GEOCHEMISTRY

The shipboard geochemistry program at Site 1151 included (1) analyses of volatile hydrocarbons; (2) determinations of abundances of inorganic carbon, total carbon, total sulfur, 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.)

Headspace gas analyses at Site 1151 indicate that methane concentrations are between 0.4% to 6% with an average concentration of ~2%. Ethane concentrations fluctuate between 1 and 13 ppmv and typically are ~4 ppmv. Methane/ethane ratios are consistent throughout the studied interval with values of ~4400 (Fig. F26; Table T12, also available in ASCII format). Other hydrocarbon gases are below the detection limit.

No trend is present in the distribution of carbonate abundances with depth at Site 1151 (Fig. F27). Abundances range from 0.08 to 79 wt%, with an average value of 3.3 wt%. Most values, however, fall between 2 and 4 wt% with excursions having peak values up to 15 wt%. Low carbonate abundances are in agreement with trace to few occurrences of calcareous nannofossils in the sediments (see "Biostratigraphy").

Abundance of organic carbon (C_org) fluctuates between 0.2 and 1.4 wt%, with an average value of about 0.9 wt% (Fig. F27). C_org values tend to decrease with depth in the studied interval from 1.2 wt% at the top to about 0.4 wt%. C_org/N ratios range from 2.5 to 11.5 with typical values of ~7 (Fig. F27). The distribution of C_org/N shows an irregular pattern with depth, although a general decreasing trend is observed. Values decrease from ~9 at the top to ~4 at the final depth. Total sulfur abundances irregularly fluctuate between 0.35 and 1.5 wt%, with an average value of 0.85 wt% (Fig. F27).

Salinity and chlorinity exhibit, in general, a similar decreasing trend with depth (Fig. F28). Salinity gradually decreases with depth from a value of ~32 at the top of the borehole to a value of 18 at ~900 mbsf. Below this depth, salinity remains constant at 18 to the final depth (Table T13, also available in ASCII format). Chlorinity concentrations remain constant at ~500 mM in the upper 200 m of the borehole and then steadily decrease to 320 mM at the final depth.

Alkalinity values gradually decrease downhole in the upper 200 m from 31 to 17 mM and then increase to 25 mM at ~450 mbsf (Fig. F28). Below this depth, values decrease steadily downhole to 2 mM at the bottom of the borehole. Dissolved sulfate (SO₄^2-) concentrations exhibit values lower than 2 mM throughout the borehole (Fig. F28). Values probably higher than 1 mM are the result of seawater contamination. Ammonium (NH₄⁺) concentrations increase downhole from 3.9 to 6.4 mM in the upper 450 m and then gradually decrease to about 1.5 mM at the final depth of the borehole (Fig. F28).

Concentrations of dissolved calcium (Ca²⁺) in pore waters slightly increase downhole from values of ~3 mM at the top to ~4 mM at 600 mbsf (Fig. F28). Below this depth, Ca²⁺ concentrations steadily increase to 18 mM at the bottom of the borehole.

Concentrations of dissolved magnesium (Mg²⁺) remain constant at ~25 mM in the upper 400 m of the borehole and then steadily decrease to 4 mM at the bottom of the borehole (Fig. F28).

Concentrations of dissolved strontium (Sr²⁺) increase from ~115 to ~130 µM in the upper 250 m of the borehole and then decline to 100 µM at 550 mbsf. Below this depth, Sr²⁺ concentrations increase downhole to reach a maximum value of 232 µM at the bottom of the borehole (Fig. F28).

Concentrations of dissolved sodium (Na⁺) remain relatively constant at a value of 425 mM in the upper 300 m of the borehole and then decrease to ~240 mM at the bottom of the borehole (Fig. F28). Concentrations of dissolved potassium (K⁺) steadily decrease downhole from ~10.5 mM at the top of the borehole to ~5 mM at the final depth.

Concentrations of dissolved lithium (Li⁺) gradually increase from 20 to 480 µM in the upper 850 m of the borehole and then decrease to ~280 µM at the bottom of the borehole (Fig. F28).

Several geochemical parameters exhibit similar distributions with depth. K⁺, Na⁺, Mg²⁺, chlorinity, and salinity show a characteristic decreasing trend with depth. Decreasing K⁺ concentrations are probably the result of the incorporation of potassium during the diagenetic formation of illite from smectite. Depletion of Mg²⁺ concentrations with depth are probably caused by dolomite formation and/or diagenetic transformation of volcanic ash.

The cause for the decreasing trend in salinity and chlorinity at Site 1151 is still unknown. Several hypothesis can, however, explain the observed trend. Influx of modern meteoric waters is unlikely since the shoreline is more than 100 km from the site. Similarly, influx from aquifers with ancient meteoric waters is also unlikely because sediments deposited in shallow environments are absent at Site 1151. Hemipelagic accumulation is characteristic throughout the entire studied stratigraphic sequence (see "Lithostratigraphy"). Another possible source of freshwater causing dilution of pore-water salts is in situ dewatering of smectite and biogenic opal. This possibility is also unlikely because the amount of smectites and opal present in sediments at Site 1151 is not enough to account for the observed 60% decrease in chlorinity. The only plausible scenario causing a decreasing trend in salinity and chlorinity is an upwelling of less-saline waters probably formed as a result of compaction dewatering at deep intervals.

Sediments at Site 1151 exhibit relatively high abundances of organic matter (OM) with characteristic low (<10) C_org/N ratios suggesting that most of the OM is marine in origin (Tyson, 1995). It is possible, however, that these ratios are higher since total nitrogen in sediments includes both organic and inorganic sources. The contribution of these inorganic sources to the total nitrogen content is generally low and typically corresponds to inorganic ammonium incorporated in the interlayer position of smectites (e.g., Muller, 1977). High abundances of marine OM in the studied sediments are the result of high productivity rates prevailing in the eutrophic Oyashio Current (Handa and Tanoue, 1980) and enhanced preservation caused by the high sedimentation rates estimated at the site (see "Sedimentation Rates"). Degradation of OM is rapid under the oxic conditions usually prevailing at the seafloor. High sedimentation rates promote the rapid removal of OM from the oxic zone and limit pore waters from exchanging oxygen with bottom waters, thereby enhancing the amount of OM present in the sediments.

Sulfate concentrations at Site 1151 are very low (<2 mM) in the studied sediments. A characteristic exponential decreasing trend observed in most marine sediments is not observed at Site 1151, probably because the upper 70 m was not cored with a RCB system in Hole 1151A and there was not sufficient time to analyze samples from the APC-cored holes. As at Site 1150 (see "Geochemistry" in the "Site 1150" chapter), sulfate concentrations are probably rapidly depleted in the sediments at Site 1151 as a result of active sulfate-reducing bacterial activity induced by an abundant supply of labile OM. A shallow sulfate-reducing zone can additionally be caused by limited infiltration of sulfate into the sediments under high sedimentation rates as a result of the rapid removal of sediments from the water/sediment interface (Berner, 1980). The presence of significant amounts of methane in the first core retrieved is evidence for a shallow sulfate-reducing zone at Site 1151 because methane is generated by methanogens immediately below the sulfate-reducing zone (e.g., Claypool and Kaplan, 1974). The bacterial origin of the methane is supported by the relatively high methane/ethane ratios present at Site 1151. Ratios higher than 1000 are typical of sediments with large methanogen activity (Claypool and Kaplan, 1974).