| IODP Proceedings Volume contents Search | |||||||||
 | |||||||||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||||||||
|
doi:10.2204/iodp.proc.346.107.2015 GeochemistrySite U1426 is located on the Oki Ridge (see “Background and objectives”) at the position of Site 798 (Shipboard Scientific Party, 1990). It is also proximal to the positions of two relevant piston cores. Piston Core KH-77-3-L4 was taken ~16 km to the northeast, and an extensive array of interstitial water species was measured on this 4.95 m long core to define geochemical environments for deep-sea sediment of the marginal basin (Masuzawa and Kitano, 1983). Piston Core KH-79-3-PC3 was taken ~54 km to the east; several microfossil assemblage and stable isotope records were generated from this 9.93 m long core and used to reconstruct the basin’s late Quaternary history (Oba et al., 1991). Piston Core KH-77-3-L4 shows very steep gradients in certain pore water species. In particular, alkalinity and dissolved NH4+ increases from ~2.1 mM and ~0 µm at the seafloor to ~30 mM and ~1000 µm at 4.95 mbsf, respectively, whereas dissolved SO42– decreases from ~28 mM at the seafloor to 8 mM at 4.95 mbsf. In addition, dissolved HS– concentrations begin to increase at ~3 mbsf. These gradients have been ascribed to decomposition of organic carbon through organoclastic sulfate reduction, expressed as (Froelich et al., 1979; Masuzawa and Kitano, 1983)
Interstitial water profiles generated at Site 798 (Shipboard Scientific Party, 1990) extend the piston core trends deeper. Alkalinity and dissolved NH4+ both continue to increase, exceeding 75 mM and ~8000 µm by 100 mbsf. Dissolved SO42– continues to drop, reaching 0 mM somewhere between 5.75 and 9.85 mbsf. Similar to conclusions reached by Masuzawa and Kitano (1983), the ODP Leg 128 shipboard scientists attributed the changes in alkalinity, NH4+, and SO42– to “bacterial degradation of organic matter primarily by sulfate reduction” (Shipboard Scientific Party, 1990). At Site 798, however, headspace gas concentrations begin to increase deeper than 5.75 mbsf, and numerous gas cracks and gas voids begin to appear deeper than 18 mbsf (Shipboard Scientific Party, 1990). The composition of volatile hydrocarbons is >99.5% CH4 (Shipboard Scientific Party, 1990), which strongly suggests a microbial origin for the gas (Hedberg, 1974). Available evidence suggests extreme CH4 concentrations at Site 798, which leads to an alternative explanation for the geochemical profiles (Burdige, 2011). Rapid accumulation of sediment with a high organic carbon content leads to methanogenesis at depth (Ferry and Lessner, 2008):
CH4 and other products generated at depth then return toward the seafloor through advection, diffusion, or both. However, across a sulfate–methane transition (SMT) within the upper few meters of sediment, CH4 reacts with SO42– via anaerobic oxidation of methane (AOM) (Reeburgh, 1976; Borowski et al., 1996):
In summary, the rise in alkalinity and NH4+ and the loss of SO42– with depth may have little to do with bacterial degradation of organic matter by sulfate reduction. Instead, they reflect the combined effects of deep methanogenesis, shallow AOM, and mass transport of dissolved components (e.g., Burdige, 2011; Chatterjee et al., 2011). The distinct potential pathways for carbon flow at Site 798 can be distinguished through flux-based calculations, numerical modeling, and detailed interstitial water profiles across the upper 20–30 m of sediment (Moore et al., 2004; Snyder et al., 2007; Chatterjee et al., 2011). As good low-resolution interstitial water profiles were generated over 517 m at Site 798 (Shipboard Scientific Party, 1990), we modified our basic Expedition 346 geochemistry sampling program (see “Geochemistry” in the “Methods” chapter [Tada et al., 2015b]) significantly for Site U1426. Our strategy was to collect a minimal number of samples from deep holes (Holes U1426A and U1426C) but gather a large number of samples from a short “geochemistry” hole (Hole U1426B). Our sampling and analytical effort could then be placed toward addressing six questions, which arise from considerations of AOM but cannot be answered with existing data at Site 798 (Fig. F27):
Sample summaryThe geochemistry team collected and analyzed a range of samples, especially targeting the shallow sediment recovered in Hole U1426B. The samples included the following (Tables T10, T11, T12, T13, T14, T15):
Carbonate and organic carbonThe distance between samples for solid-phase analyses varies considerably with depth at Site U1426. Nevertheless, the carbonate, carbon, and nitrogen profiles (Tables T10, T11; Fig. F28) are consistent with those produced at Site 798 (Shipboard Scientific Party, 1990). Sediment at Site U1426 contains relatively high amounts of CaCO3 (up to 28.6 wt%) compared to material recovered at other sites cored in the northern portion of the marginal basin during Expedition 346 (e.g., Fig. F25 in the “Site U1424” chapter [Tada et al., 2015d]). For lithologic Unit I (0–283.78 m CSF-A) (see “Lithostratigraphy”), carbonate contents are highest in Subunit IA and seem to decrease in Subunit IB. The upper 35 m CSF-A in Hole U1426B was examined at a much higher sample resolution. This suite of 23 samples averages 8 wt% carbonate with wide variance from 0.8 to 28.6 wt%. The carbonate content of Unit II (283.78–396.7 m CSF-A) (see “Lithostratigraphy”) is <3 wt% except for one value of 12 wt% at ~295 m CSF-A. The relatively high carbonate values at Site U1426 (i.e., typically >5 wt%) are consistent with micropaleontological observations (see “Biostratigraphy”), which indicate moderate abundance of calcareous microfossils such as foraminifers and nannofossils. Total organic carbon (TOC) contents average 1.9 wt% at Site U1426, which is high for marine sediment (Müller and Suess, 1979; Emerson and Hedges, 1988). The variance in TOC contents is also relatively small, ranging between 0.7 and 2.5 wt%. However, the higher resolution sampling of Hole U1426B values alternate between intervals of high TOC contents (5–15 and 25–30 m CSF-A) and low TOC contents (3–5, 15–25, and 30–35 m CSF-A). From the top of Unit II to the base of the drilled site, TOC contents average ~1.5 wt% and decrease slightly with depth. We could not address the origin and preservation state of the sedimentary organic matter because the Source Rock Analyzer (Rock-Eval) remained inoperable. The mean value of total nitrogen (TN) is relatively low at 0.28 wt%. The maximum content (0.38 wt%) is in the uppermost sample, whereas the minimum content (0.17 wt%) is at 32.3 m CSF-A. The downhole trend in TN follows that for TOC, except in the middle portion of the record (100–260 m CSF-A). The TN and TOC values of the high-resolution samples from Hole U1426B have a strong positive relationship (r2 = 0.96). We note an issue with TN measurements at Site U1426. Compared to results at Site 798 (Shipboard Scientific Party, 1990), samples from Site U1426 appear consistently richer in TN. However, the difference does not exceed the error for TN analysis within the present shipboard Chemistry Laboratory (±0.06 wt%, 1σ, at Site U1426, as determined by repeat analyses of an internal standard). Thus, any discrepancies could relate to different analytical procedures and equipment. For example, the furnace temperature in the laboratory differed between the two cruises (1000°C for Leg 128 and 950°C for Expedition 346). The C:N values at Site U1426 are probably more reliable than for previous sites drilled during Expedition 346. This is because the higher organic matter contents reduce the influence of analytical error (calculated as ±0.17% for C). For sediment at Site U1426, the C:N (atom) ratio averages 7.3 and varies between 4.1 and 11.1. Manganese and ironThe dissolved Mn and Fe profiles at Site U1426 (Table T12; Fig. F29) are somewhat similar to those generated at previously drilled sites during Expedition 346. This includes apparent offsets in Fe concentration between IW-Sq and IW-Rh samples. The distinguishing aspect of Site U1426 is that the Mn and Fe concentration maxima in shallowly buried sediment lie very near the seafloor. Dissolved Mn concentration is 6 µM in the mudline sample. This is expectedly low, although higher than at previous sites. The uppermost interstitial water sample, from 0.05 m CSF-A below the apparent seafloor, has an Mn concentration of 90 µM. The maximum Mn concentration, 100 µM, occurs at 0.15 m CSF-A (Fig. F29). Deeper than 0.15 m CSF-A, Mn concentrations steadily decrease, falling below the detection limit (0.6 µM) at 6.8 m CSF-A. Dissolved Mn concentrations remain near zero for deeper samples, although we have not yet examined samples deeper than 34.7 m CSF-A. At Site 798, dissolved Mn concentration was 28 µM at 1.45 mbsf and 7 µM or less deeper than 4.5 mbsf (Shipboard Scientific Party, 1990). Dissolved Fe is below the detection limit (1 µM) in the mudline sample, similar to that at previously drilled locations. The maximum dissolved Fe concentration of ~30 µM is found in the shallowest sample at 0.05 m CSF-A. Downhole from this depth, dissolved Fe decreases steeply to below detection limit at 0.4 m CSF-A. Dissolved Fe concentrations between 0.4 and 34.7 m CSF-A vary according to sampling method. For IW-Sq samples, Fe concentrations are generally low but erratic (0–27 µM), whereas for IW-Rh samples, Fe concentrations are consistently below the detection limit. The shallowest Rhizon was inserted at 0.05 m CSF-A in Hole U1426B. Unfortunately, after correlation with other holes at this site, the depth of this sample appears to be at 0.32 m CCSF-A (as defined in the “Methods” chapter [Tada et al., 2015b]) (0.05 m CSF-A). Thus, the top of Hole U1426B may be missing or compressed. This may explain the absence of clearly defined metal peaks in uppermost sediment, including the double peaks in dissolved Fe found at Sites U1422, U1424, and U1425. Alternatively, the extremely high organic matter flux at this location leads to very rapid dissolution of metal oxides, and even the Rhizon sampling is not close enough to resolve distinct Mn and Fe peaks in interstitial water. Dissolved Mn concentrations decrease below the detection limit at approximately the same depth as a major inflection in the alkalinity profile (below). In any case, alkalinity is very high at Site U1426, and formation of Mn-bearing carbonates, such as rhodochrosite, may be removing Mn from interstitial water. This is discussed in previous site chapter geochemistry sections. The Fe maximum in the upper 1 m below the seafloor can be explained by microbially mediated reactions reducing Fe oxides during degradation of organic matter (e.g., Froelich et al., 1979). Interestingly however, and similar to other sites examined during Expedition 346, the Fe maximum occurs at a shallower depth than the Mn maximum. This is different than expected from the canonical model for oxidation of organic matter, which has Fe oxides being used as an electron acceptor after Mn oxides are no longer available (Froelich et al., 1979). The Fe story may be complicated because of the presence of several different Fe-bearing solids and because we have not identified the origin of different Fe concentrations in closely spaced IW-Sq and IW-Rh samples. We elaborate on this matter at the end of this section. Alkalinity, ammonium, and phosphateThe profiles of alkalinity, dissolved NH4+, and PO43– at Site U1426 are consistent with those at Site 798 (Shipboard Scientific Party, 1990). All three constituents increase from low values in the mudline sample to very high concentrations at depth (Table T12; Fig. F30). The maximum alkalinity, NH4+, and PO43– concentrations are ~84 mM, 11,400 µM, and 297 µM, respectively. Clearly, at this location microbes consume large amounts of solid organic carbon, which releases HCO3–, NH4+, and PO43– to interstitial water. Over the upper 15 m, numerous IW-Sq and IW-Rh samples were examined for alkalinity and NH4+ (Fig. F31). In general, the two sampling techniques give similar values. Subtle differences in values between the two techniques are not considered meaningful because it is difficult to evaluate the cause without systematically reconstructing the time history of sampling and analyses. The same is true with the scatter in the IW-Rh samples. In particular, 23 Rhizons were inserted into Core 346-U1426B-2H at almost the same time. However, the collected water samples were analyzed individually and randomly over almost an entire day afterward. The important aspects of the high-resolution interstitial water alkalinity and NH4+ profiles (Fig. F31) are twofold. The alkalinity profile has an obvious concave downward inflection at 8 m CSF-A. The NH4+ profile increases almost linearly. As outlined in the overview, both observations support focused consumption of CH4 and SO42– across a thin depth horizon. Volatile hydrocarbonsAs expected from descriptions of sediment recovered at Site 798 (Shipboard Scientific Party, 1990), significant gas expansion has impacted most sediment cores at Site U1426. Gas expansion cracks began at ~18 m CSF-A, and many gas voids appeared deeper than this depth. Methane was the dominant hydrocarbon gas (>99.9%) in all HS samples from Site U1426 (Table T13). The same is true for VAC gas samples (Table T14). The headspace CH4 profile at Site U1426 (Fig. F32) has several components. CH4 concentrations are <30 ppmv over the upper 4.5 m CSF-A. Between this depth and 7.7 m CSF-A, CH4 concentrations begin to rise rapidly. CH4 concentration is 2,300 ppmv at 7.7 m CSF-A and climbs to a maximum of 36,000 ppmv at 13.7 m CSF-A. Deeper than 13.7 m CSF-A, CH4 values rapidly decrease to 11,900 ppmv by ~21–30 m CSF-A and continue to decrease, albeit with high scatter, to the bottom of the drilled site. Ethane concentrations in HS samples from Hole U1426A range between 0 and 40 ppmv and generally decrease with depth. However, the decrease in C2H6 concentration is less rapid than for CH4. Consequently, the C1/C2 ratio generally decreases with depth (Fig. F32). The ratio is more than 1000 across the upper 30 m CSF-A but rapidly decreases to 400 at ~100 m CSF-A. Deeper than this depth, the C1/C2 ratio gradually decreases, dipping to 124 at the base of the drilled site. Gas concentrations were determined using the HS technique at Site 798. Between 12 and 35 mbsf, CH4 concentration is much higher (50,000–90,000 ppmv) than that measured at Site U1426. Accordingly, the measured C1/C2 ratio of HS samples is much higher in shallowly buried sediment at Site 798. Here it is important to emphasize that, once the solubility of a gas has been surpassed during core recovery, gas concentration determined by the HS technique is no longer easily interpretable (e.g., Paull et al., 2000). Methane concentrations in cores from this location are probably greater than solubility at 1 atm pressure by 20 m CSF-A. Indeed, this is why gas cracks and gas voids begin to occur in sediment cores. The most important aspects of the HS gas profiles are that the C1/C2 ratio is very high and that concentrations increase rapidly at approximately the same depth as the inflection in alkalinity. The first observation strongly suggests that CH4 generated at depth has come through fermentation, as expected for microbial fermentation. The second observation strongly suggests that an upward flux of CH4 drives AOM at ~8 m CSF-A. Sulfate, sulfide, and bariumThat AOM occurs across an SMT at ~8 m CSF-A can be demonstrated with profiles of dissolved SO42– and HS– (Table T12; Fig. F33). Dissolved SO42– is 28.1 mM in the mudline samples, which is close to the 28.2 mM expected for Japan Sea Proper Water (JSPW) (Table T10 in the “Methods” chapter [Tada et al., 2015b]). From the seafloor, SO42– rapidly decreases to ~3.2 mM at ~8 m CSF-A and then to ~2.0 mM at 13 m CSF-A. The initial decrease is almost linear and terminates at the depth of the alkalinity peak. Deeper than 12 m CSF-A, SO42– concentrations are below the detection limit. By contrast, dissolved HS– concentration is below the detection limit (~5 µM) at the seafloor, but forms a maximum in shallowly buried sediment. Starting at ~1.1 m CSF-A, HS– concentrations rapidly increase, reaching a maximum value of 6032 µM at ~8 m CSF-A, or just deeper than the depth of the SMT. Deeper than ~8 m CSF-A, HS– concentrations decrease to below the detection limit at ~35 m CSF-A. The linear decrease and kink in the dissolved SO42– profile strongly indicate major consumption of SO42– at the SMT. The maximum in dissolved HS– strongly indicates production of HS– at the SMT. In theory, the loss of SO42– at the SMT should equal the production of HS–. However, some amount of sulfide escaped as H2S gas during core recovery. Nevertheless, the coincident inflections of alkalinity, CH4, and SO42– and the peak in HS–, all at ~8.0 CSF-A, are overwhelming evidence for AOM at this location. A sharp SMT in shallowly buried sediment of deepwater depth should affect sedimentary barium cycling significantly (Dickens, 2001). The reasons have been provided in other site chapters and in several publications, as cited previously. The dissolved Ba profile (Fig. F34) suggests this to be the case at Site U1426. Ba concentrations of the mudline sample and Rhizon samples from the upper 6.5 m (CSF-A) of sediment are <13 µM. Deeper than 7 m CSF-A, dissolved Ba begins to increase, reaching 1200 µM at ~34 m CSF-A. Dissolved Ba concentrations were measured on samples from Site 798, although at low resolution (von Breymann et al., 1992). These authors show an increase in dissolved Ba starting somewhere shallower than 9.9 m CSF-A and 121 µM at ~37 m CSF-A. Dissolved Fe concentrations in the Rhizon samples decrease to below the detection limit within 50 cm CSF-A shallower than the increase in HS– concentrations. This was expected because the solubility of FeS minerals is extremely low (Schippers and Jørgensen, 2002). The answers to the six questions outlined above are “yes” and were predicted before drilling commenced. This is because the chemistry of interstitial water at Site U1426 appears similar to that at other drill sites with substantial amounts of CH4 at depth and a sharp SMT in the upper 10 or so meters below the seafloor. An interesting example is ODP Site 1230 on the Peru margin (Shipboard Scientific Party, 2003). At this site, a prominent SMT occurs at 8 mbsf, and the alkalinity, NH4+, CH4, SO42–, HS–, and Ba profiles are all fairly comparable to those at Site U1426. Three other characteristics of interstitial water chemistry at Site 1230 are high Br– concentrations, yellow interstitial water, and shallow carbonate precipitation. We discuss the similarities and their causes in the “Site U1427” chapter (Tada et al., 2015f) but document these characteristics below. BromideThe new ion chromatograph in the Geochemistry Laboratory allows determination of dissolved Br– concentrations. However, the precision of Br– on this instrument appears to be ~0.02 mM. Prior to Site U1426, the range in dissolved Br– with respect to depth at sites drilled during Expedition 346 has been only minimally greater than this precision. As a result, downhole Br– profiles have shown considerable scatter (e.g., “Site U1422” chapter [Tada et al., 2015c]). By contrast, the Br– profile is fairly clean at Site U1426 (Table T12; Fig. F35) because the concentrations are significantly higher than those at previous sites. Br– concentration of the mudline samples is 0.83 mM. This is comparable to the 0.84 mM inferred for JSPW (Table T10 in the “Methods” chapter [Tada et al., 2015b]). Below the seafloor, dissolved Br– steadily increases, reaching 1.25 mM at 421 m CCSF-A. Such Br– concentrations are much higher than at most drill sites and signify substantial degradation of organic matter over time (Gieskes and Mahn, 2007). Absorbance/YellownessA total of 99 interstitial water samples collected at Site U1426 were analyzed for their “yellowness” by measuring absorbance at two wavelengths: 325 and 227 nm (Table T15). Three samples were also measured twice to assess reproducibility. Absorbance of interstitial water samples, at least for the wavelengths examined, changes significantly with depth at Site U1426 (Fig. F36). Similar to previously drilled sites during Expedition 346, absorbance is very low in the mudline sample but rises rapidly beneath the seafloor. Absorbance reaches a maximum for both wavelengths by 30 m CSF-A, although the peak is shallower for measurements at 227 nm. For both wavelengths, the absorbance profile is smooth, except for several outliers. Similar to observations at Site U1425 (see “Geochemistry” in the “Site U1425” chapter [Tada et al., 2015e]), absorbance at 325 nm also decreases more rapidly with depth than at 227 nm. The yellowness of marine water has been ascribed to dissolved organic carbon, as noted in other chapters. In general, absorbance of interstitial water at Site U1426 is greater than that of samples from Site U1425 but less than that of samples from Site U1427. This is similar to the relative concentrations of alkalinity, NH4+, and Br– in interstitial water of these three sites. Calcium, magnesium, and strontiumInterstitial water chemistry data from Site 798 depicts the overall deep hole trends of Ca and Mg. Thus, for Expedition 346 we focused on producing higher resolution data in the upper 40 m, with a few analyses performed at deeper depths (Fig. F37) to match Site U1426 data with Site 798 data (Table T12). Ca concentrations decrease from ~10 mM near the seafloor to ~5 mM at 10 m CSF-A (Fig. F38). Ca concentration continues to decrease, reaching a minimum from ~35 to ~50 m CSF-A. Deeper than 50 m CSF-A, calcium concentrations begin to increase gradually, returning to ~10 mM at 393 m CSF-A, the deepest sample recovered (Fig. F37). The Mg concentration profile has a similar shape to Ca in the upper 10 m but then shows an opposite trend deeper than 10 m CSF-A (Figs. F37, F38). Mg concentration is ~52 mM near the seafloor and decreases to ~45 mM at 8 m CSF-A. Unlike the continued decrease observed in Ca concentrations, Mg concentrations increase, reaching a relative maximum of ~52 mM between ~35 and ~90 m CSF-A. Mg concentrations then gradually decrease with depth to ~14 mM at 393 m CSF-A. Data from Site 798 show that the increase in Ca and the decrease in Mg at depth continue until ~450 mbsf, where the concentrations are equal. The deeper trends are driven by uptake of Mg and release of Ca during reaction with basalt. The removal of Ca and Mg at shallower depths suggests the presence of dolomite formation. Chlorinity and sodiumAlthough at very low resolution, the Cl– profile at Site U1426 (Table T12; Fig. F39) exhibits aspects observed at all Expedition 346 sites drilled so far during this expedition. Cl– concentration of the mudline sample is 542 mM, which compares well to 545 mM inferred for JSPW (Table T10 in the “Methods” chapter [Tada et al., 2015b]). Below the seafloor, Cl– concentrations are generally lower than that of present-day bottom water, and the lowest value (526 mM) is found between 10 and 40 m CSF-A. The Cl– profile appears to resemble that constructed at Site 798 (Shipboard Scientific Party, 1990), although neither are very detailed. Sodium concentrations at this site show scatter, but overall trends are still apparent. At the mudline, concentration is between ~460 and 480 mM and decreases to 420–440 mM at 5 m CSF-A. Concentrations begin to increase gradually deeper than 5 m CSF-A to between 485 and 495 mM in the deepest samples (Fig. F39). The cause and importance of low-salinity interstitial water have been discussed in previous chapters of this volume. PotassiumPotassium concentration is ~10.5 mM at the seafloor (Table T12; Fig. F39). There is a slight increase in K concentrations between 1 and 4 m CSF-A to ~12 mM; however, the K maximum is not as prominent as at other sites drilled during Expedition 346. The continuing downcore trend shows K concentrations increasing to 15 mM at 295 m CSF-A. The deepest samples analyzed show a decrease in K concentrations to 12.4 mM at 393 m CSF-A, and comparing these data with values from Site 798 shows that K concentrations continue to decrease to ~7 mM at ~500 mbsf. The abrupt increase in K concentration just below the seafloor has been observed at other sites and has multiple possible origins. It could be from mineral dissolution or cation exchange reactions involving ammonium or other phases. It also could represent an effect of temperature (Sayles and Manheim, 1975). Variations in K concentrations throughout the sediment column suggest a change in clay mineralogy with transformation of smectite to illite or other diagenetic reactions incorporating potassium. The decline of potassium concentrations at depth is likely from the uptake of potassium during basalt alteration (Fig. F39). Boron and lithiumThe dissolved B profile in Hole U1426B (Table T12; Fig. F40) shows limited variation but considerable scatter over the upper 40 m. B concentrations range between 415 and 630 µM, but there are no real trends in the data. The dissolved Li profile (Fig. F40) has an overall concave upward curvature. In general, Li concentrations increase from 26 µM at the mudline to 52 µM at 34.7 m CSF-A (37.5 m CCSF-A). This is similar to observations made at Site 798 (Shipboard Scientific Party, 1990). However, the high-resolution Li concentrations reach a minimum of ~22 mM near the SMT. SilicaDissolved H4SiO4 was not analyzed using the spectrophotometer because the basic profile is already known from work at Site 798 (Shipboard Scientific Party, 1990). However, dissolved Si was measured by inductively coupled plasma–atomic emission spectroscopy (ICP-AES), although only on samples from Hole U1426B to ~34 m CSF-A. The dissolved Si profile (Table T12; Fig. F40) gives an interesting perspective to the discussion. The mudline sample has a dissolved Si concentration of 168 µM. By 0.05 m CSF-A (0.32 m CCSF-A), however, Si concentrations devilishly jump to 666 µM at 0.05 m CSF-A. Deeper than the uppermost 1 m, dissolved Si gradually rises, although with apparent oscillations between that range from ~567 to 807 µM. Data from Site 798 show Si values in this range for the upper 40 m (Shipboard Scientific Party, 1990). They also show Si concentrations increasing as high as 1700 µM until ~450 mbsf, where concentrations rapidly decline at the opal-A/opal-CT boundary. Rhizon commentaryTwo problems were encountered with Rhizon sampling of cores from Hole U1426B. First, water flow into Rhizons decreased significantly for many intervals in Cores 346-U1426B-3H and 4H. Water flow typically decreases with sedimentary depth, presumably because of sediment compaction, reduced porosity, and lower permeability. However, only 2 of 20 Rhizons inserted into Core 346-U1426B-4H extracted >9 mL of water after 4.5–5 h. We think that voids from gas expansion, drill holes into the core liner to relieve gas pressure, or both, may have slowed water flow considerably. It was also later recognized that using new syringes provided a better suction and reduced extraction time. Second, and as also discussed for Site U1425 (see “Geochemistry” in the “Site U1425” chapter [Tada et al., 2015e]), the current Geochemistry Laboratory is not equipped at present to handle very large numbers of water samples arriving over a short time interval. Thus, 120 water samples arriving from Hole U1426B within <5 h proved to be challenging but not impossible. A recurring issue during Expedition 346 is the concentration discrepancy for some species between IW-Sq and IW-Rh samples. Consistent offsets are found in the measured concentrations of certain dissolved metals from samples closely spaced in depth. One possibility is that the squeezers, the Rhizons, or both, modify the chemistry through contamination. To assess this possibility, a series of blanks were prepared and included with samples in ICP-AES runs. A squeezer blank was prepared by partially filling a squeezer with 18.2 MΩ water, letting the water sit for 1 h, and hand cranking the piston to press water through an interior filter, a wire mesh, a 0.45 µm filter, and into a syringe. Three Rhizons were soaked in an acid-cleaned beaker filled with 18.2 MΩ water for 1 h and then used to pass 10 mL of the water into a syringe. Three additional Rhizons were soaked in an acid-cleaned beaker filled with synthetic seawater (NaCl + H2O) for 1 h and then used to pass 10 mL of this fluid into a syringe. Two of the three 18.2 MΩ water Rhizon blanks and two of the three synthetic seawater Rhizon blanks were acidified with nitric acid, exactly like samples are prepared. All these blanks, plus a pure 18.2 MΩ water blank and two pure synthetic seawater blanks were then stored in the cryogenic vials used for samples and eventually analyzed by ICP-AES along with interstitial water samples. The concentrations of all elements analyzed were below detection for both blank squeezer and blank Rhizon samples. Preliminary conclusionsSediment at Site U1426 is characterized by relatively high contents of organic carbon (~2 wt%) and carbonate (up to 28.6 wt%) through the upper few hundred meters below the seafloor. Rapid increases in dissolved Mn and Fe over the uppermost few meters indicate reactions between the organic matter and metal oxides. Deeper in the sediment column, other reactions involving organic carbon, especially fermentation, lead to very high concentrations of alkalinity, ammonium, and phosphate. Microbial methanogenesis also produces very large amounts of CH4. The low C1/C2 ratio and the occurrence of propane at depth suggest possible thermogenic gas near the bottom of the drilled site. However, the effects of degassing on measured gas concentration needs serious consideration. High CH4 concentrations at depth lead to an upward flux of CH4, which reacts with dissolved SO42–, diffusing downward from the seafloor through AOM. The consequence is a prominent SMT that occurs at ~8 m CSF-A. A major convex upward inflection in alkalinity, a constant concentration gradient in NH4+, and a peak in dissolved HS– at this depth all indicate that the SMT indeed derives from AOM. Intense microbial activity and shallow methane cycling impacts other interstitial water chemistry at Site U1426. Samples have high Br– concentrations and yellow color at depth. Addition of alkalinity at the SMT drives authigenic carbonate precipitation. In many regards, Site U1426 resembles other drill sites with high CH4 concentrations at depth. The profiles of Cl– and dissolved Si are similar to those at other sites in the marginal sea and likely reflect the influence of fresher bottom water in the past and silica diagenesis, respectively. |
|||||||||
