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doi:10.2204/iodp.proc.329.104.2011

Biogeochemistry

The biogeochemistry program at Site U1366 continued to focus on samples for chemical analysis of

• Interstitial water,

• Solid carbon- and nitrogen-containing phases, and

• Gas phases of biogeochemical relevance to the cruise objectives.

Sixty-six interstitial water samples were collected by whole-round squeezing and 66 were collected by Rhizon samplers (for NO₃ analyses). Dissolved oxygen was measured using electrode and optode techniques (296 total measurements). Twenty-three samples were collected for methane quantification, 66 samples for hydrogen analyses, and 35 for solid organic phase analyses. Six interstitial water samples were also collected by whole-round squeezing for shore-based ¹⁴C analysis.

Because of the very thin sediment cover (25–30 m; see “Lithostratigraphy”) and because of the difficulty in drilling unconsolidated sediment containing several chert layers, we analyzed sediment from Hole U1366C (for dissolved O₂) and Holes U1366D and U1366F (all other measurements) to ensure adequate coverage of all sediment depths. Accordingly, some of the following discussion addresses interhole correlation and interpretations of hole quality (e.g., potential for flow-in to have occurred through certain depths). We also compare interstitial water data from cores that were stored for lengthy periods of time (in some cases >12 h) prior to extracting the interstitial water to data from cores that were stored for only few hours prior to squeezing or taking Rhizon samples.

Dissolved oxygen

Measurements of dissolved oxygen (O₂) using optode and electrode methods were made separately or in combination on Holes U1366B, U1366C, U1366D, and U1366F. Optode measurements of dissolved oxygen were made (after sampling in the core refrigerator on the Hold Deck) on intact 1.5 m core sections from Cores 329-U1366C-1H through 3H and 329-U1366F-4H and on 20–70 cm long whole-round sections from Cores 329-U1366F-1H through 3H. With the exception of Sections 329-U1366F-4H-4 and 4H-5, in which higher resolution optode measurements were made (see below), optode determinations of oxygen concentration were obtained at 25–50 cm intervals. Electrode measurements, using two individually calibrated electrode/picoammeter setups, were performed on 1.5 m sections in 20 cm intervals from Cores 329-U1366C-1H through 3H and similarly on 1.5 m sections from Core 329-U1366B-2H. The first core of Hole U1366B was damaged, and no electrode measurements were attempted. For Cores 329-U1366D-1H and 2H and Cores 329-U1366F-1H through 3H, electrode measurements were performed on whole-round sections remaining after biogeochemical and microbiological sampling.

The optode and electrode measurements show that dissolved oxygen penetrates through the entire sediment column to the basalt (Fig. F30; Tables T5, T6). In Holes U1366C and U1366F, optode measurements were highly reproducible between holes and show that oxygen declines from ~180 µM in the near-surface sediment to ~165 µM at 4 mbsf. At greater depths, the concentration declines linearly toward the basement, indicating a net flux for dissolved oxygen in the underlying basalt (Fig. F30A). Oxygen concentrations remain >110 µM throughout the sediment column, except at two depths at the base of the sediment column in Hole U1366F (29.75–29.77 and 29.86 mbsf). At these depths, the lowest oxygen concentrations were, respectively, 57 and 84 µM. These low values are bracketed by higher concentrations (110–111 µM). The cause of the low concentrations is not understood; however, their reproducibility and their position near the basalt/sediment interface are consistent with them having resulted from optode insertion into partially oxidized basalt clasts in the altered red groundmass.

Electrode measurements in Hole U1366C show a similar decrease from near-surface concentrations of ~180 to nearly 100 µM at 25.3 mbsf. Oxygen profiles from the upper 0–10 m exhibit a steep decrease (see profile from Hole U1366C in Fig. F30C) of 40 to 50 µM and are indicative of aerobic microbial respiration in the lithologic Unit I sediments. Measured concentrations of dissolved oxygen (optode and electrode) were also similar to dissolved oxygen profiles obtained in the uppermost 8 mbsf during the site survey cruise (D’Hondt et al., 2009; Fischer et al., 2009). Moreover, the dissolved oxygen profile in Hole U1366C nearly matches the profiles in Holes U1365A and U1365B (see Fig. F56 in the “Site U1365” chapter [Expedition 329 Scientists, 2011b]). Below 10 mbsf, dissolved oxygen concentrations decrease toward the basement, which suggests a net flux of dissolved oxygen to the basement.

Overall, the dissolved oxygen profiles measured using electrodes are similar throughout all the investigated holes, but some hole-to-hole and method-to-method variability is evident. In particular, O₂ concentrations measured by electrode appear to be generally lower for Hole U1366D and generally higher for Hole U1366F than concentrations measured in Hole U1366C by electrode or optode or concentrations measured in Hole U1366F by optode. Although the cause(s) of these differences are not clear, their ultimate cause may be related to core handling; dissolved oxygen analyses of Holes U1366D and U1366F were limited to the short core pieces that remained after intensive sampling for biogeochemistry and microbiology whole-round samples. Oxygen analyses of the short core pieces were only performed in centers of pieces to minimize edge effects. Adverse effects of whole-round sampling are not observed in the optode measurements on the short core pieces from Section 329-U1366F-1H-3. Based on the good correspondence between electrode and optode measurements of Hole U1366C and optode measurements of Hole U1366F, we infer that the discrepancies between optode and electrode measurements for Hole U1366F may be due to the several-hour delay in taking the electrode measurements. The only exception is the lowermost 6 m of Core 329-U1366F-4H, which was analyzed by optode, but not electrode, prior to whole-round sampling.

Dissolved hydrogen and methane

Dissolved hydrogen gas (H₂) concentration was quantified in 66 samples (0.50–29.5 mbsf) collected in the ship’s core refrigerator from Holes U1366D and U1366F. Based on the average of 13 blanks, the detection limit at this site was calculated to be 5.2 nM. The concentration of H₂ is below the detection limit in the uppermost 7 m of the sediment column (Fig. F31; Table T7). Below 14 mbsf, H₂ concentration increases with depth to several tens of nanomolar in the lowermost sediment.

Methane concentration is below the detection limit (<1.3 µM) in all samples from Holes U1366B (IODP standard safety protocol), U1366D, and U1366F (refined protocol). The detection limit is defined here as three times the standard deviation of the blank (ambient air).

Interstitial water samples

Interstitial water was extracted from 66 whole-round samples from Holes U1366D and U1366F. High-resolution sampling of three samples per section (approximately one sample every 50 cm) was obtained (Table T8). Rhizon samples for dissolved nitrate analyses were obtained from all of the whole-round samples before squeezing.

Profiles of dissolved nitrate concentration in Holes U1366D and U1366F show good correlation between the holes (Fig. F32A). Nitrate concentration near the sediment surface (Sample 329-U1366D-1H-1, 30–40 cm) is 34.87 µM but gradually increases with depth to 40.03 µM at 28.75 mbsf (Sample 329-U1366F-4H-4, 70–80 cm). In conjunction with the consistent decrease in dissolved O₂ with increasing depth (~55 µM decrease from 3 to 30 mbsf; see Fig. F30), the increase in nitrate concentration with depth (by ~5 µM) implies oxidation of reduced nitrogen species (nitrification) derived from aerobic decomposition of organic matter. However, the increase of nitrate concentration from surface sediment to 15 mbsf (~5 µM) is relatively small compared to the increase at gyre-margin Site U1365 (~10 µM), suggesting that organic matter in the subseafloor sediment may diminish as marine primary productivity decreases toward the center of the gyre.

Dissolved phosphate concentrations decrease with increasing depth in Holes U1366D and U1366F (Fig. F32B). The peak concentration of 1.94 µM in near-surface sediment (1.35 mbsf; Sample 329-U1365D-1H-1, 130–140 cm) is ~0.7 µM lower than overlying bottom water concentrations reported for this region of the Pacific Ocean (2.6 µM; Talley, 2007). Phosphate concentration decreases to <1 µM by 15 mbsf and then decreases less steeply to ~0.5 µM at 29.36 mbsf near the basement (Sample 329-U1365F-4H-5, 40–50 cm). The pooled standard deviation is 0.070 µM (1σ) for all phosphate concentration data from both Holes U1366D and U1366F. The low concentration relative to bottom water may reflect removal by adsorption onto mineral or metal oxide surfaces. Overall, the phosphate profile indicates a flux of phosphate from the ocean to the sediment.

Concentrations of dissolved silicate are lower by 50–75 µM at Site U1366 than at the previous Site U1365 (Fig. F32C). They still, however, substantially exceed typical bottom water values (123 µM; Talley, 2007) for this region of the Pacific Ocean. Concentrations of dissolved Si range from 220 to 300 µM throughout the sediments and exhibit no clearly discernible trend with depth. A slight excursion in the dissolved Si concentration corresponds to the layers of porcellanite near 20 mbsf in lithologic Unit II (interval 329-U1365F-3H-5, 30–40 cm).

Alkalinity increases from 2.6 mM in the 0–0.1 mbsf interval to 2.8 mM at 15 mbsf in Holes U1366D and U1366F (Fig. F32D), perhaps caused by oxidation of organic matter. From 15 to 20 mbsf, alkalinity gradually decreases to 2.6 mM with depth. In the lower sediment column (20–30 mbsf), the data points scatter ~2.6–2.8 mM. Standard deviation and error for our measurements of alkalinity in standard seawater (CRM94) are 0.015 and 0.003 mM (N = 19), respectively.

Dissolved inorganic carbon (DIC) increases from 2.5 mM at 0.35 mbsf to 2.8 mM at 13.25 mbsf before decreasing to ~2.6 mM at 20 mbsf (Fig. F32E). Below 20 mbsf, DIC values scatter between 2.5 and 2.8 mM. The range in DIC values is 0.3 mM. Average standard deviation of triplicate injection of the samples is 0.018 mM. The values from Holes U1366D and U1365F are generally consistent with one another.

Dissolved sulfate concentrations are 27–29 mM throughout the entire section (Fig. F32F). Between 0.35 and 8.85 mbsf, the sulfate anomaly ratio varies <0.5% (Fig. F32G). Between 10 and 28 mbsf, the sulfate anomaly exhibits a larger range, varying in an S-shaped pattern between –4% and –7%. Below 25 mbsf, sulfate increases smoothly and nearly monotonically from 27.8 to 28.1 mM, except for the deepest sample, which is offset to 29.4 mM. The variations in sulfate and sulfate anomaly are correlated with the lithologic units, suggesting that the small offsets from seawater concentrations may be due to adsorption and desorption during sample recovery and extraction.

Chloride concentrations in Holes U1366D and U1366F are continuous without offset (Fig. F32H). Concentrations at the top of the holes are indistinguishable from present-day bottom waters in this region (554 mM). Concentrations increase monotonically and reach a maximum of ~571 mM in the lowermost 5 m of Hole U1366F (a 3% increase). This increase may be due to relict glacial seawater and perhaps hydration of the underlying basement.

At Site U1366, the precision of the measurements of cations by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) are as follows, as quantified by multiple triplicate analyses of International Association for the Physical Sciences of the Oceans) IAPSO standard seawater and internal matrix matched standards:

• Ca = 1.3% of the measured value,

• Mg = 1.5% of the measured value,

• Na = 1% of the measured value,

• Sr = 0.4% of the measured value, and

• B = 0.7% of the measured value.

Accuracy of the ICP-AES results, as quantified by comparison to analysis of IAPSO standard not included in the calibration, is within precision of the measurement. Because of difficulties with the instrument, K was not analyzed. Concentrations of Fe and Mn hover near or below their respective detection limits (both 3 µM) and are plotted for illustrative purposes only. For the ion chromatography analyses, precision was as follows, as quantified by multiple triplicate analysis of IAPSO standard seawater:

• Ca = 0.77%,

• Mg = 0.25%,

• Na = 0.12%, and

• K = 0.40%.

Profiles of dissolved Ca, Mg, and Sr are closely related in their distribution (Figs. F32I, F32J, F32K).

Each of these constituents exhibits a slight and essentially constant increase with depth through the nearly 30 m of recovered material. There are two important aspects of these profiles. First, although basalt was recovered at the base of Hole U1366F, active Ca and Mg flux across this sediment/basalt interface is not evident. This circumstance is unlike Site U1365 (and many other sequences drilled by DSDP/Ocean Drilling Program/IODP) with active Mg/Ca exchange in the basaltic basement. Second, the three “whole-round stored shorter” (WSS) samples (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]) yielded concentrations of Ca, Mg, and Sr that are either higher (for Ca and Mg) or lower (for Sr) in concentration than in interstitial waters that were collected by squeezing the whole rounds stored in the core refrigerator prior to extracting the interstitial water. The Ca and Mg contrast between the WSS samples and the other samples may reflect carbonate precipitation during storage. However, the Sr contrast between the WSS samples and the samples taken in the core refrigerator is difficult to explain solely by changes in the carbonate system.

Concentrations of dissolved Na (Fig. F32L) slightly increase with depth, similar to concentrations of dissolved Ca, Mg, and Sr. The WSS samples analyzed by ICP-AES bound the higher concentration range of Na and suggest some form of Na uptake during brief sample storage, possibly by reaction with authigenic clay.

Trends in Ca and Mg profiles and the absolute values measured by the Dionex ion chromatograph agree with the ICP-AES profiles. An offset of up to 2% between ion chromatography and ICP-AES values is observed in the uppermost 13 m of the Na profile, yet the agreement in trend remains excellent. Unlike the increase observed in Na, Mg, and Ca concentrations in WSS samples measured by ICP-AES, the ion chromatography Na data and Mg and Ca data from the WSS sample pool show no deviation from the main trend.

We had analytical difficulties measuring dissolved K by ICP-AES at this site, so we only report K data from the ion chromatograph (Fig. F32M). Potassium slightly decreases with depth (~0.5 mM). The profile is linear with some scatter and no defined structure. The profile of dissolved B (Fig. F32N) is characterized by a relative maximum between 10 and 16 mbsf, superimposed on an overall increase with depth.

Concentrations of dissolved Fe are at the detection limit but are shown in Figure F32O for illustrative purposes, given the importance of Fe to biogeochemical redox processes. Concentrations of Mn are also near the detection limit, except for the interval from 10 to 16 mbsf, which shows dissolved Mn at concentrations nominally within our detection range (3 µM; Fig. F32P). This is the same depth interval that presents elevated concentrations of total carbon (Fig. F33B) and elevated B concentrations (Fig. F32N). Shore-based analyses of Fe and Mn will focus on improving our analytical determinations through this depth interval.

Concentrations of dissolved Ba and Li are below the analytical detection limit and are not discussed further here.

Solid-phase carbon and nitrogen

Contents of total nitrogen, total organic carbon (TOC), total carbon, and total inorganic carbon (TIC) were determined for 35 samples from Hole U1366D and U1366F (Fig. F33; Table T9).

Total nitrogen decreases almost linearly from 0.044 wt% at 0.01 mbsf to 0.006 wt% at 15.25 mbsf (Fig. F33A). Below 15.35 mbsf, the total nitrogen content is below the detection limit. TOC rapidly decreases from 0.20 wt% at 0.01 mbsf to 0.05 wt% at 2.35 mbsf and then gradually decreases to 0.03 wt% at 10.75 mbsf (Fig. F33B). TOC then remains at or below 0.03 wt% until basement.

Total carbon decreases from 0.20 wt% at 0.01 mbsf to 0.06 wt% at 7.35 mbsf. At greater depth, two maximums of total carbon occur, centered at 14 and 25 mbsf, reaching 0.18 wt% and 0.09 wt%, respectively.

Most samples show a very small difference between total carbon and TOC content (≤0.05 wt%, with only 10 samples ≤0.16 wt%), indicating that the contribution of TIC is small. TIC was determined independently using the coulometer system. The TIC profile exhibits the same two maximums centered at 14 and 25 mbsf as observed in the total carbon profile. The first peak reaches TIC values of 0.14 wt%, whereas the second peak only reaches 0.07 wt%. TIC values in the uppermost 5 mbsf are consistently lower (≤0.03 wt%) than in the rest of the sediment column. Calcium carbonate content computed from the TIC values indicate an average content of ∼0.15 wt% in the uppermost 5 m, whereas values reach 1.14 and 0.68 wt% at the first and second maximum, respectively (Fig. F33C).

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