RESULTS

Occurrence and Distribution of Authigenic Carbonates

Authigenic carbonate concretions were recovered from various depths in Holes 996A, 996B, 996C, 996D, and 996E (Fig. 2). Zones of poorest sediment recovery at Site 996 coincided with significant decreases in penetration rates during drilling (Paull, Matsumoto, Wallace, et al., 1996), and material recovered from those zones consisted entirely of fragments of indurated carbonates. Lithified carbonates and background sediment samples analyzed for this study are noted as type "C" and "S," respectively, in Table 1, Table 3, and Table 4. The low recovery and the relatively small size of the samples resulted in a certainly incomplete, but nevertheless unique set of samples from a site of active fluid venting.

Petrography

The carbonate-cemented deposits were classified on the basis of their level of internal brecciation and the size and nature of their intraclasts. Although the distinctive characteristics of the carbonate-cemented deposits overlap, they can be separated into three general types. All carbonate nodules (except for one) are composed predominantly of aragonite (Table 1). In addition, generally fibrous or botryoidal aragonite cements have developed in open pore space, either between intraclasts or inside the cavities of biogenic components such as foraminifers or bivalve shells (Fig. 3A, Fig. 4A-4B). Commonly, well-developed idiomorphic hexagonal crystals of aragonite incompletely fill the pore spaces.

The first type of nodule (referred to as "type 1"; Fig. 3A) shows a large number of shell fragments and rounded to subangular, micritic intraclasts (5-20 mm in size). The intraclasts consist of carbonate-cemented siltstones and mudstones with micritic cements and often exhibit a distinctly darker color than the surrounding matrix. Foraminifers and bivalve fragments are a dominant biogenic component of the nodules. Clay minerals and silt-sized quartz, feldspar, and glauconite constitute the clastic and noncarbonate components. The matrix also consists of carbonate-cemented siltstones and mudstones with both micritic and sparitic cements; both cements are composed predominately of aragonite. The microcrystalline carbonate grains never exceed 10 µm and are generally ~2 µm in size. The micrite often exhibits a peloidal texture (Fig. 3B) and did not replace detrital grains. Sparitic aragonite cements occur as radial fibrous and radiating bladed aragonite crystals and fill open pore space between intraclasts. Elongate radiating aragonite crystals are up to 0.55 mm long. Both disseminated and framboidal pyrite is common, frequently filling foraminifer tests (Fig. 3F).

A second type ("type 2") of carbonate precipitate (Fig. 3C) is similar in composition and fabric to type 1 precipitates, but contains fewer intraclasts and more shell debris. The bivalve shells are broken into small pieces (< 1 cm), so that the carbonates have the appearance of cemented shell hash (Fig. 3C). The micritic matrix displays lighter and darker areas, apparently due to differences in carbonate content. Voids between intraclasts and bivalve shells are completely or partially filled with fibrous or bladed aragonite, often showing multiple stages of mineral growth (Fig. 3D). Well-developed radiating crystals of aragonite occur where large pore spaces exist (Fig. 4A-4B).

Finally, a third type of authigenic carbonate ("type 3") has no, or only very few, intraclasts. Fibrous aragonite cements are not developed. As in types 1 and 2, silt-sized quartz, feldspar, glauconite, clay minerals, and foraminifers are the main detrital components. Pyrite is common. Aragonite occurs exclusively as microcrystalline cement, filling the pore space between the detrital sediment grains. Many samples, as well as some of the larger intraclasts, show a distinctly darker rim surrounding a lighter core (Fig. 3E). One carbonate nodule recovered from 51.6 mbsf (Hole 996E) has a similar fabric, but is cemented by dolomite and not aragonite.

Mineralogy

The carbonate content of the cemented sediments ranges from 58.3 wt% to 96.5 wt% CaCO₃ with a mean of 79.7 wt% (Table 1), which is significantly higher than the mean CaCO₃ value of background sediment (~35 wt%; Paull, Matsumoto, Wallace, et al., 1996). Based on these data, we infer that the nodules contain 23-63 wt% carbonate cement. Furthermore, aragonite is more abundant in the carbonate nodules than in the surrounding sediment, confirming the petrographic observation that aragonite is the predominating authigenic mineral at Site 996 (Fig. 5). High-Mg calcite (HMC) only occurs in minor amounts (<10%) and may be detrital in origin. Magnesium content ranges between 9 and 14 mol% MgCO₃. The only exception is Sample 164-996E-6X-CC, 11-13 cm, which is a dolomite nodule with 45 mol% MgCO₃ (Table 1).

Electron microprobe analyses of pure aragonite cements show Sr contents ranging from 5300 to 11,900 ppm (Table 2). Contents of Mg, Mn, and Fe remain below 0.1 wt%. No regular compositional zoning was recognized across the carbonate nodules.

Isotopic Composition of the Carbonates

Stable isotopes of carbon and oxygen were measured on 56 bulk samples (including both carbonate nodules and background sediment) and on 55 individually microdrilled samples from carbonate-cemented matrix, pure aragonitic cements, and from bivalve shells (Table 3, Table 4). The ¹⁸O values of bulk carbonate and sediment samples ranges between 0.2 and 4.6, representing both biogenic (foraminifers and bivalve shells) and authigenic carbonate (Fig. 6). Samples of cemented matrix and pure aragonite cements have ¹⁸O values that vary between 3.3 and 4.9 with a mean of 3.9 (Table 3). The ¹³C composition of nodules ranges from -30.5 to -48.4, and ¹³C values of bulk sediments vary between -0.3 and -33.4. The dolomite nodule from Hole 996E is unique in that it shows ¹³C and ¹⁸O values of -13.1 and -19.2 and 4.8 and 5.4, respectively.

The ⁸⁷Sr/⁸⁶Sr isotopic composition of 13 microdrilled carbonate samples, three bivalve shells, and six pore-water samples was measured from this site. The ⁸⁷Sr/⁸⁶Sr values of nine pore-water samples taken from the upper 65 m at Sites 994, 995, and 997 are reported as well (Table 4). The ⁸⁷Sr/⁸⁶Sr values of the 13 aragonite samples range between 0.709125 and 0.709206 with a mean of 0.709165. The three bivalve shells have a Sr isotopic composition of 0.709085, 0.709141, and 0.709173, respectively. The pore-water ⁸⁷Sr/⁸⁶Sr values vary between 0.709130 and 0.709204. All Sr isotope data are plotted against depth (Fig. 7) and are shown relative to the Sr isotopic reference curve for Neogene seawater after Farrell et al. (1995) and the mean ⁸⁷Sr/⁸⁶Sr ratio of pore water at the BSR (~450 mbsf) in Sites 994, 995, and 997.