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doi:10.2204/iodp.proc.303306.106.2006

Geochemistry

Volatile hydrocarbons

Methane (C₁) and ethane (C₂) contents remaining in cores were measured using the headspace technique. The results are reported in Table T23 and Figure F25.

C₁ concentrations gradually increase from 0 to 259 ppmv in the interval from 1.5 to 72 mcd and increase rapidly to a maximum value of 42,644 ppmv at 237 mcd. Ethane was detected below 133 mcd, ranging from 1.0 to a maximum value of 7.1 ppmv at 290 mcd. No higher hydrocarbons were detected.

Sedimentary geochemistry

A total of 55 samples at a sampling resolution of two per core were collected for analysis of solid-phase geochemistry (inorganic carbon and elemental C, N, and H) from Site U1306. Figure F26 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 T24.

CaCO₃ contents for Site U1306 samples are extremely low, ranging from 0.3 to 12.3 wt% (average = 3.2 wt%) (Fig. F26). TOC and total N contents at Site U1306 range from 0.1 to 1.1 wt% and from 0.03 to 0.10 wt%, respectively (Fig. F26). The mean TOC and total N contents at Site U1306 are ~0.3 wt% and ~0.06 wt%, respectively. TOC contents slightly increase downhole. No notable trend is recognized in downhole N records.

Most C/N values are low (Fig. F26) and are in the range typical of fresh algal organic matter (4–10) (Meyers, 1997). The 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 15 whole-round samples were collected from Hole U1306A for shipboard interstitial water (IW) geochemical analyses. In addition to whole-round samples, IWs were collected from small plug (~10 cm³) sediment samples for the upper ~100 mcd for shore-based studies. Results of IW analyses for Site U1306 are reported in Table T25 and Figure F27.

Chloride, sodium, pH, and boron

The chloride (Cl^–) profile at Site U1306 exhibits an increasing downhole trend (Fig. F27). Cl^– concentrations increase from 561 mM in the shallowest sample (1.5 mcd) to 581 mM in the deepest sample at 311 mcd. Pore fluid sodium (Na⁺) concentrations range from 440 to 496 mM. Na⁺ concentrations increase downhole (Fig. F27) to a maximum of 496 mM, and the Na⁺ trend is similar to that of Cl^–. pH values range from 7.0 to 7.7 with distinct fluctuations observed below 123 mcd (Fig. F27).

IW boron concentrations ranging from 411 to 559 µM are mostly higher than the seawater value of 427 µM (Fig. F27). The boron profile has two minima corresponding to pH maxima below ~80 mcd. Within the pH range of IW in this hole, boron exists mainly as boric acid (H₃BO₃), as well as borate ions. Keren and Mezuman (1981) reported that pH and temperature affect adsorption of boron by clay minerals. The adsorption distribution factor of boron is controlled by pH (Xiao and Wang, 2001). High boron concentrations relative to seawater imply desorption of boron from clay and carbonate minerals. Because carbonate contents are very low throughout this hole (Fig. F26; Table T24), the pH dependency of boron adsorption and desorption from clay minerals is the most likely explanation of the antithetic trend between boron and pH in IWs.

Alkalinity, sulfate, ammonium, and dissolved silica

In the upper ~92 mcd at Site U1306, alkalinity increases steadily with depth from 3.9 to 18.7 mM (Fig. F27), which reflects the effects of sulfate (SO₄^2–) reduction and decomposition of organic matter. Below 157 mcd, alkalinity decreases progressively to 11.1 mM toward the base of the recovered section.

SO₄^2– concentrations decrease downhole in the upper 92.3 mcd (Fig. F27). SO₄^2– is almost depleted at 123 mcd. Interestingly, SO₄^2– increases slightly between 157 and 280 mcd to a maximum of 1.3 mM at 217 mcd.

Ammonium (NH₄⁺) steadily increases from 55 µM at the surface to a maximum of ~2000 µM at the base of the hole. This suggests that the decomposition of organic matter and the resulting NH₄⁺ production persist below the SO₄^2– reduction zone into the zone of methanogenesis (Fig. F27).

Dissolved silica (H₄SiO₄) concentrations range from 488 to 945 µM (Fig. F27). The downhole increasing H₄SiO₄ trend may reflect varying degrees of preservation and dissolution of biogenic H₄SiO₄ in the sediments (see “Biostratigraphy”).

Calcium, strontium, and lithium

Calcium (Ca²⁺) concentrations decrease downhole from seawater values in the shallowest sediment to a minimum of 3.7 mM at 92 mcd (Fig. F27). Below this depth, Ca²⁺ concentrations substantially increase with depth. The Ca²⁺ minimum is coincident with the alkalinity maximum at the sulfate/methane interface (SMI). Methane oxidation probably stimulates carbonate precipitation through production of alkalinity:

CH₄ + SO₄^2– → HCO₃^– + HS^– + H₂O.

Strontium (Sr²⁺) concentrations decrease with depth from the seafloor to a minimum of 66.5 µM at 52 mcd, below which values increase to 95.2 µM at the base of the hole (Fig. F27). The decrease in Sr²⁺, which is generally coincident with the minimum of Ca²⁺, suggests incorporation of Sr into a diagenetic carbonate phase. The lack of large increases in Sr²⁺ with depth indicates minimal dissolution or recystallization of biogenic carbonate within the cored interval.

Lithium (Li⁺) concentrations decrease with depth from the seafloor to a minimum of 14.1 µM at 31 mcd. Consistently low Li⁺ concentrations ranging from 14.1 to 15.8 µM were observed between 31 and 123 mcd. These low Li⁺ concentrations imply uptake of Li⁺ into alteration products. Below 123 mcd, IW Li⁺ concentrations increase steadily toward the bottom of the drilled sequence at Site U1306.

Magnesium and potassium

Magnesium (Mg²⁺) concentrations gradually decrease from the seawater value of 52.7 to 36.3 mM at the base of the hole (Fig. F27). Potassium (K⁺) concentrations monotonically decrease downcore to a minimum of 7.4 mM at the base of the hole (Fig. F27).

The changes in Mg²⁺ and K⁺ concentrations are highly correlated (r² = 0.98) (Fig. F27), suggesting that both elements are being removed from pore waters by similar processes, most likely basement alteration and/or ion-exchange reactions associated with clay within or below the cored interval (Gieskes and Lawrence, 1981; De Carlo, 1992). Downhole decreases in Mg²⁺ and K⁺ and increases in Ca²⁺ are consistent with silicate basement alteration shown in Gieskes and Lawrence (1981).

Manganese, barium, and iron

Higher manganese (Mn²⁺) concentrations suggest Mn²⁺ reduction in the upper 41 mcd (27–36 µM). Mn²⁺ concentrations decrease sharply down to 52 mcd and remain at low concentrations (6.3–15.6 µM) to the base of the cored sequence at Site U1306 (Fig. F27).

IW barium (Ba²⁺) concentrations are low (0.1–0.5 µM) in the upper 62 mcd (Fig. F27). At the base of the sulfate reduction zone, Ba²⁺ concentrations increase from 0.4 to 9.9 µM. Below 92 mcd to the bottom of the section, Ba²⁺ concentrations remain relatively high (7.4–11.8 µM). These higher Ba²⁺ concentrations are probably a consequence of Ba released from skeletal debris and ash as well as local dissolution of biogenic BaSO₄ in reducing environments. Notable decreases of Ba²⁺ are recognized between 157 and 248 mcd. Because SO₄^2– concentrations are elevated in the same depth range, these decreased Ba²⁺ concentrations result from remineralization or inhibited dissolution of barite (Fig. F27).

Iron (Fe²⁺) is almost depleted in the uppermost sample. Fe²⁺ concentrations increase to a maximum of 24.4 µM at 20 mcd (Fig. F27). Below this maximum, Fe²⁺ concentrations are relatively constant, ranging from 12.0 to 15.8 µM between 31 and 92 mcd. The low IW Fe²⁺ concentrations in the upper sediments indicate sequestration of dissolved ions into a solid phase (i.e., iron sulfides), which is commonly observed in the sediments (see “Lithostratigraphy”).

Fe²⁺ concentrations fluctuate considerably below 92 mcd, with minima at 158 and 280 mcd. Interestingly, these Fe²⁺ variations are antithetic to pH fluctuations (Fig. F27). Furthermore, elevated SO₄^2– and Fe²⁺ concentrations are both centered at 217 mcd, where intervals of pyrite are described in the cores (see “Lithostratigraphy”). These observations suggest the possibility of anaerobic oxidative dissolution of pyrite (Brothers et al., 1996):

FeS₂ + 14[Fe–L]⁺ + 8H₂O → 15Fe²⁺ + 12HL^– + 2H₂L + 2SO₄^2–,

where L is any organic ligand. In this reaction, the more soluble ferric iron complexes come in contact with pyrite grains, causing oxidative dissolution, which in turn results in localized decreases in pH. Low CaCO₃ contents below 172 mcd could be associated with this lowered pH.

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