RESULTS

Relevant Shipboard Data

Pore-water chemistry, sediment mineralogy, porosity, and biostratigraphy are similar at Sites 994, 995, and 997. Therefore, all three sites are presented together as one composite section. These sedimentary sections are characterized by fairly homogeneous nannofossil-rich clays and claystones, with overall carbonate content and porosity decreasing with depth. Throughout the sedimentary section, clay minerals, calcite, and quartz are the dominant mineral components, whereas feldspar minerals and pyrite generally only make up a few percent of the bulk mineralogy. As discussed below, dolomite and siderite abundance are depth stratified.

Shipboard measurements (Paull, Matsumoto, Wallace, et al., 1996) of pore-water Ca²⁺, Mg²⁺, and Sr²⁺ concentrations reveal a dramatic decrease in the upper 40 mbsf, whereas alkalinity increases significantly over the same interval (Fig. 2). Sr²⁺ concentrations and Sr/Cl ratios fall below seawater values between 100 and 450 mbsf. This zone of low Sr²⁺ concentration overlaps the sediment zone (~200-450 mbsf) where gas hydrates were detected at these sites (Paull, Matsumoto, Wallace, et al., 1996). There is also a gradual decrease in Mg²⁺ concentrations within this sediment zone. Beneath the base of gas hydrate stability (~450 mbsf), pore-water Ca²⁺ and Sr²⁺ concentrations increase, and alkalinity decreases.

Measurements of alkalinity reach a maximum (as high as 120 mM at ~300 mbsf) within the gas hydrate-bearing section of the sediment column and decrease to values around 30 mM at the base of the hole (Fig. 2). The extremely high alkalinities observed in the gas hydrate-bearing section of the sediment column, and subsequent decrease, may be an artifact of outgassing from sediments containing gas hydrates (Paull et al., Chap. 7, this volume).

Distribution of Carbonate Minerals

Calcite is present throughout the sedimentary section and ranges from approximately 5 to 60 wt% (Paull, Matsumoto, Wallace, et al., 1996). Calcite abundance is greatest in the upper 100 m (average 20 ± 11 wt%) of the sedimentary section and generally decreases below 100 mbsf to the bottom of the hole (average 11 ± 3 wt%). Evaluation of smear slides indicates that the calcite in these sedimentary sections is generally of biogenic origin, consisting primarily of coccoliths and foraminifers (Fig. 3A).

Authigenic carbonates in the sedimentary section exhibit a clear vertical zonation (Table 3; Fig. 4). Dolomite is only present in significant quantities above 100 mbsf, whereas siderite is present in all samples below 150 mbsf. Although trace amounts (<2%) of dolomite are generally present in the upper 20 m, dolomite occurs in significant amounts (up to ~40 wt%) between 20 and 80 mbsf. Routine core descriptions, along with evaluation of both sediment smear slides (both shipboard and postcruise) and thin sections, indicate that dolomite is present as discrete microcrystalline nodules (up to 6 cm in diameter) and disseminated throughout the sedimentary section between 20 and 100 mbsf. SEM observations indicate that both disseminated and nodular dolomite consists of euhedral rhombs (Fig. 3B), and is thus dominantly authigenic. Between 100 and 120 mbsf, dolomite abundance decreases to trace amounts and is absent below 140 mbsf.

On the basis of both smear-slide observations and XRD results, siderite occurrence is sporadic between 100 and 120 mbsf. Below 120 mbsf, disseminated siderite occupies 2-4 wt% of the sediment, or is present as small (<0.5-cm diameter) microcrystalline nodules of siderite occupying up to ~30 wt% (Table 3; Fig. 4; Paull, Matsumoto, Wallace, et al., 1996). As with dolomite, petrographic and SEM observations indicate that both disseminated and nodular siderite contains euhedral, microcrystalline rhombs, and is thus authigenic in origin. Qualitative EDS analysis of siderite throughout the sedimentary section indicates that the siderite contains approximately 10% Ca, 30% Mg, and 60% Fe.

Stable Isotope Studies

Calcite ¹³C values from Sites 994, 995, and 997 range from 1.6 to -10.7 with a mean value of -0.83 ± 1.97 and a median value of -0.15 (n = 125). Calcite ¹⁸O values range from +6.0 to -2.0 with a mean value of 0.69 ± 1.34 and a median of 0.4 (n = 125). The calcite ¹³C and ¹⁸O values (Fig. 5) generally reflect isotopic values typical of biogenic carbonates in equilibrium with seawater (both ¹³C and ¹⁸0 of ~0.2). However, between 20 and 100 mbsf, ¹³C values of calcite are typically more negative (down to -7, with a mean of -1.66 [n = 44]), and ¹⁸O values become positive (up to 6, with a mean of 1.13 ± 1.78 [n = 44]). In addition, calcite associated with some siderite nodules deep in the sedimentary section (~600 mbsf) also exhibit negative ¹³C values (Table 2; Fig. 5A). In these samples, calcite with negative ¹³C values tends to have corresponding positive ¹⁸O values (Fig. 6).

The ¹³C values for dolomite range from approximately -14 to 0 with a mean of -6.2 ± 5.4 (n = 14). The ¹⁸O values of dolomite range from 5.1 to +7.1 with a mean of 2.37 ± 3.3. Only dolomites found within the upper 22 mbsf have negative ¹⁸O values; dolomites found between 22 and 100 mbsf range from 1.0 to 7.1 (Table 2; Fig. 5-Fig. 6).

The ¹³C values of siderite range from approximately 4 to 11 with a mean of 6.9 ± 1.8 (n = 31). Siderite ¹⁸O values range from 2.9 to 7.6 with a mean of 4.9 ± 1.2 (Table 2; Fig. 5-Fig. 6).

The ¹³C values of pore-water DIC range from 10 at 144 mbsf to -38 at 20.4 mbsf, and the ¹³C values of CO₂ (gas) range from 1.1 at 141 mbsf to -32.7 at 48 mbsf. From a maximum at ~140 mbsf, pore-water DIC and CO_{2 (gas)} ¹³C values decrease to ~-4 and -18, respectively (Paull et al., Chap. 7, this volume; Fig. 7).