DISCUSSION

Diagenetic Zonation of Carbonate Minerals

Evaluation of pore-fluid chemistry and the mineralogical and isotopic composition of carbonate minerals at Sites 994, 995, and 997 reveal three distinct diagenetic zones. (1) In the upper 20 m, carbonate minerals show no evidence of a diagenetic overprint. (2) Between 20 and 80 mbsf, authigenic dolomite and calcite is common. (3) At depths below 100 mbsf, calcite remains a significant sediment component (10 wt%), dolomite abundance decreases to trace amounts, and disseminated authigenic siderite becomes a pervasive carbonate phase (~2-30 wt%). Each of these diagenetic zones will be discussed individually below.

Sulfate Reduction Zone (Upper 20 mbsf)

Calcite is the predominant carbonate phase within this part of the sedimentary section and is primarily biogenic in origin. Although small amounts of dolomite are present sporadically within this zone, their negative ¹⁸O values and the positive ¹³C values (Table 2; Fig. 5-Fig. 6) suggest a detrital source. Increases in pore-water Sr²⁺ concentrations, and Sr/Ca and Mg/Ca ratios through this interval (Fig. 2) could be considered indicative of active carbonate diagenesis consistent with calcite dissolution and reprecipitation (Curtis and Coleman, 1986; Walter, 1986; Table 1). However, these linear pore-water profiles may also represent diffusion gradients between seawater and waters in an underlying "diagenetic zone" (e.g. below 20 mbsf). Sediment smear-slide observations; generally minor variations in mineralogy; calcite stable isotopic values indicative of equilibrium with seawater (rather than pore water); and distinct dolomite ¹⁸O values suggest that authigenic carbonates (at least in concentrations large enough to detect by routine measurements) are absent in the upper 20 m of the sedimentary section.

Sulfate/Methane Interface and Upper Methanogenic Zone

The negative ¹³CCaCO₃ values at, and directly below, ~20 mbsf (the sulfate/methane interface) suggests that significant carbonate precipitation commences at this level (Fig. 5, Fig. 8). Apparently, depletion of sulfate is enhanced by anaerobic methane oxidation (AMO) as methane generated at depth is consumed by reaction with sulfate at the sulfate/methane interface (Borowski et al., 1997; Borowski et al., Chap. 9, this volume). Because the isotopic signal of CH₄ is highly ¹³C-depleted, the bicarbonate produced by the consumption of CH₄ through AMO will be ¹³C-depleted as well. The production of interstitial bicarbonate (HCO₃^-) creates increased alkalinity at this interface, apparently creating an environment especially suited for carbonate precipitation (Fig. 9). Increased concentrations of DIC, decreased Ca²⁺and Mg²⁺ concentrations (Fig. 5; and Paull, Matsumoto, Wallace, et al., 1996), and low ¹³C_DIC at the sulfate/methane interface supports the occurrence of active AMO and carbonate precipitation. Carbonate samples with negative ¹³C values also have heavier ¹⁸O values consistent with authigenic precipitation (Fig. 6).

Between 20 and 80 mbsf, authigenic dolomite occurs as discrete microcrystalline nodules in low abundance as disseminated rhombs within background sediment (Fig. 3B). Our pore-water profiles (Fig. 2) do not fit any single mechanism for dolomite formation (Table 1). This suggests that dolomite formation occurs by more than one mechanism in these sediments. The most common mechanism for dolomite formation (Table 1) in organic-rich continental margin settings is through replacement of pre-existing calcite (Baker and Burns, 1985). The minimal decreases in Ca²⁺ concentrations, combined with continued decreases in Mg²⁺ do suggest that dolomite has replaced calcite (CaCO₃ + Mg²⁺ + 2HCO₃^- CaMg(CO₃)₂ + CO₂ + H₂O). However, the decrease in Mg/Ca and Sr/Ca ratios that would be required if dolomite was forming solely by the replacement of calcite is not observed. The observed increases in Sr/Ca ratios (Fig. 2) are also generally consistent with dolomite formation through direct precipitation from surrounding pore fluids (Table 1; Ca²⁺ + Mg²⁺ + 4HCO₃^- CaMg(CO₃)₂ + 2CO₂ + 2H₂O). The approximately 3:1 decrease in Mg²⁺:Ca²⁺ that is observed on the Blake Ridge (Paull, Matsumoto, Wallace et al., 1996) suggests that dolomite forms by both direct precipitation from surrounding pore fluids, and by replacement of precursor calcite.

The extent to which calcite ¹³C values fall below a baseline established for typical pelagic carbonate ¹³C and ¹⁸O values reflects the contribution of authigenic carbonate to the total carbonate pool (Paull et al., 1992). Assuming that authigenic calcite formed near the present sulfate/methane interface has a ¹³CCaCO₃ value of -36, biogenic calcite has a ¹³CCaCO₃ value of 0, and there are no other contributors to the ¹³CCaCO₃ pool, then ~3%-20% of the calcite between 20 and 50 mbsf (corresponding with measured bulk ¹³CCaCO₃ values ranging from -1.0 to -7.3) was formed near this interface.

The addition of carbon derived from methane within the total carbonate pool is, in part, controlled by the rate at which anaerobic methane oxidation operates at the sulfate/methane interface and the extent of time methane oxidation is acting on a particular sedimentary horizon (Raiswell, 1988). Significant negative excursions in the ¹³C value of carbonates at depth may, therefore, reflect paleo sulfate/methane interfaces where AMO has been focused, leaving an enhanced ¹³C-depleted signature on the bulk carbonate (Fig. 8B, Fig. 9).

The region of the sediment column where anaerobic methane oxidation takes place is associated with conditions that are optimal for carbonate precipitation (e.g., local increases in alkalinity). If sedimentation rates are high, the zone of optimal carbonate precipitation will move relatively rapidly upward through the sediment column. If the sedimentation rates slow, or there is a break in sedimentation, the sulfate/methane boundary will be stabilized at a particular level in the sediment column so that authigenic carbonate will be concentrated at, and just below, this horizon (Raiswell, 1988). Although there is no recognizable hiatus (based on lithologic or biostratigraphic evidence), decreases in the sedimentation rate over the last 6 Ma have occurred on the Blake Ridge (Fig. 10; Paull, Matsumoto, Wallace, et al., 1996). This decrease has enhanced the likelihood that diagenetic carbonates produced at the sulfate/methane interface will be concentrated along a particular horizon (Raiswell, 1988; Hicks et al., 1996). The authigenic dolomite (and calcite), found within this region of the sediment column today, is a probable consequence of the recent decrease in sedimentation rates. Furthermore, the lack of authigenic calcite and dolomite at depths below 80 m, with isotopic signals suggestive of precipitation within this uppermost methanogenic zone, may be a consequence of the higher sedimentation rates during the Pliocene and Miocene.

Gas Hydrate Zone

The gas hydrate-bearing section of the sediment column, and the region immediately above, is characterized by high alkalinity (Fig. 2). In addition, whereas both Ca²⁺ and Mg²⁺ concentrations decrease sharply in the upper 40 m of the sediment column, Mg²⁺ concentrations and Mg/Ca values continue to decrease gradually through the gas hydrate zone (200-450 mbsf), suggesting continued active carbonate diagenesis (Fig. 2). The gradual decrease in Mg²⁺ through this zone is consistent with precipitation of nonstoichiometric siderite. Magnesium and, to a lesser extent, Ca commonly substitute for Fe in marine siderite (Mozley 1989). Furthermore, qualitative EDS analysis and previous studies of siderite from the Blake Ridge (Matsumoto, 1983, 1989) indicates that siderite in this sedimentary section contains both Mg and Ca (with a Fe:Mg:Ca ~ 6:3:1). Continued decreases in Mg/Ca ratios and sharp decreases in Sr/Ca ratios in the region between 100 and 250 mbsf (Fig. 2) suggest that the solutes required for siderite formation are derived from the surrounding pore fluids (Table 1). Between 200 and 450 mbsf (within the gas hydrate zone), a continued decrease in Mg/Ca, coupled with an increase in Sr/Ca ratios, may indicate that siderite formation is also occurring by replacement of precursor calcite (Table 1). However, we see no evidence (from smear-slide, thin section, and SEM observations) that siderite is replacing precursor calcite or dolomite (Table 1; Fig. 2). Moreover, siderite ¹⁸O and ¹³C values are tightly clustered (Fig. 6) and show no isotopic relationship with calcite or dolomite in this section.

It is unclear why the authigenic carbonate minerals change from dolomite to siderite downsection. There is no obvious lithologic change, nor abrupt change in total organic carbon (TOC; Paull, Matsumoto, Wallace, et al., 1996) between the dolomite- and siderite-bearing regions of the sediment column. Although there must be a source of iron for siderite to form, there does not seem to be a clear link between siderite and the minor amounts of pyrite (and/or trace occurrence of glauconite) contained within these sediments. Because clay minerals are the dominant lithologic component in much of the sedimentary section at Sites 994, 995, and 997 (Paull, Matsumoto, Wallace, et al., 1996), the source for Fe may be iron-oxide coatings on clay minerals and other terrigenous materials (Curtis and Coleman, 1986; Matsumoto, 1989; Hicks et al., 1996). The region of the sediment column where siderite is present at the Blake Ridge is also coincident with the rapidly deposited sediments of the Pliocene and Miocene. Several authors have previously suggested that there may be an association between siderite occurrence and high rates of sedimentation (e.g. Berner, 1980; Curtis and Coleman, 1986; Mozley and Carothers, 1992; Mozley and Burns, 1993).

Authigenic siderite throughout these sedimentary sections exhibits ¹³C values that are reasonably consistent with the DIC (and CO₂ [gas]) pools between 120 and ~450 mbsf, which includes much of the gas hydrate-bearing zone (Fig. 7). The ¹³C of the DIC pool reaches a minimum at the sulfate/methane interface and then increases to a maximum between 120 and 150 mbsf, where values are as high as 10. The ¹³C values of CO₂ (gas) also parallel this trend (Fig. 7; Paull et al., Chap. 7, this volume). The ¹³C values of DIC decrease linearly below 150 mbsf to values as low as -5 (and ¹³CCaCO₃ of 18) at 750 mbsf. However, siderite values below ~450 mbsf do not match present-day ¹³C_DIC values. Furthermore, comparison of the ¹⁸O values of siderite from Sites 994, 995, and 997 to calculated equilibrium values of ¹⁸O (Carothers et al., 1988) also indicate that siderite ¹⁸O values are approximately equivalent to the predicted equilibrium values immediately above and within the gas hydrate-bearing section of the sediment column (Fig. 11). The ¹⁸O values of siderite found deeper in the sedimentary section are inconsistent with precipitation beneath the gas hydrate-bearing section of the sediment column. Therefore, the positive ¹³C and ¹⁸O values of these siderites suggest that siderite formation on the Blake Ridge is linked to the high alkalinity associated with gas hydrates in these marine sediments.