BIOGEOCHEMISTRY

At Site 1228, 49 interstitial water (IW) samples were collected from Hole 1228A to obtain high-resolution chemical profiles over 189 m, generally at three samples per core unless recovery was limited. Two samples were collected from Core 201-1228B-6H to cover an interval missed in Hole 1228A. Ten samples were collected from Hole 1228C and eleven samples from Hole 1228E to gain additional resolution over the uppermost 6.35 m.

Overall, microbially mediated reactions and the diffusion of brine from depth dominate interstitial water trends, with electron acceptors being supplied from both the top and bottom of the section. Local extremes in IW profiles indicate nonsteady-state conditions over certain depth intervals induced by changes in boundary conditions or differences in sediment composition. Microbial activity is not high enough to completely deplete IW sulfate.

Alkalinity and DIC exhibit similar profiles downhole (Fig. F5A, F5B). Alkalinity ranges from 10.8 mM in the uppermost sample to 18.8 mM at 1.5 mbsf (Table T2). It then decreases to 15.6 mM at 4.5 mbsf and increases to a second maximum of 19.4 mM at 25.3 mbsf. Alkalinity decreases toward the base of the hole, where concentrations are <4 mM. DIC concentrations range from a low of 9 mM in the upper few centimeters below seafloor to 18.7 mM at 2 mbsf. They then decrease to 15.5 mM at 4.3 mbsf and increase to 19.8 mM at 25.25 mbsf. Concentrations below this second maximum decrease toward the bottom of the hole, where concentrations are <4.1 mM. The presence of two maxima in DIC and alkalinity probably reflects nonsteady-state conditions resulting from changes in bottom-water or sediment composition (i.e., see contents of TOC, Table T7, Fig. F6B, both in the "Site 1230" chapter). The convex-upward curvatures at the bottom suggests net removal.

There are two distinct intervals in the dissolved manganese profile (Fig. F5C). From the sediment/water interface to 56.75 mbsf, concentrations of dissolved manganese are low (<0.2 µM). Below this depth, concentrations steadily rise to 8.4 µM at 189 mbsf. Thus, the dissolved manganese profile at Site 1228 lacks the prominent maximum observed at the deep-sea locations, Sites 1225 and 1226, and the scatter observed at Site 1227.

Similar to Site 1227, concentrations of dissolved Fe (Fig. F5D) remain <1.5 µM to a depth of ~112 mbsf. Below this depth, concentrations rise erratically to 44 µM. The dissolved iron profile has an inverse relationship to total dissolved sulfide (H₂S = H₂S + HS^-) (Fig. F5E); the dissolved iron is low where dissolved sulfide is high and vice versa. Most of labile iron hosted in the upper sediment column probably resides as iron sulfides. Dissolution of these phases may occur in the deeper part of the core where total sulfide drops to low concentrations and dissolved oxygen remains undetectable.

The dissolved sulfate profile exhibits features that clearly indicate nonsteady-state conditions and distinct intervals of relatively high rates of microbial respiration (Fig. F5F). Between the sediment/water interface and 1 mbsf there is a steep drop in sulfate concentrations from a seawater value (29 mM) to 17 mM by 1.4 mbsf. The gradient then decreases and is nearly linear until 25 mbsf. Below this depth, the profile reaches a minimum of 2.5 mM at 37.8 mbsf. From 53.8 mbsf to the bottom of the hole, concentrations of dissolved sulfate increase linearly, reaching 31 mM at 189 mbsf. The increase in concentrations in the deeper part of the hole is due to the diffusion of sulfate from a deeply buried brine.

Dissolved sulfide is 0.2 mM between 0 and 12 centimeters below seafloor (Fig. F5E). Sulfide concentrations rapidly increase to 3.8 mM at 1 mbsf. At greater depths, sulfide concentrations continue to increase with increasing depth but with a substantially smaller gradient. The break in the sulfide profile at 1 mbsf mirrors the sulfate profile and coincides with a peak in the concentration of DIC. A peak sulfide concentration of 6.4 mM is present at 28.2 mbsf, approximately the depth of the second maximum in DIC. Below 28.2 mbsf, sulfide concentrations linearly decrease to 0.006 mM at a depth of 132 mbsf. Concentrations of dissolved sulfide remain low (<0.006 mM) through the rest of the drilled section except for one sample at 189 mbsf (0.02 mM).

Interstitial water barium concentrations range between 0.2 and 2.1 µM (Fig. F5G). These values exceed those at deep-sea Sites 1225 and 1226 but are much lower than those at Sites 1227 and 1229. As at all other locations, dissolved barium displays an inverse relationship with sulfate. The lowest barium concentration is present at the sediment/water interface and at the bottom of the hole, where sulfate concentrations are high. The highest barium concentration is present between 28 and 38 mbsf, where sulfate concentrations reach a minimum. The solubility of barite and the distribution of sulfate probably determine the overall barium profile.

Methane was detected in all samples at Site 1228 (Table T3; Fig. F5H). Using the standard ODP headspace technique, methane concentrations are close to 2 µM at the sediment/water interface and increase to a maximum of 8.4 µM at 35-mbsf sulfate minimum. Below this depth, methane concentrations decrease to ~100 mbsf, where they stabilize at ~2 µM. Consistent with observations from the previous sites, the 1-day extraction procedure resulted in higher methane yields than the 20-min extraction used for safety purposes (Table T3). Maximum methane concentration at Site 1228 is several orders of magnitude lower than that at Site 1227. The lower methane abundances at Site 1228 are consistent with the fact that dissolved sulfate is never completely depleted in the interstitial waters at Site 1228.

Ethane was found in very low concentrations at Site 1228. Its highest values coincide with relatively high methane concentrations and low sulfate concentrations. Similar to methane, the vertical distribution of ethane in the sediment column is consistent with its biological production and consumption.

Acetate and formate concentrations were determined in 19 IW samples from Hole 1228A (Table T2; Fig. F5I). Concentrations are in the same range as those at Site 1226 (see "Biogeochemistry" in the "Site 1226" chapter). Acetate concentrations are between 0.5 and 2.5 µM, and formate concentrations are between 0.3 and 2 µM. As at previous sites, the highest concentrations of both compounds are present deeper in the section.

Ammonium concentrations are <100 µM near the sediment/water interface and increase to a local maximum of 2500 µM by 1.4 mbsf (Fig. F5J). This local maximum coincides with the local minimum in the sulfate profile and local maxima in the alkalinity and DIC profiles (Fig. F5A, F5B). Between 2 and 4.5 mbsf, ammonium concentrations decrease to 2000 µM. Below 4.5 mbsf, ammonium concentrations steadily increase to 4000 µM at 34.8 mbsf. After a change in the slope of the profile, ammonium concentrations gradually increase with depth to ~5000 µM at 186.8 mbsf.

Dissolved phosphate concentrations were determined on samples previously analyzed for alkalinity, from which interfering hydrogen sulfide had been removed by acidification and degassing (as detailed in "Biogeochemistry" in the "Site 1227" chapter). Phosphate concentrations rise sharply below the sediment/water interface, from 4 µM in bottom seawater to 47.7 µM at 0.12 mbsf (Fig. F5K). Concentrations of phosphate remain between ~42 and 49 µM in the upper 1.3 m of the sediment column but sharply decrease to 4.5 µM by 4.6 mbsf. The decrease most likely results from the onset of authigenic apatite precipitation. There is a local increase to 9.7 µM at ~8 mbsf. This is followed by a gradual decrease to <3 µM at ~45 mbsf. Except for one sample at 72.75 mbsf, values range between 1.5 and 4 µM in the lowermost 124 m of the drilled section.

Dissolved silica concentrations are 860 µM by 0.2 mbsf and reach 1017 µM by 6.3 mbsf (Fig. F5L). Silica then ranges from ~900 to 1100 µM between 6.3 and 186.8 mbsf. The profile is most likely controlled by biogenic silica solubility.

Chloride increases monotonically downhole to a concentration at 189 mbsf that is nearly twice that of seawater (Fig. F5M). This trend results from the diffusion of chloride from a deeply buried brine. The profile has two linear segments, with an apparent change in slope at 56.7 mbsf, the boundary between lithostratigraphic Units I and II (see "Lithostratigraphy"). The slope change may reflect a difference in diffusivity between the two units. A change in diffusivity is consistent with the measured variation in formation factor and porosity between the two units (see "Physical Properties"). The linear profiles constrain vertical advection velocities to <0.002 cm/yr.

Dissolved strontium concentrations increase from 90 µM near the seafloor to ~400 µM at 189 mbsf (Fig. F5N), a lower overall gradient than at Site 1227. As at Site 1227, however, the gradient changes downhole. The strontium gradient decreases with depth from 2.1 µM/m over the upper 40 mbsf to 1.0 µM/m over the lowermost 40 m of the drilled section.

Dissolved lithium concentrations increase from 27 µM near the seafloor to ~400 µM at the bottom of the hole (Fig. F5O). As with strontium, the lithium gradient is lower at Site 1228 than Site 1227. The downhole enrichments in strontium and lithium relative to chloride are, respectively, ~25%-35% smaller at Site 1228 than at Site 1227. These differences suggest that the brines at the two sites have different compositions.

Hydrogen incubations were conducted on 14 samples from Hole 1228A and 4 samples from Hole 1228E. All incubations were at 11°C (Table T4). Calculated interstitial fluid concentrations of the incubated sediments are shown in Figure F5P and fall in a narrow range, similar to those observed at Site 1227. All concentrations were between 0.16 and 0.95 nM, with most samples between ~0.2 and ~0.4 nM.

Concentrations of TOC were determined by Rock-Eval Pyrolysis (Table T7, Fig. F6B, both in the "Site 1230" chapter). TOC values at Site 1228 are strikingly variable and range from >10% to >0.5%. All values >3% are restricted to the upper 45 mbsf.