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doi:10.2204/iodp.proc.303306.107.2006

Geochemistry

Volatile hydrocarbons

Headspace gas analysis was performed as a part of the standard protocol required for shipboard safety and pollution prevention monitoring. From Hole U1307A, a total of 19 headspace samples were collected and analyzed at a sampling resolution of one per core (Table T17). Methane (C₁) concentrations decrease from 216 ppmv in the uppermost sample at 1.5 mbsf to 33 ppmv at 30 mbsf (Fig. F21). Between 30 and 63.5 mbsf, C₁ concentrations are relatively constant at ~31–35 ppmv. C₁ concentrations increase sharply downhole from 89 ppmv at 79.2 mbsf to 10,459 ppmv at 107.7 mbsf. From 107.7 mbsf toward the bottom of the recovered section, C₁ concentrations increase gradually and remain at relatively high levels ranging between 7,186 and 26,015 ppmv. Ethane (C₂) is present in sediments from 98.2 mbsf and below (Fig. F21), generally increasing downhole from 2 to 7 ppmv. No hydrocarbons higher than C₂ were detected. C₁ in the sediments at Site U1307 appears to have a biogenic origin, as indicated by the high C₁/C₂ ratios, which range between 2566 and 4023 (not shown; see Table T17 for data), and the absence of measurable higher volatile hydrocarbons.

Sedimentary geochemistry

A total of 31 sediment samples were collected for analysis of solid-phase geochemistry (inorganic carbon and elemental C, N, and S) at a sampling resolution of two per core from Hole U1307A. Figure F22 shows calcium carbonate (CaCO₃) concentrations, total organic carbon (TOC) contents, N elemental concentrations, and organic C/N ratios for Site U1307. Results of coulometric and elemental analyses are reported in Table T18.

CaCO₃ contents for Site U1307 samples are low, ranging from 0.3 to 12 wt%, except for Sample 303-U1307A-5H-6, 3–4 cm (45.5 mbsf), with 57.6 wt% CaCO₃ (Fig. F22). This sample was taken from a ~1 m thick foraminiferal sand layer (see “Lithostratigraphy”). The average value for CaCO₃ at Site U1307 is 3.8 wt% (excluding the sample from the foraminiferal sand layer at 45.5 mbsf). TOC contents at Site U1307 range between 0.01 and 0.42 wt% (average = 0.16 wt%) (Fig. F22). The mean total N content at Site U1305 is 0.04 wt% (Fig. F22). TOC and N records show no notable downhole trends. C/N values at Site U1307 are generally low, mostly ranging from 1 to 5 (Fig. F22), indicating preservation of marine organic matter in the recovered sediments. However, C/N ratios must be considered with care because the total N contents are low (i.e., <0.1 wt%). None of the analyzed samples contain measurable S.

Interstitial water chemistry

A total of 11 whole-round samples were collected from Hole U1307A for shipboard interstitial water geochemical analyses. In addition to whole-round sections, interstitial waters for shore-based studies were collected from small plug (~10 cm³) sediment samples from the upper ~100 mbsf. Results of interstitial water analyses for Site U1307 are reported in Table T19 and Figure F23.

Chloride, sodium, salinity, pH, and dissolved boron

Chloride (Cl^–) concentrations increase from 560 mM in the shallowest sample (1.5 mbsf) to 575 mM in the deepest sample (154.6 mbsf) (Fig. F23). The overall Cl^– profile at Site U1307, with changes in slope at ~20 and 80 mbsf, is similar to that at Site U1306. The downhole increasing trend in Cl^– at Site U1307 suggests hydration reactions involving clay mineral alteration in the sediments. A similar downhole Cl^– profile is reported from Site 984 in the North Atlantic (Shipboard Scientific Party, 1996). Pore fluid sodium (Na⁺) concentrations, calculated by charge balance, range from 485 to 493 mM (Fig. F23). Similar to the Cl^– profile, the highest Na⁺ concentration is recognized in the deepest sample. Salinity (not shown) decreases downhole from 35 g/kg in the uppermost sample (1.45 mbsf) to 33 g/kg at 79.15 mbsf, below which the values increase to 34 g/kg.

pH generally increases downhole in the upper 53.95 mbsf, from 7.2 to 7.9 (Fig. F23). Between 53.95 and 154.55 mbsf, pH values decrease downhole to 7.32. Interstitial boron concentrations, mostly as boric acid (H₃BO₃), range from 321 to 428 µM at Site U1307 (Fig. F23) and negatively correlate with pH (r = –0.94). Previous workers have reported that increasing boron concentrations with pH enhance the degree of boron adsorption by clay minerals (e.g., Keren and Mezuman, 1981; Palmer et al., 1987). Thus, the antithetic relationship between pore fluid pH and boron concentrations recognized at Site U1307, as well as the other Eirik Drift Sites U1305 and U1306, may be partly explained by pH control on boron adsorption by clay minerals. Along with the downhole increasing trends in interstitial water Cl^–, the antithetical pH and boron trends at the Eirik Drift sites appear to reflect reactions with the clay-rich sediments (see “Lithostratigraphy”).

Alkalinity, sulfate, ammonium, and dissolved silica

Alkalinity increases with depth from 2.9 to 10.1 mM in the upper 79.2 mbsf, followed by a slight decrease to 8.9 mM toward the lowermost sample at 154.6 mbsf (Fig. F23). Sulfate (SO₄^2–) concentrations decrease linearly in the upper 79.2 mbsf from 29.1 to 1.6 mM and remain lower than 1 mM to the deepest sample (Fig. F23). Ammonium (NH₄⁺) concentrations increase linearly (r = 0.99) over the entire recovered sequence, ranging from 11 to 1895 µM (Fig. F23).

The downhole decrease in SO₄^2– and increase in alkalinity and NH₄⁺ in the upper 79.2 mbsf at Site U1307 reflect the effects of bacterial SO₄^2– reduction. The linear SO₄^2– profile in the upper 79.2 mbsf (r = 0.99) implies diffusion of SO₄^2– through the SO₄^2– reduction zone and focused consumption of SO₄^2– at the sulfate/methane interface (SMI), probably by anaerobic methane oxidation. The depletion of SO₄^2– to ~1.6 mM at 79.2 mbsf coincides with a sharp increase in interstitial C₁ concentration (Fig. F24). Such downhole trends in the SO₄^2– and C₁ profiles are common at all the other Eirik Drift sites drilled during Expedition 303 and support the inferred biogenic origin of C₁ (Claypool and Kvenvolden, 1983; Capone and Klein, 1988). The presence of C₁ (up to 216 ppmv) above the SMI at Site U1307 indicates simultaneous SO₄^2– reduction and methanogenesis, suggesting possible consumption of noncompetitive substrates for the methanogens (Mitterer et al., 2001). The coincident alkalinity maximum at the SMI also suggests anaerobic methane oxidation because bicarbonate is a by-product of this reaction. The downhole increase in alkalinity of ~7.2 mM within the SO₄^2– reduction zone is much smaller than the magnitude expected from the degree of SO₄^2– reduction (i.e., ~55 mM), implying diagenetic consumption of alkalinity in the sediments. The continuing downhole linear trend in NH₄⁺ across the SMI at Site U1307 suggests that decomposition of organic matter and the resulting NH₄⁺ production persist into the methanogenic zone.

Dissolved silica (H₄SiO₄) concentrations generally increase downhole from 338 µM in the shallowest sample to 885 µM in the lowermost sample (average = 569 µM) (Fig. F23). The downhole variability in the dissolved H₄SiO₄ profile may reflect varying degrees of dissolution and/or the presence of biogenic silica in the sediments.

Calcium, strontium, lithium, and barium

Pore fluid calcium (Ca²⁺) concentrations increase by ~0.7 mM to 10.8 mM in the upper 20.5 mbsf and decrease steadily to 5.3 mM between 20.5 and 79.2 mbsf (Fig. F23). Below 79.2 mbsf, Ca²⁺ concentrations remain relatively constant with a slight increase toward the bottom of the recovered section. Strontium (Sr²⁺) concentrations decrease downhole in the upper 79.2 mbsf from 86.6 to 75.9 µM (Fig. F23). Below 79.2 mbsf, Sr²⁺ concentrations increase with depth to 83.0 µM in the lowermost sample. Lithium (Li⁺) concentrations decrease in the upper 23 mbsf from 22.9 to 10.4 µM and increase downhole to 23.5 µM in the deepest sample (Fig. F23).

Ca²⁺ shows an antithetic relationship to alkalinity (r = –0.97). Furthermore, the most depleted Ca²⁺ value at 79.2 mbsf (Fig. F23) coincides with the SMI (Fig. F24). This coincidence in Ca²⁺, SO₄^2–, and alkalinity suggests precipitation of carbonate and associated consumption of alkalinity within the SO₄^2– reduction zone down to ~79.2 mbsf. The similar downhole trends recognized in the Ca²⁺ and Sr²⁺ profiles within the SO₄^2– reduction zone suggest the uptake of Sr²⁺ in the same diagenetic phase.

Unlike other Eirik Drift sites cored during Expedition 303 (i.e., Sites U1305 and U1306) and Site 646 (Shipboard Scientific Party, 1987), the Ca²⁺ profile at Site U1307 does not exhibit a downhole increasing trend below the SO₄^2– reduction zone. Downhole increases in Ca²⁺ (and decreases in Mg²⁺ and K⁺) are generally interpreted as the result of the addition of Ca²⁺ ions into pore fluids associated with alteration of silicate minerals in the sediments and/or basement and subsequent diffusion (Gieskes and Lawrence, 1981). At Site U1307, the relatively constant Ca²⁺ values below the SO₄^2– reduction zone imply that the effect of silicate alteration is masked by removal of pore fluid Ca²⁺ associated with precipitation of carbonate (Fig. F23).

The downhole trend in Li⁺ (Fig. F23) does not parallel the Ca²⁺ or Sr²⁺ trends at Site U1307, although the Li/Ca and Sr/Ca ratios between 11 and 79.2 mbsf appear to correlate well (r = 0.95). This implies that Li⁺ concentrations in pore fluid are not solely controlled by diagenetic uptake by a carbonate phase. The downhole increasing trend in Li⁺ below 30 mbsf suggests that silicate phases and/or clay mineral alteration may act as a major source for Li⁺.

Interstitial water barium (Ba²⁺) concentrations are relatively low (0.3–0.7 µM) in the upper 54 mbsf (Fig. F23). Between 54 and 79.2 mbsf, Ba²⁺ concentrations increase downhole from 0.5 to 5.9 µM. The sample with elevated Ba²⁺ at 79.2 mbsf is also characterized by depleted SO₄^2–. The relatively high Ba²⁺ concentrations, up to 10.8 µM, persist to the bottom of the recovered section. As commonly observed at the other Eirik Drift sites (Sites U1305 and U1306), increasing Ba²⁺ concentrations below the SO₄^2– reduction zone reflect dissolution of barite under conditions of SO₄^2– depletion. Similar trends in interstitial water Ba²⁺ profiles are reported from regions of high-sedimentation-rate depositional environments on continental margins and are often associated with "barite fronts" at the SMI (e.g., Torres et al., 1996a, 1996b; Dickens, 2001).

Magnesium, potassium, manganese, and iron

Magnesium (Mg²⁺) and potassium (K⁺) concentrations decrease downhole at Site U1307 (Fig. F23). The total decrease throughout the cored interval is 28% for Mg and 22% for K⁺. However, it appears that the negative downhole Mg gradient is higher in the upper 79.2 mbsf (–0.17 mM/m) than the underlying sediments (0.01 mM/m). This implies a possible change in the mechanism governing the consumption of Mg²⁺ ions across the depth of alkalinity maximum. A similar trend in the Mg²⁺ profile is observed at Site U1305 where the gradient decreases between 60 and 140 mbsf but increases downhole. As inferred from interstitial Ca²⁺ and Sr²⁺ profiles, carbonate precipitation is indicated at the SMI at Site U1307. The higher gradient in the Mg²⁺ profile above this zone may reflect incorporation of ions into a carbonate mineral phase, whereas the lower gradient below 79.2 mbsf may reflect a decreased rate of removal. Although not documented at Site U1307 because of shallower penetration, the downhole decrease in pore fluid Mg²⁺ may persist in the sediments below ~155 mbsf, as observed at Site U1305, which cored to ~260 mbsf. The linear K⁺ profile at Site U1307 (r = 0.98 overall) indicates that K⁺ is being consumed by reactions within or below the cored interval, probably associated with silicate reaction (i.e., silicate diagenesis or alteration of basement) and subsequent diffusion.

Pore fluid manganese (Mn²⁺) concentrations increase sharply downhole from 0.17 µM at 1.5 mbsf to 93.5 µM at 11 mbsf (Fig. F23). Below this maximum, Mn²⁺ concentrations decrease gradually toward the bottom of the cored section to 2.7 µM. The Mn²⁺ maximum at 11 mbsf suggests Mn²⁺ production within the Mn²⁺ reduction zone. Iron (Fe²⁺) concentrations range from 0.08 µM in the uppermost sample to 19.6 µM in the deepest sample but do not show any significant downhole trend (Fig. F23). The lowest Fe²⁺ concentration at 1.5 mbsf indicates an oxidized condition evident in the other redox-sensitive element, Mn²⁺, and sediment color. In general, the overall low interstitial Fe²⁺ concentrations indicate sequestration of dissolved ions into iron sulfides, which are commonly observed in the sediments (see “Lithostratigraphy”).

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