CARBONATE REACTIONS

The sediments at Sites 998, 999, 1000, and 1001 are characterized by high concentrations of calcium carbonate, with mean values of 77.9, 58.6, 80.6, and 68.1 wt%, respectively. Consequently, the pore-water profiles for a range of species, including calcium, magnesium, strontium, and alkalinity (bicarbonate), are intimately linked to dissolution and precipitation of reactive carbonate phases (e.g., Emerson et al., 1980; Archer et al., 1989; Morse and Mackenzie, 1990). This is particularly well expressed at Site 1000 (Fig. 5), where aragonite dissolution and/or inversion to calcite and deep precipitation of calcium carbonate (for reasons outlined below) appear to be dominant factors. Although not simple relationships, these reactions are reflected at Site 1000 by the general correspondence between dissolved calcium and strontium (Sr is enriched in aragonite); however, the decrease in dissolved strontium deeper in the core is less well understood and may reflect deep precipitation of aragonite or, more likely, celestite precipitation or adsorption onto clay minerals (Shipboard Scientific Party, 1997b). Precipitation of celestite (SrSO₄) is consistent with the high levels of dissolved strontium observed at Site 1000 (Fig. 5) and would contribute to the observed downcore decrease in dissolved sulfate; however, the strontium concentration continues to drop at depths below the attainment of asymptotic pore-water sulfate concentrations. The sulfate profile is provided later in this discussion.

What is perhaps most striking at Site 1000 and most consistent with carbonate reactions occurring within the sediment is the observed relationship between dissolved calcium and magnesium (Fig. 5). Well-developed downcore increases in dissolved calcium and corresponding systematic decreases in magnesium are a common observation in pelagic sediments (Gieskes, 1981, 1983). The lack of this simple inverse relationship at Site 1000 (Fig. 6) has important implications. Simply put, inverse behavior has been linked in past studies to calcium carbonate dissolution, dissolution of volcanic glass, and carbonate and silicate reactions within the sediments (Table 1), including the formation of Mg-rich smectite and dolomite. However, the best defined inverse relationships are often attributed to low-temperature alteration of basement basalt, whereby the observed pore-water profiles dominantly reflect diffusion between the overlying seawater and the underlying basaltic crust.

Extensive effort has been made to distinguish among the various internal (i.e., within the sediments) and external sources and sinks for dissolved calcium and magnesium. For example, McDuff and Gieskes (1976) and McDuff (1981) argued that a comparison between measured values and modeled diffusion profiles would yield an indication of conservative behavior (i.e., extent of reaction within the sediment column) for both calcium and magnesium (see also Lerman, 1975; Kastner and Gieskes, 1976; Gieskes, 1983). Differences in the two profiles can be ascribed to the carbonate and silicate reactions addressed in Table 1 and approximated by deviance from a simple inverse linear relationship between concentrations of dissolved calcium and magnesium (Fig. 6). Clearly, Figure 5 and Figure 6 show that a simple inverse relationship between the two species is not observed at Site 1000, suggesting nonconservative behavior linked at least in part to carbonate diagenesis. Further details and the implications of this observation will be addressed in greater detail below in the "Basaltic Basement/Seawater Reactions" section.

The suggestion that deep calcium carbonate precipitation may be occurring at Site 1000 is well supported by a range of independent observations, including a downcore decrease in alkalinity (Fig. 7; Table 1). This site is unique in a number of respects: (1) there is an abrupt downcore transition between poorly lithified and well-lithified limestone (Fig. 7); (2) there is an interval of pronounced subsurface enrichment in volatile hydrocarbons bounded by the minimum in solid-phase calcium carbonate and the "lithification front" marking the transition into well-lithified limestone (Fig. 7, Fig. 8); (3) there is a downcore increase in TOC content (Fig. 9); and (4) the near zero asymptotic concentration of sulfate corresponds very closely with the top of the carbonate minimum zone (Miocene carbonate "crash") and the top of the hydrocarbon-enriched interval (Fig. 10). Details are provided by the Shipboard Scientific Party (1997b), including extensive discussion addressing the character and origin of the middle/late Miocene carbonate concentration/accumulation "crash," but are summarized as follows.

The enrichment in volatile hydrocarbons is thought to represent trapping of thermogenic gases below a permeability seal caused by the reduction in carbonate content. The external origin of these gases is supported by present burial depths and maximum temperature estimates based on Rock-Eval pyrolysis that argue against sufficient thermal maturation of the local organic matter (Shipboard Scientific Party, 1997b). The enrichments in hydrocarbons (dominated by methane; Fig. 8) and the greater TOC values in this portion of the core may drive enhanced rates of microbial reaction, including sulfate reduction (see Shaw and Meyers, 1996, for a discussion on methane as a carbon source for sulfate reduction). In other words, cementation defining the lithification front could be driven by bicarbonate production resulting from the oxidation of both solid and gaseous carbon compounds (Table 1). The low sulfate concentrations in the hydrocarbon zone (Fig. 10) suggest that in situ methanogenesis may be a factor; however, the broader array of hydrocarbons in the zone (C₂ and greater; Fig. 8) eliminate bacterial production as the only gas source.

The position of the cementation front may be controlled by equilibrium relationships (i.e., saturation states) or kinetic (nucleation) considerations whereby precipitation is favored by the more carbonate-rich substrate beneath the carbonate minimum. The associated cementation would give rise to the observed decreases in dissolved calcium and alkalinity, as well as strontium if aragonite is precipitated (Fig. 5, Fig. 7; Table 1). This model, invoking deep-burial microbial mediation, is speculative and not supported by preliminary ¹³C measurements of the well-cemented limestones (M. Mutti, pers. comm., 1998); however, the isotopic data may be dominated by substrate compositions (i.e., host sediments rather than the inferred "late-stage" cement). Furthermore, the striking stratigraphic correspondence among a diverse range of geochemical parameters is undeniable. It should also be noted that the upward curvature of the sulfate profile and the model results of Table 2 indicate that sulfate reduction is occurring throughout the sediment column, including maximum calculated rates in the upper 50 mbsf. In a more general sense, enhanced carbonate reactivity at Site 1000, in terms of both deep precipitation and aragonite dissolution in the upper layers, agrees with the absence of a linear inverse relationship in Figure 5 and Figure 6, where the downcore decrease in dissolved magnesium may reflect deeper reactions with basaltic crust, but the expected corresponding increase in calcium is buffered by precipitation reactions within the sediment column.