The shipboard geochemistry program at Site 1150 included (1) analyses of volatile hydrocarbons; (2) determinations of abundances of inorganic carbon, total carbon, total sulfur, and total nitrogen in sediments; and (3) measurements of salinity, alkalinity, pH, and concentrations of some dissolved anionic species in interstitial waters (for description of methods see "Geochemistry" in the "Explanatory Notes" chapter).
Gas analyses at Site 1150 indicate that most hydrocarbon gases are below detection limits, with the exception of methane and ethane. Methane (C1) concentrations range from 0.05% to 9%, whereas ethane (C2) concentrations range from undetectable values to 26 ppmv, as measured from headspace gas analyses (Table T14, also available in ASCII format). C1 and C2 concentrations show a similar distribution with depth (Fig. F50). Both concentrations irregularly increase in the upper 120 m to reach values of ~5% for C1 and 12 ppmv for C2. Concentrations then irregularly decrease to ~1% for C1 and 2 ppmv for C2 at 250 mbsf and remain at about these low values for ~300 m downhole. C1 and C2 concentrations steadily increase to reach maximum values of 9% for C1 and 26 ppmv for C2 at 1100 mbsf and then decrease to ~2% for C1 and 9 ppmv for C2 at the bottom of the borehole.
C1/C2 values abruptly decrease from 22,230 at the top to ~4200 at 60 mbsf because of the increase in ethane. Values then irregularly increase to ~6000 at ~500 mbsf. Below this depth, C1/C2 values tend to gradually decrease with depth to reach a minimum value of ~2800 at the bottom of the borehole (Fig. F50).
Free gases withdrawn from within core liners (for description of methods see "Geochemistry" in the "Explanatory Notes" chapter) show that C1 and C2 concentrations throughout the borehole are similar, ranging from 87% to 90% for C1 and 79 to 7633 ppmv for C2 (Table T15). Other hydrocarbon gases are below detection limits.
Carbonate abundances typically are of ~2-4 wt% and exhibit several excursions with values as high as 70 wt%. Most of these excursions, however, fall between 5 and 12 wt%. Values lower than 0.2 wt% are also observed in several intervals (Fig. F51). These low carbonate abundances are in agreement with few to trace occurrences of calcareous nannofossils in the sediments (see "Biostratigraphy").
Abundance of organic carbon (Corg) fluctuates between 0.5 and 1.8 wt%, with an average value of ~0.8 wt% (Fig. F51). Corg values tend to decrease in the upper 200 m from 1.8 wt% at the top to ~1 wt%. This value remains as the typical Corg abundance for ~450 m down in the borehole. Values then irregularly decrease to reach a minimum of 0.5 wt%. Corg/N ratios range from 2 to 13 with typical values of ~7 (Fig. F51). The distribution of Corg/N shows an irregular pattern in the upper 400 m with values fluctuating between 4 and 9. Below this depth, Corg/N values are more regular ranging from 6 to 9. Total sulfur abundances irregularly fluctuate between 0.5 and 1.35 wt% with an average value of 0.85 wt% (Fig. F51).
A total of 39 whole-round samples from Holes 1150A and 1150B were analyzed for pH, alkalinity, salinity, chlorinity, sodium, potassium, magnesium, calcium, sulfate, ammonium, lithium, and strontium. Results are summarized in Table T16 (also available in ASCII format).
Salinity and chlorinity exhibit in general a similar trend with depth (Fig. F52). Values gradually decrease with depth from ~34 for salinity and 550 mM for chlorinity at the top of the borehole to 30 and 500 mM at ~100 mbsf. Salinity remains constant at 30 for 350 m down in the borehole. Chlorinity concentrations remain constant for ~200 m down in the borehole and gradually increase from this depth down to ~550 mbsf, reaching values of 520 mM. Chlorinity concentrations then abruptly decrease to 350 mM at ~700 mbsf. Below this depth, chlorinity gradually decreases to ~300 mM at the bottom of the borehole. Salinity also decreases rapidly with depth from 30 at ~450 mbsf to 18 at ~800 mbsf and remains at this value to the final depth.
Alkalinity values drastically increase downhole in the upper 20 m from 24 to 49 mM (Fig. F52). Values decrease rapidly downhole to 27 mM at ~200 mbsf and remain at about this value for ~180 m. Alkalinity values then decrease to 2 mM in the remaining portion of the borehole. This downhole decrease is particularly abrupt in the interval between 470 and 600 mbsf. Dissolved sulfate concentrations (SO42-) drastically decrease from 12.3 to <0.5 mM in the upper 10 m (Fig. F52). These very low values persist down throughout the borehole. Some higher values reaching 2 mM are present, however, between 400 and 600 mbsf. These higher values are probably the result of seawater contamination. Ammonium concentrations increase from 1.6 to 9.6 mM in the upper 200 m and then gradually decrease to ~2 mM at the final depth of the borehole (Fig. F52).
Concentrations of dissolved calcium (Ca2+) in pore waters decrease slightly downhole in the upper 30 m of the core, from values of ~6 mM at the top to ~4 mM at 30 mbsf (Fig. F52). From this depth, Ca2+ concentrations gradually increase to 8.5 mM at ~300 mbsf and decline to ~5 mM at 420 mbsf. Concentrations then gradually increase with depth to reach a maximum of 21.6 mM at the bottom of the borehole.
Concentrations of dissolved magnesium (Mg2+) rapidly decrease in the upper 120 m from 49 to 26 mM, then slightly increase to 31 mM at ~350 mbsf and decline to a minimum value of 10 mM at the bottom of the borehole. This decline is particularly abrupt between 500 and 660 mbsf (Fig. F52).
Concentrations of dissolved strontium (Sr2+) increase rapidly in the upper 250 m from 92 to 198 然. Sr2+ concentrations then decline to 147 然 at ~480 mbsf and increase to reach a maximum of ~260 然 at the bottom of the borehole (Fig. F52).
Concentrations of dissolved sodium (Na+) gradually decrease downhole from 460 to 430 mM in the upper 225 m of the borehole and then increase to ~450 mM at 410 mbsf. Below this depth, Na+ concentrations decline to a minimum value of ~250 mM at the bottom of the borehole. This decline is particularly abrupt between 450 and 600 mbsf (Fig. F52).
Concentrations of dissolved potassium (K+) gradually decrease in the upper 200 m from 11 mM to 8 mM and remain at about this value for 300 m downhole in the core (Fig. F52). K+ concentrations then abruptly decrease to 6 mM at ~620 mbsf and remain at this value to the bottom of Hole 1150A. K+ concentrations in Hole 1150B exhibit a decreasing trend with depth in the remaining interval of the borehole to a minimum value of ~6 mM.
Concentrations of dissolved lithium (Li+) gradually increase from 19 to 380 然 in the upper 220 m and decrease to ~300 然 at ~400 mbsf. Below this depth, Li+ concentrations gradually increase to ~450 然 at 800 mbsf and then decline to 241 然 at the bottom of the borehole (Fig. F52).
The profiles of dissolved chemical species at Site 1150 exhibit similar trends allowing the subdivision of four characteristic geochemical intervals. The upper interval corresponds to approximately the upper 200 m. This interval is characterized by (1) the gradual decrease in alkalinity, salinity, chlorinity, Na+, K+, and Mg2+; (2) the gradual increase in Li+, Sr2+, and Ca2+; and (3) abundant methane in the sediments. The presence of gas hydrates in the sediments would explain the abundant methane and the distribution of chemical species in pore waters. In support of this preliminary interpretation, logging data indicate that this interval is also characterized by high resistivities and low density values (see "Downhole Measurements"). Similar chemical and physical characteristics have been reported for a number of gas hydrate accumulations (e.g., Hesse and Harrison, 1981; Kvenvolden and Kastner, 1990; Sloan, 1990; Malone, 1994).
This upper interval at Site 1150 also exhibits relatively high OM abundances with characteristic low (<10) C/N ratios, suggesting that most of the OM is marine in origin (Tyson, 1995). It is possible, however, that these ratios are higher because sediments, and not kerogens, were analyzed. Total nitrogen in sediments includes both organic and inorganic sources. The contribution of inorganic sources to the total nitrogen content in sediments is generally low and typically corresponds to inorganic ammonium incorporated in the interlayer position of smectites (e.g., Muller, 1977).
The relatively high content of OM in the sediments is the result of high productivity rates prevailing in the eutrophic Oyashio Current (Handa and Tanoue, 1980). In addition to high productivity rates, high OM content in the sediments can also be the result of the high sedimentation rates estimated at the site (see "Sedimentation Rates"), causing the rapid removal of OM from the oxic conditions prevailing at the seafloor, where OM is efficiently degraded by microbial activity.
The decrease in sulfate concentrations to below detection limits or very low values in the upper 10 m can be explained by the activity of sulfate-reducing bacteria. These bacteria utilize sulfate to oxidize OM, leaving reduced sulfur as a by-product. Sulfide minerals are formed as a result of the reaction of reduced sulfur with iron and probably are the main constituent in the total sulfur content determined in the sediments (see "Lithostratigraphy"). The rapid trend in decreasing sulfate concentrations in pore waters at Site 1150 can be explained by a combination of high productivity and high sedimentation rates. In areas with high productivity rates, the sulfate-reducing zone is relatively shallow due to the large amount of labile OM delivered to the sediment, promoting the rapid utilization of sulfate by microbes (Claypool and Kaplan, 1974). In addition, fast sedimentation rates result in the rapid OM burial and cause limited exchange of sulfate between pore water and seawater (Berner, 1980).
Degradation of OM in the sulfate-reducing zone can explain the increased alkalinity at Site 1150 because bicarbonate and ammonium are created during degradation of OM. An additional source of alkalinity is the silica released during alteration of either volcanic ash or biogenic opal present in the diatomaceous silty clay of the sediments (see "Lithostratigraphy"). Bicarbonate can also be generated by the oxidation of methane in the sulfate-reducing zone. This methane is originated by the partial degradation of OM by methanogens below the sulfate-reducing zone. Evidence for methanogenic activity with depth is suggested by increasing ammonium and methane gas concentrations below the minimum of sulfate at Site 1150. In addition to OM, opal and carbonates are other biogenic sources of material in the sediments. Opal alteration in the sediments yields Li+ (Gieskes, 1983), accounting for the gradual increase of Li+ concentration in pore water with depth in the upper interval. Similarly, dissolution of biogenic carbonates (Gieskes, 1983) explains the increased Ca2+ and Sr2+ values in the interval.
A second characteristic geochemical interval is between ~200 and 450 mbsf and corresponds to sediments with relatively constant or slightly changing pore-water chemistry with depth. Alkalinity, chlorinity, Na+, K+, and Mg2+ increase slightly with depth, whereas Li+, Ca2+, and Sr2+ decrease slightly with depth. The subtle increasing trend in the concentrations of dissolved chemical species could be the result of gas hydrates present in the upper interval. Formation of gas hydrates and the consequent removal of water can result in a gradient that promotes the upward and downward diffusion of dissolved chemical species (Hesse and Harrison, 1981). This migration should therefore result in increasing concentration of dissolved chemical species above and below the gas hydrates. The increased concentrations above the gas hydrates are not observed at Site 1150, probably because of dilution of pore waters with seawater.
A third interval between ~450 and 600 mbsf is characterized by an abrupt change in pore-water chemistry. Alkalinity, salinity, chlorinity, K+, Na+, and Mg2+ rapidly decrease with depth, whereas Li+, Ca2+, and Sr2+ abruptly increase. This interval probably corresponds to the boundary between two zones with different pore-water chemistry.
The deepest interval spanning depths below ~600 mbsf is characterized by a slight change in pore-water chemistry with depth. Alkalinity, salinity, chlorinity, Na+, K+, Li+, and Mg2+ exhibit low values and a decreasing trend with depth, whereas Sr2+ and Ca2+ exhibit a slight increase coupled with an excursion in C1 and C2 abundances (Fig. F50).
The origin of the pore waters in the last interval is unknown; however, several scenarios might explain the low salinity and chlorinity values. One of these scenarios is influx of meteoric waters. Site 1150 is located over 100 km from shore, making the possibility of groundwater influx unlikely. Another scenario is that the pore waters were meteoric in origin at the time of accumulation of the sediments. This is also unlikely since there is no evidence for shallower environments of accumulation. The sediments at Site 1150 are hemipelagic throughout the entire borehole (see "Lithostratigraphy"). In addition, the diatom assemblages identified in the record provide no evidence of less saline or brackish conditions at the time of accumulation (see "Biostratigraphy"). Another scenario is in situ dewatering of smectites releasing freshwater and causing the observed profiles. Two lines of evidence preclude this possibility. The amount of smectites present in the sediments is not enough (see "Lithostratigraphy") to cause the observed decrease in salinity, and the temperature prevailing at the borehole (see "Downhole Measurements") is not high enough to promote dewatering of smectites. Another scenario involves in situ dewatering of biogenic opal, but the amount present in the sediments is not enough to account for the drastic decrease in salinity and chlorinity in pore water. The only possible scenario at the moment is that less saline conditions are caused by upwelling of water with lower salinity. The source of these waters is probably related to dewatering of deeper sediments.