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doi:10.2204/iodp.proc.317.104.2011

Geochemistry and microbiology

Organic geochemistry

Shipboard organic geochemical studies of cores from Site U1352 included monitoring hydrocarbon gases, carbonate carbon, total organic carbon (TOC), total sulfur (TS), and total nitrogen (TN) and characterizing organic matter by pyrolysis assay. The procedures used in these studies are summarized in "Geochemistry and microbiology" in the "Methods" chapter. All depths in this section are reported in CSF-A.

Volatile gases

All cores recovered in sufficient quantity at Site U1352 were monitored for gaseous hydrocarbons using the headspace (HS) gas technique, and, where possible, core gas voids were analyzed using the vacuum syringe (VAC) technique (Tables T18, T19, T20; Figs. F45, F46).

Hole U1352A was sampled at relatively high frequency (every other section or at ~1.5–3.5 m intervals) with the headspace gas technique to estimate the depth of the sulfate–methane transition (SMT) and the dissolved methane versus depth gradient. Sediment gas was below detection levels in the uppermost five samples collected (1.5–8.7 m), with the first trace appearance of methane (~3 µM) occurring at 11.7 m. Comparison with dissolved sulfate measurements (see "Inorganic geochemistry") indicates that the SMT is between 15.2 and 16.6 m.

Headspace methane contents in Hole U1352A are shown in Table T18 as dissolved methane in sediment pore space (millimolar) and headspace gas (parts per million by volume). At shallow depths beneath the sulfate reduction zone, methane concentrations are initially greater than saturation at surface conditions (~2 mM), having apparently maintained some degree of supersaturation during core retrieval and sampling. Estimated methane concentrations in sediment pore space were not calculated for Holes U1352B and U1352C because of obvious gas loss prior to sampling.

Detectable ethane is present in all cores from 18.2 m and below. The composition of the gas, as expressed by the C₁/C₂ ratio (Fig. F45C), shows the expected gradual increase in relative ethane content with increasing depth and temperature. The C₁/C₂ values of the core void gases (Fig. F45C) generally parallel those of headspace gas but are offset to higher values because of greater retention of C₁ during core retrieval and sampling. The ratio of C₁/C₂ in headspace gas decreases by three orders of magnitude (from >10,000 to 60) over the total depth interval (1927 m). The decrease in C₁/C₂ is due to a gradual increase in ethane content (Fig. F45A); below ~650 m, this decrease is also due to a decrease in methane and total gas content associated with a decrease in core porosity and fluid content. However, C₁/C₂ reaches very low values (<10) from 1385 to 1393 m, with headspace methane contents of <50 ppmv. This depth also corresponds to a major discontinuity identified between 1394 and 1410 m, with an apparent hiatus of >5 m.y. (see "Lithostratigraphy" and "Biostratigraphy"). In addition, the deepest interstitial water sample (collected at 1385 m) contains 3 mM of sulfate, somewhat more than is usually observed for cores contaminated with seawater. The very low methane and C₁/C₂ values of these cores must represent either an interval in which all methane was lost or an interval in which sulfate was never depleted and methane was never generated. In a few cores below this depth, gas returns to normal, with methane contents of 3,000–22,000 ppmv and most C₁/C₂ ratios of 35–75 from 1512 to 1922 m. There is no evidence for migrated thermogenic hydrocarbons in the gas profiles.

Some headspace and all core void gas samples were analyzed for the presence of C₄₊ hydrocarbons (Tables T19, T20; Fig. F46). Headspace samples were analyzed mainly below 650 m, when the character of the cores made core void gas samples less frequent. Butanes, pentanes, and occasionally hexanes are present in most of the samples analyzed but are generally at low levels (1–100 ppmv) (Fig. F46A–F46C). C₄–C₆ hydrocarbons become more abundant with increasing burial depth (excluding the ~1300–1450 m interval that spans the hiatus), probably because of the same low-temperature thermogenic alteration of the indigenous organic matter that produces increased ethane and propane. A distinct preference for branched C₄ and C₅ isomers is present in shallow headspace samples, and this tendency diminishes somewhat with increasing depth to ~1750 m (Fig. F46F). The C₄ hydrocarbons in the deeper parts of the core have subequal amounts of normal and branched isomers (e.g., below 1700 m), but C₅ hydrocarbons continue to be dominated by i-C₅, and only the branched C₆ hydrocarbons and no n-C₆ could be detected in this depth range (Table T19; Fig. F46F). A predominance of branched alkanes was also observed in some core void gas samples from Site U1351 (at 811 and 961 m) and may be a result of organic matter being derived mainly from land plants. Core void gas samples have less of an iso-predominance (60%–80%) than headspace gases from similar depth intervals and have no depth trend.

The plot of the ratio of n-butane to the sum of n-butane and i-butane (n-C₄/[n-C₄ + i-C₄]) follows a wormlike trend with increasing depth. This trend has remarkable detail that may be related to lithostratigraphy and more precisely to formation porosity and permeability. Below 1400 m, where gas content is very low, this ratio first increases and then decreases below 1500 m from a high of ~60% n-C₄ to ~15% at 1600 m (Fig. F46F). With increasing depth, this ratio increases again to ~45% at 1700 m, below which it remains roughly constant. The n-C₅/(n-C₅ + i-C₅) ratio shows a similar but less distinct trend, with a low between 1600 and 1700 m (0%–9%) and a high of ~20% n-C₅ in the deepest sample. Explanations for these trends remain uncertain, but they most likely reflect the mixing of thermogenic equilibrium mixtures of normal- and iso-isomers (e.g., below 1800 m) with variably retained original hydrocarbons dominated by iso-isomers. Gases below 1650 m that are richer in C₄ and C₅ hydrocarbons (Fig. F46A–F46B) form a cluster at the lowest part of the n-C₄/(n-C₄ + i-C₄) ratio trend, implying that original hydrocarbons were retained in this zone and were not substantially diluted by more recently generated thermogenic gases. In contrast, the zone immediately underlying the discontinuity mentioned previously (below ~1450 m) is strongly depleted in original hydrocarbons, and gases in these sediments are diluted by small amounts of more recently generated thermogenic gas.

Table T20 and Figure F46D show ethene (C₂₌) contents measured in some void gas samples. Ethene was also detected in some headspace gases (Table T18). Ethene is common in near-surface sediments, where it is thought to be a product of biological activity (Claypool and Kvenvolden, 1983). However, short-chain unsaturated hydrocarbons are unstable under the reducing conditions of deeper sediments, so the presence of ethene and other alkenes in some of the deeper (250–690 m; especially 422–467 m) cores is somewhat unusual. Especially noteworthy is the composition of the gas sample from Section 317-U1352B-55X-1 (467 m), in which the amounts of some of the alkenes are greater than the saturated homologs (Fig. F47). Ethene and propene were quantified, but the peaks attributed to butenes and pentenes could be only tentatively identified. These samples are from depth intervals where XCB cores that jammed in the core catcher gave off a strong odor (422–467 m) and from the interval just above, where cores became continually jammed in the core barrel, requiring a switch from XCB to RCB coring. Alkenes observed in some void gas and headspace gas samples, especially the high quantities at 467 m, may be the product of high-temperature alteration of organic matter due to frictional heating during drilling, similar to that observed in previous ODP cores from Site 682 (Shipboard Scientific Party, 1988).

CO₂ concentrations in headspace and void gas samples are given in Tables T19 and T20 and illustrated in Figure F46E. Atmospheric air contains 387 ppm CO₂, but the majority of measured values exceed this significantly; furthermore, some samples contain less CO₂ than is found in air. Values obtained from core void gas samples show no clear difference from those of headspace samples. Therefore, these values likely reflect the true composition of CO₂ in the subsurface, which was formed by a mixture of microbial, diagenetic, and early thermogenic processes.

Carbon and elemental analyses

Inorganic carbon (IC), total carbon (TC), total organic carbon by difference (TOC_DIFF), TN, and TS were analyzed in 323 samples from 0 to 1924 m at Site U1352 (Table T21; Fig. F48). Carbonate fluctuates between 0.14 and 96 wt%, with lower averages of ~5 wt% in the uppermost 500 m and scattered high-carbonate (>20 wt%) samples throughout the cored sediments below ~600–700 m (Fig. F48A). The deepest samples (1700–1927 m) are characterized by very high carbonate contents (up to 96 wt%). Four samples (317-U1352C-129R-2, 104 cm; 130R-4, 103 cm; 131R-2, 96 cm; and 147R-6, 71 cm; Table T21) are carbonate-calcareous mudstone/marlstone pairs from closely interbedded strata. TC values cluster at ~0.5–1 wt%, with frequent scatter as high as 9 wt% in the uppermost 700 m. Below this depth, average values increase slightly and fluctuate between 1 and 7 wt% from 700 to 1600 m before finally reaching 12 wt% in the deepest part of the hole below 1600 m (Fig. F48B).

TOC_DIFF fluctuates between 0.1 and 1.5 wt% but is mostly <0.5 wt% and averages 0.4% (Fig. F48C). TOC_DIFF is systematically lower than the organic carbon determination given by the source rock analyzer (TOC_SRA) (Fig. F48D). A cross-plot of TC from the elemental analyzer and TOC_SRA plus IC from the coulometer shows a good correlation and indicates that the elemental analyzer gives consistently lower values by 0.3–0.5 wt% (Fig. F49), similar to Site U1351.

TN and TS contents are scattered in the range of 0.02–0.07 wt% for TN and 0–0.5 wt% for TS in the uppermost 500 m. They slightly decrease downhole to averages of ~0.01–0.02 wt% and 0.02 wt% for TN and TS, respectively (Fig. F50C–F50D). No TS content was analyzed below 1526 m because of an onboard shortage of elemental analyzer reactors, which had to be replaced every 100 samples because the vanadium pentoxide catalyst used for sulfur determination deteriorated the activated copper package in the reactor. TOC_DIFF/TN ratios generally range from 5 to 50 and tend to increase with depth to ~1600 m before slightly decreasing near the bottom of the hole. Occasional values as high as 304 were observed, reflecting very low TN contents, mostly in carbonates (Fig. F50A). TOC_DIFF/TS ratios are mostly low, ranging from 0.1 to 10, with scattered individual samples as high as 80 (Fig. F50B). TOC_DIFF/TN and TOC_DIFF/TS ratios are much higher at Site U1352 than at Site U1351.

Organic matter pyrolysis

Most of the samples used for carbon-nitrogen-sulfur analysis were also characterized by source rock analyzer (SRA) pyrolysis (Table T22; Figs. F51, F52). S₁ and S₂ slightly increase with depth from 0 to 400 m, with ranges of ~0.0–0.3 and 0.1–1.4 mg/g, respectively. Values range widely at these shallow depths, especially for S₁. Below 600 m, values cluster more tightly near 0.04 mg/g for S₁ and 0.1–0.5 mg/g for S₂ (Fig. F51A–F51B), and average values decrease slightly downhole. S₂ values at 1500–1600 m are occasionally as high as 2.4 mg/g, representing sediment layers with higher organic carbon contents. S₃ has no trend and a range of 0.1–0.9 mg/g, with occasional values as high as 1.2 mg/g (Fig. F51C). Pyrolysis carbon necessarily mirrors S₁ and S₂ (Fig. F51D).

The hydrogen index (HI) generally ranges from 10 to 100 mg/g C, and values increase with depth, reaching a maximum at ~1000 m before decreasing slightly toward the bottom of the hole. Values as high as 133 mg/g C were observed at ~1500–1600 m (Fig. F52A). The oxygen index is scattered between 10 and 100 mg/g C, and values slightly decrease with depth (Fig. F52B). T_max values (Fig. F52C) range from ~370° to 440°C, with more scatter in the uppermost 500 m of the sediment column. Values increase downhole from ~400°C at the seafloor (with much scatter) to 430°C at the bottom of the hole. The cloud of data in the uppermost 500 m tightens into a more consistent trend in deeper sediments. The production index decreases in the uppermost 700 m from an average of 30% at the seafloor to ~10% at ~1000 m and then stays constant to ~1500 m (Fig. F52D). Below 1500 m, the production index increases significantly in some samples to 20%–30%.

A modified van Krevelen diagram (Fig. F53) indicates that organic matter is of slightly better quality at Site U1352 than at Site U1351, with most samples clustering near the Type III line. The highest HI (133 mg/g C) was found in a thin Miocene calcareous mudstone from 1575 m that was interbedded with carbonates.

Preliminary interpretation of organic matter

Variations in organic matter composition are subtle and largely correlate with lithologic Units I (0–710 m) and II (710–1853 m). Unit I is interbedded mud and sand with scattered high carbonate intervals, Unit II is a marlstone with mud and sand, and Unit III is nannofossil-rich limestone. In Unit I, samples with high carbonate contents tend to have higher organic carbon contents and higher S₁ and S₂ pyrolysis yields, whereas background mud/sand sediments have relatively low organic carbon contents (TOC_DIFF = 0.4–0.5 wt%). Unit II marls generally have low organic carbon contents (TOC_DIFF = 0.5–0.6 wt%), and organic carbon does not correlate with carbonate content except at the bottom of the unit below 1700 m, where an inverse correlation is apparent (Fig. F48A, F48C). Unit III limestones contain the highest amounts of carbonate and the lowest TN at Site U1352 (Figs. F48A, F50C). Unit I probably contains organic matter that is less altered diagenetically and that volatilizes more readily during pyrolysis. Organic matter in Units II and III is more diagenetically stabilized as protokerogen, as shown in the lower part of Unit II by the elimination of some scatter in the pyrolysis response, especially in S₃ and T_max (Figs. F51C, F52C). T_max values, especially those below ~1200 m, have less scatter and are shifted to higher temperatures, consistent with the progressive elimination of the more reactive protokerogen components of organic matter (Fig. F52C).

SRA data from Site U1352 can be compared to existing source rock quality and thermal maturity data from the Canterbury Basin (Newman et al., 2000; Sykes and Funnel, 2002; Sykes, 2004; Sykes and Johansen, 2009). Source rock quality at Site U1352 is rather low, with most HI values <100 mg/g (Fig. F52A), so the organic matter is largely interpreted to be land plant or degraded marine in origin. This contrasts with deeper Pukeiwitahi Formation coals (late Campanian–early Maastrichtian) in offshore petroleum exploration wells that have considerably higher HI values (Endeavour-1: 2094–2353 m, mean HI = 210; Galleon-1: 2822–2885 m, mean HI = 250) (Sykes, 2004). It is possible that the reported HI values at Site U1352 are underestimates because it was demonstrated that the TOC_SRA values from the SRA are high relative to TOC_DIFF calculated by the difference method (see "Geochemistry and microbiology" in the "Methods" chapter).

The thermal maturity gradient defined by T_max variation at Site U1352 ranges from ~380° to 400°C in the shallowest samples to an average of ~430°C at ~1900 m (Fig. F52C). This trend is quite steep and suggests a rather high geothermal gradient. The deepest values are within the early oil-generative window, according to published T_max data that show that the onset of oil generation in the Canterbury Basin occurs at T_max = 425°C (0.65% vitrinite reflectance; coal band rank S_r = 10) (Sykes and Funnel, 2002; Sykes and Johansen, 2009). Note that this conclusion is inconsistent with the interpreted bottom-hole temperature of ~60°C at Site U1352 that was calculated using a variable heat flow determined by thermal conductivity (see "Heat flow"). However, the geochemical results are more consistent with a bottom-hole temperature of ~100°C that is achieved if a constant geothermal gradient is assumed. Supporting evidence for an early oil-generative window thermal maturity at the base of Site U1352 is that the production index increases above 20% below 1700 m (Fig. F52D), showing greater free hydrocarbons that are perhaps generated by thermal processes. However, similar T_max values are generally only reached deeper in the Canterbury Basin (>2100–2800 m in Endeavour-1 and Galleon-1; Sykes, 2004; Sykes and Johansen, 2009). One possible explanation is the known loss of section at erosional unconformities at Site U1352 (see "Lithostratigraphy"). A second possible explanation is that heat flow at Site U1352 is high because of a deeper igneous intrusion, although there is no evidence on seismic profiles at Site U1352 for igneous intrusions at depths that we believe could have had a thermal effect in the drilled hole. Newman et al. (2000) have shown evidence for anomalously high thermal maturities in the Canterbury Basin at Clipper-1 below 4000 m, which is inferred to have been the result of a thermal intrusion at depth.

SRA data obtained on board ship, especially HI and T_max, will need to be confirmed and calibrated using a second instrument before initial interpretations about source rock quality and thermal maturity at Site U1352 can be confirmed. These analyses are scheduled for early postcruise research.

Inorganic geochemistry

Interstitial water

A total of 112 interstitial water samples (Tables T23, T24, T25) were collected and analyzed at Site U1352. Thirty-one samples were taken from Hole U1352A, which was dedicated mainly to whole-round sampling for geochemistry and microbiology. Three samples were taken from Core 317-U1352B-6H, two samples were taken from each of Cores 7H through 10H, and one sample was taken per core (where recovery was sufficient) to Core 35H (294 m). Samples were taken from approximately every other core for Cores 317-U1352B-37X through 90X (783 m). Cores 317-U1352C-2R, 3R, 6R, 11R, 14R, and 18R were spot sampled from 576 to 780 m, and every second or third core was sampled to Core 40R (989 m). From Cores 317-U1352C-45R through 58R (1034–1164 m), sampling was irregular and some whole-round samples as long as 30 cm failed to yield any water for analysis. Thereafter, only a few interstitial water samples were selected, based on appearance and apparent degree of cementation. The amount of interstitial water extracted from whole-round samples decreases with depth from ~2 to 5 mL/cm in the uppermost 700 m to <1 mL/cm below 900 m (Table T23; Fig. F54). The deepest whole-round sample successfully sampled for interstitial water was from 1386 m and yielded 0.5 mL/cm.

Salinity, chloride, and pH

Salinities in samples near the seafloor are slightly lower than normal seawater at 3.3, rapidly decline to 3.0 at 28 m, and remain relatively constant at 2.9–3.1 in the rest of the section analyzed (to 1400 m) (Fig. F55B, F55D). Chloride parallels salinity measurements, with the shallowest samples having slightly lower concentrations than seawater (540 mM) and most other samples having relatively constant but scattered chloride concentrations of 520–550 mM (Fig. F55A, F55C). The isolated deepest sample has a chloride concentration of 453 mM. Measured pH values seem to vary with the dominant diagenetic process—decreasing during sulfate reduction, increasing during methanogenesis, and decreasing again, possibly because of authigenic carbonate precipitation (Fig. F56C).

Alkalinity, sulfate, ammonium, phosphate, and dissolved silica

Alkalinity increases relatively slowly from 2.8 mM at the seafloor to 9 mM at 10.1 m and then increases rapidly to a maximum of 24.2 mM at 16.6 m (Fig. F57A, F57C). Alkalinity then decreases to ~15 mM at 100 m and remains relatively constant to ~400 m. From 400 to 600 m, alkalinity declines steadily to ~2.3 and then remains in the range of 1.4–3.0 mM to the base of sampling. The sulfate decline is almost exactly the inverse of the alkalinity increase, with sulfate declining slowly over the 0–10 m interval and rapidly from 10 to 24 m (Fig. F57B, F57D). Below this depth, sulfate remains essentially at zero, except in cores contaminated with small amounts of the seawater used as a drilling fluid.

Ammonium is zero to ~7 m and then increases gradually to a shallow maximum of ~2.3 mM between 40 and 50 m (Fig. F58C). After decreasing to 1.3 mM between 50 and 80 m, ammonium increases again to a maximum of ~7 mM at 470 m before decreasing steadily to ~2 mM in the deepest samples analyzed at 1000 m. Phosphate is low at 1–4 µM from the seafloor to 8.5 m. It then increases rapidly and spikes at 92 µM at 14.8 m (Fig. F58A). After dropping back to 41 µM at 16.6 m, phosphate varies within a range of 23–59 µM to 55 m before dropping to ~7 µM at 60 m and declining steadily to essentially zero by 400 m.

Dissolved silica is present at 272 µM at 1.1 m, increases to 645 µM at 27.5 m, remains relatively constant at a range of 380–660 µM to 200 m, and then increases to a maximum of 1066 µM at 480 m (Fig. F58B). Silica declines to 215 µM at 653 m and then increases again to 831 µM at 699 m before dropping back to 258 µM in the deepest samples analyzed at 765 m.

Calcium, magnesium, and strontium

Calcium and magnesium both decrease during sulfate reduction and then continue to decrease to minimum values (1.4 mM for Ca²⁺ and 7 mM for Mg²⁺) from 300 to 400 m (Fig. F56D–F56E). Below 400 m, both major cations increase to ~20–23 mM at ~600 m, below which magnesium remains relatively constant to ~1200 m. Calcium continues to increase to just above 30 mM in the deepest samples at >1100 m. The ratio of magnesium to calcium increases from 5 to >9 in surface sediments as deep as 18 m. It then decreases to ~1.6 at 500 m before slowly declining to 0.5 at 1386 m (Fig. F56F). Strontium is initially at seawater values of ~0.1 mM and then increases slightly to 0.3 mM at 400 m before quickly increasing downhole to 2 mM at ~800 m. Strontium then decreases to ~1.7 mM at maximum depth (1386 m) (Fig. F56A). The Sr/Ca ratio increases steadily from 0.01 to a maximum of 0.15 at 480 m and then decreases again to 0.06 in the deepest sample (Fig. F56B).

Sodium, potassium, lithium, barium, silicon, boron, iron, and manganese

Sodium increases from near-seawater values of 466 mM to 515 mM at 286 m, decreases to ~440 mM at 600 m, drops to ~410 mM at 1184 m, and then decreases to 348 mM at 1386 m (Fig. F59C). Potassium decreases during sulfate reduction and then increases during the initial stages of methanogenesis, reaching a maximum of 11 mM at 144 m. Potassium then declines to ~6 mM at 550 m, declines further to ~3 mM at 600 m, spikes back up to 5.7 mM at 709 m, and finally declines to ~2 mM in the deepest samples below 900 m (Fig. F59B).

Lithium increases steadily from 25 µM at the sediment/water interface to ~50 ?M at 450 m. Lithium then increases more rapidly to a maximum of 166 µM at 700 m before declining to ~100 µM at 800–900 m (Fig. F59A). Below 900 m, lithium increases again to ~130 µM between 1000 and 1200 m before dropping to 76 µM in the deepest sample. Barium increases from ~2 to ~7 µM at the base of the SMT before gradually increasing to 19 µM at 500 m (Fig. F59D). The profile becomes more scattered below 500 m, with concentrations ranging from 7 to 30 µM. A pronounced barium maximum of 84 µM is evident at 1091 m.

Silicon has no obvious trend in the uppermost 300 m of sediment, and values are scattered between 300 and 600 µM (Fig. F58D). Below 300 m, silicon increases to a maximum of ~900 µM at 524 m, drops significantly to ~300 µM at 600 m, increases again to 1013 µM at 700 m, and finally averages ~400 µM below 800 m.

The boron profile shows a remarkable increase from seawater values of ~0.4 mM to a maximum of 5.4 mM at 1113 m before a slight decrease in the subsequent samples and a sharp drop to 1.5 mM in the deepest sample (Fig. F60A).

Manganese ranges from 3 to 9 µM, with a maximum value of 13 µM between 0 and 50 m. Below this depth, manganese declines to 2–4 µM at 100–300 m and then increases downhole to average values of 5–15 µM (Fig. F60C). Iron shows a similar trend, increasing rapidly from ~10 µM to a maximum of 34 µM at 26 m, decreasing again to 4 µM at ~300 m, and finally increasing to 20–36 µM between 500 and 1386 m (Fig. F60B).

Preliminary interpretation of diagenesis

Interstitial water geochemistry in the uppermost 20 m at Site U1352 is dominated by the two main microbially mediated diagenetic processes, sulfate reduction and methanogenesis (Fig. F61). A zone of very gradual sulfate depletion and alkalinity increase occurs in the 0–8.5 m depth interval and represents either very slow organic matter oxidation or a zone of intense bioturbation. The very low phosphate and the absence of ammonium in this interval are more consistent with bioturbation or other physical mixing of seawater than with organic matter oxidation. From 8.5 to 16.6 m, sulfate declines rapidly from 25.1 to 0 mM, whereas alkalinity (dominantly bicarbonate ions) increases to a maximum of 24.2 mM. Over this same 8.5–16.6 m depth interval, calcium and magnesium decline by a combined 15 mM, presumably because of authigenic carbonate precipitation. The amount of carbon oxidized during sulfate reduction is equivalent to the alkalinity increase plus the major cation decrease, or ~38 mM, which is 1.5 times the amount of sulfate removed (Fig. F62). This ratio of carbon oxidized to sulfate removed implies that half of the sulfate reduction at Site U1352 is the result of organic matter oxidation and the other half is the result of methane oxidation. At this 1.5 proportion of carbon added to sulfate removed, two-thirds of the carbon added would be from the oxidation of organic matter and one-third would be from the oxidation of methane (see equations in Fig. F62). These interpretations based on interstitial water chemistry could be confirmed by postcruise analysis of stable carbon isotope ratios of dissolved inorganic carbon and diagenetic carbonates. The moderate amount of phosphate in the sulfate reduction zone, especially in the sample at 14.8 m (92 µM), is consistent with some oxidation of marine organic matter because methane oxidation would generate no phosphate.

After an initial drop of 2 mM beneath the sulfate reduction zone, alkalinity remains relatively constant to ~50 m, indicating that organic matter oxidation is replenishing bicarbonate as rapidly as it is removed by methane generation and carbonate precipitation. Below 50 m, alkalinity drops in stages to ~14 mM over the 100–350 m depth interval and then drops steadily to ~3–5 mM at 500–600 m, below which it ranges from 1.4 to 3.0 mM in the rest of the sampled interval. This gradual decline in alkalinity probably represents the final stages of biological activity resulting in the oxidation of organic matter and the reduction of dissolved CO₂ to produce methane. The alkalinity decrease and the major calcium increase occur over the same interval (350–600 m), which also corresponds to a decline in the degree of preservation of calcareous microfossils (see "Biostratigraphy").

Calcium at Site U1352 generally increases to 400 m and then reaches a steady state at ~800 m. This is common for pore fluids from carbonate-dominated sections with little influence from diffusive flux below (McDuff and Gieskes, 1976) and is consistent with high carbonate throughout the cored sediments below ~600–700 m (Fig. F48A). Below 400 m, strontium and magnesium also have profiles similar to that of calcium. Upward diffusive flow from carbonate-dominated sections may cause the abrupt gradual increases in divalent cation concentrations from 400 to 800 m. Strontium reaches values as high as >20 times seawater. The dissolution of Sr-rich aragonite in the sediments is one way of explaining the increasing flux of strontium to pore fluids.

Particles of barite are one of the main carriers of barium in sediments (Dehairs et al., 1980). Barium initially shows a clear rise below the depth of the SMT. The removal of SO₄^2– increases the solubility of barite, and dissolved Ba²⁺ concentrations increase from 2 to 9 µM (Fig. F59D). The scattered profiles of silica and silicon below 400 m are possibly associated with the scatter of barium concentrations and may reflect diagenetic dissolution/transition and/or changes in paleoproductivity (Paytan et al., 1996).

Boron steadily increases in the deepest samples, where values are as high as 13 times that of seawater (Fig. F60A). The maximum boron value of 5.4 mM is very similar to that at Site U1351, although at Site U1351 the highest value occurs much shallower (at 300 m). The boron increase is possibly related to the diagenetic opal-A/opal-CT transition and microbial degradation of organic matter.

Biogenic opal is assumed to be a major source of lithium in sediments. The rapid increase and decrease of lithium concentrations from 450 to 800 m (Fig. F59A) is within a zone of highly variable silica and silicon concentrations (Fig. F58B, F58D). A fraction of lithium enrichment also may be associated with lithium in sediments and clay minerals. Lithium is easily removed from clay interlayer exchange sites because of its high hydration energy, which may account for the observed steady increase.

Microbiology

The principal shipboard microbiological objectives at Site U1352 included testing samples for contamination using PFT and a particulate tracer and starting enrichment cultures for different types of metabolisms.

Sample collection

At this site, 107 whole-round samples were collected for microbiological investigations (51 for microbial characterization, 51 for intact polar lipid analysis, and 5 for incubation tests).

Contamination tracer tests

Contamination tests were continuously conducted using water-soluble tracers (PFT) or bacteria-sized particles (fluorescent microspheres) in order to confirm the suitability of sediment samples for microbiological research. The chemical and particle tracer techniques used are described in "Geochemistry and microbiology" in the "Methods" chapter.

Water-soluble tracer

At Site U1352, PFT was continuously delivered at its maximum solubility (2 mg/mL in seawater) into cores from which microbiology whole-round samples were taken. To maximize the detection of seawater contamination from drilling, 5 cm³ sediment samples were taken. No results could be achieved because of analytical problems. In particular, gas chromatograph traces had small peaks (other than PFT) of similar retention time to that of PFT, even after cleaning or changing the column. Consequently, we decided to preserve the samples for onshore analysis by injecting 2 mL of an autoclaved 3% NaCl solution into the vials containing the sediment. The samples were then immediately frozen upside-down at –80°C until they could be shipped on dry ice.

Particulate tracer

Fluorescent microspheres were used as a particulate tracer on all cores from which whole-round samples were subsequently taken. A 1 cm³ sample of sediment was diluted 10× by a 3% NaCl/3% formalin solution for microsphere detection. Microspheres were detected in 49 of the 51 outer sediment samples (Table T26), indicating a heterogeneous distribution of microspheres along the core liner. In the inner part of the core, 40 sediment samples did not contain any microspheres, indicating that no contamination by micron-sized particles took place. In 11 sediment samples (Table T26), between 2 × 10² and 2 × 10⁵ microspheres/cm³ were counted, indicating that potential contamination from drilling fluid may have occurred in the inner part of the core during the drilling process. The bead-delivery method used at this site is the one described by Smith et al. (2000). In samples where microspheres were not detected (Sections 317-U1352A-15H-4 and 317-U1352C-87R-4), prokaryotic molecular diversity was compared to determine if contamination occurred. No difference was found in the deployment of microspheres between APC and XCB coring, but, on average, fewer microspheres were observed at the periphery of the core when RCB coring was used (Fig. F63). This can be explained by the fact that beads can potentially become diluted in the drilling fluid. Therefore, we suggest using 40 mL instead of 20 mL microsphere bags for cores retrieved with the XCB or RCB systems.

Total cell counts

Sediment samples of 1 cm³ were taken from all whole-round samples for microbial characterization and stored at 4°C in a 3% NaCl/3% formalin solution for onshore prokaryotic cell counting.

Cultivations

In Hole U1352C, 11 whole-round samples were collected below the deepest microbiology sample analyzed to date (1626 mbsf; Roussel et al., 2008). These samples are characterized by the presence of lithified layers rich in carbonate (up to 70%; see "Lithostratigraphy") alternating with dark glauconitic layers. When a glauconitic layer was present in the 10 cm long microbial characterization samples, it was sampled separately in order to determine any differences in microbial diversity. Three samples (Sections 317-U1352C-137R-3, 137R-3 [glauconite], and 148R-3) (Table T26) were inoculated on several enrichment media, as described in "Geochemistry and microbiology" in the "Methods" chapter (see Table T16 in the "Methods" chapter). All onboard enrichments were incubated near the in situ temperature (70°C) (the temperature at the bottom of the hole was estimated to be 60°C; see "Heat flow") under 5% H₂, 5% CO₂, and 90% N₂ (biogas) headspace.

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