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doi:10.2204/iodp.proc.329.109.2011

Biogeochemistry

Site U1371 is located just south of the South Pacific Gyre in a region of relatively high biological productivity compared to previous sites occupied during Expedition 329. Although it was the last site of the expedition, we were able to completely characterize the sequence. 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 outside the gyre’s edge (Site U1371);

• The extent to which the sedimentary microbial community 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.

Site U1371 provided the thickest sediment sequence of the expedition, with the deepest interstitial water sample recovered from ~130 mbsf. Unlike any other site occupied during this expedition, the enhanced productivity in the overlying water causes oxygen to be depleted at a shallow sediment depth. Thus, anaerobic processes are more important at this site than at the other Expedition 329 sites. Furthermore, a clear signal of basement-associated processes includes a deep zone ~25 m above basement showing detectable oxygen, relatively high redox potential, and gradients in related biogeochemical constituents (e.g., Mn). Through this deepest interval, a strong signal of basement alteration exists (e.g., K).

Sampling strategy

Oxygen concentrations were measured on complete core sections from Cores 329-U1371B-1H, 329-U1371C-1H, and 329-U1371H-1H, all of which were mudline or attempted mudline cores (see “Operations”). Hole U1371D recovered the complete sequence (Cores 329-U1371D-1H through 14H) and was devoted to oxygen measurements, lithostratigraphy, and physical properties. Oxygen measurements of sections from Cores 329-U1371F-11H through 14H, the deepest core of Hole U1371D, were measured before the cores were sullied by microbiological sampling.

Samples for methane, both for safety and refined analysis (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]), were obtained during sampling on the catwalk on section ends from Holes U1371D and U1371E.

Interstitial water samples were obtained through squeezing whole rounds gathered from Hole U1371E (67 samples) and through Rhizon sampling (103 samples) on sediment intervals from Holes U1371B, U1371E, and U1371H. With the exception of six interstitial water intervals from Core 329-U1371E-1H and four interstitial water intervals from Core 329-U1371E-14H cut in the core refrigerator on the Hold Deck, all of the interstitial whole-round cores were taken on the catwalk immediately after core recovery and directly delivered to the Geochemistry Laboratory for pore water squeezing. Interstitial water was extracted by Rhizon sampling after oxygen measurements on Holes U1371B and U1371H in the Geochemistry/Microbiology Laboratory cold room. Rhizon sampling of Hole U1371E occurred in the ship’s core refrigerator on the Hold Deck.

Interstitial water extraction generally was easier at Site U1371 than at other sites during this expedition (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]); we attribute this ease to increased permeability due to the greater biogenic content of the lithologic Unit I sediment (see “Lithostratigraphy”). Deeper in the section, however, within lithologic Unit II, permeability decreased and interstitial water extraction became extremely difficult. Through the deepest ~20 m, often no water was extracted by the Rhizon samplers even after 12 h of insertion and only ~20 mL from the squeezers over a ~2 h duration (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 samples for He and ¹⁴C-dissolved inorganic carbon (¹⁴C-DIC) 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. For analysis of solid-phase C and N concentrations, 41 samples were taken from Hole U1371E.

Dissolved oxygen

Dissolved oxygen (O₂) was measured using electrodes and optodes on intact 1.5 m core sections from Holes U1371B–U1371D, U1371F (electrodes only), and U1371H after delivery from the catwalk to the Geochemistry/Microbiology Laboratory cold room (Fig. F32). Electrode-based measurements were performed at 10 cm intervals (Table T8) for the uppermost 2.9, 2.4, and 5.9 m in Holes U1371B, U1371C, and U1371H, respectively. Optode measurements were conducted in 10–30 cm intervals in the uppermost 4 m in Holes U1371B, U1371C, and U1371H (Table T9).

Electrode measurements in Hole U1371D were performed at 10 cm intervals in the uppermost 4 m (Table T8), followed by one measurement every 1.5 m (one per intact core section) to 110 mbsf. Toward the sediment/basement interface, higher resolution measurements of 10–30 cm intervals were taken to 125.7 mbsf. Optode measurements in Hole U1371D were measured at 10–30 cm intervals to 4 mbsf and from 115 to 125 mbsf. In Cores 329-U1371D-2H through 6H, 2–5 measurements were made per core. The optode signal stabilized very slowly (up to 30 min) in the consolidated sediment near the basement (Cores 13H and 14H), making quantification of oxygen concentration by optode difficult.

For Holes U1371D and U1371F, oxygen was measured with two different electrodes; the data for both profiles are recorded in parallel for most of the same depth intervals (Table T8). Intact core sections of Cores 329-U1371F-11H through 14H were directly brought from the catwalk to the Geochemistry/Microbiology Laboratory cold room and measured at the intervals marked by cutting lines drawn by microbiologists before being sectioned for microbiological whole-round core sampling. Electrode measurements were performed every 1.5 m (every core section) for Cores 329-U1371F-11H and 12H, in 20–50 intervals for Sections 329-U1371F-13H-2 through 13H-7, and at 20 cm resolution for Sections 14H-1 through 14H-6 (Table T8).

Dissolved oxygen concentrations measured by electrode decrease rapidly below the seafloor (Table T8). Near-surface values are 75, 33, and 60 µM for Holes U1371B, U1371C, and U1371H, respectively. Values approach the detection limit of oxygen measured by electrodes at ~1–1.3 mbsf for Holes U1371B and U1371C. For Hole U1371H, oxygen measurements are above the detection limit throughout the entire core (0–6 mbsf); however, very high but relatively constant values in Sections 329-U1371H-1H-2 and 1H-4 relative to Sections 1H-1 and 1H-3 suggest that large section-to-section variations in calibration affected this record.

For Hole U1371D, dissolved oxygen measured by electrode declines rapidly from a near-surface value of 82 to 2.5 µM at 1 mbsf and gradually decreases to <0.1 µM at 2.7 mbsf. Below this depth, values stay at this detection limit until ~110 mbsf, at which point oxygen becomes again detectable and gradually increases to 3–4 µM toward the basaltic basement. In Cores 329-U1371F-11H through 14H, electrode-based oxygen concentration values are close to the detection limit until 113.5 mbsf. They then increase with depth to ~11 µM with ~2 µM variability.

Oxygen measured by optode decreased rapidly from 180 to <5 µM in the uppermost 50 cm below the sediment surface (Hole U1371D). Below, oxygen was present at low and decreasing concentrations to 5–6 mbsf. No oxygen was detected (detection limit ~ 0.4 µM; see Table T9) between 10 and 45 mbsf. Oxygen again appeared in low (<2 µM) concentrations above the basement (115–125 mbsf). Oxygen measurements in both Holes U1371D and U1371F indicate a diffusive flux of oxygen upward into the sediment from the basement.

Redox potential

Redox potential (mV) was measured using needle electrodes (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]) in 1–3 intervals per core section from all of Hole U1371D and the lowermost 4 cores in Hole U1371F (Cores 329-U1371F-11H through 14H). As shown in Figure F33 and Table T10, redox potentials in the uppermost ~6 mbsf are positive and only slightly decrease from 199 mV at 0.7 mbsf to 153 mV at 6.68 mbsf. These are typical values for an oxidizing environment (ZoBell, 1946). Below 7 mbsf, the redox potential quickly decreases to –208 mV at 16.9 mbsf, which indicates the presence of dissolved reduced substances in the interstitial waters. This 16.9 mbsf minimum is the first of three or four local minima in redox potential in this sediment column. Neither the peaks nor the valleys forming the pattern seem to be associated with individual core breaks or other potential handling artifacts. The pattern may be related to zones of greater biogeochemical activity within discrete layers of the sequence (see “Lithostratigraphy”).

Redox potential reaches its most negative value (–554 mV) at 87.9 mbsf. Below 100 mbsf, redox potential exhibits a linear increase toward higher redox values and reaches values similar to, or even slightly more positive (>200 mV) than, those observed in the surface sediments. This deep (deeper than 100 mbsf) increase in redox potential to highly oxidizing values was replicated in measurements performed on the lowermost four cores of Hole U1371F (Fig. F33). These observations underscore the finding that dissolved oxygen is present in the lowermost part of the Site U1371 sediments and that lithologic Unit II is in a relatively oxidizing environment.

Dissolved hydrogen and methane

Dissolved hydrogen (H₂) concentrations were quantified in 71 samples collected in Hole U1371E from 0.3 to 130.2 mbsf (Fig. F34; Table T11). The maximum H₂ concentration in the sediment column was 10.4 nM in the uppermost sample (0.3 mbsf). Hydrogen concentrations remain at or below 2 nM for most of the sediment column, followed by a slight increase near the sediment/basalt interface. Seven samples were taken in the core refrigerator on the Hold Deck, whereas the remaining samples were collected on the catwalk. Based on the average of 13 blanks, the detection limit at this site was calculated as 1.4 nM.

Methane concentrations are below the detection limit (<0.98 µM) in all 14 samples from Hole U1371E, as measured by the IODP standard safety protocol. The detection limit is defined here as three times the standard deviation of the blank (ambient air). However, with the refined protocol, one sample (Section 329-U1371E-2H-5; 15.35 mbsf) reveals a methane concentration slightly above the detection limit (6.8 µM).

Interstitial water samples

A total of 90 Rhizon samples for dissolved nitrate analyses were obtained from Holes U1371B, U1371E, and U1371H. Profiles of dissolved nitrate concentration exhibit variations between the holes (Fig. F35A; Table T12). Nitrate concentrations in Hole U1371B are not discernible even at 0.2 mbsf. Nitrate concentrations decrease to below detection limit at 1 mbsf in Hole U1371E and drop below the detection limit in Hole U1371H at 2.5 mbsf. Vertical profiles of nitrate in Hole U1371H, in which nitrate samples were taken at high resolution (10–30 cm interval) through the uppermost 6 mbsf, were similar to those reported from the site survey cruise (D’Hondt et al., 2009). The results suggest that the near-surface sediment in Holes U1371B and U1371E may not have been recovered during coring. Nitrate concentration near the sediment/water interface in Hole U1371H is 41 µM (Sample 329-U1371H-1H-1, 18–20 cm), which is ~8 µM higher than local bottom water (Talley, 2007). This increase of nitrate is in conjunction with the steep decrease of oxygen (Fig. F32) in the uppermost 0.2 mbsf and indicates aerobic respiration of organic nitrogen (nitrification) near the seafloor at this site. Below 0.2 mbsf in Hole U1371H and for the first time during Expedition 329, nitrate concentrations continuously decrease to 2.1 µM at 2.53 mbsf (Sample 1H-2, 98–100 cm), implying microbial nitrate reduction (e.g., denitrification) in the suboxic condition with oxygen concentrations below 5 µM. Another unique feature of the nitrate profile is that ~2–5 µM of nitrate was observed above the sediment/basalt interface, between 105 and 120 mbsf.

Ammonium was measured on Rhizon-sampled interstitial waters. The concentration at 0.15 mbsf in Hole U1371E is 0.35 µM (Fig. F35A). Concentrations then rise steeply with a convex-upward profile, reaching a maximum of ~55 µM between 30 and 65 mbsf. Below 65 mbsf, concentration decreases slightly to 40 µM at 97 mbsf.

The dissolved phosphate profile, measured on 55 interstitial water samples obtained through squeezing from Hole U1371E, exhibits a similar pattern as at Site U1370, but its concentration range is approximately an order of magnitude greater here at Site U1371 (Table T13; Fig. F35B). Phosphate concentrations start at 9.9 µM at 0.05 mbsf and increase with a straight gradient to 22.6 µM at 1.45 mbsf. Peak concentrations occur at 4.95 mbsf and a peak of net release into interstitial water is apparent between 4.95 and 12.65 mbsf. This peak is attributed to microbially mediated oxidation of organic matter under anoxic conditions. Below this depth, the phosphate profile shows a slight concave-upward pattern as the phosphate concentrations decline to below 3 µM at 98.15 mbsf. The lowest concentrations range between 1.2 and 1.7 µM in the lithologic Unit II sediment. Adsorption or removal into mineral phases is expected to control the phosphate flux into the deep sediments. The pooled standard deviation (1σ; see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]) on triplicate measurements of the phosphate concentration is 0.066 µM.

In contrast to the previous Expedition 329 sites, dissolved silica shows greater and increasing concentrations with depth in Hole U1371E (Table T13; Fig. F35C). The dissolved silica concentration at 0.05 mbsf is 533 µM and increases to 759 µM by 12.5 mbsf. Below this depth, concentrations remain between 700 and 750 µM until they gently increase to above 800 µM at depths below 80 mbsf. The profile within the uppermost 100 m of sediment is typical for the dissolution of amorphous silica in siliceous sediment, coupled to equilibrium control of dissolved silica concentration. However, below 101 mbsf, dissolved silica concentrations show a striking and monotonic decrease toward basement, with a concentration of 383 µM at 128 mbsf. The pooled standard deviation (1σ; see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]) for duplicate measurements is 12.8 µM.

Alkalinity sharply increases from 3.0 mM at seafloor to 3.2 mM at ~2 mbsf and gradually decreases to 2.1 mM to the bottom of the hole (Fig. F35D). The maximum value at Site U1371 was higher than at previous sites, and may be due to more active oxidation of organic matter. No obvious offset was observed between the alkalinity of the samples taken on the catwalk and those taken in the Hold Deck’s core refrigerator (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). The standard deviation and error of alkalinity measurements on standard seawater CRM104 are 0.018 and 0.005 mM (N = 14), respectively.

Dissolved inorganic carbon (DIC) increases from 2.89 mM at the sediment surface to 3.1 mM at 1.45 mbsf (Fig. F35E) and remains stable to 5.95 mbsf, where it starts to decrease, reaching 2.1 mM at 130.31 mbsf. Between 50 and 70 mbsf, DIC content again remains stable. The range in DIC values is 0.79 mM. The average standard deviation of triplicate injection of the samples is 0.023 mM.

Chloride was determined from the squeezed interstitial water samples (Table T13; Fig. F35F). The chloride concentration near the seafloor is indistinguishable from local bottom water but monotonically increases by ~12 mM at ~35 mbsf. This 2% increase may be due to relict higher salinity seawater. Below this depth, there is no significant gradient until 100 mbsf, at which point chloride concentration decreases toward 552.8 mM (at 127.65 mbsf; interval 329-U1371E-14H-4, 40–50cm).

Sulfate was determined in the squeezed interstitial water samples. Sulfate concentrations begin at the surface sediment at 28.5 mM (Fig. F35G), which is close to the 28.6 mM concentration of sulfate in local bottom water. Sulfate concentrations increase to 28.6 mM at ~4.5 mbsf and then decrease smoothly to 27.5 mM at 98 mbsf. A steeper decline follows with a concentration of 26.6 mM at 123.6 mbsf. A clearer understanding of sulfate reactivity may be seen in the plot of the sulfate anomaly (Fig. F35H; see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). The sulfate anomaly decreases monotonically from –0.5% near the surface to –7.9% at 123.6 mbsf. The gradient of sulfate near the sediment/basalt interface may reflect removal of sulfate into authigenic minerals. There also appears to be some curvature in the sulfate profile that indicates microbial sulfate reduction.

As at previous sites, cations were measured for Site U1371 by both inductively coupled plasma–atomic emission spectroscopy (ICP-AES) and ion chromatography. Because of time constraints associated with the end of the expedition, not every sample was analyzed. The precision of cation measurements by ICP-AES was as follows, 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 = 0.8% of the measured value,
• Mg = 0.4% of the measured value,
• Na = 0.6% of the measured value,
• K = 0.4% of the measured value,
• Fe = 1% of the measured value, and
• Mn = 1%.

Because of end-of-expedition instrumentation constraints, B and Sr were not analyzed. 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 = 0.8%,
• Mg = 0.5%,
• Na = 0.4%, and
• K = 0.4%.

The shape of the concentration profiles determined by ICP-AES and ion chromatography agree well. As was the case at previous sites, the absolute values of the concentrations differ by an amount that is minimally greater than the analytical precision(s). The ion chromatography data at Site U1371 are slightly higher than the ICP-AES data. This contrast is most pronounced in the relative sense for Ca and less so for Na, Mg, and K. We favor the ICP-AES data for Ca because the shallowest samples measured by ICP-AES are closer to seawater values than the Ca data determined by ion chromatography. We favor the ion chromatography data for Na because they are closer to the expected seawater value for the shallowest samples. We emphasize this is a judgment determination at this point because the cause of this discrepancy remains unclear, even though the ICP-AES and ion chromatography protocols were both rigorously calibrated against multiple replicate analyses of IAPSO standard seawater, with identical items analyzed by both instruments and with detailed determinations of analytical precision. Postcruise shore-based analyses will aim to resolve this slight ambiguity (Table T13; Figs. F35I, F35J, F35K, F35L).

Both the ion chromatography and ICP-AES profiles of dissolved Ca show a consistent and linear increase of ~1 mM to ~70 mbsf. Between ~70 mbsf and the bottom of Hole U1371E, Ca concentrations are essentially constant. From the surface sediment to ~100 mbsf, Mg concentrations remain constant before increasing by ~4 mM to basement. Like Mg, Na presents no change from the surface to ~100 mbsf before decreasing to the basement. K remains constant over the uppermost ~20 mbsf before decreasing by ~1 mM steadily to ~100 mbsf. Between ~100 mbsf and the basement, K decreases by ~2.5 mM. Decreases in Na and K concentration most likely represent uptake of these alkalis by basement alteration processes. The increase in Mg is more unusual because during basement alteration Mg is usually sequestered into authigenic clay phases. The increase in Mg may result from some type of cation exchange during alteration, perhaps with Na and/or K.

The concentration of dissolved Fe is only a few times that of its detection limit (~4 µM; Fig. F35M) and the concentration profile is consistent with the O₂-depleted nature of this site. Moreover, the observed concentrations are the highest observed during among the Expedition 329 sites. Although there is a great deal of scatter in the Fe profile, it generally mirrors the redox potential profile (Fig. F33), with local Fe minima at the top and bottom of the sediment column and local Fe maxima coinciding with the redox potential minima within the column. Postcruise analyses will address the distribution of dissolved Fe more completely.

Unlike the previous sites occupied during Expedition 329, the concentration of Mn reached relatively high values and is readily interpretable (Figs. F35N, F35O). Concentrations of Mn show a very strong increase to ~360 µM through the uppermost 3 mbsf before decreasing gradually to values of ~200 µM at ~110 mbsf. Below this depth, values of dissolved Mn decrease to ~70 µM in the deepest sample analyzed at ~128 mbsf. Strong minima in dissolved Mn concentration occur at the top and bottom of the sediment column. Weak local minima approximately correspond to the strong minima in redox potential within the sediment column (Fig. F33).

At Site U1371, only 10 interstitial water samples were processed in the Hold Deck core refrigerator. Nonetheless, comparison of the catwalk samples (squeezed immediately upon core recovery) to those samples stored in the core refrigerator on the Hold Deck shows no offset between the data sets for any cation (see “Biogeochemistry” in the “Methods” chapter [Expedition 329 Scientists, 2011a]).

Solid-phase carbon and nitrogen

Concentrations of total carbon, total organic carbon (TOC), total inorganic carbon (TIC), and total nitrogen were determined for 40 samples from Hole U1371E (Fig. F36; Table T14).

Total nitrogen shows a slow decrease from ~0.05 wt% at the seafloor to 0.005 wt% at 128.1 mbsf. TOC shows a downcore decrease similar to total nitrogen, from 0.22 wt% at the seafloor to 0.01 wt% at 128.1 mbsf. The almost linear decrease of TOC is interrupted by slightly elevated values between 50 and 60 mbsf and again between 85 and 95 mbsf (Fig. F36). Total carbon also shows a slight decrease with depth, from 0.28 wt% at 0.01 mbsf to 0.04 wt% at 128.1 mbsf. However, the total carbon profile shows more significant variations downhole, in particular with two maxima at 49.15 and 91.65 mbsf. These depths correspond to two clear minima in the redox potential profile (Fig. F33).

TIC and CaCO₃ values obtained by coulometry follow a different pattern from the total carbon content obtained from the CHNS elemental analyzer (Fig. F36). The CaCO₃ content oscillates around ~0.04 wt% in the uppermost 20 mbsf and then shows three maxima at 33.15, 49.15, and 91.65 mbsf (0.27, 0.78, and 0.52 wt%, respectively). Below 100 mbsf, CaCO₃ values gradually increases from 0.02 wt% at 101.15 mbsf to 0.25 wt% at 128.1 mbsf.

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