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doi:10.2204/iodp.proc.329.106.2011

Biogeochemistry

Site U1368 presented the thinnest sedimentary sequence targeted by Expedition 329. The sequence is dominated by carbonate-bearing sediment (see “Lithostratigraphy”) and is ~15 m thick. As at previous sites, 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 of radiolysis to support microbial activities.

To achieve these objectives, a broad range of chemical species was measured. High spatial-resolution profiles of dissolved oxygen were acquired using optodes and electrodes, headspace samples were used 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 studies.

Dissolved oxygen

Measurements of dissolved oxygen (O₂) were performed on intact 1.5 m core sections with electrodes (Table T8) on Cores 329-U1368B-1H and 2H, 329-U1368E-1H and 2H, and on the mudline core from Core 329-U1368F-1R. Measurements were typically performed at 15–20 cm intervals. Oxygen electrode measurements were also carried out on 20–50 cm long whole-round sections from Cores 329-U1368C-1H and 2H in 30–50 cm intervals after sampling in the core refrigerator on the Hold Deck of the JOIDES Resolution. Oxygen was measured by optode in intact 1.5 m core sections from Core 329-U1368B-1H (Table T9). Below 5.2 mbsf, optode measurements were difficult because of optode breakage, and no further measurements with optodes were attempted.

The dissolved oxygen profiles from the different Site U1368 holes are very similar to each other. Although optode measurements are limited to the uppermost 5 m of sediment from Hole U1368B, they are in good agreement with the electrode profiles. The oxygen profile from Hole U1368E exhibits slightly more scatter than profiles of cores from the other holes but otherwise exhibits a similar pattern of decrease with depth.

Dissolved oxygen penetrates from the seafloor to the sediment/basalt interface (Fig. F61A). Near-seafloor oxygen concentrations are between 156 and 159 µM for all holes (U1368B–U1368D and U1368F), approximating the regional bottom water oxygen concentration (Talley, 2007). The largest decline over depth in oxygen concentration was in the uppermost 2.5 mbsf, with a decrease from ~158 to ~145 µM at 2.5 mbsf (Fig. F61B). The high-resolution optode measurements from Hole U1368B and the electrode readings from Hole U1368F suggest that the only discernible curvature in oxygen concentration takes place in the uppermost 50 cm below seafloor. Below this depth, oxygen concentration declines linearly (Fig. F61A) with depth to ~140 µM at 12 mbsf.

The oxygen profiles at Site U1368 generally match the profiles from Sites U1365–U1367, except for slightly lower values caused by slightly lower bottom water oxygen concentration (Talley, 2007). Evidence of in situ oxygen consumption is limited to the topmost 50 cm below seafloor at Site U1368. The gradual decrease in oxygen concentration with depth indicates an oxygen flux towards the basaltic basement.

Dissolved hydrogen and methane

Dissolved hydrogen (H₂) concentrations were quantified in 33 samples collected from Hole U1368C (Fig. F62; Table T10). Ten samples were taken on the catwalk and 23 were taken in the core refrigerator on the Hold Deck. Analyzed depths ranged from 0.2 to 15.6 mbsf. Based on the average of 13 blanks, the detection limit at this site was calculated to be 3.6 nM. The concentration of H₂ remains below the detection limit in the uppermost 9.4 m of the sediment column (Fig. F62). Below this depth, 6 of the 13 samples were above the detection limit, with H₂ concentrations ranging from 5 to 36 nM. One of the six samples containing a detectable H₂ concentration (7 nM) was collected on the catwalk.

Methane concentrations are below the detection limit (<1.3 µM) in all samples from Holes U1368B (IODP standard safety protocol) and U1366C (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 taken from 33 whole-round samples from Hole U1368C (Table T11). The samples were obtained at the high resolution of three per section (approximately one sample every 50 cm). Eleven of the interstitial water whole-round samples were cut and taken on the catwalk and delivered immediately to the Geochemistry Laboratory for squeezing. These samples are referred to as “catwalk” samples, and aliquots for ¹⁴C analysis were taken from them. Furthermore, 32 Rhizon samples (Table T12) for dissolved nitrate analyses were obtained from a combination of the whole-round samples taken before squeezing as well as additional samples taken from other depths in the core. The Rhizon extractions were performed in the Hold Deck refrigerated core storage area (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). All profiles are presented in Figure F63.

Nitrate concentration near the sediment surface (Sample 329-U1368C-1H-1, 10–20 cm) was 35.84 µM (Fig. F63A). This concentration is slightly higher than that found at Sites U1365–U1367. The nitrate concentration profile subsequently shows virtually no change with increasing depth. In comparison with the previous sites that exhibit increasing nitrate concentrations to greater depths, this lack of change with depth at Site U1368 suggests that the organic matter flux to the sediment is extremely low at the center of the oligotrophic gyre.

Like the pattern exhibited at the previous Expedition 329 sites, the distribution of phosphate exhibits a decline in concentration from near-seawater values with increasing depth (Fig. F63B). The concentration in the surface sediment at 0.05 mbsf (Sample 329-U1368C-1H-1, 0–10 cm) is 2.29 µM, which is very close to the deepwater concentration for the South Pacific Ocean for this region of the Pacific Ocean (Talley, 2007). Phosphate concentrations decrease to <1.0 µM at 5.5 mbsf. With the exception of two values that depart from a smoothly decreasing profile (Samples 329-U1368C-1H-4, 50–60 cm, and 100–110 cm), the concentration of phosphate remains below or close to 1.0 µM to 15.44 mbsf. This value is slightly greater than the lowest values observed in the previous sites (typically close to 0.5 µM). Below 13 mbsf (in Sections 329-U1368C-2H-5 and 2H-6), phosphate begins to increase with increasing depth and attains a concentration of 1.60 µM at 15.48 mbsf (329-U1368C-2H-6, 0–15 cm). Whether this increase at the bottom of Hole U1365C is related to flow in the basalt aquifer remains to be tested. The pooled standard deviation (1σ), based on 2 runs of 3 replicate measurements of each interstitial water sample (n = 6), is 0.13 µM (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). Measured phosphate concentrations in immediately squeezed catwalk samples were indistinguishable from samples processed is the normal fashion.

Dissolved silicate concentrations exhibit no clearly discernible trend with depth, have an average value of 245 µM, and vary ~23 µM (1σ) around this mean value (Fig. F63C). No difference is apparent in dissolved silicate concentration between those samples squeezed immediately (catwalk) and the normally processed samples.

Alkalinity and dissolved inorganic carbon (DIC) behave similarly with depth in the interstitial water of Site U1368. Alkalinity starts between 2.6 and 2.8 mM in surface sediment and then monotonously decreases with depth. At the bottom of the sequence (15.5 mbsf), the values are between 2.4 and 2.6 mM. Alkalinities of interstitial water samples stored in the core refrigerator on the Hold Deck were consistently lower than those of the rapidly squeezed catwalk samples, which indicates carbonate precipitation in sediment before interstitial water squeezing. Standard deviation and error of alkalinity measurements on standard seawater (CRM94) are 0.018 and 0.005 mM (N = 11), respectively. DIC increases from 2.62 mM at the sediment surface to a broad peak of ~2.7 mM from 4.45 to 9.45 mbsf. The values then decrease to 2.4 mM to at 15 mbsf. The range in DIC values is 0.3 mM, whereas the average standard deviation of triplicate injection of the samples is 0.015 mM. Catwalk samples that were squeezed right away show higher values than samples that spent time in the cold room before being squeezed (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). The average difference between those two set of samples is ~0.15 mM. This shift could be caused by at least two different (but coupled) processes, calcium carbonate precipitation or CO₂ degassing.

Chloride concentrations were determined by ion chromatography. All samples were analyzed in duplicate. Based on the pooled standard deviation of duplicate analyses, the 66% (1σ) confidence limit is 0.08%. Concentrations in Hole U1368C are indistinguishable from present-day bottom water in this region (554.4 mM). The average measured value is 544.3 mM with a standard deviation of 0.7 mM. The homogeneity reflects the relative stability of seawater salinity over the past few thousand years and the relatively short diffusion timescale for a sequence that is 15 m thick, as well as the limited influence of hydration/dehydration reactions.

Sulfate concentrations were also 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 anomaly 66% (1σ) confidence limit is 0.05%. There is no difference in the sulfate anomaly between samples squeezed immediately (catwalk) and normally processed samples. Measured sulfate concentrations are <28.6 mM in the entire section (the concentration in local bottom water). The range in the sulfate anomaly (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]) is –1.3% to –4.5% (Fig. F63G). The depletion relative to seawater may be due to adsorption onto sediment during sample recovery and extraction. The sulfate anomaly increases back to –0.7% within the bottommost depths of Site U1368 (14.55–15.48 mbsf; 329-U1368C-2H-5, 50–60 cm, to 2H-6, 0–10 cm). This increase in sulfate concentration (both absolute and relative to chloride) mirrors the increase of phosphate concentration at these depths, but the underlying cause is not clear.

As at previous sites, cations were measured by both ICP-AES and ion chromatography. Samples were analyzed in duplicate (same solution twice in two separate analytical batches). The precision of the measurements of cations by ICP-AES was, as quantified by multiple triplicate and quadruplicate analyses of International Association for the Physical Sciences of the Oceans (IAPSO) standard seawater and internal matrix matched standards,

• Ca = 0.6% of the measured value,
• Mg = 0.7% of the measured value,
• Na = 0.5% of the measured value, and
• K = 0.6% of the measured value.

Analyzing the samples in duplicate did not appreciably change the precision of the measurements for Ca or Mg but significantly improved the precision of the measurement of Na and K. Accuracy of the ICP-AES results, as quantified by comparison to multiple replicate analyses of IAPSO not included in the calibration, was within precision of the measurement. For the ion chromatography analyses, precision (pooled standard deviation, 1σ) was

• Ca = 0.7%,
• Mg = 0.3%,
• Na = 0.3%, and
• K = 0.7%.

The shape of the concentration profiles determined by ICP-AES and ion chromatography agree well, although the absolute values of the concentrations differ, with the ion chromatography data being higher than the ICP-AES data (Figs. F63I, F63J, F63K, F63L; Table T11). This contrast is most pronounced for Ca and less so for Na, Mg, and K. Despite the ICP-AES and ion chromatography protocols both being rigorously calibrated against multiple replicate analyses of IAPSO, with identical items being analyzed by both instruments and detailed determinations of analytical precision, the cause of this discrepancy remains unclear. Postcruise shore-based analyses will aim to resolve the ambiguity.

The profile of dissolved Ca (Fig. F63I) shows a slight decrease with depth, from near-surface values approximating that of typical seawater (10.5 mM) to values closer to ~10 mM at depth. Ca concentrations increase through the deepest four samples closest to the basalt. Overall, Mg shows no significant change in concentration with depth. The profiles of Na and K also show no significant change with depth. Over such a short sequence, the lack of significant concentration changes is not surprising.

Comparison of the catwalk samples (squeezed immediately upon core recovery) to those samples stored in the cold room on the Hold Deck (see “Microbiology” in the “Methods” chapter [Expedition 329 Scientists, 2011a]) shows no offset between the data sets for any cation (Figs. F63I, F63J, F63K, F63L).

Solid-phase carbon and nitrogen

Contents of total carbon, total organic carbon (TOC), total inorganic carbon (TIC), and total nitrogen were determined for 15 samples from Hole U1368B (Fig. F64; Table T13). Weight percent CaCO₃ was calculated from TIC.

Total nitrogen content is 0.014 wt% at the seafloor but below the detection limit at greater depths (below 0.1 mbsf; Fig. F64A). TOC rapidly decreases from 0.085 wt% at the seafloor to 0.03 wt% at 0.46 mbsf and remains low thereafter (Fig. F64B). Total carbon content obtained with the CHNS analyzer shows a curved profile with a maximum of 10.51 wt% at 4.36 mbsf. At the seafloor, total carbon content is 9.31 wt%; just above basement it is 7.33 wt%.

The TIC and CaCO₃ values obtained by coulometry show the same pattern as total carbon content obtained from the CHNS elemental analyzer. CaCO₃ content varies between 70.8 and 87.4 wt% in the uppermost 4.36 mbsf and gradually decreases to 61.3 wt% at 13.98 mbsf (Fig. F64C). TIC content varies between 8.5 and 10.5 wt% in the uppermost 4.36 mbsf and gradually decreases to 7.4 wt% at 13.98 mbsf.

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