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doi:10.2204/iodp.proc.318.105.2011

Geochemistry and microbiology

Organic geochemistry

Gas analysis was performed typically once per core from Hole U1357A using the methods described in “Geochemistry and microbiology” in the “Methods” chapter.

Methane concentrations varied by four orders of magnitude downhole, from undetected to ~45,000 ppmv (Fig. F29). Levels increase from 5,000 ppmv to a maximum of 43,000 ppmv between 0 and 20 mbsf, decrease to 10,000 ppmv at 50 mbsf, and mostly vary between 5,000 and <20,000 ppmv downhole. Ethane concentrations were negligible (<1 ppmv).

Inorganic geochemistry

Bulk sediments

Ninety-six sediment samples from Hole U1357A 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 T6). Samples were collected in close collaboration with the sedimentology group to sample the main lithologies represented. Sampling density was approximately one sample every other section (0.43–185.7 mbsf).

CaCO₃ (in weight percent) was determined on all sediment samples by coulometric methods (see “Geochemistry and microbiology” in the “Methods” chapter). CaCO₃ contents vary between 1 and 3 wt% for most of the core (Fig. F30). A distinct carbonate-rich layer with CaCO₃ > 9 wt% was found at 126.34 mbsf. CaCO₃ content increases slightly below ~177 mbsf and reaches a maximum of 25 wt% in the bottom diamict sample (318-U1357A-21X-CC, 7–9 cm).

Carbon, nitrogen, and sulfur contents were measured on 19 samples (Fig. F31). For most of the core (0.2–152 mbsf), nitrogen and sulfur concentrations are very low (0.20–0.26 and 0.4–0.7 wt%, respectively). Carbon concentrations vary between 1.4 and 2.6 wt%, yielding total organic carbon (TOC) contents of 1 to 2 wt% and C/N ratios of 7–12. Below ~152 mbsf, nitrogen, carbon, and sulfur contents decrease even further. Only the lowermost sample (185.67 mbsf; diamict) stands out, with carbon contents of 3.5 wt%, equivalent to ~0.5 wt% TOC. No obvious differences of carbon, nitrogen, and sulfur concentrations between light and dark laminations were found.

The concentrations of silicon, titanium, aluminum, calcium, magnesium, sodium, potassium, phosphorus, strontium, barium, vanadium, scandium, and cobalt were obtained for 41 bulk sediment samples by inductively coupled plasma–atomic emission spectrophotometry. Representative results are shown in Figure F32 and data are reported in Table T6. Three distinct geochemical intervals can be distinguished: (1) an upper interval (0 to ~175.5 mbsf), (2) a lower interval (~175.5–184.2 mbsf), and (3) the lowest sample analyzed (~185.67 mbsf). These geochemical intervals broadly correlate with lithostratigraphic Units I (laminated “pure” diatom ooze; 0–170.25 mbsf), II (clay-bearing to clay-rich olive-green diatom ooze with distinct laminations; 170.25–185.65 mbsf), and III (highly lithified carbonate-cemented gravelly mudstone (diamict); 186.46–186.6 mbsf) (see “Lithostratigraphy”).

The majority of the diatom ooze samples analyzed between 0 and 175.5 mbsf show relatively narrow ranges of elemental concentrations. Silicon dioxide values are high (76–91 wt%) and accompanied by low-concentration levels of TiO₂ (<0.3 wt%), Al₂O₃ (<5.6 wt%), K₂O (<1.5 wt%), CaO (<2.5 wt%), and V (<34 ppm), indicating very low terrigenous contributions to the bulk sediment assemblage. Outliers to this uniform geochemical interval are the carbonate-enriched layer described above (126.34–126.37 mbsf) and the topmost sample (no salt correction was done).

In contrast to the uniform values described for most of the recovered cores from Hole U1357A, the lowermost 10 m show very different geochemical characteristics, pointing to a higher and more variable abundance of terrigenous material (i.e., higher and more scattered values in SiO₂, TiO₂, Al₂O₃, K₂O, and Co). This finding is in agreement with the lithostratigraphy and our own observations during grinding and homogenization (i.e., first macroscopic occurrence of detrital grains and small dropstones from 175 mbsf downhole).

The diamict recovered at the very bottom of Hole U1357A shows the most extreme geochemical values observed at Site U1357 (low SiO₂ and high Al₂O₃, TiO₂, MgO, CaO, V, and Sr).

Diagenetic mineral formation

During core description, several 1–2 mm thick layers of a clear, glassy mineral oriented parallel to the bedding were observed (Sample 318-U1357A-8H-8, 54–55 cm), as well as at several other deeper levels in Hole U1357A (Fig. F11). This mineral has been identified by XRD analyses as struvite (NH₄MgPO₄·6H₂O), an authigenic phosphate mineral that is commonly formed by bacterial biomineralization in anoxic sediments in the presence of ammonium (see “Pore water chemistry”). Although no shipboard geochemical analyses were carried out on struvite minerals, their occurrence is likely to be of interest in terms of both biogeochemistry and carbonate preservation/overgrowth at Site U1357. González-Muñoz et al. (2008) suggested that struvite formation by Idiomarina sp. bacteria strains at modern seawater salinities initiates secondary Ca-Mg carbonate production. If such a process is determined to occur at Site U1357, it may impact the utility of foraminifers found in Hole U1357A for age dating and/or paleoclimatic interpretations based on stable isotope geochemistry.

Pore water chemistry

Forty-three interstitial water samples were taken in close collaboration with the microbiology samples from the uppermost 20 m (0.1–20.35 mbsf) of Hole U1357C (Fig. F33). Shipboard chemical analyses included pH, salinity, chlorinity, alkalinity, dissolved inorganic carbon (DIC), sulfate, phosphate, ammonium, sodium, potassium, magnesium, calcium, boron, manganese, strontium, barium, and silica measurements. The data are provided in Table T7 and are shown in Figures F34, F35, F36, and F37.

Because the topmost cores recovered from Site U1357 were very soupy, extremely porous, and gas-expanded further downhole, near-surface mixing processes and seawater infiltration at core tops was inevitable. In the pore water chemistry data, seawater contamination was recognized by unusually high sulfate contents at all core tops (see “pH, salinity, chloride, and sodium”). We marked these seawater-infiltrated or mixed intervals as shaded bars in Figures F34, F35, F36, and F37 and refrained from any interpretations of the corresponding data (Table T7). Cited values for average seawater composition throughout the text are taken from Millero and Sohn (1992) and Broecker and Peng (1982).

pH, salinity, chloride, and sodium

The pH of pore waters ranges from 7.8 to 7.5, with all values lower than average seawater (pH = 8.1). The pH profile shows some variations for the uppermost 7 mbsf, with pH values from 7.8 to 7.6. Below 7 mbsf, pH decreases progressively to ~7.5 at 20 mbsf (Fig. F34).

Chloride measured by ion chromatography displays a complex trend with decreasing values from 559 to 539 mM in the uppermost 6.2 mbsf and a progressive increase from 539 to 551 mM between 5 and 20 mbsf (Fig. F34). It should be noted that chloride concentrations obtained by titration are not featured in this report, as they are not solely representative of the chloride content itself. The AgNO₃ used for the titration reacted with hydrogen sulfides (H₂S) present in the pore waters, yielding erroneous results of decreasing chloride concentration with depth.

Sulfate, ammonium, dissolved inorganic carbon, alkalinity, and phosphate

Sulfate contents in interstitial waters are below detection limit, except for samples around the top of all three cores (1–5 mM at 0.1–0.5, 9.4–11.4, and 19.4–20.3 mbsf) and samples between 1.8 and 2.4 mbsf (Fig. F35).

Ammonium increases almost linearly from near-surface values of 900 to 4500 µM at 18 mbsf. Such high ammonium concentrations reflect a vigorous degradation of organic matter by biotic activity, supported by covariation of ammonium with DIC values (values increase from 39.8 to 79.6 mM at depth) (Fig. F35). The combination of sulfate-free interstitial waters with very high levels of ammonium in the uppermost 20 mbsf suggests that this part of the core is located in the carbon dioxide (methanic) reduction zone.

Alkalinity follows a trend similar to ammonium, with a gradual increase from 50 mM at the surface to 88 mM at ~18 mbsf. However, measured phosphate concentrations in interstitial waters show a more complicated pattern, with pronounced variations between 286 and 610 µM (Fig. F35). This could indicate phosphate removal from pore waters, potentially related with dissolution and/or precipitation of authigenic phases such as the mineral struvite (see discussion above).

Magnesium and calcium

Magnesium and calcium follow a parallel trend with highly variable concentration levels in the uppermost 6.2 mbsf (Mg = ~53.5 to ~55 mM, Ca = 10.67–10.98 mM) (Fig. F36). Between 6.2 and 10.5 mbsf, magnesium and calcium contents increase progressively, reaching 56.5 and 11.5 mM, respectively. From 10.5 to 20.3 mbsf, magnesium concentrations in the interstitial waters vary from 56.5 to 55.7 mM and calcium concentrations range from 11.33 to 11.59 mM.

Potassium, strontium, silica, boron, barium, and phosphorus

Potassium concentrations in interstitial waters show a gradual decrease downcore from a minimum value of 10.6 mM in the shallowest sample (0.1 mbsf) to a maximum value of 12.3 mM at 15.4 mbsf (Fig. F37). All of these values are higher than average seawater (10.2 mM). Two data points at 1.2 and 3.7 mbsf fall outside the described data range (9.7 and 9.8 mM).

Concentrations of silica and strontium in interstitial waters follow parallel patterns, with highly variable values between 386 and 506 µM and 21 to 33 µM, respectively. Two pronounced excursions to lower values can be observed at ~12 mbsf and from 16 to 19 msbf (Fig. F37). Boron values oscillate between 244 and 334 µM, levels that are lower than the average seawater value of 416 µM (Fig. F37). In the lower part of the profile, B values show two pronounced maxima that are anticorrelated to Si and Sr and may be related to an early dissolution and/or recrystalization of opal-A (Bidle and Azam, 1999) and/or specific surface area and sorption changes.

Phosphorous concentrations on the other hand are positively correlated to boron in the lower part of the profile (i.e., below 10 mbsf) and are highly variable in the upper part of the profile (as is the case for B, Sr, and Si) (Fig. F37). An important note for the final interpretation of the P, Si, Sr, K, and B data may be to consider that each interstitial water sample is integrating over several distinct centimeter-scale laminations in the diatom ooze. Hence different levels of oxidation, and their relative representation in each pore water sample, could govern the two apparent levels of concentrations below and above ~320 µM.

Finally, barium concentrations in interstitial waters are generally low and show a convex pattern from 0 to 10 mbsf, with concentrations from 5 to 20 µM. From 10 to 20.3 mbsf, barium shows more fluctuating values, which are not clearly correlated to other elemental maxima and minima described above (Fig. F37).

Microbiology

Microbiological sampling was conducted in Hole U1357C in concert with pore water sampling as highlighted in Figure F33. A high-resolution sampling plan was implemented for the uppermost 20 m of the hole, where 10 cm whole rounds were reserved for onshore phospholipid analyses and molecular 16S rRNA sequencing. Beyond 20 mbsf, 5 cm³ samples were taken for onshore molecular 16S rRNA sequencing.

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