Previous   |    Next

doi:10.2204/iodp.proc.314315316.124.2009

Inorganic geochemistry

Interstitial water geochemistry

A total of 54 whole-round sections were collected for interstitial water analyses from Site C0002 (31 samples from Hole C0002B; 23 samples from Hole C0002D) (Table T14). Routine samples were collected at a frequency of approximately one whole-round sample per core according to core condition except for the first two cores just below the seafloor (2 samples each from Cores 315-C0002D-1H and 2H). To obtain enough interstitial water for onboard and shore-based analyses, 20 to 40 cm long sections were squeezed.

Sulfate, alkalinity, phosphate, and ammonium

Sulfate concentration decreases linearly with depth from a near seawater value to nearly 0 mM at 9 m CSF. This sulfate gradient of ~3.4 mM/m results in a flux of 1.6 × 10^–3 mmol/cm²/y using Fick’s first law of diffusion with a diffusion coefficient of sulfate of 5.8 × 10^–6 cm²/s at 5°C and sediment porosity of 0.65 (Reeburgh, 1976; Borowski et al., 1996) (Fig. F20). This flux of sulfate should be equal to the flux of dissolved methane from below, similar to the case at Site C0001. However, the flux at Site C0002 is ~1.3 times higher than that at Site C0001. Sulfate concentrations in general are between 1.0 and 4.0 mM in Unit II in samples from Hole C0002B, well above the analytical detection limit (Fig. F20) and concentrations in interstitial water samples from Hole C0002D (generally <0.6 mM) that were retrieved from below the sulfate-methane transition (SMT). Although the potential contamination of drilling fluid during core recovery cannot be precluded because of very low water content in the whole-round core sections from Hole C0002B, observations of high-sulfate fluids in greater sections from previous studies (e.g., ODP Sites 1173 and 1174 and Hole 1176A) may suggest input of such sulfate-rich deep fluids into the cored interval.

Alkalinity and ammonium concentrations reach higher values than those observed at Site C0001. Although these concentrations steeply decrease in Unit II, levels are much higher than those at Site C0001 throughout the entire interval with the exception of one sample at 160 m CSF. Phosphate concentrations are also higher at Site C0002 than at Site C0001. There is a local phosphate maxima associated with the alkalinity maxima, yet there is a deeper more concentrated maximum near the bottom of Unit I. Below this maxima, phosphate concentrations decrease, reaching levels that are below the detection limit of shipboard analyses, ~3.3 µM, at ~600 m CSF (Table T14). Clearly the products of organic matter degradation show a substantial difference between diagenetic processes at Sites C0001 and C0002. These products remain deep within the cored section. For example, alkalinity and ammonium concentrations are 10 and 3.5 mM, respectively, at the bottom layer of Unit II between 800 and 830 m CSF. This lithologic boundary may represent a change in diagenetic rates.

Halogens (salinity, Cl, and Br)

Salinity and chlorine concentrations consistently decrease downhole in Unit I with some samples with substantially lower values in the depth interval between 120 and 200 m CSF. These lower values or dilutions are ascribed to the dissociation of gas hydrate during core recovery (e.g., Hesse, 2003), which are consistent with resistivity logs collected during Expedition 314. Br concentrations are diluted at the same depths as the chlorine data; however, they exceed 1200 µM at 86 m CSF. Given the biophilic nature of bromine, this profile indicates strong inputs from organic sources, probably associated in part with a gas hydrate. Salinity and concentrations of Cl and Br consistently increase downhole in the lower part of Unit II (Hole C0002D), reaching 480 mM for Cl and 950 mM for Br at ~810 m CSF. Below this depth, values slightly decrease toward the bottom of the hole (Fig. F20). At the base of the hole, Cl concentration is 456 mM, which is well below the seawater value (559 mM). In contrast, Br concentration at the base of the hole remains larger than the seawater value (840 µM), reflecting sources from organic matter degradation at depth (Fig. F20).

Major cations

Interstitial water sodium concentrations in Unit I are relatively constant at seawater values (480 mM) in Hole C0002D, except in gas hydrate–bearing sediments. Concentrations drop from the base of Hole C0002D to the first sampled depth in Hole C0002B. Here, there is a slight increase in Na concentrations with depth in the lower part of Unit II, reaching almost steady values around 430 mM at 807 m CSF (Fig. F21). Potassium and calcium concentration profiles are similar to those at Site C0001 in Unit I. Concentrations of these elements in Hole C0002B increase slightly toward ~800 m CSF at the base of Unit II. Below this depth, potassium concentrations decrease to the lowest observed level, ~2 mM. In contrast, calcium concentrations increase to the highest observed level, ~20 mM, at the bottom of Site C0002 (Fig. F21). These changes are attributable to the progressive reaction between interstitial water and clay minerals through Units III and IV or deeper, probably a result of smectite illitization. Similarly, the magnesium concentration profile is very similar to that at Site C0001.

Minor cations

Although silica and boron concentration profiles in Unit I are comparable to those at Site C0001, values are more scattered particularly below Unit II. The silica profile shows a local maxima at ~915 m CSF in Unit III before decreasing in Unit IV (Fig. F21). More pronounced maxima in the same interval are found in the concentration profiles of lithium, strontium, and barium, reflecting the liberation of these minor elements from the clay mineral phase during diagenesis within this interval (Figs. F21, F22). Manganese concentrations are relatively constant with depth, even in Unit III; however, there is a rapid concentration increase within Unit IV below 922 m CSF to >12 µM. In contrast, iron concentration drops to <20 µM in Unit IV (Fig. F22). These distributions may reflect cycling of iron manganese minerals.

Trace elements

As observed at Site C0001, transition metals, except copper, have similar profiles to manganese and iron, depending on the redox condition of host sediments. Molybdenum concentration gradually increases in Unit I with relatively large variation at the boundary between Units I and II and is constant at ~200 nM in Units II and III with manganese and peaks at >830 nM in Unit IV (Fig. F22). Concentrations of vanadium and zinc are largely scattered in Unit I at the same level as observed at Site C0001. Within the deepest Unit IV, Zn and Fe concentrations are depleted relative to the overlying Unit III. In contrast, Mn, Mo, and Cu concentrations are higher in Unit IV relative to Unit III, indicating Zn and Fe removal from and Mn, Mo, and Cu liberation to the interstitial water. Concentrations of rubidium and cesium are well correlated with those of potassium, lithium, and strontium below Unit II (Fig. F22). Maxima of rubidium and cesium are observed at the base of Unit II. Concentrations of these elements decrease in Unit III, resulting from reaction with clay minerals as indicated by other major and minor cation profiles. Lead concentration is constant at greater depths, whereas uranium concentration shows an increase within Unit IV (Fig. F22). Uranium concentrations are sensitive to redox conditions and show a similar trend to that of Mn in this unit.

δ¹⁸O and δD

Although overall variations of δ¹⁸O and δD in Unit I are smaller than those at Site C0001 (Fig. F23), positive excursions are observed at depths where ion concentrations are diluted because of gas hydrate dissociation. Water molecules composing gas hydrates are enriched in ¹⁸O and deuterium (D); therefore, more positive δ¹⁸O and δD values are expected when gas hydrates dissociate during core recovery and sample handling. Trends in δ¹⁸O and δD values with depth in Units II–IV are similar to those at Site C0001, likely resulting from pore water reactions with clay minerals.

Top of page   |    Previous   |    Next