GEOCHEMISTRY

Volatile Hydrocarbons

As a part of the shipboard safety and pollution program, volatile hydrocarbons (methane, ethane, and propane) were measured in the sediments of Holes 1093A and 1093D using the standard ODP headspace sampling techniques (Table T12; Fig. F18). Headspace methane concentrations at Site 1093 range from 2 to 19 parts per million by volume (ppmv), with a trend toward higher methane at depth. Ethane, propane, and other higher molecular weight hydrocarbons were not observed.

Interstitial Water Chemistry

Shipboard chemical analyses of the interstitial water from Site 1093 followed the procedures for Sites 1088-1092. A total of 64 interstitial water samples were taken from Holes 1093A and 1093D to a depth of 483 mbsf (Table T13; Fig. F19). Samples were taken from every core section (except the bottom section of each core) of the upper 63 mbsf in Hole 1093A; the first two sections of Core 177-1093A-7H were omitted because of split core liners. Sampling in Hole 1093A continued at a frequency of one sample per core to 185 mbsf, and then every other core to 255 mbsf. The sampling from Hole 1093D extends the profile to 498 mbsf, but whole-round samples were only taken where there was sufficient recovery. The depth offset between Hole 1093A and 1093D profiles at around 240-255 mbsf is ~15 m. In many cases, however, truly continuous profiles of dissolved species cannot be constructed from the combination of profiles from the two holes because of the poor recovery in Hole 1093D, and unusual values observed in some chemical species from the bottom of Hole 1093A. Porcellanite fragments were observed in the tops of Cores 177-1093D-28X, 46X, and 50X (see "Lithostratigraphy"). Although probably not in place, these fragments indicate porcellanite layers higher in the section, and these may have contributed somewhat to the poor recovery observed in the intervals from ~260 to 370 mbsf and from ~380 to 425 mbsf.

For almost all the dissolved species, the trends in Site 1093 are remarkably similar to those observed at Site 1091. The high-resolution sampling of the upper 60 mbsf created an especially detailed Cl^- profile that is identical (within analytical uncertainty) to its lower resolution counterpart from Site 1091. Both sites are characterized by a well-defined Cl^- maximum in the interval from 50 to 60 mbsf. If caused by glacial increases in whole-ocean salinity, the similarity in Cl^- peak depth implies comparable diffusivities at the two sites. As at Site 1091, the high sedimentation rate and the presence of diatom mats may be responsible for creating such a distinct Cl^- maximum.

The lack of significant carbonate deposition (except in certain specific horizons; see "Lithostratigraphy") suggests that the major cation profiles are largely independent of carbonate dissolution and recrystallization. In the upper 190 m of the section, both dissolved Ca⁺² and Mg⁺² decrease very slightly, perhaps because of carbonate precipitation driven by increases in alkalinity. Between 190 and 280 mbsf, both Ca⁺² and Mg⁺² concentrations increase abruptly. Neither Cl^- nor salinity increase significantly over the same interval, and there is no distinct change in lithology. We, therefore, have no immediate explanation for the Ca⁺² and Mg⁺² trends between 190 and 280 mbsf, and the profiles must be reproduced with additional shore-based analyses before we can verify these data. However, as it stands, these trends in Ca⁺² and Mg⁺² are responsible for a very large excursion in the Na⁺ profile, which was determined from charge balance calculations. Below ~280 mbsf, Ca⁺² increases and Mg⁺² decreases more unambiguously downhole, with a Mg/Ca slope of about -2. This behavior is characteristic of basaltic ash alteration (Baker, 1986), and is consistent with the volcanic detritus and glass shards observed in smear slides (see "Lithostratigraphy"). Sr⁺² concentrations increase only very slightly to ~400 mbsf, consistent with the general lack of carbonate sediments. However, there is a rather abrupt increase in Sr⁺² below 400 mbsf that accompanies the increase in CaCO₃ observed in the lower part of the section at Site 1093 (see Fig. F20, and "Solid Phase Analysis").

Site 1093 can be classified as slightly reducing based on the consistent presence of H₂S (as judged by scent), except in the deepest part of the section below ~380 mbsf. However, as at Site 1091, sulfate concentrations decrease only gradually throughout the section. In fact, the entire redox environment and history at Site 1093 are apparently very similar to that of Site 1091. In particular, phosphate is characterized by a prominent maximum in the upper 20 mbsf, with a gradual decline below. Alkalinity shows a broad maximum from ~150 to 200 mbsf, and ammonium increases steadily to values of nearly 1500 µM at ~300 mbsf. The Mn⁺² profile largely mimics that of phosphate, whereas Fe⁺² shows a small maximum just below the base of the phosphate maximum. Mn⁺² and Fe⁺² increase in the bottom of the section.

All of these features are shared with Site 1091 (though the scale of the trends may differ between sites in some cases) and must reflect unique conditions associated with the burial of diatom organic matter (see "Geochemistry" in the "Site 1091" chapter). The presence of moderately high sulfate despite rapid sedimentation, and the relatively shallow dissolved phosphate maximum (compared with the maxima in alkalinity and ammonium at depth), suggest that at least two different organic pools are remineralized. One rapidly degradable pool may exist in the uppermost sediments, whereas a second refractory pool, perhaps internal to the opal structure, probably degrades more slowly deeper in the sedimentary column. Furthermore, the significant dissolved Mn and Fe concentrations observed downhole imply substantial mobility, which, in turn, suggests that the redox zone may be migrating and/or changing cyclically with climatically induced fluctuations in the diatom-rain rate.

Solid Phase Analysis

The shipboard solid phase analysis at Site 1093 consisted of measurements of inorganic carbon, total carbon, total nitrogen (TN), and total sulfur (TS) throughout the section cored in Holes 1093A and 1093D (Table T14; Fig. F20; see "Explanatory Notes" chapter for methods). CaCO₃ contents at Site 1093 were generally low, ranging from 0 to 56.9 wt% and averaging ~9.2 wt% throughout the section. Slightly higher CaCO₃ values were observed from 400 to 480 mbsf in the late Pliocene section (Fig. F20).

Total organic carbon (TOC) contents vary between 0.34 and 1.21 wt%, with an average value of 0.77 wt% in the Pliocene-Pleistocene section at Site 1093. This average TOC value is slightly higher than the 0.65 wt% average observed in the sediments from the Pleistocene section at Site 1091, and the 0.43 wt% average in the Pliocene-Pleistocene section at Site 1089. TS concentrations vary between 0 and 1.04 wt%. TN contents range from 0.01 to 0.12 wt%. The lower TN contents observed in Hole 1093D (Fig. F20) are probably an underestimate, because the measurements of these samples were performed under poor analytical conditions (an air leak in the carbon-nitrogen-sulfur analyzer). Therefore, TOC/TN values are likely also overestimated in the sediments at Hole 1093D (Fig. F20). At Hole 1093A, however, TOC/TN values vary between 5.4 and 22.9 with an average of 11.3, which is a value that is intermediate between unaltered algal organic matter (5-8) and fresh land-plant material (25-35) (e.g., Emerson and Hedges, 1988; Meyers, 1994). These higher TOC/TN values suggest that the organic materials in the sediments at Site 1093 may have been derived from both marine and terrigenous sources (Fig. F21). However, significant fluctuations in TOC/TN values covary with both organic carbon and CaCO₃ contents and, because of the analytical problems that are clearly apparent in Hole 1093D samples, additional shore-based analyses are required before any firm conclusions can be drawn. Pyrolysis analyses were not performed because of the relatively organic-carbon-poor nature of the sediments.