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doi:10.2204/iodp.proc.317.106.2011

Geochemistry and microbiology

Organic geochemistry

Shipboard organic geochemical studies of cores from Holes U1354A–U1354C included monitoring hydrocarbon gases, carbonate carbon, total carbon (TC), organic carbon, and total nitrogen (TN). 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 U1354A and U1354C were monitored for the presence of gaseous hydrocarbons using the headspace (HS) gas technique (Table T15). In Hole U1354A, only background amounts (1–3 ppmv) of hydrocarbons were detected, with the exception of the 33–75 m depth interval, where headspace methane increases to a peak of 23 ppmv at 46 m and then decreases to background levels below 75 m (Fig. F27). HS samples from sediments with slightly elevated methane also contain trace amounts (0.7–1.3 ppmv) of ethane (Fig. F27).

Small amounts of gas were recovered from air enclosed in the core liner of Cores 317-U1354A-14H and 317-U1354B-14H (Table T16). These samples were reported as core void gas (VAC) samples, but were not from an actual gas void within the core. These samples contain methane (3.0–4.8 ppmv) slightly above background levels; no ethane was detectable.

HS samples from Hole U1354C also contain only background levels of methane to ~200 m, where methane concentrations begin to increase, eventually leveling off in the range of 12,000–22,000 ppmv to the deepest sample in Hole U1354C at 375 m. All samples from Hole U1354C with elevated methane concentrations also contain 2–29 ppmv ethane. CO₁ is present in HS samples below 250 m at concentrations significantly above atmospheric levels (Fig. F27). C₃₊ hydrocarbons were not detected in samples from Site U1354.

Carbon and elemental analyses

The results of inorganic carbon (IC), carbonate, TC, total organic carbon by difference (TOC_DIFF), TN, and TOC_DIFF/TN analyses on selected sediment samples from Site U1354 are given in Table T17. Only 18 sediment samples were analyzed because of time constraints at the end of the expedition. TC, TN, TOC_DIFF, and calcium carbonate contents are plotted against burial depth in Figure F28. Calcium carbonate contents range from 1.3 to 52 wt% in sediments analyzed to burial depths of 81 m (Fig. F28A). TC (as at Site U1353) has a profile remarkably similar to that of carbonate content, with no trend but with the highest values (>5 wt%) clustered in the 73–76 m depth range. TN amounts are very low and range from 0.004 to 0.08 wt%, having no apparent trend with depth (Fig. F28C).

Organic carbon ranges from 0.02 to 1.1 wt% (Fig. F28D), with the highest value at 50 m. The ratio of TOC_DIFF/TN generally decreases with depth, with the exception of the high-carbonate samples in the 73–76 m depth interval (Fig. F28E).

Inorganic geochemistry

A total of 69 interstitial water samples (Tables T18, T19) were collected and analyzed at Site U1354. Hole U1354A, which was dedicated mainly to whole-round sampling for geochemistry, was sampled at an average frequency of one sample per 1.7 m down to 84 m. Hole U1354C was sampled less frequently (one sample per core, where recovery allowed) from 82 to 319 m. Interstitial water chemistry is plotted versus depth in Figures F29–F34.

Salinity, chloride, sodium, and pH

Salinities in samples near the seafloor are about normal seawater values of 3.4 and decline to 3.3 over the 17–38 m depth interval (Fig. F29A). From 38 to 150 m, salinity gradually increases to slightly above seawater (3.6) and then fluctuates to lower values at ~210 m before climbing to 3.8 in the bottom three samples. Chloride (Fig. F29B) and sodium (Fig. F29C) do not parallel salinity measurements, but both increase to ~15% above seawater values over the 0–84 m depth interval. From 84 to 240 m, chloride and sodium values scatter between 580 and 640 mM and 520 and 560 mM, respectively, and do not significantly vary with depth. Sodium and chloride increase significantly in the bottom two samples in parallel with the increase in salinity (Fig. F29). Measured pH values scatter between 7.1 and 7.8, with no consistent depth trend (Fig. F29D).

Calcium, magnesium, and strontium

Calcium and magnesium both decrease below a 0–3.7 m chemically homogeneous zone to minima of 5.7 mM at 22 m for calcium and 30.7 mM at 54 m for magnesium (Fig. F30A–F30B). Calcium then increases to 13.1 mM and magnesium increases to 41.1 mM at 178 m. Between 178 m and the next sampled depth at 205 m, a major discontinuity in the calcium concentration profile is evident, with calcium decreasing by 34% to 8.6 mM and then rising gradually to 15.6 mM in the deepest sample at 318 m. Magnesium also decreases in the same depth interval, but not as dramatically. The Mg/Ca ratio increases from 5.1 in near-surface sediments to 6.3 at the depth of the calcium minimum, decreases to 3.1 at 178 m, increases to 4.3 at 215 m, and then drops gradually to 2.4 at 318 m (Fig. F30C).

Strontium rapidly increases between 5.1 and 17.6 m from seawater values to a maximum of 440 µM before gradually increasing toward 600 µM at ~60 m (Fig. F30D). Thereafter, strontium concentrations vary between 518 and 622 µM. The Sr/Ca ratio increases rapidly from seawater values in near-surface sediments to a maximum of ~0.075 at 34–41 m before gradually decreasing again to ~0.046 at 178 m (Fig. F30E). The Sr/Ca ratio increases to 0.067 in the calcium minimum zone, before decreasing to 0.034 in the deepest sample.

Alkalinity, sulfate, and phosphate

Alkalinity is relatively constant over a near-surface (0–3.7 m) chemically homogeneous zone and then increases somewhat steeply from 3.1 mM at 3.7 m to 8.8 mM at 12.2 m. Alkalinity fluctuates between 8.7 and 10.3 mM to 50 m (Fig. F30F) and then slowly decreases to ~2.2 mM at 318 m, with the exception of samples from Sections 317-U1354C-18X-2 (205.20 m) and 19X-2 (214.70 m), which show elevated alkalinities of 3.8 and 4.2 mM, respectively. Sulfate decreases rapidly beneath the chemically homogeneous zone from 28 mM at 3.7 m to zero at 34.2 m. Sulfate remains at or near zero to ~59 m and then gradually increases to 16.4 mM at 178 m (Fig. F31A). Sulfate returns to zero by the next sampled depth at 205 m, which corresponds with the depth of the second increases in methane and alkalinity.

Phosphate increases from 4.7 µM at 1.3 m to 13.8  µM at 9.7 m, remains high to 21 m, and then decreases steadily to 1.4 µM at 79 m (Fig. F31B). Deeper samples have phosphate concentrations <1.5 µM and a generally uniform trend, except that samples at 205.2 and 214.7 m have slightly higher phosphate than adjacent samples.

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

Potassium decreases steadily from ~11 mM, slightly above seawater values, to 2.9 mM at 235 m, with a slight perturbation in the samples at 165 and 178 m (Fig. F32A). Potassium then rises again to 4.7 mM at 318 m. Barium rapidly increases to 2.5 µM at 23.6 m and then varies at ~2.3 µM between 23.6 and 57.6 m (Fig. F32B). Barium increases to 3.0 µM at 67.8 m and then decreases to 1.2 µM at 111 m, with a spike to 2.6 µM at 97 m. Thereafter, barium remains constant before increasing below 150 m to a maximum of 7.9 µM in the deepest sample at 318 m, and with a spike to 3 µM at 165 m.

Lithium initially increases from seawater values in the shallowest sample to 41.1 µM at 19.2 m and then remains relatively constant to 32.6 m (Fig. F32C). Below 32.6 m, lithium begins to decrease from 41.8 µM to a minimum of 19.9 µM at 104 m. It then increases with depth, with a perturbation in the trend between 160 and 200 m. The deepest sample contains the maximum concentration of 86.6 µM.

Silicon concentrations increase from 188 to 595 µM in the uppermost 12 m and then decrease to ~400 µM (Fig. F32D). From 17.6 to 100 m, silicon mainly varies between 350 and 550 µM, with some concentration peaks as high as 664 µM (Fig. F32D). From 100 to 165 m, silicon gradually decreases to 221 µM and then remains nearly constant to the deepest sample.

Boron concentrations are at seawater values of ~0.4 mM in the shallowest samples and gradually increase in a fairly even fashion to 1.26 mM in the deepest sample (Fig. F33A).

Both iron and manganese concentrations are largely scattered between 9 and 22 µM and 3 and 9 µM, respectively (Fig. F33B–F33C). Iron and manganese tightly co-vary throughout the cored interval at Site U1354. Iron has a significantly high concentration of 27.6 µM at 56.2 m, which is not mirrored by the manganese profile (Fig. F33B–F33C). Below 85 m, both iron and manganese have relatively constant values, except that manganese is elevated relative to iron in the three deepest samples (Fig. F33B–F33C).

Preliminary interpretation of diagenesis

Interstitial water geochemistry in the uppermost 80 m at Site U1354 is dominated by a zone of complete sulfate depletion from 34 to 60 m (Figs. F34, F35). The boundaries of this zone are characterized by inflections in the magnetic susceptibility data (see Fig. F19). Where sulfate is zero, methane begins to build up. It then decreases to near background concentrations at 60 m, the depth at which sulfate reappears in the cores. The apparent carbon–sulfur stoichiometry of sulfate reduction is significantly different above and below the zone of sulfate depletion (Fig. F35). In the 0–34 m depth interval above the region of zero sulfate, the ratio of [alkalinity added plus cations (Ca²⁺ and Mg²⁺) removed] to sulfate removed is 1.3:1, consistent with sulfate reduction being fueled by about one-third organic matter oxidation and two-thirds anaerobic methane oxidation. In the 60–178 m depth interval, the same ratio is 0.98:1, indicating that sulfate reduction is driven almost exclusively by anaerobic methane oxidation (Fig. F35). The very low phosphate throughout the deeper zone and the somewhat higher phosphate in the shallower sulfate reduction zone is consistent with small quantities of phosphate being generated by organic-matter oxidation in the 0–34 m depth interval. The methane oxidation occurring in the deeper sulfate reduction zone generates no phosphate ions. However, below 84 m in Hole U1354C, the slopes of the sulfate, alkalinity, calcium, and magnesium depth profiles all change slightly, and the exact relationship developed in Hole U1354A may not apply for the deeper interval in Hole U1354C.

A hiatus is present at ~75 m, marked by a change in sedimentation rate from 210 m/m.y. above to 93 m/m.y. below, which is followed by a further decrease to 45 m/m.y. below another hiatus at ~128–190 m. Below 190 m, the sedimentation rate increases dramatically to 400 m/m.y. to the bottom of the hole (see Fig. F29 in the "Expedition 317 summary" chapter). The change in sedimentation rate below 190 m is coincidental with the second appearance of methane at Site U1354 at 200 m. Apparently, sediments within the 74–190 m depth interval were deposited at a rate slow enough to permit continuous replenishment of dissolved sulfate by diffusion from overlying seawater, thereby preventing methanogenesis. A review of methane occurrence at Deep Sea Drilling Project (DSDP)/ODP/IODP sites indicates that ~40–50 m/m.y. is the minimum sedimentation rate required for sulfate depletion and methanogenesis to occur.

One notable aspect of the shallow pore water chemistry profiles at Site U1354 is the lack of a low-salinity zone like that seen at ~50 m at the more near-shore Site U1353. This may help clarify the origin of this low-salinity zone. The water depth at Site U1353 is 85 m, compared with 110 m at Site U1354. Global sea level was ~125 m below today's sea level at the Last Glacial Maximum ~20,000 years ago (Fairbanks, 1989), so both site locations may have experienced periods of complete emergence. Therefore, the lack of less saline water at Site U1354 and the presence of less saline water at Site U1353 is more likely explained by the modern intrusion of meteoric water from land than by the historic remains of freshwater emplaced when the shelf was emergent.

Other changes in interstitial water chemistry at Site U1354 are possibly related to carbonate diagenesis and contributions from deeper basinal brines. The main decreases in dissolved calcium and magnesium occur within the depth intervals characterized by sulfate reduction, methanogenesis, and anaerobic methane oxidation. These processes are commonly associated with precipitation of authigenic carbonates with distinct carbon isotopic compositions. The increases in sodium and chloride from 0 to 60 m, which are possibly related to an influx of saline fluid, may also account for some of the other changes seen at Site U1354, such as increases in barium, lithium, and boron with depth.

Alternative sources of lithium could relate to ion-exchange or desorption reactions on authigenic clays and the transformation of biogenic opal to opal-A. The increase in lithium does not correspond to the increase in silica. This relationship was also observed at the other sites and may suggest a rather subtle influence of biogenic opal and that the major source of lithium is the diagenesis of lithium-rich clay minerals. The boron increase with depth may also be related to the diagenetic opal transition and microbial degradation of organic matter. The rapid barium increase in the sulfate reduction zone may reflect barite dissolution that resulted from enhanced barite solubility.

Microbiology

No microbiological experiments were carried out and no microbiological samples were recovered at Site U1354.

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