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doi:10.2204/iodp.proc.318.107.2011

Geochemistry and microbiology

Organic geochemistry

Gas analysis was performed once per core from Holes U1359A–U1359D using the methods described in “Geochemistry and microbiology” in the “Methods” chapter.

Methane concentrations varied by four orders of magnitude downhole (from undetected to ~43,000 ppmv) (Fig. F34). Concentrations increase from undetected at 90 mbsf to ~15,000 ppmv at 200 mbsf. From 200 to 586 mbsf, concentrations average ~15,000 ppmv, with large variations from 6,000 to 43,000 ppmv superimposed on the mean. Ethane was detected below 100 mbsf, but C₁/C₂ levels were high throughout the hole (Fig. F34).

Inorganic geochemistry

Bulk sediments

Seventy-one samples from Holes U1359A–U1359D were taken for analyses of weight percent carbonate, carbon, nitrogen, and sulfur content, as well as major and trace element analyses (silicon, titanium, aluminum, iron, manganese, calcium, magnesium, sodium, potassium, phosphorus, strontium, barium, vanadium, scandium, and cobalt) (Table T13). Samples were collected in close collaboration with the sedimentology group to sample the main lithologies represented.

CaCO₃ (in weight percent) was determined on all sediment samples by coulometric methods (see “Geochemistry and microbiology” in the “Methods” chapter). CaCO₃ contents for most samples vary between <1 and 3.2 wt% (Fig. F35). A distinct carbonate-rich layer, with CaCO₃ contents of 39.7 wt%, was found at 372.45 mbsf and corresponds to a minor lithology of diatom-bearing nannofossil ooze (see “Lithostratigraphy”).

Carbon, nitrogen, and sulfur contents were measured on 17 selected samples covering the full range of CaCO₃ contents. Except for two samples at 81.44 and 372.45 mbsf (Samples 318-U1359B-9H-5, 122–124 cm, and 318-U1359D-13R-4, 71–73 cm), which represent minor lithologies (i.e., sand layer and diatom-bearing nannofossil ooze), all samples show very low carbon (<0.9 wt%), nitrogen (<0.05 wt%), and sulfur concentrations (<0.2 wt%). Calculated total organic carbon contents are <0.6 wt% and C/N ratios vary between 6 and 21 (outliers: sand layer = C/N ratio of 82; nannofossil ooze layer = C/N ratio of 390) (Fig. F36).

The concentrations of silicon, titanium, aluminum, iron, manganese, calcium, magnesium, sodium, potassium, phosphorus, strontium, barium, vanadium, scandium, and cobalt were obtained for all 71 bulk sediment samples by inductively coupled plasma–atomic emission spectrophotometry (ICP-AES). Representative results are shown in Figure F37 and data are reported in Table T13.

Four geochemical intervals can be distinguished:

1. An upper interval (0 to ~200 mbsf),

2. A transitional interval between ~210 and ~310 mbsf,

3. An interval from ~310 to 536 mbsf, and

4. A lower interval containing the deepest six samples analyzed (547.39–594.79 mbsf).

The uppermost geochemical interval (0 to ~210 mbsf) is characterized by large concentration ranges for most elements. Silicon dioxide values vary from 63.4 to 77.4 wt% and are positively correlated with CaO (1.0–2.5 wt%) and Ba concentrations (494–1158 ppm), indicating association with biogenic phases. Aluminum, titanium, potassium, and iron oxides reveal large fluctuations (9.5–16.6, 0.6–0.9, 2.1–4.1, and 3.4–8.7 wt%, respectively), which have an inverse correlation with SiO₂ that points to an association with detrital mineral phases. All ranges are reported neglecting the sand layer (minor lithology), sampled at 81.44 mbsf, which shows anomalously low Al₂O₃, TiO₂, K₂O, and Ba values (Fig. F37). One striking feature in the uppermost interval is a gradual increase in Ba concentrations from 747 ppm at 91.1 mbsf to 1041 ppm at 113.65 mbsf, which coincides with an interval of low magnetic susceptibility and high diatom productivity (see “Paleomagnetism” and “Biostratigraphy”).

The second interval, from ~210 to ~310 mbsf, can be described as transitional. Some elements display clearly defined changes (i.e., TiO₂), whereas others continue to show fluctuating concentrations (e.g., SiO₂, K₂O, and Al₂O₃), though sometimes with decreased amplitudes (i.e., Fe₂O₃ and SiO₂). This transitional interval broadly correlates with the lithostratigraphic interpretation of a transition from levee-type deposits by low-turbidity currents (lithostratigraphic Units I and II; bottom depth = 247.1 mbsf) and deposits from an environment dominated by contour currents and/or saline density flows (below 247.1 mbsf; see “Lithostratigraphy”).

The third interval (~310–535.68 mbsf) is characterized by significantly reduced variability in most elemental concentrations (omitting the diatom-bearing nannofossil ooze layer at 372.45 mbsf) and a change in elemental correlations. Silicon dioxide concentrations vary between 66.2 and 75.5 wt% and are now inversely correlated with Al₂O₃ (11.1–14.9 wt%), TiO₂ (0.6–0.7 wt%), K₂O (2.8–3.8 wt%), Ba (470–602 ppm), CaO (1.4–3.1 wt%), and Fe₂O₃ (4.0–6.9 wt%) concentrations (Fig. F37).

The lowermost interval (547.39–594.79 mbsf) displays a notable shift in the different elemental patterns while maintaining the general elemental correlations described for the third interval. Silicon dioxide values show a pattern of increasing values, by ~7 wt%, before decreasing again by ~6 wt%. This trend is mirrored by Al₂O₃, TiO₂, K₂O, Fe₂O₃, and Ba.

The chemical index of alteration (CIA = [Al₂O₃/(Al₂O₃ + CaO* + K₂O + Na₂O)] × 100, where CaO* represents the CaO fixed in silicate minerals; Nesbitt and Young, 1982) (Fig. F37) shows highly variable values (47–63) between 0 and ~310 mbsf (first and second geochemical intervals). These values are comparable to values observed for the upper parts of Hole U1356A and imply environmental conditions dominated by physical weathering of terrigenous material (e.g., Passchier and Krissek, 2008; see also “Geochemistry and microbiology” in the “Site U1356” chapter). In the third interval (~310–535.68 mbsf), CIA values are constant between 55 and 58, indicating less variability in continental weathering. Lower values occur again in the lowermost samples.

Pore water chemistry

Fifty-one interstitial water samples were taken in close collaboration with the microbiology samples from the top ~20 m (0.1–20.1 mbsf) of cores from Hole U1359B (Fig. F9; Table T14). Samples were stored in the onboard cold room (<8°C) and squeezed as quickly as possible (~48 h until the last sample was squeezed). However, because of the large number of interstitial water samples collected in a short amount of time, the uppermost sample from each 1.5 m section and the fifth sample from the uppermost five cores were prioritized for squeezing to obtain a minimally compromised, coarse initial interstitial water profile. These first 18 samples were all squeezed within 12 h of sampling. Shipboard chemical analyses on all interstitial water samples included pH, salinity, chlorinity, alkalinity, dissolved inorganic carbon (DIC), sulfate, phosphate, ammonium, nitrite and nitrate, sodium, potassium, magnesium, calcium, boron, manganese, iron, strontium, barium, and silica measurements. The data are provided in Table T14 and shown in Figures F38, F39, and F40. Throughout the following text, cited values for average seawater composition are taken from Millero and Sohn (1992) and Broecker and Peng (1982).

Sulfate, alkalinity, dissolved organic carbon, pH, and magnesium

Dissolved sulfate concentrations within the top 2.4 m of the core are indistinguishable from seawater values (~28 mM). From 2.4 to 20.1 mbsf, sulfate contents decrease linearly, reaching a minimum value of 22.9 mM at 19 mbsf (Fig. F38). Magnesium contents follow a parallel trend of decreasing values. Absolute concentrations, however, only change by ~10%, from 50.1 to 44.6 mM. The overall decreasing pattern of Mg in interstitial waters is punctuated by two positive excursions at 5.4 and 15.5 mbsf (72.3 and 71.8 mM, respectively). Sulfate depletion is further negatively correlated with pH, alkalinity, and DIC (Fig. F38). In detail, pH increases downhole from 7.0 to 7.67, with an excursion to more acidic values of 6.76 at 1.8 mbsf coinciding with the onset of manganese and phosphate enrichment (see section below). Alkalinity increases throughout the profile from 0.1 to 5.4 (0.1–20.1 mbsf). Between 15.5 and 18.0 mbsf, alkalinity fluctuates between 5.2 and 4.2. This behavior is paralleled by the profile of DIC.

All previously described observations are consistent with active diagenesis of buried organic matter within the SRZ:

Significant levels of sulfate at the bottom of the observed profile (~23 mM at 20.1 mbsf) imply that the sampled interval did not reach the carbon dioxide (methanic) reduction zone (see “Pore water chemistry” in the “Site U1357” chapter for contrasting behavior).

Manganese, phosphate, ammonium, and silica

Dissolved manganese concentrations are below the detection limit of the ICP-AES for the top 1.8 mbsf. Below this depth, manganese levels increase steadily until a relative maximum is reached at 8 mbsf (108 µM). Between 8 and 20.1 mbsf, manganese concentrations oscillate between 62 and 112 µM (Fig. F39). The increase of manganese is most likely caused by the reduction of manganese during organic matter decay in the suboxic zone of the sediment column (i.e., where reduction of nitrate, Mn, and Fe happens; Froelich et al., 1979). Phosphate concentrations show a less smooth pattern than manganese concentrations but also increase from low values at shallow depths (0–2.2 mbsf; 0.8–3.5 µM) to higher values, reaching a relative maximum of 12.8 µM at 7.4 mbsf. Below 7.4 mbsf, phosphate values vary between 4.9 and 13.6 µM (Fig. F39). The correlation between phosphate and manganese in the upper part of the interstitial water profile suggests a common origin related to the microbial degradation of organic matter. Contrasting trends below 7.4 mbsf may be the result of different affinities of Mn and phosphate for absorption onto particle surfaces and/or coprecipitation with minerals formed in situ. The scatter in the phosphate data can be partially explained by the timing of pore water squeezing. As explained above, the top sample from each of the 13 sections plus 5 samples from lower in the uppermost five sections were squeezed first. The data corresponding to these 18 samples are shown as open circles in Figure F39, suggesting that phosphate concentrations decline when samples are stored too long prior to squeezing.

Dissolved silica displays a more complex pattern, with high values (539–570 µM) for the uppermost 0.9 mbsf and lower values (480–533 µM) between 1.1 and 7.2 mbsf. After a sharp increase in Si concentrations, values are scattered around 540 mM from 8 to 14.5 mbsf. The lowest part of the profile shows a large scatter in Si concentrations (Fig. F39). In general, variations in silica could be related to early diagenetic clay mineral reactions, but such an interpretation will need to be confirmed by future studies. Ammonium concentrations show no general trend and fluctuate between 70.8 and 181.5 µM along the profile. A slight increase in ammonium concentrations at 2.2 mbsf appears to coincide with increased phosphate and manganese concentrations.

Nitrate, nitrite, boron, strontium, and chlorine

The sum of nitrite and nitrate concentrations (NO_x) in interstitial waters decrease overall from values of 4.6 µM at 0.1 mbsf to 3.7 µM at 7 mbsf (Fig. F40). The same interval, however, also contains major oscillations to absolute values of 5.0 µM at 2 mbsf and 1.2 µM at 2.2 mbsf. From 7 to 20.1 mbsf, NO_x concentrations are invariable around 4 µM. In general, reduction of nitrate to nitrite should follow reduction of manganese and sulfate during the mineralization of organic matter. The observed decrease in total concentrations of nitrite and nitrate in the topmost meter of the core is in line with this statement, although concentrations are very low throughout the profile. Dissolved strontium, boron, and chloride concentrations show fluctuations throughout the investigated uppermost 20 m of the sediment column centered around absolute values of 33, 339, and 560 µM, respectively (Fig. F40).

Microbiology

Microbiological sampling was conducted in Hole U1359B and supported with pore water sampling (Fig. F9). Fifty-two 10 cm whole rounds were taken from the top 20 m and frozen at –80°C for onshore phospholipid analyses and molecular 16S rRNA sequencing. Between 20 and 200 mbsf, seventeen 5 cm³ samples were taken and preserved for onshore molecular 16S rRNA sequencing.

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