Previous   |    Next

doi:10.2204/iodp.proc.314315316.123.2009

Inorganic geochemistry

Interstitial water geochemistry

A total of 48 whole-round sections were collected for interstitial water analyses (14 samples from Hole C0001E, 16 samples from Hole C0001F, and 18 samples from Hole C0001H) (Table T18). One whole-round section was cut from each core when possible, with the exception of the first core sampled near the seawater/​sediment interface. Here three samples (Core 315-C0001E-1H) were retrieved for interstitial water extraction. To obtain enough interstitial water for shipboard and shore-based analyses, 20 to 30 cm long sections were squeezed from samples retrieved from Holes C0001E and C0001F. Because sediment from Hole C0001H is more consolidated, ~45 cm long sections were used for pore water extraction.

Sulfate reduction and diagenesis of organic matter (alkalinity, phosphate, and ammonium)

The sulfate-methane transition (SMT) is a fundamental diagenetic boundary within many marine sediments. Microbial sulfate reducers utilize sulfate dissolved in interstitial water to oxidize sedimentary organic matter above the SMT. Below the SMT, methanogens actively generate methane as a byproduct of respiration. Microbially mediated anaerobic oxidation of methane at the SMT balances the downward flux of sulfate with the upward diffusive flux of methane following the generalized equation (Reeburgh, 1976; Borowski et al., 1996)

CH₄ + SO₄^2– → HCO₃^– + HS^– + H₂O.

Sulfate concentrations decrease monotonically from the seawater value of ~28 mM to nearly 0 mM at ~14 m CSF at Site C0001. The flux of sulfate to the SMT, which is equal to the methane flux from below, is assessed using an estimate for the sulfate concentration-depth gradient and Fick’s first law of diffusion:

J = D₀ϕ³(δC/δX),

where

• D₀ = diffusion coefficient for sulfate (5.8 × 10^–6 cm²/s at 5°C),
• ϕ = sediment porosity (0.65), and
• δC/δX = sulfate concentration gradient with depth (2.3 mM/m), based on the interval between Samples 315-C0001E-1H-4 and 2H-4.

The calculated sulfate flux is 1.2 × 10^–3 mmol/cm²/y (Fig. F37). Below the SMT, sulfate concentrations generally remain below 0.3 mM with a decreasing trend to the bottom of the hole. Sulfate concentrations that appear above 0.3 mM are likely contaminated with seawater during sampling and extraction procedures. The lack of significant sulfate with depth precludes the possibility of deep-sourced fluids containing sulfate at Site C0001.

Alkalinity and dissolved phosphate concentrations consistently increase to maximum values at 28–37 m CSF (Fig. F37). These increases in concentration result from microbial sulfate reduction of sedimentary organic matter (e.g., Berner, 1980). At greater depths, interstitial water alkalinity decreases, consistent with the removal of alkalinity by carbonate precipitation. This precipitation also affects Ca concentrations. Similarly, phosphate concentrations decrease with depth below the maximum, resulting from the removal from solution by carbonate or oxide phases.

Dissolved ammonium concentrations reach a maximum (~3 mM) between 105 and 175 m CSF, well below the alkalinity and phosphate maxima (Fig. F37). This increase in ammonium is a byproduct of microbial respiration. The decrease in ammonium with depth reflects the uptake of ammonium onto clay minerals by ion exchange, although alkalinity and phosphate concentrations progressively decrease downhole. Particularly in the upper accretionary unit below 200 m CSF, ammonium concentrations remain uniform below the transition between 200 and 225 m CSF.

Halogen concentrations (Cl and Br)

Chloride concentrations increase from a seafloor value of ~554 mM to 557 mM at 9 m CSF. They then decrease to ~545 mM at 100 m CSF at a local minima and then increase nearly monotonically below this interval (Fig. F37). Chloride is one of the most conservative tracers in interstitial water in marginal settings with the exception of hydrate-related processes. The ~4% dilution of the interstitial water at 100 m CSF and the several other anomalously low values at 263, 400, and 440 m CSF likely reflect an input of freshwater from the dissociation of gas hydrate. Although the dissociation of gas hydrate is the most likely freshwater source in this area, no evidence of gas hydrates, such as low-temperature anomalies, bubbling on the core surface, high-resistivity anomalies in LWD records, or recovery of visible gas hydrates, was observed at Site C0001.

Br concentrations in pore water are often affected by diagenetic processes. The decomposition of organic matter at depth liberates bromine to interstitial waters. As noted above in the case of chloride, bromine concentrations are also affected by dilution, presumably from the dissociation of gas hydrate. However, the small changes (~4%) observed above are at the level of the analytical noise in the data, indicating that diagenetic processes dominate in Br interstitial water profiles in this setting. Br concentrations generally increase with depth to 168 m CSF, below which concentrations remain ~1100 mM. This change coincides with the transition in ammonium concentrations from a parabolic shape to a constant value, suggesting that there is a change in the nature of microbial processes at this depth, possibly because of maturation of the available organic materials.

Major cations

Na concentrations increase from ~450 mM to a tight range from 470 to 490 mM. Within this 4% range, concentrations are scattered because of analytical uncertainties; nevertheless, there is a general trend of increasing concentrations with depth (Fig. F38). This increase is partially accounted for by the Cl increase and partially attributed to exchange reactions with clays. A fraction of these exchange reactions could be associated with ammonium. Other exchange reactions with K and Ca are likely. For example, the steady decrease in K concentrations with depth results from exchange reactions with clay mineral phases (Fig. F38).

Ca concentrations decrease in the upper 28 m of the sediment column where sulfate reduction occurs. Here, the extensive production of alkalinity during sulfate reduction results in the precipitation of carbonates and the removal of Ca from interstitial water. Below this depth, Ca concentrations increase; however, the increase is not as great as the decrease observed in the Mg data (Fig. F38). Seawater interactions with volcanics in the absence of high alkalinity generally show a 1:1 exchange between Mg and Ca. Below ~150 m CSF, Mg and Ca data form a linear trend with a slope of –0.74, indicating that even at depth carbonate precipitation occurs, controlling Ca concentrations and alkalinity.

Minor elements

Dissolved silica concentrations increase rapidly in the upper 4 m CSF in response to the dissolution of diatoms (Fig. F38). The steady increase in concentration with depth probably reflects thermal equilibrium with biogenic silicate phases, given the higher solubility at higher temperatures with depth. In contrast, B concentrations decrease systematically with depth, being removed from interstitial water. Li concentrations increase systematically with depth below the Li minima observed at 28 m CSF (Fig. F38). Similar to the case for B, Li concentration-depth profiles are likely governed by reactions with clay minerals.

Mn concentrations increase linearly to 5 µM at 250 m CSF. Below this depth, Mn concentrations increase rapidly to a local maximum of 12.3 µM at 301 m CSF and then decrease to 6.5 µM at 354 m CSF (Fig. F38). Mn concentrations then increase to 12 µM in the lowermost sections at ~440 m CSF. The high Mn concentration in the shallowest sample is likely the result of the dissolution of oxyhydroxide phases, resulting from microbial reactions. The increase in Mn concentrations downcore and the profile shape in general are probably a function of microbial processes and the deposit of Mn-rich carbonates or as-yet undetermined trace mineral phases. Similarly, the Fe concentration-depth profile shows an initial increase from seawater values, presumably resulting from microbial processes. Fe concentrations decrease to 28 m CSF and then increase linearly to ~100 m CSF (Fig. F38). Below 100 m CSF, the overall trend is increasing Fe concentrations with depth, yet there is substantial scatter in the data. This scatter could be related to isolated pockets where iron is liberated from organic material and subsequently reduced and removed into sulfide-rich phases or may be attributed to contamination and oxidation/​removal during sample processing.

Sr and Ba concentrations increase with depth to ~230 m CSF, at which point there is an abrupt change in both concentration-depth trends (Fig. F39). Sr concentrations in the uppermost 8 m CSF are likely controlled by carbonate precipitation in an interval in which the alkalinity rises to its maximum. Increasing Sr and decreasing alkalinity with depth correlate to the apparent scatter in both data sets. These data are consistent with the precipitation of Sr-bearing carbonates, but artifacts caused during sample processing cannot be precluded. Ba concentrations in the upper 9 m are low because barium sulfate is insoluble and any available sulfate rapidly removes Ba from interstitial water. In contrast, beneath the sulfate reduction zone where sulfate concentrations are generally <0.3 mM, Ba concentrations increase, reaching a maximum value of 403 µM at 223 m CSF. This increase with depth is nearly monotonic with the exception of a decrease in a sand layer at 203 m CSF. Beneath the 230 m CSF interval, Ba appears to behave erratically except for the fact that it remains closely correlated with Sr in every interval. Within this interval, sulfate and Ba concentrations are within the same range (~50–300 µM) and there is an inverse correlation between sulfate (from contamination) and Ba concentrations. This correlation suggests that the scatter in the Ba data reflect contamination issues resulting from drilling and sampling procedures.

Trace elements

Concentrations of the transition metals Mo, V, Zn, and Cu were measured by inductively coupled plasma–mass spectrometry (ICP-MS) aboard the ship. Ten of these samples were then analyzed on shore (Table T18). This combined data set of transition metal profiles is affected by redox conditions within the sediment column. For example, Mo concentrations decrease in the upper 9 m, likely caused by microbial processes. Below this interval, the Mo concentration-depth profile is similar to that of Mn and to a lesser degree Fe (Fig. F39). In contrast, Zn concentrations increase with depth similar to those of Mo. The general V concentration-depth trend is one of decreasing concentration with depth to 11 nM at the base of the core. Similarly, the Cu concentration-depth profile is not consistently correlated to any other dissolved species that was measured. Instead, the Cu profile shows a remarkable degree of mobilization to 6160 µM at 253 m CSF, probably influenced by microbial processes.

Rb, CS, Pb, and U concentrations were measured with ICP-MS on board the ship, and 10 samples that included Y were measured ashore. Rb concentrations decrease rapidly in the uppermost 28 m of the sediment section, similar to the K concentration-depth profile (Fig. F39). This initial removal of K results from clay-interstitial water interactions. At greater depths the decrease may reflect thermal equilibrium with clay minerals. Cs concentrations are almost twice the value of bottom seawater. Between 200 and 220 m CSF, Cs concentrations drop from 3.5 to 1.7 nM. A similar but smaller offset is observed in Rb data at this depth. Such gradients, if real, cannot be supported over long periods of time because diffusion tends to smooth such anomalies. Pb concentrations are generally <2 nM but have local maxima with the highest measured concentration of 10.1 nM Pb at 175 m CSF (Fig. F39). Uranium concentrations are depleted overall, relative to bottom seawater values of ~14 nM, with a significant degree of scatter in measured values (Fig. F39). There are numerous distinct increases in U concentrations; the two largest, which increase with the exception of the first sampled depth, are at 150 (6.0 nM) and 217 m CSF (5.5 nM). The mechanism that produces these higher values cannot be determined at this time. Y concentrations in pore water are generally greater than that of seawater (0.3 pM), but no trends are evident in the small data set (Fig. F40).

δ¹⁸O and δD of interstitial water

Both δ¹⁸O and δD values decrease from seawater values (~0‰) to –2.5‰ and –8‰, respectively, at the bottom of Unit I (Fig. F40). These decreasing trends likely result from the interaction of interstitial water with clay minerals. Such decreases, particularly the large decrease in δD values, were attributed to membrane filtration of clay minerals. In contrast, δ¹⁸O values continue to decrease in Unit II; however, δD values remain constant at around –10‰. Throughout this unit there is an increase in the chlorinity, possibly resulting from hydration reactions that may then cause the resulting decrease in the δ¹⁸O values. There is not enough carbonate to have an affect on δ¹⁸O, considering the huge amount of oxygen in interstitial water.

Top of page   |    Previous   |    Next