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doi:10.2204/iodp.proc.303306.108.2006

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 35 headspace samples from Hole U1308A were analyzed at a sampling resolution of one per core (Table T31). Methane (C₁) is the only hydrocarbon detected at this site. C₁ concentrations in Hole U1308A are low and constant, ranging from 1.9 to 4.0 ppmv. The maximum C₁ concentration is 4.0 ppmv at 137 mcd (Fig. F20).

Sedimentary geochemistry

A total of 57 samples were collected for analysis of solid-phase geochemistry (inorganic carbon and elemental C, N, and H) from Site U1308, with a sample spacing of two per core. Figure F21 shows calcium carbonate (CaCO₃) concentrations, total organic carbon (TOC) contents, total N concentrations, and organic C/N ratios. Results of coulometric and elemental analyses are reported in Table T32.

CaCO₃ contents for Site U1308 samples range from 12.1 to 94.2 wt% (average = 70.4 wt%) (Fig. F21). CaCO₃ contents fluctuate considerably from the shallowest sediment to 165 mcd and are consistently high (>64 wt%) below this depth. Below 272 mcd, CaCO₃ contents are higher than 90 wt%. TOC and total N contents range from 0 to 1.5 wt% and from 0 to 0.13 wt%, respectively (Fig. F21). The mean TOC and total N contents at Site U1308 are ~0.3 wt% and ~0.04 wt%, respectively. TOC and total N are below detection limit for several samples (Table T32).

Most C/N values range between 1 and 56 (Fig. F21). Average C/N values of 15 indicate contribution of terrigenous organic matter. Low C/N values of <4 are probably an artifact of low TOC concentrations combined with the tendency of clay minerals to adsorb ammonium ions generated during degradation of organic matter (Müller, 1977).

Interstitial water chemistry

A total of 16 whole-round samples were collected from Hole U1308A for shipboard interstitial water geochemical analyses. In addition to whole-round samples, interstitial waters were collected from small plug (~10 cm³) sediment samples for the upper ~100 mcd for shore-based studies. Results of interstitial water analyses for Site U1308 are reported in Table T33 and Figure F22.

Chloride, sodium, pH, and boron

Chloride (Cl^–) concentrations increase abruptly from 562 mM at 21 mcd to 568 mM at 33 mcd. Below 33 mcd, Cl^– concentrations decrease downhole to 563 mM at the base of the cored interval. Pore fluid sodium (Na⁺) concentrations range from 482 to 500 mM (Fig. F22). Unlike Cl^–, Na⁺ concentrations increase downhole to 215 mcd. pH values range from 6.7 to 7.3 at Site U1308 (Fig. F22). Interstitial waters are slightly acidic below 159 mcd.

Interstitial water boron concentrations range from 425 to 549 µM and are near the seawater value of 427 µM or higher (Fig. F22). Slight decreases in boron concentrations are observed in the upper 66 mcd. Below this depth, boron concentrations increase with depth toward the base of the recovered section. Boron concentrations seem to reflect pH variations, as at Sites U1306 and U1307. These increasing boron concentrations are primarily caused by desorption from clay minerals under low pH conditions (Fig. F21; Table T32).

Alkalinity, sulfate, ammonium, and dissolved silica

Alkalinity increases steadily from 3.0 mM at the surface to 6.1 mM at 66 mcd. Below 66 mcd, alkalinity varies in a narrow range (5.3–6.6 mM) (Fig. F22). Sulfate (SO₄^2–) concentrations decrease downhole in the upper 33 mcd (Fig. F22) and then more gradually to the base of the recovered section reaching a minimum of 9 mM. Ammonium (NH₄⁺) steadily increases from 24 µM at the surface to ~1000 µM at 185 mcd (Fig. F22). Dissolved silica (H₄SiO₄) concentrations range from 397 to 1215 µM (Fig. F22). The increase in H₄SiO₄ concentrations downhole reflects varying degrees of dissolution of biogenic silica in the sediments and reaches a maximum concentration of 1215 µM at 272 mcd. The maximum H₄SiO₄ concentration coincides with poorest preservation and abundance of siliceous microfossils (see “Biostratigraphy”).

Calcium, strontium, and lithium

Calcium (Ca²⁺) concentrations decrease downhole from seawater values for the uppermost sediments to a minimum of 6.8 mM at 126 mcd (Fig. F22). Below this depth, Ca²⁺ concentrations substantially increase with depth. At the base of the cored interval, Ca²⁺ concentrations reach 16.4 mM exceeding the standard seawater concentration (10.5–10.55 mM). The shallow decrease in Ca²⁺ at 126 mcd is likely caused by diagenetic carbonate precipitation because alkalinity reaches a maximum at the corresponding depth. However, the elevated Ca values at depth cannot be attributed to calcite dissolution (see Sr discussion) but rather are likely the result of sedimentary silicate mineral or basement alteration below the drilled interval (e.g., Gieskes and Lawrence, 1981) (i.e., recrystallization; see Sr discussion). Hence, the increase in Ca²⁺ at depth is not the result of carbonate dissolution.

Strontium (Sr²⁺) concentrations gradually increase with depth from the seafloor to a maximum of 1592 µM at the base of Hole U1308A (Fig. F22). Sr/Ca ratios reach a maximum of 115 at 215 mcd (Fig. F22). Below this maximum, Sr/Ca ratios remain high to the base of the cored interval. The elevated Sr values and Sr/Ca ratios are interpreted to be the result of dissolution and reprecipitation of biogenic carbonates (i.e., recrystallization), during which Sr²⁺ is expelled and released to interstitial water. The elevated Sr values cannot be the result of pure dissolution of CaCO₃. Dissolution of biogenic carbonates would not result in increasing Sr/Ca ratios because Ca is much more abundant than Sr in carbonates (Baker et al., 1982).

Lithium (Li⁺) concentrations vary little in the uppermost 43 mcd (Fig. F22). Both precipitation of calcite and alteration of volcanic material can remove Li⁺ from interstitial water. Based on the calculation using a typical Li/Ca molar ratio of 20 × 10^–6 in marine carbonates, interstitial water Li⁺ concentrations should only decrease at most by 0.2 µM because of precipitation of calcite. Hence, Li⁺ concentrations in the upper 43 mcd imply uptake of Li⁺ into silicate alteration products. At greater depth (>185 mcd), dissolved Li⁺ concentrations increase sharply downcore. A maximum Li⁺ concentration of 322 µM was observed at the base of the cored sequence at Site U1308. Several explanations for Li⁺ enrichment in deep interstitial water have been proposed. These include hydrothermal alteration of underlying basalt (Martin et al., 1991; Stoffyn-Egli and MacKenzie, 1984), alteration or ion exchange with clays (Kimura et al., 1997), and diagenetic alteration of biogenic opal A (Gieskes, 1981). High Li⁺ concentrations near the base of the hole cannot be explained conclusively with available data. However, the minimal opal A content in the sediment points toward alteration of silicate minerals or basement.

Magnesium and potassium

Magnesium (Mg²⁺) concentrations progressively decrease from seawater values of 51.7 to 24.4 mM at the base of the hole (Fig. F22). Potassium (K⁺) concentrations also decrease downcore to a minimum of 8.6 mM at the base of the hole (Fig. F22). These decreasing trends are presumably attributed to clay mineral diagenesis during which Mg²⁺ and K⁺ are consumed and Ca²⁺ is produced (Gieskes, 1973) and/or diffusional communication with basement alteration (Gieskes and Lawrence, 1981).

Manganese, barium, and iron

Manganese (Mn²⁺) concentrations decrease sharply to 33 mcd and remain low throughout the cored interval (Fig. F22). Iron (Fe²⁺) is almost depleted in the uppermost sample (Fig. F22). Below the surface, the Fe²⁺ contents fluctuate between values of 12 and 30 µM. Interstitial water barium (Ba²⁺) concentrations are low (~0.6 µM) throughout the section and show a slight increasing trend with depth (Fig. F22). The average Ba²⁺ concentration in interstitial water at Site U1308 is 0.3 µM.

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