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doi:10.2204/iodp.proc.303306.105.2006

Geochemistry

Volatile hydrocarbons

Concentrations of interstitial gases were routinely monitored in Hole U1305A sediments according to shipboard safety and pollution prevention protocols. A total of 30 headspace samples were collected and analyzed at a sampling resolution of one per core (Table T21). Within the upper 48.4 mbsf, methane (C1) is present only in minor concentrations (4–19 ppmv) that are slightly higher than background levels (Fig. F24). C1 concentrations increase sharply (from 19 to 1816 ppmv) downhole between 48.4 and 57.9 mbsf. Below 57.9 mbsf, C1 concentrations increase steadily to 33,877 ppmv at 114.9 mbsf. From 114.9 mbsf to the bottom of the recovered section, C1 concentrations remain at relatively high values, ranging between 25,586 and 46,032 ppmv. Ethane (C2) is present in sediments at 76.9 mbsf and below (Fig. F24), generally increasing downhole from 2 to 14 ppmv. No hydrocarbons higher than C2 were detected.

In general, C1/C2 ratios decrease downhole, ranging from 2698 to 7574 (not shown; see Table T21 for data). The high C1/C2 ratios and the absence of measurable higher volatile hydrocarbons indicate that C1 in the sediments at Site U1305 has a biogenic origin.

Sedimentary geochemistry

A total of 58 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 U1305A. Figure F25 shows calcium carbonate (CaCO3) concentrations, total organic carbon (TOC) contents, N elemental concentrations, and organic C/N ratios for Site U1305. Results of the coulometric and elemental analyses are reported in Table T22.

CaCO3 contents for the Site U1305 samples are generally low, ranging from 0.9 to 49 wt% (average = 12.3 wt%) (Fig. F25). Two samples with the highest carbonate contents of >40 wt% at 170 and 162.2 mbsf coincide with "silt lamina" layers that are interbedded in a silty clay matrix (see “Lithostratigraphy”). No significant downhole trend is recognized for the Site U1305 carbonate profile. TOC contents at Site U1305 range between 0.1 and 0.6 wt% (average = 0.4 wt%) (Fig. F25). The mean total N content at Site U1305 is 0.06 wt% (Fig. F25). TOC and N records do not show notable downhole trends. Most of the C/N values are 3–10 (Fig. F25), indicating preservation of marine organic matter in the recovered sediments at Site U1305. Sediment samples with elevated C/N ratios at 170 and 162.2 mbsf are also characterized by relatively high CaCO3 values (48 wt%). These samples are derived from “silt lamina” layers that are composed of inorganic carbonate and siliciclastic grains (see “Lithostratigraphy”), suggesting that the high C/N values resulted from degradation of marine organic matter or influx of terrestrial organic matter or a combination of both. However, C/N ratios must be considered with care because total N contents are low (<0.1 wt%) through the cored sequence. None of the analyzed samples contain measurable S.

Interstitial water chemistry

A total of 14 whole-round samples were collected from Hole U1305A for shipboard interstitial water geochemical analysis. In addition to whole-round sections, interstitial waters were collected from small plug (~10 cm3) sediment samples from the upper ~100 mbsf for shore-based studies. Results of interstitial water analyses for Site U1305 are reported in Table T23 and Figure F26.

Chloride, sodium, salinity, pH, and dissolved boron

The chloride (Cl) profile at Site U1305 exhibits an increasing trend downhole (Fig. F26). Cl concentrations increase from 560 mM in the shallowest sample (1.5 mbsf) to 573 mM in the deepest sample (257.4 mbsf). Pore fluid sodium (Na+) concentrations increase downhole (Fig. F26), exhibiting a trend that is similar to the chlorinity profile. The downhole increasing trends in the Cl and Na+ values below ~60 mbsf at Site U1305 suggest uptake of water molecules by diagenetic processes involving clay mineral alteration. A similar downhole chlorinide profile is reported from ODP Site 984 in the North Atlantic (Shipboard Scientific Party, 1996). Salinity (not shown) decreases downhole from 36 g/kg in the uppermost sample (1.5 mbsf) to 33 g/kg at 114.9 mbsf.

pH values generally increase downhole, ranging from 7.3 to 7.9 (Fig. F26). Boron concentrations in the interstitial water samples, mostly as boric acid (H3BO3) at the measured pH range, generally decrease downhole from 551 to 411 mM (Fig. F26). Boron and pH at Site U1305 appear to be negatively correlated (r = –0.78).

Alkalinity, sulfate, ammonium, and dissolved silica

In the upper 57.9 mbsf at Site U1305, alkalinity increases steadily with depth from 9.1 to 18.9 µM (Fig. F26). Alkalinity is unusually high in the uppermost sample, which is likely a combination of the high rates of sulfate (SO42–) reduction and the fact that the upper part of the sediment column was not recovered in Hole U1305A (see “Composite section”). Below 57.9 mbsf, alkalinity decreases to 6 mM for the deepest sample collected at 257.4 mbsf. SO42– concentrations decrease downhole linearly from 21.5 mM in the shallowest sample to 0.1 mM at 57.9 mbsf. Below this depth, SO42– values remain lower than 0.4 mM toward the bottom of the recovered sediment section (Fig. F26).

The downhole decrease in SO42– and increase in alkalinity in the upper 57.9 mbsf at Site U1305 appear to reflect the effects of microbial SO42– reduction as recognized at other sites drilled during Expedition 303. The linear SO42– profile in the upper 48.4 mbsf (r = 0.99) suggests diffusion of SO42– through the SO42– reduction zone and focused consumption of SO42– at the sulfate/methane interface (SMI), probably by anaerobic methane oxidation. The complete depletion of SO42– between 48.4 and 57.9 mbsf coincides with a sharp downhole increase in interstitial C1 concentrations (Fig. F27). Such downhole trends in the SO42– and C1 profiles support the inferred biogenic origin of C1 at Site U1305 because interstitial SO42– generally inhibits microbial methanogenesis (Claypool and Kvenvolden, 1983; Capone and Klein, 1988), although exceptions have been observed (e.g., Mitterer et al., 2001). In addition, C1 can serve as a substrate for SO42– reduction (Hinrichs et al., 1999; Boetius et al., 2000; Nauhaus et al., 2002), thereby suppressing C1 above the SMI. The coincident alkalinity maximum at the SMI also suggests anaerobic methane oxidation because bicarbonate is a byproduct of this reaction. The downhole increase in alkalinity of ~9.7 mM within the SO42– reduction zone is smaller than the magnitude expected from the degree of SO42– reduction (i.e., ~43 mM), implying diagenetic consumption of alkalinity in the sediments.

Ammonium (NH4+) concentrations increase steadily downhole over the entire drilled sequence, ranging from 0.6 to 2.5 mM (Fig. F26). No downhole change in the direction or gradient of the NH4+ trend is identified across the SO42– reduction/methanogenesis interface, suggesting that decomposition of organic matter and the resulting NH4+ production persist below the SO42– reduction zone.

Dissolved silica (H4SiO4) concentrations range from 589 to 840 µM (average = 720 µM) (Fig. F26). Although no significant trend is recognized in the H4SiO4 profile, its downhole variability may reflect varying degrees of dissolution or the presence of biogenic silica in the sediments.

Calcium, strontium, lithium, and barium

In the upper 57.9 mbsf at Site U1305, pore fluid calcium (Ca2+) concentrations decrease steadily from 7.8 mM to 2.6 mM (Fig. F26). Below 57.9 mbsf, Ca2+ concentrations increase toward the lowermost sample at 257.4 mbsf, in which the highest Ca2+ value of 9.4 mM is attained. Downhole trends in the strontium (Sr2+) and lithium (Li+) profiles are similar to that of Ca2+ at Site U1305 (Fig. F26), although the depth of the most depleted Sr2+ concentration occurs at 38.9 mbsf, 19 m shallower than the Ca2+ minimum.

Downhole variabilities in interstitial water Ca2+, Sr2+, and Li+ concentrations at Site U1305 are influenced by various diagenetic as well as diffusional processes in the sediment column. The downhole trend in Ca2+ shows an antithetic relationship to alkalinity (r = –0.96). Furthermore, the most depleted Ca2+ value at 57.9 mbsf (Fig. F26) appears to coincide with the SMI (Fig. F27). This coincidence in the Ca2+, SO42–, and alkalinity profiles suggests precipitation of carbonate and associated consumption of alkalinity within the SO42– reduction zone down to 57.9 mbsf. The parallel downhole decrease in interstitial water Sr2+ and Li+ concentrations within the SO42– reduction zone suggests their uptake in the same diagenetic phase.

Below 57.9 mbsf, interstitial water Ca2+, Sr2+, and Li+ concentrations increase steadily to the bottom of the drilled sequence at Site U1305. Ca and Sr values approach or meet seawater values at the base of the cored section. Hence, the profiles may reflect precipitation only at ~58 mbsf and diffusion from above and below. However, Li increases above seawater values and Ca and Sr may continue to increase below the cored interval, which may indicate dissolution of a mineral phase at depth (see “Magnesium, potassium, manganese, and iron” below).

Interstitial water barium (Ba2+) concentrations are relatively low (~0.3–0.9 µM) in the upper 38.9 mbsf (Fig. F26). Between 48.4 and 57.9 mbsf, Ba2+ concentrations rise sharply downhole from 3.3 to 16.6 µM. Below 57.9 mbsf to the bottom of the section, Ba2+ remains at relatively high concentrations ranging between 9.9 and 18.2 µM. The major shift in the Ba2+ profile between 48.4 and 57.9 mbsf appears to occur at the SMI, suggesting that the relatively high interstitial water Ba2+ concentrations below the SO42– reduction zone reflect dissolution of barite under conditions of SO42– 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 steadily and consistently downhole at Site U1305 (Fig. F26). By the deepest sample at 257.4 mbsf, Mg2+ decreases by ~43% to 30 mM and K+ decreases by ~41% to 7 mM from the near-seawater values in the topmost sample. The correlation of profiles between Mg2+ and K+ concentrations (r = 0.96) suggests that these ions are removed from pore waters by a similar process(es). In the upper 57.9 mbsf, it appears that the gradients of the Mg2+ and K+ profiles are shallower than those in underlying sediments, implying a possible change in mechanisms governing the consumption of Mg2+ and K+ ions across the depth of alkalinity maximum. The linear diffusional profiles of Mg2+ and K+ concentrations below 57.9 mbsf (r = 0.98 for the both profiles) indicate that these ions are being consumed in reactions within or below the cored interval, probably associated with silicate reactions (i.e., silicate diagenesis or alteration of basement). The downhole decreases in Mg2+ and K+ and increase in Ca2+ (below the SMI) are characteristic of profiles above altering basement (Gieskes and Lawrence, 1981). By comparison with Site 646 nearby, which cored deeper to 767 mbsf (Shipboard Scientific Party, 1987), alteration of basement is the likely cause of the Mg2+, K+, and lower Ca2+ profiles.

Pore fluid manganese (Mn2+) concentrations are relatively high in the upper 29.4 mbsf (21.5–32.3 µM), decrease sharply down to 57.9 mbsf, and remain at relatively low values of 4.3–8.8 µM to the bottom of the cored sequence (Fig. F26). The relatively high Mn2+ values in the upper 29.4 mbsf suggest that the samples are derived from the lowermost part of the Mn2+ reduction zone. Iron (Fe2+) concentrations range from 3.4 to 19.8 µM but do not show any significant downhole trend (Fig. F26). The low interstitial water Fe2+ concentrations indicate sequestration of dissolved ions into iron sulfides, which are observed in the sediments (see “Lithostratigraphy”).