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doi:10.2204/iodp.proc.303306.104.2006

Geochemistry

Volatile hydrocarbons

Headspace gas analysis was performed as a part of the standard protocol required for shipboard safety and pollution prevention monitoring. A total of 25 headspace samples from Hole U1304A (sampling resolution of one per core) were analyzed (Table T23; Fig. F24). Methane (C₁) is the only hydrocarbon detected at this site. The C₁ concentration in Hole U1304A is relatively constant and ranges from 2.2 to 5.2 ppmv. The maximum C₁ concentration is 5.2 ppmv at 154 mcd.

Sedimentary geochemistry

Sediment samples were collected for analysis of solid-phase geochemistry (inorganic carbon and elemental C and N) at a resolution of approximately two samples per core in Hole U1304A. Figure F25 shows calcium carbonate (CaCO₃) concentrations. Total organic carbon (TOC) content, N elemental concentrations, and C/N ratios are shown in Figure F26. Results of coulometric and elemental analyses are listed in Table T24.

CaCO₃ contents range from 4.6 to 71.3 wt% and are higher than ~30 wt% in the upper sediment intervals (Fig. F25). CaCO₃ content variability increases below 120 mcd. Extremely low CaCO₃ contents are found at 197, 228, and 260 mcd. These minima coincide with the depths of diatom ooze layers (see Fig. F9); therefore, a reasonable interpretation for low CaCO₃ content is dilution by biogenic siliceous materials.

Sedimentary TOC and total N concentrations were determined on 50 samples from Hole U1304A. TOC is characterized by extremely low concentrations for most intervals at this site (average concentration = 0.4 wt%) (Fig. F26). Total N contents at Hole U1304A range from 0.03 to 0.15 wt% (average = ~0.07 wt%) (Fig. F26). The C/N ratio, which is used to distinguish the origin of organic matter (i.e., marine, degraded marine, or terrestrial) in the sediments (Emerson and Hedges, 1988; Meyers, 1997), indicates that the organic C is mainly derived from marine organic materials produced in the upper water column (Fig. F26). Relatively high C/N values are recognized in a few intervals. Further study employing other methods is necessary to determine the origin of organic C.

Interstitial water chemistry

Interstitial water samples were extracted from 12 whole-round sediment sections and processed for routine shipboard geochemical analyses. Details of sampling procedure and analytical methods for interstitial water can be found in “Geochemistry” in the “Site U1302–U1308 methods” chapter. Filtered (0.45 µm) samples were analyzed for pH, salinity, chloride, alkalinity, sulfate (SO₄^2–), ammonium (NH₄⁺), silica (H₄SiO₄), boron (H₃BO₃), iron (Fe²⁺), manganese (Mn²⁺), and major cations (Na⁺, K⁺, Mg²⁺, Ca²⁺, Li⁺, Sr²⁺, and Ba²⁺). Concentrations of dissolved chemical elements are shown in Table T25 and Figure F27.

Chloride, sodium, and pH

Chloride (Cl^–) concentrations increase to ~566 mM in the upper 50 mcd. As at Sites U1302 and U1303, this maximum Cl^– concentration at 48 mcd implies the remnant of higher salinity bottom water masses during the Last Glacial Maximum (e.g., Adkins and Schrag, 2003), although a smooth diffusional profile is not observed with discrete maxima at 92 and 186 mcd. Sodium (Na⁺) values range from 481 to 496 mM (Fig. F27). The pH profile at Site U1304 increases with depth, ranging from 7.4 to 7.7 (Fig. F27).

Dissolved silica, alkalinity, sulfate, and ammonium

Dissolved silica concentrations increase with depth from 623 µM near the seafloor to 912 µM at ~60 mcd (Fig. F27). The elevated dissolved H₄SiO₄ at Site U1304 is most likely derived from dissolution of biogenic silica in these diatom-rich sediments (see “Lithostratigraphy” and “Biostratigraphy”).

Alkalinity monotonically increases with depth from 4.5 to 19.3 mM in the upper 155 mcd, followed by relatively constant values downcore (Fig. F27). In contrast to alkalinity, the sulfate profile shows a progressive decrease from 27.3 mM near the seafloor to <3 mM at the base of the cored interval (Fig. F27). Ammonium increases with depth from 169 µM in the shallowest sample to a maximum of 1400 µM at 243 mcd (Fig. F27). The downhole increases in alkalinity and NH₄⁺ and the decrease in SO₄^2– most likely represent oxidation of organic materials through SO₄^2– reduction (Fig. F27):

1/53(CH₂O)₁₀₆ (NH₃)₁₆ H₃PO₄ + SO₄^2–

CO₂ + HCO₃^– + HS^– + 16/53NH₃ + 1/53H₃PO₄ + H₂O.

In this equation, 2 moles of alkalinity (i.e., HCO₃^–) are produced for every 1 mole of SO₄^2– that is reduced.

The increase in alkalinity is not consistent with the amount of reduced SO₄^2–. Assuming seawater concentrations of both alkalinity (2.4 mM) and SO₄^2– (~28 mM) as initial values, consumption of 24 mM of SO₄^2– at 186 mcd should result in an increase of ~48 mM of alkalinity (Fig. F27). This deficit suggests alkalinity is being consumed by another reaction, which is most likely CaCO₃ precipitation (Morse and Mackenzie, 1990).

Calcium, lithium, strontium, magnesium, and potassium

Pore water calcium (Ca²⁺) concentrations decline from approximately the seawater value of ~10.3 mM to 2.7 mM within the upper 60 mcd and decrease gradually downcore (Fig. F27). The ~75% decrease in Ca values is mostly explained by precipitation of CaCO₃, which is consistent with the inferred alkalinity consumption in the waters (Fig. F27).

Lithium (Li⁺) concentrations at Site U1304 are lower than the seawater value of 28 µM throughout the cored interval (Fig. F27). These low Li⁺ concentrations indicate uptake of Li⁺ into alteration products. The downcore Li⁺ concentration profile shows two minima at 48 and 123 mcd. Gravel counts are relatively high at these depths (see “Lithostratigraphy”). Li⁺ depletion in interstitial water has been linked to the presence of altered volcanic material in the sediment column (Stoffyn-Egli and Mackenzie, 1984). However, the highest Li⁺ concentrations are seen below 190 mcd, coincident with intervals of thick diatom ooze. Accordingly, the minima in the interstitial water most likely imply incorporation of Li⁺ into altered lithogenic material.

Dissolved strontium (Sr²⁺) contents in the uppermost sediments are close to the seawater value of 87 µM. Similar to Ca²⁺, the Sr²⁺ profile is primarily characterized by a decreasing trend, although it has a minimum concentration at 154 mcd (Fig. F27). The decrease in Sr²⁺ may indicate incorporation into diagenetic carbonate. However, like the Li⁺ profile, the minimum Sr²⁺ zones are clearly identified at the depths centered at 58 and 123–154 mcd (Fig. F27). Additionally, both Sr²⁺/Ca²⁺ and Li⁺/Ca²⁺ have distinct minima at 154 mcd (Fig. F27). This evidence suggests that some diagenetic process affects Li⁺ and Sr²⁺ concentrations in interstitial water. Thus, Sr²⁺ may also be incorporated into authigenic minerals other than secondary precipitated CaCO₃. In addition, the very low Sr²⁺ concentrations indicate that dissolution and recrystallization of biogenic carbonate are not important processes in the cored interval.

As for Sites U1302 and U1303, magnesium (Mg²⁺) and potassium (K⁺) concentrations show correlating profiles (Fig. F27). Mg²⁺ concentrations in the uppermost sediments are lower than the seawater value of 52.7 mM. In contrast, K⁺ concentrations in the same depth interval are higher than the seawater value of 10.1 mM. Mg²⁺ and K⁺ are probably being consumed in a common diagenetic process (e.g., reaction with silicate minerals).

Manganese, iron, boron, and barium

Reduction of Mn oxides is indicated by a sharp decrease in manganese (Mn²⁺) concentrations from the shallowest sample to 16 mcd (Fig. F27). Below ~16 mcd, Mn²⁺ concentrations decrease moderately downhole, reaching a minimum concentration of ~2.8 µM at 90 mcd. The downhole Mn²⁺ profile suggests that Mn²⁺ reduction occurs in the upper 40 m (Fig. F27).

Iron (Fe²⁺) concentrations are very low, ranging from 4.3 to 31.3 µM (Fig. F27). The maximum concentration of 31.3 µM is found at 27 mcd. There are two additional maxima at 123 and 212 mcd. Because of low concentrations, it is difficult to interpret the latter two Fe²⁺ maxima. Boron (H₃BO₃) concentrations in the interstitial water samples (Table T25) show a wide variety of values, ranging from 435 to 607 µM. Barium (Ba²⁺) concentrations (Table T25) monotonically increase with depth and are very low (<2.0 µM), which is expected due to barite insolubility with the sulfate reduction zone.

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