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Site U1369 is located approximately midway between the center of the South Pacific Gyre and its southern edge. Together with the previous Site U1368, it possesses one of the two thinnest sedimentary sequences (~16–20 m thick) cored during Expedition 329. The sediment sequence is dominated by zeolitic metalliferous clay (see “Lithostratigraphy”).

The biogeochemistry group performed extensive sampling and analytical activities to address questions regarding how biogeochemical parameters in the sediment and pore water vary from gyre center (Site U1368) to midway to the southern gyre edge (Site U1369) with oceanographic factors such as ocean productivity and sedimentation rate.

Sediment samples for biogeochemical study were taken mostly from Hole U1369C. Dissolved oxygen concentrations were measured on all cores from Holes U1369B and U1369E using both electrode and optode techniques (177 total measurements) (Tables T4, T5). Thirty-three samples were collected for hydrogen analyses (Table T6), and six samples were collected for methane quantification. Twenty-three interstitial water samples were collected by whole-round squeezing (including 12 samples on the catwalk) (Table T7), and 21 samples were collected by Rhizon sampling (for nitrate and onshore analyses of nitrogen stable isotope) (Table T8). Sixteen samples were taken for solid-phase carbon and nitrogen analyses (Table T9).

Dissolved oxygen

Dissolved oxygen (O2) was measured by optodes and electrodes in intact 1.5 m core sections from Cores 329-U1369B-1H and 2H. Further measurements using electrodes were performed on the 20–110 cm long whole-round section pieces remaining after biogeochemistry and microbiology sampling (Cores 329-U1369C-1H and 2H and 329-U1369E-1H and 2H). Measurements on Hole U1369B samples were performed in 10 (electrode) and 10–20 cm (optode) depth intervals in the uppermost 3 m and in 20 (electrode) and 50 cm (optode) intervals below this depth. Electrode measurements in Holes U1369C and U1369E were performed at 15–50 cm intervals in the remaining whole rounds (Table T4).

Dissolved oxygen penetrates from the seafloor to the sediment/basalt interface at Site U1369. Very good agreement exists between optode and electrode measurements in the uppermost 3 m in Hole U1369B (Fig. F27). However, the near-seafloor oxygen concentrations measured by electrode are higher in the uppermost 3 m of Hole U1369C (170–204 µM) than in Hole U1369B (158 µM) (Fig. F27). This larger value measured at the surface in Hole U1369C approximates the regional bottom water oxygen concentration (Talley, 2007). Otherwise, the oxygen profiles from Holes U1369C and U1369E are similar to those from Hole U1369B.

Oxygen concentrations (both electrode and optode) generally decline very gradually with sediment depth (Tables T4, T5; Fig. F27). This linear decrease with depth is consistent with a very slight flux of oxygen toward the basalt/sediment interface.

Dissolved hydrogen and methane

Dissolved hydrogen (H2) concentration was quantified in 33 samples collected from Hole U1369C (Fig. F28; Table T6). Seven samples were taken on the catwalk and 26 samples were taken in the core refrigerator on the Hold Deck. The depths analyzed ranged from 0.2 to 15.7 mbsf. Based on the average of 13 blanks, the detection limit at this site is 1.3 nM. Concentration of H2 is consistently low, ranging from 1.3 to 6.3 nM in the sediment column. Ten of the 33 samples were below the detection limit.

Methane concentrations are below the detection limit (<0.98 µM) in all six samples from Hole U1369B (both the IODP standard safety protocol and the refined protocol). The detection limit is defined here as three times the standard deviation of the blank (ambient air).

Interstitial water samples

Twenty-one Rhizon samples for dissolved nitrate analyses were obtained from the whole-round samples of Hole U1369C. Two samples per section (approximate intervals from 0.5 to 1 m) were obtained. The standard deviation of duplicate analyses is 0.4%. Nitrate concentration is 38 µM in the shallowest sample (0.35 mbsf; Sample 329-U1369C-1H-1, 30–40 cm) (Fig. F29A; Table T8). This value is ~5 µM higher than inferred local bottom water (Talley, 2007). At greater depth, concentrations increase slightly but continuously to 40.5 µM at 14.35 mbsf (Sample 329-U1369C-2H-6, 80–90 cm). The offset from bottom water and the continued increase are most likely due to mineralization of organic matter. However, the nitrate value at 15.30 mbsf (Sample 329-U1369C-2H-7, 25–35 cm) is above the general profile trend; contamination of this sample is suspected.

Phosphate was measured on 23 interstitial water samples obtained through squeezing and on an additional 20 interstitial water samples obtained with Rhizon samplers. The concentration of phosphate exhibits a clear peak in the uppermost 2 m of sediment, increasing from 1.78 µM (Sample 329-U1369C-1H-1, 5–15 cm) to a peak of 2.27 µM at 2.95 mbsf (Sample 1H-2, 140–150 cm) (Fig. F29B; Table T7). Below 3 mbsf, the concentration of phosphate decreases with a concave-upward profile to 10 mbsf, at which point mean phosphate concentration becomes constant with depth (0.95 µM). The pooled standard deviation (1σ) based on three replicate measurements of each interstitial water sample is 0.05 µM (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). No significant difference was noted between samples taken on the catwalk and samples taken later in the core refrigerator on the Hold Deck. Phosphate concentrations obtained with the Rhizon sampling method (Table T8) deviate from the smooth profile obtained with the squeezed samples. Whereas the Rhizon phosphate values follow the same broad trend in concentration with depth as the squeezed samples, they deviate from the phosphate profile obtained from the squeezed interstitial water samples towards greater concentrations of 0.9 µM at 9.45 mbsf (Sample 329-U1369C-2H-3, 90–100 cm) and 0.8 µM at 13.85 mbsf (Sample 2H-6, 30–70 cm). The reason for this difference between the Rhizon samples and the squeezed samples is not clear, but blank tests showed that contamination by the Rhizon samplers is not the cause. The variable nature and sharp peaks in the Rhizon sampling–derived profile suggest that Rhizon sampling itself may induce artifacts. Rhizon sampling times can be up to 6 h in duration (see Fig. F11 in the “Methods” chapter [Expedition 329 Scientists, 2011a]).

Dissolved silicate concentrations exhibit a slight increase with increasing depth, from ~245 µM near the surface to nearly 300 µM at depth (Fig. F29C). These concentrations are similar to those obtained at Sites U1367 and U1368. Pooled standard deviation for duplicate measurements is 5 µM.

Alkalinity is ~2.5 mM (Fig. F29D) in surface seafloor sediment and gradually increases to 2.9 mM at 16 mbsf (the base of the sediment column). This slight increase may be attributed to consumption of organic matter. No obvious offset was observed between alkalinity of interstitial water samples and immediately squeezed “catwalk” samples. Standard deviation and error of alkalinity measurements on standard seawater CRM94 are 0.034 and 0.010 mM (N = 11), respectively.

Dissolved inorganic carbon (DIC) increases from 2.43 mM at the sediment surface to 2.87 mM at 15.4 mbsf (Fig. F29E). Two minima are observed at 3.95 mbsf (2.39 mM) and 12.95 mbsf (2.66 mM). The range in DIC values is 0.48 mM. Average standard deviation of triplicate injection of the samples is 0.019 mM. Catwalk samples do not show as large a deviation from samples stored longer as was observed at previous sites (e.g. Site U1368).

Sulfate was determined for the squeezed interstitial water samples and the Rhizon samples. The trends from the two data sets are consistent with each other (Tables T7, T8). Sulfate concentrations are <28.6 mM throughout the entire section (28.6 mM is inferred to be the concentration in local bottom water) (Fig. F29F). The range in the sulfate anomaly (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]) is from –1.5% to –5.7% (Table T7), with the value generally decreasing with depth (Fig. F29G). The variation in the sulfate anomaly may be due to adsorption onto sediment during sample recovery and extraction, as well as uptake into the underlying basalt.

Chloride was determined in the squeezed interstitial water and Rhizon samples (Tables T7, T8). The Rhizon samples showed a general offset of 0.2% from the squeezed samples but with the same trend as the squeezed samples. Three Rhizon samples appear to be compromised and fall off the trend toward higher values. The general offset is likely caused by evaporation, as the Rhizon aliquots were smaller and stored in vials with relatively more headspace. The chloride concentration near the seafloor is indistinguishable from the inferred local bottom water (Fig. F29H). Below this depth, concentration monotonically increases by ~2 mM to 10 mbsf. Below this depth, there is no significant gradient. This increase may be due to relict higher salinity seawater from the Last Glacial Maximum or to hydration of the underlying igneous crust.

As at previous sites, cations were measured at Site U1369 by both inductively coupled plasma–atomic emission spectroscopy (ICP-AES) and ion chromatography. For the ICP-AES analyses, samples were measured in duplicate (same solution twice in two separate analytical batches) with the exception of Sr, which was measured once. The precision of cation measurements 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 = 0.7% of the measured value,
  • Mg = 0.7% of the measured value,
  • Na = 0.6% of the measured value,
  • K = 0.7% of the measured value,
  • Fe = 3% of the measured value,
  • Mn = 2% of the measured value,
  • B = 0.9% of the measured value, and
  • Sr = 0.9% 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 and the trace elements. Accuracy of the ICP-AES results, as quantified by comparison to multiple replicate analyses of IAPSO seawater 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 = 0.2%, and
  • K = 0.3%.

The shape of the concentration profiles determined by ICP-AES and ion chromatography agree well. The absolute values of the concentrations differ by a very slight amount that is minimally greater than the respective analytical precision, as was the case at Site U1368. Again, the ion chromatography data at Site U1369 are slightly higher than the ICP-AES data. This contrast is most pronounced for Ca and less so for Na, Mg, and K. Although the ICP-AES and ion chromatography protocols were both rigorously calibrated against multiple replicate analyses of IAPSO standard, with identical items analyzed by both instruments and with detailed determinations of analytical precision, the cause of this discrepancy remains unclear. Postcruise shore-based analyses will aim to resolve this slight ambiguity.

The profile of dissolved Ca shows a slight decrease with depth, although it is difficult to resolve because of the variability with depth (Fig. F29I). Ca concentrations are lower than that of typical seawater (10.5 mM). Mg decreases by ~1–2 mM through the 16 m of the profile (Fig. F29J). This probably reflects clay alteration rather than uptake by basalt alteration, given the near constancy (or decrease) of the Ca profile. The Na profile shows no significant change with depth, whereas that of K shows structure that is confirmed by both the ICP-AES and ion chromatography analyses (Figs. F29K, F29L). Boron concentrations are higher than typical seawater (Fig. F29M) and are constant to ~6 mbsf, after which they steadily increase to values near 530 µM. This increase broadly follows the increase in chloride, and the concentrations are well correlated (r = 0.86, p < 0.001).

Concentrations of dissolved Fe and Mn are above detection limits (both ~1 µM), are reproducible, and co-vary (Figs. F29N, F29O). Because it is difficult to explain the presence of appreciable amounts of dissolved Fe and Mn in these oxygenated pore waters, the Fe and Mn data may record a very fine particulate or colloidal phase that is an artifact of the squeezing process. Sr appears constant with depth (Fig. F29P).

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

Solid-phase carbon and nitrogen

Contents of total carbon, total organic carbon (TOC), total inorganic carbon (TIC), and total nitrogen were determined for 16 samples from Hole U1369B (Fig. F30; Table T9). Total nitrogen rapidly decreases from 0.070 wt% at the seafloor to 0.026 wt% at 2.86 mbsf, followed by a smaller decrease toward the basement, reaching 0.010 wt% at 15.45 mbsf. TOC also rapidly decreases from 0.19 wt% at 0.11 mbsf to 0.07 wt% at 2.86 mbsf, gently decreasing thereafter to 0.03 wt% at 15.45 mbsf. Total carbon decreases from 0.24 wt% at 0.11 mbsf to 0.08 wt% at 4.36 mbsf and then is 0.06 wt% to basement (15.45 mbsf).

The TIC values obtained by coulometry follow a pattern very different from the total carbon content obtained from the CHNS elemental analyzer. Calculated CaCO3 content increases from 0.07 wt% at 0.11 mbsf to 0.35 wt% at 15.45 mbsf. TIC content increases from 0.01 wt% at 0.11 mbsf to 0.04 wt% at 15.45 mbsf.