BIOGEOCHEMISTRY

The interstitial water (IW) sampling strategy at Site 1231 was similar to that at previous Leg 201 sites. A moderately high resolution of three samples per core in Hole 1231B was followed by more detailed sampling in Hole 1231E, targeting intervals of particular geochemical relevance. We collected a total of 67 IW samples from Holes 1231B and 1231E. Geochemical data are consistent with very low microbial activity in lithostratigraphic Units I and II and extremely low activity in Unit III (see "Lithostratigraphy"). In addition, there is evidence of the upward advection of nitrate- and oxygen-containing fluid through the section from the basement.

Alkalinity and DIC have very similar profiles downhole (Fig. F3A, F3B; Table T3). The total amplitudes of alkalinity and DIC variations at Site 1231 are similar to those from Site 1225 and are significantly lower than those from Site 1226. Alkalinity increases from 2.9 mM in the 0- to 0.10-mbsf interval to 3.6 mM at 26.8 mbsf, then is uniform until reaching a small peak of 4.0 mM at 55.3 mbsf. Concentrations decrease gradually to 2.9 mM at the bottom of Hole 1231B. The range in alkalinity values is 1.1 mM. DIC values increase from 2.9 mM in the 0- to 0.10-mbsf interval to 3.4 mM at 16.2 mbsf, then are uniform until reaching a small peak of 3.7 mM at 55.3 mbsf. DIC concentrations then decrease gradually to 2.9 mM at the bottom of Hole 1231B. The range in DIC values is 0.8 mM.

Oxygen measurements were conducted by microelectrode (see "Biogeochemistry" in the "Explanatory Notes" chapter) on Cores 201-1231D-1H, 12H, and 13H (Table T4). Oxygen was detected in the upper 0 to 60 cm of Core 201-1231D-1H and decreased to background levels below that depth interval. The relatively deep penetration of oxygen into the upper 60 cm of the hole is consistent with the low microbial activity of the site. Oxygen was also detected in Core 201-1231D-13H and the bottom section of Core 12H, decreasing with distance above basement, with the shallowest depth of measurable oxygen being 3.8 m from the bottom of the hole. Although these microelectrode data are semiquantitative, they do indicate that oxygen is present in the lowermost section. This oxygen is presumably transported from underlying crustal water.

Concentrations of dissolved nitrate were determined in 23 IW samples from Holes 1231B and 1231E (Table T3; Fig. F3C). Concentrations up to ~18 µM were detected near the bottom of Holes 1231B and 1231E. This finding is similar to observations at Site 1225 and may reflect fluid flow in the basement and possibly in the lower sediment column at Site 1231. A maximum nitrate concentration of 39.3 µM was measured at a depth of 10 cm in Hole 1231E, slightly higher than the maximum concentration of 33.1 µM measured at 0.37 mbsf at Site 1225. Dissolved nitrate was undetectable in Section 201-1231B-5H-5 at 39.25 mbsf and increased gradually with depth along a smooth gradient in Hole 1231B. High-resolution sampling in Hole 1231E revealed a more erratic but still downward-increasing profile of dissolved nitrate with depth. No obvious cause could be identified for the offset between the two profiles from the lower sections of Holes 1231B and 1231E (located ~20 m apart).

The high-resolution dissolved manganese profile (Fig. F3D) shows a steep increase in manganese concentrations from <1 µM near the sediment/water interface to 60 µM at ~6 mbsf. After rising less steeply to 26 mbsf, manganese concentrations increase to 119 µM at ~36 mbsf and level off until ~47 mbsf. Dissolved manganese then declines to 0.28 µM at ~69 mbsf (Sample 201-1231B-8H-5, 135-150 cm). Concentrations remain <0.80 µM from this depth to the base of the hole at 119 mbsf. The dissolved manganese profile differs from those at the other Leg 201 sites by having a broad concentration peak in the upper 60 m of the core. This peak probably results from the microbial reduction of manganese oxide phases. As at Site 1226, the overall dissolved manganese profile may reflect sediment composition and the burial history of authigenic manganese oxides.

The dissolved iron profile (Fig. F3E) can be divided into two main zones. From just below the sediment/water interface to ~35 mbsf, iron concentrations are consistently high (3-35 µM), although, as at Sites 1225 and 1226, with considerable scatter. Dissolved iron concentrations then remain <3 µM to the bottom of the hole, except for a small peak of 5 µM centered at 74 mbsf. Microbial reduction of iron oxides probably occurs in the upper 35 mbsf; the high iron concentrations maintained over a broad zone because of the absence of dissolved oxygen and sulfides. Following work at Site 1226, a possible explanation for the variability in dissolved iron concentrations was the inclusion of solid, iron-rich particles <0.10 µm in size (e.g., magnetite) into some interstitial water samples. To address this possibility, a series of closely spaced samples from Hole 1231E were passed through 0.45-µm (standard) and 0.02-µm filters. However, iron concentrations for all of these dual analyses are identical within analytical precision (Table T5). If solid iron contributes to the variation in dissolved iron, these particles are <0.02 µm in diameter.

Dissolved sulfate concentrations decrease nearly linearly from near seawater concentration at the sediment surface (28.9 mM) to 26.6 mM at 113.75 mbsf (Table T3; Fig. F3F). Slight negative inflections of ~0.7 mM from the linear trend fall close to the lower boundaries of lithostratigraphic Unit II and Subunit IIIA (56 and 65 mbsf, respectively). Two end-member scenarios can be invoked to explain the profile: (1) sulfate is reduced at a very low rate within the sediment column or (2) water depleted in sulfate relative to seawater is flowing through the basement.

Dissolved strontium concentrations (Fig. F3G) show little downhole variance, averaging 87.4 µM with a standard deviation of 1.9 µM. Dissolved lithium concentrations (Fig. F3H) also change only subtly between the seafloor and basement. Lithium concentrations are significantly higher than seawater value in the uppermost sediment (34 µM at 0 mbsf), decrease to 25 µM by ~5 mbsf, and remain between 25 and 29 µM until ~100 mbsf. Below this depth, lithium increases erratically to 40.3 µM ~5 m above basement. Concentrations of barium range between 0.4 and 1.4 µM from 1 to 10 mbsf. Above and below this interval, dissolved barium is <0.4 µM (Table T3; Fig. F3I).

Dissolved sulfide (H₂S = H₂S + HS^-) concentrations are below the detection limit (<0.0002 mM) in all samples analyzed at Site 1231 (Table T3; Fig. F3J).

We analyzed the concentrations of acetate and formate in 12 IW samples from Hole 1231B (Table T3; Fig. F3K, F3L). With the exception of Site 1230, concentrations were generally higher than those at other Leg 201 sites and span a range from 1.3 to 13.5 µM (acetate) and 1.5 to 19.2 µM (formate). The concentrations of acetate and formate are low at the top and bottom of the cored interval, with a broad peak in concentration between 20 and 80 mbsf.

Hydrogen incubations were conducted on 13 samples from Hole 1231B (Table T6). Calculated interstitial fluid concentration of the incubated sediments is shown in Figure F3M. The hydrogen concentrations in the upper 25 m at Site 1231 range from 29 to 102 nM and are the highest recorded during Leg 201. The concentrations measured at 28 mbsf and below are among the lowest. The hydrogen profile is very similar in structure to the dissolved iron profile from Hole 1231B. All incubations were conducted at 4°C. Site 1231 exhibited the lowest and highest hydrogen concentrations of all the sites examined during Leg 201. Concentrations at depths shallower than 43.8 mbsf rose from 6.6 to 102 nM, whereas at greater depths concentrations were between 0.04 and 0.22 nM.

Determination of methane concentrations led to two main observations. First, methane concentrations obtained according to the ODP standard safety protocol are extremely low throughout the sediment column and are analytically indistinguishable from results obtained at Site 1225 from the 20-min and 24-hr extraction procedures (Table T7; see also "Gas Analyses" in "Biogeochemistry" in the "Explanatory Notes" chapter for details on analytical procedure). Second, following prolonged extraction, methane yields from the upper 43 mbsf increased significantly, reaching concentrations >15 µM (Fig. F3N). At the transition to lithostratigraphic Subunit IIB (44 mbsf), methane concentrations abruptly decline to trace levels. The mechanism underlying the binding of methane to the sediment and its distribution in the fluid and solid phase has to be resolved in future work, but our observations are consistent with adsorption of methane on sediment particles.

Ammonium concentrations increase steeply in the upper 5 m of Hole 1231B and reach a maximum of 36 µM at 20 mbsf (Table T3; Fig. F3O). Ammonium declines steadily below this maximum toward the bottom of Hole 1231B, with concentrations of only 0.6-1.2 µM measured at 113.7 mbsf. The overall shape of the profile reflects higher metabolic activity in the uppermost ~20 mbsf of the section.

Phosphate concentrations rise to a maximum of 11 µM at 6.2 mbsf and then decline gradually to fairly constant low values of ~1-2.5 µM in the lower two-thirds of the hole (Fig. F3P; Table T3).

Maximum silica concentrations are present in the top 30 mbsf and reach 688 µM (Table T3; Fig. F3Q). This peak coincides with a high content of fine-grained biogenic silica (opal-A) in lithostratigraphic Unit I (see "Lithostratigraphy"). In Unit II, the biogenic silica is diluted by a higher abundance of siliciclastic silt and clay. In Unit II the presence of zeolites has been detected visually and by XRD (see "Mineralogy" in "Lithostratigraphy"). These zeolites may be a sink for dissolved silica. In the nannofossil ooze of Unit III, the silica is uniformly distributed with concentrations between 300 and 350 µM.

Chloride concentrations are indistinguishable from bottom seawater values near the sediment/water interface and monotonically increase by 2.8% to a depth of 12.2 mbsf (Table T3; Fig. F3R). This trend is due to the upward transport of chloride into low-salinity interglacial seawater. The depth of intrusion of the low-salinity interglacial signature is consistent with an upward advection of ~0.2 cm/yr. A number of other measurements are also consistent with upward advection. These include the nearly invariant strontium profile, the external interval of dissolved nitrate and oxygen in the lower part of the section, and an indication of overpressure at the base of the hole (see "Downhole Tools")