BIOGEOCHEMISTRY
The 115 IW samples for Site 1230 (Table T2) were collected (1) to define chemical gradients, especially around the anaerobic oxidation of methane (AOM) zone and gas hydrate intervals, and (2) to augment routinely measured species with nonroutine analyses of biogeochemically important species. A total of 73 IW samples were taken from Hole 1230A at a resolution of three per core in order to delineate key chemical horizons and to collect sufficient water for the numerous shore-based requests. From Hole 1230B, 32 IW samples, including 17 samples from Core 201-1230B-2H, were taken. This sampling was primarily aimed at constructing high-resolution chemical profiles across the AOM zone as determined by sulfate measurements in Hole 1230A. Eleven samples were taken from Hole 1230C to provide a chemostratigraphic link across the AOM zone between Holes 1230B and 1230C, the latter sampled mainly for microbiology. A WSTP sample was also collected at 10 m above the seafloor.
The most apparent change in interstitial water composition at Site 1230 is color (Fig. F3A). Interstitial water becomes yellow at many Deep Sea Drilling Project (DSDP) and ODP sites drilled into organic-rich sequences. At Site 808 (Nankai accretionary prism), one of the few locations where this yellowness has been quantified, the color probably correlates to dissolved organic carbon (You et al., 1993). Interstitial water at the top of the Site 1230 sequence is clear but rapidly becomes yellow with depth, reaching a golden color (>0.50 JWBL) by 40 mbsf. Below ~160 mbsf, the water begins to clear. Interstitial water below 220 mbsf is nearly clear (<0.15 JWBL).
Broadly tracking the yellowness of water, the profiles of alkalinity and DIC (Fig. F3B, F3C) covary (r2 > 0.98). Alkalinity increases sharply from 2.7 mM at the seafloor to 100 mM at ~30 mbsf and less rapidly to a broad maximum of 150-157 mM between 100 and 150 mbsf. Alkalinity then decreases to ~40 mM at 250 mbsf. DIC increases rapidly from 2.6 mM at the seafloor to 100 mM at ~30 mbsf and less rapidly to a broad maximum of 155-163 mM between 100 and 150 mbsf. DIC then decreases to between 50 and 80 mM below 250 mbsf. Both alkalinity and DIC concentrations are slightly lower in depth intervals where gas hydrates were present or suspected (Cores 201-1230A-15H, 18H, and 19H). The alkalinity profile is similar to that reported for the upper 250 mbsf at Site 685 (Shipboard Scientific Party, 1988). The large increase in alkalinity and DIC at Site 1230 probably corresponds to an interval of intense methane production.
Dissolved silica concentrations rapidly rise from ~500 µM near the sediment/water interface to ~900 µM at 1.5 mbsf (Fig. F3D). From this depth to ~250 mbsf, silica concentrations show considerable scatter between 750 and 1050 µM. Concentrations appear to rise slightly at the bottom of the hole. Biogenic opal solubility probably controls the overall shape of the silica profile. However, as in gas-charged sediments at the Blake Ridge (Paull et al., 1996, p. 131), the dissolved silica content of individual samples may have decreased during whole-round storage prior to squeezing.
Ammonium increases downhole from 1000 µM within 1 m of the sediment/water interface to a broad subsurface maximum of 35,000-40,000 µM between 100 and 200 mbsf (Fig. F3E). Ammonium concentrations then decrease toward the bottom of the hole. Four main processes probably control the shape of the overall ammonium profile: organic matter degradation, microbial uptake, diffusion, and adsorption onto clays.
As highlighted during ODP Leg 112 (Suess, von Huene, et al., 1988), maximum concentrations of dissolved phosphate are >500 µM at this location. These are among the highest values reported for an ODP/DSDP location. Our phosphate profile (Fig. F3F) generally traces the low-resolution record reported by Leg 112 scientists but contains some unexplained deviations. Phosphate concentrations rise steadily and steeply from near zero at the sediment/water interface to 400 µM by 20 mbsf. Below this depth, unlike at Site 685, dissolved phosphate concentrations show considerable scatter, with values ranging between 0 and 660 µM. As with silica, the interval of scatter broadly coincides with an inferred depth range of extreme degassing during core recovery and processing.
Dissolved strontium concentrations increase rapidly from 82 µM at the sediment/water interface to 105 µM at 20 mbsf (Fig. F3G). Below this depth, strontium slowly increases to 150 µM at ~210 mbsf. However, several excursions to lower values are superimposed on this general concentration gradient. At ~250 mbsf, strontium concentrations again rise quickly, reaching 235 µM at 270 mbsf. The overall shape of the downhole strontium profile mimics the low-resolution profile obtained at Site 685 (Kastner et al., 1990), except that the absolute values often differ by as much as 20%. The mean strontium concentration measured in six samples of International Association for the Physical Sciences of the Ocean (IAPSO) standard seawater was 88 ± 1 µM, which compared well to the certified value of 87 µM. The scattered low values between 50 and 90 and between 125 and 150 mbsf may indicate samples that were affected by carbonate precipitation prior to squeezing. Alternatively, some of the excursions may reflect dilution by dissociated gas hydrate.
Dissolved lithium concentrations remain close to bottom-water concentration (28 µM) until ~25 mbsf (Fig. F3H). With the exception of several deviations to lower concentrations near 80 and 150 mbsf, lithium then steadily rises to 215 µM at ~170 mbsf. Below this depth, lithium rapidly increases to 670 µM at ~270 mbsf. Carbonate recrystallization at shallow depths, a process that incorporates lithium, probably causes the significant curvature in the upper 50 m. Lithium-rich waters at depth also exert a strong control on the overall profile, although the source of these waters is unknown.
The downhole chloride profile at Site 1230 (Fig. F3I) exhibits three features characteristic of chloride profiles in sediment sequences with significant gas hydrate (e.g., Cascadia Margin [Westbrook, Carson, Musgrave, et al., 1994] and Blake Ridge [Paull, Matsumoto, Wallace, et al., 1996]). Chloride concentrations increase from bottom-water concentration (555 mM) to a shallow subsurface maximum of 568 mM at 20 mbsf. Concentrations then decrease rapidly to reach a baseline value, which lies between 550 and 555 mM at Site 1230. Superimposed on this baseline are significant deviations to lower values. For example, Sample 201-1230-19H-1, 135-150 cm, has a chloride concentration of 388 mM. A commonly held explanation for this characteristic chloride profile includes four components (Hesse et al., 1985; Egeberg and Dickens, 1999; Davie and Buffett, 2001): (1) gas hydrate formation at shallow depth excludes chloride (and other dissolved ions), which then advect or diffuse to water of lower concentration, including overlying bottom water; (2) burial of sediment eventually brings the solid gas hydrate to depths where it is no longer stable and dissociates, releasing freshwater and lowering the average chloride concentration of the sediment column; (3) dissociation of gas hydrate in discrete horizons during core recovery leads to anomalous freshening beyond baseline concentration; and (4) finally, the drop in seawater chloride since the last glacial maximum followed by diffusion decreases chloride concentration in the uppermost sediment. Interestingly, the shallow chloride maximum is more pronounced and the baseline concentration is higher at Site 1230 than at sites on the Blake Ridge, one of the few other locations with gas hydrates and high-resolution IW profiles. These characteristics may indicate stronger upward flow and a lower total amount of gas hydrate at Site 1230 than the 0.2 mm/yr and average 5% occupancy of pore space inferred for the Blake Ridge (Egeberg and Dickens, 1999). Alternatively, the location of the chloride peak may reflect the top of the hydrate zone. It should be noted that the chloride concentrations reported here include bromide and iodide, which together are >3 mM at this location (Martin et al., 1993).
Recent literature has emphasized that sulfate profiles are steep and nearly linear in regions with significant upward transport of methane and gas hydrates (Borowski et al., 1996, 1999; Hoehler et al., 2000). Sulfate concentrations were determined for the WSTP sample and 44 IW samples from shallow depth. In all three holes, sulfate decreases from typical seawater concentrations at the seafloor (28.9 mM in the WSTP sample) to <0.80 mM at shallow depth (Fig. F3J). In Hole 1230B, where high-resolution sampling was conducted, sulfate drops almost linearly with depth (R2 > 0.998) to 0.64 mM at 9.0 mbsf. This steep, nearly linear decrease of 3.2 mM/m strongly suggests that AOM consumes a large fraction of the sulfate over a thin zone (Borowski et al., 1996; Niewöhner et al., 1998; Fossing et al., 2000). This inference is supported by the steep increase in headspace methane concentration beginning at ~7 mbsf (Fig. F3K).
For samples from Holes 1230B and 1230C, the preparation of samples was changed to maximize analytical sensitivity at low sulfate concentrations. Interestingly, these analyses show that the steep sulfate gradient in shallow sediment does not extend to zero sulfate concentration immediately below the sulfate/methane interface, as purported to occur elsewhere. Instead, sulfate decreases gradually below 9.0 mbsf, reaching zero concentration at ~30 mbsf (Table T2). Addition of sulfate during the squeezing process (e.g., oxidation of through sulfides) was excluded as an explanation for these anomalous sulfate values because samples processed similarly but obtained from deeper depths at Site 1230 or from other sites of Leg 201 had zero sulfate. Hence, upon recognizing this phenomenon in Hole 1230B, two hypotheses were forwarded: (1) oxidation of sulfide during storage adds sulfate to the water or (2) a solid sulfate-bearing phase (e.g., barite, below) slowly dissolves upon entering sulfate-depleted water. To address the first hypothesis, IW samples from Hole 1230C were squeezed in opposite order from those in Hole 1230B and analyzed within minutes. Despite this approach, IW samples from similar depths in Holes 1230B and 1230C (after correcting for core offsets, below) contain similar sulfate. Additionally, three samples of squeezed water from Hole 1230A were reanalyzed after 4 days to examine the potential effect of oxidation (Table T3). No significant oxidation was observed over this 4-day period (~0.15 mM compound to <0.2 mM). Minor amounts of sulfate are probably present over an extended depth interval below the main AOM zone, although we cannot rigorously support barite dissolution as an explanation based on available information.
Interstitial water barium (Fig. F3L) spans a wide concentration range at Site 1230, as somewhat expected from limited analyses at Site 685 (von Breymann et al., 1990). Dissolved barium is between 0.4 and 10 µM in the uppermost 7 m, a concentration range that already exceeds that at most ODP sites examined for this element. Beginning at ~7 mbsf, which coincides with the increase in headspace methane (Fig. F3K), barium concentrations rise rapidly to 265 µM at ~20 mbsf. Below this depth, the barium concentrations steadily decrease, resulting in barium concentrations of ~400 µM between 50 and 150 mbsf. From this depth to 270 mbsf, dissolved barium climbs to nearly 1200 µM. To our knowledge, this is the highest concentration ever reported in a deep marine borehole. Superimposed on this overall profile is a series of excursions to lower values. Several processes likely contribute to the complex barium profile at Site 1230. First, the occasional barium data that lie well below the general trend probably reflect dilution by gas hydrate dissociation or, in some cases (e.g., Sample 201-1230A-8H-1, 135-150 cm), contamination by borehole water. Given the low Ksp of barite (~10-10) (Monnin, 1999) and the exceptional barium concentrations, even a trace incursion of sulfate will lead to barite precipitation. Second, dissolved barium (and strontium, above) appears to be precipitating into some solid phase between 50 and 150 mbsf. Third, and most importantly, extreme dissolution and precipitation of barite around the sulfate/methane interface (Torres et al., 1996; Dickens, 2001) has led to the incredibly high dissolved barium concentrations at Site 1230. Solid barite, associated with primary productivity in the water column, is deposited on the seafloor and buried over time into the sulfate-depleted zone, where it dissolves. Dissolved barium then advects or diffuses upward to the sulfate-rich zone, where it precipitates. Shallow sediment at Site 685 contains high solid-phase barium concentrations (~1000 ppm) that are likely present as barite (von Breymann et al., 1990). Apparently, at Site 1230, the extreme input of barite over time has led to high interstitial water barium concentrations.
The total dissolved sulfide (
H2S = H2S + HS-) profile at Site 1230 (Fig. F3M) can be divided into two distinct zones: a shallow zone between 0 and 25 mbsf, where sulfide is generally high, and a deep zone between 25 and 268 mbsf, where sulfide is low. Sulfide is first detected (0.0054 mM) at a depth of 0.04 mbsf. Below this depth, sulfide increases steeply and linearly with increasing depth, reaching maximum concentrations (to 10.9 mM) over a narrow depth zone from 7 to 9 mbsf. Below this peak, sulfide concentrations decrease to <0.002 mM at 27 mbsf. The sulfide peak is nearly symmetrical in the uppermost 18 m of sediment but shows concave-upward curvature between 17 and 25 mbsf, suggesting uptake of sulfide into the solid phase within this layer. Below 25 mbsf, sulfide concentrations remain low, usually <0.001 mM.
As at the other sites of Leg 201, the dissolved iron profile at Site 1230 (Fig. F3N) shows considerable downhole scatter but with an obvious association to sulfide. Except in the top 1.5 m, where they decrease from ~19 to 3 µM, dissolved iron concentrations are low (<5 µM) between the sediment/water interface and 25 mbsf. Beginning at this depth, dissolved iron abruptly increases, ranging between 5 and 60 µM until 200 mbsf. Dissolved iron concentrations are <10 µM below 200 mbsf. The overall dissolved iron profile likely reflects the dissolution and precipitation of solid iron phases. Iron oxides and hydroxides are deposited on the seafloor. Within the upper few centimeters, these phases dissolve, releasing iron, which then precipitates as one or more iron sulfide minerals. In sediment deeper than 25 mbsf, where neither sulfide nor oxygen is present, iron-bearing solids again dissolve, releasing iron to the interstitial water. These solids could be aluminosilicates, oxides that have escaped shallow diagenesis, or sulfides entering sulfide-depleted water.
Although dissolved manganese concentrations are generally low (<8 µM) at Site 1230, the downhole profile (Fig. F3O) displays two intriguing features. First, after increasing to a peak of 7.3 µM at 0.65 mbsf, dissolved manganese concentrations sharply decline to below detection limit (0.1 µM) at 5 mbsf. Second, below this depth dissolved manganese concentrations oscillate to form several well-defined peaks and troughs with amplitudes of ~1.5 µM. Manganese oxides probably dissolve in the uppermost 1 m of sediment, releasing dissolved manganese to the interstitial water, which is subsequently reduced by microorganisms. In the interval of high sulfide between 1 and 25 mbsf, sulfide phases may incorporate manganese. Below 25 mbsf, there is a balance between dissolution of manganese-bearing solids, again either aluminosilicates, oxides, or sulfides and precipitation.
Concentrations of acetate and formate were determined for 68 samples from Holes 1230A and 1230B, including a water sample obtained from sediment containing a high portion of gas hydrate (Table T2; Fig. F3P, F3Q). Acetate concentrations range from 3.3 to 220 µM and significantly exceed those at previous Leg 201 sites. In general, the acetate profile shows three concentration zones. From the sediment/water interface to ~13 mbsf, concentrations are mostly <20 µM. Between 13 and 133 mbsf, acetate concentrations range from 20 to 60 µM. Acetate concentrations then increase sharply below this depth, dropping below 100 µM in only a few samples. It is noteworthy that the sudden increase in acetate concentrations occurs in an interval with relatively high amounts of methane hydrate (see "Gas Hydrate"), although a potential link is not obvious. Acetate is depleted in water released from dissociated gas hydrate (see "Gas Hydrate"). In contrast to the acetate profile, formate concentrations display significantly less variation downhole. Formate concentrations range from 2.9 to 41 µM, with almost all values <10 µM. Formate concentrations display a slight trend to higher values with increasing depth.
Hydrogen incubations were conducted on 18 samples from Hole 1230A and 5 samples from Hole 1230C. All samples were incubated at 4°C, close to the in situ sediment temperature (see Table T4 for details). Hydrogen concentrations ranged from 0.07 to 1.4 nM and did not display a systemic downhole trend.
Headspace methane concentrations, as determined by analyses after 7 days at 22°C, are generally higher than those at previous sites (Table T5; Fig. F3K). Methane concentrations are <100 µM between the sediment/water interface and ~7 mbsf. Below this depth, where sulfate drops to <0.80 mM, methane concentrations rise sharply, reaching 7000 µM by ~18 mbsf. Of all sites studied during Leg 201, Site 1230 displays the steepest sulfate gradient in shallow sediment. Below 18 mbsf, methane concentrations determined by the headspace technique slowly and erratically decrease downhole. The methane profile between 0 and 18 mbsf may represent in situ methane concentrations. However, as highlighted by methane-rich gas voids in sediment cores (Table T6; see "Lithostratigraphy"), the scattered methane profile below 18 mbsf undoubtedly reflects differential gas loss during core recovery, the intensity perhaps linked to lithology. The in situ methane concentrations are probably >300 mM in lower parts of the sediment sequence at Site 1230, as indicated by results from the PCS (see Dickens et al., this volume). As at all sites of Leg 201, methane concentrations derived from the safety methane protocol are generally lower than those obtained from the more time-consuming extraction in NaOH solution (see "Biogeochemistry" in the "Explanatory Notes" chapter).
Ethane and propane comprise trace gas components in the majority of headspace samples from Site 1230 (Table T5; Fig. F3S). Interestingly, there is a clear downhole change in the amount of these gases. Ethane and propane concentrations are <1 µM in the top 200 mbsf, excluding one sample at 26.60 mbsf (Sample 201-1230A-4H-2, 130-135 cm) that yielded unusually high concentrations of both gases during analysis according to the safety protocol (171.7 ppm ethane and 287.1 ppm propane; ~35 and 55 µM). Below 220 mbsf, ethane and propane concentrations increase drastically, reaching 12 µM, although their abundances do not correlate in detail. The apparent increase in ethane and propane at depth may indicate an interval of in situ production by microbial activity or upward flow of gas.
High-resolution recovery of the well-characterized AOM zone at Site 1230 represents an important goal of Leg 201. However, studies and sampling of this interval need to account for meter-scale depth offsets between holes that often occur during drilling operations (e.g., Hagelberg et al., 1992). In the absence of obvious physical property changes to correlate, we have aligned Holes 1230A, 1230B, and 1230C using the dissolved sulfate and barium profiles (Fig. F4). This correlation (Fig. F5), based on a continuous Hole1230B, implies a 0.5-m gap between Cores 201-1230A-1H and 2H and missing or compressed mudlines in Holes 1230A and 1230B.
Rock-Eval pyrolysis was determined on selected sediment samples from Site 1230 as well as other Leg 201 sites for comparison. These data from all sites are presented and discussed here for brevity. Sediment samples from Site 1228 are from the IW squeeze cakes; all other samples are from split cores.
The results of Rock-Eval pyrolysis (Table T7; Fig. F6) show that sediments at most Leg 201 sites contain thermally immature organic carbon, with total organic carbon (TOC) concentrations often >1%. TOC concentrations at Site 1230 range from 0.7% to 5.2%, with most values falling between 2% and 4%. Hydrogen index (HI) values at Sites 1227-1229 are generally high and consistent with organic matter being primarily derived from marine algae (Tissot and Welte, 1984). As a group, Site 1228 samples have the highest HI and Site 1230 samples have the lowest HI. All samples but one from Site 1231 have TOC <0.5%.
