.8 mM at 71.55 m CSF-A. From this depth, K concentrations decrease steadily to the bottom of the hole, reaching 10.1 mM at ~242 m CSF-A.

The increase in K concentrations immediately below the seafloor exists at all sites drilled during Expedition 346 and may reflect a temperature effect or cation exchange during authigenic mineral formation. The decrease in K with depth perhaps results from further reactions with ash and basalt (Murray et al., 1992).

Boron and lithium

At the mudline, B concentrations are 410 µM in Hole U1430A and 422 µM in Hole U1430B (Table T10; Fig. F39). The Rhizon samples show that B concentrations increase to 567 µM at 0.99 m CSF-A and remain near that concentration until 5.04 m CSF-A. Deeper than 5.04 m CSF-A, B begins to decrease to 467 µM at 7.32 m CSF-A. The deeper B profile shows scatter but an overall decrease with depth to 453 µM at ~242 m CSF-A. B may be affected by pH at depth.

Li concentrations at the mudline are 25 µM in Hole U1430A and 27 µM in Hole U1430B. Li concentrations increase steadily and reach a maximum of 254 µM in the deepest sample at 241.55 m CSF-A. The Li profile shares some similarities with the Si profile, including inflection points at 33.55, 81.05, and 194.45 m CSF-A.

Silica

Dissolved Si concentrations were measured by inductively coupled plasma–atomic emission spectroscopy (ICP-AES). Although these values are not as precise as H₄SiO₄ concentrations determined using the spectrophotometer, comparative results at other sites during Expedition 346 suggest that this approach renders a fairly accurate Si profile. Si concentrations in the mudline samples are 175 µM in Hole U1430A and 302 µM in Hole U1430B. Immediately below the seafloor (0.05–1.34 m CSF-A), Si concentrations fluctuate between 490.06 and 781.22 µM. Overall, however, Si concentrations steadily increase downhole, almost reaching 1300 µM in the deepest sample (241.55 m CSF-A) (Fig. F39). There are hints in the sedimentary sequence at the deepest depth of opal-CT (see “Lithostratigraphy”), although the lack of a precipitous decline in dissolved silica suggests that the opal-A/opal-CT boundary was not penetrated at this site (Kastner et al., 1977).

Chlorinity and sodium

Chloride and Na concentrations are 545 and 468 mM, respectively, in the mudline sample from Hole U1430A, which compare to 551 and 473 mM, respectively (Table T10; Fig. F40), in inferred JSPW (see “Geochemistry” in the “Methods” chapter [Tada et al., 2015b]). Interstitial water concentrations of both elements decrease significantly over the upper 50 m below the seafloor and return to concentrations similar to bottom water over the next 185 m. Less saline water in shallowly buried sediment is a common theme across the marginal sea.

Bromide

Br^– concentrations of the mudline samples are 0.83 and 0.81 mM in Holes U1430A and U1430B, respectively. These values bracket the 82 mM expected for JSPW (see “Geochemistry” in the “Methods” chapter [Tada et al., 2015b]). Below the seafloor, Br^– concentrations range between 0.82 and 0.85 mM for the entire sequence (Table T10; Fig. F40). Thus, unlike all sites previously drilled during Expedition 346, there is a complete disconnect between Br^–, alkalinity, and NH₄⁺ concentrations. Bromide concentrations are essentially the same as bottom water at Site U1430 because there is very little organic matter diagenesis.

Preliminary conclusions

Site U1430 ended up being a challenging site to analyze at the end of a cruise with little time before reaching the final port call. It is perhaps the most intriguing site drilled during Expedition 346. All evidence suggests that Fe sulfide oxidation operating in an unexpected environment—the base of an in situ organic-rich marine sediment sequence. This would require a deep source of O₂ to the sediment. One possibility is that water carrying oxygen is flowing along porous diatom horizons or along the sediment/acoustic basement interface. Additionally, water could flow through a thick ash and sand layer in the deepest part of the sequence.

Top of page   |    Previous   |    Next