GEOCHEMISTRY

Volatile Hydrocarbons

As part of the shipboard safety and pollution program, volatile hydrocarbons (methane, ethane, and propane) were measured in the sediments of Site 1089 from every core using the standard ODP headspace sampling techniques. Results are presented in Table T14 and Figure F23. Headspace methane (C₁) concentrations increase rapidly from 30 to 19,000 parts per million by volume (ppmv) between 40 and 70 mbsf (Fig. F23). Below this depth, methane concentrations are generally higher and relatively constant. Low concentrations (1 ppmv) of headspace ethane (C₂) were detected between 164 and 239 mbsf. The C₁/C₂ values are extremely high, suggesting that the methane is largely of microbial origin. Higher molecular weight hydrocarbon gases (C₄-C₆) are not observed. Methanogenesis begins at around 50 mbsf, as clearly shown by the sharp increase in methane concentrations and the contemporaneous sharp disappearance of sulfate in interstitial water (Fig. F23). This inverse correlation strongly suggests that the methane results from methanogenic bacterial activity.

Interstitial Water Chemistry

Shipboard chemical analyses of the interstitial water from sediments at Site 1089 included measurements of salinity, pH, alkalinity, chlorinity, calcium, magnesium, sulfate, silica, phosphate, ammonium, strontium, iron, manganese, and lithium (see "Explanatory Notes" chapter for more details on methods). The results from the shipboard analyses are presented in Table T15 and Figure F24. The results represent 17 interstitial water samples from Hole 1089A to a depth of 213 mbsf and eight samples from Hole 1089B from 201 to 259 mbsf; the total of 25 samples is considered to represent a single continuous profile.

Salinity decreases moderately downhole from 34.5 to a single low value of 33 at 259 mbsf. Chlorinity increases by about 0.9% in the uppermost 30 mbsf and then remains relatively constant throughout the rest of the section. The initial increase is significant relative to the precision of the measurement (0.2%) and likely results from downward diffusion of higher salinity waters associated with glaciations (McDuff, 1985). This increase is not observed in salinity because of the poor resolution in the refractometer measurements.

Site 1089 is characterized by reducing sediments, as indicated by the disappearance of sulfate by 50 mbsf, and high methane concentrations deeper in the section (Figs. F23, F24). Hydrogen sulfide was not apparent by smell, but gas bubbles, presumably methane, were often observed during the squeezing process in samples below 50 mbsf.

Sulfate decreases rapidly from 23 mM at 4 mbsf to near zero at 50 mbsf. The reducing conditions associated with sulfate reduction in the upper 50 m of the section exert strong control over the interstitial water profiles of several species measured during shipboard analyses, including alkalinity, Ca⁺², Fe⁺², PO₄^-3, NH₄⁺, and to a lesser degree, Sr⁺² and Li⁺. Alkalinity increases from 10 to 42 mM, ammonium increases from 670 to 2500 µM, and phosphate increases from 67 to 190 µM. Fe⁺² concentrations average ~6 µM in the sulfate reduction zone, but then rise sharply to concentrations averaging ~17 µM in the methanogenic zone (below 50 mbsf). The Fe⁺² values should be interpreted with caution because it is not possible to collect, clean, and squeeze sediment samples for interstitial water analysis under oxygen-free conditions while maintaining timely processing of samples through the shipboard laboratory. Nevertheless, the iron values reported here are probably at least qualitative reflections of in situ interstitial water levels. The rapid increases in alkalinity, ammonium, and phosphate are direct consequences of organic-matter diagenesis associated with sulfate reduction. The presence of measurable Fe⁺², together with the absence of dissolved sulfides, suggests that sedimentation rates at this site are rapid relative to sulfate reduction rates. Therefore, at least some of the reactive iron (oxy)hydroxides are buried throughout the primary iron reduction zone (not sampled here, i.e., between 0 and 4 mbsf) and also through the sulfate reduction zone. The presence of relatively high dissolved Mn⁺² levels of as much as 69 µM in the sulfate reduction zone lends support to the interpretation that burial rates of iron (and manganese) oxides exceed microbial reduction of these metals.

Ca⁺² shows a dramatic decrease in the sulfate reduction zone from 7.9 mM at 4 mbsf (bottom-water Ca⁺² 10.5 mM) to 1.9 mM at 50 mbsf. We suggest two possible reasons for this rapid decrease in Ca⁺² in the sulfate reduction zone: (1) calcite precipitation caused by the rapid increase in alkalinity resulting from sulfate reduction (Kastner et al., 1990), and (2) authigenic gypsum formation, which is commonly associated with reducing conditions (Criddle, 1974; Siesser and Rogers, 1976; Briskin and Schrieber, 1978; Schnitker et al., 1980; all as cited in Rothwell, 1989). Mg⁺² remains relatively constant over the uppermost 90 m at concentrations of ~50 mM, a value not much below the bottom-water Mg⁺² concentrations of ~54 mM. This behavior creates abnormally high Mg/Ca values in the interstitial water (the maximum ratio at 50 mbsf is nearly 30; for comparison, typical Mg/Ca values in Sites 1088 and 1090 reach a maximum of ~5). These high values are atypical of carbonate precipitation, even in sabkha environments where elevated Mg/Ca values (of ~10) are commonly observed. However, Ca⁺² loss through gypsum formation in the upper 50 m is consistent with the continued small increase in alkalinity below 50 mbsf without corresponding increases in phosphate. Careful reinspection of the smear slides from Hole 1089B did, in fact, identify the presence of gypsum in Cores 177-1089B-6H and 7H (see "Lithostratigraphy").

Sr⁺² shows complex behavior in the sulfate reduction zone, with Sr⁺² concentrations of ~75 to 80 µM over the uppermost 30 m, slightly below bottom-water concentrations of ~87 µM. Sr⁺² increases rapidly from 30 to 60 mbsf, and more gradually downhole. The Ca⁺², Mg⁺², and Sr⁺² concentrations below 100 m are consistent with dissolution of biogenic calcite and interaction with basalt. The behavior of Sr⁺² in the uppermost 30 m is more complex, suggesting multiple competing reactions controlling Sr⁺² concentrations in the sulfate reduction zone.

Li⁺ concentrations are constant at 24 µM in the uppermost 50 m, somewhat below bottom-water concentrations of ~27 µM; below 50 mbsf, Li⁺ increases with depth to concentrations of 230 µM at 259 mbsf. The curvature in the profile and the fact that Li⁺ concentrations are less than bottom-water concentrations in the upper 50 m suggest that Li⁺ is consumed in the upper 100 to 150 m of the sediments; clearly there is a deep source, perhaps interaction with basement or alteration of volcanic materials.

Solid Phase Analysis

The shipboard solid phase analysis at Site 1089 consisted of measurements of inorganic carbon, total nitrogen (TN), total carbon, and total sulfur (TS) (see "Explanatory Notes" chapter for methods). The results from Holes 1089A and 1089B are presented in Table T16 and Figure F25. Calcium carbonate (CaCO₃) contents in Hole 1089A range from 0.6 to 69.3 wt%, with an average value of 27.0 wt%. Those in Hole 1089B (the more complete hole) also fluctuate between 0.4 and 64.1 wt%, with an average value of 20.7 wt%. These high-amplitude fluctuations in CaCO₃ contents correspond to alternating intervals of light nannofossil ooze and nannofossil-poor terrigenous sediment (see "Lithostratigraphy"). The results suggest that the variations of CaCO₃ delineate glacial-interglacial cycles in carbonate deposition and preservation since the late Pliocene. CaCO₃ contents are slightly higher above 150 mbsf than deeper in the section. This trend may be related to a gradual increase in sedimentation rates in the upper part of the record.

Total organic carbon (TOC) contents vary between 0 and 0.82 wt%, with an average value of 0.43 wt%. Somewhat higher TOC values (>0.5 wt%) are present above 140 mbsf, in the upper part of Hole 1089B. TN contents are generally low (0.0-0.17 wt%). TS values vary between 0 and 1.82 wt%. TOC/TN values vary between 0.5 and 5.7, indicating a predominance of marine organic material.