BASALTIC BASEMENT/SEAWATER REACTIONS

In contrast to the complexities at Site 1000, the remaining three sites show significantly more robust inverse relationships between calcium and magnesium in the pore waters (Fig. 6), suggesting strong controls linked to alteration of basaltic basement and arguing for sediment pore-water profiles that correspondingly reflect diffusion between the overlying seawater and underlying basaltic crust. This relationship is best expressed in the striking mirror-image profiles for dissolved calcium and magnesium at Site 1001 (Fig. 11). The calcium and magnesium profiles for Site 999 (not shown; see Shipboard Scientific Party, 1997a) and the weaker linear relationship in Figure 6 suggest that sediment reactions may play a more significant role, although nonconservative behavior is undoubtedly a factor at all three sites (see "Alteration of Volcanic Ash" section, below). For example, despite the strong inverse relationship and the mirror-image profiles between calcium and magnesium displayed in Figure 6 and Figure 12, respectively, the upward curvature of the Site 998 profiles suggests that much of the calcium and magnesium variation may be because of reactions in the sediment. Concentrations of dissolved strontium at Sites 998, 999, and 1001 (see Fig. 11, Fig. 12) (Shipboard Scientific Party, 1997a) appear to be controlled by a combination of carbonate reactions, alteration of basaltic basement, and likely reactions involving interbedded and dispersed volcanic ash, although 87Sr/86Sr relationships are required to resolve the relative contributions (Gieskes, 1981, 1983; Gieskes et al., 1998).

In terms of low-temperature interactions between seawater and basaltic basement (see Gieskes, 1981, 1983; Thompson, 1983; Berner and Berner, 1996), the critical reactions include the decomposition (hydrolysis/dissolution) of basaltic glass, calcic plagioclase, and olivine—which liberate Ca2+, Mg2+, Fe2+, bicarbonate, and silica—and the rapid precipitation of smectite that serves as an essential sink for Mg2+ (e.g., Seyfried and Bischoff, 1981; Staudigel and Hart, 1983) (Table 1). It is also certain that the formation of additional silicate phases (clay minerals and zeolites) in association with basalt alteration acts as an important sink (and source) for a wide range of cations. The downcore increase in calcium is the most striking pore-water manifestation of basalt alteration at Site 1001 (Fig. 11), with increases greater than those at Sites 998 and 999 by factors of ~4 and 2.5, respectively. This relationship attests to the strong basement alteration signal in the pore waters at Site 1001 (Fig. 6). The corresponding decrease in alkalinity is thought to reflect the precipitation of calcium carbonate as abundant calcite veins present throughout the altered basalt (Table 1), although mass-balance confirmation was not attempted, and the associated Ca2+ sink is overwhelmed by the basalt source term. General styles of basalt alteration at Site 1001, including the precipitated vein calcite, are documented in Shipboard Scientific Party (1997c).

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