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doi:10.2204/iodp.proc.329.108.2011

Biogeochemistry

Site U1370 sediment comprises 60–70 m of mostly zeolitic metalliferous pelagic clay and metalliferous pelagic clay (see “Lithostratigraphy”) deposited on 74 to 80 Ma basement at the southern edge of the oligotrophic South Pacific Gyre. Extensive sampling and analyses were performed to address

• How biogeochemical parameters in the sediment and interstitial water vary with oceanographic factors, such as ocean productivity and sedimentation rate, from gyre center (Site U1368) to the southern gyre edge (Site U1370);

• The extent to which the microbial community within the sediments may be supplied with electron donors by water radiolysis; and

• How sediment-basement exchange and potential activities in the basaltic basement vary with basement age and hydrologic regime from ridge crest to abyssal plain.

Given its relatively thick sedimentary sequence, Site U1370 also presented an opportunity to obtain interstitial water profiles for study of deepwater nutrient and oxygen variations subsequent to the Last Glacial Maximum.

Oxygen concentrations were profiled on complete sections from Holes U1370B (a mudline core; see “Operations”) and U1370D and from intact section intervals that remained after biogeochemical and microbiological sampling on Holes U1370E and U1370F. Samples for methane (both safety and refined; see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]) were obtained during catwalk sampling on core ends from Holes U1370B and U1370D. Interstitial water samples were obtained through squeezing (54 samples) and Rhizon sampling (70 samples) on sediment intervals from Holes U1370B (Rhizon only), U1370E, and U1370F. With the exception of six interstitial water intervals from Core 329-U1370E-1H cut in the Hold Deck core refrigerator, all of the interstitial water whole-round cores were taken on the catwalk after core recovery and delivered to the Geochemistry Laboratory for interstitial water squeezing.

Interstitial water was extracted by Rhizon sampling after oxygen measurements on Core 329-U1370B-1H in the Geochemistry/Microbiology Laboratory cold room. Otherwise, Rhizon sampling took place in the ship’s core refrigerator on the Hold Deck. Because of the low permeability of the samples from Site U1370, extraction using the Rhizon samplers often continued for up to 12 h and recovery was sometimes as low as 4 mL (see Fig. F11 in the “Methods” chapter [Expedition 329 Scientists, 2011a]). Syringe sampling for dissolved hydrogen analysis was coupled to interstitial water whole-round sampling. Separate interstitial water samples for He and ¹⁴C-dissolved inorganic carbon were also cut on the catwalk and delivered directly to the container laboratory on the deck above the bridge for immediate squeezing and sampling. These samples never entered the interior of the ship. Poor recovery during coring operations in Hole U1370E necessitated the addition of samples from Hole U1370F. For analysis of solid-phase carbon and nitrogen concentrations, 44 samples from Holes U1370E and U1370F were obtained. An additional 8 samples from Hole U1370D were analyzed to characterize lithologic Unit II (nannofossil ooze) and the clay that lay immediately above and below it.

Dissolved oxygen

Dissolved oxygen (O₂) was measured using optodes and electrodes on intact 1.5 m core sections from Holes U1370B and U1370D after delivery from the catwalk to the Geochemistry/Microbiology Laboratory cold room. Additionally, dissolved oxygen was determined using both optode and electrode measurements on intact core section intervals remaining after biogeochemistry and microbiological sampling (Hole U1370E) had taken place in the Hold Deck’s core refrigerator. Electrode measurements in Holes U1370B and U1370D were performed at 20–30 cm intervals (Table T7) and optode measurements were performed at 10–50 cm depth intervals (Table T8) for the uppermost 3 mbsf. At greater depths, measurements were performed at 20 (electrode) and 50 cm (optode) intervals. Electrode measurements in Holes U1370E and U1369F were performed at 20–50 cm intervals in the remaining whole rounds (Table T7).

Dissolved oxygen concentration profiles obtained with both optode and electrode methods show that the dissolved oxygen profile at Site U1370 is strikingly different from the previous sites. The dynamic range of oxygen concentration measurements captures both oxygen consumption in the sediment and deep fluxes of oxygen toward the basaltic crust. Oxygen concentrations in Holes U1370B and U1370D range from 122.4 to 149.7 µM in sediment from surface to 0.2 mbsf. Oxygen concentrations decrease to 80.9 µM at 7.8 mbsf (Hole U1370B electrode) (Table T7; Fig. F31B) and 88.3 µM at 7.6 mbsf (Hole U1370B optode) (Table T8; Fig. F31A). Explanation of the decrease in the uppermost 5–10 m of sediment as caused by organic oxidation is consistent with steep increases in phosphate and nitrate concentrations through these depths. Below 10 mbsf, oxygen concentrations continue to decrease with a gradual concave-upward pattern to 20 mbsf (except for data from a disturbed core of U1370B [18–22 mbsf]). Between 20 and 30 mbsf, the dissolved oxygen profile exhibits a deep zone where concave-upward curvature of the profile is at a maximum in subsurface depths. Processes controlling the oxygen profile above 40 mbsf are attributed to oxygen consumption by aerobic respiration of sedimentary microbes.

Below 40 mbsf, dissolved oxygen measurements decrease monotonically from ~10 µM to a few micromolar at the sediment/basalt interface (~6 µM from 67 to 68 mbsf, as measured by optode [Table T8; Fig. F31C]; ~1 µM from 64 to 68 mbsf, as measured by electrode [Table T7; Fig. F31D]). The relatively high oxygen concentrations below 69.3 mbsf (Fig. F31D) are consistent with visual evidence of flow-in (see “Lithostratigraphy”). Since the sediment/basement interface in Hole U1370D was tagged by the drill bit at 78.2 mbsf (see “Operations”), the total sediment column thickness slightly exceeds the apparent depth of the hole. This excess is consistent with flow-in of sediment in the deepest core.

The very slight offset between optode and electrode measurements may be attributed to the challenges of measuring extremely small concentrations of oxygen with microsensors in these sediments. Most important is that both methods show that oxygen concentrations monotonically decrease with the same gradient toward the basement. This suggests that oxygen concentration in the lower sediment column is controlled by a continued flux toward the underlying basalt crust.

Dissolved hydrogen and methane

Dissolved hydrogen (H₂) concentration was quantified for 45 samples collected from Hole U1370E and 13 samples from Hole U1370F (Fig. F32; Table T9). The depths analyzed ranged from 0.4 to 65.0 mbsf. Five samples were taken during subsampling in the core refrigerator on the Hold Deck, whereas the remaining samples were collected on the catwalk immediately after core recovery. After describing the split core, Core 329-U1370E-8H was determined to be disturbed; consequently, H₂ concentrations in samples taken from this core cannot be interpreted as in situ. Based on the average of 13 blanks, the detection limit at this site is 2.3 nM. The concentration of hydrogen in all but one sample (329-U1370E-9H-1, 135–140 cm) is below the detection limit. The H₂ concentration in this sample at 63.5 mbsf is 2.7 nM.

Methane concentrations are below the detection limit (<0.98 µM) in all eight samples from Hole U1370B (one sample) and Hole U1370D (seven samples), both for 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

A total of 70 Rhizon samples for dissolved nitrate analyses were obtained from Holes U1370B, U1370E, and U1370F (Table T10). Profiles of dissolved nitrate concentration are well correlated between these holes (Fig. F33A). Nitrate concentration near the seafloor (Section 329-U1370B-1H-1; 0.45 mbsf) is 33.86 µM but sharply increases to 41.5 µM at 1 mbsf (Section 1H-1; 0.95 mbsf) and then gradually increases to 51 µM at 20 mbsf (Sample 329-U1370E-3H-3, 60–70 cm). The increase from surface sediment to 20 mbsf is greater than at previous Expedition 329 sites (U1365–U1369), consistent with the organic nitrogen flux to the sediment being greater at this site, which is located at the more productive margin of the gyre. The increase in nitrate exhibits Redfield stoichiometry with the decrease in oxygen concentration. Between 20 and 40 mbsf, nitrate concentration remains relatively constant but decreases with increasing depth to 45 µM at 64.85 mbsf (Sample 329-U1370E-9H-2, 120–130 cm). The pooled relative standard deviation (1σ) on random duplicate runs is 1.3%.

Phosphate was measured on 53 interstitial water samples obtained through squeezing from Holes U1370E and U1370F. The pooled standard deviation (1σ) on triplicate measurements of the phosphate concentration is 0.10 µM. Similar to the previous Site U1369 on the southern transect of Expedition 329, phosphate concentrations exhibit a subsurface peak before declining with depth (Table T11; Fig. F33B). At Site U1370, phosphate concentrations increase from a near-surface concentration of 1.94 µM at 0.05 mbsf to 2.88 µM at 4.4 mbsf (Sample 329-U1370E-1H-3, 40–50 cm). The surface increase in phosphate has a Redfield stoichiometry with nitrate (16N:1P), suggesting that the release is due to the degradation of marine organic matter. Below this peak, phosphate concentrations decrease with a concave-upward profile to slightly less than 1 µM below 35 mbsf. Phosphate concentrations scatter between 0.5 and 1 µM from 35 mbsf to basement. A slight decrease in phosphate with depth is apparent below 35 mbsf.

The concentration of dissolved silica near the seafloor is 205 µM (Sample 329-U1370E-1H-1, 5–15 cm) (Table T11; Fig. F33C). Dissolved silica concentration in Hole U1370E varies in the uppermost three sections of Core 1H, from 205 to 288 µM (Sample 329-U1370E-1H-1, 90–100 cm). However, the samples that exhibit relatively high dissolved silica concentrations are from the six interstitial water whole-round intervals that were not cut on the catwalk but in the ship’s core refrigerator on the Hold Deck (Samples 329-U1370E-1H-1, 43–53 cm; 1H-1, 90–100 cm; 1H-2, 40–50 cm; 1H-2, 90–100 cm; 1H-3, 40–50 cm; and 1H-3, 90–100 cm). Otherwise, dissolved silica concentrations show a general increase to 240 µM at 10 mbsf. Below 20 mbsf, dissolved silica decreases slightly to concentrations generally close to 170 µM at 60 mbsf. A slight increase may be observed in the dissolved silica concentrations to 228 µM near basement at 65 mbsf (Sample 329-U1370E-9H-2, 140–150 cm). Pooled standard deviation for duplicate measurements is 4 µM.

Alkalinity and dissolved inorganic carbon (DIC) in interstitial water behave similarly with depth (Table T11; Figs. F33D, F33E). Alkalinity increases from 2.2 mM in the 0–0.1 mbsf interval to 2.6 mM at ~15 mbsf and then gradually decreases to 2.15 mM between 15 and 65 mbsf. No difference in alkalinity was observed between the interstitial samples cut in the ship’s Hold Deck core storage and the samples squeezed immediately after delivery from the catwalk. Standard deviation and error of alkalinity measurements on standard seawater CRM94 are 0.019 and 0.005 mM (N = 16), respectively.

DIC varies around 2.45 mM in the uppermost 6 mbsf and then increases toward a maximum of 2.56 mM at 14.88 mbsf (Table T11; Fig. F33E). Below this maximum, values decrease downhole, reaching 2.17 mM at 63.15 mbsf (Sample 329-U1370F-7H-6, 140–150 cm). The pattern and values of DIC at Site U1370 are very similar to those observed at Site U1365. The range in DIC values is 0.49 mM. Average standard deviation of triplicate injection of the samples is 0.023 mM. Values from catwalk samples fall within the values of samples stored longer.

Chloride was determined from the squeezed interstitial water samples (Table T11; Fig. F33F). The chloride concentration near the seafloor is indistinguishable from inferred local bottom water (Talley, 2007) but monotonically increases by ~12 mM to 2 mbsf. This 2% increase may be due to relict higher salinity seawater from the Last Glacial Maximum. Below this depth, there is no significant gradient.

Sulfate was determined in the squeezed interstitial water samples. Sulfate concentrations begin at the surface sediment at 28.32 mM (Fig. F33G), which is close to the inferred 28.6 mM concentration of sulfate in local bottom water. Sulfate concentrations exhibit a slight decline in concentration deeper than 20 mbsf. The sulfate anomaly (Fig. F33H; see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]) decreases from –1.26% near the surface to –6.15% at 63.15 mbsf. Loss of sulfate may reflect the downward flux of sulfate to the underlying basalt due to removal of sulfate into authigenic minerals during basalt weathering. Local excursions in the sulfate anomaly to values of –9% are attributed to coring and sampling artifacts.

As at previous sites, cations were measured at Site U1370 by both inductively coupled plasma–atomic emission spectroscopy (ICP-AES) and ion chromatography. 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 = 1.0% of the measured value,

• Mg = 0.7% of the measured value,

• Na = 1.5% of the measured value,

• K = 3.0% of the measured value,

• Fe = 7% of the measured value,

• Mn = 2% of the measured value,

• B = 2.0% of the measured value, and

• Sr = 0.6% of the measured value.

Accuracy of the ICP-AES results, as quantified by comparison to multiple replicate analyses of IAPSO standard seawater not included in the calibration, was within precision of the measurement. For the ion chromatography analyses, precision (pooled standard deviation, 1σ) was

• Ca = 0.6%,

• Mg = 0.3%,

• Na = 0.3%, and

• K = 0.2%.

The shapes of the concentration profiles determined by ICP-AES and ion chromatography agree very well with no appreciable offsets between the two data sets (Table T11; Figs. F33I, F33J, F33K–F33O, F33P). The ion chromatography profiles, because of their greater precision, tend to be smoother, but the results track together well through local minima and maxima for those species measured by both techniques.

The profiles of dissolved Ca show a consistent and linear increase with depth of ~1.5 mM (Fig. F33I). Concentrations of Mg are either constant or appear to very slightly increase from the surface to ~55 mbsf, below which depth they decrease by ~3 mM to the basement (Fig. F33J). Sr shows no change with depth (Fig. F33P), whereas K shows a marked decrease of ~2 mM from the surface to the basement (Fig. F33L). The constancy of Sr indicates that there is no appreciable ongoing carbonate recrystallization, which is not surprising considering the very low CaCO₃ values (see below) at this site. Because there is no carbonate recrystallization, the general Ca increase with depth likely reflects release of Ca from basement during alteration. The general decrease in K with depth (Fig. F33L) is also consistent with basement alteration. The Na concentration profile did not change significantly with increasing depth (Fig. F33K). Boron concentrations are greater than in typical seawater (Fig. F33M) and may also decrease through the deepest 5 m of sediment closest to the basalt.

Concentrations of dissolved Fe and Mn are above their detection limits (both ~2 µM) and are reproducible (Figs. F33N, F33O). Because it is difficult to explain the presence of appreciable amounts of dissolved Fe and Mn in oxygenated pore water, the Fe and Mn may be recording a very fine particulate phase that is an artifact of the squeezing process. However, the Mn concentration profile does exhibit a slight convex-upward increase over the uppermost 10 m of sediment, coinciding with the enhanced organic carbon mineralization observed at the same depths.

At Site U1370, only six interstitial water samples were processed through the Hold Deck’s core refrigerator (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). Nonetheless, comparison of the catwalk samples (squeezed immediately upon core recovery) to those samples stored in the Hold Deck’s core refrigerator shows no offset between the data sets for any cation.

Solid-phase carbon and nitrogen

Concentrations of total carbon, total organic carbon (TOC), total inorganic carbon (TIC), and total nitrogen were determined for 44 samples from Holes U1370E and U1370F (Fig. F34; Table T12). In addition, 8 samples from Hole U1370D were analyzed to characterize lithologic Unit II (i.e., the nannofossil ooze).

Total nitrogen shows a rapid decrease from 0.079 wt% at 0.06 mbsf to 0.055 wt% at 0.91 mbsf. It then remains relatively stable to 2.95 mbsf (0.052 wt%), where a second more gentle decrease occurs until 17.15 mbsf (0.014 wt%). From 17.15 mbsf to the basement, total nitrogen shows a very slight decrease with depth, reaching 0.002 wt% at 65.05 mbsf (Fig. F34A). TOC shows similar features, with a rapid decrease from 0.25 wt% at 0.06 mbsf to 0.12 wt% at 0.91 mbsf, followed by little change to 2.95 mbsf (0.10 wt%) and gently decreasing thereafter to 0.02 wt% at 65.05 mbsf (Fig. F34B). Total carbon also shows similar features, decreasing from 0.30 wt% at 0.06 mbsf to 0.13 wt% at 0.91 mbsf and then remaining stable until 2.95 mbsf (0.11 wt%), finally decreasing downcore to 0.02 wt% at 50.65 mbsf. However, total carbon concentrations slightly increase from 55 to 65 mbsf, reaching a maximum of 0.07 wt%. In addition, one data point from Hole U1370F shows a high value of 0.33 wt% at 63.15 mbsf, possibly resulting from mixing with the nannofossil ooze that was only successfully recovered in Hole U1370D.

TIC and CaCO₃ values obtained by coulometry follow a different pattern from the total carbon content obtained from the CHNS elemental analyzer. CaCO₃ (TIC) content shows a very slight increase downcore, varying around 0.04 wt% in the uppermost 15 mbsf and then around 0.10 wt% to 50 mbsf. Below 50 mbsf, the TIC and CaCO₃ values mimic the total carbon values obtained by coulometry. In particular, the anomalous sample from Hole U1370F records a CaCO₃ content of 3.52 wt% at 63.15 mbsf (Fig. F34C). The nannofossil ooze recovered from Hole U1370D contains up to 90.8 wt% CaCO₃. One white-colored interval shows slightly lower values (86.3 wt% CaCO₃ at 63.97 mbsf).

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