GEOCHEMISTRY

Concentrations of headspace gases were routinely monitored in Hole 1239A sediments according to shipboard safety and pollution prevention considerations. Methane was first detected at levels above laboratory blank in a headspace gas sample at 8.1 mcd and increased to >30,000 ppmv by 112.8 mcd (Fig. F26; Table T13). Below this depth, methane concentrations remained high throughout the hole, ranging between 4200 and 66,600 ppmv. Small amounts of ethane were detected, varying between 1.2 and 19.0 ppmv. Higher molecular weight hydrocarbons such as butane were first detected by odor. Natural gas analyses confirmed that small amounts of C₃-C₅ hydrocarbons were present in the sediments, in the interval between ~231 and 451 mcd (Table T13), and determined that gas composition was dominated by CO₂ and methane.

The temperature gradient at Site 1239 is too low to allow thermogenic methane production, which can also produce heavier hydrocarbons. However, heavier hydrocarbons (at least C₂-C₄) can be microbially produced in low concentrations together with C₁ (Vogel et al., 1982; Wiesenburg et al., 1985). In addition, the high methane/ethane ratio (Fig. F26), which is representative of microbial gases, indicates that the methane is of biogenic origin. This is also supported by the disappearance of interstitial sulfate at approximately the same depth where methane content begins to increase. The presence of interstitial sulfate inhibits methanogenesis in marine sediments (Claypool and Kvenvolden, 1983).

We collected 42 interstitial water samples from Hole 1239A for shipboard analyses and an additional 31 interstitial water samples at a frequency of one per section for the upper 60 mbsf for shore-based analyses. Chemical gradients at this site (Table T14; Fig. F27) reflect the influence of organic matter oxidation, the dissolution of biogenic silica and its reprecipitation in authigenic phases, the effects of authigenic calcite precipitation, and the diffusive influence of basalt alteration processes. The depth zone of the major hiatus (519.3-527.7 mcd; see "Biostratigraphy") is not marked by significant changes in interstitial water chemistry.

Chlorinity averages 559 mM throughout and increases from 553 mM at 1.5 mcd to >560 mM by 29.1 mcd (Fig. F27). Salinity, measured refractively as total dissolved solids, ranges from 31 to 35 (Table T14). Sodium concentrations measured by inductively coupled plasma-atomic emission spectrophotometry averaged 1.7% lower than those estimated by charge balance reported here (Table T14). Sodium concentrations parallel chlorinity, with a total range from 467 to 500 mM.

Organic matter diagenesis, driven by microbially mediated oxidation reactions, influences the interstitial water chemistry. Sulfate decreases to below the detection limit (1.0 mM) by 70.8 mcd, coincident with the increase in methane and approaching the zone of higher organic carbon contents starting at 85 mcd. Total sulfate depletion at this site is in contrast to the partial sulfate depletion seen at Site 1238, at deeper water depth. Alkalinity increases to >28 mM from 91.7 to 123.2 mcd, consistent with alkalinity generation from sulfate reduction, then declines to 3.6 mM at 553.9 mcd. Alkalinity consumption is both from authigenic mineral precipitation within the sediments and from the diffusive influence of basalt alteration reactions.

Dissolved manganese concentrations are low at this site, with values of 1.2-1.3 µM from 1.5 to 8.0 mcd, indicative of limited suboxic oxidation of organic carbon by manganese reduction, then decrease to low values, often below the detection limit (0.1 µM). Manganese increases from 0.5 µM at 513.8 mcd to 3.8 µM at 553.9 mcd from the influence of basalt alteration reactions at greater depth. Dissolved iron has a complex profile with depth, with multiple peaks >10-20 µM.

Phosphate concentrations increase from 5.5 µM at 1.5 mcd to >20 µM from 60.6 to 112.7 mcd. A sharp one-point maximum of 37 µM at 91.7 mcd is analytically robust but may be an artifact related to the acidification of the samples required to deal with the presence of hydrogen sulfide and/or the possible release of scavenged phosphate from fine particles. Ammonium concentrations increase from below the detection limit (0.2 mM) at 1.5 mcd to >6 mM from 133.2 to 377.5 mcd. The increases in phosphate and ammonium result from the oxidation of organic matter, whereas the persistence of high ammonium concentrations throughout indicates more limited sinks in authigenic phases for ammonium than for phosphate.

Dissolved silicate increases from 545 µM at 1.5 mcd to >1800 µM from 482.4 to 503.4 mcd, followed by a sharp decrease in silicate to 1319 µM at 553.9 mcd. Diatoms decrease rapidly >519.3 mcd, and the sediment is barren of diatoms in Sample 202-1239-55X-CC (see "Diatoms" in "Biostratigraphy"). The increase in dissolved silicate with increasing depth is consistent with temperature-controlled solubility for biogenic opal at this site (see "Operations"). Site 1238 has a larger thermal gradient (~12.7°C/100 m) than Site 1239 (~8.8°C/100 m). The silicate increase with depth for Site 1239 is less steep than that observed at Site 1238, although peak silicate values are similar and are reached at ~45°C. The increase in solubility of biogenic opal with increasing temperature appears to control the dissolved silicate concentrations until the biogenic opal supply in sediments is exhausted and precipitation of biogenic opal alteration products controls interstitial water composition, with cherts serving as the ultimate sink for dissolved silicate. However, unlike at Site 1238, no cherts were observed at Site 1239.

Barium concentrations increase steeply with the major decline in sulfate, from 1 µM at depths <60.6 mcd to generally >1400 µM from 102.3 to 194.0 mcd, and then decrease to 162-167 µM from 220.8 to 241.4 mcd, with continued, more minor variations downcore. Site 1239 has the highest interstitial water barium concentrations of all previous Leg 202 sites. Sites with limited or no sulfate reduction (Sites 1236, 1237, and 1238) have the lowest barium concentrations. Sites with sulfate reduction complete at shallower depths than at Site 1239 (Sites 1232, 1233, 1234, and 1235) have barium enrichments, but these are limited in depth range and maximum concentration. This indicates that the dissolution of barite, driven by decreasing sulfate, affects barium concentrations and that the depth range of sulfate reduction, along with surface productivity, affects the signal of barite remobilization in interstitial water.

Calcium concentrations decrease from 10.1 mM at 1.5 mcd to 5.5 mM from 70.8 to 81.7 mcd, just below the depth of sulfate depletion and shallower than the maximum alkalinity values, then increase to 15.3 mM at 532.9 mcd. The shallow part of this profile is controlled by authigenic calcite precipitation driven by the alkalinity increase, and the deeper portion reflects the diffusive influence of basalt alteration.

Magnesium concentrations increase slightly in the shallowest samples, probably from ion exchange with the sediments driven by the ammonium increase, decrease steeply from 51.5 mM at 18.8 mcd to 38.7 mM at 70.8 mcd, and then decrease more slowly to 17.4 mM at 532.9 mcd. Magnesium concentrations increase in the deepest two samples. Magnesium/calcium ratios increase from 5.0 at 1.5 mcd to 7.3 at 50.5 mcd then decrease to 1.1 in the deepest samples (Fig. F27). The increase in magnesium/calcium in the shallower sediments, driven by the decrease in calcium, indicates that calcite precipitation is the dominant authigenic mineralization reaction, consuming calcium and alkalinity. Below this zone, the increase in calcium from the calcium minimum at 81.7 to 532.9 mcd (~2.4 mM/100 m) is correlated to the decrease in magnesium (approximately -4.9 mM/100 m), consistent with control of these profiles by the diffusive influence of basalt alteration reactions.

Boron concentrations increase from 469 µM at 1.5 mcd to 777 µM at 532.9 mcd then have a small decline in the deepest two samples. Lithium concentrations increase from 24 µM at 1.5 mcd to >100 µM from 163.4 to 398.5 mcd then decrease to 71 µM at 553.9 mcd. The middepth maximum requires a source of lithium from the sediments, whereas basement alteration reactions at low temperatures are a sink for lithium. Strontium concentrations increase with depth from 87 µM at 1.5 mcd to 483 µM at 532.9 mcd then decline somewhat in the deepest samples. This profile does not resemble those dominated by the influence of biogenic calcite recrystallization but does indicate a source of dissolved strontium at depth in this site. Potassium decreases with depth from 11.9 mM at 1.5 mcd to 6.6 mM at 544.9 mcd. Low-temperature basalt alteration reactions are a sink for dissolved potassium, and this profile can be explained primarily by the diffusive influence of basalt alteration reactions.

Inorganic carbon (IC), total carbon (TC), and total nitrogen (TN) were determined on sediment samples from Hole 1239A (Table T15). Organic matter carbon/nitrogen ratios were employed to characterize the organic matter. Calcium carbonate and TOC records at Site 1239 are similar to those at deeper water Site 1238.

Calcium carbonate and TOC concentrations reflect the overall uniformity of the sediment lithology at Site 1239 (see "Lithostratigraphy"). Calcium carbonate concentrations range between 16.0 and 86.6 wt% (average = 59.0 wt%) (Table T15; Fig. F28). In the uppermost ~220 mcd, calcium carbonate contents average ~52 wt%, with large amplitude variations of >10 wt%. Calcium carbonate concentrations reach a minimum at 85.5-115.0 mcd, with values typically <50 wt% and as low as ~30 wt%. At depths >220 mcd, calcium carbonate concentrations generally increase downhole, which may result from the migration of the site toward more coastal conditions with a greater delivery of siliciclastic material that diluted the biogenic components.

TOC concentrations range between 0.3 and 4.4 wt% (average = 1.5 wt%) (Table T15; Fig. F28). In the uppermost 84.0 mcd, TOC contents average 1.3 wt%. TOC concentrations reach a maximum with values >2 wt% at 87.9-125.5 mcd. TOC concentrations generally decrease with greater depth to <0.5 wt% with amplitude variations of 0.5-2.5 wt%. Variations in TN concentrations are similar to those in TOC. The TOC and the calcium carbonate profiles are negatively correlated. For instance, the calcium carbonate minimum corresponds to the TOC maximum, possibly indicating a dilution effect between the two. However, TOC concentrations calculated on a carbonate-free basis (CFB) reflect the same variability as the TOC concentrations (Fig. F28), indicating that most of the TOC variations are not driven by changes in calcium carbonate concentrations. This is also supported by the lack of a strong linear relationship (r² = 0.17) between calcium carbonate and TOC concentrations (Fig. F29). The increase in biogenic opal as indicated by smear slide data and low bulk density values, together with a high organic carbon flux, may explain the observed calcium carbonate minimum.

TOC/TN ratios vary between 4.3 and 31.2 (Table T15; Fig. F28) (average = 9.6). In the uppermost 310 mcd, TOC/TN values are around a mean value of ~8.5, which is typical of marine algal material (Bordovskiy, 1965; Emerson and Hedges, 1988; Meyers, 1997). At greater depths, the TOC/TN ratios vary with high amplitudes between 4.3 and 31.2. High TOC/TN values correspond generally to higher TOC (except at 327.3 mcd) and lower TN contents. These variations are interpreted to reflect an input of terrestrial organic matter, which is able to significantly elevate TOC/TN ratios. Although the input of land-derived organic matter may have dominated the organic sedimentation in some intervals at depths >310 mcd, it always remained low, as indicated by the generally low TOC contents in that interval. These interpretations will have to be confirmed by shore-based analyses.

It seems therefore that most of the changes in sedimentary TOC and calcium carbonate contents result from a change in the trophic conditions in the overlying waters rather than from preservation. Periods of higher productivity are associated with higher TOC contents and generally correspond to increased diatom abundance (Thalassiothrix ooze; see "Diatoms" in "Biostratigraphy"). This is particularly the case for the organic matter- and diatom-rich interval between ~90 and 110 mcd, which was deposited during the Pliocene-Pleistocene transition. This organic- and diatom-rich "event" was also found at Site 1238 correlating to the same period.