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doi:10.2204/iodp.proc.314315316.136.2009

Inorganic geochemistry

The main objectives of the geochemical program at Site C0008 are to

• Characterize in situ biogeochemical reactions;
• Constrain in situ inorganic diagenetic reactions with depth;
• Identify potential deep-sourced fluids within fault zones and other permeable horizons, as well as fluid-sediment reactions at the source; and
• Constrain the subsurface hydrology, including fluid flow pathways and possible transport mechanisms.

A total of 67 whole-round samples were collected for interstitial water analyses at Site C0008 (35 from Hole C0008A, 1 from Hole C0008B, and 31 from Hole C0008C). Whole-round lengths ranged from 6.5 to 42.5 cm with larger subsamples collected from cores recovered deeper within the hole where the sediments were more consolidated.

Samples were collected at a higher spatial resolution in the uppermost 30 m of Holes C0008A and C0008C to define the sulfate-methane transition (SMT) in order to constrain the relative importance of anaerobic oxidation of methane (AOM) versus ordinary organic matter oxidation in producing the observed profile at this site, as well as for future geochemical and microbiological studies. Below Cores 316-C0008A-3H and 316-C0008C-3H, one sample was taken per core. The main geochemical objectives below the SMT were to identify the main in situ diagenetic reactions in the sediment section and any exotic/​deeper-sourced fluids along permeable horizons and zones of deformation. This was done through analysis of dissolved elements that reflect inorganic fluid-rock reactions or microbially mediated reactions. Pore fluid chemical compositions and δ¹⁸O isotopic ratios are reported in Tables T11 and T12 and plotted as a function of depth in Figures F24, F25, F26, F27, F28, F29, and F30. Samples from Sections 316-C0008C-11H-2 and 14H-6 are not included in the figures. These cores recovered more sediment than was penetrated and had clear flow structures associated with suction during HPCS coring; thus, the curated depths are not accurate (see “Operations”). Furthermore, some of the ESCS cores were extremely difficult to clean prior to processing, especially within more coarse-grained lithologies. All of the contaminated samples were collected below the SMT, where in situ SO₄ concentration is expected to be zero. Thus, these few samples that were slightly contaminated by drilling fluid exhibit slightly elevated SO₄ values below the SMT. Because Site C0008 was cored at the end of the expedition, there was insufficient time to correct the chemical data for drill water contamination. Thus, only uncorrected concentrations are presented in Tables T11, T12, and T13.

Because of the paucity of argon at the time of drilling Site C0008, which is used as a carrier gas for both inductively coupled plasma–atomic emission spectroscopy (ICP-AES) and inductively coupled plasma–mass spectrometry (ICP-MS) analyses, the ICP-MS measurement program was halted for the remainder of Expedition 316. This stoppage ensured that there was enough argon available to measure the major and minor element concentrations via ICP-AES. Trace element analyses were performed onshore via ICP-MS for Hole C0008C. In addition, δ¹⁸O in Holes C0008A and C0008C was analyzed via isotope ratio mass spectrometry. Trace metal and δ¹⁸O data are tablulated in Table T13.

Salinity, chloride, and sodium

Pore fluid salinity rapidly decreases in the upper 15.5 m of Hole C0008A (Fig. F24A). From 15.0 to 87.0 m CSF, salinity slightly increases but starts decreasing again below 87.0 m CSF, accompanied by two negative excursions at 120.3 and 136.3 m CSF. These excursions coincide with similar excursions in Cl and Br (Figs. F24C, F26C). Below this depth, pore fluid salinity remains relatively constant to the Subunit IA/IB boundary. At 202.4 m CSF, salinity increases slightly to 33.28 at 267.0 m CSF. The decrease in salinity in the upper ~30 m of the sediment section reflects active sulfate reduction and the subsequent precipitation of authigenic carbonates consuming pore fluid SO₄, Ca, alkalinity, and Mg within this interval (Fig. F27).

Pore fluid salinity in Hole C0008C decreases from seawater value at 1.5 m CSF to 32.33 at 56.0 m CSF. Below this depth, salinity is scattered with pronounced negative excursions at 86, 95, 96, 102, 135, 150, 161 and 167 m CSF (Fig. F24E). At the bottom of Hole C0008C, salinity is depleted by ~9% relative to modern seawater value. The negative excursions in the salinity profile are the result of gas hydrate dissociation during the core recovery process.

Chloride increases rapidly in the upper ~30 m of Hole C0008A and then remains relatively constant between 30 and 94 m CSF with a maximum of 585 mM (~5% higher than modern seawater value). Cl decreases from ~80 to 140 m CSF. At 120.3 and 136.3 m CSF, two negative Cl excursions (552 and 536 mM) are detected. During core recovery, gas hydrates dissociate, releasing freshwater into the pore spaces, thus diluting pore fluid Cl. The two negative excursions in the Cl profile clearly indicate gas hydrate dissociation during core recovery, with Cl values within these horizons being ~1% and 4% lower than modern seawater value, respectively. The steady increase in Cl in the upper part of Hole C0008A and below 150 m CSF may reflect either active gas hydrate formation or in situ ash alteration, both of which consume water and increase pore fluid Cl concentration.

Chloride in Hole C0008C increases rapidly in the upper 7 m of the sediment section from 553 mM to 563 mM (Fig. F24F). Cl continues to increase with depth below 7 m CSF to a lesser extent. At 38.1 m CSF, a Cl maximum of 569.7 mM was observed, below which Cl decreases and becomes highly variable between 71 and 172 m CSF. Pronounced negative Cl excursions are observed at 73, 86, 95, 96, 102, 135, 144, 150, 161, and 167 m CSF, with a minimum recorded Cl value of 293 mM (48% of modern seawater value). These negative Cl excursions likely reflect pore fluid freshening because of the dissociation of gas hydrates. This is further substantiated by the coincidence of Cl minima with minima in Br, Na, K, Ca, Mg, NH₄, Li, B, Sr, Ba, and H₄SiO₄ (Figs. F24, F25, F26, F27, F28) and maxima in δ¹⁸O (Fig. F30). The core was scanned with an infrared (IR) camera in the core cutting area, and particularly cold intervals were sampled as potential gas hydrate–bearing samples. Some of the observed concentration minima correlate well with methane maxima (see “Organic geochemistry”). The gas hydrate–bearing horizons are mainly associated with ash and, to a lesser extent, coarse sand layers (see “Lithology”).

Dissolved sodium increases in the upper ~40 m of the sediment section of Hole C0008A. At 58.7 m CSF, a Na maximum is detected (523 mM) and remains elevated to 114.0 m CSF. This broad zone of elevated Na mimics the broad Cl maximum. Below 114.0 m CSF are two negative Na excursions at 120.3 and 136.3 m CSF. These negative anomalies coincide with a minimum in pore fluid salinity and Cl, Br, and Sr and likely reflect the localized occurrence of gas hydrate. Below the two gas hydrate occurrences, Na increases to 525 mM at TD. Dissolved sodium in Hole C0008C increases above seawater value in the upper 70 m of the sediment section (Fig. F24G). Between 71 and 172 m CSF, Na shows seven discrete negative anomalies associated with gas hydrate–bearing horizons. At the bottom of Hole C0008C, Na is 512 mM (~7% enriched relative to modern seawater value).

Pore fluid constituents controlled by microbially mediated reactions

Sulfate and alkalinity

In Hole C0008A, sulfate decreases monotonically from 23.86 mM at 1.7 m CSF to 2.27 mM at 8.5 m CSF (Fig. F25B). At 15.5 m CSF, sulfate is below detection limit. Headspace methane starts to increase at ~5–6 m CSF (see “Organic geochemistry”), suggesting that the SMT is somewhere between 5 and 10 m CSF. Below the SMT, sulfate remains depleted with excursions up to 5 mM (Table T11). These samples were collected using the ESCS, and the elevated sulfate reflects minor contamination by seawater circulating in the borehole during drilling. Pore fluid data were not corrected for drill water contamination at this site.

The sulfate profile in Hole C0008C is similar to that of Hole C0008A, but the decrease in the upper part of the sediment section is more rapid. Sulfate decreases rapidly from 24.7 mM at 1.5 m CSF to 4.6 mM at 4.4 m CSF. At 6.9 m CSF, sulfate is totally depleted, suggesting an SMT depth of 4.4–6.9 m CSF (Fig. F25F). Sulfate remains depleted below the SMT with excursions as high as 3.9 mM because of minor contamination of ESCS cores by drilling fluid. The SMT in Holes C0008A and C0008C was penetrated at much shallower depths than in the other holes drilled during Expedition 316. Methane concentration at this site was also elevated with respect to previous sites (see “Organic geochemistry”). Overall, the shallow SMT at this site indicates that the upward methane flux is higher and the rates of AOM are greater in this region of the margin. It is very likely that this high methane flux is the main reason for the shallow occurrence of appreciable gas hydrate concentration at this site.

Pore fluid alkalinity increases rapidly in the upper part of Hole C0008A and reaches a maximum of 22.33 mM at 8.5 m CSF, just above the SMT (Fig. F25C). Below the SMT, alkalinity decreases steadily and reaches 5.24 mM at the bottom of the hole. In Hole C0008C, pore fluid alkalinity increases sharply from 6.2 mM at 1.5 m CSF to 22.8 mM at 6.9 m CSF, coinciding with the depth of sulfate depletion (Fig. F25G). Below the SMT, alkalinity displays an overall decreasing trend with negative excursions coinciding with gas hydrate–bearing horizons. The elevated alkalinity in the upper 10 m in Holes C0008A and C0008C reflects rapid alkalinity production in the region of intense sulfate reduction likely occurring through both organic matter oxidation and AOM. The relative extent of AOM will be constrained by shore-based δ¹³C-DIC analyses.

Ammonium, phosphate, and bromide

Dissolved ammonium increases monotonically in Hole C0008A from 1.7 to 58.7 m CSF. The dissolved ammonium profile then exhibits a broad maximum of ~4700 µM between 60.0 and 105.1 m CSF (Fig. F26A), below which ammonium decreases. In Hole C0008C, dissolved ammonium increases steadily from 232 µM at 1.5 m CSF to 3902 µM at 70.8 m CSF (Fig. F26D). Dissolved ammonium is scattered below this depth and the overall trend is decreasing. Local ammonium minima coincide with ash and coarse sand layers where gas hydrates occur. The initial rapid increase in ammonium in both holes is the result of microbially mediated decomposition of organic matter. The decline with depth likely reflects decreasing metabolic rates, thus declining ammonium production, as well as NH₄ sorption on clay minerals.

Phosphate in Hole C0008A increases sharply in the upper part of the sediment section, peaking at 38.8 m CSF. Below this maximum, PO₄ decreases (Fig. F26B). Below 150.0 m CSF, PO₄ varies slightly and remains close to the detection limit. At 120.3 and 136.3 m CSF, two local minima are observed; these coincide with minima in salinity, Cl, Br, and Sr (Figs. F24A, F24B, F28C). Phosphate in Hole C0008C is generally lower than in Hole C0008A. Phosphate increases rapidly from 11 µM at 1.5 m CSF to 38 µM at 4.4 m CSF and then suddenly decreases before starting to increase again, reaching a maximum of 44 µm at 38.1 m CSF (Fig. F26E). Below this maximum, phosphate generally decreases to 5 µM at the bottom of the hole. The initial rapid increase in phosphate in Holes C0008A and C0008C reflects active organic matter decomposition, which occurs in the zone of most active sulfate reduction. This zone extends deeper in Hole C0008A than in Hole C0008C. The decreasing and low phosphate in both holes below the depths of maximum concentration is likely controlled by the solubility of apatite, which is a major sink for phosphate.

Bromide in marine interstitial waters is sensitive to organic matter diagenesis with elevated concentrations reflecting marine organic matter decomposition. Dissolved bromide in Hole C0008A (Fig. F26C) increases in the upper 100 m of the sediment section, below which dissolved Br remains relatively constant with local minima reflecting gas hydrate dissociation during core recovery. In Hole C0008C, Br increases in the upper 70 m of the sediment section, below which Br is highly variable with negative concentration excursions coinciding with the gas hydrate–bearing intervals. The overall elevated background Br indicates that Br production is mainly the result of decomposition of marine organic matter and, unlike the other sites drilled during Expedition 316, a much lower amount of terrestrial organic matter degradation. This is consistent with the sediment carbon/​nitrogen ratio within the depth interval spanning Holes C0008A and C0008C, which indicates that organic matter in the sediment section is primarily marine in origin (see “Organic geochemistry”).

Major cations (Ca, Mg, and K)

Calcium in Hole C0008A decreases monotonically from 1.7 to 8.5 m CSF, below which the profile reverses and Ca starts to increase (Fig. F27A). The initial decreasing trend reflects Ca consumption during authigenic carbonate formation in the zone of most intense sulfate reduction. The increase in Ca with depth is consistent with progressive ash alteration and carbonate diagenesis downhole. The calcium profile for Hole C0008C resembles the Hole C0008A profile, with a rapid decrease followed by an overall increasing trend with depth (Fig. F27E). However, superimposed on this increasing trend are pronounced Ca minima coinciding with gas hydrate occurrences.

Magnesium decreases throughout Hole C0008A (Fig. F27B). The decrease is more rapid in the upper ~10 m of the sediment section. This zone of rapid Mg depletion coincides with the SO₄ reduction zone and the SMT, indicating some precipitation of Mg with authigenic carbonates within this depth interval, as well as uptake in clay minerals. The general decrease in Mg indicates Mg uptake in hydrous silicate minerals (mainly clays) formed during volcanic ash alteration. Magnesium in Hole C0008A decreases rapidly from near-seawater values to 23.58 mM at 22.1 m CSF, below which Mg decreases more slowly (Fig. F27F). Between 71 and 172 m CSF, Mg varies with negative excursions similar to those seen in many of the other dissolved elements.

The potassium profile for Hole C0008A varies yet decreases with depth (Fig. F27C). Potassium in Hole C0008C is similar to that in Hole C0008A, but the scatter with depth is more pronounced in Hole C0008C and coincides with gas hydrate–bearing horizons (Fig. F27G). The higher than seawater concentration in the upper ~20 m of Holes C0008A and C0008C is likely a sampling artifact resulting from pressure changes during core recovery and ion exchange with NH₄ on clay surfaces. The overall decline in K likely reflects the uptake of dissolved potassium in authigenic zeolites formed during the alteration of volcanic ash and feldspars.

Minor elements (B, Li, H₄SiO₄, Sr, Ba, Mn, and Fe)

Boron in Hole C0008A decreases from 515 µM at 1.7 m CSF to 330 µM at 49.4 m CSF (Fig. F28B). Between 50 and 140 m CSF, B varies and is lower than modern seawater value. From 143.2 m CSF, B increases with depth (Fig. F28B) to ~30% higher than modern seawater value at 215 m CSF. In Hole C0008C, boron does not decrease as rapidly as in Hole C0008A (Fig. F28E), but there is an overall decreasing trend from 504 µM at 1.5 m CSF to 350 µM at 73.1 m CSF. Below this depth, the trend reverts and increases overall. Superimposed on this increasing trend are several negative B excursions, corresponding to gas hydrate–bearing layers. The increase in B at the base of Holes C0008A and C0008C to values as high as 859 µM was not observed at the other sites drilled during Expedition 316 and will be the focus of postcruise research at this site.

Dissolved lithium in Hole C0008A decreases relatively rapidly from seawater value at 1.7 m CSF to 13.8 µM at 38.8 m CSF. Below this depth, Li gradually increases to seawater value (Fig. F28A). Between 120.3 and 143.2 m CSF, Li is higher than modern seawater value. Below this local maximum, Li remains relatively constant and then increases slightly from the Subunit IA/IB boundary to 31.1 µM at TD. Lithium in Hole C0008C is generally lower than in Hole C0008A and below seawater value throughout the hole (Fig. F28D). Li decreases rapidly from 22.9 µM at 1.5 m CSF to 16.3 µM at 6.9 m CSF. Minimum concentration between 71 and 172 m CSF is observed where gas hydrates are inferred.

Dissolved silica is higher than modern seawater value throughout Hole C0008A and displays an overall scattered profile with pronounced minima at 49, 77, and 202 m CSF and local maxima at 23, 87, 155, and 193 m CSF. In Hole C0008C, Si is relatively constant in the upper 50 m of the sediment section. However, between 60 and 172 m CSF, dissolved silica is scattered (Fig. F27H). The general trend in both holes is likely caused by opal diagenesis, and variations in the Si profile likely reflect other varying dominant silicate diagenetic reactions, as well as gas hydrate dissociation during core recovery.

Strontium in Hole C0008A rapidly decreases in the uppermost few meters of the sediment section before starting to increase, reaching a maximum of 96 µM at 49.4 m CSF (Fig. F28C). Below this depth, Sr decreases and then increases slightly below 200 m CSF. Superimposed on this decreasing trend are two negative excursions, coinciding with similar minima in Cl, Na, PO₄, and salinity and maxima in δ¹⁸O. Strontium is lower than modern seawater value throughout Hole C0008C (Fig. F28F) and remains relatively constant in the upper 70 m of the sediment section. Below this depth, seven Sr minima are associated with gas hydrate occurrences.

Dissolved barium in Hole C0008A is close to modern seawater value in the upper 10 m of the sediment section, below which Ba increases, reaching a broad maximum of 58 µM between 77 and 94 m CSF. At ~100 m CSF, the profile reverses and Ba decreases (Fig. F25D). A local minimum at 120.3 m CSF coincides with minima in Cl, Na, Sr, PO₄, and salinity and maxima in δ¹⁸O, indicating intervals of gas hydrate occurrence. The increase in Ba below the SMT is likely the result of barite dissolution, and the variability in the dissolved Ba profile is likely controlled by the abundance of barite in the sediment. Dissolved Ba rapidly increases from near-seawater value at 1.5 m CSF to 113 µM at 6.9 m CSF in Hole C0008C (Fig. F25H). Below this maximum, Ba decreases rapidly to 63 µM at 22.1 m CSF. Thereafter, the decrease rate is slower, and at the bottom of the hole, Ba reaches 20 µM. The rapid increase from 4.4 to 6.9 m CSF coincides with the inferred SMT depth. Below the SMT, barite becomes unstable and thus a significant amount of Ba²⁺ is released to the pore fluids. The more extensive increase in Hole C0008C is probably due to a greater amount of barite dissolution at this interval.

Dissolved manganese rapidly decreases in Hole C0008A from 6.44 µM at 1.7 m CSF to 0.63 µM at 5.5 m CSF (Fig. F29A). Below this rapid decline, Mn remains relatively constant until 29.3 m CSF, where Mn starts increasing rapidly, reaching a local maximum at 49.4 m CSF (4.67 µM). Mn displays an overall increase between 50 and 175 m CSF and reaches a second local maximum at 173.8 m CSF. Toward the bottom of Hole C0008A Mn is scattered. The elevated concentration in the upper ~6 m at this site reflects MnO₂ reduction within a depth interval where MnO₂ is a favorable and important electron acceptor for microbially mediated organic matter decomposition. A remarkably similar manganese profile is observed in Hole C0008C (Fig. F29E), but the scattered behavior is more pronounced in Hole C0008C than in Hole C0008A, and toward the bottom of Hole C0008C, the Mn profile is characterized by an overall decreasing trend.

Dissolved iron in Hole C0008A is close to the detection limit in the upper 40 m of the sediment section. Fe increases in some horizons with depth and approaches the detection limit toward the bottom of Hole C0008A. In Hole C0008C, Fe rapidly decreases in the upper part of the sediment section. Considerable mobility is displayed at various horizons throughout Hole C0008C (Fig. F29F).

Trace elements (Rb, Cs, V, Cu, Zn, Mo, Pb, U, and Y)

Because of the paucity of argon (which is used as a carrier gas for ICP-MS analyses) toward the end of the expedition, the trace metal program was halted before coring at Site C0008. Trace element concentrations were subsequently analyzed shore based via ICP-MS; however, there was only time to analyze the samples from Hole C0008C before publication of the Expedition Report.

Rb concentration decreases from above seawater values to 0.83 nM at 38.1 m CSF. Below this depth, Rb remains relatively constant to TD, except for concentration minima at 73.0, 86.0, 96.0, 150.0, and 161.4 m CSF (Fig. F27I). These Rb excursions are the result of gas hydrate dissociation during core recovery. Cs generally decreases with depth from 2.2 nM at 1.5 m CSF to 1.81 nM at 47.9 m CSF (Fig. F27J), with a concentration maxima of 2.8 nM at 12.3 m CSF. Below ~50 m CSF, Cs concentration generally increases to 3.4 nM at TD. Superimposed on this increasing trend are concentration minima at 86.0, 96.0, 150.0, and 161.0 m CSF that are associated with gas hydrate–bearing intervals (see “Summary”). Molybdenum is relatively low and variable from 1.5 to 83.4 m CSF, ranging from 7.7 to 172 µM (Fig. F29C). Below this depth, concentrations are generally higher and more variable to TD, ranging from 33 to 435 µM.

Copper is generally low and relatively constant with depth, except for six peaks at 12.3, 38.1, 82.9, 96.0, 150.0, and 167.0 m CSF. Some of these peaks correspond with gas hydrate occurrences, and they likely reflect contamination of the pore fluid sample with drilling fluid that invaded voids left by the dissociating hydrate, as observed in the sulfate profile (Figs. F29I, F25). Zinc concentration is variable and ranges from 0.37 to 6465 nM (Fig. F29J). Vanadium initially increases from 28 nM at 1.5 m CSF to 40 nM at 4.4 m CSF (Fig. F29K). Below this depth, concentrations generally remain below 20 nM with concentration maxima at 83, 96, and 161 m CSF associated with intervals of gas hydrate occurrence.

Uranium generally decreases from 6.1 nM at 1.5 m CSF to 0.4 nM at 56.0 m CSF. Below this depth, background concentration is slightly higher, ~1 µM, to the base of the hole with concentration maxima up to 6.3 nM at 88.0, 103.0, and 161.2 m CSF (Fig. F29G). Lead concentration is relatively low in Hole C0008C. Background concentration ranges from 0.3 to 6 nM (Fig. F29D). Superimposed on the low background concentration are three peaks at 22.1, 47.9, and 83.4 m CSF. Yttrium generally decreases with depth from 2.8 pM at 4.4 m CSF to 1.2 pM at TD (Fig. F29H). There are two concentration maxima of 4.2 pM at 88.2 m CSF and 3.1 pM at 102.8 m CSF.

δ¹⁸O

Pore fluid δ¹⁸O ratios in Holes C0008A and C0008C were measured on shore. In Hole C0008A, δ¹⁸O decreases relatively rapidly from –0.08‰ at 1.5 m CSF to –3.12‰ at 68.5 m CSF (Fig. F30). Below this depth, δ¹⁸O continues to decrease, but the change in δ¹⁸O with depth decreases reaching –4.25‰ at 164.3 m CSF. From this depth, δ¹⁸O increases to –3.61‰ at TD. There are no variations in the δ¹⁸O profile where there are anomalies in the Cl concentration profile at 120 and 136.3 m CSF. When gas hydrates form, H₂¹⁸O is preferentially incorporated in the hydrate structure and the pore fluids become depleted in δ¹⁸O. When gas hydrates dissociate, H₂¹⁸O is released back to the pore fluids and they become enriched in δ¹⁸O. It is possible that gas hydrate concentrations were too low, as indicated by the Cl, Br, and Na profiles, to impact the δ¹⁸O profile upon dissociation at these depths.

In Hole C0008C, δ¹⁸O decreases with depth from –0.27‰ at 1.5 m CSF to –3.77‰ at 70.8 m CSF. Below this depth, background δ¹⁸O remains relatively constant. Superimposed on the background δ¹⁸O profile from ~71 m CSF to TD are five positive excursions at 83, 86, 96, 150, and 161 m CSF that coincide with Cl concentration minima (Fig. F30). The positive excursions in the δ¹⁸O profile at Hole C0008C confirm the presence of gas hydrate within these depth intervals.

Summary

The primary features of the pore fluid geochemical profiles collected in Hole C0008 are (1) the relatively shallow SMT, (2) evidence of disseminated gas hydrate and two occurrences of gas hydrates at low concentration in Hole C0008A, and (3) disseminated gas hydrates with seven localized occurrences of elevated pore space gas hydrate concentration in Hole C0008C.

Sulfate reduction is relatively rapid in the upper 9 m of the sediment section in Hole C0008A and in the upper 4 m in Hole C0008C, likely indicating a much higher upward methane flux at Site C0008 in comparison with other sites drilled during Expedition 316. This elevated methane flux is likely one of the main factors contributing to the elevated occurrence of gas hydrate at this site. There is evidence of localized occurrences of gas hydrate in Holes C0008A and C0008C. The localized gas hydrate occurrences in Hole C0008A are manifested by two sharp and local Cl minima detected at ~120 and 136 m CSF with Cl values being ~1.5% and 4% less than modern seawater value, respectively. These Cl minima coincide with minima in Na, PO₄, Sr, and salinity at the same depths. In Hole C0008C, the occurrences of gas hydrate–bearing horizons were even greater than in Hole C0008A, and these horizons contain higher gas hydrate concentration. Between 70 and 172 m CSF in Hole C0008C, most of the dissolved elements display scattered behavior with discrete excursions to fresher values with respect to modern seawater, which clearly indicates the existence of gas hydrate–bearing intervals. The sampled gas hydrate–bearing horizons are associated with ash and coarse-grained sand layers.

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