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doi:10.2204/iodp.proc.317.105.2011

Geochemistry and microbiology

Organic geochemistry

Shipboard organic geochemical studies of cores from Holes U1353A and U1353B included monitoring hydrocarbon gases, carbonate carbon, organic carbon, and total nitrogen (TN) and characterizing organic matter with the source rock analyzer (SRA). The procedures used in these studies are summarized in "Geochemistry and microbiology" in the "Methods" chapter. All depths in this section are reported in m CSF-A.

Volatile gases

All cores recovered from Holes U1353A and U1353B were monitored for gaseous hydrocarbons using the headspace (HS) gas technique (Table T12). No significant amounts of hydrocarbons above background laboratory air (1.7–2.4 ppmv) were detected in the uppermost 350 m. Three samples from deeper in Hole U1353B contain 5–32 ppmv methane but no higher hydrocarbons (Fig. F32).

Small amounts of gas were recovered from air enclosed in the core liners of four cores from Holes U1353A and U1353B (Table T13). These samples were reported as core void gas (VAC) samples, but they did not come from actual gas voids within the cores. Two of these samples (Sections 317-U1353B-9H-4 [61.75 m] and 10H-4 [67.35 m]) contain methane slightly above background levels, one (Section 10H-3 [65.87 m]) contains higher amounts of methane with a trace of ethane, and one (Section 317-U1353A-5H-5 [32.93 m]) contains 717 ppmv of methane with hydrocarbons to C₄ and significantly more CO₂ than is found in air. The C₁/C₂ ratios of the two samples that contain ethane are 163 and 284, respectively.

Carbon and elemental analyses

The results of inorganic carbon (IC), carbonate, total carbon (TC), total organic carbon by difference (TOC_DIFF), TN, and TOC_DIFF/TN analyses on selected sediment samples from Hole U1353B are given in Table T14. In total, 82 sediment samples were analyzed. TC, TN, TOC, and calcium carbonate contents are plotted against burial depth in Figure F33. Calcium carbonate contents range from 2 to 29 wt% in Pleistocene–Holocene shelf sediments at burial depths <70 m (Fig. F33A). Below 100 m there is a background of low calcium carbonate (0.5–2 wt%) with occasional high values (10–57 wt%). Between 150 and 250 m, carbonate values scatter widely between 0.3 and 57 wt% and then decrease slightly to values ranging between 0.4 and 8 wt% below 270 m, the only exception being Sample 317-U1353B-92X-CC (547.1 m), which has a carbonate content of 39 wt%. TC has a profile remarkably similar to that of carbonate content, with average values generally decreasing downhole and frequent scatter as high as 7.3 wt% above 250 m (Fig. F33B). TC values below 250 m are more uniform and average ~0.3 wt% (with the exception of the above-mentioned sample at 547.1 m). TN amounts are very low and range from 0.003 to 0.08 wt%, with an apparent decrease with depth (Fig. F33C).

TOC generally decreases with depth (Fig. F33D–F33E). Surface sediments are characterized by organic carbon contents of ~0.8 wt% (TOC_DIFF) and decrease to ~0.1–0.2 wt% in the deeper layers below 250 m. TOC values determined by the source rock analyzer (TOC_SRA) are systematically higher than corresponding values determined by the difference method (TOC_DIFF) (Fig. F33E). A cross-plot of TC from the elemental analyzer and the sum of TOC_SRA plus IC from the coulometer confirms this observation (Fig. F34). The ratio of TOC_DIFF/TN decreases with depth, reflecting the faster decrease with depth of TOC compared to TN (Fig. F33F).

Organic matter pyrolysis

All of the samples used for CNS analysis were also characterized by SRA pyrolysis (Table T15; Figs. F35, F36). S₁ and S₂ rapidly decrease with depth from 0 to 60 m with ranges of ~0.05–0.25 and 0.1–1.2 mg/g, respectively (Fig. F35A–F35B). Below 60 m, the decrease is more gradual, and values are ~0.05 and 0.1–0.2 mg/g, respectively. S₃ decreases with depth in the uppermost 250 m with a range of 0.05–3.8 mg/g (Fig. F35C). Pyrolysis carbon necessarily mirrors S₁ and S₂ and decreases with depth from 0.1 mg/g at the sediment surface to 0.01–0.02 mg/g below 400 m (Fig. F35D). The hydrogen index ranges from 17 to 127 mg/g C, with higher values in the 0–250 m depth interval (Fig. F36A). The oxygen index ranges from 5 to 240 mg/g C, with no apparent trend (Fig. F36B). Average T_max values decrease, with temperatures of 415°–420°C in the 0–8.2 m depth interval and 370°–422°C at the bottom of Hole U1353B (Fig. F36C). Production index values show a reverse trend and increase downhole from ~18% in near-surface sediments to 30%–34% at depths of 500–600 m (Fig. F36D).

A modified van Krevelen diagram (Fig. F37) indicates poor-quality organic matter at Site U1353, with most samples clustering near the Type IV kerogen line, similar to Site U1351.

Preliminary interpretation of organic matter

Organic matter content appears to decrease with depth over the uppermost 100 m and remains relatively low throughout the remainder of the sampled section. This probably represents active biological oxidation and roughly correlates with intervals of increased alkalinity and decreased sulfate (see "Inorganic geochemistry"). Pyrolysis results suggest a largely terrestrial plant origin for organic matter, but TOC_DIFF/TN values suggest some marine influence.

Inorganic geochemistry

A total of 53 interstitial water samples (Tables T16, T17) were collected and analyzed at Site U1353. Thirty-five samples were taken to 56 m in Hole U1353A, which was dedicated mainly to whole-round sampling for geochemistry and microbiology. Two samples were taken from each of Cores 317-U1353B-9H, 10H, and 11H. Below Core 11H (72 m), sample spacing was largely dictated by core recovery. Only one whole-round core sample for interstitial water analyses was collected below 318 m (Section 317-U1353B-96X-1 [585.8 m]). Interstitial water chemistry is plotted versus depth in Figures F38, F39, F40, F41, and F42.

Salinity, chloride, sodium, and pH

Salinities in samples near the seafloor are slightly lower than normal seawater at 3.4 and rapidly decline to 2.4 at 38.6 m (Fig. F38A). Thereafter, salinity remains relatively constant at 2.4–2.5 to 63.4 m and then increases gradually to 3.4 at 178.4 m. Below this sample, salinity remains constant to 318.5 m, but the deepest sample has a somewhat elevated salinity at 3.8 (Fig. F38A). Chloride (Fig. F38B) and sodium (Fig. F38C) almost exactly parallel salinity measurements. Measured pH values range between ~7.2 and 8.0 with no depth trend (Fig. F38D).

Calcium, magnesium, and strontium

Calcium and magnesium both decrease from the surface sediments to the salinity minimum at ~47 m and then increase below this depth (Fig. F39A–F39B). Calcium increases to >15 mM below 150 m, considerably above seawater values. In contrast, the magnesium increase below the salinity minimum is only to ~44 mM, showing a net depletion relative to seawater. The Mg/Ca ratio decreases from 5 in the surface sediments to 2.4 in the deepest sample (Fig. F39C). Strontium increases in the uppermost 150 m, reaching 0.44 mM, and then decreases to 0.26 mM at 319 m (Fig. F39D). In the deepest section of Site U1353, strontium increases again to 0.53 mM. The Sr/Ca ratio initially increases from seawater values of ~0.01 to 0.03 at 60 m. It then decreases from a maximum of 0.032 at 115 m to 0.016 at 319 m (Fig. F39E). The deepest sample has an elevated Sr/Ca ratio of 0.03.

Alkalinity, sulfate, ammonium, phosphate, and dissolved silica

Alkalinity increases relatively steeply from 3.2 mM at 1.2 m to 6.5 mM at 6.7 m. It then fluctuates between 5.5 and 6.3 mM to 60 m (Fig. F39F). Alkalinity then slowly decreases to ~2.3 mM at 115 m and below. The decline in sulfate appears to be more related to the salinity decrease than to sulfate reduction. After dropping to a minimum of 15.5 mM at 44.1 m, sulfate increases slowly to ~24 mM at 115 m and remains relatively constant to 319 m (Fig. F40A).

Ammonium increases from 0.13 mM at 1.2 m to 1.2 mM at 29 m and then remains relatively constant to 38.6 m (Fig. F40B). Ammonium then decreases to ~0.6 mM at 60 m, below which it remains roughly constant. Phosphate increases from 3.7 µM at 1.2 m to 7.9 µM at 6.7 m and then decreases steadily throughout the rest of the sampled interval to <0.1 µM at 178 m (Fig. F40C).

Dissolved silica is present at 196 µM at 1.2 m, increases to 555 µM at 12.2 m, and then fluctuates between 300 and 629 µM to 72 m (Fig. F40D). Below 72 m, dissolved silica decreases rapidly and stays relatively constant at <210 µM.

Potassium, barium, lithium, silicon, boron, iron, and manganese

Potassium decreases from ~11 mM, slightly above seawater, to 4.7 mM at 55 m (Fig. F41A). This decrease closely matches the salinity decline. Thereafter, potassium varies little with additional depth.

Barium rapidly increases between 1.2 and 24.1 m from seawater values to a high of 1.4 µM and then remains relatively constant at ~1.1 µM to 253 m (Fig. F41B). Thereafter, barium increases to 3.0 µM, reaching a maximum of 4.0 µM at 586 m in the deepest sample.

Lithium increases throughout the cored interval at Site U1353 from seawater values of ~24 µM in the shallowest samples to a maximum of 112 µM in the deepest sample (Fig. F41C).

Silicon generally shows trends similar to dissolved silica. Silicon fluctuates in the uppermost 72 m before decreasing rapidly to 135 m (Fig. F41D).

Boron concentrations are at seawater values of ~420 µM in the shallowest samples and decrease to 342 µM at 30 m (Fig. F42A) before gradually increasing to a maximum of 614 µM in the deepest sample.

Iron and manganese show similar trends and tightly co-vary (Fig. F42B–F42C). Above 70 m, iron and manganese concentrations are largely scattered between 12 and 39 µM and 5 and 17 µM, respectively. Below 70 m, both iron and manganese have relatively constant values, except manganese is elevated relative to iron in the three deepest samples (Fig. F42B–F42C).

Preliminary interpretation of diagenesis

Interstitial water geochemistry in the uppermost 150 m at Site U1353 is dominated by the salinity minimum at ~50 m (Figs. F38, F43). The presence of this less saline lens can be explained either by the modern intrusion of meteoric water from land or by the historic remains of freshwater that was emplaced when the shelf was emergent and is now being slowly replaced by the downward diffusion of seawater. A slight alkalinity increase in the 0–7 m depth interval probably represents some degree of sulfate reduction. Sulfate is never depleted and methane is not present above background levels. Either methanogenesis did not occur in these sediments, or previously generated methane was lost when the shelf was emergent or was oxidized when sulfate was replenished by diffusion after a subsequent sea level rise.

Alkalinity, sulfate, calcium, and magnesium were normalized to chloride, which is here presumed to be a nonreactive ionic species, so as to evaluate changes due to reaction rather than dilution (Table T18). The ratio of chloride surface concentration to measured concentration was used as a normalization factor for every data point, assuming that chloride levels should be constant at surface concentrations over the depth interval of Site U1353 when no dilution by fresher water had taken place. In Figure F44, normalized concentrations are shown along with observed concentrations. Chloride-normalized alkalinity has a somewhat larger increase from 3.2 mM near the seafloor to 9 mM at 54 m. Chloride-normalized sulfate has a depletion of 8 mM from the sediment surface to the concentration minimum over the 38–44 m depth interval. This decrease does not exactly match the alkalinity increase, indicating that sulfate reduction has occurred at Site U1353 and that alkalinity must also be affected by other processes, probably carbonate precipitation and dissolution. Chloride-normalized calcium decreases very slightly in the 0–8 m depth interval but increases in stages below that depth, leveling off at ~16 mM (Fig. F44). The relative increase in chloride-normalized calcium is likely related to the dissolution of calcareous microfossils. In contrast, chloride-normalized magnesium declines steadily over the 10–135 m depth interval, leveling off at ~45 mM.

A small positive increase in boron with increasing depth may indicate the release of a desorbable boron fraction and the degradation of organic matter in the pore waters. The increase in lithium potentially also reflects desorption reactions. The increase in barium at the same time as the decrease in sulfate possibly relates to the dissolution of barite.

Microbiology

Sample collection

Shipboard microbiological studies of cores from Site U1353 included recovering whole-round sediment samples for intact polar lipid analyses and incubation tests, as previously described in "Geochemistry and microbiology" in the "Methods" chapter. Microbiological sediment samples were only taken from the hole dedicated to whole-round sampling (Hole U1353A). Eight whole-round samples were collected for microbiological investigations.

Contamination tracer tests

All whole-round sediment samples were tested for seawater contamination using a particulate tracer. At this site, water-soluble perfluoromethylcyclohexane tracer (PFT) was not employed.

Particulate tracer

Fluorescent microspheres were used as a particulate tracer on all cores from which whole-round samples were subsequently taken. For each microbiology sample, two subsamples (1 cm³) were collected on the catwalk with a 3 mL cut-off syringe from the periphery and center of the core. A total of 16 samples were collected for contamination tests. Table T19 shows data for potential micrometer-sized particle contamination that could occur during the drilling process. All samples contained large amounts of microspheres on their outside surfaces, showing a heterogeneous distribution of microspheres along the core liner. Five samples (317-U1353A-2H-1 [6.3 m], 3H-1 [10.1 m], 4H-1 [19 m], 5H-1 [27.3 m], and 6H-1 [35 m]; see Table T19) showed high contamination levels (up to 5.8 × 10³ beads/cm³) on the center part of the cores, which can be explained by the sampling technique, despite the fact that a 1 cm thick layer was removed during sampling of the inner part of the core on the catwalk. The high water content of cores can sometimes increase the contamination level in the inner part of the cores, particularly in the case of near-surface sediments. Three samples (317-U1353A-1H-1 [0.8 m], 7H-1 [42 m], and 8H-1 [48.8 m]; see Table T19) did not contain any microspheres in the center part of the cores, which indicates that no contamination occurred during the drilling operations.

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