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doi:10.2204/iodp.proc.329.103.2011

Biogeochemistry

Site U1365 is located over the oldest basement of all the sites drilled during Expedition 329. Onboard measurements and sample processing were designed

• To document metabolic activities and evaluate chemical fluxes between the sediment and the underlying basalt basement,

• To evaluate the potential of radiolysis to support microbial metabolism, and

• To document deep ocean chemistry over the last glacial–interglacial interval.

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 collected for hydrogen and methane analyses, and interstitial waters were obtained for analysis using Ti Manheim squeezers and Rhizon pore fluid samplers. Additional sediment samples for solid-phase nitrogen and carbon determination were taken, and separate whole-round intervals were sectioned and squeezed for ³He/⁴He and ¹⁴C analyses, both to be determined in shore-based laboratories. Lithologic Unit IV basement (see “Lithostratigraphy”) was not sampled, nor was the chert-bearing Unit II.

Dissolved oxygen

Dissolved oxygen measurements were conducted on intact 1.5 m core sections from Cores 329-U1365A-1H through 5H above the chert layer (lithologic Unit II) (see “Lithostratigraphy”) and from Cores 23H through 25H below Unit II. Optode and electrode measurements were performed simultaneously on the same core sections. Within the uppermost 3 m of sediment (Sections 329-U1365A-1H-1 through 1H-3) and in Unit III (Cores 23H-1 through 25H-3), electrode measurements were conducted at 10 cm intervals and optode measurements were conducted at 30–50 cm intervals. In all other Hole U1365A core sections, measurements were typically at 15–20 cm intervals with electrodes and 50–75 cm intervals with optodes.

Additional optode and electrode measurements were made on 30–50 cm long whole-round sections obtained from the microbiological sampling cold room (the core refrigerator on the Hold Deck of the JOIDES Resolution; see “Microbiology” in the “Methods” chapter [Expedition 329 Scientists, 2011]) on Cores 329-U1365B-1H through 5H, 8H, and 9H. Whole-round samples from Hole U1365B underwent extensive handling in the core refrigerator in the Hold Deck before they were brought to the cold room in the Geochemistry/Microbiology Laboratory to thermally equilibrate. Electrode measurements were performed on Cores 1H through 4H above lithologic Unit II (chert) and on Cores 8H and 9H below Unit II (Unit III). Optode measurements were performed on Cores 1H and 3H through 5H. Data from optode and electrode measurements are shown in Tables T12 and T13. Optode and electrode methods gave similar results (Figs. F56, F57). Concentrations in the uppermost 6.85 m are consistent with those acquired during the site survey (D’Hondt et al., 2009). Dissolved oxygen was present in all measured samples (Fig. F56). Thus, oxygen penetrates through the entire sediment column to the sediment/basalt interface (75.1 mbsf). Oxygen concentration exhibits the steepest decline from ~190 µM in the near-surface sediment to 120–140 µM at 12 mbsf. From this depth to 20 mbsf, oxygen decreases slightly to 100–120 µM and then remains constant at 100–120 µM to the top of Unit II (44 mbsf; bottom of Core 329-U1365A-5H). In Unit III (64–75 mbsf), concentrations remain constant between 60 and 80 µM. Oxygen concentrations in Unit III are 40–60 µM lower than in the Unit I sediment overlying the chert (Unit II). The chert layer (Unit II) appears to pose a diffusive barrier to dissolved oxygen between Units I and III.

Electrode oxygen measurements taken at the very top and bottom of core sections (10 cm from cut edge) often showed spurious readings inferred to be caused by alteration of the core sections during drilling and handling. The piston corer did not achieve full stroke for Core 329-U1365A-3H, allowing flow-in of disturbed sediment on retrieval. This was clearly reflected in the scatter in oxygen concentrations to higher values over this interval in both electrode and optode oxygen measurements (Fig. F56). Cores 329-U1365A-5H and 329-U1365B-5H were also suspected to be compromised during coring. The increase in oxygen concentration values toward the lower part of Cores 329-U1365A-5H and 329-U1365B-5H, near the chert layer in Unit II, is attributed to drilling disturbance. Overall, there was good agreement between the profiles measured from Holes U1365A and U1365B, after removing from consideration the oxygen measurements that we ascribe to drilling and handling disturbance (Fig. F57A).

Optode and electrode data show very similar oxygen concentration profiles but with slight offsets (Fig. F57). Optode measurements show a smooth profile, whereas electrode measures showed some scatter (±5 µM). Although electrode measurements were performed with four different electrodes that were individually calibrated, no detectable differences or offsets are attributed to individual electrodes. In Unit I of Hole U1365A (0–44 mbsf), the optode profile shows slightly more curvature with lower values in the central part compared to the electrode profile. In Unit III, the average optode values are 13 µM lower than average electrode values.

Dissolved hydrogen and methane

Dissolved hydrogen (H₂) concentrations were quantified in 78 samples collected in the core refrigerator on the Hold Deck (Table T14). The depths analyzed range from 0.50 to 75.61 mbsf. Based on the average of 13 blanks, the detection limit at this site was calculated to be 2.8 nM. The concentration of H₂ remained below the detection limit in the upper 25.45 m of the sediment column (Fig. F58). Below this depth, there was a prominent peak centered at 29.95 mbsf with a maximum concentration of 30.5 nM. At 33.45 mbsf, the concentration of H₂ was again below the level of detection. Of the 32 samples below the peak, most (~78%) were below the detection limit. Those above the detection limit ranged from 0.1 to 5.8 nM and exhibited no apparent spatial pattern.

Methane concentrations are below the detection limit (<1.3 µM) in all samples from Holes U1365A (IODP standard safety protocol) and U1365B (refined protocol). The detection limit is defined here as three times the standard deviation of the blank (ambient air).

Interstitial water samples

Interstitial water whole-round samples were taken from 102 intervals from Holes U1365B and U1365C. Resolution of sampling was generally two samples per section (or approximately every 65 cm) in undisturbed core material. A higher resolution of three samples per section was obtained in Sections 329-U1365B-1H-1 through 1H-2 and in Core 329-U1365B-9H. Rhizon interstitial water samples were obtained from all but five whole-round intervals dedicated to squeezing. All shipboard interstitial water chemistry results from Site U1365 are listed in Table T15 and shown in Figure F59.

Nitrate concentration measured on the Rhizon-sampled interstitial waters increases from 35.46 µM at 0.85 mbsf (Sample 329-U1365B-1H-1, 80–90 cm) to 40.05 µM at 6.95 mbsf (Sample 2H-2, 130–140 cm) (Fig. F59A). This increase is similar to that found during the previous survey cruise for the surface sediments (D’Hondt et al., 2009). Below 15 mbsf, the concentration of nitrate remains constant at 42–43 µM (15 mbsf), increasing with depth to 45.35 µM at 40 mbsf. The nitrate increase to 15 mbsf may indicate oxidation of reduced nitrogen in this highly oxygenated environment. Within lithologic Unit III, nitrate concentrations are lower than in Unit I and range from 37.51 µM (Sample 8H-3, 45–55 cm) at 66.50 m to 31.98 µM at 76.70 m (Sample 9H-3, 30–40 cm).

The increase in nitrate concentration within Unit I exhibits a Redfield-like stoichiometry to the decrease in dissolved oxygen. This relationship suggests that the nitrate concentration increase (to 11 µM above seawater values of 33 µM for this region of the Pacific Ocean) (Talley, 2007) may be attributed to the oxidation of reduced nitrogen species (e.g., nitrification) derived from the aerobic remineralization of organic matter in the sediments. The Redfield relationship does not hold in Unit III, where lower dissolved oxygen concentrations are not mirrored by stoichiometrically higher nitrate concentrations. The nitrate concentrations in this lowermost sediment are close to those of modern bottom water. Neither nitrate concentrations nor oxygen concentrations exhibit a discernible gradient in Unit III, which suggests that diffusion is relatively limited between Unit I and Unit III and between Unit III and the underlying basalt.

In contrast to the downhole distribution of nitrate, phosphate concentrations (Fig. F59B) decrease monotonically with depth, from near bottom water seawater concentrations (2.00–2.23 µM) at 0.35–3.35 mbsf (Samples 329-U1365B-1H-1, 30–40 cm, to 1H-3, 30–40 cm) to 0.85 µM at 20.30 mbsf (Sample 3H-4, 130–140 cm). Below 20 mbsf, phosphate concentrations decrease slightly to values of 0.75 µM at 42.18 mbsf (Sample 5H-7, 53–63 cm). The concentration of phosphate in Unit III ranges between 0.4 and 0.6 µM. The standard deviation of the analytical means pooled from all of the measurements (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011]) was 0.06 µM (1σ) for the phosphate concentration profile. Overall, within a few meters centered at any given depth, the phosphate data scatter by ±0.1 µM around a mean value. Given the organic matter degradation indicated by the nitrate, oxygen, and dissolved inorganic carbon profiles (see below), an increase in phosphate concentration of ~0.6 µM would be expected if dissolved phosphate built up in Redfield stoichiometry with the other species in the interstitial fluid of lower Unit I. Consequently, the observed decrease suggests that dissolved phosphate is removed into or adsorbed onto a presently unidentified authigenic phase.

Concentrations of dissolved silica (Fig. F59C) substantially exceed typical bottom water values (123 µM) (Talley, 2007) for this region of the Pacific Ocean. Concentrations in the uppermost 20 m of Unit I range from 320 to 360 µM (Samples 329-U1365B-1H-1, 30–40 cm, through 3H-5, 65–75 cm). Below this depth, dissolved Si increases to 588 µM at the bottom of Unit I at 42.18 mbsf (Sample 5H-7, 53–63 cm), where the chert begins. In Unit III (below the chert of Unit II), the dissolved Si concentration decreases from 646 µM at 65.70 mbsf (Sample 8H-2, 65–75 cm) to 576 µM just above the basalt/sediment interface (Sample 9H-6, 40–50 cm). Solubilities of dominant silica minerals most likely control the concentration of dissolved silica and account for Si concentrations that are three times or more those of bottom water dissolved Si. Moreover, there are apparent upward and downward fluxes of dissolved Si away from the chert-bearing Unit II.

Alkalinity and dissolved inorganic carbon (DIC) behave similarly with depth in the interstitial water of Site U1365. Alkalinity (Fig. F59D) sharply increases from 2.2 mM in the 0–0.1 mbsf interval to 2.6 mM at ~10 mbsf and then gradually decreases to ~2.3 mM at the top of the chert-bearing Unit II (~40 mbsf). In Unit III, the concentrations continue to decrease to 2.0 mM at the bottom of the sediment (75.50 mbsf). After staying relatively constant through the uppermost 8 m of the sediment column, DIC increases to a maximum of 2.60 mM at 9.30 mbsf, before decreasing more gently toward a low of 2.3 mM at 40 mbsf, above the chert (Fig. F59E). Below the chert-bearing Unit II, DIC concentrations are between 1.9 and 2.0 mM. Average standard deviation of triplicate injection of the samples is 0.018 mM. Sample 329-U1365B-4H-2, 130–140 cm (25.95 mbsf), shows large standard deviation (0.115 mM). The values from Holes U1365B and U1365C are consistent with one another.

Sulfate (Fig. F59F) concentrations generally decrease from near–bottom seawater concentrations of 28.3 mM at 0.35 mbsf to 27.5 mM at 37.8 mbsf. Below the chert layer, sulfate exhibits a steeper decline with depth, with sulfate depleted to ~25.3 mM near the basement. The sulfate/chloride ratio can be measured more precisely than the sulfate concentration, and the ratio is not affected by changes in salinity caused by changes in ocean volume and hydration reactions. Thus, the sulfate anomaly is a more sensitive indicator of sulfate reactivity. The sulfate anomaly decreases by more than 13% at 74.40 mbsf (Sample 329-U1365B-9H-5, 130–140 cm;). The reason for this decrease in this organic-poor, oxygen-rich sediment may be precipitation of calcium sulfate in the underlying basement or in the deeper part of the sediment column.

Chloride concentrations (Fig. F59G) increase from near-seafloor concentrations of 556–565 mM at 25 mbsf. The increase is attributed to remnant higher salinity water from the Last Glacial Maximum. The maintenance of this chloride profile curvature also indicates that diffusion dominates transport within Unit I.

At Site U1365, the cations Ca, Mg, Na, and K were measured using the Dionex IC and the Leeman ICP-AES (Table T15). There is excellent agreement between the two data sets in terms of trends, even in detail, although the absolute value of some of the values appears offset by 5% of the measured value(s). Whether this contrast reflects differences in calibration is not presently clear. The elements Ca, Mg, and Sr are involved to varying degrees in carbonate dissolution and reprecipitation reactions, as well as clay mineral diagenesis (Fig. F59H–F59J). At Site U1365, the major feature in the Ca profile is the notably higher concentration in Unit III compared to Unit II. Below the diffusive barrier of the Unit II chert, Ca is 2–4 mM higher than in the shallower Unit II. Both data sets show a slight increase in Ca with depth from 0 to 42 mbsf. Mg concentration slightly increases with depth over the uppermost 40 mbsf and is slightly lower below the chert. All data show good agreement between Holes U1365B and U1365C. Although both Ca and Mg indicate some low-temperature alteration of the underlying basalt (Mg exchanging for Ca), the nearness to seawater concentrations of both Ca and Mg in Unit III suggests that the interstitial water in Unit III is not highly evolved relative to seawater.

Concentrations of Sr slightly increase with depth, with no significant change in trend deeper in Unit III. The Sr data from Hole U1365C exhibit significant scatter relative to the data from Hole U1365B. This is the only dissolved constituent that shows such a notable contrast between Hole U1365B and Hole U1365C.

Sodium, potassium, and boron are primarily involved in clay mineral diagenesis and cation exchange reactions (Fig. F59L–F59M). The K profiles, as measured by both ion chromatography and ICP-AES, show consistent structure, with a relative minimum at ~15 mbsf and a local maximum at 25 mbsf. The more highly resolved ion chromatography profile also shows another minimum at 35 mbsf. The sinuous nature of these profiles with depth may reflect subtle differences in the composition of the solid sediment, and postcruise analyses may shed further light on the processes that control their distributions. In Unit III, K concentrations are lower than in the above Unit II (chert). Although it is difficult to discern whether this is a real trend, there appear to be lower concentrations of B in Unit II, immediately above the chert. In Unit III, B appears constant (Fig. F59M).

The concentration of Mn (Fig. F59N) is largely below the detection limit (<4–5 µM), although a number of samples from 25 to 35 mbsf record values slightly greater than the detection limit. Fe (not shown) fell below the detection limit of 5 µM. The Fe and Mn concentrations will be measured again during shore-based research to further study their distributions. Ba and Li (not shown) are entirely below their respective detection limits.

Solid-phase carbon and nitrogen

Contents of total organic carbon (TOC), total carbon, and total nitrogen were determined for 38 samples from Hole U1365A (Table T16; Fig. F60). TOC rapidly decreases from 0.31 wt% at 0.04 mbsf to 0.03 wt% at 5.6 mbsf and then shows relatively higher values (up to 0.11 wt%) between 7.4 and 18.3 mbsf. Below 20 mbsf, the values are below 0.05 wt%. Replicate analysis of selected samples (N = 10) shows a small difference (up to 0.01%) for 9 samples and a relatively large difference (0.03%) in 1 sample (329-U1365A-2H-2, 116–117 cm).

Total nitrogen also rapidly decreases from 0.051 wt% at 0.04 mbsf to 0.010 wt% at 5.6 mbsf and then remains relatively stable around 0.01 wt%. Below 38 mbsf, the total nitrogen content is less than the detection limit. Replicate analysis of selected samples (N = 7) shows only small differences (maximum of 0.002%). Total carbon content of samples from Site U1365 had to be corrected for carbon contamination associated with the vanadium pentoxide reagent. Correction values are between 0.009% and 0.021%, depending on sample weight during analysis. Replicate analysis of selected samples without vanadium pentoxide shows a good correspondence with the corrected total carbon values. Most samples show a very small difference between total carbon and TOC content (≤0.02%, with only 3 samples ≤0.04%), indicating that the contribution of total inorganic carbon (TIC) is small. The accurate determination of such small TIC content values was impossible using the coulometer system, which showed higher blank variation than the assumed TIC values (i.e., TIC would be below the detection limit). Overall, the decrease in TOC and total nitrogen from very low concentrations in the near-surface sediments to still lower concentrations at depth is consistent with the aerobic remineralization of organic matter in the uppermost 20 m of sediment.

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