BIOGEOCHEMISTRY

Site 1229 is characterized by the presence of a deep brine that introduces sulfate at depth. The interstitial water (IW) sampling scheme for Site 1229 was designed to recover dissolved components at high spatial resolution along the transition from seawater to subsurface brine. Additional sampling was targeted at key biogeochemical intervals including the uppermost sediment column and two sulfate-methane transitions.

A total of 106 IW samples were obtained from Site 1229. From Hole 1229A, 52 samples were collected at an average resolution of two to three per core, except in Cores 201-1229A-9H, 10H, 11H, and 12H, where resolution was increased to five to seven samples per core (about one per section), and in Cores 6H and 7H, where no samples were recovered between 42.9 and 60.2 mbsf. Sampling in Hole 1229D included a bottom-water sample (WSTP) and high-resolution coverage (five to seven samples per core) of the first five cores between 0 and 40.7 mbsf. Two samples per core were collected between 40.7 mbsf and the bottom of the hole at 107.6 mbsf, except in the case of the intervals between 77.8 and 84.3 mbsf and 96.8 and 106.3 mbsf, where samples were not available.

As at previous Leg 201 sites, we determined concentrations of important electron donor/acceptor species and microbial metabolites including volatile fatty acids (acetate and formate), methane, ethane, propane, hydrogen, ammonium, phosphate, DIC, sulfate, iron, and manganese.

Alkalinity and DIC have similar profiles. As at Site 1228, alkalinity and DIC have two maxima, one between 1 and 2 mbsf and a second maximum at ~30 mbsf. Recognition of the maximum between 1 and 2 mbsf was achieved through high-resolution interstitial water sampling, which included eight samples in Section 201-1229D-1H-1 (Fig. F5A, F5B).

Alkalinity in the upper 2 mbsf ranges from 12.6 mM between 0.12 and 0.25 mbsf to 21.6-22.0 mM between 1 and 2 mbsf (Table T2; Fig. F5A). Alkalinity decreases to 14.4 mM at 16 mbsf before increasing again to a second maximum of 19.1 mM at 31 mbsf. Alkalinity then decreases gradually to <6 mM at the bottom of Hole 1229A.

The DIC profile is very similar to the alkalinity profile, with two maxima at ~2 and 30-40 mbsf (Fig. F5B). In the upper 2 m of the sediment column, DIC ranges from a low value of 11.6 mM in the upper few centimeters to 21 mM at 1.35 mbsf. DIC concentrations then decrease to 13.3 mM at 15.7 mbsf before increasing again to a second maximum of 20-21 mM over a depth range from 28 to 54 mbsf. They then decrease gradually to 6.8 mM at the bottom of Hole 1229A.

Sulfate concentrations were determined for 102 interstitial water samples from Holes 1229A and 1229D. Sulfate declines rapidly in the upper 3 m of Hole 1229D and reaches a local minimum of 14.1 mM at 3.25 mbsf. This local minimum is followed by a rise to 15.3 mM at 6 mbsf and is then followed by a more gradual steady decline to 0 mM by ~38 mbsf (Fig. F5C). Sulfate then reappears at 90.2 mbsf and increases steadily toward a final measured value of 38.0 mM at the bottom of Hole 1229A.

Dissolved sulfide (H₂S = H₂S + HS^-) exhibits a sinuous profile at Site 1229, consistent with the profiles of other interstitial water reactants and products, including sulfate and DIC (Fig. F5E). Sulfide concentrations are present at 2.34 mM in the uppermost 12 cm and increase steeply to a peak concentration of 6.37 mM at 2.34 mbsf. The sulfide profile exhibits a minimum of 4.5-5 mM centered at 14 mbsf, followed by a broad maximum of 5.8-6 mM between 22 and 38 mbsf. The sulfide profile broadly correlates with the DIC profile n the uppermost 50 mbsf. These profiles collectively indicate distinct zones of maximal sulfate reduction at 2 mbsf and 25-35 mbsf. The deeper zone is coincident with the shallow sulfate-methane transition zone. Sulfide concentrations decrease to 2 mM over the sulfate-free methane-rich zone between 66 and 80 mbsf, which suggests removal of sulfide in this zone. Low scattered values of sulfide in the interval from 77 to 82 mbsf are considered to be a sampling artifact. Slightly elevated concentrations are present between 86 and 97 mbsf, where values reach 3.0 mM. Below this, sulfide concentrations decrease linearly with increasing depth, approaching a concentration of <0.07 mM by 166 mbsf and <0.001 mM at 186 mbsf.

As at other sites drilled during Leg 201, IW barium concentrations at Site 1229 (Fig. F5F) vary antithetically with sulfate concentrations. Barium is close to the detection limit (~0.1 µM) at the sediment/water interface, where sulfate is high, but sharply rises to 19 µM at 42 mbsf, where sulfate is below detection limit. Across the sulfate-depleted zone at Site 1229 from 42 to 86 mbsf, barium concentrations range between 16.5 and 19 µM. Below this interval, sulfate concentrations steadily rise and dissolved barium concentrations decrease to 0.5 µM at 156.75 mbsf. The overall barium profile at Site 1229 most likely is controlled by barite solubility. Interestingly, however, barium concentrations at the bottom of the hole exceed those at the top, despite higher sulfate concentrations at the bottom.

With the exception of one sample at 76.85 mbsf, the dissolved manganese concentrations at Site 1229 (Fig. F5G) are low (<2 µM) over the upper 115 mbsf. Below this depth, dissolved manganese concentrations rise to >6 µM at 166.25 mbsf. Site 1229 is located at the upper boundary of a strong oxygen minimum zone.

The low dissolved manganese concentrations may reflect a lack of solid manganese inputs because they are reduced in the water column or in the upper few centimeters of the seafloor. Interestingly, however, lightly elevated concentrations of dissolved manganese (~2 µM), centered at ~69 and 92 mbsf, suggest reduction of some manganese-bearing phase in the middle of the sediment sequence. The sample with anomalously high manganese (and iron) concentrations (Sample 201-1229A-9H-6, 95-110 cm) may be affected by an unknown artifact or contamination.

The dissolved iron profile (Fig. F5H) shows a high degree of scatter but generally low values (<2 µM) down to ~125 mbsf, with one exception. Between 78 and 92 mbsf, the iron concentrations of several samples are >10 µM. Iron concentrations are also relatively high (>5 µM) below 125 mbsf. Most labile iron in the sediment column at Site 1229 has probably been precipitated as sulfides.

Dissolved strontium concentrations (Fig. F5I) rise significantly from 90 µM at the seafloor to 360 µM at 186 mbsf. As at Sites 1227 and 1228, the strontium gradient of Site 1229 decreases with depth, changing from 2.1 µM/m over the upper 100 m to 0.6 µM/m over the lower 90 m. Interestingly, of the three sites, the curvature to a smaller gradient occurs deepest and is most pronounced at Site 1229. The steep strontium gradients and curvature imply both a diagenetic release of strontium in the cored section and a flux of strontium from deep brines to shallow sediment and seawater. The similarity in the strontium concentration gradients at Sites 1228 and 1229 suggests that brines beneath these two holes are similar in chemical composition.

Dissolved lithium concentrations (Fig. F5J) rise from 27 µM at the seafloor to 184 µM at the bottom of the hole. However, unlike at Sites 1227 and 1228, this ~1.0-µM/m increase in lithium is less than the increase in strontium. The cause of the range in lithium gradients is unknown but, in contrast to the strontium gradients, may suggest differences in diagenetic exchange and in the composition of the source brine. Nonetheless, as at the other sites, a substantial flux of lithium occurs from deep brine to the shallow sediment and seawater.

The ammonium profile in the upper 43 m at Site 1229 is very similar to that observed at Site 1228. It has two distinct maxima, one between 1.3 and 4.3 mbsf (as at Site 1228) and a second broader maximum (~5000 µM) centered between ~50 and 75 mbsf (Fig. F5K). Below 75 mbsf there is a linear decrease to ~4000 µM at 159.75 mbsf.

As at Sites 1227 and 1228, the dissolved phosphate concentration was determined on splits of 55 IW samples (46 from Hole 1229A and 9 from Hole 1229D) that were previously analyzed for alkalinity in order to overcome chemical interferences from hydrogen sulfide (see "Biogeochemistry" in the "Site 1227" chapter).

The upper 10 m of the phosphate concentration profile in Hole 1229D (Fig. F5M) is very similar to the Site 1228 profile, with maximum concentrations of ~42 µM in the upper 2 mbsf and a decline to a local minimum of 6.6 µM at 12.2 mbsf. Below 12.2 mbsf, phosphate concentrations increase toward a second local maximum of 12.7 µM at 31.25 mbsf. Between ~40 and 71 mbsf, phosphate concentrations decline gradually to 4-5 µM at a depth of 160 mbsf and then increase to 7.8 µM at 186.2 mbsf. The sharp decrease above 12 mbsf and relatively small range below is consistent with control of phosphate by apatite solubility below this depth.

Dissolved silica concentrations in Holes 1229A and 1229D are fairly constant by 18 mbsf and range between ~950 and 1050 µM over most of the site (Fig. F5M). These values likely reflect control by biogenic silica solubility in these diatomaceous sediments.

Chloride exhibits a regular and steady increase with depth, as at Sites 1227 and 1228 (Fig. F5D). Chloride concentrations range from 555.7 mM near the sediment/water interface to a maximum of 1208.2 mM at 186.2 mbsf at the bottom of Hole 1229A.

The concentrations of acetate and formate were analyzed in 57 IW samples from Holes 1229A and 1229D (Table T2; Fig. F5N, F5O). Overall, the concentrations of both compounds are similar to those at Site 1227 and higher than those at Site 1228. Acetate concentrations range from 0.7 to 11.9 µM, and formate concentrations range from 0.6 to 12.1 µM. Maximum concentrations of both acids are present in the two sulfate-methane transition zones (~40 and ~90 mbsf) and close to the bottom of Hole 1229A. Concentrations in other depth intervals of Site 1229 are commonly low, with the majority of values <2 µM.

Methane was detected in all samples at Site 1229 (Table T3; Fig. F5P). In addition, ethane and propane were found in the majority of samples (Table T3; Fig. F5Q, F5R). As noted in the chapters for previous sites, increasing extraction times led to increased yields (Table T3). In the following discussion of methane concentrations in a stratigraphic context, we will discuss in detail the data series from the 8-day extraction of samples from Hole 1229A, which yielded consistently higher values than shorter extraction.

Concentrations of methane are 1.5 µM near the sediment/water interface and increase to ~80 µM at 21.70 mbsf. There is an almost threefold increase in methane concentrations over the next 2 m. Between 23.35 and 37.9 mbsf, methane concentrations are fairly uniform at levels slightly >200 µM. The depth interval around 37.9 mbsf (upper shaded bar in Fig. F5P) coincides with the transition zone, where sulfate concentrations decrease to undetectable levels. From 37.9 to 41 mbsf, methane concentrations increase sharply to ~800 µM but then drop again to <400 µM at 43.65 mbsf. This somewhat unusual decrease in the upper portion of the "methanogenic" sediment is associated with an erosional layer that marks the boundary between lithostratigraphic Subunits 1A and 1B. Poor recovery in this interval precluded methane analyses until a depth of 60.2 mbsf, where its concentration reaches 1260 µM. Concentrations of methane remain high to a depth of 76.80 mbsf, with the majority of values in excess of 1000 µM. Below 85.20 mbsf, concentrations decrease sharply from slightly above 500 µM to 17 µM at 93.40 mbsf within the second sulfate-methane transition zone created by the sulfate-rich brine (lower shaded bar in Fig. F5P). Below the second sulfate-methane transition zone, methane concentrations are relatively low (between 13 and 102 µM).

The concentrations of ethane and propane are <1 µM in the top 100 mbsf at Hole 1229A. In more deeply buried sediments, concentrations of both compounds peak at ~120 and 160 mbsf, with ethane values being 2 and 2.5 µM, respectively (Fig. F5Q). Propane concentrations in those intervals are slightly higher (Fig. F5R). Concentrations of both gases drop noticeably at the more deeply buried transition zone at ~90 mbsf, whereas the relative decrease at the upper transition zone appears less pronounced.

Hydrogen incubations were conducted on 20 samples from Hole 1229A and 10 samples from Hole 1229D (Table T4; Fig. F5S). Concentrations range between ~0.1 and ~4 nM, with most samples between ~0.1 and ~0.5. It is interesting to note that the highest-concentration samples were in the sulfate-methane transition zone.