INTRODUCTION

One of the objectives of Ocean Drilling Program (ODP) Leg 164 was to establish the diagenetic effects associated with gas hydrate formation and decomposition. Because gas hydrates are not preserved in cores or in exposed outcrops, it is necessary to find diagenetic "fingerprints" (or proxies) to identify sediments that formerly contained gas hydrate. The physical and chemical characteristics of sedimentary sections currently containing gas hydrates provide distinct diagenetic environments that may promote the precipitation and preservation of carbonate minerals. Our study, therefore, focuses on the distribution of carbonate minerals in three gas hydrate-bearing sedimentary sections (Sites 994, 995, and 997) drilled during ODP Leg 164 on the crest of the Blake Ridge, offshore southeastern North America (Fig. 1).

Early diagenesis of marine sediment is commonly dominated by the oxidation of sedimentary organic matter (e.g. Berner, 1980). This degradation of organic matter takes place in a vertical succession of biogeochemical zones, with sulfate reduction, methane oxidation (anaerobic and aerobic methane oxidation), fermentation, and methanogenesis being the dominant diagenetic processes (Claypool and Kaplan, 1974; Berner, 1980; Reeburgh, 1983). In normal marine settings, sulfate is consumed by two principal diagenetic reactions: (1) microbially mediated oxidation of organic matter by sulfate reduction (2CH₂O + SO₄^2- 2HCO₃^- + H₂S), and (2) microbially mediated anaerobic methane oxidation (CH₄ + SO₄^2- HCO₃^- + HS^- + H₂O). These microbially mediated "net" reactions increase alkalinity (by production of bicarbonate [HCO₃^-]) and may stimulate carbonate precipitation (Berner 1980; Baker and Burns, 1985). Furthermore, enhanced carbonate precipitation is predicted at the sulfate/methane boundary where focused anaerobic methane oxidation may produce sharp increases in pore-water alkalinity (Raiswell, 1988; Blair and Aller, 1995). Deeper in the sedimentary section, carbonate precipitation may be associated with methane production and gas hydrate formation (Claypool and Threlkeld, 1983; Kastner et al., 1990) and decomposition (Matsumoto, 1983; Matsumoto, 1989). The resulting carbonate minerals will reflect the isotopic signature of the dissolved inorganic carbon (DIC) pool in which they formed.

Authigenic carbonates, with carbon derived from methane, are known from numerous continental margin localities where methane-rich fluids seep to the sediment-water interface (e.g. Ritger et al., 1987; Hovland et al., 1987; Paull et al., 1992; Jørgensen, 1992; Roberts and Aharon, 1994; Paull et al., 1995; Nähr et al., Chap. 29, this volume). Modern methane-derived carbonates, however, have not been definitively identified in sedimentary sections where diffusive processes dominate.

Diagenetic dolomite is common in methane-bearing continental margin sediments (Baker and Burns, 1985; Kastner et al., 1990) where high alkalinities are common. The calcium and magnesium required for dolomite formation is generally provided by diffusion from overlying seawater and replacement of precursor calcite. Where calcium becomes limited, other carbonate minerals such as siderite or rhodochrosite will form (Baker and Burns, 1985; Compton and Seiver, 1986; Kastner et al., 1990). Furthermore, previous studies of the gas hydrate-bearing sediments on the Blake Ridge suggest that there is a link between formation of authigenic siderite and gas hydrate decomposition (Matsumoto, 1983, 1989).

Siderite (FeCO₃) formation is favored in anoxic environments where activities of dissolved sulfide are extremely low, and iron and bicarbonate activities are high (Taylor and Curtis, 1995); conditions that are present within the upper methanogenic and gas hydrate zones at the Blake Ridge (Paull, Matsumoto, Wallace, et al., 1996). Although a linkage between authigenic siderite formation and gas hydrate occurrence has been made only at the Blake Ridge (Lancelot and Ewing, 1972; Matsumoto, 1989), siderite occurrences are common in the methanogenic zone in other continental margin settings (Mozley and Carothers, 1992; Hicks, et al., 1996; Burns, 1997).

Changes in interstitial fluid compositions, particularly in dissolved Ca²⁺, Mg²⁺, and Sr²⁺ concentrations, are potentially useful indicators of carbonate diagenesis (Kastner et al., 1990). The principal sources for the solutes needed for diagenetic precipitation of carbonates include skeletal calcite (Ca²⁺, Mg²⁺, and HCO₃^-), seawater (Ca²⁺, Mg²⁺), clay minerals (Ca²⁺, Mg²⁺, and Fe²⁺), the oxidation of sulfide minerals (Fe²⁺), and organic matter degradation (HCO₃^-; Curtis and Coleman, 1986). Although the interstitial chemical environment controls carbonate diagenetic processes, the specific mechanism by which calcite, dolomite, or siderite precipitates (or dissolves) will produce predictable changes in interstitial Ca²⁺, Mg²⁺, and Sr²⁺ (Table 1).

The comparison of sediment properties such as mineralogy, texture, chemistry, and stable isotope measurements to current pore fluid chemistry in this study provides insight into the origin of carbonate minerals in the Blake Ridge sediments. Because drilling at Sites 994, 995, and 997 (700-750 mbsf; Paull, Matsumoto, Wallace, et al., 1996) penetrated the gas hydrate-bearing section of the sediment column (200-450 mbsf), well into the underlying gas hydrate-free sediments, the diagenetic environment within each of these diagenetic zones can be evaluated.