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doi:10.2204/iodp.proc.329.105.2011

Biogeochemistry

Site U1367 is underlain by 33.5 Ma basaltic basement and has a thinner sediment cover than Sites U1365 and U1366. Onboard measurements and sample processing continued to focus on understanding

• Microbially mediated chemical processes,

• Chemical fluxes between the sediment and the underlying basalt, and

• The potential for radiolysis to support microbial metabolism.

To achieve these objectives, a broad range of chemical species was measured. High-resolution profiles of dissolved oxygen were acquired using optodes and electrodes, headspace samples were taken for hydrogen analyses, and interstitial waters were obtained for analysis using Ti Manheim squeezers and Rhizon pore fluid samplers. Additional sediment samples were taken for solid-phase nitrogen and carbon determination, and separate whole-round intervals were sectioned and squeezed for postexpedition ¹⁴C analysis of interstitial waters.

Dissolved oxygen

Measurements of dissolved oxygen (O₂) were performed with both optodes and electrodes in Hole U1367B and only with electrodes in Holes U1367C and U1367E and on the first core of Hole U1367F (Tables T9, T10), which was drilled to recover basalt. Dissolved O₂ was measured on intact 1.5 m core sections from Cores 329-U1367B-1H through 3H and 329-U1367E-1H through 3H. Oxygen electrode measurements were also performed on 20–70 cm long whole-round sections from Cores 329-U1367C-1H through 3H after sampling in the ship’s core refrigerator on the Hold Deck. For Holes U1367B and U1367E, electrode measurements were typically performed at 15–20 cm intervals. Electrode measurements were obtained using two individually calibrated electrodes and picoammeter. Optode measurements in Hole U1367B were obtained at 10 cm intervals for the first meter and 20–50 cm intervals below 1 m. Below 16 mbsf, optode measurements became difficult because of sediment stiffness and were stopped after three optodes were broken in the process of obtaining one data point at 18.8 mbsf. Electrode measurements in Hole U1367B were performed to 21.8 mbsf and in Hole U1367E to 24 mbsf in 15–20 cm intervals. Sections 329-U1367F-1R-1 and 1R-2 were also measured using electrodes at 10 cm intervals.

Oxygen measurements in Hole U1367B show that dissolved O₂ penetrates from the seafloor to the basalt (Fig. F35A). The steepest decline in O₂ content was near the seafloor, with a decrease from ~170 at the top to 145 µM at 3 mbsf for the O₂ electrode data and from ~160 to 135 µM at 1 mbsf for the optode data. Values obtained by optode and electrode show the same depth pattern, although the optode-based O₂ profile appears smoother. At greater depths, the concentration generally declines in a monotonic manner (Fig. F35). In Hole U1367B, O₂ concentrations decline to ~81 µM at 21 mbsf, just above the basalt interface. The O₂ concentration profiles in Holes U1367B and U1367E diverge slightly with depth. These differences may partly reflect the depth of the underlying basalt in the two holes, which differ by a few meters despite the short horizontal distance between drilling locations (see “Lithostratigraphy” for differences in basement depths). Dissolved O₂ concentrations decrease toward the bottom of all holes, which indicates a net flux of dissolved O₂ toward the basement. In Hole U1367B, the O₂ gradient may steepen slightly as basement is approached, possibly caused by changes in physical properties (see “Physical properties”).

All O₂ profiles from the uppermost 10 mbsf exhibit gradients similar to those of the short (5 m) profile obtained during the site survey cruise (D’Hondt et al., 2009; Fischer et al., 2009). The O₂ profiles at Site U1367 generally match the profiles from Sites U1365 and U1366, except for a shift to lower values caused by a difference in deepwater O₂ concentration of ~200 µM at Sites U1365 and U1366 and ~170 µM at Site U1367 (Talley, 2007).

Dissolved hydrogen and methane

Dissolved hydrogen (H₂) concentration was quantified in 48 samples (Table T11). The 38 samples collected from Hole U1367C were taken in the core refrigerator on the Hold Deck. Three of the 10 samples from Hole U1367D (Samples 329-U1367D-3H-3, 135–140 cm; 3H-4, 135–140 cm; and 3H-5, 135–140 cm) were taken on the catwalk prior to moving the core sections to the core refrigerator. The sample depths ranged from 0.50 to 24.5 mbsf. Based on the average of 13 blanks, the detection limit (mean plus three times the standard deviation) at this site is 2.2 nM. The concentration of H₂ remained below the detection limit in the uppermost 4.4 m of the sediment column (Fig. F36). At greater depths, H₂ concentration was often above the detection limit but varied from depth to depth. The maximum concentration of 36 nM was at 9.4 mbsf. All three samples collected on the catwalk were below the detection limit (2.2 nM).

Methane concentrations are below the detection limit (<1.3 µM) in all samples from Hole U1367B (IODP standard safety protocol). At Site U1367, no samples were taken for the refined protocol. The detection limit is defined here as three times the standard deviation of the blank above the average for the blank (ambient air).

Interstitial water samples

Interstitial water was extracted from 65 whole-round samples from Holes U1367C and U1366D. We took these samples at a spacing of three per section (~1 sample every 50 cm). Nine of the interstitial water whole-round samples were cut and taken on the catwalk and delivered immediately to the Geochemistry Laboratory for squeezing. We extracted water by squeezing for postexpedition ¹⁴C analysis from these samples. We obtained 54 Rhizon samples for dissolved nitrate analyses from each of the whole-round samples before squeezing the whole round for further geochemical analysis.

Profiles of dissolved nitrate concentration exhibit good correlation between Holes U1367C and U1367D (Fig. F37A). Nitrate concentration near the seafloor (Sample 329-U1367C-1H-1, 0–10 cm) was 34.97 µM, similar to concentrations at Sites U1365 and U1366, and increased very slightly with depth to 36.68 µM at 1.55 mbsf (Sample 329-U1367C-1H-2, 0–10 cm). Below that depth, nitrate concentration remains relatively constant to 24.45 mbsf (36.91 µM; Sample 329-U1367D-3H-6, 0–10 cm). The decreased concentration with depth at Site U1367, as compared to Sites U1365 and U1366, is consistent with diminishment of organic-fueled respiration in subseafloor sediment as marine productivity decreases toward the center of the gyre.

Phosphate concentrations of interstitial water from Holes U1367C and U1367D decrease with increasing depth in the uppermost 10 mbsf (Fig. F37B). Peak phosphate concentration (2.06 µM) in the near-surface sediment (0.55 mbsf; Sample 329-U1365C-1H-1, 50–60 cm) is ~0.3 µM lower than the concentration expected in the overlying bottom water at this location (Talley, 2007). The pooled standard deviation (1σ) based on three individual measurements of each interstitial water sample is 0.07 µM (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). The phosphate gradient at Site U1367, in comparison to the previous sites, also appears to be slightly steeper. Below 10 mbsf, phosphate concentrations increase with increasing depth, although considerable scatter in the data is evident. The reasons for the increased variability are not clear.

Dissolved silicate concentrations vary between 260 and 300 µM throughout the sediment at Site U1367 and exhibit no discernible trend with depth (Fig. F37C). The pooled standard deviation (1σ) based on duplicate measurements of each interstitial water sample is 4 µM (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). Silica concentrations are similar to concentrations observed at Site U1366.

Alkalinity (Fig. F37D) and dissolved inorganic carbon (DIC) (Fig. F37E) behave similarly with depth in the interstitial water of Site U1367. Alkalinity gradually increases from 2.7 to 3.0 mM in the 0–5 mbsf interval and then sharply decreases to 2.6 mM at 5.5 mbsf (the lithologic transition from metalliferous clay to carbonate ooze). From 5 to 25 mbsf, alkalinity appears constant or slightly decreases with depth. Alkalinity of interstitial water samples taken in the core storage refrigerator on the Hold Deck is relatively low compared to alkalinity of the rapidly squeezed ¹⁴C samples, which indicates carbonate precipitation in sediment before interstitial water squeezing. Standard deviation and error of alkalinity measurements on standard seawater (CRM94) are 0.064 and 0.015 mM (N = 19), respectively.

In the uppermost 5 m of the sediment column, DIC increases from 2.62 to 2.95 mM (Fig. F37E). Like alkalinity, it then sharply decreases to 2.53 mM at 5.55 mbsf (the lithologic transition from metalliferous clay to carbonate ooze). Below 5.55 mbsf, DIC values are between 2.44 and 2.83 mM. This scatter could be caused by calcium carbonate precipitation, as suggested by higher DIC values from samples that were squeezed more rapidly. Average standard deviation of triplicate injection of the samples is 0.022 mM. The values from Holes U1367C and U1367D are generally consistent.

Sulfate concentrations (Fig. F37F) were determined by ion chromatography. All samples were analyzed in duplicate. Based on the pooled standard deviation of duplicate analyses, the 66% confidence limit is 0.09%, whereas the sulfate/chloride ratio 66% confidence limit is 0.05%. Measured sulfate concentrations are less than those of inferred local bottom water (28.6 mM) in the entire section. Between 1.05 and 3.05 mbsf, they vary less than ±0.15% in the sulfate anomaly, although depleted from the seawater ratio by ~5% (Fig. F37G). From 3.05 mbsf to the bottom of the holes, a larger range varies between 3% and 8% depletion. The multiple minima and maxima suggest that variability in the sulfate concentration is dominated by adsorption and desorption during sample recovery and extraction.

Chloride concentrations (Fig. F37H) were determined by ion chromatography. All samples were analyzed in duplicate. Based on the pooled standard deviation of duplicate analyses, the 66% confidence limit is 0.08%. The sulfate and chloride data from Holes U1367C and U1367D are continuous without offset (Figs. F37F, F37H). Concentrations at the top of the hole are indistinguishable from present-day bottom waters in this region (554 mM). Concentrations generally increase monotonically with depth to ~11 mbsf. From this depth to the bottom of the holes, chloride varies narrowly (560 ± 1 mM). This increase is most likely due to relict glacial seawater and possibly hydration of the underlying basement. The increase is approximately one-third of that observed at Sites U1365 and U1366.

At Site U1367, cations were measured using the Dionex ion chromatograph and the Leeman ICP-AES system (Table T12). For the ion chromatography analyses, precision was, as quantified by multiple (N = 4) triplicate analysis of International Association for the Physical Sciences of the Oceans (IAPSO) standard seawater,

• Ca = 0.96% of the measured value,

• Mg = 0.34% of the measured value,

• Na = 0.45% of the measured value, and

• K = 1.5% of the measured value.

The precision of the measurements of cations by ICP-AES was, as quantified by multiple triplicate analyses of IAPSO standard seawater and internal matrix-matched standards,

• Ca = 0.7% of the measured value,

• Mg = 0.6% of the measured value,

• Na = 2.2% of the measured value,

• K = 1% of the measured value,

• Sr = 0.5% of the measured value,

• B = 0.7% of the measured value,

• Fe = 0.5% of the measured value, and

• Mn = 0.5% of the measured value.

Accuracy of the ICP-AES results, as quantified by comparison to analysis of IAPSO not included in the calibration, was within precision of the measurement. Concentrations of Ba, Fe, and Mn were measured to be at or minimally above their detection limits (both ~1 µM), and thus the data are not tabulated. Slightly elevated Mn concentrations may be in the shallow portion of the sediment column, but this will need to be verified by postcruise analyses.

Profiles of dissolved Ca, Mg, and Sr show only slight variation throughout the short depth interval recovered at Site U1367 (Figs. F37I, F37J, F37M). Ca values start near typical seawater (10.5 mM) and show some local minima and maxima in samples gathered in the hold-level core refrigerator and the samples gathered on the catwalk. Dissolved Mg concentrations may slightly decrease with increasing depth, perhaps reflecting slight Mg uptake into the underlying basalt. Higher precision shore-based analyses will determine whether this weak trend is real or not. As with Ca, concentrations of Sr appear constant with depth.

The profile of K concentration (Fig. F37K) shows a pattern of slight increases and decreases similar to the Ca profile, although the local minima/maxima are at different depths. The K profile most significantly presents a local minima at ~11 mbsf, when plotted on a scale that shows it. This decrease is also observed in the profile of dissolved B (Table T12), and both decreases suggest the presence of clay alteration reactions through that depth range. Both ion chromatography and ICP-AES data sets are in excellent agreement in terms of trends and absolute values. Because of Dionex ion chromatograph calibration difficulties, ion chromatography values for K were only recorded for the uppermost 11 m of this profile.

Solid-phase carbon and nitrogen

Concentrations of total carbon, total organic carbon (TOC), total inorganic carbon (TIC), and total nitrogen were determined for 20 samples from Hole U1367B (Fig. F38; Table T13).

Total nitrogen decreases from 0.029 wt% at the seafloor to below the detection limit at 6.98 mbsf. TOC rapidly decreases from 0.17 wt% at the seafloor to 0.02 wt% at 6.98 mbsf, remaining low thereafter. Total carbon decreases in the first 45 cm below seafloor from 2.79 to 0.19 wt%. It then varies around 0.15 wt% to 5 mbsf, where it shows a dramatic increase to values between 9.19 and 11.16 wt% in the carbonate ooze between 5 mbsf and basement. The combined total nitrogen and TOC data suggest that the microbially degradable pool of organic matter disappears within the uppermost 7 m of sediment. The surface remineralization of this pool is also consistent with the steeper oxygen concentration gradients in the surface sediments. TOC below this depth may consist mostly of black carbon that does not undergo aerobic remineralization.

Carbonate values obtained by coulometry follow the same pattern as the TC content obtained from the CHNS elemental analyzer. CaCO₃ content decreases from 21.04 wt% at the seafloor to 0.8 wt% at 0.45 mbsf. CaCO₃ content then varies around 1 wt% to 5 mbsf, where it increases to values between 79 and 94.6 wt% until basement.

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